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  • Actively controlled local drug delivery using conductive polymer-based devices
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-03
    Christopher A. R. Chapman, Estelle A. Cuttaz, Josef A. Goding, Rylie A. Green

    Localized and actively controlled delivery of drugs presents an opportunity for improving bioavailability, therapeutic efficacy, and long-term treatment of injury or disease. Conductive polymer (CP) based systems present a unique opportunity for using inherent electrochemical and actuating properties to ensure that drugs are delivered or retained using charge controlled mechanisms. A number of CP formats have been explored spanning CP films, composites of CPs with polymeric carriers, and organic electronic ion pumps (OEIPs). Each of these designs can be used to deliver drugs with ionic properties that take advantage of the doping and dedoping characteristics of CPs during electrical pulsing or cycling. However, CP composites that use actuation and OEIPs are emerging technologies that can better address the need for the delivery of a wide range of drugs with varying net charge properties. These systems also allow a high drug loading profile, and with an appropriate configuration, they can use additional electrodes to drive drugs into the tissues. There are also innovative opportunities in the delivery of multiple drug types with varying charge properties that can be individually addressed. The future of CP based drug delivery systems will be strongly influenced by translational challenges including the need for regulatory approvals prior to the use of these novel material platforms in the clinic. Multidisciplinary collaboration will be critical to driving technology development and creating a new paradigm in personalized bioelectronic delivery of therapeutics.

    更新日期:2020-01-10
  • Perspective: Toward highly stable electroluminescent quantum dot light-emitting devices in the visible range
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-06
    Tyler Davidson-Hall, Hany Aziz

    With significant improvements in external quantum efficiency (EQE) and stability for red, green, and blue devices over the past decade, the future of electroluminescent quantum dot light-emitting devices (QDLEDs) is bright. State-of-the-art QDLEDs have achieved >30% EQE and a >2 000 000 h electroluminescence half-life for an initial luminance of 100 cd m−2, rivaling those of organic light-emitting devices. To date, most of the improvements in QDLED performance have been primarily achieved via advancements in QD synthesis and design that aim at reducing Auger recombination and improving the balance between electron and hole concentrations in the emissive QD layer. However, recent work is starting to reveal the critical role that other device layers, as well as interlayer interfaces, play in limiting QDLED stability. Degradation within the organic hole transport layer (HTL) and near the QD/HTL interface has recently been found to lead to the formation of nonradiative recombination centers that quench excitons in the emissive QD layer and contribute to QDLED failure over time. Looking forward, minimizing degradation in the charge transport layers will likely be crucial for the realization of highly stable QDLEDs and this perspective provides potential avenues to achieve these enhancements. In particular, tailoring the QD energy levels via material selection or interfacial dipoles may reduce charge carrier accumulation in the transport layers and replacing the organic HTL with an inorganic alternative may be an effective approach to circumvent the inherent susceptibility of organic semiconductors to exciton-induced degradation.

    更新日期:2020-01-10
  • Orientation-selective elliptic optical vortex array
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    Y. K. Wang, H. X. Ma, L. H. Zhu, Y. P. Tai, X. Z. Li

    We propose an orientation-selective elliptic optical vortex array (OS-EOVA). Using multicoordinate (namely, polar, Cartesian, and elliptic) transformations, three kinds of operations applied on optical vortex elements (including location, rotation, and stretching) were executed to obtain the desired orientation in the observed plane. Then, exploiting the reverse design technique, the above-mentioned operations were mapped onto the initial execution plane via Fourier transform. Based on this, 1D and 2D OS-EOVAs were generated experimentally and the existence of optical vortices was verified. Specific OS-EOVAs were designed, possessing antenna array orientation as well as radial and azimuthal orientation. Compared to existing OVAs, the OS-EOVA provides an additional modulated dimension, i.e., orientation. This technique will open up some potential applications, such as complex manipulation of multiparticle systems and fabrication of micromaterials with orientation.

    更新日期:2020-01-10
  • Temperature profile and transient response of thermally tunable ridge waveguides with laterally supported suspension
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-03
    Fei Duan, Kai Chen, Siyi Wang, Lan Wei, Yonglin Yu, Dayan Ban

    Using the thermoreflectance imaging method, the temperature profile and transient response of thermally tunable ridge waveguides with laterally supported suspension are investigated. This method has a high accuracy in the temperature measurement. The experimental data convincingly confirm a uniform temperature distribution along the waveguide except the initial 30 μm long sections near the two longitudinal edges. The 10%–90% rising time and 90%–10% falling time of the device transient thermal response are also measured to be ∼48 μs and ∼44 μs, respectively, regardless of different waveguide lengths and at different heating powers. In addition, the delay time of the waveguide transient thermal response is revealed to be 1.3 μs by comparison between experiment and simulation.

    更新日期:2020-01-10
  • Ultra-high sensitive trace gas detection based on light-induced thermoelastic spectroscopy and a custom quartz tuning fork
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-07
    Yufei Ma, Ying He, Pietro Patimisco, Angelo Sampaolo, Shunda Qiao, Xin Yu, Frank K. Tittel, Vincenzo Spagnolo

    A highly sensitive trace gas sensor based on light-induced thermoelastic spectroscopy (LITES) and a custom quartz tuning fork (QTF) is reported. The QTF has a T-shaped prong geometry and grooves carved on the prongs' surface, allowing a reduction of both the resonance frequency and the electrical resistance but retaining a high resonance quality factor. The base of the QTF prongs is the area maximizing the light-induced thermoelastic effect. The front surface of this area was left uncoated to allow laser transmission through the quartz, while on the back side of the QTF, a gold film was coated to back-reflect the laser beam and further enhance the light absorption inside the crystal. Acetylene (C2H2) was chosen as the target gas to test and validate the LITES sensor. We demonstrated that the sensor response scales linearly with the laser power incident on the prong base, and the optimum signal to noise ratio was obtained at an optical power of 4 mW. A minimum detection limit of ∼325 ppb was achieved at an integration time of 1 s, corresponding to a normalized noise equivalent absorption coefficient of 9.16 × 10−10 cm−1W/√Hz, nearly one order of magnitude better with respect to the value obtained with a standard 32.768 kHz QTF-based LITES sensor under the same experimental conditions.

    更新日期:2020-01-10
  • Acoustoelectric charge transport at the LaAlO3/SrTiO3interface
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-03
    Y. Uzun, A. E. M. Smink, M. P. de Jong, H. Hilgenkamp, W. G. van der Wiel

    The two-dimensional electron system (2DES) formed at the interface of LaAlO3 (LAO) and SrTiO3 (STO), both band insulators in bulk, exhibits properties not easily attainable in conventional electronic materials. The extreme shallowness of the 2DES, only a few nanometers below the surface, opens up unique possibilities such as tunneling spectroscopy, local electronic sensing, and in situ patterning by manipulating the surface properties. It is particularly tempting to manipulate the charge carriers with surface acoustic wave (SAW) phonons, which are confined to the surface. However, the absence of intrinsic piezoelectricity in both LAO and STO complicates the electric generation of SAWs, as well as the induction of an acoustoelectric current. Here, we present robust acoustoelectric coupling between SAWs and the LAO/STO 2DES by using electrostriction in STO, induced by a dc electric field. Electromechanical coupling to the carriers is provided by phonon-induced modulation of the 2DES potential well, leading to SAW-induced carrier transport. The ability to control charge carriers with SAWs brings the versatile LAO/STO 2DES into reach of quantum acoustics, opening possibilities to study the interplay of nanoscale mechanical waves and the rich physics exhibited by nonpiezoelectric complex oxides, including superconductivity, magnetism, and correlated insulator states.

    更新日期:2020-01-10
  • Singularities and topologically protected states in twisted bilayer graphene
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-08
    Qirong Yao, Xingchen Chen, Rik van Bremen, Kai Sotthewes, Harold J. W. Zandvliet

    We have studied the structural and electronic properties of twisted bilayer graphene by scanning tunneling microscopy (STM). For twist angles in the range of about 1° to 4°, the twisted bilayer graphene possesses two Van Hove singularities in the vicinity of the Fermi level. We use the exact location of these Van Hove singularities to determine the twist angle dependent interlayer hopping energy. For a twist angle of 0.6°, we found a hexagonal network of topologically protected one-dimensional channels that run along the boundaries of the AB/BA domains. The electric field in the tunnel junction is responsible for the breaking of the symmetry of the AB and BA domains and the development of the hexagonal network of topologically protected states. The latter shows that the electric field in the tunneling junction can significantly affect the topological nature of two-dimensional materials, and therefore, one should be cautious when interpreting scanning tunneling microscopy and spectroscopy experiments of this class of materials.

    更新日期:2020-01-10
  • Rare-earth and precious-metal free Cu-based metallic glasses with superior glass-forming ability and processability
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    Jaskaran Singh Saini, Collin Palian, Fuqiong Lei, Alexander Dyall, Nicholas AuYeung, Ryan McQuade, Sanu K. Gupta, David P. Cann, Donghua Xu

    Metallic glasses have many fascinating properties such as theoretical-limit approaching strength, high wear and corrosion resistance, and extremely smooth surfaces enabling high-performance thin film diodes. Copper-based metallic glasses are of particular interest due to the low cost and attractive characteristics of the base element, but the copper-based metallic glasses known to date rely on rare-earth or precious metals that have limited supplies and relatively high cost, in order to achieve good glass-forming ability. Here, we report our discovery of a series of rare-earth and precious-metal free copper-based metallic glasses formulated as Cu46Zr47−xHfxAl7 (8 ≤ x ≤ 20, at. %), which possess superior glass-forming ability and processability. Fully amorphous ingots with a large cross section of 15 (all x) to 28.5 mm (x = 13.5) diameter can be readily produced by a simple water quenching method. The Cu46Zr33.5Hf13.5Al7 alloy, in particular, presents the highest glass-forming ability and processability of all known copper-based metallic glasses. The adjustable Hf content in the composition offers a range of physical properties (e.g., density) that can be chosen based on applications. We also discuss the physical origin of the superior glass-forming ability and processability in these copper-based metallic glasses.

    更新日期:2020-01-10
  • Lateral straggling of implanted aluminum in 4H-SiC
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    J. Müting, V. Bobal, T. Neset Sky, L. Vines, U. Grossner

    The implantation of aluminum into 4H-SiC is studied using secondary ion mass spectrometry. In particular, two-dimensional concentration profiles are obtained, which allow the investigation of lateral straggling and its dependence on the crystallographic orientation. A high dose, medium energy aluminum implantation is studied in great detail. It shows an asymmetric distribution due to the 4 °-off axis growth of the epitaxial layer. The lateral straggling is found to be in the range of several micrometers for a concentration of 1 × 10 15   cm − 3, which is contrary to the expectation given by most simulation studies. Implantations performed at different orientations support the idea that lateral straggling highly depends on the particular channeling opening.

    更新日期:2020-01-10
  • Artificial synaptic transistor with solution processed InOxchannel and AlOxsolid electrolyte gate
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    Xiaoci Liang, Zhenwen Li, Ling Liu, Shujian Chen, Xinzhong Wang, Yanli Pei

    In this Letter, three-terminal transistor-based artificial synapses are proposed that are simply constructed with a solution-processed InOx channel and AlOx electrolyte gate. Paired pulse facilitation and short-term potentiation (STP) are realized and modulated by adjusting the amplitude, duration, and interval time of the spiking pulses. Furthermore, the STP is transferred to long-term potentiation (LTP) by increasing the pulse amplitude and number. In addition, spike-timing-dependent plasticity is demonstrated. The high density hydrogen in low temperature processed AlOx is adsorbed on InOx electrostatically or electrochemically, which plays a role in synaptic behaviors. This study provides useful information to understand neuromorphic devices based on solution processed oxide dielectrics and oxide semiconductors.

    更新日期:2020-01-10
  • Development of 4H-SiC Schottky np diode with high blocking voltage and ultralow on-resistance
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-03
    Kazutoshi Kojima, Hajime Okumura

    A Schottky np diode (SNPD) was fabricated on a 4H-SiC C-face epitaxial wafer, and its I-V characteristics were investigated. The diode showed a high blocking voltage of 300 V and ultralow on-resistance of 0.18 m Ω cm2 at a forward bias of 2.4 V. This value is almost the same as the resistance of the 4H-SiC bulk substrate, indicating that the resistance of the drift layer is almost zero and does not contribute to the observed on-resistance. As the temperature was increased, the forward I-V curve moved in parallel as the built-in bias shifted to a lower voltage due to the reduction in the barrier height of the np junction. This means that the resistance of SNPD above built-in bias is the same regardless of temperature. These results suggest that the diode is different from the conventional Schottky diodes and pn diodes. This diode has great potential for devices with both ultralow on-resistance and high blocking voltage.

    更新日期:2020-01-10
  • Bowing of transport gap in hybrid halide perovskite alloys (CH3NH3Sn1−xPbxI3): Which band is responsible?
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-03
    Salma Khatun, Abhishek Maiti, Amlan J. Pal

    Apart from being a unique material for efficient solar cells, hybrid halide perovskites possess more mysteries than ever. An anomalous bandgap behavior in CH3NH3Sn1−xPbxI3 alloys has been reported recently [Hao et al., J. Am. Chem. Soc. 136, 8094 (2014)], in which the composition-dependent optical bandgap follows nonmonotonic and nonlinear characteristics instead of a linear trend or Vegard's law; the bandgap of the intermediate compounds was lower than that of the end members. In this article, we study composition-dependent conduction and valence band energies through scanning tunneling spectroscopy to deliberate on the role of the two bands in the bandgap bowing phenomenon and the underlying mechanism. We observe a nonlinear behavior of the two bands with respect to the alloy composition, leading to an anomalous behavior in the transport gap as well. We confirm that two competing events, namely, a spin–orbit coupling parameter appearing due to inclusion of a high-Z material and structural distortion affecting molecular orbitals responsible for the formation of the valence and the conduction bands, result in bandgap bowing in CH3NH3Sn1−xPbxI3 alloys.

    更新日期:2020-01-10
  • Charge transfer dynamics of the CdTe quantum dots fluorescence quenching induced by ferrous (II) ions
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-06
    Licai Hao, Xuanhu Chen, Dongyang Liu, Yue Bian, Weikang Zhao, Kun Tang, Rong Zhang, Youdou Zheng, Shulin Gu

    Size dependence of glutathione capped CdTe quantum dots (GSH-CdTe QDs) on the sensitivity and selectivity in the fluorometric detection of ferrous (II) ions (Fe2+) has been systematically investigated. Smaller-size QDs show higher sensitivity in the detection of Fe2+, resulting in higher quenching efficiency and red shift of the fluorescence peak of QDs. Stern–Volmer plots indicate that the charge transfer model can be employed to account for the observed fluorescence quenching effect. Fe2+ is bound to the surface of QDs by GSH and excited electrons are transferred from QDs to Fe2+, which facilitates a nonradiative recombination process and a decrease in the PL efficiency. In addition, the results from time resolved photoluminescence and a confocal scanning fluorescence microscope have shown that smaller-size QDs have a faster decrease in the fluorescence lifetime compared with that of larger-size QDs with Fe2+ addition, suggesting that the fast charge transfer in smaller-size QDs should be responsible for the observed fluorescence quenching effect. This Letter provides a comprehensive understanding of the mechanism of the fluorescence for the CdTe QDs quenched by Fe2+.

    更新日期:2020-01-10
  • Web buckle-mediated room-temperature ferromagnetism in strained MoS2thin films
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    Hongtao Ren, Lei Zhang, Gang Xiang

    Ferromagnetism of two-dimensional (2D) materials mediated by strain engineering has been extensively studied in theoretical calculations. However, due to the difficulty of introducing strain into 2D materials, experimental research has always been a challenge. We have fabricated MoS2 thin films using polymer assisted deposition and have observed strain-induced ferromagnetism in buckled MoS2 films. After buckling, the saturated magnetization (Ms) of buckled films at 300 K (0.486 emu·g−1) is enhanced 7.5 times compared to that of flat films (0.065 emu·g−1), while the linear temperature coefficient (χT) of buckled MoS2 films for E12g mode of Raman spectra is reduced to one third. Our results suggest that biaxial tensile strain plays a significant role in modulating magnetism, which may provide a feasible way for the fabrication and study of strain-related spintronic devices.

    更新日期:2020-01-10
  • Thermomagnetic control of spintronic THz emission enabled by ferrimagnets
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    Mario Fix, Robert Schneider, Jannis Bensmann, Steffen Michaelis de Vasconcellos, Rudolf Bratschitsch, Manfred Albrecht

    In this Letter, we present a material system with two ferrimagnetic GdxFe100-x layers where the relative orientation of the Fe magnetic moments can be set by temperature in the presence of an external magnetic field. We demonstrate that, depending on the relative alignment of the Fe moments, the spintronic emitter system can be either in a high- or in a low-amplitude terahertz emitting state. Nonmagnetic metal layers with opposite spin Hall angles were utilized for further improvement of the efficiency. This study opens a route for an efficient type of spintronic terahertz emitter system based on the ferrimagnetic properties of rare earth-3d transition metal alloys, which allows switching the emission state from high to low power.

    更新日期:2020-01-10
  • Field driven recovery of the collective spin dynamics of the chiral soliton lattice
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    F. J. T. Goncalves, Y. Shimamoto, T. Sogo, G. W. Paterson, Y. Kousaka, Y. Togawa

    We investigate the magnetic field dependence of the spin excitation spectra of the chiral soliton lattice (CSL) in the helimagnet CrNb 3 S 6, by means of microwave resonance spectroscopy. The CSL is a prototype of a noncollinear spin system that forms periodically over a macroscopic length scale. Following the field initialization of the CSL, we found three collective resonance modes over an exceptionally wide frequency range. Upon further reducing the magnetic field toward 0 T, the spectral weight of these collective modes was disrupted by the emergence of additional resonances whose Kittel-like field dependence was linked to coexisting field polarized magnetic domains. The collective behavior at a macroscopic level was only recovered upon reaching the helical magnetic state at 0 T. The magnetic history of this noncollinear spin system can be utilized to control microwave absorption, with potential use in magnon-driven devices.

    更新日期:2020-01-10
  • Detection of spin-orbit torque with spin rotation symmetry
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-06
    Tao Wang, Sergi Lendinez, M. Benjamin Jungfleisch, James Kolodzey, John Q. Xiao, Xin Fan

    It has been recently demonstrated that spin–orbit coupling in ferromagnetic metals can generate spin current with symmetries different from the conventional spin Hall effect in nonmagnetic metals. The generated spin current can induce a spin–orbit torque on a neighboring magnetic layer with spin rotation symmetry. In this paper, we introduce a set of tools to measure this effect in a perpendicularly magnetized film, by using the second-order planar Hall effect method and spin-torque ferromagnetic resonance spectroscopy. These results are comparable to those detected by the polar magneto-optic Kerr effect technique.

    更新日期:2020-01-10
  • Long-range coupling interaction between a non-magnetic transition metal capping layer and a neighboring magnetic layer
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-08
    L. Wang, W. T. Xu, W. L. Zhao, G. Li, Y. X. Huang, A. X. Li, Y. M. Liu

    In nanoscale magnetic multilayers, capping layers are often used to protect the underlying magnetic layers from oxidation. However, little research has investigated possible long-range coupling interactions between nonmagnetic transition metal (TM) capping layers and neighboring magnetic layers. In this paper, the temperature (T) dependence of the magnetic moment of different thicknesses of cobalt (Co) was studied in a tantalum (Ta)/Co/TM trilayer structure with four TM capping layers, where the TMs were Ta, Chromium (Cr), titanium (Ti), and zirconium (Zr), respectively. It was found that the capping layer had a large effect on the phase-transition behavior and thermal stability of the Co layer. In the Ta and Cr layers, the T-dependence of Co magnetic moment showed nonmonotonic behavior, and in the Ti and Zr layers, the Co M-T curve exhibited very few effects of the capping layer. We attribute this phenomenon to the long-range coupling between the Co and TM layers. Furthermore, the coupling mechanism was linked to the indirect magnetic exchange coupling in Co/TM multilayers, similar to the Ruderman-Kittel-Kasuya-Yoshida coupling. The results of this work will support further development of the understanding of the coupling between the 3d ferromagnetic (FM) metal and nonmagnetic TM at nanoscales. Relative to potential applications, it will inspire us to rediscover the role of both the TM capping layer and buffer nonmagnetic layer in FM/TM multilayers, especially for nanoscale magnetic multilayers with spin-dependent effects, such as spin valves, spin halls, spin transfer torque, and spin–orbit coupling, which are in widespread use in the manufacture of various spintronics devices.

    更新日期:2020-01-10
  • Role of temperature-dependent electron trapping dynamics in the optically driven nanodomain transformation in a PbTiO3/SrTiO3superlattice
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-09
    Joonkyu Park, Youngjun Ahn, Jack A. Tilka, Hyeon Jun Lee, Anastasios Pateras, Mohammed H. Yusuf, Matthew Dawber, Haidan Wen, Paul G. Evans

    The spontaneously formed striped polarization nanodomain configuration of a PbTiO3/SrTiO3 superlattice transforms to a uniform polarization state under the above-bandgap illumination with a time dependence varying with the intensity of optical illumination and a well-defined threshold intensity. The recovery after the end of illumination occurs over a temperature-dependent period of tens of seconds at room temperature and shorter times at elevated temperatures. A model in which the screening of the depolarization field depends on the population of trapped electrons correctly predicts the observed temperature and optical intensity dependence.

    更新日期:2020-01-10
  • Monolayer GaN excitonic deep ultraviolet light emitting diodes
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    Y. Wu, X. Liu, P. Wang, D. A. Laleyan, K. Sun, Y. Sun, C. Ahn, M. Kira, E. Kioupakis, Z. Mi

    We report on the molecular beam epitaxy and characterization of monolayer GaN embedded in N-polar AlN nanowire structures. Deep ultraviolet emission from 4.85 to 5.25 eV is measured by varying the AlN barrier thickness. Detailed optical measurements and direct correlation with first-principles calculations based on density functional and many-body perturbation theory suggest that charge carrier recombination occurs predominantly via excitons in the extremely confined monolayer GaN/AlN heterostructures, with exciton binding energy exceeding 200 meV. We have further demonstrated deep ultraviolet light-emitting diodes (LEDs) with the incorporation of single and double monolayer GaN, which operate at 238 and 270 nm, respectively. These unique deep ultraviolet LEDs exhibit highly stable emission and a small turn-on voltage around 5 V.

    更新日期:2020-01-10
  • Broadband modulation of subwavelength topological interface states in a one-dimensional acoustic system
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    Zhenyu Wang, Degang Zhao, Jinlong Luo, Rongli Wang, Hai Yang

    The acoustic analogy of topological insulators is a hot field of materials research. On one-dimensional acoustic systems, many researchers have lately paid their attention to the applications of the Su-Schrieffer-Heeger (SSH) model, which can support topologically nontrivial phases. In this paper, we design a supercell composed of two identical hollow cylinders with a side split immersed in the air background. The supercell is arranged in a line to form a SSH model, which has three bandgaps including two zone-folding-induced gaps and a local resonant gap in the subwavelength region. By analyzing the eigenstates and calculating the Zak phases, we find that a topological phase transition takes place only in the zone-folding-induced gaps when we rotate the split-cylinders. Thus, a finite-size array, made of two sublattices with distinct topological properties, inevitably produces topological interface states on their interface. In addition, we demonstrate that the topological interface states can be adjusted in a wide frequency range by rotating the cylinders that control the coupling strength between two split-cylinders in one supercell. These tunable topological interface states may have potential application prospects in wave filtering, wave detecting, and so on.

    更新日期:2020-01-10
  • Role of finite-size effect in BiFeO3nanoparticles to enhance ferromagnetism and microwave absorption
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-03
    Yang Hong, Jun Li, Han Bai, Zhenjia Song, Guangmao Li, Ming Wang, Zhongxiang Zhou

    Exploring the performance of nanoscale multiferroic materials is a significant challenge because the physical properties of these materials vary dramatically on this scale. In the present work, BiFeO3 (BFO) nanoparticles are synthesized by a facile sol-gel method, producing homogeneous spherical nanoparticles whose Raman modes and binding energies coincide with pure rhombohedral perovskite BFO. The dramatic magnetic properties of nanoscale BFO with diameters in the range of 50–130 nm are explored by analyzing the temperature and field dependence of magnetization, and detailed zero-field cooling and field cooling magnetization curves show that interparticle interactions play a constructive role in increasing the magnetic response. Moreover, BFO nanoparticles have two absorption regions in the range of 2–18 GHz, and the minimum reflection loss is −18 dB. The finite-size effect is discussed as the primary mechanism for enhancing the ferromagnetism and microwave absorption, and the results provide a feasible route for designing multifunctional materials.

    更新日期:2020-01-10
  • Sensing anisotropic stresses with ferromagnetic nanowires
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-06
    L. Forzani, A. M. Gennaro, R. R. Koropecki, C. A. Ramos

    We have measured the temperature variation of the magnetic anisotropy of Ni nanowires (Ni NWs) embedded in freestanding porous anodized aluminum oxide membranes, using DC magnetometry and ferromagnetic resonance. Both techniques show a significant reduction of the uniaxial anisotropy with decreasing temperature. This decrease can be explained by magnetoelastic effects, as Ni NWs are subjected to stress due to the difference in thermal expansion coefficients between the nanocomposite materials. Matching our experimental findings with previously measured thermal strains along the Ni NW axis led us to estimate the perpendicular stress. Thus, we postulate the Ni NWs as nanometric differential stress sensors.

    更新日期:2020-01-10
  • Coulomb blockade in monolithic and monocrystalline Al-Ge-Al nanowire heterostructures
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-07
    M. Sistani, J. Delaforce, K. Bharadwaj, M. Luong, J. Nacenta Mendivil, N. Roch, M. den Hertog, R. B. G. Kramer, O. Buisson, A. Lugstein, C. Naud

    We report the realization of Ge single-hole transistors based on Al-Ge-Al nanowire (NW) heterostructures. The formation of these axial structures is enabled by a thermally induced exchange reaction at 350 °C between the initial Ge NW and Al contact pads, leading to a monolithic and monocrystalline Al-Ge-Al NW. The 25 nm-diameter Ge segment is a quasi-1D hole channel. Its length is defined by two abrupt Al-Ge Schottky tunnel barriers. At low temperatures, the device shows a single hole transistor signature with well pronounced Coulomb oscillations. The barrier strength between the Ge segment and the Al leads can be tuned as a function of the gate voltage VG. It leads to a zero conductance at VG= 0 V to a few quantum conductance at VG= –15 V. When the gate voltage increases from –5 V to –3 V, the charging energy is extracted and it varies from 0.39 meV to 2.42 meV.

    更新日期:2020-01-10
  • Two-dimensional high-quality Ag/Py magnetoplasmonic crystals
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-10
    A. R. Pomozov, A. L. Chekhov, I. A. Rodionov, A. S. Baburin, E. S. Lotkov, M. P. Temiryazeva, K. N. Afanasyev, A. V. Baryshev, T. V. Murzina

    We demonstrate a perspective approach for the fabrication of functional high-quality magnetoplasmonic crystals based on a 2D periodical perforated silver film covered by a thin layer of ferromagnetic metal (Permalloy). The wavelength-angular spectra of the 2D crystals reveal a large number of high-quality resonant features associated with the excitation of surface plasmon-polariton modes of various orders. Due to the presence of a ferromagnetic material on both plasmonic interfaces, pronounced magnetic effects are observed for all excitations and are influenced by the coupling between various modes. The suggested magnetoplasmonic crystal composition with high-quality resonant optical and magneto-optical properties gives perspective for the control over the light propagation as well as for sensor applications.

    更新日期:2020-01-10
  • Unique resistive switching phenomena exhibiting both filament-type and interface-type switching in Ti/Pr0.7Ca0.3MnO3−δ/Pt ReRAM cells
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    Naoki Kanegami, Yusuke Nishi, Tsunenobu Kimoto

    The current-voltage characteristics of resistive random-access memory cells with Ti/Pr0.7Ca0.3MnO3−δ (PCMO)/Pt stack structures were investigated. The PCMO layer on Pt had a mixed polycrystalline and amorphous structure. The cells displayed interface-type and filament-type resistive switching (RS) depending on the PCMO layer thickness. The interface-type RS was attributed to the migration of oxygen ions, which caused a redox reaction at the Ti/PCMO interface and the formation of a TiOx layer. For filament-type RS, a forming process occurred and this indicated the formation of a conductive filament in the PCMO layer. After forming, the cells showed bipolar and continuous RS similar to interface-type RS. This indicated that both the formation of a conductive filament in the PCMO layer and the redox reaction at the Ti/PCMO interface occur in the same cell. Finally, a qualitative model for the observed RS phenomenon is discussed based on conventional interface-type RS.

    更新日期:2020-01-10
  • Hydrogen behavior under X-ray irradiation for a-IGZO thin film transistors
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    Dong-Gyu Kim, Tae-Kwon Lee, Kwon-Shik Park, Youn-Gyoung Chang, Kyong-Joo Han, Duck-Kyun Choi

    We studied hydrogen (H) behavior in amorphous In-Ga-Zn-O (a-IGZO) films under X-ray irradiation by evaluating the threshold voltage (VTH) shift in a-IGZO thin film transistors (TFTs) with different H concentrations in the active layers. We fabricated three types of a-IGZO TFTs: (i) one without a buffer layer and postannealed in N2, (ii) one with a H-resolved buffer layer and postannealed in N2, and (iii) one with a H-resolved buffer layer and postannealed in a mixture of N2 and H2. All three TFTs showed a negative VTH shift after 100 Gy of X-ray exposure. The degree of VTH shift correlated with an increase in conductivity, which, in turn, corresponds to the H concentration in the active layer of the as-fabricated TFTs. Based on spectroscopic ellipsometry analysis, we confirmed a large increase in the donorlike H related D1 state after X-ray irradiation in high-H concentration a-IGZO films. In addition, an increase in the number of H2 molecules in a-IGZO films after X-ray irradiation was observed via thermal desorption spectroscopy analysis. Therefore, we conclude that the increase in conductivity and/or the resulting negative VTH shift in a-IGZO TFTs during X-ray irradiation can be attributed not only to the state transition from acceptorlike to donorlike H in the as-prepared a-IGZO but also to the incorporation of additional H radicals generated by X-ray irradiation.

    更新日期:2020-01-10
  • High temperature (300 °C) ALD grown Al2O3on hydrogen terminated diamond: Band offset and electrical properties of the MOSFETs
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    Zeyang Ren, Dandan Lv, Jiamin Xu, Jinfeng Zhang, Jincheng Zhang, Kai Su, Chunfu Zhang, Yue Hao

    Hydrogen-terminated diamond (H-diamond) metal-oxide-semiconductor field effect transistors (MOSFETs) were fabricated on a polycrystalline diamond substrate. The device has a gate length of 2 μm and uses Al2O3 grown by atomic layer deposition at 300 °C as a gate dielectric and passivation layer. The Al2O3/H-diamond interfacial band configuration was investigated by X-ray photoelectron spectroscopy, and a large valence band offset (3.28 eV) that is very suitable for p-channel H-diamond FETs was observed. Meanwhile, the measured O/Al ratio hints that there are Oi or VAl defects in the Al2O3 dielectric, which can work as an acceptorlike transfer doping material on a H-diamond surface. The device delivers the maximum saturation drain current of over 200 mA/mm, which is the highest for 2-μm H-diamond MOSFETs with the gate dielectric or passivation layer grown at 300 °C or higher temperature. The ultrahigh on/off ratio of 1010 and ultralow gate leakage current of below 10−12 A have been achieved. The high device performance is ascribed to the ultrahigh carrier density, good interface characteristics, and device processes. In addition, the transient drain current response of the device can follow the gate voltage switching on/off pulse at a frequency from 100 kHz to 1 MHz, which indicates the potential of the H-diamond FETs in power switch applications.

    更新日期:2020-01-10
  • Demonstration of a tunable antenna-coupled intersubband terahertz (TACIT) mixer
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    C. Yoo, M. Huang, J. H. Kawamura, K. W. West, L. N. Pfeiffer, B. S. Karasik, M. S. Sherwin

    A fast, voltage-tunable terahertz mixer based on the intersubband transition of a high-mobility 2-dimensional electron gas has been fabricated from a single 40 nm GaAs-AlGaAs square quantum well heterostructure. The device is called a Tunable Antenna-Coupled Intersubband Terahertz mixer and shows tunability of the detection frequency from 2.52 to 3.11 THz with small (<1 V) top gate and bottom gate voltage biases. Mixing at 2.52 THz has been observed at 60 K with a −3dB intermediate frequency bandwidth exceeding 6 GHz.

    更新日期:2020-01-10
  • Low-barrier Mott diodes with near-surface polarization-inducedδ-doping
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    N. V. Vostokov, M. N. Drozdov, O. I. Khrykin, P. A. Yunin, V. I. Shashkin

    The possibility of a controlled decrease in the effective height of the Schottky (Mott) barrier to the AlGaN/GaN (Ga-face polarity) heterostructure due to the modification of the shape of the barrier by the electric field of the polarization charge arising in the plane of the heterojunction because of the jump in electric polarization is experimentally shown. A decrease in the effective barrier height is related to an increase in the role of electron tunneling through the barrier. The effective barrier height can be controlled by varying the thickness and chemical composition of the AlGaN layer and choosing the metal of the barrier contact. Test low-barrier Mott Ti/AlGaN/GaN diodes demonstrating high values of the ampere-watt sensitivity (9 A/W) for a low specific differential resistance (4 × 10–4 Ω⋅cm2) at zero bias have been manufactured.

    更新日期:2020-01-10
  • Characteristic investigation of highly oriented Hf0.5Zr0.5O2thin-film resistive memory devices
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-06
    Xiaobing Yan, Zuoao Xiao, Chao Lu

    In this work, characteristics of highly oriented Hf0.5Zr0.5O2 (HZO) thin-film resistive memory devices are investigated. The x-ray diffraction analysis indicates that the (−111) plane is the preferred orientation of HZO films, which is consistent with the prediction of two-dimensional crystal nucleus theory. Compared with semirandom HZO thin-film devices, the highly oriented (−111) HZO film exhibits excellent resistive switching behavior and superior retention time of up to 105 s with negligible performance degradation. Besides, highly oriented (−111) HZO films show a lower threshold of switching voltage, faster response time, and multilevel storage capability. Furthermore, the highly oriented (−111) HZO films can achieve better biosynaptic functions and plasticity. This study reveals that controlling the orientation of HZO thin films can promote and facilitate high-quality resistive memory devices.

    更新日期:2020-01-10
  • Selective ablation of cancer cells with low intensity pulsed ultrasound
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-07
    David R. Mittelstein, Jian Ye, Erika F. Schibber, Ankita Roychoudhury, Leyre Troyas Martinez, M. Houman Fekrazad, Michael Ortiz, Peter P. Lee, Mikhail G. Shapiro, Morteza Gharib

    Ultrasound can be focused into deep tissues with millimeter precision to perform noninvasive ablative therapy for diseases such as cancer. In most cases, this ablation uses high intensity ultrasound to deposit nonselective thermal or mechanical energy at the ultrasound focus, damaging both healthy bystander tissue and cancer cells. Here, we describe an alternative low intensity (ISPTA < 5 W/cm2) pulsed ultrasound approach that leverages the distinct mechanical properties of neoplastic cells to achieve inherent cancer selectivity. We show that ultrasound applied at a frequency of 0.5–0.67 MHz and a pulse duration of >20 ms causes selective disruption of a panel of breast, colon, and leukemia cancer cell models in suspension without significantly damaging healthy immune or red blood cells. Mechanistic experiments reveal that the formation of acoustic standing waves and the emergence of cell-seeded cavitation lead to cytoskeletal disruption, expression of apoptotic markers, and cell death. The inherent selectivity of this low intensity pulsed ultrasound approach offers a potentially safer and thus more broadly applicable alternative to nonselective high intensity ultrasound ablation.

    更新日期:2020-01-10
  • Thermoacoustic endoscopy
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-07
    Xiao Liang, Heng Guo, Qiang Liu, Changfeng Wu, Yubin Gong, Lei Xi

    In this study, we developed microwave-induced thermoacoustic endoscopy (TAE), which employs a high-repetition-rate pulsed microwave generator for external excitation and a side-view focused ultrasound transducer for internal acoustic detection. The system yields a lateral resolution of 1.5 mm and an axial resolution of 0.35 mm. The penetration depths of saline-containing tube (5% NaCl) and tumor lesions in biological tissues are 9 and 6 cm under current experimental conditions, respectively. To improve the signal-to-noise ratio (SNR) of the reconstructed image and eliminate the off-focus distortion of the transducer, we applied the synthetic aperture focusing technique (SAFT) and coherence weighting factor into the reconstruction algorithm. Additionally, we carried out in vivo rat experiments to evaluate clinical feasibility of this technique. We could clearly distinguish multiple tumor lesions embedded inside the rat abdomen from the surrounding normal tissues.

    更新日期:2020-01-10
  • Analytical model for light modulating impedance spectroscopy (LIMIS) in all-solid-state p-n junction solar cells at open-circuit
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-02
    Osbel Almora, Daniel Miravet, Gebhard J. Matt, Germà Garcia-Belmonte, Christoph J. Brabec

    Potentiostatic impedance spectroscopy (IS) is a well-known tool for characterization of materials and electronic devices. It can be complemented by numerical simulation strategies relying on drift-diffusion equations without any equivalent circuit-based assumptions. This implies the time-dependent solutions of the transport equations under small perturbation of the external bias applied as a boundary condition at the electrodes. However, in the case of photosensitive devices, a small light perturbation modulates the generation rate along the absorber bulk. This work then approaches a set of analytical solutions for the signals of IS and intensity modulated photocurrent and photovoltage spectroscopies, intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS), respectively, from one-sided p-n junction solar cells at the open-circuit. Subsequently, a photoimpedance signal named “light intensity modulated impedance spectroscopy” (LIMIS = IMVS/IMPS) is analytically simulated, and its difference with respect to IS suggests a correlation with the surface charge carrier recombination velocity. This is an illustrative result and the starting point for future more realistic numerical simulations.

    更新日期:2020-01-10
  • Fractional-order electric double-layer capacitors with tunable low-frequency impedance phase angle and energy storage capabilities
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-09
    Anis Allagui, Halima Alnaqbi, Ahmed S. Elwakil, Zafar Said, Ahmed A. Hachicha, Chunlei Wang, Mohammad A. Abdelkareem

    In this study, we report the electrical response of two sets of solid-state fractional-order electrochemical capacitors for which the low-frequency impedance phase angle can be tuned from − 69 ° to − 7 °. The configuration makes use of a gel electrolyte in which carbonaceous additives (graphite or reduced graphene oxide) are dispersed at different proportions. Such a disordered electrolyte structure results in subdiffusive charge transport and thus a frequency dispersive capacitive-resistive behavior typical of a constant phase element, which can be useful for both frequency applications and energy storage purposes.

    更新日期:2020-01-10
  • Shannon entropy and quantitative time irreversibility for different and even contradictory aspects of complex systems
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-03
    Wenpo Yao, Wenli Yao, Dezhong Yao, Daqing Guo, Jun Wang

    The Shannon entropy and quantitative time irreversibility (qTIR) are statistical quantifiers that are widely used for characterizing complex processes. However, the differences and associations between them have not been subjected to detailed investigation. In this Letter, we report a comparative analysis of the Shannon entropy and qTIR using model series and real-world heartbeats. We find that the permutation-based Shannon entropy (PEn) and time irreversibility (PYs) detect nonlinearities in the model series differently according to the surrogate theory. Moreover, PEn and PYs, based on either the original or the equal-value permutation, give contradictory results for congestive heart failure cases and healthy young and elderly heartbeats. PEn quantifies the complexity by calculating the amount of mean information, whereas PYs measures the probabilistic differences among symmetric nonequilibrium distributions, and these yield different or even contradictory outcomes. Our findings demonstrate the statistical associations between the Shannon entropy and qTIR, contribute to more reliable elucidation of the nonlinear dynamics of heartbeats, and improve our understanding of the complexity and nonequilibrium nature of complex systems.

    更新日期:2020-01-10
  • Effect of electron concentration on electrical conductivity inin situAl-TiB2nanocomposites
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-03
    Shuaihang Pan, Jie Yuan, Peng Zhang, Maximilian Sokoluk, Gongcheng Yao, Xiaochun Li

    Electrical conductivity is important for the applications of metals containing nanoparticles, and a thorough understanding of how nanoparticles affect their electrical conductivity is much needed. In this paper, an in situ Al-TiB2 nanocomposite is used as a model system to study its electrical behavior from 10–300 K with Hall scanning up to ± 6   T. By experimentally identifying the respective contributions from the nanoparticle size, grain boundaries, dislocation density, and nanoparticle volume percentage, it suggests that a low volume percent of TiB2 nanoparticles can reduce the electron concentration significantly to decrease the electrical conductivity of the Al-TiB2 nanocomposites, while yielding less effect on the electron mobility. Moreover, the results show that the intrinsically enhanced electron-phonon interaction and the interfacial bound states by TiB2 nanoparticles play a role in lowering the electron concentration. This understanding of how nanoparticles affect the electrical conductivity provides useful insights into the rational design and optimization of metal matrix nanocomposites for numerous applications.

    更新日期:2020-01-10
  • Plenoptic x-ray microscopy
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-01-06
    Katarzyna M. Sowa, Marcin P. Kujda, Paweł Korecki

    Plenoptic cameras use arrays of microlenses to capture multiple views of the same scene in a single compound image. They enable refocusing on different planes and depth estimation. However, until now, all types of plenoptic computational imaging processes have been limited to visible light. We demonstrate an x-ray plenoptic microscope that uses a concentrating microcapillary array instead of a microlens array and can simultaneously acquire from one hundred to one thousand x-ray projections of imaged volumes that are located in the focal spot region of the microcapillary array. Hence, tomographic slices at various depths near the focal plane can be reconstructed in a way similar to tomosynthesis but from a single x-ray exposure. The microscope enables the depth-resolved imaging of small subvolumes in large samples and can be used for the imaging of weakly absorbing artificial and biological objects by means of propagation phase-contrast.

    更新日期:2020-01-10
  • Near ultraviolet enhanced 4H-SiC Schottky diode
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-23
    Yang Shen, Andrew H. Jones, Yuan Yuan, Jiyuan Zheng, Yiwei Peng, Brenda VanMil, Kimberley Olver, Anand V. Sampath, Cory Parker, Elizabeth Opila, Joe C. Campbell

    Silicon carbide Schottky diodes with thick i-regions are reported. Compared with previously reported p-i-n photodiodes, a shift of the absorption peak from 270 nm to 350 nm was observed. The responsivity curves of the Schottky diode are modeled and compared with the experimental data.

    更新日期:2019-12-31
  • High figure-of-merit NEMS thermal detectors based on 50-nm thick AlN nano-plate resonators
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-23
    Zhenyun Qian, Vageeswar Rajaram, Sungho Kang, Matteo Rinaldi

    This paper reports on the demonstration of ultrafast (thermal time constant, τ ∼ 166 μs) and high resolution (noise equivalent power, NEP ∼ 549 pW/Hz1/2) thermal detectors based on high quality factor 50-nm thick aluminum nitride (AlN) piezoelectric resonant nanoplates. Here we show that by employing nanoscale (30 nm) aluminum anchors, both high thermal resistance (Rth ∼ 1.1 × 106 K/W) and high quality factor (Q ∼ 1000) can be achieved in greatly scaled AlN nanoplate resonators. Furthermore, the absorptance of such ultrathin AlN resonators was characterized, in mid-wavelength infrared region showing an average absorptance of ∼36% from 2.75 μm to 6.25 μm. These unique features were exploited for the experimental demonstration of AlN NEMS resonant thermal detectors with greatly reduced thermal capacitance and over doubled figure of merit [FoM = 1/(NEP × τ)] compared to what was previously achieved by the same technology.

    更新日期:2019-12-31
  • Terahertz intersubband absorption of GaN/AlGaN step quantum wells grown by MOVPE on Si(111) and Si(110) substrates
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-26
    A. Jollivet, M. Tchernycheva, V. Trinité, E. Frayssinet, P. De Mierry, Y. Cordier, F. H. Julien

    We demonstrate terahertz intersubband absorptions in nitride step quantum wells (SQWs) grown by metal organic vapor phase epitaxy simultaneously on two different substrate orientations [Si(111) and Si(110)]. The structure of the SQWs consists of a 3 nm thick Al0.1Ga0.9N barrier, a 3 nm thick GaN well, and an Al0.05Ga0.95N step barrier with various thicknesses. This structure design has been optimized to approach a flatband potential in the wells to allow for an intersubband absorption in the terahertz frequency range and to maximize the optical dipole moment. Structural characterizations prove the high quality of the samples. Intersubband absorptions at frequencies of 5.6 THz (λ ≈ 54 μm), 7 THz (43 μm), and 8.9 THz (34 μm) are observed at 77 K on both substrate orientations. The observed absorption frequencies are in excellent agreement with calculations accounting for the depolarization shift induced by the electron concentration in the wells.

    更新日期:2019-12-31
  • Light-activated electroforming in ITO/ZnO/p-Si resistive switching devices
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-26
    O. Blázquez, J. L. Frieiro, J. López-Vidrier, C. Guillaume, X. Portier, C. Labbé, S. Hernández, B. Garrido

    We report on light-activated electroforming of ZnO/p-Si heterojunction memristors with transparent indium tin oxide as the top electrode. Light-generated electron-hole pairs in the p-type substrate are separated by the external electric field and electrons are injected into the active ZnO layer. The additional application of voltage pulses allows achieving different resistance states that end up in the realization of the low resistance state (LRS). This process requires much less voltage compared to dark conditions, thus avoiding undesired current overshoots and achieving a self-compliant device. The transport mechanisms governing each resistance state are studied and discussed. An evolution from an electrode-limited to a space charge-limited transport is observed along the electroforming process before reaching the LRS, which is ascribed to the progressive formation of conductive paths that consequently induce the growth of conductive nanofilaments through the ZnO layer.

    更新日期:2019-12-31
  • Properties of self-mixing interference in terahertz distributed feedback quantum cascade lasers
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-26
    Lei Ge, Ning Yang, Jian Wang, Yuanyuan Li, Weidong Chu, Suqing Duan, Yan Xie

    The properties of self-mixing interference in terahertz distributed feedback quantum cascade lasers (THz DFB-QCLs) were studied by coupled wave theory and the multimode rate equation method. Under weak self-mixing optical feedback, the response behaviors of the DFB-QCLs are similar to those of Fabry–Pérot lasers and can be described by the Lang–Kobayashi equations with a modified feedback parameter. The amplitudes of the self-mixing power and frequency signals decrease with the increasing DFB coupling factor. Under strong feedback, new side modes arise, which broaden the laser linewidth. The number of side modes decreases with the increasing DFB coupling factor. The simulation also demonstrates that the maximum power mode of a THz DFB-QCL under strong self-mixing feedback is the minimum linewidth mode. These results support the development of self-mixing interferometers based on DFB-QCLs.

    更新日期:2019-12-31
  • Dynamic study of phase transition in Bi2O3epitaxial film induced by electrolyte gating
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-23
    Lele Fan, Yiyu Zhu, Zhiqiang Wang, Sihan Zhao, Zhonghu Liu, Lei Zhu, Xiaosan Wang, Qinfang Zhang

    Electrolyte gating with ionic liquids has been broadly applied in various fields in recent years. However, it remains under debate since defect-controlling and electrochemical doping are conventionally disputed to interpret the corresponding mechanism. In this work, we provide the synergistic mechanism that oxygen vacancy migration and element-doping together drive the formation of metallization. The prepared Bi2O3 films experienced insulator-metal transition and structural transformation by field-induced ionic liquid. The consequent structural transition in the Bi2O3 film was dynamically monitored by XRD, and the results indicated that an extraordinary metal Bi phase was formed during the electrolyte gating process, which was further verified by HR-TEM and XPS. Our current findings will boost the development of electrolyte gating and bring insight into other metal oxides in ionic liquid gating experiment.

    更新日期:2019-12-31
  • In-situ stress modulated ferroelectric photovoltaic effect in cluster-assembled TbFe2/Bi5Ti3FeO15heterostructural films
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-26
    Yulong Bai, Bo Yang, Shifeng Zhao

    TbFe2/Bi5Ti3FeO15 heterostructural films were prepared by inserting cluster-assembled TbFe2 microdiscs into a Bi5Ti3FeO15 matrix using low energy cluster beam deposition combined with sol-gel methods. The phase structure, ferroelectric properties, bandgap, photovoltaic spectral response, and performances of the ferroelectric photovoltaic effect were modulated by the in situ stress driven by magnetostriction of TbFe2 clusters under external magnetic fields. The short-circuit current, open-circuit voltage, and power conversation efficient increase with the in situ stress, reaching 0.026 mA/cm2, 9.5 V, and 5.88 × 10−2%, respectively, under a maximum in-stress of 0.075 GPa. So the high open-circuit voltage above bandgap is attributed to the distinct bandgap shifting and the effective separation of photogenerated electron-hole pairs derived from the in situ stress induced large built-in field. The in situ stress dominated symmetry breaking contributes to the improvement of the power conversation coefficient. The in situ dynamic internal stress provides a high efficient approach to modulate and improve ferroelectric photovoltaic effects.

    更新日期:2019-12-31
  • Impact of substrate induced band tail states on the electronic and optical properties of MoS2
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-30
    J. Klein, A. Kerelsky, M. Lorke, M. Florian, F. Sigger, J. Kiemle, M. C. Reuter, T. Taniguchi, K. Watanabe, J. J. Finley, A. N. Pasupathy, A. W. Holleitner, F. M. Ross, U. Wurstbauer

    Substrate, environment, and lattice imperfections have a strong impact on the local electronic structure and the optical properties of atomically thin transition metal dichalcogenides. We find by a comparative study of MoS2 on SiO2 and hexagonal boron nitride (hBN) using scanning tunneling spectroscopy (STS) measurements that the apparent bandgap of MoS2 on SiO2 is significantly reduced compared to MoS2 on hBN. The bandgap energies as well as the exciton binding energies determined from all-optical measurements are very similar for MoS2 on SiO2 and hBN. This discrepancy is found to be caused by a substantial amount of band tail states near the conduction band edge of MoS2 supported by SiO2. The presence of those states impacts the local density of states in STS measurements and can be linked to a broad red-shifted photoluminescence peak and a higher charge carrier density that are all strongly diminished or even absent using high quality hBN substrates. By taking into account the substrate effects, we obtain a quasiparticle gap that is in excellent agreement with optical absorbance spectra and we deduce an exciton binding energy of about 0.53 eV on SiO2 and 0.44 eV on hBN.

    更新日期:2019-12-31
  • Mott insulator to metal transition driven by oxygen incorporation in epitaxial LaTiO3films
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-30
    T. T. Zhang, C. Y. Gu, Z. W. Mao, X. F. Chen, Z. B. Gu, P. Wang, Y. F. Nie, X. Q. Pan

    Lanthanum titanate, LaTiO3, is an antiferromagnetic Mott insulator with a Ti 3d1 electronic configuration and exhibits an intriguing metallic state in its epitaxial film grown on the SrTiO3 substrate. Here, we explore the driving force of the Mott insulator to metal transition in LaTiO3 epitaxial films by a systematic study of the film growth conditions and biaxial strain using reactive molecular beam epitaxy. Within the achievable range (up to −2.4%) of the biaxial compressive strain in our study, we found that the oxygen incorporation plays a more crucial role than the biaxial epitaxial strain in the Mott insulator to metal transition in LaTiO3 films.

    更新日期:2019-12-31
  • Single-layer LaBr2: Two-dimensional valleytronic semiconductor with spontaneous spin and valley polarizations
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-31
    Pei Zhao, Yandong Ma, Chengan Lei, Hao Wang, Baibiao Huang, Ying Dai

    The current focus of valleytronics research lies in how to produce valley polarization. Although many schemes have been broadly studied, spontaneous valley polarization is rarely explored. Here, we report the discovery of a two-dimensional material with the long-pursued spontaneous spin and valley polarizations. Using first-principles calculations, we reveal that single-layer LaBr2 is dynamically and thermally stable, which could be exfoliated from its bulk material. Single-layer LaBr2 is found to be a compelling two-dimensional ferromagnetic semiconductor. More interestingly, we show that single-layer LaBr2 harbors the extremely rare intrinsic valley polarization, owing to the coexistence of inversion symmetry and time-reversal symmetry breakings. Its spontaneous valley polarization reaches 33 meV, sizable enough for operating room-temperature valleytronic physics. Our work thus provides a promising material for experimental studies and practical applications of two-dimensional spintronics and valleytronics.

    更新日期:2019-12-31
  • Physical mechanism on the suppression of dynamic resistance degradation by multi-mesa-channel in AlGaN/GaN high electron mobility transistors
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-23
    Kailin Ren, Yung C. Liang, Chih-Fang Huang

    In this letter, the suppression of dynamic on-state resistance (RON) degradation for faster dynamic RON recovery is achieved by the multimesa-channel (MMC) structure in AlGaN/GaN high electron mobility transistors. The measurement results are discussed with the physical mechanisms investigated. The initial transient RON degradation is reduced in the MMC structure, resulting from the lower peak electric field around the drain-side gate edge in the trigate structure compared to that in a planar device. The faster dynamic RON recovery in MMC devices is attributed to the quick emission of electrons at sidewall traps of shallower energy levels. The energy levels of dominant traps at the sidewall and top interfaces are found to be 0.26 eV and 0.37 eV below the conduction band edge, respectively, verified by Technology Computer Aided Design simulations in agreement with the measurement data.

    更新日期:2019-12-31
  • Fine structure of another blue luminescence band in undoped GaN
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-23
    Michael A. Reshchikov

    A blue luminescence band, labeled BL3, has been found in undoped GaN samples grown by hydride vapor phase epitaxy. It has a maximum at 2.8 eV and a phonon-related fine structure at its high-energy side. The zero-phonon line of this band consists of a duplet with two sharp lines at 3.0071 and 3.0147 eV. Three phonon modes, including the LO mode with an energy of 91.3 meV and two pseudolocal phonon modes with energies of 39.6 and 68.2 meV, form the characteristic fine structure of the BL3 band. The BL3 band is attributed to internal transitions from excited states located near the conduction band to the 0/+ transition level of unknown defect. The defect is preliminarily identified as the RY3 center, which is also responsible for bright red-yellow luminescence bands in the studied samples.

    更新日期:2019-12-31
  • Zeeman spin-splitting in the (010) β-Ga2O3two-dimensional electron gas
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-23
    Adam T. Neal, Yuewei Zhang, Said Elhamri, Siddharth Rajan, Shin Mou

    Through magnetotransport measurements and analysis of the observed Shubnikov de-Haas oscillations in (010) (AlxGa1-x)2O3/Ga2O3 heterostructures, spin-splitting of the Landau levels in the (010) Ga2O3 two-dimensional electron gas (2DEG) has been studied. Analysis indicates that the spin-splitting results from the Zeeman effect. By fitting both the first and second harmonics of the oscillations as a function of magnetic field, we determine the magnitude of the Zeeman splitting to be 0.4 ħωc, with a corresponding effective g-factor of 2.7, for the magnetic field perpendicular to the 2DEG.

    更新日期:2019-12-31
  • Room-temperature ferromagnetism inC+-implanted AlN films
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-23
    R. Ye, J. D. Liu, H. J. Zhang, B. J. Ye

    Diluted magnetic semiconductors (DMSs) have numerous potential applications, particularly in spintronics. Therefore, the search for advanced DMSs has been a critical task for a long time. In this work, room-temperature ferromagnetism is observed in the C +-implanted AlN films with C + doses of 5 × 10 16 ( AlN : C 5 × 10 16) and 2 × 10 17   cm − 2 ( AlN : C 2 × 10 17). AlN : C 2 × 10 17 exhibits a saturation magnetization of ∼0.104 emu/g, nearly 1.5 times that of AlN : C 5 × 10 16. X-ray diffraction and X-ray photoelectron spectroscopy (XPS) measurements reveal that the implanted C + ions occupy the interstitial lattice sites and substitute at the sites of Al atoms. XPS and Doppler broadening of positron annihilation radiation measurements demonstrate the existence of the Al-vacancy related defects in the C +-implanted AlN films. First-principles calculations indicate that the ferromagnetism in AlN : C 5 × 10 16 and AlN : C 2 × 10 17 is mainly originated from defect complexes involving interstitial C atoms and Al vacancies, which have the lowest formation energy among AlN:C defects containing C atoms and Al vacancies. This work provides a feasible route to develop advanced DMSs.

    更新日期:2019-12-31
  • A long-lived capacitively shunted flux qubit embedded in a 3D cavity
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-23
    Leonid V. Abdurakhimov, Imran Mahboob, Hiraku Toida, Kousuke Kakuyanagi, Shiro Saito

    We report the experimental realization of a 3D capacitively shunt superconducting flux qubit with long coherence times. At the optimal flux bias point, the qubit demonstrates energy relaxation times in the range of 60–90 μs and a Hahn-echo coherence time of about 80 μs, which can be further improved by dynamical decoupling. Qubit energy relaxation can be attributed to quasiparticle tunneling and unwanted two-level-system defects, while qubit dephasing is caused by flux noise away from the optimal point. Our results show that 3D c-shunt flux qubits demonstrate improved performance over other types of flux qubits, which is advantageous for applications such as quantum magnetometry and spin sensing.

    更新日期:2019-12-31
  • Influence of electric polarization on Coulomb blockade in a super-paraelectric clusters assembly
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-26
    Brice Kengni-Zanguim, Nabil Najjari, Bernard Doudin, Yves Henry, David Halley

    We study electron transport in an assembly of epitaxial Cr2O3 nanoparticles embedded in a MgO tunnel barrier: an unusual variation in the Coulomb blockade charging energy is observed as a function of the size of the clusters. In striking contrast to the expected increase in charging energy when decreasing the cluster size, an almost constant behavior is observed. We argue here that the spontaneous superparaelectric moment carried by the cluster core is the origin of this unusual behavior since it drives the dielectric constant in this cluster assembly. This phenomenon could be exploited to fabricate devices with single valued Coulomb blockade energy despite a statistical dispersion in the cluster size.

    更新日期:2019-12-31
  • Quantum paraelectricity to dipolar glass transition in Sc dopedBaFe12O19single crystals
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-26
    L. H. Yin, R. R. Zhang, J. Yang, P. Tong, W. H. Song, J. M. Dai, X. B. Zhu, Y. P. Sun

    We report a quantum paraelectric (QP) to dipolar glass (DG) phase transition induced by Sc substitution in ferrimagnetic hexaferrite BaFe 12 − x Sc x O 19 ( 0 ≤ x ≤ 2.4) single crystals. The DG behavior is revealed via anisotropic dielectric spectroscopy, polarization, and specific heat measurements. The phase evolution from QP to DG ( 0.5 ≤ x ≤ 1.2) and finally to the nonpolar state ( x ≥ 2.4) with increasing x is found to be due to the competition between the enhanced polar instability by lattice volume expansion and disordered/frustrated dipole-dipole interaction induced by the partial replacement of Sc 3 + for Fe 3 + in the bipyramids. Our results suggest further experimental clues to realize possible multiferroicity with strong displacive ferroelectricity in BaFe 12 O 19 and other magnetic materials.

    更新日期:2019-12-31
  • Fundamental limits to the transfer of energy harvested from ferroelectric materials under shock loading
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-26
    Sergey I. Shkuratov, Jason Baird, Vladimir G. Antipov, Jay B. Chase

    Ferroelectrics are capable of producing megawatt power levels under shock loading due to stress-induced phase transformations, resulting in depolarization of the ferroelectric materials. This power can be used for generation of high voltages, high currents, or ultrahigh-power electromagnetic radiation. The results are reported herein on an experimental study of limitations on energy harvested from shocked Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 and PbZr0.52Ti0.48O3 ferroelectrics and transferred to external electrical systems. The experimental results indicate that one of the limits to the energy transfer is electric breakdown that occurs within ferroelectric specimens during shock wave transit and depolarization, interrupting the energy transfer process and resulting in energy losses. It was revealed that the mechanism for breakdown in shocked ferroelectrics differs depending on the energy transfer time range, making a significant impact on the energy transfer process. High-speed photography and analysis of outputs for the two ferroelectrics indicate that for energy transfer times exceeding eight microseconds, the mechanical fragmentation of the ferroelectric material caused by the shock and resulting release waves following the shock wave front plays an important part in the breakdown process, while a thermal runaway dominates the breakdown at shorter energy transfer times. The heretofore disregarded mechanism of electric breakdown of the mechanically fragmented dielectric media is an unavoidable time-limiting factor for energy transfer from ferroelectrics under shock loading. The results obtained in this study are important for understanding the behavior of ferroelectrics during shock wave transit under high electric fields and for ultrahigh-power applications of ferroelectric materials.

    更新日期:2019-12-31
  • Graphene-based thermal repeater
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-26
    Ming-Jian He, Hong Qi, Ya-Tao Ren, Yi-Jun Zhao, Mauro Antezza

    In this Letter, we have demonstrated the possibility to efficiently relay the radiative heat flux between two nanoparticles by opening a smooth channel for heat transfer. By coating the nanoparticles with a silica shell and modifying the substrate with multilayered graphene sheets, respectively, the localized phonon polaritons excited near the nanoparticles can couple with the multiple surface plasmon polaritons near the substrate to realize the heat relay at a long distance. The heat transfer can be enhanced by more than six orders of magnitude, and the relay distance can be as high as 35 times in the far-field regime. This work may provide a way to realize the energy modulation or thermal communications especially at long distances.

    更新日期:2019-12-31
  • Thickness-dependent ultrafast nonlinear absorption properties of PtSe2films with both semiconducting and semimetallic phases
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-26
    Xin Zhao, Fang Liu, Dongqi Liu, Xiao-Qing Yan, Changfu Huo, Wangwei Hui, Junfang Xie, Qing Ye, Chengcheng Guo, Yang Yao, Zhi-Bo Liu, Jian-Guo Tian

    The bandgap, electrical, and optical properties of PtSe2 depend dramatically on the vertical stacking and fabrication method. Here, we study the nonlinear absorption properties of the PtSe2 films composed of both semiconducting and semimetallic phases in a single film. These PtSe2 films exhibit remarkable thickness-dependent saturable absorption for femtosecond pulses at 400 nm and 800 nm. The saturation intensities decrease with the increase in the film thickness due to the accompanied increase in the semimetallic component and are much smaller than the reported values of PtSe2 synthesized by thermally assisted conversion. The saturable absorption characteristics are confirmed by time-resolved spectroscopies. The nonlinear refractive indexes of these PtSe2 films should be smaller than 1 × 10–12 cm2/W. Our results imply that the optical nonlinearities of PtSe2 could be flexibly tuned by the synthesis method and thickness.

    更新日期:2019-12-31
  • Nonlinear optical effect of interlayer charge transfer in a van der Waals heterostructure
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-26
    Peng Yao, Dawei He, Peymon Zereshki, Yongsheng Wang, Hui Zhao

    The recently discovered two-dimensional materials can be used to fabricate multilayer heterostructures. Interlayer charge transfer is a key process in such heterostructures as it can enable emergent optoelectronic properties. Efficient interlayer charge transfer in van der Waals heterostructures has been observed by femtosecond transient absorption and steady-state optical spectroscopy measurements, based on measuring the interlayer carrier distribution. Here, we show that a second harmonic generation process allows direct probing of the electric field induced by the charge transfer. An ultrashort laser pulse was used to excite electrons and holes in a MoS2/WS2 heterostructure. The separation of the electrons and holes from the two monolayers generates an electric field, which enables the generation of the second harmonic of an incident fundamental pulse. We further studied the time evolution of this electric field by measuring the second harmonic signal as a function of the time delay between the pump and the fundamental pulses. The result agrees well with the dynamics revealed by a transient absorption measurement. These results provide direct evidence of interlayer charge transfer and demonstrate an all-optical method of studying charge transfer and induced electric fields in two-dimensional materials. Furthermore, this effect, if large enough, could be utilized in optical devices based on 2D heterostructures with nonlinear optical responses controllable by interlayer charge transfer.

    更新日期:2019-12-31
  • Significant coercivity enhancement at low temperatures in magnetically oriented cobalt ferrite nanoparticles
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2019-12-26
    Pablo Tancredi, Patricia C. Rivas-Rojas, Oscar Moscoso-Londoño, Diego Muraca, Marcelo Knobel, Leandro M. Socolovsky

    The present work describes a synthesis and characterization strategy employed to study the magnetic anisotropic properties of a diluted nanoparticulate system. The system under analysis is composed of monodisperse and highly crystalline 16 nm Co0.5Fe2.5O4 nanoparticles (NPs), homogenously dispersed in 1-octadecene. Owing to the liquid nature of the matrix at room temperature, the relative orientation of the nanoparticle easy axis can be controlled by an external magnetic field, enabling us to measure how the magnetic properties are modified by the alignment of the particles within the sample. In turn, by employing this strategy, we have found a significant hardness and squareness enhancement of the hysteresis loop in the magnetically oriented system, with the coercive field reaching a value as high as 30.2 kOe at low temperatures. In addition, the magnetic behavior associated with the system under study was supported by additional magnetic measurements, which were ascribed to different events expected to take place throughout the sample characterization, such as the melting process of the 1-octadecene matrix or the NP relaxation under the Brownian mechanism at high temperatures.

    更新日期:2019-12-31
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