Characterization of an active metasurface using terahertz ellipsometry Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 Nicholas Karl, Martin S. Heimbeck, Henry O. Everitt, Hou-Tong Chen, Antoinette J. Taylor, Igal Brener, Alexander Benz, John L. Reno, Rajind Mendis, Daniel M. Mittleman
Switchable metasurfaces fabricated on a doped epi-layer have become an important platform for developing techniques to control terahertz (THz) radiation, as a DC bias can modulate the transmission characteristics of the metasurface. To model and understand this performance in new device configurations accurately, a quantitative understanding of the bias-dependent surface characteristics is required. We perform THz variable angle spectroscopic ellipsometry on a switchable metasurface as a function of DC bias. By comparing these data with numerical simulations, we extract a model for the response of the metasurface at any bias value. Using this model, we predict a giant bias-induced phase modulation in a guided wave configuration. These predictions are in qualitative agreement with our measurements, offering a route to efficient modulation of THz signals.
Optofluidic trapping and delivery of massive mesoscopic matters using mobile vortex array Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 Jianxin Yang, Zongbao Li, Haiyan Wang, Debin Zhu, Xiang Cai, Yupeng Cheng, Mingyu Chen, Xiaowen Hu, Xiaobo Xing
The realization of directional and controllable delivery of massive mesoscopic matters is of great significance in the field of microfluidics. Here, the mobile thermocapillary vortex array has achieved the enrichment and transport of massive mesoscopic matters in free or limited space. The ability of the vortex array to confine objects in the center ensures the controllability of particle trajectory. We also simulated the delivery process to reveal the stability of the mobile vortex. Owing to the distance between the vortex center and the heat source, the method provides the ability to protect trapped matters, including organisms and living cells. The mobile vortex array has opened the exciting possibilities of realizing that bridges the gap between remote optofluidics and lab on a chip.
Stabilization of microcrystal λ-Ti3O5 at room temperature by aluminum-ion doping Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 Zujia Shen, Qiwu Shi, Wanxia Huang, Bo Huang, Mingzhe Wang, Junzheng Gao, Yanli Shi, Tiecheng Lu
λ-Ti3O5 is an intriguing phase-transition material that has been proposed to be metastable and has emerged at room temperature only in the form of nanocrystals. In this work, λ-Ti3O5 was stabilized to room temperature in the form of microcrystals by aluminum (Al)-ion doping. Al entered the Ti3O5 lattice in the substitutional mode, which reduced the threshold temperature (Tc) of the β-λ phase transition in Ti3O5 and maintained a λ-phase Ti3O5 at room temperature. Al doping caused a significant decrease in resistivity of Ti3O5, which corresponds to a semiconductor-metal transition that is induced by Al-ion doping. We have developed a mechanism to fabricate λ-Ti3O5 by ion doping and have provided a fundamental foundation for a more available application of λ-Ti3O5 in smart optoelectronic devices.
Stimulated emission from HgCdTe quantum well heterostructures at wavelengths up to 19.5 μm Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 S. V. Morozov, V. V. Rumyantsev, M. A. Fadeev, M. S. Zholudev, K. E. Kudryavtsev, A. V. Antonov, A. M. Kadykov, A. A. Dubinov, N. N. Mikhailov, S. A. Dvoretsky, V. I. Gavrilenko
We report on stimulated emission at wavelengths up to 19.5 μm from HgTe/HgCdTe quantum well heterostructures with wide‐gap HgCdTe dielectric waveguide, grown by molecular beam epitaxy on GaAs(013) substrates. The mitigation of Auger processes in structures under study is exemplified, and the promising routes towards the 20–50 μm wavelength range, where HgCdTe lasers may be competitive to the prominent emitters, are discussed.
Weak interlayer dependence of lattice thermal conductivity on stacking thickness of penta-graphene Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 Fancy Qian Wang, Jie Liu, Xiaoyin Li, Qian Wang, Yoshiyuki Kawazoe
Penta-graphene (PG), as a novel carbon allotrope, has attracted considerable attention because of its unique atomic structure and outstanding intrinsic properties. Here, we systematically investigate the effect of layer numbers on the lattice thermal conductivity of the stacked PG structures by solving exactly the linearized phonon Boltzmann transport equation combined with first-principles calculations. We find that the lattice thermal conductivity of the stacked PG is insensitive to the number of layers, which is in sharp contrast to that of graphene. Such a layer-independent thermal conductivity is attributed to the buckled structure of PG which breaks the two-dimensional selection rule of three-phonon scattering and the weak van der Waals interlayer interactions that hardly have any effect on the lattice thermal conductivity. This mechanism can be generalized to other van der Waals layered materials with buckled or puckled structures, which may also show the layer-independent lattice thermal conductivity.
Formation process of skyrmion lattice domain boundaries: The role of grain boundaries Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 H. Nakajima, A. Kotani, M. Mochizuki, K. Harada, S. Mori
We report on the formation process of skyrmion lattice (SkL) domain boundaries in FeGe using Lorentz transmission electron microscopy and small-angle electron diffraction. We observed that grain boundaries and edges play an important role in the formation of SkL domain boundaries; The SkL domain boundary is stabilized at the intersection of two grains. A micromagnetic simulation using the Landau−Lifshitz−Gilbert equation revealed that the SkL domains separated by a boundary represent the lowest energy configuration. Conversely, in a wide area, SkL domain boundaries were not formed and SkL domains with different orientations rotated to form a single SkL domain.
Ultra-low voltage control of magnetic properties in amorphous MgO Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 Jiajun Guo, Liqian Wu, Shuxia Ren, Xin Kang, Wei Chen, Xu Zhao
We report ultra-low voltage controlled magnetic properties in an amorphous MgO (a-MgO) thin film. The intrinsic magnetization of MgO can be decreased by about 57.5% by the application of a positive bias voltage while increased by about 56.7% by a negative bias, at an ultralow voltage of just 0.2 V. More interestingly, this ultralow voltage also induces a strong magnetic anisotropy in the a-MgO film. Further analysis indicates that the migration of O2− ions under an electric field results in a change in the Mg/O ratio and the redistribution of Mg vacancies, thus leading to the change in the magnetic properties of the film. The control of room temperature magnetic properties at ultralow voltages may find applications in multifunctional memory and ultralow-power consumption spintronics.
Collective spin waves in arrays of permalloy nanowires with single-side periodically modulated width Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 G. Gubbiotti, L. L. Xiong, F. Montoncello, A. O. Adeyeye
We have experimentally and numerically investigated the dispersion of collective spin waves propagating through arrays of longitudinally magnetized nanowires (NWs) with a periodically modulated width. Two nanowire arrays with single-side modulation and different periodicities of modulation were studied and compared to the nanowires with a homogeneous width. The spin-wave dispersion, measured up to the third Brillouin zone of the reciprocal space, revealed the presence of two dispersive modes for the width-modulated NWs, whose amplitude of the magnonic band depends on the modulation periodicity, and a set of nondispersive modes at higher frequency. These findings are different from those observed in homogeneous width NWs where only the lowest mode exhibits sizeable dispersion. The measured spin-wave dispersion has been satisfactorily reproduced by means of the dynamical matrix method. The results presented in this work are important in view of the possible realization of tunable frequency magnonic devices.
Ultra-low damping in lift-off structured yttrium iron garnet thin films Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 A. Krysztofik, L. E. Coy, P. Kuświk, K. Załęski, H. Głowiński, J. Dubowik
We show that using maskless photolithography and the lift-off technique, patterned yttrium iron garnet thin films possessing ultra-low Gilbert damping can be accomplished. The films of 70 nm thickness were grown on (001)-oriented gadolinium gallium garnet by means of pulsed laser deposition, and they exhibit high crystalline quality, low surface roughness, and the effective magnetization of 127 emu/cm3. The Gilbert damping parameter is as low as 5×10−4. The obtained structures have well-defined sharp edges which along with good structural and magnetic film properties pave a path in the fabrication of high-quality magnonic circuits and oxide-based spintronic devices.
NbRe as candidate material for fast single photon detection Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 M. Caputo, C. Cirillo, C. Attanasio
The suitability of NbRe as a promising material for the design of Superconducting Single Photon Detectors is investigated in order to lower both the minimum detectable photon energy and the recovery time of the devices. Both the low values determined for the quasiparticle relaxation time, τE, and its weak temperature dependence are desirable in the design of fast single photon detectors. Both properties can be further improved by coupling NbRe with a ferromagnetic layer, as demonstrated by estimating the characteristic relaxation rates in NbRe/CuNi bilayers.
Ultrafast dynamics of coherent optical phonon correlated with the antiferromagnetic transition in a hexagonal YMnO3 epitaxial film Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 Takayuki Hasegawa, Norifumi Fujimura, Masaaki Nakayama
We report on the observation of the coherent optical phonon in a hexagonal YMnO3 epitaxial film using a reflection-type pump-probe technique at various temperatures, excitation powers, and energies. We detected an oscillatory structure with a frequency of ∼5.1 THz, which is assigned to the coherent optical phonon with A1 symmetry, in a time-domain signal. It was found that the coherent optical phonon can be observed at temperatures from 10 K to room temperature, while the oscillation amplitude markedly decreases with an increase in temperature around ∼70 K corresponding to the Néel temperature. The temperature dependence of the oscillation amplitude indicates that the coherent optical phonon is sensitive to the spin-lattice coupling connected with the antiferromagnetic transition.
All-optical lithography process for contacting nanometer precision donor devices Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 D. R. Ward, M. T. Marshall, D. M. Campbell, T. M. Lu, J. C. Koepke, D. A. Scrymgeour, E. Bussmann, S. Misra
We describe an all-optical lithography process that can make electrical contact to nanometer-precision donor devices fabricated in silicon using scanning tunneling microscopy (STM). This is accomplished by implementing a cleaning procedure in the STM that allows the integration of metal alignment marks and ion-implanted contacts at the wafer level. Low-temperature transport measurements of a patterned device establish the viability of the process.
Near-infrared localized surface plasmon resonance of self-growing W-doped VO2 nanoparticles at room temperature Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 Kazutaka Nishikawa, Yoshihiro Kishida, Kota Ito, Shin-ichi Tamura, Yasuhiko Takeda
Nanoparticles (NPs) of vanadium dioxide (VO2) in the metal state exhibit localized surface plasmon resonance (LSPR) at 1200–1600 nm, which fills the gap between the absorption ranges of silicon and the LSPR of conventional transparent conductor NPs (ZnO:Al, In2O3:Sn, etc.). However, two issues of the lithographic process for NP formation and the metal-insulator transition temperature (69 °C) higher than room temperature have made it difficult to use VO2 NPs for applications such as energy conversion devices, near infrared (NIR) light detectors, and bio-therapy. In this study, we developed a self-growing process for tungsten (W)-doped VO2 NPs that are in the metal state at room temperature, using sputter deposition and post-lamp annealing. The changes in the LSPR peak wavelengths with the NP size were well controlled by changing the deposited film thickness and oxygen pressure during the post-annealing treatment. The presented results resolve the difficulties of using the metal-insulator transition material VO2 for practical NIR utilization.
Computational passive imaging of thermal sources with a leaky chaotic cavity Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 Ariel Christopher Tondo Yoya, Benjamin Fuchs, Matthieu Davy
Computational imaging techniques are of great interest to simplify the architecture of imaging devices since random illuminations of a scene enable its reconstruction from few measurements by solving an inverse problem. Here, we present a passive system for imaging of thermal sources in the microwave range from the cross-correlation of noise signals recorded by only two channels. The channels are attached to a high Q-factor chaotic cavity with a leaky aperture on its front side. The spatial distribution of noise sources is encoded onto the broadband spectrum of the cross-correlation and can be reconstructed from the sensing matrix mapping the uncorrelated far-field speckle patterns of the cavity onto a set of frequencies. We demonstrate imaging of localized and extended thermal sources and show that the polarization of those radiations can be discriminated. Moreover, we exhibit the effectiveness of the proposed system as a compressive imaging device which exploits the natural randomness of the speckle patterns. We believe that these results are a promising step for the design of real time and low cost microwave radiometers.
Voltage-tunable acoustic metasheet with highly asymmetric surfaces Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-06 Songwen Xiao, Suet To Tang, Z. Yang
We report the experimental demonstration of a voltage-tunable acoustic metasheet device with two highly asymmetric surfaces, made by combining two decorated membrane resonators (DMRs) separated by a sealed air column. The front surface of the metasheet is impedance matched to air and perfectly absorbing, while the back surface is hard and totally reflecting. When a suitable DC voltage is applied to the back side of the DMR via proper electrodes, the back surface of the metasheet can be tuned to impedance matched to air and perfectly absorbing, while the front surface is totally reflecting. The metasheet also exhibits high transmission contrast around two frequencies. The tunability of the reflection is over 23 dB at 388 Hz and that of the transmission is over 33 dB at 240 Hz and 590 Hz with 600 V of applied voltage.
Sub-band-gap absorption in Ga2O3 Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-03 Hartwin Peelaers, Chris G. Van de Walle
β-Ga2O3 is a transparent conducting oxide that, due to its large bandgap of 4.8 eV, exhibits transparency into the UV. However, the free carriers that enable the conductivity can absorb light. We study the effect of free carriers on the properties of Ga2O3 using hybrid density functional theory. The presence of free carriers leads to sub-band-gap absorption and a Burstein-Moss shift in the onset of absorption. We find that for a concentration of 1020 carriers, the Fermi level is located 0.23 eV above the conduction-band minimum. This leads to an increase in the electron effective mass from 0.27–0.28 me to 0.35–0.37 me and a sub-band-gap absorption band with a peak value of 0.6 × 103 cm–1 at 3.37 eV for light polarized along the x or z direction. Both across-the-gap and free-carrier absorption depend strongly on the polarization of the incoming light. We also provide parametrizations of the conduction-band shape and the effective mass as a function of the Fermi level.
Self-mode-locked AlGaInP-VECSEL Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-03 R. Bek, M. Großmann, H. Kahle, M. Koch, A. Rahimi-Iman, M. Jetter, P. Michler
We report the mode-locked operation of an AlGaInP-based semiconductor disk laser without a saturable absorber. The active region containing 20 GaInP quantum wells is used in a linear cavity with a curved outcoupling mirror. The gain chip is optically pumped by a 532 nm laser, and mode-locking is achieved by carefully adjusting the pump spot size. For a pump power of 6.8 W, an average output power of up to 30 mW is reached at a laser wavelength of 666 nm. The pulsed emission is characterized using a fast oscilloscope and a spectrum analyzer, demonstrating stable single-pulse operation at a repetition rate of 3.5 GHz. Intensity autocorrelation measurements reveal a FWHM pulse duration of 22 ps with an additional coherence peak on top, indicating noise-like pulses. The frequency spectrum, as well as the Gaussian beam profile and the measured beam propagation factor below 1.1, shows no influence of higher order transverse modes contributing to the mode-locked operation.
Multilevel storage device based on domain-wall motion in a magnetic tunnel junction Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-03 Jialin Cai, Bin Fang, Chao Wang, Zhongming Zeng
We report on a multilevel storage device based on a magnetic tunnel junction (MTJ). Six different resistance states have been observed by controlling domain wall motion in the free layer of a MTJ. It is realized by pinning the domain wall at different positions in the free layer with a special geometric structure. The resistance states can be modulated with the application of an external magnetic field or a d.c. The experimental results are well explained by micromagnetic simulation. The results suggest that our design is expected to have applications in magnetic memory and neuromorphic systems.
Spin pumping and probe in permalloy dots-topological insulator bilayers Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-03 H. C. Han, Y. S. Chen, M. D. Davydova, P. N. Petrov, P. N. Skirdkov, J. G. Lin, J. C. Wu, J. C. A. Huang, K. A. Zvezdin, A. K. Zvezdin
We present a ferromagnetic resonance (FMR) spin pumping experiment at room temperature in periodic arrays of permalloy nanodots of different radii deposited onto a 3D topological insulator Bi2Se3 film. We measure the dc voltage signal generated by spin-to-charge conversion of the pumped spin current due to the spin-orbit coupling in the bulk of Bi2Se3. In the nanostructured samples, two resonance peaks are observed, associated with Kittel and inhomogeneous edge modes, respectively. This more complex modal composition in comparison to continuous systems may provide additional advantages for development of prospective spintronic devices. We support our experimental results by theoretical calculations, which are based on micromagnetic modeling of the magnetization dynamics under FMR excitation in a nanodot. A numerical approach to the calculation of the spin-pumping voltage is proposed, and the efficiency of spin-to-charge conversion is estimated for two nanostructured samples with different dot sizes.
Quantitative angle-insensitive flow measurement using relative standard deviation OCT Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-31 Jiang Zhu, Buyun Zhang, Li Qi, Ling Wang, Qiang Yang, Zhuqing Zhu, Tiancheng Huo, Zhongping Chen
Incorporating different data processing methods, optical coherence tomography (OCT) has the ability for high-resolution angiography and quantitative flow velocity measurements. However, OCT angiography cannot provide quantitative information of flow velocities, and the velocity measurement based on Doppler OCT requires the determination of Doppler angles, which is a challenge in a complex vascular network. In this study, we report on a relative standard deviation OCT (RSD-OCT) method which provides both vascular network mapping and quantitative information for flow velocities within a wide range of Doppler angles. The RSD values are angle-insensitive within a wide range of angles, and a nearly linear relationship was found between the RSD values and the flow velocities. The RSD-OCT measurement in a rat cortex shows that it can quantify the blood flow velocities as well as map the vascular network in vivo.
Plasmon-enhanced optical nonlinearity for femtosecond all-optical switching Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02 Kuidong Wang, Long Chen, Haijuan Zhang, Hui-Hsin Hsiao, Din Ping Tsai, Jie Chen
Ultrafast all-optical switching in metals can be an efficient way for high-speed active photonic devices. However, with the improvement in modulation speed, typically by reducing the optical switching pulse width from picoseconds to femtoseconds, the nonlinear optical response of the metal will decrease significantly, which hinders the realization of the sufficient modulation depth at femtosecond optical control. Here, by combining two optical nonlinear enhancement effects of surface plasmon polaritons, including their extreme sensitivity to refractive index change and their capability to induce strong localized optical fields, we have achieved an ∼50-times enhancement in the modulation depth simultaneously with a switching time of ∼75-fs. Such enhancement was found to be independent of the control intensity, which sets a basis for the future application of femtosecond switching at a minimum power.
Room temperature 2D electron gas at the (001)-SrTiO3 surface Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-31 Sara Gonzalez, Claire Mathieu, Olivier Copie, Vitaliy Feyer, Claus M. Schneider, Nicholas Barrett
Functional oxides and phenomena such as a 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post-CMOS electronics. Non-invasive techniques are required to study the surface chemistry and electronic structure, underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy to chemistry and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. We present results demonstrating a room temperature 2DEG at the (001)-SrTiO3 surface. The 2DEG is switched on by soft X-ray irradiation.
Ultrasmooth metal thin films on curved fused silica by laser polishing Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-31 Gloria Anemone, Christian Weingarten, Amjad Al Taleb, Carlos Prieto, Daniel Farías
The fabrication of atomically smooth metal films on supporting oxides is a quite demanding task, since most physical vapor deposition methods used on metals do not work properly on oxide substrates. Here, we report an alternative procedure, based on performing laser polishing of a fused silica substrate before depositing the metallic thin film. This reduces the RMS surface roughness of fused silica by ca. 33%, and increases the maximum grain size of the metallic film from 200 nm to 1200 nm. The method has been applied to a fused silica parabolic lens, which has been coated with a graphene-terminated Ru thin film. The reduction of surface roughness caused by laser polishing leads to the formation of ultrasmooth Ru thin films. Crystallinity and subnanometer roughness of the metal coating are demonstrated by the observation of He diffraction from a macroscopically curved surface.
Atomic scale study of surface orientations and energies of Ti2O3 crystals Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-01 Meng Gu, Zhiguo Wang, Chongmin Wang, Jianming Zheng
For nanostructured particles, the faceting planes and their terminating chemical species are two critical factors that govern their chemical behavior. The surface atomistic structure and termination of Ti2O3 crystals were analyzed using atomic-scale aberration-corrected scanning transmission electron microscopy (STEM) combined with density functional theory (DFT) calculations. STEM imaging reveals that the Ti2O3 crystals are most often faceted along (001), (012), (−114), and (1–20) planes. The DFT calculation indicates that the (012) surface with TiO-termination has the lowest cleavage energy and correspondingly the lowest surface energy, indicating that (012) will be the most stable and prevalent surfaces in Ti2O3 nanocrystals. These observations provide insights for exploring the interfacial process involving Ti2O3 nanoparticles.
Tunable actuation of dielectric elastomer by electromechanical loading rates Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-01 Guorui Li, Mingqi Zhang, Xiangping Chen, Xuxu Yang, Tuck-Whye Wong, Tiefeng Li, Zhilong Huang
Dielectric elastomer (DE) membranes are able to self-deform with the application of an electric field through the thickness direction. In comparison to conventional rigid counterparts, soft actuators using DE provide a variety of advantages such as high compliance, low noise, and light weight. As one of the challenges in the development of DE actuating devices, tuning the electromechanical actuating behavior is crucial in order to achieve demanded loading paths and to avoid electromechanical failures. In this paper, our experimental results show that the electromechanical loading conditions affect the actuating behaviors of the DE. The electrical actuating force can be tuned by 29.4% with the control of the electrical charging rate. In addition, controllable actuations have been investigated by the mechanical model in manipulating the electromechanical loading rate. The calculated results agree well with the experimental data. Lastly, it is believed that the mechanisms of controlling the electromechanical loading rate may serve as a guide for the design of DE devices and high performance soft robots in the near future.
Investigation of high density two-dimensional electron gas in Zn-polar BeMgZnO/ZnO heterostructures Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-01 K. Ding, M. B. Ullah, V. Avrutin, Ü. Özgür, H. Morkoç
Zn-polar BeMgZnO/ZnO heterostructures grown by molecular beam epitaxy on high resistivity GaN templates producing high-density two-dimensional electron gas (2DEG) are investigated. This is motivated by the need to reach plasmon-longitudinal optical (LO) phonon resonance for attaining minimum LO phonon lifetime. Achievement of high 2DEG concentration in MgZnO/ZnO heterostructures requires growth of the MgZnO barrier at relatively low temperatures, which compromises the ternary quality that in turn hinders potential field effect transistor performance. When this ternary is alloyed further with BeO, the sign of strain in the BeMgZnO barrier on ZnO switches from compressive to tensile, making the piezoelectric and spontaneous polarizations to be additive in the BeMgZnO/ZnO heterostructures much like the Ga-polar AlGaN/GaN heterostructures. As a result, a 2DEG concentration of 1.2 × 1013 cm−2 is achieved in the Be0.03Mg0.41Zn0.56O/ZnO heterostructure. For comparison, a 2DEG concentration of 7.7 × 1012 cm−2 requires 2% Be and 26% Mg in the barrier, whereas the same in the MgZnO/ZnO system would require incorporation of more than 40% Mg into the barrier, which necessitates very low growth temperatures. Our results are consistent with the demands on achieving short LO phonon lifetimes through plasmon-LO phonon resonance for high carrier velocity.
1.3 μm single-photon emission from strain-coupled bilayer of InAs/GaAs quantum dots at the temperature up to 120 K Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-01 Yongzhou Xue, Zesheng Chen, Haiqiao Ni, Zhichuan Niu, Desheng Jiang, Xiuming Dou, Baoquan Sun
We report on 1.3 μm single-photon emission based on a self-assembled strain-coupled bilayer of InAs quantum dots (QDs) embedded in a micropillar Bragg cavity at temperature of liquid nitrogen or even as high as 120 K. The obtained single-photon flux into the first lens of the collection optics is 4.2 × 106 and 3.3 × 106/s at 82 and 120 K, respectively, corresponding to a second-order correlation function at zero delay times of 0.27(2) and 0.28(3). This work reports on the significant effect of the micropillar cavity-related enhancement of QD emission and demonstrates an opportunity to employ telecom band single-photon emitters at liquid nitrogen or even higher temperature.
Valence band splitting in bulk dilute bismides Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-01 Lars C. Bannow, Stefan C. Badescu, Jörg Hader, Jerome V. Moloney, Stephan W. Koch
The electronic structure of bulk GaAs1−xBix systems for different atomic configurations and Bi concentrations is calculated using density functional theory. The results show a Bi-induced splitting between the light-hole and heavy-hole bands at the Γ-point. We find a good agreement between our calculated splittings and experimental data. The magnitude of the splitting strongly depends on the local arrangement of the Bi atoms but not on the uni-directional lattice constant of the supercell. The additional influence of external strain due to epitaxial growth on GaAs substrates is studied by fixing the in-plane lattice constants.
Non-local electrical spin injection and detection in germanium at room temperature Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-31 F. Rortais, C. Vergnaud, A. Marty, L. Vila, J.-P. Attané, J. Widiez, C. Zucchetti, F. Bottegoni, H. Jaffrès, J.-M. George, M. Jamet
Non-local carrier injection/detection schemes lie at the very foundation of information manipulation in integrated systems. This paradigm consists in controlling with an external signal the channel where charge carriers flow between a “source” and a well separated “drain.” The next generation electronics may operate on the spin of carriers in addition to their charge and germanium appears as the best hosting material to develop such a platform for its compatibility with mainstream silicon technology and the predicted long electron spin lifetime at room temperature. In this letter, we demonstrate injection of pure spin currents (i.e., with no associated transport of electric charges) in germanium, combined with non-local spin detection at 10 K and room temperature. For this purpose, we used a lateral spin valve with epitaxially grown magnetic tunnel junctions as spin injector and spin detector. The non-local magnetoresistance signal is clearly visible and reaches ≈15 mΩ at room temperature. The electron spin lifetime and diffusion length are 500 ps and 1 μm, respectively, the spin injection efficiency being as high as 27%. This result paves the way for the realization of full germanium spintronic devices at room temperature.
Moving towards the magnetoelectric graphene transistor Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-01 Shi Cao, Zhiyong Xiao, Chun-Pui Kwan, Kai Zhang, Jonathan P. Bird, Lu Wang, Wai-Ning Mei, Xia Hong, P. A. Dowben
The interfacial charge transfer between mechanically exfoliated few-layer graphene and Cr2O3 (0001) surfaces has been investigated. Electrostatic force microscopy and Kelvin probe force microscopy studies point to hole doping of few-layer graphene, with up to a 150 meV shift in the Fermi level, an aspect that is confirmed by Raman spectroscopy. Density functional theory calculations furthermore confirm the p-type nature of the graphene/chromia interface and suggest that the chromia is able to induce a significant carrier spin polarization in the graphene layer. A large magnetoelectrically controlled magneto-resistance can therefore be anticipated in transistor structures based on this system, a finding important for developing graphene-based spintronic applications.
Magnetic compensation-induced sign reversal of exchange bias in a multi-glass perovskite SmFeO3 Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-01 Chandan De, Ajaya K. Nayak, Michael Nicklas, A. Sundaresan
We report an unusual sign reversal of exchange bias (EB) across a magnetic compensation point in an orthorhombic perovskite SmFeO3. A conventional negative EB with a positive vertical magnetization shift is observed below a cluster-glass freezing temperature (Tg ∼ 150 K). Upon further lowering of the temperature, the EB disappears at the magnetic compensation point before reversing its sign to a positive exchange bias below 4 K. The EB effect originates from an interfacial exchange interaction within a cluster glass phase, whereas its sign reversal arises from the reversal of the direction of the net magnetic moment as a result of dominance of Sm3+ over Fe3+ below the compensation temperature. The existence of a multi-glass state is demonstrated by ac-susceptibility and electrical permittivity measurements. A phenomenological model is presented to understand the EB effect and its sign reversal across the compensation point.
Photo-spin voltaic effect and photo-magnetoresistance in proximized platinum Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02 D. Li, A. Ruotolo
Spin orbit coupling in heavy metals allows the conversion of unpolarized light into an open-circuit voltage. We experimentally prove that this photo-spin voltaic effect is due to photo-excitation of carriers in the proximized layer and can exist for light in the visible range. While carrying out the experiment, we discovered that, in closed-circuit conditions, the anisotropic magnetoresistance of the proximized metal is a function of the light intensity. We name this effect photo-magnetoresistance. A magneto-transport model is presented that describes the change in magnetoresistance as a function of the light intensity.
Enhanced magnetization and anisotropy in Mn-Ga thin films grown on LSAT Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02 J. Karel, F. Casoli, L. Nasi, P. Lupo, R. Sahoo, B. Ernst, A. Markou, A. Kalache, R. Cabassi, F. Albertini, C. Felser
Epitaxial thin films of MnxGa1−x (x = 0.70, 0.74) grown on single crystal (LaAlO3)0.3(Sr2TaAlO6)0.7 [LSAT] substrates exhibit an enhanced magnetic moment and magnetic anisotropy in comparison to films of the same composition grown epitaxially on SrTiO3 [STO] single crystal substrates. Atomic and magnetic force microscopy revealed films exhibiting uniform grains and magnetic domain structures, with only minor differences between the films grown on different substrates. High resolution transmission electron microscopy on the x = 0.74 sample grown on LSAT showed a well-ordered, faceted film structure with the tetragonal c-axis oriented out of the film plane. Further, misfit dislocations, accommodating the lattice mismatch, were evidenced at the film/substrate interface. The out of plane c lattice parameter is larger for all x in the films grown on LSAT, due to the smaller substrate lattice parameter compared to STO. The increase in c generates a larger distortion of the tetragonal lattice which promotes the enhanced magnetization and magnetocrystalline anisotropy. These results indicate that LSAT is a promising substrate for realizing highly tailored magnetic properties for future spintronic applications not only in MnxGa1−x but also in the broader class of tetragonal Mn-Z-Ga (Z = transition metal) materials.
Magnetic memory effect at room temperature in exchange coupled NiFe2O4-NiO nanogranular system Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02 Zhaoming Tian, Longmeng Xu, Yuxia Gao, Songliu Yuan, Zhengcai Xia
Compared to the low temperature memory effect observed in magnetic nanoparticles (NPs), here we report a room temperature memory effect in a Ferrimagnetic (FiM)-Antiferromagnetic exchange coupled NiFe2O4-NiO nanogranular system, which is experimentally studied by different protocols of dc magnetization relaxation measurements below the blocking temperature TB = 345 K. The interfacial exchange coupling between the FiM NiFe2O4 clusters and the spin-glassy like phase is proposed to provide an additional anisotropic energy, leading to the enhancement of the magnetic memory effect up to room temperature. The observed memory effect is discussed based on the multiple distribution of energy barriers for both the FiM NPs and interfacial magnetic exchange anisotropy.
Hot-deformed Nd-Fe-B magnet with macroscopic composite structure Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02
Hot-deformed Nd-Fe-B magnets with a larger maximum energy product at higher operating temperature are desirable in a wide range of applications but are very challenging to realize in a common “single phase” structure. Here, we show the macroscopic structural design in hot-deformed magnets by using two kinds of melt-spun powders with/without heavy rare earth. Higher coercivity with a remarkably improved maximum energy product is obtained in the separated multilayer magnet. We find that the multilayer structure can improve the c-axis alignment of platelet-shaped grains in each layer and propose the possible interlayer's long-range magnetic interaction explaining the recoil loop open of designed magnets, coupled with visible field-induced domain evolution. This experimental approach reveals exciting applications of structural design in ultrafine-grained hot-deformed magnets.
Silicene spintronics: Fe(111)/silicene system for efficient spin injection Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02 Jiaqi Zhou, Arnaud Bournel, Yin Wang, Xiaoyang Lin, Yue Zhang, Weisheng Zhao
Silicene is an emerging 2D material with advantages of high carrier mobility, compatibility with the silicon-based semiconductor industry, and the tunable gap by a vertical electrical field due to the buckling structure. In this work, we report a first-principles investigation on the spin injection system, which consists of a Fe(111)/silicene stack as the spin injector and pure silicene as the spin channel. An extremely high spin injection efficiency (SIE) close to 100% is achieved. The partial density of states of Fe layers in the Fe(111)/silicene stack shows that spin-down states dominate above the Fermi level, resulting in a negligible spin-up current and high SIE. The transmission spectra have been investigated to analyze the spin-resolved properties. The spin injection system based on silicene is promising for the efficient silicon-based spintronics devices such as switching transistors.
Structure, transition temperature, and magnetoresistance of titanium-doped lanthanum barium manganite epilayers onto STO 001 substrates Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02 Aurelian Catalin Galca, Marwène Oumezzine, Aurel Leca, Cristina Florentina Chirila, Victor Kuncser, Andrei Kuncser, Corneliu Ghica, Iuliana Pasuk, Mohamed Oumezzine
We have developed a thin film structure with a maximum magnetoresistance effect (MRE) at room temperature, which is one of the operating requirements for many applications. It is shown that La0.67Ba0.33Ti0.02Mn0.98O3 epilayers obtained by pulsed laser deposition onto (001) SrTiO3 single crystal substrates exhibit the highest MRE, Δ R / R ( H ) ≈ 150 % or Δ R / R ( 0 ) ≈ 60 % under 5 T, at 300 K, a temperature near to the corresponding Curie temperature (TC). Both doping with a tiny amount of titanium and induced stress due to lattice mismatch between the thin film and the substrate contribute to a decrease in TC as compared to the pristine compound and therefore to the decrease in the temperature where the highest MRE is recorded.
Spin-orbit torques and Dzyaloshinskii-Moriya interaction in PtMn/[Co/Ni] heterostructures Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-03 S. DuttaGupta, T. Kanemura, C. Zhang, A. Kurenkov, S. Fukami, H. Ohno
Antiferromagnet (AFM)/ferromagnet (FM) heterostructures with broken inversion symmetry are perceived to open new opportunities for nonvolatile spintronic devices. Previous studies of such systems have demonstrated an emergence of spin-orbit torques (SOTs) in the heterostructures which are strong enough to bring about magnetization reversal. The impact of broken inversion symmetry and spin-orbit coupling also leads to an emergence of the Dzyaloshinskii-Moriya interaction (DMI) which governs the magnetic configuration and magnetization reversal. In this work, we study the SOT-induced effective fields and DMI in a heterostructure with an antiferromagnetic PtMn layer and a ferromagnetic [Co/Ni] multilayer and compare the results with a reference Pt/[Co/Ni] system. Magnetotransport measurements reveal the same sign and similar magnitude of SOT-induced effective fields for the two systems while current-induced domain wall motion measurements under in-plane fields reveal the opposite sign and smaller magnitude of DMI at the PtMn/[Co/Ni] interface compared to the Pt/[Co/Ni]. The obtained results offer in-depth information concerning the manifestations of spin-orbit interactions in AFM/FM systems, which is key to understanding of static magnetic configuration and magnetization reversal for their possible applications in antiferromagnetic spintronics.
Enhanced stability of magnetoelectric gyrators under high power conditions Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-31 Chung Ming Leung, Xin Zhuang, Min Gao, Xiao Tang, Junran Xu, Jiefang Li, Jitao Zhang, G. Srinivasan, D. Viehland
In this study, three different coil-based magnetoelectric (ME) gyrators of different geometries, including gyrators with high power output, have been designed and characterized. These included two magnetostrictive/piezoelectric/magnetostrictive (M-P-M) and one piezoelectric/magnetostrictive/piezoelectric (P-M-P) type ME gyrators, which consisted of nickel zinc ferrite (NZFO) and lead zirconate titanate (PZT) ceramic plates. Compared with M-P-M ME gyrators, the P-M-P ones exhibited a higher power efficiency (η) of 85% when operated at resonance under an optimal magnetic bias field (HBias) of 40 Oe at low power conditions. It retained a relatively high efficiency of η = 79% under a high input power density of 2.87 W/cm3. A low reduction in the magnetomechanical coupling and mechanical quality (k33,m and Qm) factors of the NZFO ferrite layer in the ME gyrator explains the resilience of the P-M-P type structure with increasing power drive. The findings open the possibility of using ME gyrators in high power applications.
Insights into antiferroelectrics from first-order reversal curves Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-01 Michael Hoffmann, Tony Schenk, Milan Pešić, Uwe Schroeder, Thomas Mikolajick
Antiferroelectric (AFE) HfO2 and ZrO2 based thin films are promising for energy and low power computing related applications. Here, we investigate 10 nm thin AFE Si:HfO2 films by means of first-order reversal curves (FORCs). Polarization-voltage, capacitance-voltage, and X-ray diffraction measurements confirm typical AFE behavior originating from the tetragonal phase. FORC analysis reveals two oppositely biased switching density peaks with a narrow distribution of coercive fields around 0.23 MV/cm, which is at least 4 times lower than that in typical ferroelectric HfO2 and ZrO2 films. The distributions along the internal bias field axis are much broader compared to the distribution of coercive fields. The exceptional stability of the switching density magnitude and coercive fields for up to 108 electric field cycles is demonstrated. Only small reductions of the internal bias fields are observed with cycling. These results highlight pathways towards improved cycling stability and variability of ferroelectric HfO2 and ZrO2 based devices as well as AFE supercapacitors with enhanced efficiency and energy storage density.
Sensing flexural motion of a photonic crystal membrane with InGaAs quantum dots Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-30 S. G. Carter, A. S. Bracker, M. K. Yakes, M. K. Zalalutdinov, M. Kim, C. S. Kim, C. Czarnocki, M. Scheibner, D. Gammon
Optical coupling between quantum dots and photonic crystal cavities and waveguides has been studied for many years in order to explore interesting physics and to advance quantum technologies. Here, we demonstrate strain-based coupling between mechanical motion of a photonic crystal membrane and embedded single InGaAs quantum dots. The response to high frequency mechanical vibration is measured for a series of quantum dots along the length of a photonic crystal waveguide for several flexural modes by optically driving the membrane while measuring high resolution time-resolved photoluminescence. The position-dependent response is similar to the measured and calculated displacement profile of the membrane but falls off less rapidly at higher frequencies. These results indicate potential for nanoscale strain sensing with high bandwidth and sensitivity.
Selective electroless plating of 3D-printed plastic structures for three-dimensional microwave metamaterials Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02 Atsushi Ishikawa, Taiki Kato, Nobuyuki Takeyasu, Kazuhiro Fujimori, Kenji Tsuruta
A technique of selective electroless plating onto PLA-ABS (Polylactic Acid-Acrylonitrile Butadiene Styrene) composite structures fabricated by three-dimensional (3D) printing is demonstrated to construct 3D microwave metamaterials. The reducing activity of the PLA surface is selectively enhanced by the chemical modification involving Sn2+ in a simple wet process, thereby forming a highly conductive Ag-plated membrane only onto the PLA surface. The fabricated metamaterial composed of Ag-plated PLA and non-plated ABS parts is characterized experimentally and numerically to demonstrate the important bi-anisotropic microwave responses arising from the 3D nature of metallodielectric structures. Our approach based on a simple wet chemical process allows for the creation of highly complex 3D metal-insulator structures, thus paving the way toward the sophisticated microwave applications of the 3D printing technology.
Field-effect enhanced triboelectric colloidal quantum dot flexible sensor Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02 Lingju Meng, Qiwei Xu, Shicheng Fan, Carson R. Dick, Xihua Wang
Flexible electronics, which is of great importance as fundamental sensor and communication technologies for many internet-of-things applications, has established a huge market encroaching into the trillion-dollar market of solid state electronics. For the capability of being processed by printing or spraying, colloidal quantum dots (CQDs) play an increasingly important role in flexible electronics. Although the electrical properties of CQD thin-films are expected to be stable on flexible substrates, their electrical performance could be tuned for applications in flexible touch sensors. Here, we report CQD touch sensors employing polydimethylsiloxane (PDMS) triboelectric films. The electrical response of touching activity is enhanced by incorporating CQD field-effect transistors into the device architecture. Thanks to the use of the CQD thin film as a current amplifier, the field-effect CQD touch sensor shows a fast response to various touching materials, even being bent to a large curvature. It also shows a much higher output current density compared to a PDMS triboelectric touch sensor.
Localization effects in the disordered Ta interlayer of multilayer Ta–FeNi films: Evidence from dc transport and spectroscopic ellipsometry study Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02 N. N. Kovaleva, D. Chvostova, O. Pacherova, L. Fekete, K. I. Kugel, F. A. Pudonin, A. Dejneka
Using dc transport and wide-band spectroscopic ellipsometry techniques, we study localization effects in the disordered metallic Ta interlayer of different thicknesses in the multilayer films (MLFs) (Ta–FeNi)N grown by rf sputtering deposition. In the grown MLFs, the FeNi layer was 0.52 nm thick, while the Ta layer thickness varied between 1.2 and 4.6 nm. The Ta layer dielectric function was extracted from the Drude-Lorentz simulation. The dc transport study of the MLFs implies non-metallic ( d ρ / d T < 0 ) behavior, with negative temperature coefficient of resistivity (TCR). The TCR absolute value increases upon increasing the Ta interlayer thickness, indicating enhanced electron localization. With that, the free charge carrier Drude response decreases. Moreover, the pronounced changes occur in the extended spectral range, involving the higher-energy Lorentz bands. The Drude dc conductivity drops below the weak localization limit for the thick Ta layer. The global band structure reconstruction may indicate the formation of a nearly localized many-body electron state.
High resolution magnetic field energy imaging of the magnetic recording head by A-MFM with Co-GdOx super-paramagnetic tip Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-03 Pawan Kumar, Yudai Suzuki, Yongze Cao, Satoru Yoshimura, Hitoshi Saito
In this letter, the concept of a high-resolution magnetic field energy imaging technique is demonstrated by a high susceptibility superparamagnetic Co-GdOx magnetic force microscopy (MFM) tip for a perpendicular magnetic recording head with alternating magnetic force microscopy (A-MFM). The distribution of the magnetic energy gradient from the perpendicular recording head is imaged by the Co-GdOx superparamagnetic tip and compared with magnetic field imaging by the FePt-MgO hard magnetic tip. The Fourier analysis of the A-MFM amplitude images revealed enhancement in a spatial resolution of 13 nm by the Co-GdOx superparamagnetic tip as compared to 17 nm by the state-of-the-art FePt-MgO hard magnetic tip. The magnetic dipolar nature and short range force character of magnetic energy imaging by the Co-GdOx superparamagnetic tip showed high performance, confirmed by the tip transfer function analysis as compared to the monopole type FePt-MgO hard magnetic tip. The proposed technique opens an opportunity for the development of advanced high-resolution magnetic energy based imaging methods and development of the high-resolution MFM tips.
Dynamics of direct X-ray detection processes in high-Z Bi2O3 nanoparticles-loaded PFO polymer-based diodes Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-30 A. Ciavatti, T. Cramer, M. Carroli, L. Basiricò, R. Fuhrer, D. M. De Leeuw, B. Fraboni
Semiconducting polymer based X-ray detectors doped with high-Z nanoparticles hold the promise to combine mechanical flexibility and large-area processing with a high X-ray stopping power and sensitivity. Currently, a lack of understanding of how nanoparticle doping impacts the detector dynamics impedes the optimization of such detectors. Here, we study direct X-ray radiation detectors based on the semiconducting polymer poly(9,9-dioctyfluorene) blended with Bismuth(III)oxide (Bi2O3) nanoparticles (NPs). Pure polymer diodes show a high mobility of 1.3 × 10−5 cm2/V s, a low leakage current of 200 nA/cm2 at −80 V, and a high rectifying factor up to 3 × 105 that allow us to compare the X-ray response of a polymer detector in charge-injection conditions (forward bias) and in charge-collection conditions (reverse bias), together with the impact of NP-loading in the two operation regimes. When operated in reverse bias, the detectors reach the state of the art sensitivity of 24 μC/Gy cm2, providing a fast photoresponse. In forward operation, a slower detection dynamics but improved sensitivity (up to 450 ± 150 nC/Gy) due to conductive gain is observed. High-Z NP doping increases the X-ray absorption, but higher NP loadings lead to a strong reduction of charge-carrier injection and transport due to a strong impact on the semiconductor morphology. Finally, the time response of optimized detectors showed a cut-off frequency up to 200 Hz. Taking advantage of such a fast dynamic response, we demonstrate an X-ray based velocity tracking system.
Low-voltage operating flexible ferroelectric organic field-effect transistor nonvolatile memory with a vertical phase separation P(VDF-TrFE-CTFE)/PS dielectric Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-30 Meili Xu, Lanyi Xiang, Ting Xu, Wei Wang, Wenfa Xie, Dayu Zhou
Future flexible electronic systems require memory devices combining low-power operation and mechanical bendability. However, high programming/erasing voltages, which are universally needed to switch the storage states in previously reported ferroelectric organic field-effect transistor (Fe-OFET) nonvolatile memories (NVMs), severely prevent their practical applications. In this work, we develop a route to achieve a low-voltage operating flexible Fe-OFET NVM. Utilizing vertical phase separation, an ultrathin self-organized poly(styrene) (PS) buffering layer covers the surface of the ferroelectric polymer layer by one-step spin-coating from their blending solution. The ferroelectric polymer with a low coercive field contributes to low-voltage operation in the Fe-OFET NVM. The polymer PS contributes to the improvement of mobility, attributing to screening the charge scattering and decreasing the surface roughness. As a result, a high performance flexible Fe-OFET NVM is achieved at the low P/E voltages of ±10 V, with a mobility larger than 0.2 cm2 V−1 s−1, a reliable P/E endurance over 150 cycles, stable data storage retention capability over 104 s, and excellent mechanical bending durability with a slight performance degradation after 1000 repetitive tensile bending cycles at a curvature radius of 5.5 mm.
Analysis of the threshold switching mechanism of a Te–SbO selector device for crosspoint nonvolatile memory applications Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-30 Young Seok Kim, Ji Woon Park, Jong Ho Lee, In Ah Choi, Jaeyeong Heo, Hyeong Joon Kim
The threshold switching mechanism of Te–SbO thin films with a unique microstructure in which a Te nanocluster is present in the SbO matrix is analyzed. During the electro-forming process, amorphous Te filaments are formed in the Te nanocluster. However, unlike conventional Ovonic threshold switching (TS) selector devices, it has been demonstrated that the off-current flows along the filament. Numerical calculations show that the off-current is due to the trap present in the filament. We also observed changes in TS parameters through controls in the strength or volume of the filaments.
Revealing the recombination dynamics in squaraine-based bulk heterojunction solar cells Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-30 Dorothea Scheunemann, Oliver Kolloge, Sebastian Wilken, Majvor Mack, Jürgen Parisi, Matthias Schulz, Arne Lützen, Manuela Schiek
We combine steady-state with transient optoelectronic characterization methods to understand the operation of photovoltaic devices based on a benchmark model squaraine blended with a fullerene acceptor. These devices suffer from a gradual decrease in the fill factor when increasing the active layer thickness and incident light intensity. Using transient photocurrent, transient photovoltage, and bias-assisted charge extraction measurements, we show that the fill factor deteriorates due to slow charge carrier collection competing with bimolecular recombination. Under normal operating conditions, we find a bimolecular recombination rate constant of ∼10–17 m3 s−1, which corresponds to a reduction of one to two orders of magnitude compared to the Langevin model.
Synchronization of a micromechanical oscillator in different regimes of electromechanical nonlinearity Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-30 P. Taheri-Tehrani, M. Defoort, D. A. Horsley
In this letter, we investigate the dynamics of injection-locking a nonlinear micromechanical oscillator operating in different regimes of electromechanical nonlinearity to an external tone generated by a secondary oscillator. The micromechanical oscillator exhibits a combination of mechanical and electrostatic nonlinearities that were tuned using a bias voltage to adjust the relative importance of third-order and fifth-order stiffness nonlinearities. While it is well-known that third-order stiffness (Duffing) nonlinearity results in a synchronization range that increases with an oscillator's amplitude, little is known about the impact of other nonlinearities. We show that when using Duffing nonlinearity cancellation, higher order nonlinearities dominate, the synchronization range is smaller but has a greater rate-of-increase with oscillation amplitude. When both mechanical stiffness-hardening and electrostatic stiffness-softening nonlinearities are present, the frequency response follows an “s-curve” and, unlike the other conditions, the synchronization range does not increase monotonically with amplitude but instead reaches a minimum when both nonlinearities have similar magnitude. We develop a nonlinear resonator model and show that this model achieves good quantitative prediction of the measured synchronization range in all nonlinear operating regimes studied.
Improved resistive switching reliability by using dual-layer nanoporous carbon structure Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-31 Ye Tao, Xuhong Li, Zhongqiang Wang, Haiyang Xu, Wentao Ding, Jiangang Ma, Yichun Liu
We optimized the diameter and microgeometry of preformed conductive filaments (CFs) to improve the switching reliability of copper/nanoporous amorphous carbon (a-C)/platinum memory devices. Forming-free devices were obtained because of the introduction of preformed CFs into the nanoporous layer during the copper electrode evaporation process. The switching fluctuation decreased with the increasing preformed CF size in a certain range; however, the device with stronger preformed CFs suffered from high current in the first RESET process. Furthermore, to achieve both high switching uniformity and low power consumption, a dual-layer structure was proposed to regulate the microgeometry of preformed CFs. Compared with those of a pristine device and single-layer nanoporous device, the fluctuation of high/low resistance values was further suppressed to 26% and 21%, respectively. In addition, Resistive random access memory (RRAM) devices exhibited a fast switching speed (<50 ns), excellent endurance (>105 cycles), and long retention time (>105 s at 85 °C). These results reveal the key role of preformed CF optimization in resistive switching performance improvement, providing an effective approach to develop high-performance RRAM devices.
Mutual 3:1 subharmonic synchronization in a micromachined silicon disk resonator Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-01 Parsa Taheri-Tehrani, Andrea Guerrieri, Martial Defoort, Attilio Frangi, David A. Horsley
We demonstrate synchronization between two intrinsically coupled oscillators that are created from two distinct vibration modes of a single micromachined disk resonator. The modes have a 3:1 subharmonic frequency relationship and cubic, non-dissipative electromechanical coupling between the modes enables their two frequencies to synchronize. Our experimental implementation allows the frequency of the lower frequency oscillator to be independently controlled from that of the higher frequency oscillator, enabling study of the synchronization dynamics. We find close quantitative agreement between the experimental behavior and an analytical coupled-oscillator model as a function of the energy in the two oscillators. We demonstrate that the synchronization range increases when the lower frequency oscillator is strongly driven and when the higher frequency oscillator is weakly driven. This result suggests that synchronization can be applied to the frequency-selective detection of weak signals and other mechanical signal processing functions.
Quantification of intergranular exchange coupling in CoPtCr-based perpendicular recording media via ferromagnetic resonance measurements Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02 Daniel Richardson, Kumar Srinivasan, Sidney Katz, Mingzhong Wu
Intergranular exchange fields in CoPtCr granular media materials were quantified through ferromagnetic resonance measurements in various magnetic states. The data indicate that the exchange field in CoPtCr granular films with no oxide segregant is comparable to the saturation magnetization of the films. With an introduction of a SiO2 segregant, however, the exchange field decreases. A 30% volume fraction of the segregant reduces the strength of the intergranular exchange coupling to zero.
Measurement of subcell depletion layer capacitances in multijunction solar cells Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-02 M. Rutzinger, M. Salzberger, A. Gerhard, H. Nesswetter, P. Lugli, C. G. Zimmermann
A method for measuring subcell capacitance voltage (C–V) in a multijunction solar cell is introduced. The subcell of interest is illuminated by a monochromatic light pulse with a ns rise time. The subcell capacitance is calculated from the measured rise time of the solar cell voltage. The effect of optical coupling is eliminated by introducing a high intensity bias illumination to all subcells below the one measured. The method is verified by comparing the subcell capacitance obtained from four junction solar cells with the results from corresponding component cells, which can be measured using well-established methods. From the C–V curves, the built-in voltage and the base layer doping density for each subcell are calculated.
Tracing molecular dephasing in biological tissue Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-31 M. Mokim, C. Carruba, F. Ganikhanov
We demonstrate the quantitative spectroscopic characterization and imaging of biological tissue using coherent time-domain microscopy with a femtosecond resolution. We identify tissue constituents and perform dephasing time (T2) measurements of characteristic Raman active vibrations. This was shown in subcutaneous mouse fat embedded within collagen rich areas of the dermis and the muscle connective tissue. The demonstrated equivalent spectral resolution (<0.3 cm−1) is an order of magnitude better compared to commonly used frequency-domain methods for characterization of biological media. This provides with the important dimensions and parameters in biological media characterization and can become an effective tool in detecting minute changes in the bio-molecular composition and environment that is critical for molecular level diagnosis.
A microfluidic needle for sampling and delivery of chemical signals by segmented flows Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-31 Shilun Feng, Guozhen Liu, Lianmei Jiang, Yonggang Zhu, Ewa M. Goldys, David W. Inglis
We have developed a microfluidic needle-like device that can extract and deliver nanoliter samples. The device consists of a T-junction to form segmented flows, parallel channels to and from the needle tip, and seven hydrophilic capillaries at the tip that form a phase-extraction region. The main microchannel is hydrophobic and carries segmented flows of water-in-oil. The hydrophilic capillaries transport the aqueous phase with a nearly zero pressure gradient but require a pressure gradient of 19 kPa for mineral oil to invade and flow through. Using this device, we demonstrate the delivery of nanoliter droplets and demonstrate sampling through the formation of droplets at the tip of our device. During sampling, we recorded the fluorescence intensities of the droplets formed at the tip while varying the concentration of dye outside the tip. We measured a chemical signal response time of approximately 3 s. The linear relationship between the recorded fluorescence intensity of samples and the external dye concentration (10–40 μg/ml) indicates that this device is capable of performing quantitative, real-time measurements of rapidly varying chemical signals.
Mg shallow doping effects on the ac magnetic self-heating characteristics of γ-Fe2O3 superparamagnetic nanoparticles for highly efficient hyperthermia Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-01 Jung-tak Jang, Seongtae Bae
The effects of Mg doping on the magnetic and AC self-heating temperature rising characteristics of γ-Fe2O3 superparamagnetic nanoparticles (SPNPs) were investigated for hyperthermia applications in biomedicine. The doping concentration of nonmagnetic Mg2+ cation was systematically controlled from 0 to 0.15 at. % in Mgx-γFe2O3 SPNPs during chemically and thermally modified one-pot thermal decomposition synthesis under bubbling O2/Ar gas mixture. It was empirically observed that the saturation magnetization (Ms) and the out-of-phase magnetic susceptibility ( χ m ″ ) of Mgx-γFe2O3 SPNPs were increased by increasing the Mg2+ cation doping concentration from 0.05 to 0.13 at. %. Correspondingly, the AC magnetically induced self-heating temperature (Tac,max) in solid state and the intrinsic loss power in water were increased up to 184 °C and 14.2 nH m2 kg−1 (Mgx-γFe2O3, x = 0.13), respectively, at the biologically and physiologically safe range of AC magnetic field (Happl × fappl = 1.2 × 109 A m−1 s−1). All the chemically and physically analyzed results confirmed that the dramatically improved AC magnetic induction heating characteristics and the magnetic properties of Mgx-γFe2O3 SPNPs (x = 0.13) are primarily due to the significantly enhanced magnetic susceptibility (particularly, χ m ″ ) and the improved AC/DC magnetic softness (lower AC/DC magnetic anisotropy) resulting from the systematically controlled nonmagnetic Mg2+ cation concentrations and distributions (occupation ratio) in the Fe vacancy sites of γ-Fe2O3 (approximately 12% vacancy), instead of typically well-known Fe3O4 (no vacancy) SPNPs. The cell viability and biocompatibility with U87 MG cell lines demonstrated that Mgx-γFe2O3 SPNPs (x = 0.13) has promising bio-feasibility for hyperthermia agent applications.
Self-assembled nanotextures impart broadband transparency to glass windows and solar cell encapsulants Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-30 Andreas C. Liapis, Atikur Rahman, Charles T. Black
Most optoelectronic components and consumer display devices require glass or plastic covers for protection against the environment. Optical reflections from these encapsulation layers can degrade the device performance or lessen the user experience. Here, we use a highly scalable self-assembly based approach to texture glass surfaces at the nanoscale, reducing reflections by such an extent so as to make the glass essentially invisible. Our nanotextures provide broadband antireflection spanning visible and infrared wavelengths (450–2500 nm) that is effective even at large angles of incidence. This technology can be used to improve the performance of photovoltaic devices by eliminating reflection losses, which can be as much as 8% for glass encapsulated cells. In contrast, solar cells encapsulated with nanotextured glass generate the same photocurrent as when operated without a cover. Ultra-transparent windows having surface nanotextures on both sides can withstand three times more optical fluence than commercial broadband antireflection coatings, making them useful for pulsed laser applications.
High efficiency and non-Richardson thermionics in three dimensional Dirac materials Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-10-30 Sunchao Huang, Matthew Sanderson, Yan Zhang, Chao Zhang
Three dimensional (3D) topological materials have a linear energy dispersion and exhibit many electronic properties superior to conventional materials such as fast response times, high mobility, and chiral transport. In this work, we demonstrate that 3D Dirac materials also have advantages over conventional semiconductors and graphene in thermionic applications. The low emission current suffered in graphene due to the vanishing density of states is enhanced by an increased group velocity in 3D Dirac materials. Furthermore, the thermal energy carried by electrons in 3D Dirac materials is twice of that in conventional materials with a parabolic electron energy dispersion. As a result, 3D Dirac materials have the best thermal efficiency or coefficient of performance when compared to conventional semiconductors and graphene. The generalized Richardson-Dushman law in 3D Dirac materials is derived. The law exhibits the interplay of the reduced density of states and enhanced emission velocity.
Bi3.25La0.75Ti3O12 thin film capacitors for energy storage applications Appl. Phys. Lett. (IF 3.411) Pub Date : 2017-11-01 B. B. Yang, M. Y. Guo, D. P. Song, X. W. Tang, R. H. Wei, L. Hu, J. Yang, W. H. Song, J. M. Dai, X. J. Lou, X. B. Zhu, Y. P. Sun
Environmentally benign Bi3.25La0.75Ti3O12 (BLTO) thin film capacitors were prepared by a cost effective chemical solution deposition method for high energy density storage device applications. Low annealing temperature annealed BLTO thin films showed very slim hysteresis loops with high maximum and small remnant polarization values. Increasing the applied electric field to 2040 kV/cm, the optimized BLTO thin films show a high recoverable energy density of 44.7 J/cm3 and an energy efficiency of 78.4% at room temperature. Additionally, the BLTO thin film capacitors exhibited excellent fatigue endurance after 4 × 108 cycles and a good thermal stability up to 140 °C, proving their strong potential for high energy density storage and conversion applications.
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