High-efficiency optical terahertz modulation of aligned Ag nanowires on a Si substrate Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-12 Gyuseok Lee, Inhee Maeng, Chul Kang, Myoung-Kyu Oh, Chul-Sik Kee
High-efficiency optical modulation of a terahertz pulse transmitted through aligned silver nanowires on a silicon substrate is demonstrated. Without optical excitation, the terahertz pulses mostly pass through the silver nanowires. However, an optically excited sample significantly modulates the transmittance compared with an excited silicon substrate. The enhanced modulation efficiency is explained by the redistribution effect of photo-carriers due to the nanowires. The simple structure of metal nanowires on a semiconductor substrate could be useful in implementing optically tunable terahertz wave modulators.
Reconfigurable generation and measurement of mutually unbiased bases for time-bin qudits Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-14 Joseph M. Lukens, Nurul T. Islam, Charles Ci Wen Lim, Daniel J. Gauthier
We propose a method for implementing mutually unbiased generation and measurement of time-bin qudits using a cascade of electro-optic phase modulator–coded fiber Bragg grating pairs. Our approach requires only a single spatial mode and can switch rapidly between basis choices. We obtain explicit solutions for dimensions d = 2, 3, and 4 that realize all d + 1 possible mutually unbiased bases and analyze the performance of our approach in quantum key distribution. Given its practicality and compatibility with current technology, our approach provides a promising springboard for scalable processing of high-dimensional time-bin states.
Room temperature operation of InxGa1−xSb/InAs type-II quantum well infrared photodetectors grown by MOCVD Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-14 D. H. Wu, Y. Y. Zhang, M. Razeghi
We demonstrate room temperature operation of In0.5Ga0.5Sb/InAs type-II quantum well photodetectors on an InAs substrate grown by metal-organic chemical vapor deposition. At 300 K, the detector exhibits a dark current density of 0.12 A/cm2 and a peak responsivity of 0.72 A/W corresponding to a quantum efficiency of 23.3%, with the calculated specific detectivity of 2.4 × 109 cm Hz1/2/W at 3.81 μm.
Enhancement of slope efficiency and output power in GaN-based vertical-cavity surface-emitting lasers with a SiO2-buried lateral index guide Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Masaru Kuramoto, Seiichiro Kobayashi, Takanobu Akagi, Komei Tazawa, Kazufumi Tanaka, Tatsuma Saito, Tetsuya Takeuchi
We have achieved a high output power of 6 mW from a 441 nm GaN-based vertical-cavity surface-emitting laser (VCSEL) under continuous wave (CW) operation, by reducing both the internal loss and the reflectivity of the front cavity mirror. A preliminary analysis of the internal loss revealed an enormously high transverse radiation loss in a conventional GaN-based VCSEL without lateral optical confinement (LOC). Introducing an LOC structure enhanced the slope efficiency by a factor of 4.7, with a further improvement to a factor of 6.7 upon reducing the front mirror reflectivity. The result was a slope efficiency of 0.87 W/A and an external differential quantum efficiency of 32% under pulsed operation. A flip-chip-bonded VCSEL also exhibited a high slope efficiency of 0.64 W/A and an external differential quantum efficiency of 23% for the front-side output under CW operation. The reflectivity of the cavity mirror was adjusted by varying the number of AlInN/GaN distributed Bragg reflector pairs from 46 to 42, corresponding to reflectivity values from 99.8% to 99.5%. These results demonstrate that a combination of internal loss reduction and cavity mirror control is a very effective way of obtaining a high output GaN-based VCSEL.
Ultrafast electric phase control of a single exciton qubit Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Alex Widhalm, Amlan Mukherjee, Sebastian Krehs, Nandlal Sharma, Peter Kölling, Andreas Thiede, Dirk Reuter, Jens Förstner, Artur Zrenner
We report on the coherent phase manipulation of quantum dot excitons by electric means. For our experiments, we use a low capacitance single quantum dot photodiode which is electrically controlled by a custom designed SiGe:C BiCMOS chip. The phase manipulation is performed and quantified in a Ramsey experiment, where ultrafast transient detuning of the exciton energy is performed synchronous to double pulse π/2 ps laser excitation. We are able to demonstrate electrically controlled phase manipulations with magnitudes up to 3π within 100 ps which is below the dephasing time of the quantum dot exciton.
Continuous-wave operation of m-plane GaN-based vertical-cavity surface-emitting lasers with a tunnel junction intracavity contact Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Charles A. Forman, SeungGeun Lee, Erin C. Young, Jared A. Kearns, Daniel A. Cohen, John T. Leonard, Tal Margalith, Steven P. DenBaars, Shuji Nakamura
We have achieved continuous-wave (CW) operation of an optically polarized m-plane GaN-based vertical-cavity surface-emitting laser (VCSEL) with an ion implanted current aperture, a tunnel junction intracavity contact, and a dual dielectric distributed Bragg reflector design. The reported VCSEL has 2 quantum wells, with a 14 nm quantum well width, 1 nm barriers, a 5 nm electron-blocking layer, and a 23 λ total cavity thickness. The thermal performance was improved by increasing the cavity length and using Au-In solid-liquid interdiffusion bonding, which led to lasing under CW operation for over 20 min. Lasing wavelengths under pulsed operation were observed at 406 nm, 412 nm, and 419 nm. Only the latter two modes appeared under CW operation due to the redshifted gain at higher temperatures. The peak output powers for a 6 μ m aperture VCSEL under CW and pulsed operation were 140 μ W and 700 μ W, respectively. The fundamental transverse mode was observed without the presence of filamentary lasing. The thermal impedance was estimated to be ∼1400 °C/W for a 6 μ m aperture 23 λ VCSEL.
Three-dimensional spatially curved local Bessel beams generated by metasurface Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Dawei Liu, Jiawen Wu, Bo Cheng, Hongliang Li
We propose a reflective metasurface based on an artificial admittance modulation surface to generate three-dimensional spatially curved beams. The phase acquisition utilized to modulate this sinusoidally varying surface admittance combines the enveloping theory of differential geometry and the method for producing two-dimensional Bessel beams. The metasurface is fabricated, and the comparison between the full-wave simulations and experimental results demonstrates good performance of three-dimensional spatially curved beams generated by the metasurface.
Comparison of microrings and microdisks for high-speed optical modulation in silicon photonics Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Zhoufeng Ying, Zheng Wang, Zheng Zhao, Shounak Dhar, David Z. Pan, Richard Soref, Ray T. Chen
The past several decades have witnessed the gradual transition from electrical to optical interconnects, ranging from long-haul telecommunication to chip-to-chip interconnects. As one type of key component in integrated optical interconnect and high-performance computing, optical modulators have been well developed these past few years, including ultrahigh-speed microring and microdisk modulators. In this paper, a comparison between microring and microdisk modulators is well analyzed in terms of dimensions, static and dynamic power consumption, and fabrication tolerance. The results show that microdisks have advantages over microrings in these aspects, which gives instructions to the chip design of high-density integrated systems for optical interconnects and optical computing.
Magnetic-field-dependent slow light in strontium atom-cavity system Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Zeng-Xing Liu, Bao Wang, Cui Kong, Hao Xiong, Ying Wu
Realizing and controlling a long-lived slow light is of fundamental importance in physics and may find applications in quantum router and quantum information processing. In this work, we propose a feasible scheme to realize the slow light in a strontium atom-cavity system, in which the value of group delay can be continuously adjusted within a range of different Zeeman splittings and vacuum Rabi frequencies by varying the applied static magnetic field and the atom number instead of a strong coherent field. In our scheme, the major limitations of the slow-light structure, namely, dispersion and loss, can be effectively resolved, and so our scheme may help to achieve the practical application of slow light relevant to the optical communication network.
Independence of surface morphology and reconstruction during the thermal preparation of perovskite oxide surfaces Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Maren Jäger, Ali Teker, Jochen Mannhart, Wolfgang Braun
Using a CO2 laser to directly heat the crystals from the back side, SrTiO3 substrates may be thermally prepared in situ for epitaxy without the need for ex-situ etching and annealing. We find that the formation of large terraces with straight steps at 900–1100 °C is independent of the formation of the ideal surface reconstruction for epitaxy, which requires temperatures in excess of 1200 °C to complete. The process may be universal, at least for perovskite oxide surfaces, as it also works, at different temperatures, for LaAlO3 and NdGaO3, two other widely used oxide substrate materials.
Surface-initiated phase transition in solid hydrogen under the high-pressure compression Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Haile Lei, Wei Lin, Kai Wang, Xibo Li
The large-scale molecular dynamics simulations have been performed to understand the microscopic mechanism governing the phase transition of solid hydrogen under the high-pressure compression. These results demonstrate that the face-centered-cubic-to-hexagonal close-packed phase transition is initiated first at the surfaces at a much lower pressure than in the volume and then extends gradually from the surface to volume in the solid hydrogen. The infrared spectra from the surface are revealed to exhibit a different pressure-dependent feature from those of the volume during the high-pressure compression. It is thus deduced that the weakening intramolecular H-H bonds are always accompanied by hardening surface phonons through strengthening the intermolecular H2-H2 coupling at the surfaces with respect to the counterparts in the volume at high pressures. This is just opposite to the conventional atomic crystals, in which the surface phonons are softening. The high-pressure compression has further been predicted to force the atoms or molecules to spray out of surface to degrade the pressure. These results provide a glimpse of structural properties of solid hydrogen at the early stage during the high-pressure compression.
Mechanical response of CH3NH3PbI3 nanowires Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-12 L. Ćirić, K. Ashby, T. Abadie, M. Spina, M. Duchamp, B. Náfrádi, M. Kollár, L. Forró, E. Horváth
We report a systematic study of the mechanical response of methylammonium lead triiodide CH3NH3PbI3 nanowires by employing bending measurements using atomic force microscope on suspended wires over photo-lithographically patterned channels. Force-deflection curves measured at room temperature give a Young's modulus between 2 and 14 GPa. This broad range of values is attributed to the variations in the microcrystalline texture of halide perovskite nanowires. The mechanical response of a highly crystalline nanowire is linear with force and has a brittle character. The braking modulus of 48 ± 20 MPa corresponds to 100 μm of radius of curvature of the nanowires, rendering them much better structures for flexible devices than spin coated films. The measured moduli decrease rapidly if the NW is exposed to water vapor.
Trochoidal X-ray Vector Radiography: Directional dark-field without grating stepping Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-12 Y. Sharma, S. Bachche, M. Kageyama, M. Kuribayashi, F. Pfeiffer, T. Lasser, A. Momose
X-ray Vector Radiography (XVR) is an imaging technique that reveals the orientations of sub-pixel sized structures within a sample. Several dark-field radiographs are acquired by rotating the sample around the beam propagation direction and stepping one of the gratings to several positions for every pose of the sample in an X-ray grating interferometry setup. In this letter, we present a method of performing XVR of a continuously moving sample without the need of any grating motion. We reconstruct the orientations within a sample by analyzing the change in the background moire fringes caused by the sample moving and simultaneously rotating in plane (trochoidal trajectory) across the detector field-of-view. Avoiding the motion of gratings provides significant advantages in terms of stability and repeatability, while the continuous motion of the sample makes this kind of system adaptable for industrial applications such as the scanning of samples on a conveyor belt. Being the first step in the direction of utilizing advanced sample trajectories to replace grating motion, this work also lays the foundations for a full three dimensional reconstruction of scattering function without grating motion.
Quantitative evaluation of the mechanical strength of titanium/composite bonding using laser-generated shock waves Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-14 M. Ducousso, S. Bardy, Y. Rouchausse, T. Bergara, F. Jenson, L. Berthe, L. Videau, N. Cuvillier
Intense acoustic shock waves were applied to evaluate the mechanical strength of structural epoxy bonds between a TA6V4 titanium alloy and a 3D woven carbon/epoxy composite material. Two bond types with different mechanical strengths were obtained from two different adhesive reticulations, at 50% and 90% of conversion, resulting in longitudinal static strengths of 10 and 39 MPa and transverse strengths of 15 and 35 MPa, respectively. The GPa shock waves were generated using ns-scale intense laser pulses and reaction principles to a confined plasma expansion. Simulations taking into account the laser–matter interaction, plasma relaxation, and non-linear shock wave propagation were conducted to aid interpretation of the experiments. Good correlations were obtained between the experiments and the simulation and between different measurement methods of the mechanical strength (normalized tests vs laser-generated shock waves). Such results open the door toward certification of structural bonding.
Photoacoustic technique to measure temperature effects on microbubble viscoelastic properties Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-14 Jordan S. Lum, David M. Stobbe, Mark A. Borden, Todd W. Murray
Phospholipid-coated microbubbles are being developed for several biomedical applications, but little is known about the effect of temperature on the viscoelastic properties of the shell. Here, we report on the use of a photoacoustic technique to study the shell properties of individual microbubbles as a function of temperature. The microbubbles were driven into small-amplitude oscillations by ultrasound waves generated from the absorption of an intensity-modulated infrared laser, and these oscillations were detected by forward-light scattering of a second blue laser. The drive laser modulation frequency was swept to determine the resonant response of 2–4 μm radius microbubbles. Lipid shell elasticity and viscosity were determined by modeling the microbubble response as a linear harmonic oscillator. The results from slow heating showed a linear decrease in elasticity and viscosity between 21 and 53 °C and a corresponding increase in the maximum oscillation amplitude. Rapid heating to 38 °C, on the other hand, showed a transient response in the viscoelastic properties, suggesting shell rupture and reformation during microbubble growth and subsequent dissolution. These effects are important for biomedical applications, which require warming of the microbubbles to body temperature.
Temperature-dependent and optimized thermal emission by spheres Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-14 K.L. Nguyen, O. Merchiers, P.-O. Chapuis
We investigate the temperature and size dependencies of thermal emission by homogeneous spheres as a function of their dielectric properties. Different power laws obtained in this work show that the emitted power can depart strongly from the usual fourth power of temperature given by Planck's law and from the square or the cube of the radius. We also show how to optimize the thermal emission by selecting permittivities leading to resonances, which allow for the so-called super-Planckian regime. These results will be useful as spheres, i.e. the simplest finite objects, are often considered as building blocks of more complex objects.
Structural evaluation of reduced graphene oxide in graphene oxide during ion irradiation: X-ray absorption spectroscopy and in-situ sheet resistance studies Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 K. Saravanan, G. Jayalakshmi, K. Suresh, B. Sundaravel, B. K. Panigrahi, D. M. Phase
We report the structural evolution of reduced graphene oxide (rGO) in graphene oxide (GO) flakes during 1 MeV Si+ ion irradiation. In-situ electrical resistivity measurements facilitate monitoring the sheet resistance with the increase in the fluence. The electrical sheet resistance of the GO flake shows the exponential decay behaviour with the increasing ion fluence. Raman spectra of the GO flake reveal the increase in the ID/IG ratio, indicating restoration of the sp2 network upon irradiation. The C/O ratio estimated from resonant Rutherford backscattering spectrometry analysis directly evidenced the reduction of oxygen moieties upon irradiation. C K–edge X-ray absorption near edge structure spectra reveal the restoration of C=C sp2–hybridized carbon atoms and the removal of oxygen-containing functional groups in the GO flake. STM data reveal the higher conductance in the rGO regime in comparison with the regime, where the oxygen functional groups are present. The experimental investigation demonstrates that the ion irradiation can be employed for efficient reduction of GO with tunable electrical and structural properties.
Effects of stress on neighboring laser written waveguides in gallium lanthanum sulfide Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Romina Diener, Stefan Nolte, Thomas Pertsch, Stefano Minardi
We investigate an observed lack of excitation symmetry of discrete diffraction patterns in periodic arrays of waveguides written by ultrafast laser inscription (ULI) in gallium lanthanum sulfide glasses. We found experimentally that successive waveguides written with identical parameters are detuned from the previous one by Δβ ∼ 0.2–0.5 mm−1. We show that by varying the writing speed of successive waveguides, we increase the symmetry of the array and reduce the detuning by a factor of 2. After careful analysis of possible physical causes, observations suggest that the density of the laser irradiated material is affected by long range stresses induced by ULI.
Plane shock loading on mono- and nano-crystalline silicon carbide Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Paulo S. Branicio, Jingyun Zhang, José P. Rino, Aiichiro Nakano, Rajiv K. Kalia, Priya Vashishta
The understanding of the nanoscale mechanisms of shock damage and failure in SiC is essential for its application in effective and damage tolerant coatings. We use molecular-dynamics simulations to investigate the shock properties of 3C-SiC along low-index crystallographic directions and in nanocrystalline samples with 5 nm and 10 nm grain sizes. The predicted Hugoniot in the particle velocity range of 0.1 km/s–6.0 km/s agrees well with experimental data. The shock response transitions from elastic to plastic, predominantly deformation twinning, to structural transformation to the rock-salt phase. The predicted strengths from 12.3 to 30.9 GPa, at the Hugoniot elastic limit, are in excellent agreement with experimental data.
Symmetry blockade and its breakdown in energy equipartition of square graphene resonators Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-16 Yisen Wang, Zhigang Zhu, Yong Zhang, Liang Huang
The interaction between flexural modes due to nonlinear potentials is critical to heat conductivity and mechanical vibration of two dimensional materials such as graphene. Much effort has been devoted to understanding the underlying mechanism. In this paper, we examine solely the out-of-plane flexural modes and identify their energy flow pathway during the equipartition process. In particular, the modes are grouped into four classes by their distinct symmetries. The couplings are significantly larger within a class than between classes, forming symmetry blockades. As a result, the energy first flows to the modes in the same symmetry class. Breakdown of the symmetry blockade, i.e., inter-class energy flow, starts when the displacement profile becomes complex and the inter-class couplings bear nonneglectable values. The equipartition time follows the stretched exponential law and survives in the thermodynamic limit. These results bring fundamental understandings to the Fermi-Pasta-Ulam problem in two dimensional systems with complex potentials and reveal clearly the physical picture of dynamical interactions between the flexural modes, which will be crucial to the understanding of their contribution in high thermal conductivity and mechanism of energy dissipation that may intrinsically limit the quality factor of the resonator.
Preparation of indium tin oxide contact to n-CdZnTe gamma-ray detector Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-14 Leqi Li, Yadong Xu, Binbin Zhang, Aoqiu Wang, Jiangpeng Dong, Hui Yu, Wanqi Jie
The nonmetal electrode material Indium Tin Oxide (ITO) has advantages of excellent conductivity, higher adhesion, and interface stability, showing potential to replace the metallic contacts for fabrication of CdZnTe (CZT) X/γ-ray detectors. In this work, high quality ITO electrodes for n-type CZT crystals were prepared by magnetron sputtering under a sputtering power of 75 W and a sputtering pressure of 0.6 Pa. A low dark current of ∼1 nA is achieved for the 5 × 5 × 2 mm3 ITO/CZT/ITO planar device under 100 V bias. The characteristics of Schottky contact are presented in the room temperature I-V curves, which are similar to those of the Au contact detectors. Based on the thermoelectric emission theory, the contact barrier and resistance of ITO electrodes are evaluated to be 0.902–0.939 eV and 0.87–3.56 × 108 Ω, respectively, which are consistent with the values of the Au electrodes. The ITO/CZT/ITO structure detector exhibits a superior energy resolution of 6.5% illuminated by the uncollimated 241Am @59.5 keV γ-ray source, which is comparable to the CZT detector with Au electrodes.
Cyclable and non-volatile electric field control of magnetism in BiFeO3 based magnetoelectric heterostructures Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-12 C. Daumont, J. Wolfman, C. Autret-Lambert, P. Andreazza, B. Negulescu
Room temperature manipulation of the ferromagnetic state via an electric field is investigated in Ni/BiFe0.95Mn0.05O3 thin film heterostructures. A 600% increase in the magnetic coercive field of the Ni layer is observed at the initial DC electrical poling of the ferroelectric BiFe0.95Mn0.05O3 layer. The magnetoelectric effect is remanent, and the magnetic coercive field can be modulated between a low value and a high value by successively switching the ferroelectric polarization. After the initial poling, the coercive field difference is decreased by subsequent back and forth switching. However, the magnetic bi-stability is preserved at least up to 250 cycles, which is promising for spintronic applications.
3D multilevel spin transfer torque devices Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-14 J. Hong, M. Stone, B. Navarrete, K. Luongo, Q. Zheng, Z. Yuan, K. Xia, N. Xu, J. Bokor, L. You, S. Khizroev
Spin-transfer torque magnetic tunneling junction devices capable of a multilevel three-dimensional (3D) information processing are studied in the sub-20-nm size range. The devices are built using He+ and Ne+ focused ion beam etching. It has been demonstrated that due to their extreme scalability and energy efficiency, these devices can significantly reduce the device footprint compared to the modern CMOS approaches and add advanced features in a 3D stack with a sub-20-nm size using a spin polarized current.
Thickness driven spin reorientation transition of epitaxial LaCrO3 films Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Junho Park, Dong-Hwan Kim, Doopyo Lee, Kyung-Tae Ko, Jong Hyun Song, Jae-Young Kim, Tae-Yeong Koo, Seung Ran Lee, Jae-Hoon Park
We grew fully strained epitaxial LaCrO3 (LCO) films on SrTiO3(001) under layer-by-layer control up to the film thickness of t = 130 nm using a pulsed laser deposition method. The spin axis of the antiferromagnetic LCO film was systematically examined as a function of t by using Cr L2,3-edge x-ray magnetic linear dichroism (XMLD). The XMLD results manifest a spin reorientation transition (SRT) across a transition thickness of tT ∼ 60 nm. This SRT is well explained in terms of two competing magnetic anisotropy energies of the surface/interface (KS) and the LCO film itself (KV).
Magnetic skyrmion bubble motion driven by surface acoustic waves Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Rabindra Nepal, Utkan Güngördü, Alexey A. Kovalev
We study the dynamical control of a magnetic skyrmion bubble by using counter-propagating surface acoustic waves (SAWs) in a ferromagnet. First, we determine the bubble mass and derive the force due to SAWs acting on a magnetic bubble using Thiele's method. The force that pushes the bubble is proportional to the strain gradient for the major strain component. We then study the dynamical pinning and motion of magnetic bubbles by SAWs in a nanowire. In a disk geometry, we propose a SAWs-driven skyrmion bubble oscillator with two resonant frequencies.
Controlling the electric charge of gold nanoplatelets on an insulator by field emission nc-AFM Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-12 Bulent Baris, Mohanad Alchaar, Janak Prasad, Sébastien Gauthier, Erik Dujardin, David Martrou
Charging of 2D Au nanoplatelets deposited on an insulating SiO2 substrate to or from the tip of a non-contact atomic force microscope (nc-AFM) is demonstrated. Charge transfer is controlled by monitoring the resonance frequency shift Δf(V) during the bias voltage ramp V applied to the tip-back electrode junction. The onset of charge transfer is revealed by a transition from a capacitive parabolic behavior to a constant Δf(V) region for both polarities. An analytical model, based on charging by electron field emission, shows that the field-emitted current saturates shortly after the onset of the charging, due to the limiting effect of the charge-induced rise of the Au platelet potential. The value of this current plateau depends only on the rate of the bias voltage ramp and on the value of the platelet/SiO2/back electrode capacitance. This analysis is confirmed by numerical simulations based on a virtual nc-AFM model that faithfully matches the experimental data. Our charging protocol could be used to tune the potential of the platelets at the single charge level.
Imaging nanoscale spatial modulation of a relativistic electron beam with a MeV ultrafast electron microscope Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-12 Chao Lu, Tao Jiang, Shengguang Liu, Rui Wang, Lingrong Zhao, Pengfei Zhu, Yaqi Liu, Jun Xu, Dapeng Yu, Weishi Wan, Yimei Zhu, Dao Xiang, Jie Zhang
An accelerator-based MeV ultrafast electron microscope (MUEM) has been proposed as a promising tool to the study structural dynamics at the nanometer spatial scale and the picosecond temporal scale. Here, we report experimental tests of a prototype MUEM where high quality images with nanoscale fine structures were recorded with a pulsed ∼3 MeV picosecond electron beam. The temporal and spatial resolutions of the MUEM operating in the single-shot mode are about 4 ps (FWHM) and 100 nm (FWHM), corresponding to a temporal-spatial resolution of 4 × 10−19 s m, about 2 orders of magnitude higher than that achieved with state-of-the-art single-shot keV UEM. Using this instrument, we offer the demonstration of visualizing the nanoscale periodic spatial modulation of an electron beam, which may be converted into longitudinal density modulation through emittance exchange to enable production of high-power coherent radiation at short wavelengths. Our results mark a great step towards single-shot nanometer-resolution MUEMs and compact intense x-ray sources that may have widespread applications in many areas of science.
Thermally assisted nanosecond laser generation of ferric nanoparticles Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-12 K. Kurselis, V. Kozheshkurt, R. Kiyan, B. Chichkov, L. Sajti
A technique to increase nanosecond laser based production of ferric nanoparticles by elevating temperature of the iron target and controlling its surface exposure to oxygen is reported. High power near-infrared laser ablation of the iron target heated up to 600 °C enhances the particle generation efficiency by more than tenfold exceeding 6 μg/J. Temporal and thermal dependencies of the particle generation process indicate correlation of this enhancement with the oxidative processes that take place on the iron surface during the per spot interpulse delay. Nanoparticles, produced using the heat-assisted ablation technique, are examined using scanning electron and transmission electron microscopy confirming the presence of 1–100 nm nanoparticles with an exponential size distribution that contain multiple randomly oriented magnetite nanocrystallites. The described process enables the application of high power lasers and facilitates precise, uniform, and controllable direct deposition of ferric nanoparticle coatings at the industry-relevant rates.
Prompt increase of ultrashort laser pulse transmission through thin silver films Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-14 S. G. Bezhanov, P. A. Danilov, A. V. Klekovkin, S. I. Kudryashov, A. A. Rudenko, S. A. Uryupin
We study experimentally and numerically the increase in ultrashort laser pulse transmissivity through thin silver films caused by the heating of electrons. Low to moderate energy femtosecond laser pulse transmission measurements through 40–125 nm thickness silver films were carried out. We compare the experimental data with the values of transmitted fraction of energy obtained by solving the equations for the field together with the two-temperature model. The measured values were fitted with sufficient accuracy by varying the electron-electron collision frequency whose exact values are usually poorly known. Since transmissivity experiences more pronounced changes with the increase in temperature compared to reflectivity, we suggest this technique for studying the properties of nonequilibrium metals.
Optically pumped lasing of an electrically active hybrid OLED-microcavity Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-14 S. Meister, R. Brückner, M. Sudzius, H. Fröb, K. Leo
Highly conductive electrodes are a prerequisite for electrically pumped organic lasers. We investigate the influence of very thin metal contacts in an electrically active organic microcavity. We test different deposition techniques and seed layers to decrease the thickness of the metal layers and reduce possibly harmful absorption. For such very thin contacts, the spectral position of the modes is modeled by simulated modes using the transfer-matrix-algorithm. The input-output characteristics of the device without, with bottom, with top, and with both metal layer(s) are recorded. These measurements allow us to understand and improve the impact on the functionality. With these results and the help of a theoretical approximation, we determine the minimal current density needed to reach the lasing threshold for electrical pumping in this sample structure.
Hybrid organic/inorganic position-sensitive detectors based on PEDOT:PSS/n-Si Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Mohammad Javadi, Mahdiyeh Gholami, Hadis Torbatiyan, Yaser Abdi
Various configurations like p-n junctions, metal-semiconductor Schottky barriers, and metal-oxide-semiconductor structures have been widely used in position-sensitive detectors. In this report, we propose a PEDOT:PSS/n-Si heterojunction as a hybrid organic/inorganic configuration for position-sensitive detectors. The influence of the thickness of the PEDOT:PSS layer, the wavelength of incident light, and the intensity of illumination on the device performance are investigated. The hybrid PSD exhibits very high sensitivity (>100 mV/mm), excellent nonlinearity (<3%), and a response correlation coefficient (>0.995) with a response time of <4 ms to the inhomogeneous IR illumination. The presented hybrid configuration also benefits from a straightforward low-temperature fabrication process. These advantages of the PEDOT:PSS/n-Si heterojunction are very promising for developing a new class of position-sensitive detectors based on the hybrid organic/inorganic junctions.
High-efficiency water-loaded microwave antenna in ultra-high-frequency band Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-12 Zilun Gong, Chris Bartone, Fuyi Yang, Jie Yao
High-index dielectrics are widely used in microwave antennas to control the radiation characteristics. Liquid water, with a high dielectric index at microwave frequency, is an interesting material to achieving tunable functionalities. Here, we demonstrate a water-loaded microwave antenna system that has high loss-tolerance and wideband tunability enabled by fluidity. Our simulation and experimental results show that the resonance frequency can be effectively tuned by the size of loading water. Furthermore, the antenna systems with water loading can achieve high radiation efficiency (>90%) in the ultra-high-frequency (0.3–3 GHz) band. This work brings about opportunities in realistic tunable microwave antenna designs enabled by liquid.
Study of additive manufactured microwave cavities for pulsed optically pumped atomic clock applications Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-16 C. Affolderbach, W. Moreno, A. E. Ivanov, T. Debogovic, M. Pellaton, A. K. Skrivervik, E. de Rijk, G. Mileti
Additive manufacturing (AM) of passive microwave components is of high interest for the cost-effective and rapid prototyping or manufacture of devices with complex geometries. Here, we present an experimental study on the properties of recently demonstrated microwave resonator cavities manufactured by AM, in view of their applications to high-performance compact atomic clocks. The microwave cavities employ a loop-gap geometry using six electrodes. The critical electrode structures were manufactured monolithically using two different approaches: Stereolithography (SLA) of a polymer followed by metal coating and Selective Laser Melting (SLM) of aluminum. The tested microwave cavities show the desired TE011-like resonant mode at the Rb clock frequency of ≈6.835 GHz, with a microwave magnetic field highly parallel to the quantization axis across the vapor cell. When operated in an atomic clock setup, the measured atomic Rabi oscillations are comparable to those observed for conventionally manufactured cavities and indicate a good uniformity of the field amplitude across the vapor cell. Employing a time-domain Ramsey scheme on one of the SLA cavities, high-contrast (34%) Ramsey fringes are observed for the Rb clock transition, along with a narrow (166 Hz linewidth) central fringe. The measured clock stability of 2.2 × 10−13 τ−1/2 up to the integration time of 30 s is comparable to the current state-of-the-art stabilities of compact vapor-cell clocks based on conventional microwave cavities and thus demonstrates the feasibility of the approach.
Strategy to overcome recombination limited photocurrent generation in CsPbX3 nanocrystal arrays Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-16 Wasim J. Mir, Clément Livache, Nicolas Goubet, Bertille Martinez, Amardeep Jagtap, Audrey Chu, Nathan Coutard, Hervé Cruguel, Thierry Barisien, Sandrine Ithurria, Angshuman Nag, Benoit Dubertret, Abdelkarim Ouerghi, Mathieu G. Silly, Emmanuel Lhuillier
We discuss the transport properties of CsPbBrxI3−x perovskite nanocrystal arrays as a model ensemble system of caesium lead halide-based perovskite nanocrystal arrays. While this material is very promising for the design of light emitting diodes, laser, and solar cells, very little work has been devoted to the basic understanding of their (photo)conductive properties in an ensemble system. By combining DC and time-resolved photocurrent measurements, we demonstrate fast photodetection with time response below 2 ns. The photocurrent generation in perovskite nanocrystal-based arrays is limited by fast bimolecular recombination of the material, which limits the lifetime of the photogenerated electron-hole pairs. We propose to use nanotrench electrodes as a strategy to ensure that the device size fits within the obtained diffusion length of the material in order to boost the transport efficiency and thus observe an enhancement of the photoresponse by a factor of 1000.
Stable and simple quantitative phase-contrast imaging by Fresnel biprism Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-15 Samira Ebrahimi, Masoomeh Dashtdar, Emilio Sánchez-Ortiga, Manuel Martínez-Corral, Bahram Javidi
Digital holographic (DH) microscopy has grown into a powerful nondestructive technique for the real-time study of living cells including dynamic membrane changes and cell fluctuations in nanometer and sub-nanometer scales. The conventional DH microscopy configurations require a separately generated coherent reference wave that results in a low phase stability and a necessity to precisely adjust the intensity ratio between two overlapping beams. In this work, we present a compact, simple, and very stable common-path DH microscope, employing a self-referencing configuration. The microscope is implemented by a diode laser as the source and a Fresnel biprism for splitting and recombining the beams simultaneously. In the overlapping area, linear interference fringes with high contrast are produced. The frequency of the interference pattern could be easily adjusted by displacement of the biprism along the optical axis without a decrease in fringe contrast. To evaluate the validity of the method, the spatial noise and temporal stability of the setup are compared with the common off-axis DH microscope based on a Mach-Zehnder interferometer. It is shown that the proposed technique has low mechanical noise as well as superb temporal stability with sub-nanometer precision without any external vibration isolation. The higher temporal stability improves the capabilities of the microscope for studying micro-object fluctuations, particularly in the case of biological specimens. Experimental results are presented using red blood cells and silica microspheres to demonstrate the system performance.
Growth of self-textured Ga3+-substituted Li7La3Zr2O12 ceramics by solid state reaction and their significant enhancement in ionic conductivity Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-14 Shiying Qin, Xiaohong Zhu, Yue Jiang, Ming'en Ling, Zhiwei Hu, Jiliang Zhu
A highly self-textured Ga2O3-substituted Li7La3Zr2O12 (LLZO-Ga) solid electrolyte with a nominal composition of Li6.55Ga0.15La3Zr2O12 is obtained by a simple and low-cost solid-state reaction technique, requiring no seed crystals to achieve grain orientation. The as-prepared self-textured LLZO-Ga shows a strong (420) preferred orientation with a high Lotgering factor of 0.91. Coherently, a terrace-shaped microstructure consisting of many parallel layers, indicating a two-dimensional-like growth mode, is clearly observed in the self-textured sample. As a result, the highly self-textured garnet-type lithium-ion conducting solid electrolyte of LLZO-Ga exhibits an extremely high ionic conductivity, reaching a state-of-the-art level of 2.06 × 10−3 S cm−1 at room temperature (25 °C) and thus shedding light on an important strategy for improving the structure and ionic conductivity of solid electrolytes.
Thermal imaging of high power diode lasers subject to back-irradiance Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-07 C. Li, K. P. Pipe, C. Cao, P. Thiagarajan, R. J. Deri, P. O. Leisher
CCD-based thermoreflectance imaging and finite element modeling are used to study the two-dimensional (2D) temperature profile of a junction-down broad-area diode laser facet subject to back-irradiance. By determining the temperature rise in the active region (ΔΤAR) at different diode laser optical powers, back-irradiance reflectance levels, and back-irradiance spot locations, we find that ΔΤAR increases by nearly a factor of three when the back-irradiance spot is centered in the absorbing substrate approximately 5 μm away from the active region, a distance roughly equal to half of the back-irradiance spot FWHM (9 μm). This corroborates prior work studying the relationship between the back-irradiance spot location and catastrophic optical damage, suggesting a strong thermal basis for reduced laser lifetime in the presence of back-irradiance for diode lasers fabricated on absorbing substrates.
Hydrogen ion induced ultralow wear of PEEK under extreme load Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-05 Shuai Yan, Anying Wang, Jixiong Fei, Zhenyang Wang, Xiaofeng Zhang, Bin Lin
As a high-performance engineering polymer, poly(ether ether ketone) (PEEK) is a perfect candidate material for applications under extreme working conditions. However, its high wear rate greatly shortens its service life. In this study, ultralow friction and wear between PEEK and silicon nitride (Si3N4) under extreme-load conditions (with a mean contact pressure above 100 MPa) are found in acid lubricating solutions. Both friction and wear decrease sharply with decreasing pH. At pH = 1, the friction coefficient decreases by an order of magnitude and the wear rate of the PEEK decreases by two orders of magnitude compared to the results with water lubrication. These reductions in friction and wear occur for different speed, load, and surface roughness conditions. The underlying mechanism can be attributed to the formation of hydrogen-ion-induced electrical double layers on the surfaces of PEEK and Si3N4. The combined effect of the resulting repulsive force, electro-viscosity, and low shear strength of the water layer dramatically reduces both friction and wear.
STM-induced light emission enhanced by weakly coupled organic ad-layers Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-07 M. C. Cottin, E. Ekici, C. A. Bobisch
We analyze the light emission induced by the tunneling current flowing in a scanning tunneling microscopy experiment. In particular, we study the influence of organic ad-layers on the light emission on the initial monolayer of bismuth (Bi) on Cu(111) in comparison to the well-known case of organic ad-layers on Ag(111). On the Bi/Cu(111)-surface, we find that the scanning tunneling microscopy-induced light emission is considerably enhanced if an organic layer, e.g., the fullerene C60 or the perylene derivate perylene-tetracarboxylic-dianhydride, is introduced into the tip-sample junction. The enhancement can be correlated with a peculiarly weak interaction between the adsorbed molecules and the underlying Bi/Cu(111) substrate as compared to the Ag(111) substrate. This allows us to efficiently enhance and tune the coupling of the tunneling current to localized excitations of the tip-sample junction, which in turn couple to radiative decay channels.
Thermally conductive tough flexible elastomers as composite of slide-ring materials and surface modified boron nitride particles via plasma in solution Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-05 Taku Goto, Masaki Iida, Helen Tan, Chang Liu, Koichi Mayumi, Rina Maeda, Koichi Kitahara, Kazuto Hatakeyama, Tsuyohito Ito, Yoshiki Shimizu, Hideaki Yokoyama, Kaoru Kimura, Kohzo Ito, Yukiya Hakuta, Kazuo Terashima
We have developed a thermally conductive flexible elastomer as a composite material with slide-ring (SR) materials and boron nitride (BN) particles surface-modified via plasma in solution. This composite shows excellent properties as a flexible insulator for thermal management. Surface modification of BN particles using plasma in solution increases the tensile strength, extension ratio at break, toughness, and rubber characteristics of the composites, compared to SR and non-modified BN, while the Young's modulus values are identical. Furthermore, the thermal conductivity also improved as a result of plasma surface modification.
Metamaterials for optical Bragg accelerators Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-05 Adi Hanuka, Elron Goldemberg, Almog Zilka, Levi Schächter
We present a systematic study of the advantages of using optical artificial materials in designing periodic structures for laser-driven accelerators. As a case study, we investigate the electromagnetic properties of a Bragg waveguide, with its alternating layers being composed of artificial materials. The layers can be optimized to maximize the structure's properties. We show that when the structure's eigenmode interacts with free electrons, the maximum efficiency is nearly four times higher than in configurations that rely on natural materials. As a result, accelerators and radiation sources may be miniaturized significantly.
Hybrid reflections from multiple x-ray scattering in epitaxial bismuth telluride topological insulator films Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-06 Sérgio L. Morelhão, Stefan Kycia, Samuel Netzke, Celso I. Fornari, Paulo H. O. Rappl, Eduardo Abramof
Epitaxial films of bismuth telluride topological insulators have received increasing attention due to their potential applications in spintronic and quantum computation. One of the most important properties of epitaxial films is the presence of interface defects due to the lateral lattice mismatch since electrically active defects can drastically compromise device performance. By describing hybrid reflections in hexagonal bismuth telluride films on cubic substrates, in-plane lattice mismatches were characterized with accuracy at least 20 times better than using other X-ray diffraction methods, providing clear evidence of 0.007% lateral lattice mismatch, consistent with stress relaxation associated with van der Waals gaps in the film structure.
Tunable Fano resonator using multilayer graphene in the near-infrared region Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-08 Chaobiao Zhou, Guoqin Liu, Guoxun Ban, Shiyu Li, Qingzhong Huang, Jinsong Xia, Yi Wang, Mingsheng Zhan
Fano resonance (FR) holds promising applications for high performance optoelectronic devices due to its strong enhancement of light-matter interactions. In this work, we experimentally demonstrate a tunable FR in a photonic crystal nanoresonator (PCR), including the effects of structural parameters and graphene nanosheets with different layer numbers. The results show that the intensity and position of Fano peaks can be tuned via altering the lattice constant and the hole radius of PCR due to the variation of the effective refractive index. More importantly, we experimentally study the interaction between sharp FR with multilayer graphene. The results indicate that the FR transmission spectrum can be efficiently adjusted with the layer number of graphene, and the largest change in transmission (∼44%) is achieved with three-layer graphene because of high conductivity. These consequences may lead to efficient and tunable electro-optical modulators, biosensors, and optical switches in the near-infrared region.
Double negative acoustic metastructure for attenuation of acoustic emissions Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-08 Sanjay Kumar, Pulak Bhushan, Om Prakash, Shantanu Bhattacharya
Acoustic metamaterials hold great potential for attenuation of low frequency acoustic emissions. However, a fundamental challenge is achieving high transmission loss over a broad frequency range. In this work, we report a double negative acoustic metastructure for absorption of low frequency acoustic emissions in an aircraft. This is achieved by utilizing a periodic array of hexagonal cells interconnected with a neck and mounted with an elastic membrane on both ends. An average transmission loss of 56 dB under 500 Hz and an overall absorption of over 48% have been realized experimentally. The negative mass density is derived from the dipolar resonances created as a result of the in-phase movement of the membranes. Further, the negative bulk modulus is ascribed to the combined effect of out-of-phase acceleration of the membranes and the Helmholtz resonator. The proposed metastructure enables absorption of low frequency acoustic emissions with improved functionality that is highly desirable for varied applications.
Rhodium doped InGaAs: A superior ultrafast photoconductor Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-06 R. B. Kohlhaas, B. Globisch, S. Nellen, L. Liebermeister, M. Schell, P. Richter, M. Koch, M. P. Semtsiv, W. T. Masselink
The properties of rhodium (Rh) as a deep-level dopant in InGaAs lattice matched to InP grown by molecular beam epitaxy are investigated. When InGaAs:Rh is used as an ultrafast photoconductor, carrier lifetimes as short as 100 fs for optically excited electrons are measured. Rh doping compensates free carriers so that a near intrinsic carrier concentration can be achieved. At the same time, InGaAs:Rh exhibits a large electron mobility of 1000 cm2/V s. Therefore, this material is a very promising candidate for application as a semi-insulating layer, THz antenna, or semiconductor saturable absorber mirror.
Distinct light emission from two-dimensional electron gas at a lattice-matched InAlN/AlGaN heterointerface Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-08 Lei Li, Daiki Hosomi, Yuta Miyachi, Makoto Miyoshi, Takashi Egawa
We report on distinct light emission from two-dimensional electron gas (2DEG) at a lattice-matched (LM) In0.12Al0.88N/Al0.21Ga0.79N heterointerface. The recombination between the electrons in the 2DEG in the ground state E1 and photoexcited holes in the Al0.21Ga0.79N layer was identified. In contrast to GaN channel-based heterostructures (HSs), larger activation energy of the 2DEG-related emission from LM In0.12Al0.88N/Al0.21Ga0.79N HS was obtained to be approximately 17 meV, which enables the distinguished 2DEG photoluminescence (PL) peak to be more thermally stable. Moreover, the existence of the 2DEG accelerates the reduction of the PL lifetime of the emission from Al0.21Ga0.79N. Compared to the general 2DEG PL feature with a broad recombination band in GaN channel-based HSs, the improved emission characteristics of the 2DEG in the In0.12Al0.88N/Al0.21Ga0.79N HS were attributed to electron localization in a deep triangular potential well, large 2DEG density induced by the In0.12Al0.88N layer, and the improvement of the interfacial crystal quality due to the lattice match between In0.12Al0.88N and Al0.21Ga0.79N layers. These findings provide important insight into understanding the InAlN-based HSs and will be potentially useful to advance the electronic and photonic applications for group-III nitrides.
High quality Al2O3/(100) oxygen-terminated diamond interface for MOSFETs fabrication Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-09 T. T. Pham, M. Gutiérrez, C. Masante, N. Rouger, D. Eon, E. Gheeraert, D. Araùjo, J. Pernot
In this letter, we report on the improvement of gate controlled Al2O3/(100) boron doped (B-doped) oxygen-terminated diamond (O-diamond) Metal Oxide Semiconductor Capacitors using 40 nm thick Al2O3 deposited by Atomic Layer Deposition at 380 °C and then annealed at 500 °C in vacuum conditions. The high quality of Al2O3 and an Al2O3/diamond interface is verified thanks to electrical measurements and Transmission Electron Microscopy (TEM) measurements. A density of interface states lower than 1012 eV−1 cm−2 is measured from the flat-band regime to the depletion regime. The shift of the flat-band voltage and the leakage current through the oxide are significantly reduced in good agreement with the mono-crystalline character of the Al2O3 layer revealed by TEM.
Unusual negative magnetoresistance in Bi2Se3–ySy topological insulator under perpendicular magnetic field Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-05 Rahul Singh, Vinod K. Gangwar, D. D. Daga, Abhishek Singh, A. K. Ghosh, Manoranjan Kumar, A. Lakhani, Rajeev Singh, Sandip Chatterjee
The magneto-transport properties of Bi2Se3–ySy were investigated. Magnetoresistance (MR) decreases with an increase in the S content, and finally, for 7% (i.e., y = 0.21) S doping, the magnetoresistance becomes negative. This negative MR is unusual as it is observed when a magnetic field is applied in the perpendicular direction to the plane of the sample. The magneto-transport behavior shows the Shubnikov–de Haas (SdH) oscillation, indicating the coexistence of surface and bulk states. The negative MR has been attributed to the non-trivial bulk conduction.
Electrical tuning of the band alignment and magnetoconductance in an n-type ferromagnetic semiconductor (In,Fe)As-based spin-Esaki diode Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-07 Le Duc Anh, Pham Nam Hai, Masaaki Tanaka
We report a strong bias dependence of the magnetoconductance (MC) of a spin-Esaki diode composed of n+-type ferromagnetic semiconductor (FMS) (In,Fe)As and p+-type Be doped InAs grown on a p+-InAs (001) substrate by molecular beam epitaxy. When the bias voltage V is increased above 450 mV in the forward bias, we found that the MC, measured at 3.5 K under a magnetic field H of 1 T in the in-plane  direction, changes its sign from positive to negative and its magnitude rises rapidly from 0.5% at V < 450 mV to −7.4% at V = 650 mV. Furthermore, the MC magnitude decreases as cos2(θ) when rotating H from the in-plane  direction to the perpendicular  direction, where θ is the angle between H and the  axis. Using a two-fluid model, we explain both the magnitude and the anisotropy of the MC based on the evolution of the spin-Esaki diode's band profile with V. This analysis provides insights into the density of states and spin-polarization of the conduction band and the Fe-related impurity band in n-type FMS (In,Fe)As.
Asymmetric and partial injection locking of a three-terminal spin-torque oscillator Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-09 Emilie Jué, Matthew R. Pufall, William H. Rippard
We measure the injection locking of a three-terminal spin-torque oscillator (STO) excited by spin-orbit torque. The device consists of a magnetic tunnel junction on top of a Pt wire. A DC and an AC current are applied through the Pt wire to induce the oscillations and injection lock the STO, respectively. The injection locking is studied at fMW ≈ f0 or at fMW ≈ 2f0, where fMW is the microwave frequency and f0 is the free running frequency of the STO. The frequency response is qualitatively different from the injection locking in STOs generally reported experimentally and theoretically. Whereas typical phase-locking behavior is observed at fMW ≈ 2f0, the injection locking at fMW ≈ f0 is only partial and exhibits a strongly asymmetric frequency response. Defining the frequency deviation range as the frequency range where the STO differs from its free running frequency, we show that the asymmetric interaction is characterized by a pulling effect present on only one side of the frequency deviation range, the presence of a sideband inside the frequency deviation range, and an interaction of the STO with the microwave current that is wider than the frequency deviation range.
A thermodynamic potential, energy storage performances, and electrocaloric effects of Ba1-xSrxTiO3 single crystals Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-05 Y. H. Huang, J. J Wang, T. N. Yang, Y. J. Wu, X. M. Chen, L. Q. Chen
A thermodynamic potential for Ba1-xSrxTiO3 solid solutions is developed, and the corresponding thermodynamic properties of Ba1-xSrxTiO3 single crystals are calculated. The predicted temperature-composition phase diagram from the thermodynamic potential agrees well with the experimental measurements. Based on this potential, the energy storage performances and electrocaloric effects of Ba1-xSrxTiO3 single crystals are obtained using the phase-field method. It is found that there is an optimal Sr concentration which maximizes the discharged energy density of a Ba1-xSrxTiO3 single crystal under an applied electric field. The electrocaloric effects of Ba0.8Sr0.2TiO3, Ba0.7Sr0.3TiO3, Ba0.6Sr0.4TiO3, and Ba0.5Sr0.5TiO3 single crystals are also predicted, from which the corresponding optimal temperatures are identified.
Evolution of ferroelectric HfO2 in ultrathin region down to 3 nm Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-06 Xuan Tian, Shigehisa Shibayama, Tomonori Nishimura, Takeaki Yajima, Shinji Migita, Akira Toriumi
The ferroelectric properties of ultrathin Y-doped HfO2 films were investigated. Ferroelectricity was demonstrated experimentally in 3 nm-thick Y-doped HfO2 via direct detection of displacement currents during polarization switching. The dependence on the HfO2 thickness within the 30 to 3 nm range revealed that the ferroelectric properties decrease rapidly below a critical thickness. In the ultrathin HfO2 region, methods such as higher Y doping or metal capping annealing were required to further stabilize the ferroelectric phase. These methods could be used to enhance the switchable polarization (Psw) to 35 μC/cm2 in 5 nm- and 10 μC/cm2 in 3 nm-thick Y-doped HfO2. This paper indicates that HfO2 ferroelectricity is scalable even in the ultrathin region.
Dipolar glass-like dielectric response of nanocrystalline Sr0.95Nd0.05Fe12-xScxO19 hexaferrites Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-06 Andrzej Hilczer, Szymon Łoś, Zbigniew Trybuła, Katarzyna Pasińska, Adam Pietraszko
Recently reported magnetic quantum paraelectric properties in M-type hexaferrite single crystals have encouraged us to study the dielectric response of SrFe12O19 nanocrystallites down to the temperature of 10 K. As Sc-induced multiferroicity, promising for electromagnetic control, has been reported in bulk and films of hexaferrites, we also studied the size effect in dielectric response of Sr0.95Nd0.05Fe12-xScxO19 nanocrystallites with x = 0.36, 1.08, and 1.56. The nanopowders were obtained by citric method and the phase purity and the microstructure were controlled using X-ray diffraction and scanning electron microscopy. No clear evidence of quantum paraelectric behavior has been observed in temperature variation of dielectric permittivity of SrFe12O19 nanopowder. In the case of Nd-stabilized Sc-doped nanocrystallites, a low-temperature dielectric relaxation, similar to that in dipolar glasses, has been discovered. Activation energy of 62.5 meV was obtained for the lowest doping level and a modest increase in the energy was found at higher Sc concentrations. We relate the low-temperature relaxation in Sc-doped hexaferrite nanopowder to dielectric displacive polarization at the 4e Wyckoff sites modified by Sc-ions substituting the ferric ions in 4f2 and 12k positions.
Temperature-dependent phase transition in barium titanate crystals probed by second harmonic generation Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-08 Jiesu Wang, Kuijuan Jin, Hongbao Yao, Junxing Gu, Xiulai Xu, Chen Ge, Can Wang, Meng He, Guozhen Yang
More and more evidence points out the coexistence of displacive and order-disorder dynamics in the phase transition of barium titanate. Here, we report an initial state determined phase transition in barium titanate by applying second harmonic generation technology and piezoresponse force microscopy (PFM). The out-of-plane PFM results of these barium titanate crystals show the increase in domain walls in the surfaces after annealing, leading to the increase in the second harmonic signal measured. This work directly revealed how the displacive and order-disorder dominate the phase transition and what the role is that the domain wall plays in this process.
Bi-ferroic memristive properties of multiferroic tunnel junctions Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-08 Zheng-Dong Luo, Geanina Apachitei, Ming-Min Yang, Jonathan J. P. Peters, Ana M. Sanchez, Marin Alexe
The giant tunnelling electroresistance (TER) and memristive behaviours of ferroelectric tunnel junctions make them promising candidates for future information storage technology. Using conducting ferromagnetic layers as electrodes results in multiferroic tunnel junctions (MFTJs) which show spin dependent transport. The tunnelling magnetoresistance (TMR) of such structures can be reversibly controlled by electric pulsing owing to ferroelectric polarisation-dependent spin polarisation at the ferroelectric/ferromagnetic interface. Here, we show multilevel electric control of both TMR and TER of MFTJs, which indicates the bi-ferroic or magneto-electric memristive properties. This effect is realised by manipulating the ferroelectric domain configuration via non-volatile partial ferroelectric switching obtained by applying low voltage pulses to the junction. Through electrically modulating the ratio between up- and down-polarised ferroelectric domains, a broad range of TMR (between ∼3% and ∼30%) and TER (∼1000%) values can be achieved. The multilevel control of TMR and TER using the electric pulse tunable ferroelectric domain configuration suggests a viable way to obtain multiple state memory.
Room temperature microwave oscillations in GaN/AlN resonant tunneling diodes with peak current densities up to 220 kA/cm2 Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-05 Jimy Encomendero, Rusen Yan, Amit Verma, S. M. Islam, Vladimir Protasenko, Sergei Rouvimov, Patrick Fay, Debdeep Jena, Huili Grace Xing
We report the generation of room temperature microwave oscillations from GaN/AlN resonant tunneling diodes, which exhibit record-high peak current densities. The tunneling heterostructure grown by molecular beam epitaxy on freestanding GaN substrates comprises a thin GaN quantum well embedded between two AlN tunneling barriers. The room temperature current-voltage characteristics exhibit a record-high maximum peak current density of ∼220 kA/cm2. When biased within the negative differential conductance region, microwave oscillations are measured with a fundamental frequency of ∼0.94 GHz, generating an output power of ∼3.0 μW. Both the fundamental frequency and the output power of the oscillator are limited by the external biasing circuit. Using a small-signal equivalent circuit model, the maximum intrinsic frequency of oscillation for these diodes is predicted to be ∼200 GHz. This work represents a significant step towards microwave power generation enabled by resonant tunneling transport, an ultra-fast process that goes beyond the limitations of current III-Nitride high electron mobility transistors.
Improving the morphological stability of nickel germanide by tantalum and tungsten additions Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-06 L. Jablonka, T. Kubart, F. Gustavsson, M. Descoins, D. Mangelinck, S.-L. Zhang, Z. Zhang
To enhance the morphological stability of NiGe, a material of interest as a source drain-contact in Ge-based field effect transistors, Ta or W, is added as either an interlayer or a capping layer. The efficacy of this Ta or W addition is evaluated with pure NiGe as a reference. While interlayers increase the NiGe formation temperature, capping layers do not retard the NiGe formation. Regardless of the initial position of Ta or W, the morphological stability of NiGe against agglomeration can be improved by up to 100 °C. The improved thermal stability can be ascribed to an inhibited surface diffusion, owing to Ta or W being located on top of NiGe after annealing, as confirmed by means of transmission electron microscopy, Rutherford backscattering spectrometry, and atom probe tomography. The latter also shows a 0.3 at. % solubility of Ta in NiGe at 450 °C, while no such incorporation of W is detectable.
High precision single qubit tuning via thermo-magnetic field control Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-07 David A. Broadway, Scott E. Lillie, Nikolai Dontschuk, Alastair Stacey, Liam T. Hall, Jean-Philippe Tetienne, Lloyd C. L. Hollenberg
Precise control of the resonant frequency of a spin qubit is of fundamental importance to quantum sensing protocols. We demonstrate a control technique on a single nitrogen-vacancy (NV) centre in diamond where the applied magnetic field is modified by fine-tuning a permanent magnet's magnetisation via temperature control. Through this control mechanism, nanoscale cross-relaxation spectroscopy of both electron and nuclear spins in the vicinity of the NV centre is performed. We then show that through maintaining the magnet at a constant temperature, an order of magnitude improvement in the stability of the NV qubit frequency can be achieved. This improved stability is tested in the polarisation of a small ensemble of nearby 13C spins via resonant cross-relaxation, and the lifetime of this polarisation explored. The effectiveness and relative simplicity of this technique may find use in the realisation of portable spectroscopy and/or hyperpolarisation systems.
Ferroelectric HfZrOx-based MoS2 negative capacitance transistor with ITO capping layers for steep-slope device application Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-07 Jing Xu, Shu-Ye Jiang, Min Zhang, Hao Zhu, Lin Chen, Qing-Qing Sun, David Wei Zhang
A negative capacitance field-effect transistor (NCFET) built with hafnium-based oxide is one of the most promising candidates for low power-density devices due to the extremely steep subthreshold swing (SS) and high on-state current induced by incorporating the ferroelectric material in the gate stack. Here, we demonstrated a two-dimensional (2D) back-gate NCFET with the integration of ferroelectric HfZrOx in the gate stack and few-layer MoS2 as the channel. Instead of using the conventional TiN capping metal to form ferroelectricity in HfZrOx, the NCFET was fabricated on a thickness-optimized Al2O3/indium tin oxide (ITO)/HfZrOx/ITO/SiO2/Si stack, in which the two ITO layers sandwiching the HfZrOx film acted as the control back gate and ferroelectric gate, respectively. The thickness of each layer in the stack was engineered for distinguishable optical identification of the exfoliated 2D flakes on the surface. The NCFET exhibited small off-state current and steep switching behavior with minimum SS as low as 47 mV/dec. Such a steep-slope transistor is compatible with the standard CMOS fabrication process and is very attractive for 2D logic and sensor applications and future energy-efficient nanoelectronic devices with scaling power supply.
Rubidium distribution at atomic scale in high efficient Cu(In,Ga)Se2 thin-film solar cells Appl. Phys. Lett. (IF 3.411) Pub Date : 2018-03-09 Arantxa Vilalta-Clemente, Mohit Raghuwanshi, Sébastien Duguay, Celia Castro, Emmanuel Cadel, Philippe Pareige, Philip Jackson, Roland Wuerz, Dimitrios Hariskos, Wolfram Witte
The introduction of a rubidium fluoride post deposition treatment (RbF-PDT) for Cu(In,Ga)Se2 (CIGS) absorber layers has led to a record efficiency up to 22.6% for thin-film solar cell technology. In the present work, high efficiency CIGS samples with RbF-PDT have been investigated by atom probe tomography (APT) to reveal the atomic distribution of all alkali elements present in CIGS layers and compared with non-treated samples. A Scanning Electron Microscopy Dual beam station (Focused Ion Beam–Gas Injection System) as well as Transmission Kikuchi diffraction is used for atom probe sample preparation and localization of the grain boundaries (GBs) in the area of interest. The analysis of the 3D atomic scale APT reconstructions of CIGS samples with RbF-PDT shows that inside grains, Rb is under the detection limit, but the Na concentration is enhanced as compared to the reference sample without Rb. At the GBs, a high concentration of Rb reaching 1.5 at. % was found, and Na and K (diffusing from the glass substrate) are also segregated at GBs but at lower concentrations as compared to Rb. The intentional introduction of Rb leads to significant changes in the chemical composition of CIGS matrix and at GBs, which might contribute to improve device efficiency.
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- Acc. Chem. Res.
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