Applied Physics Letters

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American Institute of Physics / AIP
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Anharmonicity of Bi2Se3 revealed by fs transient optical spectroscopy
Volume 115, Issue 20, November 2019. We investigate the anharmonic effects in Bi2Se3 crystals using femtosecond transient optical spectroscopy at 5–280 K. The reflectivity time series consist of exponential decay due to hot carriers and decaying oscillations due to the [math] phonon vibration. Vibration frequency and dephasing time of this optical phonon mode are obtained as a function of temperature, decreasing with increasing temperature; both the red shift in frequency and the increased dephasing rate induced by heating can be well described using the anharmonicity model including lattice thermal expansion and phonon-phonon coupling.
Crosstalk in microwave SQUID multiplexers
Volume 115, Issue 20, November 2019. Low-temperature detector technologies provide extraordinary sensitivity for applications ranging from precision measurements of the cosmic microwave background to high-resolution, high-rate x-ray, and γ-ray spectroscopy. To utilize this sensitivity, new instruments are being built, and new instruments are imagined, with ever greater pixel counts, but the scale of these instruments is limited by the capability of the readout electronics. Microwave SQUID multiplexing addresses the needs of these future instruments, exploiting gigahertz of bandwidths of coaxial cables and broadband components to combine hundreds to thousands of signals on a single readout line. A key feature of any multiplexer is the level of crosstalk between input channels. This crosstalk can degrade the sensitivity of the instrument, introduce systematic error, or simply confound data analysis. In this letter, we explain the primary mechanisms of crosstalk in a microwave SQUID multiplexer, calculate and measure their magnitude, and consider their effect and methods of mitigation.
Quantitative phase imaging based on Fresnel diffraction from a phase plate
Volume 115, Issue 20, November 2019. The structural complexity and instability of many interference phase microscopy methods are the major obstacles toward high-precision phase measurement. In this vein, improving more efficient configurations as well as proposing methods are the subjects of growing interest. Here, we introduce Fresnel diffraction from a phase step to the realm of quantitative phase imaging. By employing Fresnel diffraction of a divergent (or convergent) beam of light from a plane-parallel phase plate, we provide a viable, simple, and compact platform for three-dimensional imaging of micrometer-sized specimens. The recorded diffraction pattern of the outgoing light from an imaging system in the vicinity of the plate edge can be served as a hologram, which would be analyzed via the Fourier transform method to measure the sample phase information. The period of diffraction fringes is adjustable simply by rotating the plate without the reduction of both the field of view and fringe contrast. The high stability of the presented method is affirmatively confirmed through comparison of the result with that of the conventional Mach–Zehnder based digital holographic method. Quantitative phase measurements on silica microspheres, onion skins, and red blood cells ensure the validity of the method and its ability for monitoring nanometer-scale fluctuations of living cells, particularly in real-time.
Multi-wave mixing using a single vector optical field
Volume 115, Issue 20, November 2019. The traditional multiwave mixing setups are always complicated because multiple laser beams are involved, and they are required to be aligned into certain spatial configurations to get the phase-matching condition satisfied. Here, we report on a multiwave mixing method using a single vector optical field. The spatially separated phase-matching beams are obtained after the vector light passes through a polarizer. Filtering out part(s) of them and focusing the remaining ones into the Rb vapor cell, we detected degenerate four-wave mixing, degenerate six-wave mixing, and coexisting degenerate four-wave mixing signals under different circumstances. As the beams participating in the multiwave mixing processes originate from the same phase plane of the vector laser beam, their relative positions are fixed for the setup so that this method has the potential to resist for turbulence compared with the traditional methods. Besides, the entire system is simple and easy to align because the complicated optical setup is avoided. This work provides a convenient tool for multiwave mixing generation and has great potential in spectroscopy, light squeezing, nonlinear optics, and so on.
Reduced nonradiative recombination in semipolar green-emitting III-N quantum wells with strain-reducing AlInN buffer layers
Volume 115, Issue 20, November 2019. Using strain-reducing partially relaxed AlInN buffer layers, we observe reduced nonradiative recombination in semipolar green-emitting GaInN/GaN quantum wells. Since strain is a key issue for the formation of defects that act as nonradiative recombination centers, we aim to reduce the lattice mismatch between GaInN and GaN by introducing an AlInN buffer layer that can be grown lattice-matched along one of the in-plane directions of GaN, even in the semipolar [math] orientation. With the increasing thickness, the buffer layer shows partial relaxation in one direction and thereby provides a growth template with reduced lattice mismatch for the subsequent GaInN quantum wells. Time-resolved photoluminescence measurements show reduced nonradiative recombination for the structures with a strain-reducing buffer layer.
Coulomb drag in strongly coupled quantum wells: Temperature dependence of the many-body correlations
Volume 115, Issue 20, November 2019. We investigate the effect of the temperature dependence of many-body correlations on hole–hole Coulomb drag in strongly coupled GaAs/GaAlAs double quantum wells. For arbitrary temperatures, we obtained the correlations using the classical-map hypernetted-chain approach. We compare the temperature dependence of the resulting drag resistivities [math] at different densities with [math] calculated assuming correlations fixed at zero temperature. Comparing the results with those when correlations are completely neglected, we confirm that correlations significantly increase the drag. We find that the drag becomes sensitive to the temperature dependence of [math], twice the Fermi temperature. Our results show excellent agreement with available experimental data.
Fully epitaxial magnetic tunnel junction on a silicon wafer
Volume 115, Issue 20, November 2019. We developed a fully epitaxial magnetic tunnel junction on an 8″ silicon wafer by using a mass-production sputtering apparatus and achieved a high magnetoresistance ratio exceeding 240% at room temperature. One of the key factors in this achievement is the use of a B2-type Ni-Al seed layer on the wafer as a (001)-oriented and an atomically smooth template. Another is the insertion of a thin Al layer prior to MgO sputtering as protection from plasma damage, resulting in the formation of a spinel-type single-crystal Mg-Al-O tunnel barrier after in situ annealing. This epitaxial technology for transition metals on large wafers will lead to advanced practical spintronics devices incorporating high-performance single-crystalline materials such as chemical-ordered alloys and tunnel barriers.
Tuning electrical properties and phase transitions through strain engineering in lead-free ferroelectric K0.5Na0.5NbO3-LiTaO3-CaZrO3 thin films
Volume 115, Issue 20, November 2019. The effects of epitaxial strain on the properties of 0.95(K0.49Na0.49Li0.02)(Ta0.2Nb0.8)O3-0.05CaZrO3 (KNNLT-CZ) thin films are investigated. La0.07Sr0.93SnO3 and SrRuO3 are used as bottom electrodes to provide in-plane tensile and compressive stress, respectively. Our results show that the La0.07Sr0.93SnO3-buffered KNNLT-CZ films are mostly strain-relaxed with an orthorhombic (O) and tetragonal (T) mixed phase and a tetragonality of 1.002, which have a twice remnant polarization (2Pr) of 14.29 μC/cm2, an effective piezoelectric strain coefficient (d33*) of ∼60 pm/V, and an O to T phase transition temperature (TO-T) of 140 °C, while the SrRuO3-buffered KNNLT-CZ films are only partially strain-relaxed with a pure O phase and a larger tetragonality of 1.011, resulting in an increased 2Pr value of 33.63 μC/cm2, an improved d33* value of ∼80 pm/V, and an enhanced TO-T value of 200 °C. Both films show a high Curie temperature above 380 °C and stable hysteresis loops from room temperature to 225 °C. These results highlight the feasibility to improve the performance of KNN-based materials via epitaxial strain.
A string-driven rotor for efficient energy harvesting from ultra-low frequency excitations
Volume 115, Issue 20, November 2019. This Letter reports a string-driven rotor for constructing ultralow frequency energy harvesters. Consisting of a disk-shaped rotor with a shaft, an elastic string, and an inelastic string, the proposed rotor structure can convert ultralow frequency vibrations or linear reciprocating motions to high-speed rotation of the rotor without any sophisticated transmission mechanism. On the basis of the string-driven rotor, an electromagnetic energy harvester is designed, and the corresponding theoretical model is established. Both simulation and experiments demonstrate the high output performance of the harvester under a periodic excitation with an amplitude of 5 mm and at a frequency lower than 5 Hz. The harvester also generates 6.5 mW power when driven by hand at a frequency of approximately 4 Hz. This study exhibits the exciting potential of the string-driven rotor for boosting the efficiency of harvesting energy from pervasive ultralow frequency excitations.
Unusual electric field-induced optical behaviors in cesium lead bromide perovskites
Volume 115, Issue 20, November 2019. The electric field effect on the optical properties of semiconductors is important in terms of both fundamental physics and technological applications. Here, we explored the optical behavior of cesium halide perovskites under a varied electric field (F). We revealed the intrinsically distinct photoluminescence (PL) spectral evolution between the quantum-confined perovskites and the bulk phase, indicating the different carrier recombination behaviors under F. Strong PL quenching along with significant broadening of emission linewidths was observed. Notably, the CsPbBr3 colloidal quantum dots and nanoplatelets exhibit an unusual field-induced bandgap increase, which is attributed to the weakened orbital coupling between the Pb 6s and Br 4p states with the increase in F. These results could advance their application potential in optoelectronics.
Tunable high-quality Fano resonance in coupled terahertz whispering-gallery-mode resonators
Volume 115, Issue 20, November 2019. Fano resonance is widely discussed in designing functional terahertz components, such as sensors, filters, modulators, and group delay modules. Usually, a high quality (Q) factor and flexible tunability of Fano resonance are key requirements for these applications. Here, we present tunable terahertz Fano resonance with a Q factor of 2095 at 0.439 THz in coupled terahertz whispering-gallery-mode resonators (WGMRs). Coupling between a relatively low Q (578) quartz ring and a high Q (2095) silicon ring is employed to generate high Q Fano resonance. The resonant frequency of the Fano resonance can be actively manipulated by tuning the resonant frequency of the high Q WGMR, which is achieved through utilizing an electrical thermo-optic tuning method; meanwhile, the resonance intensity of the Fano resonance can be engineered by adjusting the coupling strength between two WGMRs. This coupled-WGMR scheme delivers tunable high Q Fano resonance and may contribute to the design of high-performance configurable terahertz devices.
“Optical mill”—A tool for the massive transfer of airborne light-absorbing particles
Volume 115, Issue 20, November 2019. We present an all-optical tool for the massive transfer of airborne light-absorbing particles. A generated light sheet trap can be used as an “optical mill” for guiding particles via photophoretic forces. We show the possibility of transferring hundreds to thousands of trapped particles from one cuvette to another in a controllable manner. Two different types of particles were used for demonstration—nonspherical agglomerations of carbon nanoparticles and printer toner particles with a more regular shape. The proposed tool can be used for the transportation of light-absorbing particles, such as biological nano- and micro-objects, or for the touch-free sampling of airborne particles being measured.
Strain-driven lattice distortion and the resultant magnetic properties of La0.7Sr0.3MnO3/BaTiO3 superlattices
Volume 115, Issue 20, November 2019. We report on the artificial manipulation of interfacial magnetism in the superlattices (SLs) of ([La0.7Sr0.3MnO3]30/[BaTiO3]25)n (1 ≤ n ≤ 10) fabricated by pulsed laser deposition. The thicker 30 uc-La0.7Sr0.3MnO3 and 25 uc-BaTiO3 layers are designed as a single period of SLs in order to eliminate the interaction between two adjacent interfaces that could contribute to a polar phase transition and the corresponding magnetism. We use aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy to demonstrate that epitaxial-strain-driven lattice distortion renders the emergence of divalent Mn at the La0.7Sr0.3MnO3/BaTiO3 interfaces. The saturated magnetization decreases and the magnetic easy axis becomes more in-plane inclined as the interfacial strain of the SLs increases.
The true spectrum of tribo-generated X-rays from peeling tape
Volume 115, Issue 20, November 2019. X-rays generated through tribological processes differ from those obtained with conventional X-ray tubes in that a substantial portion of the total energy is emitted in pulses of order 10 ns in duration. The short duration of these pulses usually causes solid-state detectors to register pileup events that can make the corresponding spectrum unreliable as a characterization tool. In this work, we find that a solid angle subtended by the detector of [math] is necessary to obtain the true spectra of X-rays generated from peeling adhesive tape in a moderate vacuum. The maximum individual photon energy is found to be 30 keV, which is about half of that reported in previous studies that overlook the effects of pileup. Being able to obtain a reliable spectrum may help us understand the physical processes behind this phenomenon so that it can be optimized for present and future applications.
Atomic ordering and bond relaxation in optical spectra of self-organized InP/GaInP2 Wigner molecule structures
Volume 115, Issue 20, November 2019. We used transmission electron microscopy, Raman, and photoluminescence spectroscopy to identify the effect of CuPt-type GaP-InP atomic ordering (AO) on the structural and emission properties of self-organized (SO) InP/GaInP2 Wigner molecule (WM) quantum dot (QD) structures. We found that the correlation of AO and SO growth results in the formation of InP/GaInP2 QD/AO-domain (QD/AOD) core-shell composites. This observation shows that intrinsic WMs in this system emerge due to a strong piezoelectric field generated by AODs, which induces QD doping and a built-in magnetic field. We found that the bond relaxation of AODs leads to a decrease in the emission energy of WMs of 80 meV. The photoluminescence spectra of single WMs having an emission energy ∼1.53 eV are presented here, the lowest one reported for this system.
Suppression of substrate coupling in GaN high electron mobility transistors (HEMTs) by hole injection from the p-GaN gate
Volume 115, Issue 20, November 2019. GaN-on-Si is a lateral technology and as such it allows the integration of high voltage High Electron Mobility Transistors and low voltage devices on the same chip, thus enabling the miniaturization and reduction of parasitic inductances. Due to the fact that integrated devices share a common substrate, the performance of one device can be significantly affected by the operation of another. The choice of the substrate bias is particularly important in the integrated half-bridge, a popular topology which includes a low- and a high-side device. A grounded substrate will cause vertical stress on the high-side device, while a floating substrate will couple with the high voltage, resulting in stress on the low-side device. This is highly problematic as the devices may fail to turn on or have a significantly increased RON. In this work, we carefully investigate the substrate coupling of a high-side and low-side device via backgating measurements. We demonstrate that the unwanted RON increase in the high side device could be suppressed by hole injection from the gate, if the gate is formed of a p-type material.
Ultrafast ultrasound imaging in acoustic microbubble trapping
Volume 115, Issue 20, November 2019. The lack of actively targeted nanocarriers and a low drug concentration in lesions are two of the main problems in targeted therapy for clinical use. In this paper, an ultrasound-induced trapping and ultrafast imaging system for flowing microbubbles is proposed to increase the effective drug dose and achieve real-time positioning. A finite element method model is established to analyze microbubble tracing in a fluid flow model at 2.5 MHz, which demonstrates how the interaction of acoustic radiation force (ARF) and flowing drag force is able to trap microbubbles and move them to a specific location. This motion can be explicitly imaged and captured by ultrafast plane wave imaging with a 1D array ultrasound probe at 18 MHz. The use of this plane wave and ARF technique can be beneficial for fast localization, monitoring, and manipulation of drug delivery bubbles for targeted release at 10 mm depth in a noninvasive and feasible way. Therefore, this ultrasound technology can be a useful tool to increase the local drug concentration in an accurate location for clinical use.
Charge generation from as-electrospun polystyrene fiber mat with uncontacted/contacted electrode
Volume 115, Issue 20, November 2019. Micrometer/submicrometer electromechanical polymer fibers are promising components for wearable pressure sensors and energy harvesters because of their high mechanical flexibility, low weight, and excellent breathability. Recently, the excellent electromechanical properties of mats of the as-electrospun micrometer/submicrometer polymer fibers have been reported; however, these devices have a contacting electrode/fiber mat/electrode structure. In this study, the electrical output properties of an as-electrospun polystyrene fiber mat with one electrode not in contact and/or in contact with the fiber mat were theoretically and experimentally investigated. The electric charges were output when the uncontacted electrode approached the fiber mat, and the amount of electric charges increased monotonically during the approach even after the electrode contacted the fiber mat and indented. The amount of electric charges was determined using theoretical functions. At the maximum displacement of the electrode, the average maximum output voltage had an absolute value of 2.28 ± 0.04 V when using the electrode with an area of 5.0 × 10−5 m2, a load of 10.1 MΩ, and a displacement time of approximately 15 ms. These experimental and theoretical findings can pave the way for the development of soft, lightweight, and breathable wearable pressure sensors and energy harvesters from a variety of materials and unique structures.
Extraordinary quasi-two-dimensional magnetotransport properties of a LaAlO3/SrTiO3 heterostructure tailored with a surface TiO2 atomic sheet
Volume 115, Issue 20, November 2019. Epitaxial heterostructures of lanthanum aluminate (LaAlO3) and strontium titanate (SrTiO3) exhibit extraordinary quasi-two-dimensional magnetotransport properties at low temperatures. To elucidate the mechanisms responsible for the unique properties of these interfaces, which can guide the design of novel structures having high performances, extensive analyses of the magnetotransport properties at low temperatures are required. We report the magnetotransport properties of a LaAlO3/SrTiO3 system tailored with a topmost surface atomic sheet of titanium dioxide (TiO2). Three unit cells of LaAlO3 were deposited on a reconstructed SrTiO3(001)-([math])-R33.7° substrate, yielding LaAlO3 covered with a single-atom-thick TiO2 sheet. The high-mobility electrons confined at the LaAlO3/SrTiO3 interface provided significantly high magnetoresistance ratios of +150% and −80% under magnetic fields perpendicular and parallel to the interface, respectively. The in-plane anisotropic magnetoresistance at 4.2 K reached approximately +30%, reflecting the Rashba spin–orbit interactions of the quasi-two-dimensional electrons. A high carrier concentration at the interface realized by the capping of LaAlO3/SrTiO3 with the surface TiO2 sheet significantly contributed to the enhancement of magnetotransport properties arising from the Ti 3d orbitals.
Determination of electronic band structure by electron holography of etched-and-regrown interfaces in GaN p-i-n diodes
Volume 115, Issue 20, November 2019. The electrostatic potential variation across etched-and-regrown GaN p-i-n diodes for power electronics has been studied using electron holography in a transmission electron microscope. The potential profiles have been correlated with the composition profiles of Mg, Si, and O obtained by secondary ion mass spectroscopy. Electronic charges obtained from the potential profiles correlate well with the presence of Si and O impurities at regrown interfaces. The overlap of Mg and Si when Mg doped GaN is grown directly over an etched undoped GaN surface results in the formation of a highly doped p-n junction. The introduction of a thin undoped layer over the etched GaN surface prevents the formation of such a junction as the regrowth interface is moved away from the Mg-doped GaN, and results in diodes with improved reverse leakage currents, close to the best values of continuously grown p-i-n diodes. Potential profiles of continuously grown (not etched) p-i-n diodes are compared to those of etched-and-regrown diodes.
Tuning critical phase transition in VO2 via interfacial control of normal and shear strain
Volume 115, Issue 20, November 2019. Interface strain plays a key role in creating the emergent functional properties of heteroepitaxially correlated materials. Strain that originates from the lattice mismatch of thin films and substrates has been widely studied to support the creation of desired functionalities. However, the shear strain induced by the symmetry mismatch of heterostructures has rarely been considered. Here, we report evidence of twin domains of stabilized vanadium dioxide (VO2) epitaxial films grown on sapphire substrates with a miscut along the a-plane. A systematic investigation of lattice variations, including lattice rotations and lattice distortions, reveals that both normal strain and shear strain can be manipulated by vicinal sapphire surfaces using different miscut angles. Consequently, the critical phenomenon of metal-insulator transitions (MITs) in VO2 epitaxial films is strongly coupled with lattice variations. A significantly sharpened MIT transition, over four orders of magnitude in resistance change, is also achieved by controlling interfacial shear strain. Our results demonstrate that the degree of freedom of shear lattice deformation opens the door to fine-tune the critical properties of heterostructures of strongly correlated oxides to aid in the development of electronic devices.
Polarization mode hybridization and conversion in phononic wire waveguides
Volume 115, Issue 20, November 2019. Phononic wire waveguides of subwavelength cross sections support two orthogonal polarization modes: the out-of-plane motion dominated Rayleigh-like and the in-plane motion dominated Love-like modes, analogous to transverse-electric and transverse-magnetic modes in photonic waveguides. Due to the anisotropic elasticity of the substrate material, the polarization states of phonons propagating along certain crystallographic orientations can strongly hybridize. Here, we experimentally investigate the orientation-dependent mode hybridization in phononic wire waveguides patterned from GaN-on-sapphire thin films. Such mode hybridization allows efficient actuation of piezoelectrically inactive Love-like modes using common interdigital electrodes designed for Rayleigh-like modes and further enables on-chip polarization conversion between guided transverse modes. Both are important for on-chip implementation of complex phononic circuits.
Band alignment and band bending at α-Ga2O3/ZnO n-n isotype hetero-interface
Volume 115, Issue 20, November 2019. Understanding the electronic structures at the interfaces of wide bandgap oxide heterostructures is crucial for the rational design of oxide-based optoelectronic devices with novel functionality and improved performance. In this work, the electronic band diagram at a ZnO/α-Ga2O3 n-n isotype heterojunction is investigated by depth-profile x-ray photoemission spectroscopy (XPS). The directly measured valence-band offset is −0.61 ± 0.1 eV and a type-I (straddling gap) band alignment is formed at the ZnO/α-Ga2O3 heterointerface. As probed by the depth profile of core-levels and VB-XPS, the formation of an interfacial layer is observed due to Ga and Zn interdiffusion, where charged interfacial states result in the downward and upward band-bending at the ZnO and α-Ga2O3 sides, respectively. The influence of band bending and band discontinuity at the interface is confirmed by the rectifying characteristics in the Au/α-Ga2O3/ZnO heterojunction with electron accumulation at its interface. Taking the thermionic-field emission and band-to-band tunneling mechanisms into account, the simulated transport properties agrees well with the reported I-V characteristics of Au/α-Ga2O3/ZnO avalanche photodiode, a further validation of the deduced band alignment of the heterostructure.
Thermoelectric property of a one dimensional channel in the presence of a transverse magnetic field
Volume 115, Issue 20, November 2019. We studied the thermal conduction through a quantum point contact (QPC), defined in a GaAs-[math] As heterostructure, in the presence of a transverse magnetic field. A shift in the position of a thermo-voltage peak is observed with increasing field. The position of the thermo-voltage peak follows the Cutler-Mott relation in the small field regime (B 3.0 T). Our results suggest that additional calibration is necessary when using QPC as thermometry, especially when the transverse magnetic field is applied.
Flexible CoFeB/MgO-based magnetic tunnel junctions annealed at high temperature (≥350 °C)
Volume 115, Issue 20, November 2019. This study investigates the effect of high-temperature (350–500 °C) annealing on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) directly formed on a flexible polyimide substrate, which has superior thermal tolerance. As the annealing temperature increases, the tunnel magnetoresistance (TMR) ratio enhances and reaches up to ∼200% at an annealing temperature of 450 °C. The annealing temperature dependence is similar to that of MTJs fabricated in the same way on a thermally oxidized silicon substrate. Images taken by a scanning transmission electron microscope confirm the improvement of the crystallization of the CoFeB and MgO layers, which can be an important factor in enhancing the TMR ratio. Furthermore, the endurance of the flexible MTJ against repeated stretching of its substrate is investigated. The TMR ratio shows no change during and after a 1000-cycle application of a tensile strain larger than 1%. The high TMR ratio and strain endurance demonstrated in this study suggest that the flexible MTJ structure is a promising candidate for a future strain-sensing device.
Magnetization reversal, damping properties and magnetic anisotropy of L10-ordered FeNi thin films
Volume 115, Issue 20, November 2019. [math]-ordered magnetic alloys such as FePt, FePd, CoPt, and FeNi are well known for their large magnetocrystalline anisotropy. Among these, the [math]-FeNi alloy is an economically viable material for magnetic recording media because it does not contain rare earth and noble elements. In this work, [math]-FeNi films with three different strengths of anisotropy were fabricated by varying the deposition process in a molecular beam epitaxy system. We have investigated magnetization reversal along with domain imaging via a magneto-optic Kerr effect based microscope. It is found that in all three samples, the magnetization reversal happens via domain wall motion. Furthermore, ferromagnetic resonance spectroscopy was performed to evaluate the damping constant (α) and magnetic anisotropy. It was observed that the FeNi sample with a moderate strength of anisotropy exhibits a low value of [math]. In addition to this, it was found that the films possess a mixture of cubic and uniaxial anisotropies.
In-situ control of electrical properties of nanoelectromechanical resonators by electromigration for self-sustained oscillations
Volume 115, Issue 20, November 2019. We use electromigration for in situ control of the electrical impedance of nanoelectromechanical resonators, the vibrations of which are detected with magnetomotive detection. The resonator consists of a doubly clamped, suspended silicon nitride beam with a gold nanowire fabricated on top. A constriction is present in the gold nanowire near the middle of the beam. As fabricated, the impedance of the device is smaller than the cable impedance of 50 Ω so that the mechanical resonance of the beam appears as a minimum on a background of the reflected ac signal in a perpendicular magnetic field. We decrease the width of the junction by inducing controlled electromigration of the gold atoms near the junction. As the electrical resistance of the nanowire is increased to near 50 Ω, the reflection background is minimized. With the vibration phase accurately measured, self-sustained oscillations of the beam resonator are excited using a phase-locked loop for a wide range of phase delay between the response and the drive. By optimizing the impedance of the nanobeam, we measure all three branches of the Duffing oscillator, including the middle one that is unstable when the driving frequency is swept without the phase-locked loop. Electromigration could serve as a versatile tool to enhance the performance of nanomechanical resonators as sensors and clocks.
Tunable giant Rashba-type spin splitting in PtSe2/MoSe2 heterostructure
Volume 115, Issue 20, November 2019. We report a giant Rashba-type spin splitting in two-dimensional heterostructure PtSe2/MoSe2 with first-principles calculations. We obtain a large value of spin splitting energy 110 meV at the momentum offset k0 = 0.23 Å−1 around the [math] point, arising from the emerging strong interfacial spin-orbital coupling induced by the hybridization between PtSe2 and MoSe2. Moreover, we find that the band dispersion close to the valence band maximum around the Γ point can be well approximated by the generalized Rashba Hamiltonian [math]. It is found that the generalized Rashba constant [math] in PtSe2/MoSe2 is as large as 1.3 eV⋅Å and, importantly, ηR can be effectively tuned by biaxial strain and external out-of-plane electrical field, presenting a potential application for the spin field-effect transistor (SFET). In addition, with the spin-valley physics at [math] points in monolayer MoSe2, we propose a promising model for SFETs with optovalleytronic spin injection based on a PtSe2/MoSe2 heterostructure.
Ultralow thermal conductivity and high thermoelectric performance of Cu2Se/TiO2 nanocomposite
Volume 115, Issue 20, November 2019. In this work, nano-TiO2 particles were introduced into Cu2Se for enhancing thermoelectric (TE) properties. We found that nano-TiO2 can efficaciously decrease the thermal conductivity by increasing the phonon scattering, making it possible to enhance TE performance of Cu2Se to some extent. The minimum κ value was 0.35 W m−1 K−1, and the κL value was as low as 0.23 W m−1 K−1 for the Cu2Se/TiO2 nanocomposites. Further, a high ZT value of ∼1.6 at 1073 K and an average ZT (400 K–1073 K) value of ∼1 were obtained for the Cu2Se/TiO2 nanocomposite (the molar ratio of Ti to Se is 0.1).
Optimization of TiO2 compact layer formed by atomic layer deposition for efficient perovskite solar cells
Volume 115, Issue 20, November 2019. The microstructure of the compact TiO2 (c-TiO2) layer formed by atomic layer deposition (ALD) was investigated for optimization of organometal halide perovskite solar cells (PSCs). The ALD c-TiO2 layer has an amorphous structure alleviating performance deterioration of the PSCs caused by defects. To apply the optimized ALD c-TiO2 layer to the PSCs, an efficiency of 18.36% was achieved. It is the top record among the PSCs using a compact TiO2 layer formed by ALD.
Effect of low-frequency alternating current poling on 5-mm-thick 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 single crystals
Volume 115, Issue 19, November 2019. Alternating current (electric field) poling (ACP) was applied on [001]-oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-0.3PT) single crystal samples with dimensions of 5 × 1.25 × 1.25 mm3 (with electrodes on the 1.25 × 1.25 mm2 surfaces), and the influence of ACP frequency (fACP) was studied. Compared to those from traditional direct (electric field) poling samples, the piezoelectric coefficient (d33) and free dielectric constant (εT33/ε0) of ACP samples could gain up to a 67% increase to 3200 pC/N and 10 500, respectively. The influence of fACP was studied on two main aspects: saturated properties and dynamic saturation process. In general, ACP samples with lower fACP had higher saturated d33, εT33/ε0, and coupling factor k33, as well as lower dielectric loss and faster saturation speed. The ACP dynamics during the saturation process were studied by measuring the polarization-vs-electric field hysteresis loops (P-E loops). The P-E loops illustrated that the coercive field of ACP samples could be further tuned from 1.84 kV/cm to 3.03 kV/cm by changing fACP (0.1–10 Hz). This work demonstrated the enormous potential of ACP optimization in relaxor-PT single crystal-based low-frequency transducer applications.
Strong edge-induced ferromagnetism in sputtered MoS2 film treated by post-annealing
Volume 115, Issue 19, November 2019. We report edge-induced ferromagnetism in a sputtered molybdenum disulfide (MoS2) film having a long whole-edge length, with the effects of crystallinity improvement including edge reconstruction by sulfur vapor and argon annealing. Strong edge-induced ferromagnetism was observed by annealing, and its saturation magnetization of 13–26 emu/cc was larger than that of a chemical-vapor deposition sample with edge-induced ferromagnetism, as reported previously. Whereas both the annealing steps improved the crystallinity of the sputtered MoS2 film, argon annealing significantly enhanced the ferromagnetism. We conclude that the difference of the ferromagnetism enhancement between the sulfur and argon annealing steps is attributed to the edge reconstruction shape, which depends on the sulfur chemical potential.
Anisotropy-driven thermal conductivity switching and thermal hysteresis in a ferroelectric
Volume 115, Issue 19, November 2019. We present a theoretical proposal for the design of a thermal switch based on the anisotropy of the thermal conductivity of PbTiO3 and the possibility to rotate the ferroelectric polarization with an external electric field. Our calculations are based on an iterative solution of the phonon Boltzmann Transport Equation and rely on interatomic force constants computed within an efficient second-principles density functional theory scheme. We also characterize the hysteresis cycle of the thermal conductivity in the presence of an applied electric field and show that the response time would be limited by the speed of the ferroelectric switch itself and thus the switch can operate in the high-frequency regime.
Phenomenological model of piezoelectric energy harvesting from galloping oscillations
Volume 115, Issue 19, November 2019. We present an experimentally validated phenomenological model that directly predicts levels of energy harvested from large oscillations of an object subjected to wake galloping. This model is superior to currently used quasisteady models, which are applicable only at low-reduced frequencies and small amplitudes. In the model, the damping controlled instability and manifestation of limit cycle oscillations due to nonlinearities are represented by linear and nonlinear damping terms. The model coefficients are then identified from measurements using analytical expressions obtained by implementing the method of multiple scales. The validation is performed by comparing time series and spectra obtained from the model and experiments.
Effect of glycerol on the mechanical and temperature-sensing properties of pectin films
Volume 115, Issue 19, November 2019. Temperature-sensitive films embedded in electronic skins (e-skins) can provide temperature feedback to robots, high-tech prostheses, and wearable devices for health care monitoring. Pectin-based films have shown a temperature response at least two orders of magnitude higher than previously reported temperature-sensing materials. However, they are not easily stretchable and tearable, which limit their applications as e-skins that require repetitive bending and mechanical stresses. Here, we show how the addition of glycerol as a plasticizer in the fabrication of pectin-based films improves their mechanical properties. We report how the enhancement of the mechanical performance is accompanied by a decrease in the temperature responsivity. Through thermogravimetric analysis, we show that this reduction in responsivity can be associated with water retention due to the addition of the plasticizer. The link between the water content and the temperature response demonstrates that a dehydrated status of pectin is crucial to record its high temperature responsivity. Combining electrical and thermal characterization with the tensile strength test, we estimate the optimal concentration of glycerol for improving the mechanical properties without compromising the temperature response of the pectin films.
Pressure-enhanced electronic coupling of highly passivated quantum dot films to improve photovoltaic performance
Volume 115, Issue 19, November 2019. PbS colloidal quantum dot solar cells (CQDSCs) have recently achieved remarkable performance enhancement due to the development of the phase-transfer ligand exchange (PTLE) method. However, the lack of compact packing of the PTLE-passivated CQDs impairs the interdot electronic coupling and thereby severely restricts further improvement in performance. To address this electronic coupling issue, we report a simple yet effective process of external pressure (0–2 MPa). We find that the interdot distance is reduced after the application of the pressure. Both optical and electrical measurements clearly demonstrate that the distance reduction can effectively strengthen the interdot electronic coupling, thus promoting the carrier transport of the CQD layer. However, too much pressure (>2 MPa) could accelerate the detrimental carrier recombination processes of CQDSCs. Accordingly, by optimizing the carrier transport and recombination processes, we achieve the maximum power conversion efficiency of 8.2% with a moderate pressure of 1.5 MPa, which is 25.5% higher than the solar cell without the external pressure. This effective strategy of external pressure could also be applied to other CQD-based optoelectronic devices to realize a better device performance.
Ohmic contact to AlN:Si using graded AlGaN contact layer
Volume 115, Issue 19, November 2019. We formed a graded-AlGaN contact layer to improve the Ohmic characteristics of Si-doped AlN. Linear I-V characteristics were obtained for AlN with the graded-AlGaN layer, and the current was three orders of magnitude larger than that for AlN without the one. The specific contact resistivity decreased with the increasing thickness of the graded-AlGaN layer. This was probably due to a reduction in the three-dimensional negative charge density induced by the polarization charge in the graded AlGaN layer. A minimum contact resistivity of 1.4 [math] [math] was obtained for a 330-nm-thick graded-AlGaN layer. To obtain the Ohmic contact, the Si-dopant concentration ([math]) should be larger than the negative fixed charge density ([math]) induced by the polarization charge. However, the heavily doped graded-AlGaN layer ([math] cm−3) became semi-insulating due to self-compensation. The results indicated that reducing [math] by relaxing the compositional slope in the graded layer can improve the Ohmic characteristics.
The effect of phase purification on photovoltaic performance of perovskite solar cells
Volume 115, Issue 19, November 2019. Organic-inorganic hybrid perovskite solar cells (PSCs) have witnessed a rapid rising in power conversion efficiency (PCE) over the past few years; however, they still suffer from recombination loss via interface defects in perovskite films. In this study, we implement an efficient phase purification strategy by incorporating isopropyl alcohol (IPA) post-treatment of perovskite films that reduces defect states and improves charge transport. It is found that the DMSO-PbI2-MAI complex in the perovskite film is eliminated after IPA post-treatment. A suit of opto-electric characterizations demonstrates that the nonradiative recombination is greatly diminished, and charge extraction is effectively boosted in the modified perovskite films. The perovskite solar cells with phase-pure MAPbI3 achieve an impressively larger PCE of 18.78% than that of 17.1% for the control devices. Our work presents a facile and efficient path to performance improvement of PSCs.
Effect of vacancies on thermoelectric properties of β-CuAgSe studied by positron annihilation
Volume 115, Issue 19, November 2019. CuAgSe is a promising thermoelectric material due to its superionicity. In this work, β-Cu1−xAg1−ySe (x = 0, 0.02, and 0.04; y = 0, 0.02, and 0.04) samples are synthesized by solid-state reaction method. The vacancies in samples are characterized by positron annihilation spectroscopy. Thereafter, the effects of vacancies on thermoelectric properties are investigated. The positron annihilation results reveal that Ag vacancies exist in the Ag-deficient samples (β-CuAg0.98Se and β-Cu0.98Ag0.98Se) but also in the Cu-deficient samples (β-Cu0.96AgSe and β-Cu0.98AgSe). For the Cu-deficient samples, the existence of Ag vacancies is attributed to the formation of impurity phases. For the nonstoichiometric samples, the vacancies are responsible for the decrease in the Seebeck coefficient in the temperature range from 300 to 400 K. However, for β-CuAgSe, no decrease in the Seebeck coefficient is observed due to the lack of extra holes, and electrons are still the majority carriers. For CuAgSe, the ZT value is mainly determined by the Seebeck coefficient. Therefore, for the nonstoichiometric samples, the ZT value reduces drastically with increasing temperature and drops to nearly zero at 400 K. In contrast, with the temperature increasing from 300 to 450 K, the ZT value of β-CuAgSe goes up from 0.4 to 0.5.
Physical reservoir computing using magnetic skyrmion memristor and spin torque nano-oscillator
Volume 115, Issue 19, November 2019. Spintronic nanodevices have ultrafast nonlinear dynamic and recurrence behaviors on a nanosecond scale that promises to enable a high-performance spintronic reservoir computing (RC) system. Here, two physical RC systems based on one single magnetic skyrmion memristor (MSM) and 24 spin-torque nano-oscillators (STNOs) are numerically modeled to process image classification task and nonlinear dynamic system prediction, respectively. Based on the nonlinear responses of the MSM and STNO with current pulse stimulation, our results demonstrate that the MSM-based RC system exhibits excellent performance on image classification, while the STNO-based RC system does well in solving the complex unknown nonlinear dynamic problems, e.g., a second-order nonlinear dynamic system and NARMA10. Our result and analysis of the current-dependent nonlinear dynamic properties of the MSM and STNO provide the strategy to optimize the experimental parameters in building the better spintronic-based brainlike devices for machine learning based computing.
DC measurement of dressed states in a coupled 100 GHz resonator system using a single quasiparticle transistor as a sensitive microwave detector
Volume 115, Issue 19, November 2019. We report on the on-chip detection of microwaves in the frequency range around 100 GHz. For the purpose of detection, we employ a discrete transport channel triggered in a superconducting single-electron transistor by photon-assisted tunneling of quasiparticles. The technique is applied to observe the spectrum of the dressed states of a model circuit quantum electrodynamics system consisting of a superconducting coplanar resonator coupled to a Josephson oscillator. The dressed states appear as typical resonance anticrossing exhibiting, in our case, an expectedly wide frequency splitting corresponding to the Jaynes–Cummings coupling strength, [math] 10 GHz. Due to the high decay rate, [math] 20[math]40 GHz, in the very transparent Josephson junctions used, the strong coupling limit, [math], which is required for qubit operation, is not achieved, and the photon population in the resonator is low, [math] 1. Remarkably, the continuous readout of the low population states demonstrates the high microwave sensitivity of the detector.
Perovskite light-emitting diodes for uniform eight-segment displays
Volume 115, Issue 19, November 2019. With the development of the display technology, there are higher requirements for color saturation and vividness. As a base unit, light-emitting diode (LED) has become a primary object of study. Perovskite quantum dots with high photoluminescence quantum yield, narrow emission, and wide color gamut have attracted special attention in recent years. Although LEDs based on perovskite exhibit excellent performance, there are few applications of perovskite LEDs in display. Herein, LEDs based on CsPbI3 perovskite were used as basic units for fabricating eight-segment LED displays. The LEDs exhibited a luminance of 780 cd/m2 and an external quantum efficiency of 6%. The current-density and luminance vs driving voltage confirms the uniformity of these LEDs in the eight segments, which will promote the application of perovskite nanocrystalline LEDs in the field of full color display.
Single-layer CdPSe3: A promising thermoelectric material persisting in high temperatures
Volume 115, Issue 19, November 2019. Searching for two-dimensional (2D) functional semiconductors with excellent performance is a central issue in the field of 2D materials. Using the first-principles calculation combined with the Boltzmann transport theory, we survey the thermodynamic stabilities, electronic transports, and thermoelectric performances of single-layer (1L-) CdPSe3, which is a transition-metal phosphorus trichalcogenide. Through an investigation of the cleavage energy, we reveal that an isolation into 1L-CdPSe3 from the bulk form is guaranteed, which is in addition thermodynamically stable, as confirmed by both the first-principles molecular dynamics and the phonon spectrum. Electron and hole mobilities of 1L-CdPSe3 are calculated and found to be ∼390 and ∼300 cm2 V−1 s−1, respectively. The lattice thermal conductivity of 1L-CdPSe3 is shown to be as low as ∼1.25 W m−1 K−1 at room temperature. Finally, the thermoelectric figure of merit of 1L-CdPSe3 is calculated to be ∼1.2 under the p-type optimal doping at a high temperature (1200 K). This suggests that 1L-CdPSe3 could be a promising candidate for pursuing an excellent thermoelectric functionality, in particular, valid even at high temperatures.
A robust actively-tunable perfect sound absorber
Volume 115, Issue 19, November 2019. Perfect sound absorption (PSA), producing an absorption coefficient of 1, can be achieved based upon the coherent interaction of acoustic waves, while it can merely be achieved within a narrow frequency-band due to critical impedance matching conditions. Here, we theoretically and experimentally study self-modulation and active-tunability in PSA created on account of a membrane-covered cavity. It is observed that due to the nonlinearity of the membrane, the frequency of PSA increases with the intensities of input acoustic waves, exhibiting a self-modulation property. Furthermore, we design an apparatus to control the elasticity of the membrane via four electromagnets, and thus, the PSA frequency can be freely and effectively adjusted by changing the direct-current driving voltage of the electromagnets. Despite the critical impedance matching conditions, the absorption coefficient achieved in our system holds at 1 when the PSA frequency is freely changed, which exhibits strong robustness in active-tunability.
Quartz tuning fork—A potential low temperature thermometer in high magnetic fields
Volume 115, Issue 19, November 2019. We present the performance of commercial quartz tuning forks (QTFs) operating at resonance frequencies of 32 kHz, 77 kHz, and 100 kHz in the temperature range below 1 K and in high magnetic fields up to 7.5 T. We show that characteristics of the quartz tuning forks, in particular, the normalized QTF resonance frequency, manifest a universal temperature dependence, which is independent of the magnetic field strength. This feature makes the QTFs very promising low temperature thermometers in high magnetic fields in the temperature range below 1 K having the B/T ratio up to 1000. We also discuss the physical origin of the observed dependencies.
A double-beam piezo-magneto-elastic wind energy harvester for improving the galloping-based energy harvesting
Volume 115, Issue 19, November 2019. This study investigates the performance of a double-beam piezo-magneto-elastic wind energy harvester (DBPME-WEH) when exhibiting a galloping-based energy harvesting regime under wind excitation. The DBPME-WEH comprises two piezoelectric beams, each of which supports a prism bluff body embedded with a magnet at the tip. The magnets are oriented to repulse each other to introduce a bistable nonlinearity. Wind tunnel tests were conducted to compare performances of the DBPME-WEH and a double-beam piezoelectric wind energy harvester (DBP-WEH) that does not comprise the magnet-induced nonlinearity. The results reveal that compared to the DBP-WEH, the critical wind speed to activate the galloping vibration of DBPME-WEH can be reduced up to 41.9%. Thus, the results corroborate the significant performance enhancement by the DBPME-WEH. It can also be found that the distance of the two magnets affects the performance and the distance that achieves the weakly bistable nonlinearity is beneficial to energy harvesting in reducing the critical wind speed and improving the output voltage.
Plasma formation and relaxation dynamics in fused silica driven by femtosecond short-wavelength infrared laser pulses
Volume 115, Issue 19, November 2019. Laser-induced plasma formation and subsequent relaxation in dielectric solids is the precursor to structural modifications serving as the basis for direct laser writing of functional optical micro- and nanostructures. Based on an experimental arrangement combining a time-resolved transmission measurement with a cross-phase modulation measurement, we isolate the plasma formation and relaxation dynamics in the bulk of amorphous fused silica excited by femtosecond short-wavelength infrared ([math] 2100 nm) laser pulses. Whereas the relaxation time of the generated subcritical electron-hole plasma was so far assumed to be constant, our findings indicate an intensity-dependent relaxation time. We attribute this intensity dependence to vibrational activation of the medium, leading to detrapping of trapped carriers and a reduced trapping probability.
Polaronic nature of a muonium-related paramagnetic center in SrTiO3
Volume 115, Issue 19, November 2019. The hyperfine features and thermal stability of a muonium (Mu)-related paramagnetic center were investigated in SrTiO3 perovskite titanate via muon spin rotation spectroscopy. The hyperfine coupling tensor of the paramagnetic center was found to have prominent dipolar characteristics, indicating that the electron spin density is dominantly distributed on a Ti site to form a small polaron near an ionized Mu+ donor. Based on a hydrogen-Mu analogy, interstitial hydrogen is also expected to form such a polaronic center in the dilute doping limit. The small activation energy of 30(3) meV found for the thermal dissociation of the Mu+-polaron complex suggests that the strain energy required to distort the lattice is comparable to the electronic energy gained by localizing the electron.
Spatially resolved optical excitation of mechanical modes in graphene NEMS
Volume 115, Issue 19, November 2019. Emerging applications in nanoelectromechanical systems (NEMS) made from two-dimensional (2D) materials demand simultaneous imaging and selective actuation of the mechanical modes. Focused optical probes to measure and actuate motion offer a possible solution, but their lateral spatial resolution must be better than the size of the resonator. While optical interferometry is known to have excellent spatial resolution, the spatial resolution of the focused, laser-based optical driving is not currently known. Here, we combine separately scanned interferometry and optical drive probes to map the motion and forces on a suspended graphene nanomechanical resonator. By analyzing these maps with a force density model, we determine that the optical drive force has a spatial resolution on the order of the size of the focused laser spot. Using the optical force probe, we demonstrate the selective actuation and suppression of a pair of orthogonal antisymmetric mechanical modes of the graphene resonator. Our results offer a powerful approach to image and actuate any arbitrary high-order mode of a 2D NEMS.
Thermal transport and energy dissipation in two-dimensional Bi2O2Se
Volume 115, Issue 19, November 2019. Thermal transport and energy dissipation are important for a material in both thermoelectric and electronic devices. Here, we investigate the lateral and interfacial thermal transport of two-dimensional (2D) Bi2O2Se by Raman spectroscopy. It is found that thin Bi2O2Se flakes have a low in-plane thermal conductivity while maintaining an appropriate interfacial thermal conductance. The in-plane thermal conductivity of Bi2O2Se decreases with decreasing thickness, to as low as 0.92 ± 0.18 W⋅m−1⋅K−1 at a thickness of ∼8 nm. Such a low thermal conductivity is derived from the low phonon group velocity, strong anharmonicity, and large surface scattering of acoustic phonons of the Bi2O2Se thin layer. Simultaneously, thinner Bi2O2Se presents a higher thermal dissipation to the substrate than the thicker counterparts in the device. The interfacial thermal conductance increases with decreasing thickness, and reaches ∼21 MW⋅m−2⋅K−1 at ∼8 nm. These results provide critical information for the design of thermoelectric devices with high figures of merit and electronics with low-power consumption based on 2D materials.
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