Journal of Applied Physics

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ISSN / EISSN : 0021-8979 / 1089-7550
Published by: AIP Publishing (10.1063)
Total articles ≅ 143,695
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Alexander Y. Choi, Iretomiwa Esho, Bekari Gabritchidze, ,
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0063331

Abstract:
Cryogenic low-noise amplifiers based on high electron mobility transistors (HEMTs) are widely used in applications such as radio astronomy, deep space communications, and quantum computing. Consequently, the physical mechanisms governing the microwave noise figure are of practical interest. In particular, the magnitude of the contribution of thermal noise from the gate at cryogenic temperatures remains unclear owing to a lack of experimental measurements of thermal resistance under these conditions. Here, we report measurements of gate junction temperature and thermal resistance in a HEMT at cryogenic and room temperatures using Schottky thermometry. At temperatures ∼20 K, we observe a nonlinear trend of thermal resistance vs power that is consistent with heat dissipation by phonon radiation. Based on this finding, we consider heat transport by phonon radiation at the low-noise bias and liquid helium temperatures and estimate that the thermal noise from the gate is several times larger than previously assumed owing to self-heating. We conclude that without improvements in thermal management, self-heating results in a practical lower limit for microwave noise figure of HEMTs at cryogenic temperatures.
Wei Li, Xiangguo Zeng, Yehui Cui, Junfeng Xiao, Han Zhao,
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0052468

Abstract:
This work established a two-dimensional mathematical model to evaluate the thermofluidynamic behavior of a newly developed double-layered cylindrical ZrCo-based hydride bed during the hydrogenation reaction process. Numerical simulations by using COMSOL Multiphysics were conducted to solve the governing partial differential equations associated with chemical reaction and thermal transfer. Importantly, the local thermal nonequilibrium theory was applied to analyze the heat transfer between the ZrCo particle and hydrogen. Detailed analysis revealed the effects of operating conditions, material thermophysical properties, and the bed configuration, in addition to the ZrCo hydride particle sizes on hydrogen recovery characteristics. The simulation results indicated that increasing the heat transfer coefficient, reducing the coolant temperature, improving the thermal conductivity of the metal hydride, using the thinner hydride layer, and being equipped with copper fins were more beneficial to accelerate the heat transfer rate and the hydrogen charging rate of the metal hydride bed (MHB). Furthermore, along the radial direction, tremendous temperature gradient and distribution of absorbed hydrogen were found in the hydrogenated zones. The present model can effectively characterize the reaction kinetic mechanisms of ZrCo hydriding process as to further promote the practical application of the proposed MHB designs.
, Xingyu Zhang, Steven L. Richardson, Efthimios Kaxiras
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0061396

Abstract:
The nitrogen-vacancy (NV) lattice defect in diamond, consisting of an N substitutional atom and an adjacent C vacancy, is commonly observed in two charge states, negative (NV−) and neutral (NV0). The NV− defect exhibits spin state-dependent fluorescence and is, therefore, amenable to optical methods for spin-state readout, while the NV0 is not. Hence, the NV− defect is much more useful for quantum sensing and quantum information processing. However, only NV0 electroluminescence has been observed, even from centers showing NV− in photoluminescence. In the present work, we use first-principles electronic structure calculations to determine adiabatic charge transition levels for the conversion of NV− to NV0 in the presence of substitutional N or P impurities, which provide the charge of the NV− center. We find that the adiabatic charge transition levels in the presence of such impurities lie at energies close to or lower than the zero-phonon line of the NV− center and that these energies only decrease as the concentration of N donors increases. This work, therefore, elucidates the absence of observed electroluminescence from the NV− and proposes a path toward observation of the phenomenon.
, A. F. Zinovieva, , , V. I. Vdovin, , L. I. Fedina, O. M. Borodavchenko, V. D. Zhivulko, A. V. Mudryi
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0063592

Abstract:
The photoluminescence (PL) of Ge/Si nanostructures synthesized by using Ge+ ion bombardment is studied. The structure represents a Si substrate with GeSi nanoclusters created by 80 keV Ge implantation with a fluence of ∼1015 ions/cm2 and subsequent thermal annealing. The PL measurements confirm the advantage of Ge/Si structures synthesized using Ge+ ion bombardment over the usual epitaxial structures with GeSi quantum dots. The presence of defects produced by Ge implantation results in pronounced PL at telecom wavelengths up to room temperature. The results provide a basis for creating efficient light emitters compatible with the existing Si technology.
S. Saib, , Ertugˇrul Karaca, S. Bagˇcı, H. M. Tütüncü, G. P. Srivastava
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0060200

Abstract:
Ab initio pseudopotential calculations have made for the structural, electronic, elastic, mechanical, and electron–phonon interaction properties of molybdenum borocarbide (Mo2BC) and niobium boronitride (Nb2BN) superconductors. Analysis of the structural and electronic properties reveals that the nature of bonding in both these compounds is a combination of covalent, ionic, and metallic. The near-Fermi electronic states in both compounds are occupied by the d states of transition metal atoms. The electronic density of states at the Fermi level in Mo2BC is significantly higher than that in Nb2BN. Lattice dynamical calculations verify their dynamical stability in the base-centered orthorhombic Mo2BC-type crystal structure. We find that the total electron–phonon coupling constant is equal to 0.745 for Mo2BC and 0.539 for Nb2BN. The calculated superconducting transition temperature of 7.41 K for Mo2BC and 3.50 K for Nb2BN is comparable with their experimental values of 7.2 and 4.4 K, respectively.
Yuhang Zhang, Jiejie Li, Yiqun Hu, Suhang Ding, Fuying Du,
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0065441

Abstract:
Thanks to their excellent properties such as superelasticity, high hardness, and shape memory effect, polycrystalline shape memory alloys (SMAs) have extensive applications in various engineering fields including automobile, functional materials, and aerospace. Using molecular dynamics simulations, the present paper aims to a systematic study of the fundamental tensile behavior in the nanoscale of polycrystalline B2-CuZr SMAs with mean grain sizes in the range of 6–25 nm. Effects of mean grain size, temperature, and tensile rate on mechanical properties are considered. Our results show that along with the increase in mean grain size came increases in Young's modulus, yield strength, flow stress, and ultimate tensile strength. The development of amorphous regions in the grain cores is the major deformation mode in polycrystalline CuZr SMAs with larger grain sizes, while the grain boundary sliding and grain rotation for smaller grain sizes. Besides, an increased temperature results in mechanical performance degradation and the temperature sensitivity of mechanical properties does not depend on the mean grain size. Our work would lay the groundwork for the optimization of the mechanical properties of polycrystalline SMAs as well as serving as a useful theoretical guideline for their practical engineering applications.
S. C. Scholten, A. J. Healey, I. O. Robertson, G. J. Abrahams, ,
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0066733

Abstract:
A dense layer of nitrogen-vacancy (NV) centers near the surface of a diamond can be interrogated in a widefield optical microscope to produce spatially resolved maps of local quantities such as magnetic field, electric field, and lattice strain, providing potentially valuable information about a sample or device placed in proximity. Since the first experimental realization of such a widefield NV microscope in 2010, the technology has seen rapid development and demonstration of applications in various areas across condensed matter physics, geoscience, and biology. This Perspective analyzes the strengths and shortcomings of widefield NV microscopy in order to identify the most promising applications and guide future development. We begin with a brief review of quantum sensing with ensembles of NV centers and the experimental implementation of widefield NV microscopy. We then compare this technology to alternative microscopy techniques commonly employed to probe magnetic materials and charge flow distributions. Current limitations in spatial resolution, measurement accuracy, magnetic sensitivity, operating conditions, and ease of use are discussed. Finally, we identify the technological advances that solve the aforementioned limitations and argue that their implementation would result in a practical, accessible, high-throughput widefield NV microscope.
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0062523

Abstract:
In this Perspective, I raise the question whether the terms high entropy ceramics, high entropy nitrides, high entropy oxides, high entropy borides, etc., are meaningful considering the magnitude of the calculated configurational entropy. Here, the origin of Boltzmann's equation is reviewed and the implications for application are discussed. This back to the roots’ excursion may be helpful for approaching an answer to the question raised in the title and for re-evaluating literature reports connecting superior performance with configurational entropy of so-called high entropy ceramics.
Maxwell Li, , Ashok Pokhrel, Arjun Sapkota, Claudia Mewes, , Di Xiao, , Vincent Sokalski
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0056100

Abstract:
We examine the substructures of magnetic domain walls (DWs) in [Pt/(Co/Ni)M/Ir]N multi-layers using a combination of micromagnetic theory and Lorentz transmission electron microscopy. Thermal stability calculations of Q=±1 substructures [2π vertical Bloch lines and DW skyrmions] were performed using a geodesic nudged elastic band model, which supports their metastability at room temperature. Experimental variation in strength of the interfacial Dzyaloshinskii–Moriya interaction and film thickness reveals conditions under which these substructures are present and enables the formation of a magnetic phase diagram. Reduced thickness is found to favor Q=±1 substructures likely due to the suppression of hybrid DWs. The results from this study provide an important framework for examining 1D DW substructures in chiral magnetic materials.
Shangradhanva E. Vasisth,
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0062811

Abstract:
Highly dense, energy-efficient, and fast neuromorphic architectures emulating the computational abilities of the brain use memristors to emulate synapses in the analog or digital systems. Core–shell nanowires provide us with new opportunities for neuromorphic hardware integration. In this work, we have fabricated core–shell nanowires using a combination of bottom-up and top-down techniques. Additionally, we have demonstrated eightwise and counter-eightwise bipolar resistive switching (BRS). Remarkably, for the first time along with BRS, we have also demonstrated complementary resistive switching (CRS) in core–shell nanowires. Here, Pt was used as the conductive core and HfO2 as the memristive shell with Ti as the top electrode. The resistive switching properties were characterized by I–V curves and pulse operation modes. The cycling endurance in the BRS mode was 1000 cycles with an off–on ratio of ∼13 and resistance was retained for 104 s. Additionally, the compliance current used to form the nanowire in the BRS mode influenced the CRS operation by lowering the peak operating current. Additionally, current density–electric field analysis performed to determine charge conduction mechanisms revealed that the wires exhibit a thermionic emission mechanism in the high resistance state and Ohmic conduction mechanism in the low resistance state during the BRS mode of operation and hopping conduction mechanism in state 0 and space-charge-limited conduction mechanism in state 1 during the CRS mode of operation. This observed versatility in the mode of operation makes core–shell nanowires of significant interest for use as synaptic elements in neuromorphic network architectures.
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