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Low Temperature Spin Seebeck Effect in Non-Magnetic Vanadium Dioxide

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arXiv 2023年

作者： Luo, Renjie Legvold, Tanner J. Chen, Liyang Navarro, Henry Basaran, Ali C. Hong, Deshun Liu, Changjiang Bhattacharya, Anand Schuller, Ivan K. Natelson, Douglas Department of Physics and Astronomy Rice University HoustonTX77005 United States Applied Physics Graduate Program Rice Quantum Institute Rice University HoustonTX77005 United States Department of Physics Center for Advanced Nanoscience University of California La Jolla San DiegoCA92093 United States Materials Science Division Argonne National Laboratory LemontIL60439 United States Department of Electrical and Computer Engineering Department of Materials Science and NanoEngineering Rice University HoustonTX77005 United States

The spin Seebeck effect (SSE) is sensitive to thermally driven magnetic excitations in magnetic insulators. Vanadium dioxide in its insulating low temperature phase is expected to lack magnetic degrees of freedom, as vanadium atoms are thought to form singlets upon dimerization of the vanadium chains. Instead, we find a paramagnetic SSE response in VO2 films that grows as the temperature decreases below 50 K. The field and temperature dependent SSE voltage is qualitatively consistent with a general model of paramagnetic SSE response and inconsistent with triplet spin transport. Quantitative estimates find a spin Seebeck coefficient comparable in magnitude to that observed in strongly magnetic materials. The microscopic nature of the magnetic excitations in VO2 requires further examination. Copyright © 2023, The Authors. All rights reserved.

关键词： Paramagnetism

Biophysics of amplitude-modulated giga-hertz electromagnetic waves stimulation

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Biophysics of amplitude-modulated giga-hertz electromagnetic...

Asilomar Conference on Signals, Systems & Computers

作者： Fatima Ahsan Aravind C. Govindaraju Robert M. Raphael Taiyun Chi Sameer A. Sheth Wayne Goodman Behnaam Aazhang Department of Electrical and Computer Engineering Rice University Houston TX USA Applied Physics Graduate Program Smalley-Curly Institute Rice University Houston TX Department of Bioengineering Rice University Houston TX USA Department of Neurosurgery Baylor College of Medicine Houston TX USA Menninger Department of Psychiatry and Behavioral Sciences Baylor College of Medicine Houston TX USA

This paper presents the biophysical modeling for neuronal stimulation caused by amplitude-modulated giga-hertz (GHz) electromagnetic (EM) waves. We model a 1D cable equation for electrical signal propagation in a vestibular ganglion neuronal fiber and use Hodgkin-Huxley formalism to model voltage-dependent channel conductances. We find out that for a given stimulation signal strength, the GHz waveform amplitude-modulated at a low frequency of Δf causes neuronal firing with the frequency of Δf whereas the continuous wave GHz did not elicit any neuronal firing. Moreover, the voltage-dependent channel gating dynamics of the Δf amplitude-modulated GHz stimulation matches that of a simple low frequency stimulation at Δf. Additionally, we find that the modulation depth controls the firing rate of the neurons. The significance of this property is that the amplitude modulation caused by the interference of two similar high frequency signals could be leveraged to focally excite neurons at depth without stimulating overlying cortical regions. These theoretical predictions based on the Hodgkin-Huxley model can later be tested experimentally.

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Positive- and negative-frequency noise from an ensemble of two-level fluctuators

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Physical Review Research 2021年第1期3卷 013045-013045页

作者： Xinyuan You Aashish A. Clerk Jens Koch Northwestern-Fermilab Center for Applied Physics and Superconducting Technologies Northwestern University Evanston Illinois 60208 USA Graduate Program in Applied Physics Northwestern University Evanston Illinois 60208 USA Pritzker School of Molecular Engineering University of Chicago 5640 South Ellis Avenue Chicago Illinois 60637 USA Department of Physics and Astronomy Northwestern University Evanston Illinois 60208 USA

The analysis of charge noise based on the Bloch-Redfield treatment of an ensemble of dissipative two-level fluctuators generally results in a violation of the fluctuation-dissipation theorem. The standard Markov approximation (when applied to the two-level fluctuators coupled to a bath) can be identified as the main origin of this failure. The resulting decoherence rates only involve the bath response at the fluctuator frequency, and thus completely neglect the effects of frequency broadening. A systematic and computationally convenient way to overcome this issue is to employ the spectator-qubit method: by coupling an auxiliary qubit to the two-level fluctuator ensemble, an analytical approximation for S(ω) fully consistent with the fluctuation-dissipation theorem can be obtained. We discuss the resulting characteristics of the noise which exhibits distinct behavior over several frequency ranges, including a 1/f to 1/f2 crossover with a T3 temperature dependence of the crossover frequency.

关键词： Open quantum systems & decoherence Quantum fluctuations & noise Quantum information with solid state qubits Josephson junctions Superconducting qubits Thin films

Unifying (Machine) Vision via Counterfactual World Modeling

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arXiv 2023年

作者： Bear, Daniel M. Feigelis, Kevin Chen, Honglin Lee, Wanhee Venkatesh, Rahul Kotar, Klemen Durango, Alex Yamins, Daniel L.K. Department of Psychology Stanford University StanfordCA94305 United States Department of Physics Stanford University StanfordCA94305 United States Department of Computer Science Stanford University StanfordCA94305 United States Department of Applied Physics Stanford University StanfordCA94305 United States Neurosciences Graduate Program Stanford University StanfordCA94305 United States Wu Tsai Neurosciences Institute Stanford University StanfordCA94305 United States

Leading approaches in machine vision employ different architectures for different tasks, trained on costly task-specific labeled datasets. This complexity has held back progress in areas, such as robotics, where robust task-general perception remains a bottleneck. In contrast, "foundation models" of natural language have shown how large pre-trained neural networks can provide zero-shot solutions to a broad spectrum of apparently distinct tasks. Here we introduce Counterfactual World Modeling (CWM), a framework for constructing a visual foundation model: a unified, unsupervised network that can be prompted to perform a wide variety of visual computations. CWM has two key components, which resolve the core issues that have hindered application of the foundation model concept to vision. The first is structured masking, a generalization of masked prediction methods that encourages a prediction model to capture the low-dimensional structure in visual data. The model thereby factors the key physical components of a scene and exposes an interface to them via small sets of visual tokens. This in turn enables CWM’s second main idea – counterfactual prompting – the observation that many apparently distinct visual representations can be computed, in a zero-shot manner, by comparing the prediction model’s output on real inputs versus slightly modified ("counterfactual") inputs. We show that CWM generates high-quality readouts on real-world images and videos for a diversity of tasks, including estimation of keypoints, optical flow, occlusions, object segments, and relative depth. Taken together, our results show that CWM is a promising path to unifying the manifold strands of machine vision in a conceptually simple foundation. © 2023, CC BY.

关键词： Computer vision

Temperature Dependence of the Anisotropy of the Infrared Dielectric Properties and Phonon-Plasmon Coupling in N-Doped 4h-Sic

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SSRN

SSRN 2023年

作者： Kazan, Michel Zgheib, C. Bruyant, A. Wu, K.-T. El-Helou, Y. Rahbany, N. Chahal, J. Department of Physics American University of Beirut PO Box 11-0236 Riad El-Solh Beirut1107-2020 Lebanon Light Nanomaterials and Nanotechnology University of Technology of Troyes CNRS ERL 7004 Troyes10004 France Department of Physics National Central University Taoyuan32001 Taiwan Molecular Science and Technology Program Taiwan International Graduate Program Institute of Atomic and Molecular Science Academia Sinica Taipei10617 Taiwan Faculty of Natural Sciences & Applied Sciences Notre Dame University-Louaize PO Box 72 Zouk Mosbeh Lebanon

Polarized infrared reflectivity spectroscopy was used to measure the temperature dependence of the anisotropy of the infrared dielectric properties and coupling between phonon and plasmon modes in n-doped 4H-SiC. The advantage of polarized infrared spectroscopy over the commonly used Raman spectroscopy is that it enables dielectric, plasmonic, and transport properties to be probed along and perpendicular to the c-axis, which is impossible with Raman spectroscopy because of its selection rules. In addition, the infrared spectrum, comparatively rare in Raman spectroscopy, can be described at high temperatures using simple classical theory, providing more precise information on the material’s properties. Analysis of the s- and p-polarized infrared reflectivity spectra of an n-doped 4H-SiC substrate shows that the strength and damping of the oscillators, the response of the bound electrons to infrared excitation, the effective ion mass, and the transverse frequency and anharmonicity of the phonons of n-doped 4H-SiC are temperature-sensitive and strongly anisotropic. It also shows that the plasmonic properties of n-doped 4H-SiC, namely the relaxation time and collective oscillation frequency of free electrons, are also highly anisotropic and, owing to the long relaxation time of free electrons in the direction parallel to the c-axis, only the axial longitudinal phonon-plasmon modes split into low-frequency and high-frequency modes at temperatures above 700 K. © 2023, The Authors. All rights reserved.

关键词： Anisotropy

Local and global measures of the shear moduli of jammed disk packings

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arXiv 2023年

作者： Zhang, Shiyun Jin, Weiwei Wang, Dong Xu, Ding Zhang, Jerry Shattuck, Mark D. O’Hern, Corey S. Department of Physics University of Science and Technology of China Anhui Hefei230026 China Department of Mechanical Engineering and Materials Science Yale University New HavenCT06520 United States Benjamin Levich Institute and Physics Department The City College of New York New YorkNY10031 United States Department of Physics Yale University New HavenCT06520 United States Department of Applied Physics Yale University New HavenCT06520 United States Graduate Program in Computational Biology and Bioinformatics Yale University New HavenCT06520 United States

Strain-controlled isotropic compression gives rise to jammed packings of repulsive, frictionless disks with either positive or negative global shear moduli. We carry out computational studies to understand the contributions of the negative shear moduli to the mechanical response of jammed disk packings. We first decompose the ensemble-averaged, global shear modulus as hGi = (1 − F−)hG+i + F−hG−i, where F− is the fraction of jammed packings with negative shear moduli and hG+i and hG−i are the average values from packings with positive and negative moduli, respectively. We show that hG+i and h|G−|i obey different power-law scaling relations above and below pN2 ∼ 1. For pN2 > 1, both hG+iN and h|G−|iN ∼ (pN2)β, where β ∼ 0.5 for repulsive linear spring interactions. Despite this, hGiN ∼ (pN2)β0 with β0 & 0.5 due to the contributions from packings with negative shear moduli. We show further that the probability distribution of global shear moduli P(G) collapses at fixed pN2 and different values of p and N. We calculate analytically that P(G) is a Gamma distribution in the pN2 1 limit. As pN2 increases, the skewness of P(G) decreases and P(G) becomes a skew-normal distribution with negative skewness in the pN2 1 limit. We also partition jammed disk packings into subsystems using Delanunay triangulation of the disk centers to calculate local shear moduli. We show that the local shear moduli defined from groups of adjacent triangles can be negative even when G > 0. The spatial correlation function of local shear moduli C(~r) displays weak correlations for pn2sub −2, where nsub is the number of particles within each subsystem. However, C(~r) begins to develop long-ranged spatial correlations with four-fold angular symmetry for pn2sub & 10−2 © 2023, CC BY-NC-ND.

关键词： Elastic moduli

Angle-resolved photoemission spectroscopy with an in situ tunable magnetic field

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arXiv 2023年

作者： Huang, Jianwei Yue, Ziqin Baydin, Andrey Zhu, Hanyu Nojiri, Hiroyuki Kono, Junichiro He, Yu Yi, Ming Department of Physics and Astronomy Rice University HoustonTX77005 United States Applied Physics Graduate Program Smalley-Curl Institute Rice University HoustonTX77005 United States Department of Electrical and Computer Engineering Rice University HoustonTX77005 United States Smalley–Curl Institute Rice University HoustonTX77005 United States Department of Materials Science and NanoEngineering Rice University HoustonTX77005 United States Institute for Materials Research Tohoku University Katahira 2-1-1 Sendai980-8577 Japan Department of Applied Physics Yale University New HavenCT06511 United States

Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for probing the momentum-resolved single-particle spectral function of materials. Historically, in situ magnetic fields have been carefully avoided as they are detrimental to the control of photoelectron trajectory during the photoelectron detection process. However, magnetic field is an important experimental knob for both probing and tuning symmetry-breaking phases and electronic topology in quantum materials. In this paper, we introduce an easily implementable method for realizing an in situ tunable magnetic field at the sample position in an ARPES experiment and analyze magnetic field induced artifacts in ARPES data. Specifically, we identified and quantified three distinct extrinsic effects of a magnetic field: Constant energy contour rotation, emission angle contraction, and momentum broadening. We examined these effects in three prototypical quantum materials, i.e., a topological insulator (Bi2Se3), an iron-based superconductor (LiFeAs), and a cuprate superconductor (Pb-Bi2Sr2CuO6+x), and demonstrate the feasibility of ARPES measurements in the presence of a controllable magnetic field. Our studies lay the foundation for the future development of the technique and interpretation of ARPES measurements of field-tunable quantum phases. Copyright © 2023, The Authors. All rights reserved.

关键词： Photoelectrons

Absence of superconductivity in micrometer-sized ɛ-NbN single crystals

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Physical Review B 2022年第17期105卷 174512-174512页

作者： Fan-Yun Chiu Tsu-Lien Hung Cheng-Yen Liu Ya-Hsin Pai Wei-Liang Chien Peng-Jen Chen Ting-Kuo Lee Ming-Jye Wang Min-Nan Ou Chang C. Tsuei Maw-Kuen Wu Chia-Seng Chang Yang-Yuan Chen Department of Physics National Taiwan University Taipei 106319 Taiwan Republic of China Institute of Physics Academia Sinica Taipei 115201 Taiwan Republic of China Nano science and Technology program Taiwan International Graduate Program Taipei 115201 Taiwan Republic of China Department of Materials Science and Engineering National Taiwan University Taipei 106319 Taiwan Republic of China Department of Mechanical Engineering City University of Hong Kong Kowloon Hong Kong SAR China Hong Kong Institute for Advanced Study City University of Hong Kong Kowloon Hong Kong SAR China Department of Physics National Sun Yat-sen University Kaohsiung 804201 Taiwan Republic of China Institute of Astronomy and Astrophysics Academia Sinica Taipei 106319 Taiwan Republic of China Graduate Institute of Applied Physics National Chengchi University Taipei 116011 Taiwan Republic of China

It is important to study the properties of high quality single crystal in order to resolve the issue of an interesting material in which certain debatable fundamental properties exist. However, it is unfortunate that a sizable single crystal for experimental measurements is not always available. NbN is one of the examples; it has attracted scientific and engineering interest due to its diverse physical properties and a variety of structural phases. Until now superconductivity is only observed in cubic δ- and tetragonal γ-NbN but not in hexagonal ɛ-NbN. Recently, Zou et al. reported the observation of superconductivity with Tc∼11.6K in a hexagonal ɛ-NbN based on the measurement on a multiphase powder specimen. In order to resolve the issue, the work used the electron backscattering diffraction technique to characterize phases of micron-size NbN crystals from commercial powders and measure their transport properties. Our results unambiguously confirm that the hexagonal ɛ-NbN phase is not superconducting.

关键词： Electrical conductivity Specific heat Superconductors

Spin Seebeck effect at low temperatures in the nominally paramagnetic insulating state of vanadium dioxide

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arXiv 2022年

作者： Luo, Renjie Zhao, Xuanhan Chen, Liyang Legvold, Tanner J. Navarro, Henry Schuller, Ivan K. Natelson, Douglas Department of Physics and Astronomy Rice University HoustonTX77005 United States Applied Physics Graduate Program Rice University HoustonTX77005 United States Department of Physics Center for Advanced Nanoscience University of California-San Diego La Jolla CA92093 United States Department of Electrical and Computer Engineering Department of Materials Science and NanoEngineering Rice University HoustonTX77005 United States

The low temperature monoclinic, insulating phase of vanadium dioxide is ordinarily considered nonmagnetic, with dimerized vanadium atoms forming spin singlets, though paramagnetic response is seen at low temperatures. We find a nonlocal spin Seebeck signal in VO2 films that appears below 30 K and which increases with decreasing temperature. The spin Seebeck response has a non-hysteretic dependence on in-plane external magnetic field. This paramagnetic spin Seebeck response is discussed in terms of prior findings on paramagnetic spin Seebeck effects and expected magnetic excitations of the monoclinic ground state. Copyright © 2022, The Authors. All rights reserved.

关键词： Vanadium dioxide