Open Access Publications

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Open access publications by faculty, postdocs, and graduate students in the Department of Physics and Astronomy.


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Now showing 1 - 20 of 57
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    Observation of ultrafast ballistic orbital transport
    (Nature Nanotechnology, 2023-08-07) Jungfleisch, M. Benjamin
    Terahertz emission spectroscopy reveals long-distance ballistic orbital-angular-momentum transport in tungsten. While most electronic devices so far are based on the electron’s charge or its spin degree of freedom, electrons can also carry orbital angular momentum. Orbitronics (orbital electronics), which focuses on the electron’s orbital angular momentum1, is much less explored than the field of spintronics, especially at terahertz (THz) frequencies2,3. However, orbitronics promises higher-density information transfer over longer distances in many materials than would be possible with spin currents. Furthermore, utilizing the electron’s orbital angular momentum L offers distinct advantages: (1) orbital current is an emergent property from Bloch states in a solid, comprising many atoms and, hence, orbital angular momentum transfer can be arbitrarily large1, whereas the spin angular momentum S of one electron is limited to 1/2h. This may hinder efficient transport and control of information in spintronic devices. (2) The conversion of orbital angular momentum to charge currents does not rely on spin–orbit coupling, suggesting that many more materials could potentially be harnessed for interfacing angular-momentum-based devices with charge-based devices4. Despite these advantages, it has been experimentally challenging to unambiguously distinguish L and S transport and their conversion into charge currents. Furthermore, it has been unclear if L transport could be used similarly to S transport at ultrafast timescales, potentially leading to efficient THz devices5,6.
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    Probing Ion Configurations in the KcsA Selectivity Filter with Single-Isotope Labels and 2D IR Spectroscopy
    (Journal of the American Chemical Society, 2023-08-23) Ryan, Matthew J.; Gao, Lujia; Valiyaveetil, Francis I.; Zanni, Martin T.; Kananenka, Alexei A.
    The potassium ion (K+) configurations of the selectivity filter of the KcsA ion channel protein are investigated with two-dimensional infrared (2D IR) spectroscopy of amide I vibrations. Single 13C–18O isotope labels are used, for the first time, to selectively probe the S1/S2 or S2/S3 binding sites in the selectivity filter. These binding sites have the largest differences in ion occupancy in two competing K+ transport mechanisms: soft-knock and hard-knock. According to the former, water molecules alternate between K+ ions in the selectivity filter while the latter assumes that K+ ions occupy the adjacent sites. Molecular dynamics simulations and computational spectroscopy are employed to interpret experimental 2D IR spectra. We find that in the closed conductive state of the KcsA channel, K+ ions do not occupy adjacent binding sites. The experimental data is consistent with simulated 2D IR spectra of soft-knock ion configurations. In contrast, the simulated spectra for the hard-knock ion configurations do not reproduce the experimental results. 2D IR spectra of the hard-knock mechanism have lower frequencies, homogeneous 2D lineshapes, and multiple peaks. In contrast, ion configurations of the soft-knock model produce 2D IR spectra with a single peak at a higher frequency and inhomogeneous lineshape. We conclude that under equilibrium conditions, in the absence of transmembrane voltage, both water and K+ ions occupy the selectivity filter of the KcsA channel in the closed conductive state. The ion configuration is central to the mechanism of ion transport through potassium channels.
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    Electron energy dissipation in a magnetotail reconnection region
    (Physics of Plasmas, 2023-08-08) Burch, J. L.; Genestreti, K. J.; Heuer, S. V.; Chasapis, A.; Torbert, R. B.; Gershman, D. J.; Bandyopadhyay, R.; Pollock, C. J.; Matthaeus, W. H.; Nakamura, T. K. M.; Egedal, J.
    The four Magnetospheric Multiscale spacecraft encountered a reconnection region in the Earth's magnetospheric tail on 11 July 2017. Previous publications have reported characteristics of the electron diffusion region, including its aspect ratio, the reconnection electric field, plasma wave generation from electron beams in its vicinity, and energetic particles in the Earthward exhaust. This paper reports on the investigation of conversion of electromagnetic energy to electron kinetic energy (by J·E) and the ensuing conversion of electron beam energy to electron thermal energy via the pressure–strain interaction. The main result is that omnidirectional, compressive dissipation of electron energy dominates in the positive J·E region, while incompressive parallel dissipation dominates in the inflow region where J·E is small. The existence of parallel electric fields in the inflow region supports previous suggestions that electron trapping by these fields contributes to the parallel dissipation. All of the results are reproduced quantitatively within a factor of two with a 2.5-D particle-in-cell simulation.
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    The Trans-Heliospheric Survey
    (Astronomy & Astrophysics, 2023-07-24) Maruca, Bennett A.; Qudsi, Ramiz A.; Alterman, B. L.; Walsh, Brian M.; Korreck, Kelly E.; Verscharen, Daniel; Bandyopadhyay, Riddhi; Chhiber, Rohit; Chasapis, Alexandros; Parashar, Tulasi N.; Matthaeus, William H.; Goldstein, Melvyn L.
    Context. Though the solar wind is characterized by spatial and temporal variability across a wide range of scales, long-term averages of in situ measurements have revealed clear radial trends: changes in average values of basic plasma parameters (e.g., density, temperature, and speed) and a magnetic field with a distance from the Sun. Aims. To establish our current understanding of the solar wind's average expansion through the heliosphere, data from multiple spacecraft needed to be combined and standardized into a single dataset. Methods. In this study, data from twelve heliospheric and planetary spacecraft - Parker Solar Probe (PSP), Helios 1 and 2, Mariner 2 and 10, Ulysses, Cassini, Pioneer 10 and 11, New Horizons, and Voyager 1 and 2 - were compiled into a dataset spanning over three orders of magnitude in heliocentric distance. To avoid introducing artifacts into this composite dataset, special attention was given to the solar cycle, spacecraft heliocentric elevation, and instrument calibration. Results. The radial trend in each parameter was found to be generally well described by a power-law fit, though up to two break points were identified in each fit. Conclusions. These radial trends are publicly released here to benefit research groups in the validation of global heliospheric simulations and in the development of new deep-space missions such as Interstellar Probe.
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    Winds and magnetospheres from stars and planets: similarities and differences
    (Proceedings of the International Astronomical Union, 2023-08-16) Owocki, Stan
    Both stars and planets can lose mass through an expansive wind outflow, often constrained or channeled by magnetic fields that form a surrounding magnetosphere. The very strong winds of massive stars are understood to be driven by line-scattering of the star’s radiative momentum, while in the Sun and even lower-mass stars a much weaker mass loss arises from the thermal expansion of a mechanically heated corona. In exoplanets around such low-mass stars, the radiative heating and wind interaction can lead to thermal expansion or mechanical ablation of their atmospheres. Stellar magnetospheres result from the internal trapping of the wind outflow, while planetary magnetospheres are typically shaped by the external impact from the star’s wind. But in both cases the stressing can drive magnetic reconnection that results in observable signatures such as X-ray flares and radio outbursts. This review will aim to give an overview of the underlying physics of these processes with emphasis on their similarities and distinctions for stars vs. planets.
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    Getting started: How a supersonic stellar wind is initiated from a hydrostatic surface
    (Proceedings of the International Astronomical Union, 2022-11-30) Owocki, Stan
    Most of a star’s mass is bound in a hydrostatic equilibrium in which pressure balances gravity. But if at some near-surface layer additional outward forces overcome gravity, this can transition to a supersonic, outflowing wind, with the sonic point, where the outward force cancels gravity, marking the division between hydrostatic atmosphere and wind outflow. This talk will review general issues with such transonic initiation of a stellar wind outflow, and how this helps set the wind mass loss rate. The main discussion contrasts the flow initiation in four prominent classes of steady-state winds: (1) the pressure-driven coronal wind of the sun and other cool stars; (2) line-driven winds from OB stars; (3) a two-stage initiation model for the much denser winds from Wolf-Rayet (WR) stars; and (4) the slow “overflow” mass loss from highly evolved giant stars. A follow on discussion briefly reviews eruptive mass loss, with particular focus on the giant eruption of η Carinae.
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    Calculations of multipole transitions in Sn II for kilonova analysis
    (The European Physical Journal D, 2023-07-03) Bondarev, A. I.; Gillanders, J. H.; Cheung, C.; Safronova, M. S.; Fritzsche, S.
    We use the method that combines linearized coupled-cluster and configuration interaction approaches for calculating energy levels and multipole transition probabilities in singly ionized tin ions. We show that our calculated energies agree very well with the experimental data. We present probabilities of magnetic dipole and electric quadrupole transitions and use them for the analysis of the AT2017gfo kilonova emission spectra. This study demonstrates the importance and utility of accurate atomic data for forbidden transitions in the examination of future kilonova events.
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    Patellofemoral Joint Loading Progression Across 35 Weightbearing Rehabilitation Exercises and Activities of Daily Living
    (American Journal of Sports Medicine, 2023-06-05) Song, Ke; Silva, Rodrigo Scattone; Hullfish, Todd J.; Silbernagel, Karin Grävare; Baxter, Josh R.
    Background: Exercises that provide progressive therapeutic loading are a central component of patellofemoral pain rehabilitation, but quantitative evidence on patellofemoral joint loading is scarce for a majority of common weightbearing rehabilitation exercises. Purpose: To define a loading index to quantify, compare, rank, and categorize overall loading levels in the patellofemoral joint across 35 types of weightbearing rehabilitation exercises and activities of daily living. Study Design: Descriptive laboratory study. Methods: Model-estimated knee flexion angles and extension moments based on motion capture and ground-reaction force data were used to quantify patellofemoral joint loading in 20 healthy participants who performed each exercise. A loading index was computed via a weighted sum of loading peak and cumulative loading impulse for each exercise. The 35 rehabilitation exercises and daily living activities were then ranked and categorized into low, moderate, and high “loading tiers” according to the loading index. Results: Overall patellofemoral loading levels varied substantially across the exercises and activities, with loading peak ranging from 0.6 times body weight during walking to 8.2 times body weight during single-leg decline squat. Most rehabilitation exercises generated a moderate level of patellofemoral joint loading. Few weightbearing exercises provided low-level loading that resembled walking or high-level loading with both high magnitude and duration. Exercises with high knee flexion tended to generate higher patellofemoral joint loading compared with high-intensity exercises. Conclusion: This study quantified patellofemoral joint loading across a large collection of weightbearing exercises in the same cohort. Clinical Relevance: The visualized loading index ranks and modifiable worksheet may assist clinicians in planning patient-specific exercise programs for patellofemoral pain rehabilitation.
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    Assessment of Directionally Solidified Eutectic Sm–Fe(Co)–Ti Alloys as Permanent Magnet Materials
    (IEEE Transactions on Magnetics, 2023-05-29) Gabay, Alexander M.; Han, Chaoya; Ni, Chaoying; Hadjipanayis, George C.
    Sm–Fe–Ti and Sm–Fe 0.8 Co 0.2 –Ti alloys were prepared via arc-melting and directionally solidified on a water-cooled copper hearth. The as-solidified alloys featured cells of the Sm(Fe,Co,Ti) 12 –Ti(Fe,Co) 2+δ –(α-Fe) lamellar eutectic. The lamellae of Sm(Fe,Co,Ti) 12 phase with a crystal structure of the ThMn12 type were less than 0.2 μm thick, and had their [001] easy-magnetization directions oriented along the temperature gradient of the solidification. The eutectic microstructure led to an increased coercivity, especially in the Co-added alloys. Below 250 °C, this coercivity was found not to vary much with temperature with a temperature coefficient of -0.18 %/°C. However, the modest absolute values, reaching only 0.7 kOe, are insufficient for utilization of the directionally solidified alloys as anisotropic permanent magnets.
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    Quantum classical approach to spin and charge pumping and the ensuing radiation in terahertz spintronics: Example of the ultrafast light-driven Weyl antiferromagnet Mn3Sn
    (Physical Review B, 2023-05-15) Suresh, Abhin; Nikolić, Branislav K.
    The interaction of a femtosecond laser pulse with magnetic materials has been intensely studied for more than two decades in order to understand ultrafast demagnetization in single magnetic layers or terahertz emission from their bilayers with nonmagnetic spin-orbit (SO) materials. However, in contrast to well-understood spin and charge pumping by dynamical magnetization in spintronic systems driven by microwaves or current injection, analogous processes in light-driven magnets and radiation emitted by them remain largely unexplained due to the multiscale nature of the problem. Here we develop a multiscale quantum-classical formalism—where conduction electrons are described by quantum master equation (QME) of the Lindblad type, classical dynamics of local magnetization is described by the Landau-Lifshitz-Gilbert (LLG) equation, and incoming light is described by classical vector potential, while outgoing electromagnetic radiation is computed using the Jefimenko equations for retarded electric and magnetic fields—and apply it to a bilayer of antiferromagnetic Weyl semimetal Mn3Sn, hosting noncollinear local magnetization, and SO-coupled nonmagnetic material. Our QME+LLG+Jefimenko scheme makes it possible to understand how a femtosecond laser pulse directly generates spin and charge pumping and electromagnetic radiation by the Mn3Sn layer, including both odd and even high harmonics (of the pulse center frequency) up to order n≤7. The directly pumped spin current then exerts spin torque on local magnetization whose dynamics, in turn, pumps additional spin and charge currents radiating in the terahertz range. By switching on and off LLG dynamics and SO couplings, we unravel which microscopic mechanism contributes the most to emitted terahertz radiation—charge pumping by local magnetization of Mn3Sn in the presence of it own SO coupling is far more important than standardly assumed (for other types of magnetic layers) spin pumping and subsequent spin-to-charge conversion within the adjacent nonmagnetic SO-coupled material.
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    Combinational Vibration Modes in H2O/HDO/D2O Mixtures Detected Thanks to the Superior Sensitivity of Femtosecond Stimulated Raman Scattering
    (Journal of Physical Chemistry B, 2023-06-01) Pastorczak, Marcin; Duk, Katsiaryna; Shahab, Samaneh; Kananenka, Alexei A.
    Overtones and combinational modes frequently play essential roles in ultrafast vibrational energy relaxation in liquid water. However, these modes are very weak and often overlap with fundamental modes, particularly in isotopologues mixtures. We measured VV and HV Raman spectra of H2O and D2O mixtures with femtosecond stimulated Raman scattering (FSRS) and compared the results with calculated spectra. Specifically, we observed the mode at around 1850 cm–1 and assigned it to H–O–D bend + rocking libration. Second, we found that the H–O–D bend overtone band and the OD stretch + rocking libration combination band contribute to the band located between 2850 and 3050 cm–1. Furthermore, we assigned the broad band located between 4000 and 4200 cm–1 to be composed of combinational modes of high-frequency OH stretching modes with predominantly twisting and rocking librations. These results should help in a proper interpretation of Raman spectra of aqueous systems as well as in the identification of vibrational relaxation pathways in isotopically diluted water.
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    Spin currents with unusual spin orientations in noncollinear Weyl antiferromagnetic Mn3Sn
    (Physical Review Materials, 2023-03-10) Wang, Xinhao; Hossain, Mohammad Tomal; Thapaliya, T. R.; Khadka, Durga; Lendinez, Sergi; Chen, Hang; Doty, Matthew F.; Jungfleisch, M. Benjamin; Huang, S. X.; Fan, Xin; Xiao, John Q.
    There are intensive efforts to search for mechanisms that lead to spin-orbit torque with unusual spin orientation, particularly out-of-plane spin orientation which can efficiently switch perpendicular magnetizations. Such a phenomenon has been observed in materials with low structural symmetry, ferromagnetic materials, and antiferromagnets with noncollinear spin structures. Here, we demonstrate the observation of, in addition to out-of-plane spin orientation, spin orientation along the charge current direction in Mn3Sn, a noncollinear antiferromagnet and Weyl semimetal. The mechanism arises from noncollinear spin structure with spin-orbit coupling and it can be viewed as spin rotation around the octupole moment, the lowest order of cluster multipole moment pertaining to the Mn3Sn crystal group.
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    Three-Dimensional Magnetic Reconnection Spreading in Current Sheets of Non-Uniform Thickness
    (Journal of Geophysical Research: Space Physics, 2023-03-09) Arencibia, Milton; Cassak, Paul A.; Shay, Michael A.; Qiu, Jiong; Petrinec, Steven M.; Liang, Haoming
    Magnetic reconnection in naturally occurring and laboratory settings often begins locally and elongates, or spreads, in the direction perpendicular to the plane of reconnection. Previous work has largely focused on current sheets with a uniform thickness, for which the predicted spreading speed for anti-parallel reconnection is the local speed of the current carriers. We derive a scaling theory of three-dimensional (3D) spreading of collisionless anti-parallel reconnection in a current sheet with its thickness varying in the out-of-plane direction, both for spreading from a thinner to thicker region and a thicker to thinner region. We derive an expression for calculating the time it takes for spreading to occur for a current sheet with a given profile of its thickness. A key result is that when reconnection spreads from a thinner to a thicker region, the spreading speed in the thicker region is slower than both the Alfvén speed and the speed of the local current carriers by a factor of the ratio of thin to thick current sheet thicknesses. This is important because magnetospheric and solar observations have previously measured the spreading speed to be slower than previously predicted, so the present mechanism might explain this feature. We confirm the theory via a parametric study using 3D two-fluid numerical simulations. We use the prediction to calculate the time scale for reconnection spreading in Earth's magnetotail during geomagnetic activity. The results are also potentially important for understanding reconnection spreading in solar flares and the dayside magnetopause of Earth and other planets. Key Points: - We derive a theory of three-dimensional spreading of collisionless anti-parallel reconnection in current sheets with non-uniform thickness - Spreading from a thinner to a thicker current sheet occurs slower than local electron and Alfvén speeds, a key prediction of the theory - We apply the theory to reconnection spreading in Earth's magnetotail and discuss potential implications for solar flare ribbons Plain Language Summary: Magnetic reconnection is fundamental process in plasmas that converts magnetic energy into kinetic and thermal energy and is known to mediate eruptive solar flares and geomagnetic substorms that create the northern lights. The x-line where magnetic reconnection occurs can elongate or spread over time in the direction normal to the plane of reconnection, and this trait has been observed in the laboratory, Earth's magnetosphere, and is thought to be related to the elongation of chromospheric ribbons during solar flares. This study presents a scaling theory of the three-dimensional (3D) spreading of anti-parallel magnetic reconnection in current sheets with thickness varying in the out-of-plane direction. A key result is that when reconnection spreads from a thinner to a thicker region, the spreading speed in the thicker region is slower than expected. This is important because magnetospheric and solar observations have observed slower spreading speeds than previously predicted, so the present mechanism might explain this feature. We confirm the theory with 3D numerical simulations and use the prediction to calculate the time scale for reconnection spreading in Earth's magnetotail during geomagnetic activity.
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    Kinetic Scale Magnetic Reconnection with a Turbulent Forcing: Particle-in-cell Simulations
    (The Astrophysical Journal, 2023-01-31) Lu, San; Lu, Quanming; Wang, Rongsheng; Li, Xinmin; Gao, Xinliang; Huang, Kai; Sun, Haomin; Yang, Yan; Artemyev, Anton V.; An, Xin; Jia, Yingdong
    Turbulent magnetic reconnection has been observed by spacecraft to occur commonly in terrestrial magnetosphere and the solar wind, providing a new scenario of kinetic scale magnetic reconnection. Here by imposing a turbulent forcing on ions in particle-in-cell simulations, we simulate kinetic scale turbulent magnetic reconnection. We find formation of fluctuated electric and magnetic fields and filamentary currents in the diffusion region. Reconnection rate does not change much compared to that in laminar Hall reconnection. At the X-line, the electric and magnetic fields both exhibit a double power-law spectrum with a spectral break near local lower-hybrid frequency. The energy conversion rate is high in turbulent reconnection, leading to significant electron acceleration at the X-line. The accelerated electrons form a power-law spectrum in the high energy range, with a power-law index of about 3.7, much harder than one can obtain in laminar reconnection.
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    High-performance HZO/InAlN/GaN MISHEMTs for Ka-band application
    (Semiconductor Science and Technology, 2023-02-01) Cui, Peng; Moser, Neil; Chen, Hang; Xiao, John Q.; Chabak, Kelson D.; Zeng, Yuping
    This paper reports on the demonstration of microwave power performance at 30 GHz on InAlN/GaN metal–insulator–semiconductor high electron mobility transistor (MISHEMT) on silicon substrate by using the Hf0.5Zr0.5O2 (HZO) as a gate dielectric. Compared with Schottky gate HEMT, the MISHEMT with a gate length (LG) of 50 nm presents a significantly enhanced performance with an ON/OFF current ratio (ION/IOFF) of 9.3 × 107, a subthreshold swing of 130 mV dec−1, a low drain-induced barrier lowing of 45 mV V−1, and a breakdown voltage of 35 V. RF characterizations reveal a current gain cutoff frequency (fT) of 155 GHz and a maximum oscillation frequency (fmax) of 250 GHz, resulting in high (fT × fmax)1/2 of 197 GHz and the record high Johnson's figure-of-merit (JFOM = fT × BV) of 5.4 THz V among the reported GaN MISHEMTs on Si. The power performance at 30 GHz exhibits a maximum output power of 1.36 W mm−1, a maximum power gain of 12.3 dB, and a peak power-added efficiency of 21%, demonstrating the great potential of HZO/InAlN/GaN MISHEMTs for the Ka-band application.
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    Polarizability, Stark shifts, and field ionization of highly charged ions in ultraintense lasers
    (Physical Review A, 2023-03-06) Jones, Evan C.; Andreula, Zachery P.; Walker, Barry C.
    We have calculated the polarization and Stark-shifted binding energy for ultraintense lasers interacting with highly charged ions across the periodic table from beryllium to uranium at intensities up to 1022Wcm−2. The induced dipole and Stark shifts for the bound states can be as large as 0.1ea0 and 50Eh. Calculations of tunneling show the impacts of polarization and Stark shifts on the ionization rate are significant but counteracting. The work resolves a long-standing question of how field-free derivations of the tunneling response for highly charged ions have been quantitatively successful in relativistic, ultrahigh-intensity experiments. Using a scaling relationship, the results can be generalized to give the induced electric dipole for any species across an intensity range from 1015 to 1022Wcm−2.
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    Comparing spin injection in Fe75Co25/Bi2Te3 at GHz and optical excitations
    (Applied Physics Letters, 2023-02-13) Sharma, Vinay; Nepal, Rajeev; Wu, Weipeng; Pogue, E. A.; Kumar, Ravinder; Kolagani, Rajeswari; Gundlach, Lars; Jungfleisch, M. Benjamin; Budhani, Ramesh C.
    Spin-to-charge conversion (S2CC) processes in thin-film heterostructures have attracted much attention in recent years. Here, we describe the S2CC in a 3D topological insulator Bi2Te3 interfaced with an epitaxial film of Fe75Co25. The quantification of spin-to-charge conversion is made with two complementary techniques: ferromagnetic resonance based inverse spin Hall effect (ISHE) at GHz frequencies and femtosecond light-pulse induced emission of terahertz (THz) radiation. The role of spin rectification due to extrinsic effects like anisotropic magnetoresistance (AMR) and planar Hall effects (PHE) is pronounced at the GHz timescale, whereas the THz measurements do not show any detectible signal, which could be attributed to AMR or PHE. This result may be due to (i) homodyne rectification at GHz, which is absent in THz measurements and (ii) laser-induced thermal spin current generation and magnetic dipole radiation in THz measurements, which is completely absent in GHz range. The converted charge current has been analyzed using the spin diffusion model for the ISHE. We note that regardless of the differences in timescales, the spin diffusion length in the two cases is comparable. Our results aid in understanding the role of spin pumping timescales in the generation of ISHE signals.
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    Second-harmonic signature of chiral spin structures in W/Pt/Co heterostructures with tunable magnetic anisotropy
    (Journal of Physics D: Applied Physics, 2023-03-28) Wang, Yang; Chan, Ying-Ting; Wang, Xiao; Wang, Tao; Cheng, Xuemei M.; Wu, Weida; Xiao, John Q.
    Second-harmonic Hall voltage (SHV) measurement method has been widely used to characterize the strengths of spin–orbit torques (SOTs) in heavy metal/ferromagnet thin films saturated in the single-domain regime. Here, we show that the magnetic anisotropy of a W/Pt/Co trilayer can be robustly tuned from in-plane to out-of-plane by varying W, Pt, or Co thicknesses. Moreover, in samples with easy-cone anisotropy, SHV measurements exhibit anomalous 'humps' in the multidomain regime accessed by applying a nearly out-of-plane external magnetic field. These hump features can only be explained as a result of the formation of Néel-type domain walls, efficiently driven by nevertheless small SOTs in this double heavy metal heterostructure with canceling spin Hall angles.
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    Surface viscosities of lipid bilayers determined from equilibrium molecular dynamics simulations
    (Biophysical Journal, 2023-02-03) Fitzgerald, James E. III; Venable, Richard M.; Pastor, Richard W.; Lyman, Edward R.
    Lipid membrane viscosity is critical to biological function. Bacterial cells grown in different environments alter their lipid composition in order to maintain a specific viscosity, and membrane viscosity has been linked to the rate of cellular respiration. To understand the factors that determine the viscosity of a membrane, we ran equilibrium all-atom simulations of single component lipid bilayers and calculated their viscosities. The viscosity was calculated via a Green-Kubo relation, with the stress-tensor autocorrelation function modeled by a stretched exponential function. By simulating a series of lipids at different temperatures, we establish the dependence of viscosity on several aspects of lipid chemistry, including hydrocarbon chain length, unsaturation, and backbone structure. Sphingomyelin is found to have a remarkably high viscosity, roughly 20 times that of DPPC. Furthermore, we find that inclusion of the entire range of the dispersion interaction increases viscosity by up to 140%. The simulated viscosities are similar to experimental values obtained from the rotational dynamics of small chromophores and from the diffusion of integral membrane proteins but significantly lower than recent measurements based on the deformation of giant vesicles. SIGNIFICANCE: Viscosity is a critical property of cell membranes, actively regulated and known to control the rate of reactions that require the diffusion and encounter of proteins and small molecules. However, experimental measurements span more than an order of magnitude in the obtained viscosity depending on the technique and analysis. Extensive simulations of membrane viscosity are presented in order to make progress toward a unified understanding of membrane viscosity.
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    3D MHD models of the centrifugal magnetosphere from a massive star with an oblique dipole field Get access Arrow
    (Monthly Notices of the Royal Astronomical Society, 2023-02-01) ud-Doula, Asif; Owocki, Stanley P.; Russell, Christopher; Gagné, Marc; Daley-Yates, Simon
    We present results from new self-consistent 3D magnetohydrodynamics (MHD) simulations of the magnetospheres from massive stars with a dipole magnetic axis that has a non-zero obliquity angle (β) to the star’s rotation axis. As an initial direct application, we compare the global structure of co-rotating discs for nearly aligned (β = 5°) versus half-oblique (β = 45°) models, both with moderately rapid rotation (∼0.5 critical). We find that accumulation surfaces broadly resemble the forms predicted by the analytical rigidly rotating magnetosphere model, but the mass buildup to near the critical level for centrifugal breakout against magnetic confinement distorts the field from the imposed initial dipole. This leads to an associated warping of the accumulation surface towards the rotational equator, with the highest density concentrated in wings centred on the intersection between the magnetic and rotational equators. These MHD models can be used to synthesize rotational modulation of photometric absorption and H α emission for a direct comparison with observations.
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