Nonmagnetic semiconductor spin devices
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semiconductor spin devices magnetic fields structu↗
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Nonmagnetic heterostructure spin devices
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inversion asymmetry↗
High‐Asymmetry Metasurface: A New Solution for Terahertz Resonance via Active Learning‐Augmented Diffusion Model
Terahertz (THz) metamaterials with high‐figure‐of‐merit (high‐FoM) performance resonance are essential for advancing sensors, detectors, and imagers. Conventional designs focus on symmetric or low‐asymmetry geometric structures, leaving high‐asymmetry designs largely unexplored due to the inefficiency of trial‐and‐error‐based rational design. Recent deep learning techniques offer automation and acceleration but are constrained by the need for large datasets inherent to their data‐driven nature. Here, a novel prior knowledge‐guided generative model augmented by a physics‐constrained active learning mechanism to design high‐asymmetry metamaterials. An advanced diffusion model learns features from a small set of classical structures with high‐FoM THz resonance and generates new high‐asymmetry structures. To mitigate the limited number of classical structures, the generated high‐asymmetry structures are actively selected and integrated into the initial training dataset based on their physical characteristics. Experimental results demonstrate the superior resonance performance of the generated high‐asymmetry metamaterials over classical designs, exhibiting improvements exceeding 30% in key resonance metrics. Remarkably, this performance is attained using only 68 classical structures as the initial training dataset, significantly reducing the data requirements for deep learning‐based metamaterial design. The proposed scheme for generating high‐asymmetry structures provides a new effective and efficient solution for high‐FoM resonance, expanding applications in high‐sensitivity THz metadevices.
diffusion model↗
The influence of NO and ClO variations at twilight on the interpretation of solar occultation measurements
Measurement of short-lived photochemically-produced species in the stratosphere by solar occultation is difficult because the rapid variation of such species near the terminator introduces ambiguities in interpreting the measured absorption in terms of meaningful atmospheric abundances. These variations produce tangent path concentrations that are asymmetric relative to the tangent point, as opposed to the symmetrical distribution usually assumed in most inversion algorithms. Neglect of this asymmetry may yield an inverted profile that deviates significantly from the true sunset/sunrise profile. In the present paper, the influence of this effect on solar occultation measurements of ClO and NO is examined. The results show that average inhomogeneity factors, which measure the concentration variation along the tangent path and which can be calculated from a photochemical model, can indicate which species require more careful data analysis.
Boughner, R.↗
First Evidence of Axial Shape Asymmetry and Configuration Coexistence in Zn 74 : Suggestion for a Northern Extension of the N = 40 Island of Inversion
The excited states of N=44 74 Zn were investigated via γ-ray spectroscopy following 74 Cu β decay. By exploiting γ-γ angular correlation analysis, the 2$_2^+$, 3$_1^+$, 0$_2^+$, and 2$_3^+$ states in 74 Zn were firmly established. The γ-ray branching and E2/M1 mixing ratios for transitions deexciting the 2$_2^+$, 3$_1^+$, and 2$_3^+$ states were measured, allowing for the extraction of relative B(E2) values. In particular, the 2$_3^+$→0$_2^+$ and 2$_3^+$→4$_1^+$ transitions were observed for the first time. Here the results show excellent agreement with new microscopic large-scale shell-model calculations, and are discussed in terms of underlying shapes, as well as the role of neutron excitations across the N=40 gap. Enhanced axial shape asymmetry (triaxiality) is suggested to characterize 74 Zn in its ground state. Furthermore, an excited K=0 band with a significantly larger softness in its shape is identified. A shore of the N=40 "island of inversion" appears to manifest above Z=26, previously thought as its northern limit in the chart of the nuclides.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Characterizing the effects of drive asymmetries, component offsets, and joint gaps in double shell capsule implosions
This work provides a numerical study of how double shell capsule deformations caused by drive asymmetries and fabrication imperfections affect implosion symmetry and neutron yield. Hydrodynamics simulations are performed in two dimensions and focus on low-mode deformations that are caused by corresponding asymmetries in the Hohlraum drive, component offsets, and ablator joint gaps. By providing a parameter study of these features, our goal is to understand the dominant sources for inner shell deformation and yield degradation. The discussed capsules are composed of an aluminum ablator with a chromium inner shell. The latter encloses a carbon-deuterium foam ball that serves as fuel. We find that for clean capsules, even-numbered low-mode asymmetries in the drive are imprinted on the ablator and smoothly transferred to the inner shell during shell collision. The resulting deformation of the inner shell is more pronounced with larger fuel radius, while the yield is inversely proportional to the amplitude of the drive asymmetry and varies by factors ≤4 in comparison with clean simulations. Capsule component offsets in the vertical direction and ablator thickness nonuniformity result in p1-type deformations of the imploding inner shell. Finally, joint gaps have the largest effect in deforming the ablator and inner shell and degrading yield. While small gap widths (1 μm) result in prolate inner shells, larger gap widths (4 μm) cause an oblate deformation. More importantly, capsules with a small outer gap (1 μm) experience a dramatic drop in yield, typically <3% of a clean simulation.
Goodarzi, S. (ORCID:000000018556489X)↗
X-ray inverse Compton emission from the radio halo of M87
M87 has been observed in the 0.2-4 KeV X-ray band using the High Resolution Imager on the Einstein Observatory, and at 1.452 GHz using the Very Large Array. The radio map showed that the halo contained prominent asymmetries to the east and southwest. The X-ray map indicated similar asymmetries, but they were imbedded in the diffuse hot gas that surrounds the core out to a radius of several arcminutes. The hot X-ray emitting gas was assumed to be spherically symmetric and could, therefore, be subtracted from the image. The resultant image was asymmetric with major lobes to the east and southwest that coincide approximately with the asymmetries in the radio halo. The data indicates that inverse Compton emission is a plausible model for the X-rays coming from the asymmetric component.
Wood, P. A. D.↗
Inverse heat transfer analysis of Bridgman crystal growth
The effects of asymmetry in furnace temperature profile and pulling velocity on the crystal interface shape are demonstrated while neglecting the latent heat of solidification. It is seen that the furnace temperature profile may be varied in order to influence the shape of the melt-crystal interface. An exact thermal analysis is then performed on the Bridgman technique by including the latent heat of solidification as a source term. The exact temperature field required for yielding a flat melt-crystal interface is obtained. The earlier observation regarding the influence of furnace temperature profile on the interface shape is confirmed and a criterion for achieving a flat interface is obtained. Various furnace temperature profiles are selected and their corresponding melt-crystal interface results are presented.
Taghavi, K.↗
Observation and interpretation of the asymmetric Stokes Q, U, and V line profiles in sunspots
We present a first approach to systematic characterization and interpretation of observed asymmetries of Stokes profiles as a function of spatial position within sunspots. Spatial maps of the full Stokes profiles of Fe I 6302.5 A gathered in large sunspots using the HAO Stokes II instrument reveal asymmetries that vary systematically across large sunspots observed during 1980. We use the inversion technique by Landolfi to extract the velocity gradients along the line of sight (LOS) which give rise to these asymmetries. The gradients derived from full Stokes profiles are in agreement with previous characterizations of the Evershed flow derived from Stokes I profiles alone (i.e., a flow increasing with depth in the atmosphere). By coupling this semiempirical gradient of velocity with a magnetic field inclination varying along the LOS, the synthesized profiles are able to mimic basic observed features of the broad-band circular polarization present in our data and observed previously by others. This characterization has magnetic field lines which become progressively more horizontal with depth in the penumbra.
Sanchez Almeida, J.↗
Electric fields and substrates dramatically accelerate spin relaxation in graphene
Electrons in graphene are theoretically expected to retain spin states much longer than most materials, making graphene a promising platform for spintronics and quantum information technologies. Here, in this work, we use first-principles density-matrix (FPDM) dynamics simulations to show that interaction with electric fields and substrates strongly enhances spin relaxation through scattering with phonons. Consequently, the relaxation time at room temperature reduces from microseconds in free-standing graphene to nanoseconds in graphene on the hexagonal boron nitride (hBN) substrate, which is the order of magnitude typically measured in experiments. Further, inversion symmetry breaking by hBN introduces a stronger asymmetry in electron and hole spin lifetimes than predicted by the conventional D'yakonov-Perel' (DP) model for spin relaxation. Deviations from the conventional DP model are stronger for in-plane spin relaxation, resulting in out-of-plane to in-plane lifetime ratios much greater than 1/2 with a maximum close to the Dirac point. These FPDM results, independent of symmetry-specific assumptions or material-dependent parameters, also validate recent modifications of the DP model to explain such deviations. Overall, our results indicate that spin-phonon relaxation in the presence of substrates may be more important in graphene than typically assumed, requiring consideration for graphene-based spin technologies at room temperature.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Satellite observations of polar mesospheric clouds by the solar backscattered ultraviolet spectral radiometer - Evidence of a solar cycle dependence
Results are presented on eight years of satellite observations of the polar mesospheric clouds (PMCs) by the SBUV spectral radiometer, showing that PMCs occur in the summertime polar cap regions of both hemispheres and that they exhibit year-to-year variability. It was also found that the increase in the PMC occurrence frequency was inversely correlated with solar activity. Two kinds of hemispherical asymmetries could be identified: (1) PMCs in the Northern Hemisphere were significantly brighter than in the Southern Hemisphere, in accordance with previous results derived from SME data; and (2) the solar cycle response in the south is more pronounced than in the north. The paper also describes the cloud detection algorithm.
Thomas, Gary E.↗
Suppression of one-dimensional weak localization by band asymmetry
Herein we investigate disorder-induced localization in metals that break time-reversal and inversion symmetries through their energy dispersion, ε k ≠ε– k , but lack Berry phases. In the perturbative regime of disorder, we show that weak localization is suppressed due to a mismatch of the Fermi velocities of left and right movers. To substantiate this analytical result, we perform quench numerics on chains shorter than the Anderson localization length ξ—the latter computed and verified to be finite using the recursive Green's function method—and find a sharp rise in the saturation value of the participation ratio due to band asymmetry, indicating a tendency to delocalize. Interestingly, for weak disorder strength η, we see a better fit to the scaling behavior ξ∝1/η 2 for asymmetric bands than conventional symmetric ones.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
An Efficient and Accurate Algorithm for Computing Grid-Averaged Solar Fluxes for Horizontally Inhomogeneous Clouds
A computationally efficient method is presented to account for the horizontal cloud inhomogeneity by using a radiatively equivalent plane parallel homogeneous (PPH) cloud. The algorithm can accurately match the calculations of the reference (rPPH) independent column approximation (ICA) results, but use only the same computational time required for a single plane parallel computation. The effective optical depth of this synthetic sPPH cloud is derived by exactly matching the direct transmission to that of the inhomogeneous ICA cloud. The ffective9 scattering asymmetry factor is found from a pre-calculated albedo inverse look-up-table that is allowed to vary over the range from -1.0 to 1.0. In the special cases of conservative scattering and total absorption, the synthetic method is exactly equivalent to the ICA, with only a small bias (about 0.2% in flux) relative to ICA due to imperfect interpolation in using the look-up tables. In principle, the ICA albedo can be approximated accurately regardless of cloud inhomogeneity. For a more complete comparison, the broadband shortwave albedo and transmission calculated from the synthetic sPPH cloud and averaged over all incident directions, have the RMS biases of 0.26% and 0.76%, respectively, for inhomogeneous clouds over a wide variation of particle size. The advantages of the synthetic PPH method are that (1) it is not required that all the cloud subcolumns have uniform microphysical characteristic, (2) it is applicable to any 1D radiative transfer scheme, and (3) it can handle arbitrary cloud optical depth distributions and an arbitrary number of cloud subcolumns with uniform computational efficiency.
cloud inhomogeneity↗
Effects of solar magnetic field on cosmic rays
Aspects of the problem of galactic cosmic ray propagation, including inversion of the solar total magnetic field and an analysis of data related to the heliomagnetic cycle are discussed. It is noted that the global structure of the solar magnetic field results in an additional flux of galactic cosmic rays generated by curvature and gradient drifts. An analysis of heliomagnetic cycle data shows that the latitudinal gradient results in a N-S asymmetry, with the amplitude of the effect growing with depth in the atmosphere. The inversion of the solar total magnetic field, drift effects, and other space distributions are found to contribute to a 22-year cycle of solar activity.
Goncher, G. A.↗
Correlation-driven electron-hole asymmetry in graphene field effect devices
Electron-hole asymmetry is a fundamental property in solids that can determine the nature of quantum phase transitions and the regime of operation for devices. The observation of electron-hole asymmetry in graphene and recently in twisted graphene and moiré heterostructures has spurred interest into whether it stems from single-particle effects or from correlations, which are core to the emergence of intriguing phases in moiré systems. Here, we report an effective way to access electron-hole asymmetry in 2D materials by directly measuring the quasiparticle self-energy in graphene/Boron Nitride field-effect devices. As the chemical potential moves from the hole to the electron-doped side, we see an increased strength of electronic correlations manifested by an increase in the band velocity and inverse quasiparticle lifetime. These results suggest that electronic correlations intrinsically drive the electron-hole asymmetry in graphene and by leveraging this asymmetry can provide alternative avenues to generate exotic phases in twisted moiré heterostructures.
36 MATERIALS SCIENCE↗
Proximity-induced equilibrium supercurrent and perfect superconducting diode effect due to band asymmetry
Herein we theoretically investigate the consequences of proximity-induced conventional superconductivity in metals that break time-reversal and inversion symmetries through their energy dispersion. We discover behaviors impossible in an isolated superconductor such as an equilibrium supercurrent that apparently violates a no-go theorem and, at suitable topological defects, nonconservation of electric charge reminiscent of the chiral anomaly. The equilibrium supercurrent is expected to be trainable by a helical electromagnetic field in the normal state. Remarkably, if the band asymmetry exceeds the critical current of the parent superconductor in appropriate units, we predict a perfect superconducting diode effect with diode coefficient unity. We propose toroidal metals such as UNi 4 B and metals with directional scalar spin chiral order as potential platforms.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Apparent temperatures of neutron-poor and neutron-rich compound nuclei
The possibility of the dependence of the nuclear caloric curve on neutron excess sets a limit on the accuracy of our knowledge of the nuclear equation of state, and thus impacts predictive capabilities of nuclear reaction and nuclear astrophysics models. To date, theoretical models have not reached consensus on the magnitude or sign on the asymmetry dependence. To provide constraints, we have measured evaporated particles and heavy residues for complete and incomplete fusion-evaporation reactions in inverse kinematics. The temperatures extracted from the observed light charged particles tend to favor higher temperatures for the neutron-rich fused systems, though they are near the limits of the systematic uncertainty. The present measurement may be used as an upper limit to constrain the asymmetry dependence of the nuclear caloric curve.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Radiative Properties of Cirrus Clouds in the Infrared (8-13 microns) Spectral Region
Atmospheric radiation in the infrared (IR) 8-13 microns spectral region contains a wealth of information that is very useful for the retrieval of ice cloud properties from aircraft or space-borne measurements. To provide the scattering and absorption properties of nonspherical ice crystals that are fundamental to the IR retrieval implementation, we use the finite-difference time domain (FDTD) method to solve for the extinction efficiency, single-scattering albedo, and the asymmetry parameter of the phase function for ice crystals smaller than 40 microns. For particles larger than this size, the improved geometric optics method (IGOM) can be employed to calculate the asymmetry parameter with an acceptable accuracy, provided that we properly account for the inhomogeneity of the refracted wave due to strong absorption inside the ice particle. A combination of the results computed from the two methods provides the asymmetry parameter for the entire practical range of particle sizes between 1 micron and 10000 microns over wavelengths ranging from 8 microns to 13 microns. For the extinction and absorption efficiency calculations, several methods including the IGOM, Mie solution for equivalent spheres (MSFES), and the anomalous diffraction theory (ADT) can lead to a substantial discontinuity in comparison with the FDTD solutions for particle sizes on the order of 40 microns. To overcome this difficulty, we have developed a novel approach called the stretched scattering potential method (SSPM). For the IR 8-13 microns spectral region, we show that SSPM is a more accurate approximation than ADT, MSFES, and IGOM. The SSPM solution can be further refined numerically. Through a combination of the FDTD and SSPM, we have computed the extinction and absorption efficiency for hexagonal ice crystals with sizes ranging from 1 to 10000 microns at 12 wavelengths between 8 and 13 microns Calculations of the cirrus bulk scattering and absorption properties are performed for 30 size distributions obtained from various field campaigns for midlatitude and tropical cirrus cloud systems. Parameterization of these bulk scattering properties is carried out by using second-order polynomial functions for the extinction efficiency and the single-scattering albedo and the power law expression for the asymmetry parameter. We note that the volume-normalized extinction coefficient can be separated into two parts: one is inversely proportional to effective size and is independent of wavelength, and the other is the wavelength-dependent effective extinction efficiency. Unlike conventional parameterization efforts, the present parameterization scheme is more accurate because only the latter part of the volume-normalized extinction coefficient is approximated in terms of an analytical expression. After averaging over size distribution, the single-scattering albedo is shown to decrease with an increase in effective size for wavelengths shorter than 10.0 microns whereas the opposite behavior is observed for longer wavelengths. The variation of the asymmetry parameter as a function of effective size is substantial when the effective size is smaller than 50 microns. For effective sizes larger than 100 microns, the asymmetry parameter approaches its asymptotic value. The results derived in this study can be useful to remote sensing applications involving IR window bands under cirrus cloud conditions.
Yang, Ping↗