Abstract
The Weyl semimetal (WSM), which hosts pairs of Weyl points and accompanying Berry curvature in momentum space near Fermi level, is expected to exhibit novel electromagnetic phenomena. ...Although the large optical/electronic responses such as nonlinear optical effects and intrinsic anomalous Hall effect (AHE) have recently been demonstrated indeed, the conclusive evidence for their topological origins has remained elusive. Here, we report the gigantic magneto-optical (MO) response arising from the topological electronic structure with intense Berry curvature in magnetic WSM Co
3
Sn
2
S
2
. The low-energy MO spectroscopy and the first-principles calculation reveal that the interband transitions on the nodal rings connected to the Weyl points show the resonance of the optical Hall conductivity and give rise to the giant intrinsic AHE in dc limit. The terahertz Faraday and infrared Kerr rotations are found to be remarkably enhanced by these resonances with topological electronic structures, demonstrating the novel low-energy optical response inherent to the magnetic WSM.
Earthquake stress drops are often estimated from far-field body wave spectra using measurements of seismic moment, corner frequency and a specific theoretical model of rupture behaviour. The most ...widely used model is from Madariaga in 1976, who performed finite-difference calculations for a singular crack radially expanding at a constant speed and showed that
, where
is spherically averaged corner frequency, β is the shear wave speed, a is the radius of the circular source and k = 0.32 and 0.21 for P and S waves, respectively, assuming the rupture speed V
r = 0.9β. Since stress in the Madariaga model is singular at the rupture front, the finite mesh size and smoothing procedures may have affected the resulting corner frequencies. Here, we investigate the behaviour of source spectra derived from dynamic models of a radially expanding rupture on a circular fault with a cohesive zone that prevents a stress singularity at the rupture front. We find that in the small-scale yielding limit where the cohesive-zone size becomes much smaller than the source dimension, P- and S-wave corner frequencies of far-field body wave spectra are systematically larger than those predicted by the Madariaga model. In particular, the model with rupture speed V
r = 0.9β shows that k = 0.38 for P waves and k = 0.26 for S waves, which are 19 and 24 per cent larger, respectively, than those of Madariaga. Thus for these ruptures, the application of the Madariaga model overestimates stress drops by a factor of 1.7. In addition, the large dependence of corner frequency on take-off angle relative to the source suggests that measurements from a small number of seismic stations are unlikely to produce unbiased estimates of spherically averaged corner frequency.
The longitudinal spin Seebeck effect has been investigated for a uniaxial antiferromagnetic insulator Cr(2)O(3), characterized by a spin-flop transition under magnetic field along the c axis. We have ...found that a temperature gradient applied normal to the Cr(2)O(3)/Pt interface induces inverse spin Hall voltage of spin-current origin in Pt, whose magnitude turns out to be always proportional to magnetization in Cr(2)O(3). The possible contribution of the anomalous Nernst effect is confirmed to be negligibly small. The above results establish that an antiferromagnetic spin wave can be an effective carrier of spin current.
Photoexcitation in solids brings about transitions of electrons/holes between different electronic bands. If the solid lacks an inversion symmetry, these electronic transitions support spontaneous ...photocurrent due to the geometric phase of the constituting electronic bands: the Berry connection. This photocurrent, termed shift current, is expected to emerge on the timescale of primary photoexcitation process. We observe ultrafast evolution of the shift current in a prototypical ferroelectric semiconductor antimony sulfur iodide (SbSI) by detecting emitted terahertz electromagnetic waves. By sweeping the excitation photon energy across the bandgap, ultrafast electron dynamics as a source of terahertz emission abruptly changes its nature, reflecting a contribution of Berry connection on interband optical transition. The shift excitation carries a net charge flow and is followed by a swing over of the electron cloud on a subpicosecond timescale. Understanding these substantive characters of the shift current with the help of first-principles calculation will pave the way for its application to ultrafast sensors and solar cells.
The motion of electrons in a solid has a profound effect on its topological properties and may result in a nonzero Berry's phase, a geometric quantum phase encoded in the system's electronic wave ...function. Despite its ubiquity, there are few experimental observations of Berry's phase of bulk states. Here, we report detection of a nontrivial π Berry's phase in the bulk Rashba semiconductor BiTel via analysis of the Shubnikov-de Haas (SdH) effect. The extremely large Rashba splitting in this material enables the separation of SdH oscillations, stemming from the spin-split inner and outer Fermi surfaces. For both Fermi surfaces, we observe a systematic π-phase shift in SdH oscillations, consistent with the theoretically predicted nontrivial π Berry's phase in Rashba systems.
We have characterized the microstructures of as-sintered and optimally post-sinter annealed Nd-rich Ga-doped Nd–Fe–B magnets by scanning electron microscopy (SEM) and aberration-corrected scanning ...transmission electron microscopy (STEM). While the Nd2Fe14B grains in the as-sintered sample with a coercivity of 0.99T are in direct contact with each other, those in the optimally annealed sample with a coercivity of 1.8T are completely enveloped by typically 10-nm-thick Nd-rich phase that contains little Fe. This strongly suggests that the Nd2Fe14B grains in the optimally annealed Nd-rich Ga-doped Nd–Fe–B magnets are exchange decoupled in contrast to those in the commercial sintered magnets.
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SUMMARY
Slip inversions of geodetic data from several large (magnitude ∼7) strike‐slip earthquakes point to coseismic slip deficit at shallow depths (<3–4 km), that is, coseismic slip appears to ...decrease towards the Earth surface. While the inferred slip distribution may be consistent with laboratory‐derived rate and state friction laws suggesting that the uppermost brittle crust may be velocity strengthening, there remains a question of how the coseismic slip deficit is accommodated throughout the earthquake cycle. The consequence of velocity‐strengthening fault friction at shallow depths is that the deficit of coseismic slip is relieved by post‐seismic afterslip and interseismic creep. However, many seismic events with inferred shallow slip deficit were not associated with either resolvable shallow interseismic creep or robust shallow afterslip. Hence, the origin of shallow ‘slip deficit’ remains uncertain. In this study, we investigate whether inelastic failure in the shallow crust due to dynamic earthquake rupture can explain the inferred deficit of shallow slip. Evidence for such failure is emerging from geologic, seismic and geodetic observations. We find that the amount of shallow slip deficit is proportional to the amount of inelastic deformation near the Earth surface. Such deformation occurs under a wide range of parameters that characterize rock strength in the upper crust. However, the largest magnitude of slip deficit in models accounting for off‐fault yielding is 2–4 times smaller than that inferred from kinematic inversions of geodetic data. To explain this discrepancy, we further explore to what extent assumptions in the kinematic inversions may bias the inferred slip distributions. Inelastic deformation in the shallow crust reduces coseismic strain near the fault, introducing an additional ‘artificial’ deficit of up to 10 per cent of the maximum slip in inversions of geodetic data that are based on purely elastic models. The largest magnitude of slip deficit in our models combined with the bias in inversions accounts for up to 25 per cent of shallow slip deficit, which is comparable, but still smaller than 30–60 per cent deficit inferred from kinematic inversions. We discuss potential mechanisms that may account for the remaining discrepancy between slip deficit predicted by elasto‐plastic rupture models and that inferred from inversions of space geodetic data.
We have investigated the effect of Ga on the microstructure and coercivity in Nd-rich Ga-doped Nd-Fe-B sintered magnets with different amount of Ga additions using focused ion beam scanning electron ...microscope (FIB/SEM), aberration corrected scanning transmission electron microscope (STEM), three dimensional atom probe (3DAP) and synchrotron X-ray diffraction. While a ferromagnetic Fe-rich amorphous phase is a dominant grain boundary phase in the Ga-free magnet, the trace addition of Ga resulted in the formation of the Nd6Fe13Ga antiferromagnetic phase at grain boundaries as well as in triple junctions after post-sinter annealing above 480 °C. In addition, non-ferromagnetic Nd-rich phases with the Ia3¯ structure and the amorphous structure were formed along the grain boundaries. The structures and chemical compositions of these three types of grain boundary phases were identified. The high coercivity is closely related with the formation of the three types of non-ferromagnetic grain boundary phase rather than the amount of the Nd6Fe13Ga triple junction phase; hence the coercivity enhancement after the optimal heat-treatment is attributed to the magnetic isolation of the Nd2Fe14B grains through the formation of the non-ferromagnetic grain boundary phases. The underlying mechanism for the formation of these grain boundary phases is discussed based on the experimental results.
•Through microstructure analysis to clarify the role of Ga in Nd-Fe-B magnet.•Formation of Nd6Fe13Ga phase instead of the Fe-rich ferromagnetic amorphous phase.•Formation of non-ferromagnetic Nd-rich phase along grain boundaries.•Magnetic isolation of the Nd2Fe14B grains leading to the high coercivity.
The magnetic skyrmion is a topologically stable spin texture in which the constituent spins point to all the directions wrapping a sphere. Generation and control of nanometric magnetic skyrmions have ...large potential, for example, reduced power consumption, in spintronics device applications. Here we show the real-space observation of a biskyrmion, as defined by a molecular form of two bound skyrmions with the total topological charge of 2, realized under magnetic field applied normal to a thin plate of a bilayered manganite with centrosymmetric structure. In terms of a Lorentz transmission electron microscopy (TEM), we have observed a distorted-triangle lattice of biskyrmion crystal, each composed of two bound skyrmions with oppositely swirling spins (magnetic helicities). Furthermore, we demonstrate that these biskyrmions can be electrically driven with orders of magnitude lower current density (<10(8) A m(-2)) than that for the conventional ferromagnetic domain walls.
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