We compare the spin-orbit interaction (SOI) in InAs nanowires grown in the conventional $\langle 0001\rangle$ crystal direction and the perpendicular $\langle 01\overline{1}0\rangle$ direction. It is theoretically shown that, for individual transverse modes, the intrinsic contribution due to the bulk inversion asymmetry of the crystal vanishes for wires in the $\langle 0001\rangle$ direction but remains finite for $\langle 01\overline{1}0\rangle$. Experimental spin-orbit scattering lengths extracted from low-temperature magnetoresistance measurements of individual nanowires yields, however, comparable values in the two cases, suggesting that the intrinsic intramode spin-orbit term is not the dominant source of the SOI. We discuss the implications for the manipulation of SOI in nanowire devices.

We have performed ${}^1\mathrm{H}$ NMR and ${}^{13}\mathrm{C}$ NMR measurements to investigate the coupling between molecular dynamics and the electronic state of ${\beta }^{\prime }$-(BEDT-TTF)${}_2{\mathrm{ICl}}_2$. From the ${}^1\mathrm{H}$ NMR measurements, we observed a frequency-dependent anomaly in the nuclear spin-lattice relaxation rate ${}^1\mathrm{T}_1^{-1}$ that originates from the slowing down of the ethylene motion. In the ${}^{13}\mathrm{C}$ NMR measurements, we found an anomaly in the linewidth of the NMR spectra at around 150 K, which is attributed to a nuclear spin-spin relaxation rate $\left({}^{13}\mathrm{T}_2\right)$ anomaly. The magnitudes of the anomalies in the linewidth and in ${}^{13}\mathrm{T}_2^{-1}$ are related to the hyperfine coupling constant. These results suggest that the ethylene motion modulates the molecular orbital of the BEDT-TTF molecules and gives rise to a difference in the orbital energy between the “staggered” and “eclipsed” conformations. We propose that significant coupling exists between the ethylene motion and the electronic state of the molecular dimer and that the ethylene dynamics can trigger the emergence of charge degrees of freedom inside the dimers and cause the dielectric anomaly in ${\beta }^{\prime }$-(BEDT-TTF)${}_2{\mathrm{ICl}}_2$.

We present results of our broadband ellipsometry measurements of the optical response of ferromagnetic ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{CoO}}_3$. Our data show that the ferromagnetic transition is accompanied by a transfer of optical spectral weight from an absorption band centered at 1.5 eV to a narrow component of the Drude-like peak. The associated reduction of the intraband kinetic energy is significantly larger than $k_{\mathrm{B}}{T}_{\mathrm{c}}$, confirming that the double exchange plays a major role in the ferromagnetism of doped cobaltites. In conjunction with results of recent theoretical studies, the temperature dependence of the Drude-like peak suggests that the double exchange is mediated by $t_{2g}$ orbitals.

The interaction between two magnetic spheres in microwave cavities is studied by Mie scattering theory beyond the magnetostatic and rotating wave approximations. We demonstrate that two spatially separated dielectric and magnetic spheres can be strongly coupled over a long distance by the electric field component of standing microwave cavity modes. The interactions split acoustical (dark) and optical (bright) modes in a way that can be mapped on a molecular orbital theory of the hydrogen molecule. Breaking the symmetry by assigning different radii to the two spheres introduces “ionic” character to the magnonic bonds. These results illustrate the coherent and controlled energy exchange between objects in microwave cavities.

We theoretically and experimentally investigate magnetization reversal and associated spin-wave dynamics of isolated threefold vertices that constitute a Kagome lattice. The three permalloy macrospins making up the vertex have an elliptical cross section and a uniform thickness. We study the dc magnetization curve and the frequency versus field curves (dispersions) of those spin-wave modes that produce the largest response. We also investigate each macrospin reversal from a dynamic perspective, by performing micromagnetic simulations of the reversal processes, and revealing their relationships to the soft-mode profile calculated at the equilibrium state immediately before reversal. The theoretical results are compared with the measured magnetization curves and ferromagnetic resonance spectra. The agreement achieved suggests that a much deeper understanding of magnetization reversal and accompanying hysteresis can be achieved by combining theoretical calculations with static and dynamic magnetization experiments.

Generating and detecting coherent high-frequency heat-carrying phonons have been topics of great interest in recent years. Although there have been successful attempts in generating and observing coherent phonons, rigorous techniques to characterize and detect phonon coherence in a crystalline material have been lagging compared to what has been achieved for photons. One main challenge is a lack of detailed understanding of how detection signals for phonons can be related to coherence. The quantum theory of photoelectric detection has greatly advanced the ability to characterize photon coherence in the past century, and a similar theory for phonon detection is necessary. Here, we reexamine the optical sideband fluorescence technique that has been used to detect high-frequency phonons in materials with optically active defects. We propose a quantum theory of phonon detection using the sideband technique and found that there are distinct differences in sideband counting statistics between thermal and coherent phonons. We further propose a second-order correlation function unique to sideband signals that allows for a rigorous distinction between thermal and coherent phonons. Our theory is relevant to a correlation measurement with nontrivial response functions at the quantum level and can potentially bridge the gap of experimentally determining phonon coherence to be on par with that of photons.

The microscopic magnetic nature of the $A$-site ordered chromium perovskites ${\mathrm{CaCu}}_3{\mathrm{Cr}}_4{\mathrm{O}}_{12}$ and ${\mathrm{LaCu}}_3{\mathrm{Cr}}_4{\mathrm{O}}_{12}$ and their solid-solution system, ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{Cu}}_3{\mathrm{Cr}}_4{\mathrm{O}}_{12}$, with $x=0.2$, 0.4, and 0.8, has been studied with muon spin rotation and relaxation (${\mu }^{+}\mathrm{SR}$) measurements down to 2 K using a powder sample. For ${\mathrm{CaCu}}_3{\mathrm{Cr}}_4{\mathrm{O}}_{12}$, ${\mu }^{+}\mathrm{SR}$ revealed the formation of static antiferromagnetic (AF) order below 122 K ($=T_{\mathrm{N}}$), although magnetization measurements showed a very small change at $T_{\mathrm{N}}$. Analyses of the internal magnetic field $H_{\mathrm{int}}$ at the muon sites, predicted with first-principles calculations, suggested $G$-type AF order as a ground state. For ${\mathrm{LaCu}}_3{\mathrm{Cr}}_4{\mathrm{O}}_{12}$ with $T_{\mathrm{N}}=225$ K, ${\mu }^{+}\mathrm{SR}$ also supported the presence of a $G$-type AF ordered state, which was recently proposed based on neutron diffraction measurements. However, the ordered Cr moments were found to change the direction at around 10 K. For ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{Cu}}_3{\mathrm{Cr}}_4{\mathrm{O}}_{12}$, both $T_{\mathrm{N}}$ and $H_{\mathrm{int}}$ at 2 K increase monotonically with $x$.

We have studied the magnetization depth profiles in a ${[}^{57}\mathrm{Fe}\left(d_{\mathrm{Fe}}\right)/\mathrm{Cr}\left(d_{\mathrm{Cr}}\right){]}_{30}$ multilayer with ultrathin Fe layers and nominal thickness of the chromium spacers $d_{\mathrm{Cr}}\approx 2.0\mathrm{nm}$ using nuclear resonance scattering of synchrotron radiation. The presence of a broad pure-magnetic half-order (1/2) Bragg reflection has been detected at zero external field. The joint fit of the reflectivity curves and Mössbauer spectra of reflectivity measured near the critical angle and at the “magnetic” peak reveals that the magnetic structure of the multilayer is formed by two spirals, one in the odd and another one in the even iron layers, with the opposite signs of rotation. The double-spiral structure starts from the surface with the almost-antiferromagnetic alignment of the adjacent Fe layers. The rotation of the two spirals leads to nearly ferromagnetic alignment of the two magnetic subsystems at some depth, where the sudden turn of the magnetic vectors by $\sim {180}^{\circ }$ (spin flop) appears, and both spirals start to rotate in opposite directions. The observation of this unusual double-spiral magnetic structure suggests that the unique properties of giant magnetoresistance devices can be further tailored using ultrathin magnetic layers.

We have studied the electronic structure of ferrimagnetic ${\mathrm{Mn}}_2\mathrm{VAl}$ single crystals by means of soft x-ray absorption spectroscopy (XAS), x-ray absorption magnetic circular dichroism (XMCD), and resonant soft x-ray inelastic scattering (RIXS). We have successfully observed the XMCD signals for all the constituent elements. The Mn $L_{2,3}$ XAS and XMCD spectra are reproduced by spectral simulations based on density-functional theory, indicating the itinerant character of the Mn $3d$ states. On the other hand, the V $3d$ electrons are rather localized since the ionic model can qualitatively explain the V $L_{2,3}$ XAS and XMCD spectra. This picture is consistent with local $dd$ excitations revealed by the V $L_3$ RIXS.

We report the investigation of the angular momentum transfer of light to photoelectrons and Auger electrons at the Ni $L_3$ absorption threshold. Upon core-level excitation by circularly polarized light, the angular momentum of light, or helicity, is transferred to the emitted photoelectron. The angular momentum of the emitted photoelectron ($m_{\mathrm{f}}$) is the sum of the helicity ($\sigma$) and the orbital magnetic quantum number of the initial state ($m_{\mathrm{i}}$). Here the quantization axis was defined as the direction along the incident light. This can be measured by the parallax shift of the forward focusing peak (FFP) direction in the photoelectron intensity angular distribution. At the absorption threshold, the excited core-level electron is promoted to a conduction-band state and the angular momentum of the light is partially transferred to Auger electrons. We measured photoelectron and Auger electron intensity angular distributions from the Ni(111) surface at the $L_3$ absorption threshold. We observed a significant angular circular dichroism of the $\left[101\right]$ FFP for the $L_3{M}_{4,5}{M}_{4,5}$ Auger electrons. Furthermore, we discovered non-negligible reversal angular circular dichroism contrasts for the triplet components in the case of the $L_3{M}_{2,3}{M}_{4,5}$ Auger electrons, suggesting that the angular momentum of light was transferred to the emitted electron in the antiparallel way.

Fracton phases are recent entrants to the roster of topological phases in three dimensions. They are characterized by subextensively divergent topological degeneracy and excitations that are constrained to move along lower-dimensional subspaces, including the eponymous fractons that are immobile in isolation. We develop correlation function diagnostics to characterize type-I fracton phases which build on their exhibiting partial deconfinement. These are inspired by similar diagnostics from standard gauge theories and utilize a generalized gauging procedure that links fracton phases to classical Ising models with subsystem symmetries. En route, we explicitly construct the space-time partition function for the plaquette Ising model which, under such gauging, maps into the $X$-cube fracton topological phase. We numerically verify our results for this model via Monte Carlo calculations.

We study the interplay of disorder and band-structure topology in a Weyl semimetal with a tilted conical spectrum around the Weyl points. The spectrum of particles is given by the eigenvalues of a non-Hermitian matrix, which contains contributions from a Weyl Hamiltonian and complex self-energy due to electron elastic scattering on disorder. We find that the tilt-induced matrix structure of the self-energy gives rise to either a flat band or a nodal line segment at the interface of the electron and hole pockets in the bulk band structure of type-II Weyl semimetals depending on the Weyl cone inclination. For the tilt in a single direction in momentum space, each Weyl point expands into a flat band lying on the plane, which is transverse to the direction of the tilt. The spectrum of the flat band is fully imaginary and is separated from the in-plane dispersive part of the spectrum by the “exceptional nodal ring” where the matrix of the Green's function in momentum-frequency space is defective. The tilt in two directions might shrink a flat band into a nodal line segment with “exceptional edge points.” We discuss the connection to the non-Hermitian topological theory.

We use a combination of Raman spectroscopy and transport measurements to study thin flakes of the type-II Weyl semimetal candidate $\mathrm{MoT}{\mathrm{e}}_2$ protected from oxidation. In contrast to bulk crystals, which undergo a phase transition from monoclinic to the inversion symmetry breaking, orthorhombic phase below $\sim 250\mathrm{K}$, we find that in moderately thin samples below $\sim 12\mathrm{nm}$, a single orthorhombic phase exists up to and beyond room temperature. This could be due to the effect of $c$-axis confinement, which lowers the energy of an out-of-plane hole band and stabilizes the orthorhombic structure. Our results suggest that Weyl nodes, predicated upon inversion symmetry breaking, may be observed in thin $\mathrm{MoT}{\mathrm{e}}_2$ at room temperature.

Many one-dimensional systems of experimental interest possess multiple bands arising from shallow confining potentials. In this paper, we study a gas of weakly interacting fermions and show that the bulk viscosity is dramatically altered by the occupation of more than one band. The reasons for this are twofold: a multichannel system is more easily displaced from equilibrium and the associated relaxation processes lead to more rapid equilibration than in the single channel case. We estimate the bulk viscosity in terms of the underlying microscopic interactions. The experimental relevance of this physics is discussed in the context of quantum wires and trapped cold atomic gases.

The robustness of quantum edge transport in InAs/GaSb quantum wells in the presence of magnetic fields raises an issue on the fate of topological phases of matter under time-reversal symmetry breaking. A peculiar band structure evolution in InAs/GaSb quantum wells is revealed: the electron subbands cross the heavy hole subbands but anticross the light hole subbands. The topologically protected band crossing point (Dirac point) of the helical edge states is pulled to be close to and even buried in the bulk valence bands when the system is in a deeply inverted regime, which is attributed to the existence of the light hole subbands. A sizable Zeeman energy gap verified by the effective $g$ factors of edge states opens at the Dirac point by an in-plane or perpendicular magnetic field; however, it can also be hidden in the bulk valance bands. This provides a plausible explanation for the recent observation on the robustness of quantum edge transport in InAs/GaSb quantum wells subjected to strong magnetic fields.

A method is introduced to stochastically generate crystal structures with defined structural characteristics. Reasonable quotient graphs for symmetric crystals are constructed using a random strategy combined with space group and graph theory. Our algorithm enables the search for large-size and complex crystal structures with a specified connectivity, such as threefold ${\mathrm{sp}}^2$ carbons, fourfold ${\mathrm{sp}}^3$ carbons, as well as mixed ${\mathrm{sp}}^2\text{−}{\mathrm{sp}}^3$ carbons. To demonstrate the method, we randomly construct initial structures adhering to space groups from 75 to 230 and a range of lattice constants, and we identify 281 new ${\mathrm{sp}}^3$ carbon crystals. First-principles optimization of these structures show that most of them are dynamically and mechanically stable and are energetically comparable to those previously proposed. Some of the new structures can be considered as candidates to explain the experimental cold compression of graphite.

We study the correlation between the magnetic reversal and the anisotropic magnetoresistance (AMR) response in magnetic hybrid structures that were created by local modification of magnetic properties induced by ion implantation. The stripe pattern has been investigated simultaneously by dual-wavelength Kerr microscopy and magnetoresistance measurements. We observe that the switching of the stripe pattern introduces an additional AMR maximum. The domain wall in between the stripes provides a positive resistance contribution, whereas domains at the stripe edges lead to an asymmetric AMR response. A method for calculating the AMR response from the quantitative Kerr micrographs is demonstrated that allows the reconstruction of the AMR value within a region of interest only.