We perform de Haas–van Alphen measurements and quasiparticle self-consistent GW (QSGW) calculations on FeS. The calculated Fermi surface (FS) consists of two hole and two electron cylinders. We observe all the eight predicted FS cross sections experimentally. With momentum-independent band-energy adjustments of less than 0.1 eV, the maximum deviation between the calculated and observed cross sections is less than 0.2% of the Brillouin zone area for $B\parallel c$. The carrier density is $\sim 0.5$ carriers/Fe. The mass enhancements are nearly uniform across the FS cylinders and moderate, $\sim 2$. The absence of a third hole cylinder with $d_{xy}$ character is favorable for the formation of a nodal superconducting gap.

Condensed matter systems realizing Weyl fermions exhibit striking phenomenology derived from their topologically protected surface states as well as chiral anomalies induced by electromagnetic fields. More recently, inhomogeneous strain or magnetization were predicted to result in chiral electric ${\mathbf{E}}_5$ and magnetic ${\mathbf{B}}_5$ fields, which modify and enrich the chiral anomaly with additional terms. In this Rapid Communication, we develop a lattice-based approach to describe the chiral anomaly, which involves Landau and pseudo-Landau levels and treats all anomalous terms on equal footing, while naturally incorporating Fermi arcs. We exemplify its potential by physically interpreting the largely overlooked role of Fermi arcs in the covariant (Fermi level) contribution to the anomaly and revisiting the factor of $1/3$ difference between the covariant and consistent (complete band) contributions to the ${\mathbf{E}}_5·{\mathbf{B}}_5$ term in the anomaly. Our framework provides a versatile tool for the analysis of anomalies in realistic lattice models as well as a source of simple physical intuition for understanding strained and magnetized inhomogeneous Weyl semimetals.

We investigate the dynamics of spin in the axially anisotropic Landau-Lifshitz field theory with a magnetic domain-wall initial condition. Employing the analytic scattering technique, we obtain the exact scattering data and reconstruct the time-evolved profile. We identify three qualitatively distinct regimes of spin transport, ranging from ballistic expansion in the easy-plane regime, absence of transport in the easy-axis regime, and logarithmically enhanced diffusion for the isotropic interaction. Our results are in perfect qualitative agreement with those found in the anisotropic quantum Heisenberg spin-$1/2$ chain, indicating a remarkable classical-quantum correspondence for macroscopic spin transport.

In magnetic wires with perpendicular anisotropy, moving a domain wall with only current or only circularly polarized light requires a high power. Here, we propose to reduce it by using both short current pulses and femtosecond laser pulses simultaneously. The wires were made out of perpendicularly magnetized film of Pt/Co/Ni/Co/Pt. The displacement of the domain wall is found to be dependent on the laser helicity. Based on a quantitative analysis of the current-induced domain-wall motion, the spin-orbit torque contribution can be neglected when compared to the spin transfer torque contribution. The effective field of the spin transfer torque is extracted from the pulsed field domain-wall measurements. Finally, our result can be described using the Fatuzzo-Labrune model and considering the effective field due to the polarized laser beam, the effective field due to spin transfer torque, and the Gaussian temperature distribution of the laser spot.

We report the transport and magnetic measurements on single crystals of ${\mathrm{CsCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$, a recently discovered double-FeAs-layer superconductor with $T_{\mathrm{c}}\sim 30$ K. We observed a large resistivity anisotropy of ${\rho }_c\left(T\right)/{\rho }_{ab}\left(T\right)\sim {10}^3$ that tends to increase with decreasing temperature. The ${\rho }_c\left(T\right)$ data exhibit a nonmetallic behavior at high temperatures, suggesting an incoherent electronic state due to the dimension crossover. The superconducting onset transition temperature in ${\rho }_{ab}\left(T\right)$ is 0.7 K higher than that in ${\rho }_c\left(T\right)$, reflecting two-dimensional (2D) superconducting fluctuations. The lower and upper critical fields also show an exceptionally high anisotropy among iron-based superconductors. The $H_{c1}^{\perp }\left(T\right)$ data are well fitted with two $s$-wave-like superconducting gaps, ${\mathrm{\Delta }}_1\left(0\right)=6.75$ meV and ${\mathrm{\Delta }}_2\left(0\right)=2.32$ meV. The interplane coherence length ${\xi }_c\left(0\right)$ is estimated to be 3.6 Å, which is remarkably smaller than the distance between conducting layers (8.6 Å), consolidating the 2D nature in the title material.

We apply the projective truncation technique to the tensor renormalization group (TRG) algorithm in order to reduce the computational cost from $O\left({\chi }^6\right)$ to $O\left({\chi }^5\right)$, where $\chi$ is the bond dimension, and propose three kinds of algorithms for demonstration. On the other hand, the technique causes a systematic error due to the incompleteness of a projector composed of isometries and in addition requires iteration steps to determine the isometries. Nevertheless, we find that the accuracy of the free energy for the Ising model on a square lattice is recovered to the level of TRG with a few iteration steps even at the critical temperature for $\chi =32$, 48, and 64.

We introduce an effective edge network theory to characterize the boundary topology of coupled edge states generated from various types of topological insulators. Two examples studied are a two-dimensional second-order topological insulator and three-dimensional topological fullerenes, which involve multileg junctions. As a consequence of bulk-edge correspondence, these edge networks can faithfully predict properties such as the energy and fractional charge related to the bound states (edge solitons) in the aforementioned systems, including several aspects that were previously complicated or obscure.

The transition-metal dichalcogenide $1T-{\mathrm{TiSe}}_2$ is a quasi-two-dimensional layered material with a phase transition towards a commensurate charge-density wave (CDW) at a critical temperature $T_c\approx 200\mathrm{K}$. The relationship between the origin of the CDW instability and the semimetallic or semiconducting character of the normal state, i.e., with the nonreconstructed Fermi-surface topology, remains elusive. By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations, we investigate $1T-{\mathrm{TiSe}}_{2-x}{\mathrm{S}}_x$ single crystals. Using STM, we first show that the long-range phase-coherent CDW state is stable against S substitutions with concentrations, at least, up to $x=0.34$. The ARPES measurements then reveal a slow but continuous decrease in the overlap between the electron and the hole ($e-h$) bands of the semimetallic normal state well reproduced by DFT and related to slight reductions of both the CDW order parameter and $T_c$. Our DFT calculations further predict a semimetal-to-semiconductor transition of the normal state at a higher critical S concentration of $x_c=0.9\pm 0.1$ that coincides with a suppressed CDW state in TiSeS as measured with STM. Finally, we rationalize the $x$ dependence of the $e-h$ band overlap in terms of isovalent substitution-induced competing chemical pressure and charge localization effects. Our study highlights the key role of the $e-h$ band overlap for the CDW instability.

We report high-frequency/high-field electron spin resonance (ESR) and high-field magnetization studies on single crystals of the bond-disordered pyrochlore ${\mathrm{NaCaCo}}_2{\mathrm{F}}_7$. Frequency- and temperature-dependent ESR investigations above the freezing temperature $T_f\sim 2.4$ K reveal the coexistence of two distinct magnetic phases. A cooperative paramagnetic phase, evidenced by a gapless excitation mode, is found along with a spin-glass phase developing below 20 K which is associated with a gapped low-energy excitation. Effective $g$ factors close to 2 are obtained for both modes, in line with pulsed high-field magnetization measurements which show an unsaturated isotropic behavior up to 58 T at 2 K. In order to describe the field-dependent magnetization in high magnetic fields, we propose an empirical model accounting for highly anisotropic ionic $g$ tensors expected for this material and taking into account the strongly competing interactions between the spins which lead to a frustrated ground state. As a detailed quantitative relation between effective $g$ factors as determined from ESR and the local $g$ tensors obtained by neutron scattering [Ross et al., Phys. Rev. B 93, 014433 (2016)] is still sought after, our work motivates further theoretical investigations of the low-energy excitations in bond-disordered pyrochlores.

In order to investigate the electronic properties of the semiconducting van der Waals ferromagnet $\mathrm{C}{\mathrm{r}}_2\mathrm{G}{\mathrm{e}}_2\mathrm{T}{\mathrm{e}}_6$ (CGT), where ferromagnetic layers are bonded through van der Waals forces, we have performed angle-resolved photoemission spectroscopy measurements and density-functional theory $(\mathrm{DFT}+U)$ calculations. The valence-band maximum at the $\mathrm{\Gamma }$ point is located $\sim 0.2\mathrm{eV}$ below the Fermi level, consistent with the semiconducting property of CGT. Comparison of the experimental density of states with the DFT calculation has suggested that Coulomb interaction between the Cr $3d$ electrons $U_{\mathrm{eff}}\sim 1.1\mathrm{eV}$. The $\mathrm{DFT}+U$ calculation indicates that magnetic coupling between Cr atoms within the layer is ferromagnetic if Coulomb $U_{\mathrm{eff}}$ is smaller than 3.0 eV and that the interlayer coupling is ferromagnetic below $U_{\mathrm{eff}}\sim 1.0\mathrm{eV}$. We therefore conclude that, for $U_{\mathrm{eff}}$ deduced by the experiment, the intralayer Cr-Cr coupling is ferromagnetic and the interlayer coupling is near the boundary between ferromagnetic and antiferromagnetic, which means experimentally deduced $U_{\mathrm{eff}}$ is consistent with the theoretical ferromagnetic condition.

We have investigated the electron spin lifetime ${\tau }_{\mathrm{S}}$ and momentum lifetime $\tau$ in a two-dimensional (2D) accumulation channel of Schottky-barrier spin metal-oxide-semiconductor field-effect transistors (spin MOSFETs). The spin MOSFETs examined in this study have Fe/Mg/MgO/Si Schottky-tunnel junctions at the source/drain and a 15-nm-thick nondegenerated Si channel with a phosphorus donor doping concentration $N_{\mathrm{D}}$ of $1\times {10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$. We estimated $\tau$ and the electron diffusion coefficient $D_e$ in the 2D accumulation channel from experimental results of a Hall-bar-type MOSFET device and self-consistent calculations using Poisson's and Schrödinger's equations. The spin MOSFETs with various channel lengths $L_{\mathrm{ch}}(=0.3-10\mu \mathrm{m})$ exhibited transistor characteristics with a high on/off ratio of $\sim {10}^6$ as well as clear spin-valve signals at 295 K. From the spin-valve signals measured with various gate electric fields $(2-5\mathrm{MV}/\mathrm{cm}), {\tau }_{\mathrm{S}}$ and the electron spin diffusion length ${\lambda }_{\mathrm{S}}$ were estimated. We found that the spin-flip rate per one momentum scattering event $\tau /{\tau }_{\mathrm{S}}$ is $\sim 1/14000$, which is almost unchanged by the gate electric field. The proportionality between ${\tau }_{\mathrm{S}}$ and $\tau$ indicates that the Elliott-Yafet mechanism is dominant in the Si 2D electron accumulation channel, and that the spin-flip rate per one phonon scattering event and that per one surface roughness scattering event are the same. Based on the Elliott-Yafet theory, there is a possibility that the spin-orbit coupling in the Si 2D accumulation channel is almost twice as strong as that in bulk Si materials.

The physics underlying spectral redshift of thermally generated surface phonon polaritons (SPhPs) observed in near-field thermal spectroscopy is investigated. Numerically exact fluctuational electrodynamics simulations of the thermal near field emitted by a silicon carbide surface scattered in the far zone by an intrinsic silicon probe show that SPhP resonance redshift is a physical phenomenon. A maximum SPhP redshift of $19\mathrm{c}{\mathrm{m}}^{-1}$ is predicted for a 200-nm-diameter hemispherical probing tip and a vacuum gap of 10 nm. Resonance redshift is mediated by electromagnetic gap modes excited in the vacuum gap separating the probe and the surface when the probing tip is much larger than the gap size. The impact of gap modes on the scattered field can be mitigated with a probing tip size approximately equal to or smaller than the vacuum gap. However, sharp probing tips induce important spectral broadening of the scattered field. It is also demonstrated that a dipole approximation with multiple reflections cannot be used for explaining the physics and predicting the amount of redshift in near-field thermal spectroscopy. This work shows that the scattered field in the far zone is a combination of the thermal near field emitted by the surface, and electromagnetic interactions between the probe and the surface. Spectroscopic analysis of near-field thermal emission thus requires a numerically exact fluctuational electrodynamics framework for modeling probe-surface interactions.

Quantum fluctuations in vacuum can exert a dissipative force on moving objects, which is known as Casimir friction. Especially, a rotating particle in the vacuum will eventually slow down due to the dissipative Casimir friction. Here, we identify a dissipationless force by examining a rotating particle near a bi-isotropic media that generally breaks parity symmetry or/and time-reversal symmetry. The direction of the dissipationless vacuum force is always parallel with the rotating axis of the particle. We therefore call this dissipationless vacuum force the axial Casimir force.

We study a capacitor made of three monolayers of transition-metal dichalcogenide separated by hexagonal boron nitride. We assume that the structure is symmetric with respect to the central layer plane. The symmetry includes the contacts: if the central layer is contacted by the negative electrode, both external layers are contacted by the positive one. As a result a strong enough voltage $V$ induces electron-hole dipoles (indirect excitons) pointing toward one of the external layers. Antiparallel dipoles attract each other at large distances. Thus, the dipoles alternate in the central plane forming a two-dimensional antiferroelectric with negative binding energy per dipole. The charging of a three-layer device is a first-order transition, and we show that if $V_1$ is the critical voltage required to create a single electron-hole pair and charge this capacitor by $e$, the macroscopic charge $Q_c=eS{n}_c$ ($S$ is the device area) enters the three-layer capacitor at a smaller critical voltage $V_c<V_1$. In other words, the differential capacitance $C\left(V\right)$ is infinite at $V=V_c$. We also show that in a contactless three-layer device, where the chemically different central layer has lower conduction and valence bands, optical excitation creates indirect excitons that attract each other, and therefore form antiferroelectric exciton droplets. Thus, the indirect exciton luminescence is redshifted compared to a two-layer device.

We present an ab initio study of MgO at high temperature and pressure, around the phase transition between the B1 and B2 phases. By means of ab initio molecular dynamic calculations, the thermal evolution of vibrational properties and thermodynamic quantities is obtained. We carefully compare our results with previous theoretical works on the phase transition curve and we analyze the differences among them. We show that anharmonic effects have been underestimated in the quasiharmonic approximation and that their inclusion in the free energy strongly straightens up the transition curve. Then, we use our B1-B2 phase boundary and our calculated Hugoniot to analyze recent decaying shock experiments on MgO. We also provide important thermodynamic quantities as the Grüneisen parameter and sound velocities and we discussed their temperature dependence.

Three-dimensional nanomagnetism is a rapidly growing field of research covering both noncollinear spin textures and curved magnetic geometries including microtubular structures. We spatially resolve the field-induced magnetization reversal of free-standing ferromagnetic microtubes utilizing multifrequency magnetic force microscopy (MFM). The microtubes are composed of Co/Pd multilayer films with perpendicular magnetic anisotropy that translates to an anisotropy with radial easy axis upon rolling-up. Simultaneously mapping the topography and the perpendicular magnetostatic force derivative, the relation between surface angle and local magnetization configuration is evaluated for a large number of locations with slopes exceeding 45 degrees. The angle-dependence of the switching field is concurrent with the Kondorsky model, i.e., the rolled-up nanomembrane behaves like a planar magnetic film with perpendicular anisotropy and a pinning dominated magnetization reversal. Additionally, we discuss methodological challenges when detecting magnetostatic force derivatives near steep surfaces.

The origin of noncollinear magnetic order in ${\mathrm{UO}}_2$ is studied by an ab initio dynamical-mean-field-theory framework in conjunction with a linear-response approach for evaluating intersite superexchange interactions between U $5f^2$ shells. The calculated quadrupole-quadruple superexchange interactions are found to unambiguously resolve the frustration of face-centered-cubic U sublattice toward stabilization of the experimentally observed noncollinear 3k-magnetic order. Therefore, the exotic 3k-antiferromagnetic order in ${\mathrm{UO}}_2$ can be accounted for by a purely electronic exchange mechanism acting in the undistorted cubic lattice structure. The quadrupolar short-range order above magnetic ordering temperature $T_N$ is found to qualitatively differ from the long-range order below $T_N$.

We investigate the twisted bilayer graphene by a two-orbital Hubbard model on the honeycomb lattice. The model is studied near $\frac{1}{4}$-band-filling by using the singular-mode functional renormalization group theory. Spin-triplet $f$-wave pairing is found from weak to moderate coupling limit of the local interactions, and is associated with the Hund's rule coupling and incommensurate spin fluctuations at moderate momenta.