Recent advances in the physics of current-driven antiferromagnetic skyrmions have observed the absence of a Magnus force. We outline the symmetry reasons for this phenomenon, and show that this cancellation will fail in the case of spin polarized currents. Pairing micromagnetic simulations with semiclassical spin wave transport theory, we demonstrate that skyrmions produce a spin-polarized transverse magnon current and that spin-polarized magnon currents can in turn produce transverse motion of antiferromagnetic skyrmions. We examine qualitative differences in the frequency dependence of the skyrmion Hall angle between ferromagnetic and antiferromagnetic cases, and close by proposing a simple skyrmion-based magnonic device for demultiplexing of spin channels.

We investigate numerically the low-energy properties of an artificial square spin system in which the nanomagnets are physically connected at the lattice vertex sites. Micromagnetic simulations performed on a single square vertex reveal that type-II vertices always have the lowest energy, in sharp contrast with what is found in lattices made of disconnected nanomagnets, for which type-I vertices are the ground-state configuration. The micromagnetic simulations also show that the energy stored at the vertex sites strongly depends on the type of magnetic domain wall formed by the four connected nanomagnets. Interestingly, the energy gap between type-I and type-II vertices can be drastically reduced by varying the geometrical parameters of the nanomagnets, such as their width and thickness. For typical widths and thicknesses achievable experimentally, we find that this energy gap is small enough to consider type-I and type-II vertices as quasidegenerate. Based on the vertex energies provided by the micromagnetic simulations, we compute the thermodynamic properties of the corresponding spin model using Monte Carlo simulations. In some cases, these properties are hardly distinguishable from those of the celebrated square ice model. Our findings then suggest that an ice physics, characterized by a massively degenerate ground-state manifold at low temperature, may be observed experimentally in a simple square lattice of connected magnetic elements. This work thus provides a route to fabricate artificial algebraic spin liquids using a purely two-dimensional geometry.

The proposed loop-current order in cuprates cannot give the observed pseudogap and the Fermi arcs because it preserves translation symmetry. A modification to a periodic arrangement of the four possible orientations of the order parameter with a large period of between about 12 to 30 lattice constants is proposed and shown in a simple and controlled calculation to give one-particle spectra with every feature as in the ARPES experiments. The results follow from (1) the currents at the boundaries of the periodic domains with similar topology as the Affleck-Marston flux phase, and (2) the mixing introduced by the boundary currents between the states near the erstwhile Fermi surface and the ghost Fermi surfaces which are displaced from it by mini-reciprocal vectors. The proposed idea can be ruled out or verified by high resolution diffraction or imaging experiments. It does not run afoul of the variety of different experiments consistent with the loop-current order as well as the theory of the marginal Fermi-liquid and $d$-wave superconductivity based on quantum-critical fluctuations of the loop-current order.

We show that an energy gap is induced in graphene by light-matter coupling to a circularly polarized photon mode in a cavity. Using many-body perturbation theory, we compute the electronic spectra which exhibit photon-dressed sidebands akin to Floquet sidebands for laser-driven materials. In contrast with Floquet topological insulators, in which a strictly quantized Hall response is induced by light only for off-resonant driving in the high-frequency limit, the photon-dressed Dirac fermions in the cavity show a quantized Hall response characterized by an integer Chern number. Specifically for graphene, we predict that a Hall conductance of $2e^2/h$ can be induced in the low-temperature limit.

In this paper, we study doped HgTe/CdTe quantum wells with Hubbard-type interaction under a perpendicular magnetic field using a lattice Bernevig-Hughes-Zhang (BHZ) model with a bulk inversion asymmetry (BIA) term. We show that the BIA term is strongly enhanced by interaction around the region when the band inversion of the topological insulator is destroyed by a magnetic field. The enhanced BIA term creates edge-like electronic states which can explain the experimentally discovered edge conductance in doped HgTe/CdTe quantum wells in a similar magnetic field regime.

Monolayer transition metal dichalcogenides (TMDs) are characterized by strong light-matter interactions due to enhanced excitonic effects, which make them exciting materials for fundamental physics and optoelectronics applications. Moreover, the valley-dependent chirality of the band structure in TMDs significantly modifies the optical selection rules for single- and multiphoton processes. Here, we propose an analytical approach for calculating the linear and nonlinear optical (NLO) responses of monolayer TMDs, including excitonic effects at low photon energies. Based on this approach, we provide an informative diagram which encompasses all excitonic selection rules. The diagram enables us to identify main transitions for the first-, second-, and third-order optical responses. As a case study, we calculate the optical conductivity and second-/third-harmonic generation responses of monolayer ${\mathrm{MoS}}_2$ and demonstrate that the analytical approach accurately reproduces the spectra obtained using the Bethe-Salpeter equation (BSE). Moreover, the analytical approach enables us to obtain valuable physical insight into the fundamental transitions responsible for individual resonances, which is not straightforward in the full BSE method. Our analytical approach can readily be extended to higher-order nonlinearities and, hence, provides a simple but accurate tool for analyzing experimental NLO spectra of monolayer TMDs.

SiC(0001)-$\left(1\times 1\right)$-H consisting of monohydride is a preferred starting surface structure for synthesis of the two-dimensional materials on SiC(0001). Here we report preparation of the SiC(0001)-$\left(1\times 1\right)$-H by atomic hydrogen exposure and structure determination of the SiC(0001)-$\left(1\times 1\right)$-H by a quantitative LEED analysis. Our data show that the SiC(0001)-$\left(1\times 1\right)$-H is indeed a bulk terminated unreconstructed SiC(0001) surface. The sample morphology was also investigated using AFM. The dominance of one of two possible inequivalent surface terminations of $4H$ polytype of SiC crystal was confirmed.

We use hydrodynamic techniques to analyze the one-dimensional propagation of solitons in gated graphene on an arbitrary uniform background current. Results are derived for both the Fermi liquid and Dirac fluid regimes. We find that these solutions satisfy the Korteweg–de Vries–Burgers equation. Viscous dissipation and Ohmic heating are included, causing the solitons to decay. Experiments are proposed to measure this decay and thereby quantify the shear viscosity in graphene.

We study the dynamics of the energy fluctuations in the Hubbard chain with strong potential disorder that preserves the SU(2) spin symmetry. Within the effective spin-only model that correctly captures relaxation in the situation of localized charges, we show that the decay of local energy correlations is suppressed and is at least marginally nonergodic, being qualitatively different from the subdiffusive dynamics and relaxation of local spins. The anomalous behavior can be traced back to the singular distribution of effective exchange couplings. Numerical results for the dynamical thermal conductivity within the full Hubbard model confirm that the energy (thermal) transport is closer to the charge dynamics, which appears to be also marginally localized in a wide parameter range.

We introduce a method to efficiently study the dynamical properties of many-body localized systems in the regime of strong disorder and weak interactions. Our method reproduces qualitatively and quantitatively the time evolution with a polynomial effort in system size and independent of the desired time scales. We use our method to study quantum information propagation, correlation functions, and temporal fluctuations in one- and two-dimensional many-body localization systems. Moreover, we outline strategies for a further systematic improvement of the accuracy and we point out relations of our method to recent attempts to simulate the time dynamics of quantum many-body systems in classical or artificial neural networks.

We show that interference between parallel currents through two quantum dots, in the presence of spin-orbit interactions and strong on-site Coulomb repulsion, leads to resonances in current noise at the corresponding Larmor frequencies. An additional resonance at the difference of Larmor frequencies is present even without spin-orbit interaction. The resonance lines have strength comparable to the background shot noise and therefore can account for the numerous observations of spin resonance in scanning tunneling microscope noise with nonpolarized leads. We solve also several other models that show similar resonances.

Angle-resolved photoemission spectroscopy (ARPES) is used to study the energy and momentum dependence of the inelastic scattering rates and the mass renormalization of charge carriers in LiFeAs at several high symmetry points in the Brillouin zone. A strong and linear-in-energy scattering rate is observed for sections of the Fermi surface having predominantly Fe $3d_{xy/yz}$ orbital character on the inner hole and on electron pockets. We assign them to hot spots with marginal Fermi liquid character inducing high antiferromagnetic and pairing susceptibilities. The outer hole pocket, with Fe $3d_{xy}$ orbital character, has a reduced but still linear in energy scattering rate. Finally, we assign sections on the middle hole pockets with Fe $3d_{xz,yz}$ orbital character and on the electron pockets with Fe $3d_{xy}$ orbital character to cold spots because there we observe a quadratic-in-energy scattering rate with Fermi-liquid behavior. These cold spots prevail the transport properties. Our results indicate a strong momentum dependence of the scattering rates. We also have indications that the scattering rates in correlated systems are fundamentally different from those in noncorrelated materials because in the former the Pauli principle is not operative. We compare our results for the scattering rates with combined density functional plus dynamical mean-field theory calculations. The work provides a generic microscopic understanding of macroscopic properties of multiorbital unconventional superconductors.

We propose a theoretical framework for the detection of order-parameter fluctuations in three dimensions using ultrafast coherent phonon spectroscopy with long-range interactions. We focus our attention on long-wavelength charge density fluctuations (plasmons), and charged nematic fluctuations where the direction of the propagation vector is fixed perpendicular to the plane of anisotropy. By treating phonons and light classically and decoupling interactions to integrate out the fermionic degrees of freedom, we arrive at an effective theory of order-parameter fluctuations about the spatially uniform saddle-point solution. We find that, due to the $(k_x^2-k_y^2)\left(B_{1g}\right)$ symmetry of the form factor appearing in the vertex, nematic fluctuations couple to light only at fourth order, unlike isotropic density fluctuations which couple at second order. Hence, to lowest order, the interaction between electrons and the electromagnetic field contributes a driving force for plasmon oscillations while it provides a frequency shift for nematic fluctuations. From the resulting coupled harmonic oscillator equations of motion, we argue that ultrafast coherent phonon spectroscopy could be a useful tool to extract and analyze various electronic properties of interest such as the frequency of the collective mode and the coupling between electrons and phonons. Specific experiments are proposed on the normal state of high-$T_c$ systems to observe the frequency shift predicted here resulting directly from orbital ordering (nematic) fluctuations. Our paper presents a new mechanism for generating coherent phonons from long-range interactions (coherent long-range interaction induced phonons) that does not require the existence of multiple bands to act as intermediary states for quasiparticles.

Weyl semimetals with time-reversal symmetry breaking are expected to show various fascinating physical behaviors, such as intrinsic giant anomalous Hall effect, chiral anomaly effect in the bulks, and Fermi arcs on the surfaces. Here we report a scanning tunneling microscopy study on the magnetic Weyl semimetal candidate ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$. According to the morphology and local density of states of the surface, we provide assignments to different surface terminations. The measured local density of states reveals a semimetal gap of $\sim 300$ mV, which is further verified as the gap in spin-minority bands using spin-resolved tunneling spectra. Additionally, signature for the nontrivial surface states around 50 mV is proposed. This is further confirmed by the observations of standing waves around a step edge of the sample. Our observations and their comparison with band-structure calculations provide direct yet timely evidence for the bulk and surface band structures of the magnetic Weyl semimetal candidate ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$.

Single crystals of $R{\mathrm{Cd}}_3{\mathrm{P}}_3$ ($R=\mathrm{La}$ and Ce) have been investigated by magnetization, electrical resistivity, the Hall coefficient, and specific heat. Magnetization measurements of ${\mathrm{CeCd}}_3{\mathrm{P}}_3$ demonstrate clear quasi-two-dimensional magnetic behavior. Electrical resistivity and Hall coefficient measurements suggest that $R{\mathrm{Cd}}_3{\mathrm{P}}_3$ compounds are low-carrier-density metallic systems, in strong contrast to an earlier study of polycrystalline material. Specific heat and electrical resistivity measurements of ${\mathrm{CeCd}}_3{\mathrm{P}}_3$ reveal a high-temperature (structural) phase transition at $T_s=127$ K and antiferromagnetic ordering below $T_N=0.41$ K. Upon applying magnetic field in the easy-plane ($H\parallel ab$) the magnetic ordering temperature increases to 0.43 K at $H\sim 15$ kOe, demonstrating partial lifting of the magnetic frustration. The large electronic specific heat persists in an unusually wide range of temperature above $T_N$ due to the frustrated spins. The observation of conventional metallic behavior in the electrical resistivity suggests that the $f$ electrons in ${\mathrm{CeCd}}_3{\mathrm{P}}_3$ undergo negligible hybridization with the conduction electrons. Thus, ${\mathrm{CeCd}}_3{\mathrm{P}}_3$ may be a model system for exploring the complex interplay between magnetic frustration and Ruderman-Kittel-Kasuya-Yosida physics on a low-carrier-density Ce triangular lattice.

${\mathrm{BaBiO}}_3$ (BBO) is well known as the parent material for the high-$T_c$ superconducting compounds ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{BiO}}_3$ and ${\mathrm{BaPb}}_{1-x}{\mathrm{Bi}}_x{\mathrm{O}}_3$. In its pristine state, BBO is a charge-ordered (CO) insulator, resulting from a static breathing distortion of the ${\mathrm{BiO}}_6$ octahedra with alternating long and short bond lengths. Recently, it has been reported that the CO state is suppressed for BBO films grown on ${\mathrm{SrTiO}}_3$ (STO) below a thickness of approximately 4 nm, possibly resulting in a metallic phase. While we do confirm structural modifications in our BBO/Nb:STO samples in this thickness range by Raman spectroscopy and electron diffraction, in situ photoemission evidences that these changes are accompanied by a Bi deficit and that the films remain insulating. We hence conclude that, in line with previous findings for the BBO/STO interface, the thickness-controlled suppression of the CO state is not purely driven by the two-dimensional confinement but rather originates from modifications of the composition and structure inherent to the epitaxial growth of BBO on ${\mathrm{SrTiO}}_3$(001).

Detailed studies of specific heat, magnetization, and magnetic torque of a single crystal of ${\mathrm{LiNiPO}}_4$ olivine are presented. Olivines attract attention as promising for application as cathodes in Li-ion batteries and exhibiting a unique set of properties. ${\mathrm{LiNiPO}}_4$ is the unique olivine, in which antiferromagnetic order develops in two steps, i.e., at 21.8 K, the second-order transition to an incommensurate phase and then, at 20.9 K, the first-order transition to a commensurate phase appears. Specific heat studies, supplemented by the “slope analysis procedure,” revealed a splitting of the specific heat anomaly accompanying the first-order transition, which suggests that actually, these are two coupled transitions, one of which can be the ferroelectric one. The specific heat was measured as a function of temperature for a series of fixed external magnetic field $B$ values. As the result, analytical equations describing the phase transition lines in the $T\text{−}B$ plane were determined and evolution of a shape of the specific heat anomalies accompanying the phase transitions, observed in the powder sample under influence of $B$, was modeled. Angular dependence of magnetic torque and of magnetization for $B$ rotating within the $a\text{−}c$ and $b\text{−}c$ crystalline planes was measured for several fixed temperature and $B$ values. Based on these results, we found a new effect, that we called “off-diagonal nonlinear magnetic susceptibility,” i.e., we found that for each main crystallographic axis $(a,b$, and $c)$, an additional component of magnetic susceptibility, proportional to the square of the perpendicular to this axis component of $B$, exists. A phenomenological model of this effect, describing the experimental results correctly, was proposed.

One of the key questions concerning frustrated lattices that has lately emerged is the role of disorder in inducing spin-liquid-like properties. In this context, the quantum kagome antiferromagnet ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_3$, which has been recently reported as the first geometrically perfect realization of the kagome lattice with negligible magnetic/nonmagnetic intersite mixing and a possible quantum-spin-liquid ground state, is of particular interest. However, contrary to previous conjectures, here we show clear evidence of bulk magnetic ordering in this compound below $T_N=15$ K by combining bulk magnetization and heat capacity measurements, and local-probe muon spin relaxation measurements. The magnetic ordering in this material is rather unconventional in several respects. First, a crossover regime where the ordered state coexists with the paramagnetic state extends down to $T_N/3$ and, second, the fluctuation crossover is shifted far below $T_N$. Moreover, persistent spin dynamics that is observed at temperatures as low as $T/T_N=1/300$ could be a sign of emergent excitations of correlated spin loops or, alternatively, a sign of fragmentation of each magnetic moment into an ordered and a fluctuating part.

Neutron scattering, ac magnetic susceptibility, and specific-heat studies have been carried out on polycrystalline $\mathrm{D}{\mathrm{y}}_2\mathrm{Z}{\mathrm{r}}_2{\mathrm{O}}_7$. Unlike the pyrochlore spin ice ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7, \mathrm{D}{\mathrm{y}}_2\mathrm{Z}{\mathrm{r}}_2{\mathrm{O}}_7$ crystallizes into the fluorite structure and the magnetic $\mathrm{D}{\mathrm{y}}^{3+}$ moments randomly reside on the corner-sharing tetrahedral sublattice with nonmagnetic Zr ions. Antiferromagnetic spin correlations develop below 10 K but remain dynamic down to 40 mK, with a significant amount of magnetic susceptibility. These correlations extend over the length of two tetrahedra edges and grow to six nearest neighbors with the application of a 20-kOe magnetic field. Magnetic heat capacity revealed a correlation peak at 2 K, but no Pauling's residual entropy was observed. We propose that the disorder precludes the development of spin-ice correlations seen in the chemically ordered ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ compound, with fluctuating spins in a disordered, liquidlike state (albeit slow) which do not freeze into a canonical spin-glass state that one might intuitively expect.

We construct and solve a classical percolation model with a phase transition that we argue acts as a proxy for the quantum many-body localization transition. The classical model is defined on a graph in the Fock space of a disordered, interacting quantum spin chain, using a convenient choice of basis. Edges of the graph represent matrix elements of the spin Hamiltonian between pairs of basis states that are expected to hybridize strongly. At weak disorder, all nodes are connected, forming a single cluster. Many separate clusters appear above a critical disorder strength, each typically having a size that is exponentially large in the number of spins but a vanishing fraction of the Fock-space dimension. We formulate a transfer matrix approach that yields an exact value $\nu =2$ for the localization length exponent, and also use complete enumeration of clusters to study the transition numerically in finite-sized systems.