We report a theory of spin-torque-induced magnetic resonance in collinear ferrimagnets, i.e., spin-torque ferrimagnetic resonance (ST-FiMR). We find that the rectified DC voltage of ST-FiMR has several distinct features as compared to spin-torque ferromagnetic resonance (ST-FMR) signals. The most important feature of the ST-FiMR signal is that its magnitude due to dampinglike spin-orbit torque is almost linearly proportional to the net spin density, and thus its sign changes near the angular momentum compensation condition. Our result suggests that the conventional ST-FMR line-shape analysis is unable to correctly estimate the magnitude and sign of spin-orbit torque for compensated ferrimagnets. Therefore, the analysis based on the ST-FiMR theory, of which a complete analytic expression is provided in this work, must be applied.

When interacting spins in condensed matter order ferromagnetically, their ground-state wave function is topologically trivial. Nonetheless, in two dimensions, ferromagnets can support spin excitations with nontrivial topology, an exotic state known as topological magnon insulator (TMI). Here, we theoretically unveil and numerically confirm a ferromagnetic state in three dimensions dubbed second-order TMI, whose hallmarks are excitations at its hinges, where facets intersect. Since ferromagnetism naturally comes with broken time-reversal symmetry, the hinge magnons are chiral, rendering backscattering impossible. Hence, they trace out three-dimensional paths about the sample unimpeded by defects and are topologically protected by the spectral gap. They are remarkably robust against disorder and highly tunable by atomic-level engineering of the sample termination. We predict that a van der Waals heterostructure built from chromium trihalide and transition metal dichalcogenide monolayers exhibits second-order magnon topology. Our findings empower magnonics, the harnessing of spin waves as information carriers, with the tools of higher-order topology, a promising route to combine low-energy information transfer free of Joule heating with three-dimensional vertical integration.

We theoretically describe the behavior of a terahertz nano-oscillator based on an anisotropic antiferromagnetic dynamical element driven by spin torque. We consider the situation when the polarization of the spin current is perpendicular to the external magnetic field applied along the anisotropy easy axis. We determine the domain of the parametric space (field, current) where the oscillator demonstrates chaotic dynamics. Characteristics of the chaotic regimes are analyzed using conventional techniques such as spectra of the Lyapunov exponents. We show that the threshold current of the chaos appearance is particularly low in the vicinity of the spin-flop transition. In this regime, we consider the mechanism of the chaos appearance in detail when the field is fixed and the current density increases. We show that the appearance of chaos is preceded by a regime of quasiperiodic dynamics on the surface of a two-frequency torus arising in phase space as a result of the Neimark-Sacker bifurcation.

We investigate nonequilibrium quantum spin systems via an exact mapping to stochastic differential equations. This description is invariant under a shift in the mean of the Gaussian noise. We show that one can extend the simulation time for real-time dynamics in one and two dimensions by a judicious choice of this shift. This can be updated dynamically in order to reduce the impact of stochastic fluctuations. We discuss the connection to drift gauges in the gauge-$P$ literature.

Recently, several experiments on ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ (LSCO) challenged the Fermi liquid picture for overdoped cuprates and stimulated intensive debates. In this work, we study the magnetotransport phenomena in such systems based on the Fermi liquid assumption. The Hall coefficient $R_H$ and magnetoresistivity ${\rho }_{xx}$ are investigated near the Van Hove singularity (VHS) $x_{\mathrm{VHS}}\approx 0.2$ across which the Fermi surface topology changes from hole- to electronlike. The main results are (1) $R_H$ drops from positive to negative values with increasing $B$ in the doping regime $x_{\mathrm{VHS}}<x\lesssim 0.3$ and (2) ${\rho }_{xx}$ grows as $B^2$ at small $B$ and saturates at large $B$, while a “nearly linear” dependence shows up in the transition regime, which is significantly enlarged near the VHS. These results can be tested by future magnetotransport experiments in overdoped LSCO to check whether the Fermi liquid picture applies or not.

We investigate the possibility of spin texture in the bilayer cuprate superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ using cluster dynamical mean-field theory. The one-band Hubbard model with a small interlayer hopping and a Rashba spin-orbit coupling is used to describe the material. The $d$-wave order parameter is not much affected by the presence of the Rashba coupling, but a small triplet component appears. We find a spin texture circulating in the same direction around $\mathbf{k}=(0,0)$ and $\mathbf{k}=(\pi ,\pi )$ and stable against the superconducting phase. We predict that the amplitude of the spin structure is strongly affected by the pseudogap phenomenon, more so than the spectral function itself.

We generalize the family of approximate momentum average methods to formulate a numerically exact, convergent hierarchy of equations whose solution provides an efficient algorithm to compute the Green's function of a particle dressed by bosons suitable in the entire parameter regime. We use this approach to extract ground-state properties and spectral functions. Our approximation-free framework, dubbed the generalized Green's function cluster expansion (GGCE), allows access to exact numerical results in the extreme adiabatic limit, where many standard methods struggle or completely fail. We showcase the performance of the method, specializing three important models of charge-boson coupling in solids and molecular complexes: the molecular Holstein model, which describes coupling between charge density and local distortions, the Peierls model, which describes modulation of charge hopping due to intersite distortions, and a more complex Holstein$+$Peierls system with couplings to two different phonon modes, paradigmatic of charge-lattice interactions in organic crystals. The GGCE serves as an efficient approach that can be systematically extended to different physical scenarios, thus providing a tool to model the frequency dependence of dressed particles in realistic settings.

We consider fermions on a honeycomb lattice supplemented by a spin invariant interaction that dynamically generates a quantum spin Hall insulator. This lattice model provides an instance of Gross-Neveu Heisenberg criticality, as realized for example by the Hubbard model on the honeycomb lattice. Using auxiliary field quantum Monte Carlo simulations we show that we can compute with unprecedented precision susceptibilities of the order parameter. In O($N$) Gross-Neveu transitions, the anomalous dimension of the bosonic mode grows as a function of $N$ such that in the large-$N$ limit it is of particular importance to consider susceptibilities rather than equal-time correlations so as to minimize contributions from the background. For the $N=3$ case, we obtain $1/\nu =1.11\left(4\right), {\eta }_{\varphi }=0.80\left(9\right)$, and ${\eta }_{\psi }=0.29\left(2\right)$, respectively, for the correlation length exponent, and the bosonic and fermionic anomalous dimensions.

Two-dimensional (2D) half-metallic materials are of great interest for their promising applications in spintronics. Although numerous 2D half metals have been proposed theoretically, rarely can they be synthesized experimentally. Here, exemplified by monolayer $\mathrm{Fe}{\mathrm{Cl}}_2$, we show three mechanisms in such quantum magnets that would cause the metal-insulator transition by using first-principles calculations. In particular, half metallicity, especially that protected by symmetry-induced degeneracies, predicted by the previous theoretical simulations could be destroyed by electron correlation, spin-orbit coupling, and further structural distortions to lower the total energy. Our work reveals the fragility of the symmetry-protected half metals upon various competing energy-lowering mechanisms, which should be taken into account for theoretically predicting and designing quantum materials with exotic functionalities.

A heterostructure consisting of a magnetic Weyl semimetal and a conventional superconductor exhibits an equilibrium current parallel to the superconductor interface and perpendicular to the magnetization. Analyzing a minimal model, which as a function of parameters may be in a trivial magnetic insulator phase, a Weyl semimetal phase, or a three-dimensional weak Chern insulator phase, we find that the equilibrium current is sensitive to the presence of surface states, such as the topological Fermi-arc states of the Weyl semimetal or the chiral surface states of the weak Chern insulator. While there is a nonzero equilibrium current in all three phases, the appearance of the surface states in the topological regime leads to a reversal of the direction of the current, compared to the current direction for the trivial magnetic insulator phase. We discuss the interpretation of the surface-state contribution to the equilibrium current as a real-space realization of the superconductivity-enabled equilibrium chiral magnetic effect of a single chirality, predicted to occur in bulk Weyl superconductors.

Orthorhombic CuMnAs has been proposed as an antiferromagnetic semimetal hosting nodal line and Dirac points around the Fermi level. We investigate relations between the topological bands and transport phenomena, i.e., the spin Hall effect and anomalous Hall effect, in orthorhombic CuMnAs with first-principles calculations combined with a symmetry analysis of the magnetic structures and of the (spin) Berry curvature. We show the nodal line gapped with spin-orbit coupling in CuMnAs dominantly generates large spin Hall conductivity in the ground state. Although the magnetic symmetry in the ground state of CuMnAs forbids the finite anomalous Hall effect, applied magnetic fields produce a significant anomalous component of the Hall conductivity with the magnetic symmetry breaking. We identify that the dominant contribution to anomalous Hall components comes from the further lifting of band degeneracy under external magnetic fields for the Bloch states generated with the splitting of nodal lines by spin-orbit coupling near the Fermi energy.

Recent experiments showed the distinct observations on the transition-metal ditelluride ${\mathrm{NiTe}}_2$ under pressure: one reported a superconducting phase transition at 12 GPa, whereas another observed a sign reversal of Hall resistivity at 16 GPa without the appearance of superconductivity. To clarify the controversial experimental phenomena, we have carried out first-principles electronic structure calculations on the compressed ${\mathrm{NiTe}}_2$ with structure searching and optimization. Our calculations show that the pressure can transform ${\mathrm{NiTe}}_2$ from a layered $P\text{−}3m1$ phase to a cubic $Pa\text{−}3$ phase at $\sim 10\mathrm{GPa}$. Meanwhile, both $P\text{−}3m1$ and $Pa\text{−}3$ phases possess nontrivial topological properties. The calculated superconducting $T_c$'s for these two phases based on the electron-phonon coupling theory both approach 0 K. Further magnetic transport calculations reveal that the sign of Hall resistance for the $Pa\text{−}3$ phase is sensitive to the pressure and the charge doping, in contrast to the case of the $P\text{−}3m1$ phase. Our theoretical predictions on the compressed ${\mathrm{NiTe}}_2$ wait for careful experimental examinations.

Constitutive relations for Lorentz media with a time-varying density of oscillators and a time-varying intrinsic frequency of oscillators are derived ab initio. For an electromagnetic wave propagating in a dynamic Lorentz medium, combinations of the wave energy and frequency (adiabatic invariants) that are conserved during slow time variations of the medium are obtained. If the oscillator density varies, the invariants differ from the known ones for nondispersive dielectrics and plasmas and have a different form for increasing and decreasing oscillator densities. In both cases, however, they predict that the frequency shift is accompanied by a decrease of the wave energy. If the intrinsic frequency varies, the invariant coincides with the one for a nondispersive dielectric and predicts the wave amplification and attenuation with the increase and decrease of the frequency, respectively. It is also shown that a medium with a decreasing density of oscillating dipoles cannot be described by a time-varying dielectric permittivity $\varepsilon \left(t\right)$ even in the nondispersive limit.

The bulk properties of nodal-line materials have been an important research topic in recent years. In this paper, we study the orbital magnetic susceptibility and the Hall conductivity of nodal-line materials using the formalism with thermal Green's functions and find characteristic singular behaviors of them. It is shown that, in the vicinity of the gapless nodal line, the orbital magnetic susceptibility shows a $\delta$-function singularity and the Hall conductivity shows a step-function behavior in their chemical potential dependencies. Furthermore, these singular behaviors are found to show strong field-angle dependencies corresponding to the orientation of the nodal line in the momentum space. These singular behaviors and strong angle dependencies will give clear evidence for the presence of the nodal line and its orientation and can be used to experimentally detect nodal-line materials.

The planar Hall effect (PHE) has remarkable advantages for applications to spintronic devices and the study of intrinsic magnetic properties because of its high signal-to-noise ratio and low thermal drift. Here, the magnetic field controllable PHE in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4/\mathrm{SrTi}{\mathrm{O}}_3$ (001) films is presented. The PHE transforms from a “sinusoidal-shaped” twofold symmetry to fourfold oscillations with decrease in the magnetic field at 35 K. Furthermore, the PHE is approximately two to three orders of magnitude larger than the giant PHE associated with ferromagnetic semiconductors as the temperature decreases. Based on subsequent measurements of the magnetic transport properties and calculations of the angle between the magnetization and current, the unexpected fourfold-symmetric PHE can be attributed to magnetocrystalline anisotropy and spin-orbital coupling. These observations of PHE facilitate a deeper understanding of the magnetic interactions in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ films and indicate potential applications in spintronic or magnetic-sensor devices.

In the study of oscillatory electron transport in high-mobility Si MOSFETs at low temperatures we observe two correlated effects in weak in-plane magnetic fields: a steep decrease of the magnetic susceptibility ${\chi }^{*}\left(H\right)$ and an increase of the concentration of mobile carriers $n\left(H\right)$. We suggest a phenomenological model of the magnetic field driven redistribution between the extended and localized electronic states that qualitatively explains both effects. We argue that the redistribution is mainly caused by magnetization of the large-spin $S\approx 2$ localized states with energies close to the Fermi energy $E_F$, coexisting with the majority Fermi liquid state. Our findings also resolve a long-standing disagreement between the experimental data on ${\chi }^{*}$ obtained in weak ($H\sim k_{B}T/{\mu }_B$) and strong ($H\sim E_F/g{\mu }_B$) magnetic fields.

Designer mode dispersions with properties such as negative or zero group velocities have applications in tailored wave propagation and energy redistribution. The modes of elastic plates can readily exhibit such phenomena by virtue of their bulk elastic properties and thickness. Here, we investigate the symmetric leaky Lamb modes within a thin flat aluminum-alloy plate submerged in water using pulsed broadband ultrasound, and experimentally characterize acoustic beaming from the plate caused by resonant transmission, which is enhanced via intrinsic negative dispersion of energy within the plate. The beaming of acoustic power occurs within a narrow frequency band in transmission over the negative group velocity region of the first-order symmetric leaky Lamb mode. Experimental characterization utilizes Fourier analysis and measurements of transmitted fields in time and space to obtain the leaky Lamb modes dispersion, which agree well with the predictions of a numerical model. This model is then used to visualize the pressure field and confirm the nature of the energy flow inside the plate. The properties of the acoustic focusing depend on the plate thickness and elastic material parameters, and may be readily controlled for a range of applications. Such phenomena may be exploited in other areas, such as in the microwave domain where designer mode dispersions can be developed using metamaterial concepts.

The modulation is analyzed from the analytical properties of zeros of the free fermionic partition function on the complex plane of wave numbers. It is shown how these properties are related to the oscillations of correlation functions. This approach can be used for analysis of phase transitions with local or nonlocal order parameters, as well as for the disorder lines. We find an infinite cascade of disorder lines at finite temperature in the quantum $XY$ chain (equivalent to free fermions). The well-known ground-state factorization on the disorder line, and consequently disentanglement, is shown to follow directly from analytical properties of this model on the complex plane. From the quantum-classical correspondence the results for the chain are used to detect the disorder lines in several frustrated two-dimensional Ising models. The present formalism can be applied to other fermionic models in two and three spatial dimensions. In particular, we find the temperature-dependent Fermi wave vector of oscillations in the degenerate gas of three-dimensional fermions, which naturally leads in the limit $T\to 0$ to the definition of the Fermi energy as the surface of quantum criticality. The modulation is a very common phenomenon, and it occurs in a large variety of models. The important point is that all these modulation transitions can be related to the complex zeros of partition functions, as done in the present study.

Crystallographic symmetries enforcing band touchings (BTs) in the Brillouin zone (BZ) have been utilized to classify and predict the topological semimetals. Though the early proposed topological semimetals contain isolated nodal points in the BZ, the proposed nodal line semimetals later could host various structures of several nodal lines/loops: Nodal chains, nodal nets or Hopf links, etc. In this work, using compatibility relations, we first list all possible high-symmetry lines (HSLs) that can be nodal lines itself, high-symmetry planes (HSPLs) that can host nodal loops, and HSPLs that are nodal surfaces for all 230 space groups (SGs), with spin-orbit coupling and time-reversal symmetry considered or not. We then show how to diagnose a nodal loop from the band crossing in an HSL, or nodal line/surface from irreducible representation (irrep) of a high-symmetry point (HSP), while the rest cases correspond to nodal points. Among our results, those essential cases, for which the nodal points/lines/loops/surfaces must exist, are highlighted since they are promising for the realizations of (nearly) ideal nodal point/line/loop/surface semimetals, as well as systems with flexible tunability owning fixed structure of topological nodal points/lines/loops/surfaces. Based on our results, SGs allowing Hopf-link structure with one straight nodal line threading a nodal loop, or two nesting nodal loops lying in two respective HSPLs, are highlighted, with the predicted materials being ${\mathrm{B}}_5{\mathrm{Pb}}_2{\mathrm{IO}}_9$ in SG 34 and ${\mathrm{SrAl}}_2{\mathrm{Au}}_3$ in SG 62, respectively. Our exhaustive results could serve as a useful guide for efficiently predicting and designing materials or artificial systems owning exotic geometric nodal structures of energy bands simply based on structure symmetries.

The two-dimensional organic superconductor $\lambda -{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ exhibits pronounced charge fluctuations below $T\approx 150$ K, in contrast to the sibling compound $\kappa -{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ that remains metallic down to milli-Kelvin. Infrared spectroscopy reveals only minor splitting in the vibrational features of the latter compound, common to other strongly dimerized $\kappa$-type salts. When the organic molecules are arranged in the $\lambda$-type pattern, however, a strong vibrational ${\nu }_{27}\left(b_{1u}\right)$ mode is present that forms a narrow doublet. Most important, when cooling $\lambda -{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ below 150 K, two weak side modes appear due to charge disproportionation that amounts to $2\delta =0.14e$. In analogy to the ${\beta }^{\prime \prime }$-type organic conductors, we propose that charge fluctuations play an important role in emerging of unconventional superconductivity in $\lambda -{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ at $T_c=4.7$ K. We discuss the possibility of a charge-density wave that coexists with the proposed spin-density-wave state.