The discovery of two-dimensional (2D) van der Waals (vdW) intrinsic magnets has opened a promising avenue to design high-performance magnetic tunnel junctions (MTJs) based on 2D materials. In this work, using first-principles calculations, it is demonstrated that bilayer $\mathrm{Co}{\mathrm{Br}}_2$ is intrinsically a magnetic semiconductor with intralayer ferromagnetic (FM) and interlayer antiferromagnetic (AFM) couplings and the interlayer AFM coupling in bilayer $\mathrm{Co}{\mathrm{Br}}_2$ is independent on the stacking orders. Moreover, using the nonequilibrium Green's function combined with density functional theory, it is found that due to the large difference between interlayer AFM and FM states of the $\mathrm{Co}{\mathrm{Br}}_2$ barrier, the conductance of spin filter (SF) vdW MTJs based on the graphene/bilayer $\mathrm{Co}{\mathrm{Br}}_2$/graphene heterostructure for the interlayer FM state of the $\mathrm{Co}{\mathrm{Br}}_2$ barrier is about 25 times that for the interlayer AFM state of the $\mathrm{Co}{\mathrm{Br}}_2$ barrier. Consequently, a high tunneling magnetoresistance (TMR) ratio of $2420\%$ is achieved in this SF-vdW MTJ at zero bias. In particular, because the current for the interlayer FM state of the $\mathrm{Co}{\mathrm{Br}}_2$ barrier rapidly increases with the increase of bias voltage, a giant TMR ratio of up to about $38000\%$ can be achieved in this SF-vdW MTJ at 0.2-V bias. Our results suggest that SF-vdW MTJs formed by the interlayer AFM barrier with variable conductivity hold great potential for developing vdW MTJs with a high TMR ratio.

We report the ground state magnetic structure and in-field magnetostrictive effects of $\mathrm{Ni}\mathrm{Ti}{\mathrm{O}}_3$ studied by means of zero-field and in-field single-crystal neutron diffraction, magnetization, and high-resolution dilatometry experiments. Zero-field neutron diffraction on $\mathrm{Ni}\mathrm{Ti}{\mathrm{O}}_3$ single crystals confirms an easy-plane antiferromagnet with a multidomain ground state. Upon application of external magnetic fields, neutron diffraction shows the evolution of domains with spins perpendicular to the applied field. The rotation of spins in the multidomain state exhibits pronounced lattice changes in the magnetostriction measurements. We see magnetization and magnetostriction measurements scale with each other in the multidomain state in accordance with phenomenological theories, revealing the strong coupling of spins to the lattice.

The anisotropic magnetoresistance (AMR) of ferromagnetic ${\mathrm{Mn}}_5{\mathrm{Ge}}_3{\mathrm{C}}_x(0\le x\le 1)$ and ${\mathrm{Mn}}_5{\mathrm{Si}}_3{\mathrm{C}}_x(0.5\le x\le 1)$ thin films was investigated and compared with density functional theory calculations from which the spin-split electronic density of states at the Fermi level and the spin polarization were obtained. The isostructural compounds exhibit different AMR behavior. While only ${\mathrm{Mn}}_5{\mathrm{Si}}_3{\mathrm{C}}_{0.5}$ shows a positive AMR ratio and a positive spin polarization, the negative AMR ratio of all other compounds is due to a negative spin polarization. The correlation between the sign of the AMR and the degree of spin polarization is in agreement with theoretical calculations of the AMR ratio indicating that the magnetoelectronic transport in both compounds is dominated by minority-spin conduction. The dominating role of minority-spin conduction remains unaffected even after incorporation of carbon into the crystalline lattice which weakens both AMR and spin polarization.

We report an experimental study of oscillatory thermal counterflow of superfluid ${}^4\mathrm{He}$ and its transition to quantum turbulence inspired by the work of Kotsubo and Swift [Phys. Rev. Lett. 62, 2604 (1989)]. We use a pair of transversally oriented second-sound sensors to provide direct proof that upon exceeding a critical heat flux, quantized vorticity is generated in the antinodes of the longitudinal resonances of the oscillating counterflow. Building on modern understanding of oscillatory flows of superfluid ${}^4\mathrm{He}$ [D. Schmoranzer et al., Phys. Rev. B 99, 054511 (2019)], we re-evaluate the original data together with ours and provide grounds for the previously unexplained temperature dependence of critical velocities. Our analysis incorporates a classical flow instability in the normal component described by the dimensionless Donnelly number, which is shown to trigger quantum turbulence at temperatures below $\approx 1.7\mathrm{K}$. This contrasts with the original interpretation based on the dynamics of quantized vortices, and we show that for oscillatory counterflow, such an approach is valid only at temperatures above $\approx 1.8\mathrm{K}$. Finally, we demonstrate that the instabilities occurring in oscillatory counterflow are governed by the same underlying physics as those in flow due to submerged oscillators and propose a unified description of high Stokes number coflow and counterflow experiments.

We systematically study the work statistics for quantum phase transition. For a quantum system approached by an anisotropic conformal field theory near the critical point, the driving protocol is divided into three different regimes for different quench rates, which reflects the competition between the frozen time and the quench timescale. In each regime, we find universal scaling behaviors in work statistics (after renormalization). It is shown that the critical exponents are determined by the space-time dimension $d$, the dynamical critical exponent $z$, the correlation-length exponent $\nu$, and the power-law protocols. These universal scalings in nonequilibrium processes may be found in quantum phase transition by measuring the Loschmidt echo or the Ramsey interferometry.

The appropriate magnetic Langevin equation, namely the inertial Landau-Lifshitz-Gilbert equation augmented by thermal noise, is written starting from an analogy with the dynamics of the magnetic dipole moment of a circular current-carrying loop (wire) viewed as a symmetric top. Hence the corresponding Fokker-Planck equation for the evolution of the probability density function in the phase space of angular velocities and orientations and its stationary solution are derived. Moreover, the inertial stochastic magnetization dynamics of ferromagnetic nanoparticles is also seen to be analogous to the stochastic dynamics of the electric dipole moment of a polar molecule visualized as a dipole lying along the axis of symmetry of a symmetric top ignoring friction about that axis–a conclusion reached by replacing electrical parameters with their magnetic analogs in the respective Langevin equations. Therefore, existing results from gyroscopic theory, as applied to dielectric relaxation of polar molecules, may with appropriate modifications be used to study inertial magnetization effects in ferromagnetic nanoparticles.

Nonergodicity of phase coexisting state is not trivial, and its understanding is yet unclear. We have investigated two prototype manganites, ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.975}{\mathrm{Al}}_{0.025}{\mathrm{O}}_3$ and ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ where ferromagnetic and antiferromagnetic phases coexist at low temperatures due to kinetic arrest of a first-order transition. We have studied the evolution of the dynamics of a kinetically arrested state with temperature and magnetic field. We also highlight a peculiar nonequilibrium behavior of the phase coexisting state that appears like the memory effect, which basically is an intrinsic property of spin glasses or related systems. If the volume fraction of the two coexisting phases is altered by changing magnetic field at a temperature $T_m$ during cooling, the phase fraction is fully or partially reacquired during heating above $T_m$ depending on which phase among the coexisting phases is the actual equilibrium phase of the system. We suggest that the memorylike response observed here is not due to competing exchange interaction, instead arises due to supercooling, superheating, and kinetic arrest of the associated transition. Such magnetic response provides an easy method of identifying the ground state of a kinetically arrested system.

We study zero-energy states of the chirally symmetric continuum model (CS-CM) of twisted bilayer graphene. The zero-energy state obeys the Dirac equation on a torus in the external non-Abelian magnetic field. These zero-energy states could form a flat band—a band where the energy is constant across the Brillouin zone. We prove that the existence of the flat band implies that the wave function of any state from the flat band has a zero and vice versa. We found a hidden flat band of unphysical states in the CS-CM that has a pole instead of a zero. Our main result is that in the basis of the flat band and hidden wave functions the flat band could be interpreted as a Landau level in the external magnetic field. From that interpretation we show the existence of extra flat bands in the magnetic field.

We derive the effective interactions between local magnetic moments which are mediated by Weyl electrons in magnetic topological semimetals. The resulting spin dynamics is governed by the induced Heisenberg, Kitaev, and Dzyaloshinskii-Moriya (DM) interactions with extended range and oscillatory dependence on the distance between the spins. These interactions are realized in multiple competing channels shaped by the multitude of Weyl nodes in the electron spectrum. Microscopic spins need to be spatially modulated with a channel-dependent wave vector in order to take advantage of the interactions. The DM vector is parallel to the displacement between the two interacting spins, and requires the presence of Weyl electron Fermi surfaces. We also derive the Weyl-induced chiral three-spin interaction in the presence of an external magnetic field. This interaction has an extended range as well, and acts upon the spatially modulated spins in various channels. Its tendency is to produce a skyrmion lattice or a chiral spin liquid which exhibits topological Hall effect. Ultimately, the theory developed here addresses magnetic dynamics in relativistic metals even when chiral magnetism is microscopically precluded. We discuss insights into the ordered state of the magnetic Weyl semimetal NdAlSi.

In many paradigmatic materials, such as transition metal dichalcogenides, the role played by the spin degrees of freedom is as important as the one played by the electron-electron interaction. Thus an accurate treatment of the two effects and of their interaction is necessary for an accurate and predictive study of the optical and electronic properties of these materials. Despite the fact that the GW-BSE approach correctly accounts for electronic correlations, the spin-orbit coupling effect is often neglected or treated perturbatively. Recently, spinorial formulations of GW-BSE have become available in different flavors in material-science codes. However, an accurate validation and comparison of different approaches is still missing. In this work, we go through the derivation of the noncollinear GW-BSE approach. The scheme is applied to transition metal dichalcogenides comparing the perturbative and full spinorial approaches. Our calculations reveal that dark-bright exciton splittings are generally improved when the spin-orbit coupling is included nonperturbatively. The exchange-driven intravalley mixing between the A and B excitons is found to play a role for Mo-based systems, being especially strong in the case of $\mathrm{Mo}{\mathrm{Se}}_2$. We finally compute the excitonic spin and use it to sharply analyze the spinorial properties of transition metal dichalcogenide excitonic states.

SmS, a prototypical intermediate valence compound, has been studied by performing high-pressure nuclear magnetic resonance measurements on a ${}^{33}\mathrm{S}$-enriched sample. The observation of an additional signal below 15–20 K above a nonmagnetic-magnetic transition pressure $P_{\mathrm{c}2}\approx 2$ GPa gives evidence of a magnetic transition. The absence of a Curie term in the Knight shift near $P_{\mathrm{c}2}$ indicates that the localized character of $4f$ electrons is entirely screened and the mechanism of the magnetic ordering is not described within a simple localized model. Simultaneously, the line shape in the magnetically ordered state is incompatible with a spin density wave order. These suggest that the magnetic order in SmS may require an understanding beyond the conventional framework for heavy fermions. The fact that hyperfine fields from the ordered moments cancel out at the S site leads us to a conclusion that the ordered phase has a type II antiferromagnetic structure.

We study a two-dimensional single-band Hubbard Hamiltonian with antisymmetric spin-orbit coupling. We argue that this is the minimal model to understand the electronic properties of locally noncentrosymmetric transition metal (TM) oxides such as ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$. Based on exact diagonalizations of small clusters and the random-phase approximation, we investigate the correlation effects on charge and magnetic order as a function of doping and of the TM-oxygen-TM bond angle $\theta$. For small doping and $\theta \lesssim {15}^{\circ }$ we find dominant commensurate in-plane antiferromagnetic fluctuations, while ferromagnetic fluctuations dominate for $\theta \gtrsim {25}^{\circ }$. Moderately strong nearest-neighbor Hubbard interactions can also stabilize a charge density wave order. Furthermore, we compare the dispersion of magnetic excitations for the hole-doped case to resonant inelastic x-ray scattering data and find good qualitative agreement.

The electronic properties of the Be(0001) surface are investigated on the atomic scale at low temperature. Utilizing laterally resolved scanning tunneling spectroscopy (STS) a pronounced surface state is identified, additionally manifested by the observation of bias-dependent standing-wave patterns. Fourier analysis reveals a parabolic dispersive behavior, being characteristic for a two-dimensional free-electron gas confined to the Be(0001) surface. Moreover, signatures of the opening of inelastic phonon-mediated tunnel channels for electrons near the Fermi level are observed by STS.

By using the single-band and multiband effective Hamiltonian for monolayer $X\mathrm{Se}$ ($X=\mathrm{In},\mathrm{Ga}$), we calculate the density-density response function of monolayer $X\mathrm{Se}$ ($X=\mathrm{In},\mathrm{Ga}$) and study its plasmon dispersion within the random-phase approximation. At long wavelengths ($q\to 0$), plasmon dispersion shows the local classical behavior $\omega ={\omega }_0\sqrt{q}$. Although, monolayer $X\mathrm{Se}$ ($X=\mathrm{In},\mathrm{Ga}$) has a nonparabolic “Mexican hat” topmost valence band dispersion which is so different from the parabolic band structure in two-dimensional electron gas, the corresponding density dependence of the plasmon energy is still of the form ${\omega }_0\propto \sqrt{n}$ ($n$ is the carrier concentration) as in a conventional two-dimensional electron gas (2DEGs). However, the Fermi energy ($\left|\mu \right|$) dependence of the plasmon energy is of the form ${\omega }_0\propto {\left|\mu \right|}^{1/4}$, which is different from the conventional 2DEGs (${\omega }_0\propto \sqrt{\left|\mu \right|}$).

We determine magnon spectra of an atomic bilayer magnet with ferromagnetic intra- and both ferro- and antiferromagnetic interlayer coupling. Analytic expressions for the full magnon band of the latter case reveal that both exchange interactions govern the fundamental magnon gap. The inter- and intralayer magnetic ordering are not independent: a stronger ferromagnetic intralayer coupling effectively strengthens the antiferromagnetic interlayer coupling as we see from comparison of two bilayer systems. The trivial topology of these exchange-anisotropy spin models without spin-orbit interaction excludes a magnon thermal Hall effect.

We point out that there are two different chiral spin liquid states on the triangular lattice and discuss the conducting states that are expected on doping them. These states labeled CSL1 and CSL2 are associated with two distinct topological orders with different edge states, although they both spontaneously break time reversal symmetry and exhibit the same quantized spin Hall conductance. While CSL1 is related to the Kalmeyer-Laughlin state, CSL2 is the $\nu =4$ member of Kitaev's 16-fold way classification. Both states are described within the Abrikosov fermion representation of spins, and the effect of doping can be accessed by introducing charged holons. On doping CSL2, condensation of charged holons leads to a topological $d + id$ superconductor. However on doping CSL1, in sharp contrast, two different scenarios can arise: first, if holons condense, a chiral metal with enlarged unit cell and finite Hall conductivity is obtained. However, in a second novel scenario, the internal magnetic flux adjusts with doping and holons form a bosonic integer quantum Hall state. Remarkably, the latter phase is identical to a $d+id$ superconductor. In this case the Mott insulator to superconductor transition is associated with a bosonic variant of the integer quantum Hall plateau transition. We connect the above two scenarios to two recent numerical studies of doped chiral spin liquids on a triangular lattice. Our work clarifies the complex relation between topological superconductors, chiral spin liquids, and quantum criticality.

We compute the effects of electronic interactions on gapless spin-3/2 excitations that in a noninteracting system emerge at a biquadratic touching of Kramers degenerate valence and conduction bands in three dimensions, also known as a Luttinger semimetal. This model can describe the low-energy physics of HgTe, gray-Sn, 227 pyrochlore iridates, and half-Heuslers. For the sake of concreteness, we only consider the short-range components of the Coulomb interaction (extended Hubbard-like model). By combining mean-field analysis with a renormalization group (RG) calculation (controlled by a “small” parameter $\epsilon$, where $\epsilon =d-2$), we construct multiple cuts of the global phase diagram of interacting spin-3/2 fermions at zero and finite temperature and chemical doping. Such phase diagrams display a rich confluence of competing orders, among which rotational symmetry breaking nematic insulators and time-reversal symmetry breaking magnetic orders (supporting Weyl quasiparticles) are the prominent candidates for excitonic phases. We also show that even repulsive interactions can be conducive to both mundane $s$-wave and topological $d$-wave pairings. The reconstructed band structure (within the mean-field approximation) inside the ordered phases allows us to organize them according to the energy (entropy) gain in the following (reverse) order: $s$-wave pairing, nematic phases, magnetic orders, and $d$-wave pairings, at zero chemical doping. However, the paired states are energetically superior over the excitonic ones for finite doping. The phase diagrams obtained from the RG analysis show that for sufficiently strong interactions, an ordered phase with higher energy (entropy) gain is realized at low (high) temperature. In addition, we establish a “selection rule” between the interaction channels and the resulting ordered phases, suggesting that repulsive short-range interactions in the magnetic (nematic) channels are conducive to the nucleation of $d$-wave ($s$-wave) pairing among spin-3/2 fermions. We believe that the proposed methodology can shed light on the global phase diagram of various two and three-dimensional interacting multiband systems, such as Dirac materials, doped topological insulators, and the like.

Modifying the wavefront of electromagnetic fields is one of the most significant operations in a variety of photonic components from spatial wave modulators and optical deflectors to focusing lenses and reflectarray nanoantennas. Metamirrors comprising dielectric and plasmonic layers back-to-back are found to serve well such a purpose of wavefront transformation for reflected waves with use of tiny scatterers placed at positions of enhanced field and by exploiting the absorbing mechanism of these bilayers. Even though the efficiency of the configurations is not high due to the presence of losses, the tailoring of the reflective wave pattern is successful given the simplicity of the considered setups. In particular, numerous optimal designs that anomalously reflect the incoming plane waves or convert them into cylindrical ones have been determined by trying and testing every combination from a large set of available media. Thus, extra degrees of freedom are provided in modeling such modules that are vital for several wavefront engineering applications.

Previously engineered wave-bearing materials including metamaterials generate their acoustic responses from a combination of passive geometrical structures possibly augmented by active elements. The passive geometrical features typically translate to uncontrolled properties outside the band of interest and are the source of adverse effects such as narrowing the operating band significantly or generating difficult-to-control instabilities due to interactions with the active elements. This paper proposes a nonlocal active metamaterial (NAM) architecture that employs a purely active approach to tailoring the acoustic field utilizing spatially separated sensors and actuators. A significant benefit of this approach is that the acoustic properties are described by analytical closed-form equations exposing the complex interplay between the various components of the material. The method is demonstrated by engineering extremely nonreciprocal wave transmission in an otherwise completely transparent subwavelength NAM. We experimentally demonstrate the remarkable flexibility of the system by electronically tuning the acoustic isolation characteristics and discuss the massive design space uncovered by the NAM approach.