We propose a different route to time-reversal invariant Weyl semimetals by employing multilayer heterostructures comprising ordinary “trivial” insulators and nontrivial insulators with pairs of protected Dirac cones on the surface. We consider both the case of weak topological insulators, where surface Dirac cones are pinned to time-reversal invariant momenta, and of topological crystalline insulators with unpinned surface Dirac cones. For both realizations we explain phenomenologically how the proposed construction leads to the emergence of a Weyl semimetal phase. We further formulate effective low-energy models for which we prove the existence of semimetallic phases with four isolated Weyl points. Finally, we discuss how the proposed design can be realized experimentally with state-of-the-art technologies.

We investigate the magneto-optical response of double Weyl semimetals whose energy dispersion is intrinsically anisotropic. We find that in the presence of a magnetic field, the most salient feature of the optical conductivity is a series of resonant peaks with the corresponding frequencies scaling linearly with the strength of the magnetic field. In addition, the optical conductivity is found to be anisotropic, with two of the three longitudinal components residing at a linear background and the remaining one at a constant background. The effects of chemical potential, temperature, impurity scattering, and particle-hole symmetry breaking on the optical conductivity are also studied.

We study theoretically the recombination within a pair of two polarons in magnetic field subject to a strong linearly polarized ac drive. Strong drive implies that the Zeeman frequencies of the pair partners are much smaller than the Rabi frequency, so that the rotating wave approximation does not apply. What makes the recombination dynamics nontrivial is that the partners recombine only when they form a singlet $S$. By admixing singlet to triplets, the drive induces the triplet recombination as well. We calculate the effective decay rate of all four spin modes. Our main finding is that, under the strong drive, the major contribution to the decay of the modes comes from short time intervals when the driving field passes through zero. When the recombination time in the absence of drive is short, fast recombination from $S$ leads to anomalously slow recombination from the other spin states of the pair. We show that, with strong drive, this recombination becomes even slower. The corresponding decay rate falls off as a power law with the amplitude of the drive.

We investigate the quantization of adiabatic charge transport in the insulating ground state of finite systems. Topological charge pumps are used in experiments as an indicator of topological order. In the thermodynamic limit, the transport can be related to a topological Berry phase and is thus strictly quantized. This is no longer true for finite systems. We derive finite-size corrections to the transport for both noninteracting and interacting systems and relate them to analytic properties of the single- and many-body Berry curvature. We find that they depend on the details of experimental realizations of the pumps. While they can be non-negligible even in large systems, a proper choice of the pumping protocol can suppress these corrections.

Interfacing a ferromagnet with a polarized ferroelectric gate generates a nonuniform, interfacial spin density coupled to the ferroelectric polarization. This coupling allows for an electric field control of the effective field acting on the magnetization. To unravel the usefulness of this interfacial magnetoelectric coupling we investigate the magnetization dynamics of a ferroelectric/ferromagnetic multilayer structure using the Landau-Lifshitz-Baryakhtar equation. The results demonstrate that the interfacial magnetoelectric coupling is utilizable as a highly localized and efficient tool for manipulating magnetism by electrical means. Ways of enhancing the strength of the interfacial coupling and/or its effects are discussed.

Motivated by the recent progress in the spin-orbit-coupled triangular lattice spin liquid candidate ${\mathrm{YbMgGaO}}_4$, we carry out a systematic projective symmetry group analysis and mean-field study of candidate $U\left(1\right)$ spin-liquid ground states. Due to the spin-orbital entanglement of the Yb moments, the space-group symmetry operation transforms both the position and the orientation of the local moments, and hence it brings different features for the projective realization of the lattice symmetries from the cases with spin-only moments. Among the eight $U\left(1\right)$ spin liquids that we find with the fermionic parton construction, only one spin-liquid state, which was proposed and analyzed by Yao Shen et al. [Nature (London) 540, 559 (2016)] and labeled as U1A00 in the present work, stands out and gives a large spinon Fermi surface and provides a consistent explanation for the spectroscopic results in ${\mathrm{YbMgGaO}}_4$. Further connection of this spinon Fermi surface $U\left(1\right)$ spin liquid with ${\mathrm{YbMgGaO}}_4$ and the future directions are discussed. Finally, our results may apply to other spin-orbit-coupled triangular lattice spin-liquid candidates, and more broadly, our general approach can be well extended to spin-orbit-coupled spin-liquid candidate materials.

The quantitative analysis of $3d$ transition metal ${\mathrm{L}}_{2,3}$ edge x-ray magnetic circular dichroism (XMCD) spectra and the related sum rules are compared for measurements with electron yield and fluorescence yield detection. Multiplet calculations on divalent ions show that fluorescence yield detected sum rule derived expectation values of ${\mathrm{L}}_z$ and ${\mathrm{S}}_z$ show noticeable deviations and detection angle dependence. We show that small deviations of the polarization dependent fluorescent decay values lead to significant deviations in the ${\mathrm{L}}_z$ and ${\mathrm{S}}_z$ sum rule values. Fe and Co experimental XMCD spectra of a supported 10 nm ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ thin film are measured simultaneously by both electron and fluorescence yield. The deviations shown in the experimental data are well explained by the calculations and are shown to mainly depend on the polarization dependent total decay. We conclude that fluorescence yield detected x-ray magnetic circular dichroism is unsuitable for quantitative analysis of the ${\mathrm{L}}_z$ and ${\mathrm{S}}_z$ sum rule values.

We study the optically induced torques in thin-film ferromagnetic layers under excitation by circularly polarized light. We study cases both with and without Rashba spin-orbit coupling using a four-band model. In the absence of Rashba spin-orbit coupling, we derive an analytic expression for the optical torques, revealing the conditions under which the torque is mostly derived from optical spin transfer torque (i.e., when the torque is along the direction of optical angular momentum), versus when the torque is derived from the inverse Faraday effect (i.e., when the torque is perpendicular to the optical angular momentum). For this model, we find the optical spin transfer torque dominates provided that the excitation energy is far away from band-edge transitions and the magnetic exchange splitting is much greater than the lifetime broadening. For the case with large Rashba spin-orbit coupling and out-of-plane magnetization, we find the torque is generally perpendicular to the photon angular momentum and is ascribed to an optical Edelstein effect.

Through analysis of variable-temperature neutron powder-diffraction data, we present solutions for the magnetic structures of ${\mathrm{SrMn}}_7{\mathrm{O}}_{12}$, ${\mathrm{CdMn}}_7{\mathrm{O}}_{12}$, and ${\mathrm{PbMn}}_7{\mathrm{O}}_{12}$ in all long-range ordered phases. The three compounds were found to have magnetic structures analogous to that reported for ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$. They all feature a higher-temperature lock-in phase with commensurate magneto-orbital coupling and a delocked multi-k magnetic ground state where incommensurate magneto-orbital coupling gives rise to a constant-moment magnetic helix with modulated spin helicity. ${\mathrm{CdMn}}_7{\mathrm{O}}_{12}$ represents a special case in which the orbital modulation is commensurate with the crystal lattice and involves stacking of fully and partially polarized orbital states. Our results provide a robust confirmation of the phenomenological model for magneto-orbital coupling previously presented for ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$. Furthermore, we show that the model is universal to the $A^{2+}$ quadruple perovskite manganites synthesized to date and that it is tunable by selection of the $A$-site ionic radius.

We consider two distant spin-$\frac{1}{2}$ particles (or qubits) and a number of interacting objects, all with the same value $S\gg 1$ of their respective spin, distributed on a one-dimensional lattice (or large-$S$ spin chain). The quantum states of the chain are constructed by linearly combining tensor products of single-spin coherent states, whose evolution is determined accordingly, i.e., via classical-like equations of motions. We show that the quantum superposition of the above product states resulting from a local interaction between the first qubit and one spin of the chain evolves so that the second qubit, after having itself interacted with another spin of the chain, can be entangled with the first qubit. Obtaining such an outcome does not imply imposing constraints on the length of the chain or the distance between the qubits, which demonstrates the possibility of generating quantum correlations at a distance by means of a macroscopic system, as far as local interactions with just a few of its components are feasible.

We study current-induced torques in ${\mathrm{WTe}}_2$/permalloy bilayers as a function of ${\mathrm{WTe}}_2$ thickness. We measure the torques using both second-harmonic Hall and spin-torque ferromagnetic resonance techniques for samples with ${\mathrm{WTe}}_2$ thicknesses that span from 16 nm down to a single monolayer. We confirm the existence of an out-of-plane antidamping torque, and we show directly that the sign of this torque component is reversed across a monolayer step in the ${\mathrm{WTe}}_2$. The magnitude of the out-of-plane antidamping torque depends only weakly on ${\mathrm{WTe}}_2$ thickness, such that even a single-monolayer ${\mathrm{WTe}}_2$ device provides a strong torque that is comparable to much thicker samples. In contrast, the out-of-plane fieldlike torque has a significant dependence on the ${\mathrm{WTe}}_2$ thickness. We demonstrate that this fieldlike component originates predominantly from the Oersted field, thereby correcting a previous inference drawn by our group based on a more limited set of samples.

We theoretically consider the observed soft gap in the proximity-induced superconducting state of semiconductor nanowires in the presence of spin-orbit coupling, Zeeman spin splitting, and tunneling leads, but in the absence of any extrinsic disorder (i.e., an ideal system). We critically consider the effects of three distinct intrinsic physical mechanisms (tunnel barrier to normal leads, temperature, and dissipation) on the phenomenology of the gap softness in the differential conductance spectroscopy of the normal-superconductor junction as a function of spin splitting and chemical potential. We find that all three mechanisms individually can produce a soft gap, leading to calculated conductance spectra qualitatively mimicking experimental results. We also show through extensive numerical simulations that the phenomenology of the soft gap is intrinsically tied to the broadening and the height of the Majorana zero-mode-induced differential conductance peak above the topological quantum phase transition point with both the soft gap and the quality of the Majorana zero mode being simultaneously affected by tunnel barrier, temperature, and dissipation. We establish that the Majorana zero-mode splitting oscillations can be suppressed by temperature or dissipation (in a similar manner) but not by the tunnel barrier. Since all three mechanisms (plus disorder, not considered in the current work) are likely to be present in any realistic nanowires, discerning the effects of various mechanisms is difficult, necessitating detailed experimental data as a function of all the system parameters, some of which (e.g., dissipation, chemical potential, tunnel barrier) may not be known experimentally. While the tunneling-induced soft-gap behavior is benign with no direct adverse effect on the Majorana topological properties with the zero-bias peak remaining quantized at $2e^2/h$, the soft gap induced by finite temperature and/or finite dissipation is detrimental to topological properties and must be avoided as much as possible.

The Bose-Einstein condensate of magnons (mBEC) that is formed at room temperature in parametrically pumped magnetic films is doubly degenerate: it is formed simultaneously in two spectral minima corresponding to the lowest-energy magnons propagating in opposite directions along the in-plane bias magnetic field. In this work the interactions of magnons in the mBEC are studied both theoretically and experimentally. It is shown by direct calculation that the magnons residing in each of the degenerate spectral minima of mBEC form a practically ideal magnon gas, as the attractive self-interaction between these magnons is very weak. At the same time, the interaction between the magnons residing in different spectral minima, corresponding to opposite directions of the magnon wave vector, is relatively strong and repulsive, leading to a repulsive total intermagnon interaction. By measuring the spectral characteristics of the mBEC it is shown that with increased magnon density the energy per magnon in the mBECs increases, thus confirming experimentally that the net intermagnon interaction in a doubly degenerate mBEC is repulsive.

Using a minimum model consisting of a magnetic quantum dot and an electron lead, we investigate spin pumping by its precessing magnetization. Focusing on the “adiabaticity,” which is quantified using a comparison between the frequency of precession and the relaxation rate of the relevant system, we investigate the role of nonadiabaticity in spin pumping by obtaining the dependence of the spin current generated on the frequency of precession using full counting statistics. This evaluation shows that the steady-state population of the quantum dot remains unchanged by the precession owing to the rotational symmetry about the axis of precession. This implies that in the adiabatic limit the spin current is absent and that spin pumping is entirely a nonadiabatic effect. We also find that the nonadiabatic spin current depends linearly on the frequency in the low-frequency regime and exhibits an oscillation in the high-frequency regime. The oscillation points to an enhancement of spin pumping by tuning the frequency of precession.

A tunable giant exchange bias effect is discovered in a family of bulk intermetallic compounds $\mathrm{YM}{\mathrm{n}}_{12-x}\mathrm{F}{\mathrm{e}}_x$. Experimental data demonstrate that the exchange bias effect originates from global interactions among ferromagnetic and antiferromagnetic sublattices but not the interfacial exchange coupling or inhomogeneous magnetic clusters. A giant exchange bias with a loop shift of up to 6.1 kOe has been observed in $\mathrm{YM}{\mathrm{n}}_{4.4}\mathrm{F}{\mathrm{e}}_{7.6}$ compound. In a narrow temperature range, the exchange bias field shows a sudden switching-off whereas the coercivity shows a sudden switching-on with increasing temperature. This unique feature indicates that the intersublattice exchange coupling is highly homogenous. Our theoretical calculations reveal this switching feature, which agrees very well with the experiments and provides insights into the physical underpinnings of the observed exchange bias and coercivity.

Previous studies have shown that bipartite Hubbard systems with inhomogeneous hopping amplitudes can exhibit higher pair-binding energies than the uniform model. Here we examine whether this result holds for systems with a more generic band structure. To this end, we use exact diagonalization and the density matrix renormalization-group method to study the $4\times 4$ Hubbard cluster and the two-leg Hubbard ladder with checkerboard-modulated nearest-neighbor hopping, $t$, and next-nearest-neighbor (diagonal) hopping, $t_d$. We find that the strongest pairing continues to occur at an intermediate level of inhomogeneity. While the maximal pair-binding energy is enhanced by a positive $t_d/t$, it is suppressed and appears at weaker repulsion strengths and lower hole concentrations when $t_d/t$ is negative. We point out a possible connection between the pairing maximum and the magnetic properties of the system.

The spin relaxation dynamics of ${\mathrm{Ce}}^{3+}$ ions in heavily cerium-doped YAG crystals is studied using pulse-electron paramagnetic resonance and time-resolved pump-probe Faraday rotation. Both techniques address the $4f$ ground state, while pump-probe Faraday rotation also provides access to the excited $5d$ state. We measure a millisecond spin-lattice relaxation time $T_1$, a microsecond spin coherence time $T_2$, and a $\sim 10$ ns inhomogeneous spin dephasing time $T_2^{*}$ for the ${\mathrm{Ce}}^{3+}$ ground state at low temperatures. The spin-lattice relaxation of ${\mathrm{Ce}}^{3+}$ ions is due to modified Raman processes involving the optical phonon mode at $\sim 125{\mathrm{cm}}^{-1}$. The relaxation at higher temperature goes through a first excited level of the ${}^2\mathrm{F}_{\frac{5}{2}}$ term at about $\hslash \omega \approx 228{\mathrm{cm}}^{-1}$. Effects provided by the hyperfine interaction of the ${\mathrm{Ce}}^{3+}$ with the ${}^{27}\mathrm{Al}$ nuclei are observed.