Using controlled ferromagnet (FM) -semiconductor (SC) interfaces in SC-based lateral spin-valve (LSV) devices, we experimentally study the effect of interfacial spin moments in FM–SC heterojunctions on spin transport in SC. First-principles calculations predict that the spin moment of FM–SC junctions can be artificially reduced by inserting $3d$ transition metal V, Cr, or Cu atomic layers between FM and SC. When all-epitaxial FM–SC Schottky-tunnel contacts with a $0.4–0.5$-nm-thick V, Cr, or Cu interfacial layer are formed, we find that the spin signals in FM–SC LSV devices are significantly decreased at 8 K. When we increase the interfacial spin moment by inserting an $\sim 0.3$-nm-thick Co layer between FM and SC, the spin signals at 8 K are significantly enhanced again. From these experiments, we conclude that the interfacial spin moments at FM–SC interfaces are one of the important factors to achieve large spin signals even in SC-based spintronic devices.

Higher-order topological insulators (HOTIs) have attracted much attention in photonics due to the tightly localized disorder-robust corner and hinge states. Here, we reveal an unconventional HOTI phase with vanishing dipole and quadrupole polarizations. This phase arises in the array of evanescently coupled waveguides hosting degenerate $s$- and $d$-type orbital modes arranged in a square lattice with four waveguides in the unit cell. As we prove, the degeneracy of the modes with the different symmetry gives rise to the nontrivial topological properties rendering the system topologically equivalent to the two copies of the anisotropic two-dimensional Su-Schrieffer-Heeger model rotated by ${90}^{\circ }$ with respect to each other and based on $s\pm d$ hybridized orbitals. We probe the unusual topology of the model by constructing a quantized Fu-Kane-type pseudospin pump. Our results introduce a route to tailor higher-order band topology leveraging both crystalline symmetries and accidental degeneracies of the different orbital modes.

Quantum dot spin valves are characterized by exchange fields which induce spin precession and generate current spin resonances even in the absence of spin splitting. Analogous effects have been studied in double quantum dots, in which the orbital degree of freedom, the pseudospin, replaces the spin in the valve configuration. We generalize, now, this setup to allow for arbitrary spin and orbital polarization of the leads, thus obtaining an even richer variety of current resonances, stemming from the precession dynamics of entangled spin and pseudospin. We observe for both vectors a delicate interplay of decoherence, pumping, and precession which can only be understood by also considering the dynamics of the spin-pseudospin correlators. The numerical results are obtained in the framework of a generalized master equation within the cotunneling approximation and are complemented by the analytics of a coherent sequential tunneling model.

Two-dimensional (2D) laser arrays are shown to be achievable at a large scale by exploiting the interplay of higher-order topological insulator (HOTI) physics and the so-called non-Hermitian skin effect (NHSE). The higher-order topology allows for the amplification and hence lasing of a single mode protected by a band gap, whereas the NHSE is introduced to compete with the topological localization of corner modes. By tuning the system parameters appropriately and pumping at only one site, a single topologically protected lasing mode delocalized across two dimensions emerges, with its power widely tunable by adjusting the pump strength. Computational studies clearly indicate that the lasing mode thus engineered is stable, and the phase difference between all lasing sites is locked at zero, even after disorder is accounted for. The total power of the designed laser array is proportional to the area of the 2D lattice accommodating a HOTI phase. Based on existing experiments, we further propose to use coupled optical ring resonators as a promising platform to realize large-scale 2D laser arrays.

In this work, we investigate the Anderson localization problems of the generalized Aubry-André model (Ganeshan-Pixley-Das Sarma's model) with an unbounded quasiperiodic potential where the parameter $\left|\alpha \right|\ge 1$. The Lyapunov exponent $\gamma \left(E\right)$ and the mobility edges $E_c$ are exactly obtained for the unbounded quasiperiodic potential. With the Lyapunov exponent, we find that there exists a critical region in the parameter $\lambda \text{−}E$ plane. The critical region consists of critical states. In comparison with localized and extended states, the fluctuation of spatial extensions of the critical states is much larger. The numerical results show that the scaling exponent of inverse participation ratio (IPR) of critical states $x\simeq 0.5$. Furthermore, it is found that the critical indices of localized length $\nu =1$ for the bounded ($\left|\alpha \right|<1$) case and $\nu =1/2$ for the unbounded ($\left|\alpha \right|\ge 1$) case. The above distinct critical indices can be used to distinguish the localized-extended from localized-critical transitions. At the end, we show that the systems with different $E$ for both cases of $\left|\alpha \right|<1$ and $\left|\alpha \right|\ge 1$ can be classified by the Lyapunov exponent $\gamma \left(E\right)$ and Avila's quantized acceleration $\omega \left(E\right)$.

We explore possibilities for control of magnons in two-dimensional heterostructures by an external electric field acting across a dielectric barrier. By performing ab initio calculations for a Fe monolayer and Fe bilayer, both suspended in vacuum and deposited on Cu(001), we demonstrate that external electric field can significantly modify magnon lifetimes and that these changes can be related to field-induced changes in layer-resolved Bloch spectral functions. For systems with more magnon dispersion branches, the gap between high- and low-energy eigenmodes varies with the external field. These effects are strongly influenced by the substrate. Considerable variability in how the magnon spectra are sensitive to the external electric field can be expected, depending on the substrate and on the thickness of the magnetic layer.

The noninteracting magnon gas description in ferromagnets breaks down at finite magnon density where momentum-conserving collisions between magnons become important. Here we present a hydrodynamic description of spin systems with global SU(2) symmetry in the ferromagnetic phase. We identify a key signature of the collision-dominated hydrodynamic regime—a magnon sound mode—which governs dynamics at low frequencies. The magnon sound mode is an excitation of the longitudinal spin component with frequencies below the spin-wave continuum in gapped ferromagnets and can be detected with recently introduced spin qubit magnetometers. We also show that, in the presence of exchange interactions with SU(2) symmetry, the ferromagnet hosts an usual hydrodynamic regime that lacks Galilean symmetry. We show that our results are relevant to ferromagnetic insulators in a finite energy/temperature window such that dipolar and magnon–phonon interactions are negligible, as well as in recent experiments in cold atomic gases.

Multiple-$Q$ magnetic orderings represent magnetic textures composed of superpositions of multiple spin density waves or spin spirals, as represented by two-dimensional skyrmion crystals and three-dimensional hedgehog lattices. Such magnetic orderings have been observed in various magnetic materials in recent years, and attracted enormous attention, especially from the viewpoint of topology and emergent electromagnetic fields originating from noncoplanar magnetic structures. Although they often exhibit successive phase transitions among different multiple-$Q$ states while changing temperature and an external magnetic field, it is not straightforward to elucidate the phase diagrams, mainly due to the lack of concise theoretical tools as well as appropriate microscopic models. Here, we provide a theoretical framework for a class of effective spin models with long-range magnetic interactions mediated by conduction electrons in magnetic metals. Our framework is based on the steepest descent method with a set of self-consistent equations that leads to exact solutions in the thermodynamic limit, and has many advantages over existing methods such as biased variational calculations and numerical Monte Carlo simulations. We develop two methods that complement each other in terms of the computational cost and the range of applications. As a demonstration, applying the framework to the models with instabilities toward triple- and hextuple-$Q$ magnetic orderings, we clarify the magnetic field–temperature phase diagrams with a variety of multiple-$Q$ phases. We find that the models exhibit interesting reentrant phase transitions where the multiple-$Q$ phases appear only at finite temperature and/or nonzero magnetic field. Furthermore, we show that the multiple-$Q$ states can be topologically nontrivial stacked skyrmion crystals or hedgehog lattices, which exhibit large net spin scalar chirality associated with nonzero skyrmion number. The results demonstrate that our framework could be a versatile tool for studying magnetic and topological phase transitions and related quantum phenomena in actual magnetic metals hosting multiple-$Q$ magnetic orderings.

Laser irradiation, as a versatile tool to tune the topological properties of electronic systems, is under intensive study. Experimentally, a laser irradiation induced anomalous Hall effect in graphene has been observed [J. W. McIver et al., Nat. Phys. 16, 38 (2020)]. Disorder is ubiquitous in real materials, and it has been shown that diagonal disorders, i.e., on-site disorder, can enhance the topological properties of time-periodically driven quantum materials [P. Titum et al., Phys. Rev. Lett. 114, 056801 (2015)]. Here, we investigate circularly polarized laser irradiated graphene with nondiagonal disorders, i.e., disordered tunneling, and find that disorder can induce nontrivial topological properties, characterized by a Bott index and the real-space Chern number. Moreover, we show that one can turn on the laser irradiation nonadiabatically to drive the disordered graphene into a nontrivial topological phase. It is a scheme which is especially interesting for experimental implementations.

Spin transport is crucial for future spintronic devices operating at bandwidths up to the terahertz range. In F|N thin-film stacks made of a ferromagnetic/ferrimagnetic layer $F$ and a normal-metal layer $N$, spin transport is mediated by (1) spin-polarized conduction electrons and/or (2) torque between electron spins. To identify a crossover from (1) to (2), we study laser-driven spin currents in $F$|Pt stacks where $F$ consists of model materials with different degrees of electrical conductivity. For the magnetic insulators yttrium iron garnet, gadolinium iron garnet (GIG) and $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, identical dynamics is observed. It arises from the terahertz interfacial spin Seebeck effect (SSE), is fully determined by the relaxation of the electrons in the metal layer, and provides a rough estimate of the spin-mixing conductance of the GIG/Pt and $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3/\mathrm{Pt}$ interfaces. Remarkably, in the half-metallic ferrimagnet ${\mathrm{Fe}}_3{\mathrm{O}}_4$ (magnetite), our measurements reveal two spin-current components with opposite direction. The slower, positive component exhibits SSE dynamics and is assigned to torque-type magnon excitation of the A- and B-spin sublattices of ${\mathrm{Fe}}_3{\mathrm{O}}_4$. The faster, negative component arises from the pyrospintronic effect and can consistently be assigned to ultrafast demagnetization of minority-spin hopping electrons. This observation supports the magneto-electronic model of ${\mathrm{Fe}}_3{\mathrm{O}}_4$. In general, our results provide a route to the contact-free separation of torque- and conduction-electron-mediated spin currents.

The celebrated Wiedemann-Franz (WF) law, which governs the relation between charge and heat transport traces back to the experimental discovery in 1853 by Wiedemann and Franz. Despite the fundamental difference of the quantum-statistical properties between fermions and bosons, the linear-in-$T$ behavior of the WF law at low temperatures has recently been found to be the universal property by the discovery of the WF law for magnon transport. However, the WF law is for the linear response, and whether or not the universal law is valid even in the nonlinear regime of Bose systems remains an open issue. Here we provide a solution to this fundamental challenge. We show that the ratio of the thermal to spin transport coefficient of magnons in topologically trivial insulating magnets exhibits a different behavior from the linear response and the universal law breaks down in the strong nonlinear regime. This finding is within experimental reach with current device and measurement technologies. Our discovery is the key ingredient in magnon-based spintronics, in the evaluation of the figure of merit for thermomagnetic conversion elements of spintronics devices.

The discovery of an intrinsic antiferromagnetic topological insulator (AFMTI) in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ has attracted intense attention, most of which lies in its electrical properties. In this paper, we report electronic, thermal, and thermoelectric transport studies of this newly found AFMTI. The temperature and magnetic field dependence of its resistivity, thermal conductivity, and Seebeck coefficient indicate strong coupling between charge, lattice, and spin degrees of freedom in this system. Furthermore, ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ exhibits a large anomalous Nernst signal, which is associated with nonzero Berry curvature of the field-induced canted antiferromagnetic state.

In this paper, we theoretically show that in a helical crystal, a current is induced by chiral phonons representing the microscopic local rotation of atoms. By treating the rotational motion as a perturbation, we calculate the time-dependent current by using the adiabatic Berry phase method. The time average of the current along the helical axis becomes finite in the metallic phase but it vanishes in the insulating phase. On the other hand, the current in the hexagonal plane changes with time, but its time average vanishes due to the threefold rotation space-time symmetry. We show that the time evolutions of the current follow the space-time symmetries of the helical systems. Moreover, we explain the reason for the vanishing of the time average of the current in the insulating phase from the aspect of the Chern number in the parameter space.

Computing superconducting properties starting from an exactly solvable model for a doped Mott insulator stands as a grand challenge. We have recently shown that this can be done starting from the Hatsugai-Kohmoto (HK) model, which can be understood generally as the minimal model that breaks the nonlocal ${\mathbb{Z}}_2$ symmetry of a Fermi liquid, thereby constituting a new quartic fixed point for Mott physics [Phillips et al., Nat. Phys. 16, 1175 (2020); Huang et al., Nat. Phys. (2022)]. In the current paper, we compute the thermodynamics, condensation energy, and electronic properties such as the NMR relaxation rate $1/T_1$ and ultrasonic attenuation rate. Key differences arise with the standard BCS analysis from a Fermi liquid: (1) the free energy exhibits a local minimum at $T_p$ where the pairing gap turns on discontinuously above a critical value of the repulsive HK interaction, thereby indicating a first-order transition; (2) a tricritical point emerges, thereby demarcating the boundary between the standard second-order superconducting transition and the novel first-order regime; (3) Mottness changes the sign of the quartic coefficient in the Landau-Ginzburg free-energy functional relative to that in BCS; (4) as this obtains in the strongly interacting regime, it is Mott physics that underlies the generic first-order transition; (5) the condensation energy exceeds that in BCS theory suggesting that multiple Mott bands might be a way of enhancing superconducting; (6) the heat-capacity jump is nonuniversal and increases with the Mott scale; (7) Mottness destroys the Hebel-Slichter peak in NMR; and (8) Mottness enhances the fall-off of the ultrasonic attenuation at the pairing temperature $T_p$. As several of these properties are observed in the cuprates, our analysis here points a way forward in computing superconducting properties of strongly correlated electron matter.

We report the synthesis and characterization of phase pure ${\mathrm{Ta}}_3\mathrm{Sb}$, a material predicted to be topological with eightfold degenerate fermionic states [Bradlyn et al., Science 353, aaf5037 (2016)] and to exhibit a large spin Hall effect [Derunova et al., Sci. Adv. 5, eaav8575 (2019)]. We observe superconductivity in ${\mathrm{Ta}}_3\mathrm{Sb}$ with $T_c\sim 0.67$ K in both electrical resistivity $\rho \left(T\right)$ and specific heat $c\left(T\right)$ measurements. Field-dependent measurements yield the superconducting phase diagram with an upper critical field of $H_{c2}\left(0\right)\sim$ 0.95 T, corresponding to a superconducting coherence length of $\xi \approx 18.6\mathrm{nm}$. The gap ratio deduced from specific heat anomaly $2{\mathrm{\Delta }}_0/k_B{T}_c$ is 3.46, a value close to the Bardeen-Cooper-Schrieffer value of 3.53. From a detailed analysis of both the transport and thermodynamic data within the Ginsburg-Landau (GL) framework, a GL parameter of $\kappa \approx 90$ is obtained, identifying ${\mathrm{Ta}}_3\mathrm{Sb}$ as an extreme type-II superconductor. The observation of superconductivity in an eightfold degenerate fermionic compound with topological surface states and predicted large spin Hall conductance positions ${\mathrm{Ta}}_3\mathrm{Sb}$ as an appealing platform to further explore exotic quantum states in multifold degenerate systems.

We investigate the formation of quasisteady states in one-dimensional pumps of interacting fermions at noninteger filling fraction, in the regime where the driving frequency and the interaction strength are small compared to the instantaneous single-particle band gap throughout the driving cycle. The system rapidly absorbs energy from the driving field and approaches a quasisteady state that locally resembles a maximal entropy state subject to the constraint of a fixed particle number in each of the system's single-particle Floquet bands. We explore the nature of this quasisteady state through one-body observables including the pumped current and natural orbital occupations, as well as the (many-body) entanglement spectrum and entropy. Potential disorder significantly reduces the amplitude of fluctuations of the quasisteady-state current around its universal value, while the lifetime of the quasisteady state remains nearly unaffected for disorder strengths up to the scale of the single-particle band gap. Interestingly, the natural orbital occupations and the entanglement entropy display patterns signifying the periodic entangling and disentangling of the system's degrees of freedom over each driving cycle. Moreover, prominent features in the system's time-dependent entanglement spectrum reveal the emergence of long timescales associated with the equilibration of many-particle correlations.

Coupling ordinary metals with topological ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ can instigate the long-range migration of the spin-polarized Dirac states. Instead, for trivial metallic $\mathrm{V}{\mathrm{Se}}_2$ films on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, topological and trivial Rashba-type interface states emerge, each strongly localized at the $\mathrm{V}{\mathrm{Se}}_2/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ interface. Their rapidly decaying spectral weights are uncovered by thickness-dependent band mappings of $\mathrm{V}{\mathrm{Se}}_2$ films and replicated by a phenomenological first-principles model of the spectral function. The results are pertinent to spintronic devices requiring spin transmission across metal/topological insulator interfaces.

The minimal ferrimagnetism by Lieb's theorem emerges on the T-shaped bipartite lattice composed of four sites, which can be realized experimentally, just as Nagaoka ferromagnetism has been demonstrated experimentally using a quartet quantum dot [J. P. Dehollain et al., Nature (London) 579, 528 (2020)]. In this paper, the Kondo effect on this ferrimagnetism is theoretically studied. The magnetic moment $S=1$ is screened in two steps by the Kondo effect and the series conductance $g_s$ is strongly suppressed to $g_s\simeq 0$, while the parallel conductance $g_p$ has the maximum value $g_p\simeq 4e^2/h$. The robustness of these properties against a parameter change toward reducing the Lieb's ferrimagnetism is also discussed, showing the scenarios for entanglement of the degrees of freedom toward the ground state.