The interface between a ferro- or ferrimagnetic insulator and a normal metal can support spin currents polarized collinear with and perpendicular to the magnetization direction. The flow of angular momentum perpendicular to the magnetization direction (“transverse” spin current) takes place via spin torque and spin pumping. The flow of angular momentum collinear with the magnetization (“longitudinal” spin current) requires the excitation of magnons. In this article we extend the existing theory of longitudinal spin transport [Bender and Tserkovnyak, Phys. Rev. B 91, 140402(R) (2015)] in the zero-frequency weak-coupling limit in two directions: We calculate the longitudinal spin conductance nonperturbatively (but in the low-frequency limit) and at finite frequency (but in the limit of low interface transparency). For the paradigmatic spintronic material system $\mathrm{YIG}|\mathrm{Pt}$, we find that nonperturbative effects lead to a longitudinal spin conductance that is ca. 40% smaller than the perturbative limit, whereas finite-frequency corrections are relevant at low temperatures $\lesssim 100\mathrm{K}$ only, when only few magnon modes are thermally occupied.

The Pr-rich end $0\le x\le 0.25$ of the alloy series ${\mathrm{Pr}}_{1-x}{\mathrm{Nd}}_x{\mathrm{Os}}_4{\mathrm{Sb}}_{12}$ has been studied using muon spin rotation and relaxation. The end compound ${\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}$ is an unconventional heavy-fermion superconductor, which exhibits a spontaneous magnetic field associated with broken time-reversal symmetry (TRS) in the superconducting phase. No such field is observed in the Nd-doped alloys for $x\ge 0.05$, indicating that TRS has been restored. The superfluid density from the vortex-lattice field distribution is insensitive to Nd concentration for $x\lesssim 0.2$, indicating that TRS restoration does not have a strong effect on the superconducting state. No ${\mathrm{Nd}}^{3+}$ static magnetism, ordered or disordered, is found down to the lowest temperatures of measurement.

The topological three-dimensional Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ has drawn great attention for the novel physics and promising applications in optoelectronic devices operating in the infrared and terahertz (THz) regimes. Among the extensive studies in the past decades, one intriguing debate is the underlined mechanism governing the nonequilibrium carrier dynamics following photoexcitation. In this study, the temperature-dependent photocarrier dynamics in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ film has been investigated with time-resolved terahertz spectroscopy. The experimental results demonstrate that photoexcitation results in abrupt increase in THz photoconductivity, and the subsequent relaxation shows a single exponential relaxation for various temperatures and pump fluences. The relaxation time increases from 4.7 ps at 5 K to 7.5 ps at 220 K, while the lifetime remains almost constant at $\sim 7.5$ ps with temperature above 220 K. A Rothwarf-Taylor model was employed to fit the temperature-dependent relaxation time, and a narrow energy gap of $\sim 35\pm 6$ meV is obtained, which occurs around the Dirac node. Our THz spectroscopy results demonstrate that the photocarrier relaxation in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ shows a semiconductorlike behavior, rather than hot-carrier scatterings in graphene and most of metals.

We present experimental and theoretical investigations for the cobalt thiospinel ${\mathrm{Co}}_3{\mathrm{S}}_{4-\delta }$. High-quality samples of ${\mathrm{Co}}_3{\mathrm{S}}_{4-\delta }$ $(\delta \sim 0.1)$ were prepared so as to measure their intrinsic properties. The measurements of magnetic susceptibility, specific heat, and electrical transport consistently indicate an antiferromagnetic transition at $\sim 60$ K, which is attributed to long-range magnetic ordering in the $\mathrm{Co}\left(A\right)$ ($A$-site Co ions) diamond sublattice. In addition, there exists a short-range magnetic ordering at around 120 K associated with the magnetic frustrations. The high-temperature magnetic susceptibility obeys Curie-Weiss law, from which a small effective magnetic moment of $1.0{\mu }_{\mathrm{B}}$ per formula unit is yielded. The main experimental results can be interpreted by the density-functional-theory calculations with a Hubbard $U$ correction of $\sim 0.5$ eV. Combining the experimental measurements with the theoretical calculations, we conclude that ${\mathrm{Co}}_3{\mathrm{S}}_4$ represents a rare example of itinerant-electron diamond-lattice antiferromagnet with moderate electron correlations.

We study the collective excitations of fractional quantum Hall states in graphene. We focus on states which allow for chiral symmetry breaking orders, specifically antiferromagnetism and charge density wave order. We investigate numerically how the collective excitation spectra depend on filling and the flux attachment scheme for two classes of variational states, the Töke-Jain sequence and the Modak-Mandal-Sengupta sequence.

We investigate the twist-angle dependence of spin-orbit coupling proximity effects and charge-to-spin conversion (CSC) in $\mathrm{graphene}/{\text{WSe}}_2$ heterostructures from first principles. The CSC is shown to strongly depend on the twist angle, with standard Rashba-Edelstein and disorder-free spin Hall efficiencies optimized near ${30}^{\circ }$ twisting. Symmetry breaking due to twisting also gives rise to an unconventional Rashba-Edelstein effect, with electrically generated nonequilibrium spin densities possessing spins collinear to the applied electric field. Our work provides a perspective on the electrical generation of spins in van der Waals heterostructures.

We reveal a topological burning glass effect, where the local response of a system exhibits a topological quantization that is enhanced by an integer due to its environmental coupling. As a paradigmatic platform for this intriguing phenomenon, we study a central spin that is quasiperiodically driven by two incommensurate frequencies, and statically coupled to $N-1$ surrounding spins. In the strong-coupling regime, the adiabatic dynamics of the total system is readily understood to imprint on the central spin an $N$-fold enhanced topological frequency conversion between the two driving frequencies. We argue that the topological burning glass effect is induced by the nonunitary dynamics of the central spin, which locally involves the collective motion of the surrounding spins. Our results are derived in the framework of adiabatic perturbation theory and fully corroborated by exact numerical simulations.

A mutual interplay between the charge and spin degrees of freedom in itinerant magnets leads to a plethora of topological spin textures, such as magnetic skyrmion and vortex crystals, in both centrosymmetric and noncentrosymmetric hosts. Meanwhile, their stabilization has been extensively studied in a system including classical localized spins. We here study a realization of the skyrmion crystal in a centrosymmetric triangular-lattice Hubbard model, where the itinerant nature of electrons plays a more significant role. By performing self-consistent mean-field calculations, we find that two types of skyrmion crystals with spatially nonuniform charge modulations appear in the ground state at zero magnetic field. Moreover, we obtain another noncoplanar vortex crystal phase without a net scalar chirality in the vicinity of the skyrmion crystal phase. We show that the latter vortex crystal exhibits a topological phase transition to a different skyrmion phase in an applied magnetic field. Our results provide a possibility of skyrmion and vortex crystals in itinerant magnets without localized moments.

Finite temperature calculations, based on ab initio molecular dynamics (AIMD) simulations, are a powerful tool able to predict material properties that cannot be deduced from ground state calculations. However, the high computational cost of AIMD limits its applicability for large or complex systems. To circumvent this limitation we introduce a method named machine learning assisted canonical sampling, which accelerates the sampling of the Born-Oppenheimer potential surface in the canonical ensemble. Based on a self-consistent variational procedure, the method iteratively trains a machine learning interatomic potential to generate configurations that approximate the canonical distribution of positions associated with the ab initio potential energy. By proving the reliability of the method on anharmonic systems, we show that the method is able to reproduce the results of AIMD with an ab initio accuracy at a fraction of its computational cost.

Ferrimagnetism in orthorhombic ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ below its Néel temperature, $T_{FN}\sim 76\mathrm{K}$ is reported to result from two inequivalent ${\mathrm{Ni}}^{2+}$ ions having different magnetic moments. However, a clear understanding of the temperature variation of its magnetization $\left[M\right(T$)] for $T>T_{FN}$ and $T<T_{FN}$ in terms of a single set of exchange parameters is still lacking. In this work, experimental results obtained from a detailed analysis of the temperature and magnetic field dependence of magnetization $\left[M\right(T,H)$], ac-magnetic susceptibility [${\chi }_{\text{ac}}(f,T,H$)], and heat-capacity [$C_P(T,H)$] measurements are combined with theoretical analysis to provide new insights into the nature of ferrimagnetism in ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. X-ray diffraction/Rietveld analysis of the prepared sample yielded the structural parameters of the orthorhombic crystal in agreement with previous studies, whereas x-ray photoelectron spectroscopy confirmed the ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Nb}}^{5+}$ electronic states in ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. Analysis of ${\chi }_{\text{ac}}\left(T\right)$ shows the paramagnetic-to-ferrimagnetic transition occurs at 76.5 K ($T_{FN}$), which increases with applied field $H$ as $T_{FN}\propto H^{0.35}$ due to the coupling of the ferromagnetic component with $H$. For $T>T_{FN}$, the ${\chi }_{dc}$ versus $T$ data are fitted to the Néel's expression for ferrimagnets, yielding the $g$-factors for the two ${\mathrm{Ni}}^{2+}$ ions as $g_A=2.47$ and $g_B=2.10$. Also, the antiferromagnetic molecular field constants between the $A$ and $B$ sublattices were evaluated as $N_{AA}=26.31, N_{BB}=8.59$, and $N_{AB}=43.06,$ which, in turn, yield the antiferromagnetic exchange parameters: $J_{AA}/{\mathrm{k}}_B=4.27$ K, $J_{BB}/{\mathrm{k}}_B=1.40$ K, and $J_{AB}/{\mathrm{k}}_B=6.98$ K. For $T<T_{FN}$, the $M$ versus $T$ data clearly show the magnetic compensation point at $T_{\text{COM}}\sim 33$ K. The mathematical model presented here using the magnitudes of $N_{AA}, N_{BB}$, and $N_{AB}$ correctly predicts the position of $T_{\text{COM}}$ as well the temperature variation of $M$ both above and below $T_{\text{COM}}$. The data of $C_P(T$) versus $T$ shows a $\lambda$-type anomaly across $T_{FN}$. After subtracting the lattice contribution, the $C_P(T$) data are fitted to $C_P=A{(T-T_N)}^{(-\alpha )}$ yielding the critical exponent $\alpha =0.14\left(0.12\right)$ for $T<T_{FN}(T>T_{FN}),$ which is a characteristic of second-order phase transition. Magnetic entropy changes determined from the $M\text{−}H$ isotherms shows that the applied field $H$ enhances the magnetic ordering for $T>T_{FN}$ and $T<T_{\text{COM}}$, but for $T_{COM}<T<T_{FN}$, the spin disorder increases with the increase in $H$. The temperature variation of the measured coercivity $H_C(T$) and remanence $M_R(T$) from 1.9 K to $T_{FN}$ initially show a decreasing trend, becoming zero at $T_{\text{COM}}$, then followed by an increase and eventually becoming zero again at $T_{FN}$.

The crystal structure, thermal properties, and magnetic state of the rare-earth intermetallic compound ${\mathrm{Ho}}_7{\mathrm{Rh}}_3$ was studied using the measurements of the AC and DC magnetic susceptibility, magnetization, specific heat and thermal expansion, as well as by neutron and synchrotron powder diffraction. Below the Néel temperature $T_{\mathrm{N}}=32$ K, an incommensurate antiferromagnetic (AFM) structure of the spin density wave (SDW) type was observed, described by the magnetic superspace group $Cmc{2}_{1.}{1}^{\prime }\left(00g\right)0sss$ and the propagation vector ${\mathbf{k}}_{\mathrm{IC}}=\left(000.389\right)$. Further cooling leads to the spin reorientation transition and squaring-up transformation of the SDW magnetic structure below $T_{t1}=22$ K. The spin reorientation transition is accompanied by the emergence of a ferromagnetic component in the incommensurate magnetic structure upon cooling below $T_{t2}=9$ K. Using neutron diffraction, the existence of a short-range antiferromagnetic order was revealed up to temperatures twice as high as $T_{\mathrm{N}}$. These short-range AFM correlations were found to affect the magnetic susceptibility and thermal expansion, while the crystal structure retains its hexagonal symmetry below and above the Néel temperature.

The recent prediction of a metastable high-symmetry $Fm\overline{3}m$ phase of ${\mathrm{LaBH}}_8$ gives hopes to reach high superconducting critical temperatures at affordable pressures among ternary hydrogen-rich compounds. Making use of first-principles calculations within density functional theory and the stochastic self-consistent harmonic approximation, we determine that ionic quantum fluctuations drive the system dynamically unstable below 77 GPa, a much higher pressure than the 45 GPa expected classically. Quantum anharmonic effects stretch the covalent B-H bond in the ${\mathrm{BH}}_8$ units of the structure, and consequently, soften all hydrogen-character modes. Above 77 GPa $Fm\overline{3}m{\mathrm{LaBH}}_8$ remains metastable, and interestingly, its superconducting critical temperature is largely enhanced by quantum anharmonic effects, reaching critical temperatures around 160 K at the verge of the dynamical instability. Our results suggest that low-pressure metastable phases with covalently bonded symmetric ${\mathrm{XH}}_8$ units will be destabilized by ionic quantum fluctuations.

Monolayer transition metal dichalcogenides (TMDs) host deeply bound excitons interacting with itinerant electrons, and as such they represent an exciting new quantum many-body Bose-Fermi mixture. Here, we demonstrate that electrons interacting with a Bose-Einstein condensate (BEC) of exciton-polaritons can realize a two-dimensional topological $p_x+i{p}_y$ superconductor. Using strong coupling Eliashberg theory, we show that this is caused by an attractive interaction mediated by the BEC, which overcompensates the repulsive Coulomb interaction between the electrons. The hybrid light-matter nature of the BEC is crucial for achieving this, since it can be used to reduce retardation effects and increase the mediated interaction in regimes important for pairing. We finally show how the great flexibility of TMDs allows one to tune the critical temperature of the topological superconducting phase to be within experimental reach.

Motivated by recent advances in modeling pseudo-Hermitian Hamiltonian (pHH) systems using superconducting qubits, we analyze their quantum dynamics subject to a small time-dependent perturbation. In particular, we develop the linear response theory formulation suitable for application to various pHH systems and compare it to the ones available in the literature. We derive analytical expressions for the generalized temporal quantum-mechanical correlation function $C\left(t\right)$ and the time-dependent dynamic susceptibility $\chi \left(t\right)\propto \text{Im}C\left(t\right)$. We apply our results to two $\mathit{PT}$-symmetric non-Hermitian quantum systems: a single qubit and two unbiased/biased qubits coupled by the exchange interaction. For both systems, we obtain the eigenvalues and eigenfunctions of the Hamiltonian and identify $\mathit{PT}$-symmetry unbroken and broken quantum phases and quantum phase transitions between them. The temporal oscillations of the dynamic susceptibility of the qubits polarization ($z$ projection of the total spin), $\chi \left(t\right)$, relate to $\mathit{ac}$-induced transitions between different eigenstates and we analyze the dependencies of the oscillation frequency and the amplitude on the gain/loss parameter $\gamma$ and the interaction strength $g$. Studying the time dependence of $\chi \left(t\right)$, we observe different types of oscillations, i.e., undamped, heavily damped, and amplified ones, related to the transitions between eigenstates with broken (unbroken) $PT$ symmetry. These predictions can be verified in the microwave transmission experiments allowing controlled simulation of the pHH systems.

A detailed investigation of ${\mathrm{BaNi}}_{\text{2}}{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_{\text{2}}$ single crystals using high-resolution thermal-expansion, heat-capacity, Young's-modulus, and resistivity measurements is presented. The phase diagram of ${\mathrm{BaNi}}_{\text{2}}{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_{\text{2}}$ is shown to be much richer than suggested by the original data of Kudo et al. [Phys. Rev. Lett. 109, 097002 (2012)]. The transition to the commensurate charge density wave (C-CDW) is always preceded by a fourfold symmetry-breaking transition associated with the long-range ordering of a strongly fluctuating unidirectional incommensurate charge density wave (I-CDW). Significant precursors above the I-CDW and C-CDW transitions are seen in the thermal expansion and resistivity and are particularly evident in the temperature dependence of the $c/a$ ratio of the lattice parameters. Heat-capacity measurements of the crystals with a higher P content and a higher critical temperature of 3.2 K uncover a Debye-like behavior of a soft-phonon mode with a very low ${\mathrm{\Theta }}_{\text{Debye}}$ of roughly 50 K. Associated with this soft phonon are unusually large thermal-expansion anomalies, resulting in logarithmically diverging uniaxial phonon Grüneisen parameters. Young's-modulus data of these higher-$T_c$ crystals exhibit a significant softening in both $B_{1g}$ and $B_{2g}$ channels, which is argued to be incompatible with nematic criticality and is rather associated with a broad phase transition to an hitherto unknown structure. Possible origins of the increase in the superconducting critical temperature with P substitution are discussed.

Competition between the double-exchange hopping and Coulomb repulsion underlies fertile phenomena in mixed-valent manganites, such as colossal magnetoresistance associated with a magnetic-field-driven insulator-metal transition (IMT). In contrast, an analogous double-exchange system, perovskite cobaltites, however, shows much smaller magnetoresistance, and the insulator-to-metal phase evolution with field stimulus remains elusive. Here, we unveil a submicrometer-scale phase separation and magnetic-field-controlled crossoverlike IMT in a tensile-strained ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Co}{\mathrm{O}}_3$ film. Our transport, magnetization, and magnetic-force-microscopy measurements reveal that, although the IMT is barely induced under an isothermal field sweep, it emerges under field cooling through a thermally activated ferromagnetic domain percolation. Such thermodynamic-history-dependent properties signify a nonergodic feature associated with the microscopic phase separation. By further comparing the transport properties among ${\mathrm{La}}_{0.7}\mathrm{A}{\mathrm{E}}_{0.3}\mathrm{Co}{\mathrm{O}}_3$ films with AE from Ca, Sr, to Ba, we reveal that the nonergodic behavior prevails in these films and that the mixed spin states are a key factor to enhance the energy barrier for domain-wall motions during domain percolation. Such a spin-state degree of freedom is absent in manganites, probably resulting in the large difference of the phase evolution kinetics in the magnetic-field-induced IMT between cobaltites and manganites.