We theoretically investigate laser-induced spin transport in metallic magnetic heterostructures using an effective spin-transport description that treats itinerant electrons and thermal magnons on an equal footing. Electron-magnon scattering is included and taken as the driving force for ultrafast demagnetization. We assume that in the low-fluence limit, the magnon system remains in a quasiequilibrium, allowing a transient nonzero magnon chemical potential. In combination with the diffusive transport equations for the itinerant electrons, the description is used to chart the full spin dynamics within the heterostructure. In agreement with recent experiments, we find that in the case the spin-current-receiving material includes an efficient spin dissipation channel, the interfacial spin current becomes directly proportional to the temporal derivative of the magnetization. Based on an analytical calculation, we discuss that other relations between the spin current and magnetization may arise in the case the spin-current-receiving material displays inefficient spin-flip scattering. Finally, we discuss the role of (interfacial) magnon transport and show that, a priori, it cannot be neglected. However, its significance strongly depends on the system parameters.

The discovery of van der Waals (vdW) ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ provides a platform for investigating the quantum anomalous Hall effect (QAHE), which is a candidate for quantum computation without dissipation. However, because of the topological requirement, the High Chern number QAHE with extended dissipation-free channels is hard to achieve in atomically thin samples, which can take full advantage of two-dimensional (2D) materials. In this work, by first-principles calculations, the variable stacking order (inherent to vdW materials) was proposed as a means of regulating the Chern number of the QAHE in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ (MBT). The interlayer stacking order in the 2D limit dominates the orbital hybridization, resulting in a tunable topological property. Furthermore, the High Chern number QAHE with $C=N$ was achieved in ultrathin MBT ($2N$ septuple layers) with alternate stacking orders, which have different topological states. This scenario was extended to the vdW heterostructure of MBT-family materials for achieving a stable, zero-field, High Chern number QAHE. Our work reveals stacking tunable topological properties in vdW magnetic topological insulators and provides an effective means of regulating the QAHE.

Starting with general semiclassical equations of motion for electrons in the presence of electric and magnetic fields, we extend the Chambers formula to include, in addition to a magnetic field, time-dependent electric fields, and bands with Berry curvature. We thereby compute the conductivity tensor ${\sigma }_{\alpha \beta }(B,\omega )$ in the presence of magnetic field for bands in two (2D) and three (3D) dimensions with Berry curvature. We focus then on several applications to magnetotransport for metals with Fermi surface topological transitions in 2D. In particular, we consider a rectangular lattice and a model related to overdoped graphene, to investigate the signatures of different types of Fermi surface topological transitions in metals in the Hall coefficient, Hall conductivity ${\sigma }_{xy}$, and longitudinal conductivity ${\sigma }_{xx}$. The behavior of those quantities as a function of frequency, when the electric field is time dependent, is also investigated. As an example of nonzero Berry curvature, we study the magnetotransport of the Haldane model within this context. In addition, we provide the linear and nonlinear electric current formula to order $E^2$.

We examine how dissipation and gate timing errors affect the fidelity of a sequence of swap gates on a chain of interacting qubits in comparison to noise in the interqubit interaction. Although interqubit interaction noise and gate timing errors are always present in any qubit platform, dissipation is a special case that can arise in multivalley semiconductor spin qubit systems, such as Si-based qubits, where dissipation may be used as a general model for valley leakage. In our Hamiltonian, each qubit is coupled via Heisenberg exchange to every other qubit in the chain, with the strength of the exchange interaction decreasing exponentially with distance between the qubits. Dissipation is modeled through the term $-i\gamma \mathbf{1}$ in the Hamiltonian, and $\gamma$ is chosen so as to be consistent with the experimentally observed intervalley tunneling in Si. We show that randomness in the dissipation parameter should have little to no effect on the swap gate fidelity in the currently fabricated Si circuits. We introduce quasistatic noise in the interqubit interaction and random gate timing error and average the fidelities over 10 000 realizations for each set of parameters. The fidelities are then plotted against $J_{\mathrm{SWAP}}$, the strength of the exchange coupling corresponding to the swap gate. We find that dissipation decreases the fidelity of the swap operation—though the effect is small compared to that of the known noise in the interqubit interaction—and that gate timing error creates an effective optimal value of $J_{\mathrm{SWAP}}$, beyond which infidelity begins to increase.

Several recent studies have shown that the anisotropy in the magnetic structure of ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ plays a significant role in stabilizing the Weyl nodes. To investigate the relationship between magnetic anisotropy and Weyl physics, we present a comparative study between ${\mathrm{EuZn}}_2{\mathrm{As}}_2$ and ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ that are isostructural but with different magnetic anisotropy. We performed structural analysis, electronic transport, and magnetization experiments on millimeter-sized single crystals of ${\mathrm{EuZn}}_2{\mathrm{As}}_2$, and compared the results to those of ${\mathrm{EuCd}}_2{\mathrm{As}}_2$. By combining the first principle calculations and neutron diffraction experiment, we identify the magnetic ground state of ${\mathrm{EuZn}}_2{\mathrm{As}}_2$ as A-type antiferromagnetic order with a transition temperature ($T_{\mathrm{N}}$ = 19.6 K) twice that of ${\mathrm{EuCd}}_2{\mathrm{As}}_2$. Like ${\mathrm{EuCd}}_2{\mathrm{As}}_2$, the negative magnetoresistance of ${\mathrm{EuZn}}_2{\mathrm{As}}_2$ is observed after suppressing the resistivity peak at $T_{\mathrm{N}}$ with increasing fields. However, the anisotropy in both transport and magnetization are much reduced in ${\mathrm{EuZn}}_2{\mathrm{As}}_2$. The difference could be ascribed to the weaker spin-orbit coupling, more localized $d$ orbitals, and a larger contribution from the Eu $s$ orbitals in the zinc compound, as suggested by the electronic band calculations. The same band structure effect could be also responsible for the observation of a smaller nonlinear anomalous Hall effect in ${\mathrm{EuZn}}_2{\mathrm{As}}_2$ compared to ${\mathrm{EuCd}}_2{\mathrm{As}}_2$.

We numerically investigate the ground state of the extended $t\text{−}J$ Hamiltonian with periodic local modulations in one dimension by using the density-matrix renormalization group method. Examining charge and spin excitation gaps, as well as the pair binding energy, with extrapolated results to the thermodynamic limit, we obtain a rich ground-state phase diagram consisting of the metallic state, the superconducting state, the phase separation, and insulating states at commensurate fillings. Compared to the homogeneous one-dimensional (1D) $t\text{−}J$ model, the superconductivity is greatly enhanced and stabilized by the flat-band structure. This superconducting state in periodically modulated chains shares similar properties with ladder systems: significant negative pair binding energy occurs, and the singlet-pairing correlation function dominates with the algebraic decay while the single-particle Green's function and spin correlation function decay exponentially. On the other hand, the superlattice structure leads to nontrivial topological nature in insulating states, characterized by different integer Chern numbers at different fillings. Due to the interplay among the topology, the interaction, and the 1D confinement, gapless edge modes show strong spin-charge separation and in different regions can relate to different collective modes, which are the charge of a single fermion, the magnon, and the singlet pair. We also find two interaction-driven topological transitions: (i) at particle filling $\rho =\frac{1}{2}$, the low-energy edge excitations change from the magnon to singlet pair, accompanied with pair formation in bulk; and (ii) at $\rho =\frac{3}{4}$, while the gapless edge mode is always the charge of a single fermion, there is a gap-closing point and a $\pi$-phase shift in the quasiparticle spectrum.

Bringing together spin, valley, topology, and layer degrees of freedom on a single platform could establish an efficient way to engineer phenomena such as valley-symmetry-broken magnetic topological insulators. Here, we combine van der Waals (vdW) layered ferromagnetic insulators (${\mathrm{CrGeTe}}_3$ and ${\mathrm{CrI}}_3$) and Kane-Mele-type topological insulators (${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ and ${\mathrm{Pd}}_2{\mathrm{HgSe}}_3$) in the form of heterobilayers. We find that the mutual interfacial coupling turns the helical nature in ${(\mathrm{Pt}/\mathrm{Pd})}_2{\mathrm{HgSe}}_3$ layers to the chiral topological nature with a large valley polarization. In particular, we identify ${(\mathrm{Pt}/\mathrm{Pd})}_2{\mathrm{HgSe}}_3/{\mathrm{CrGeTe}}_3$ and ${\mathrm{Pd}}_2{\mathrm{HgSe}}_3/{\mathrm{CrI}}_3$ as quantum anomalous Hall insulators supporting a large valley polarization platform together with sizable nontrivial global band gaps $\sim$ 32, 17, and 16 meV at the Fermi surface, respectively. Meanwhile, the appearance of considerable orbital magnetization in ${(\mathrm{Pt}/\mathrm{Pd})}_2{\mathrm{HgSe}}_3$ layers can cause a measurable optical Kerr effect in this family of materials. Furthermore, we observe the layer-type band inversion around a single valley driven by the spin-orbit coupling in the ${(\mathrm{Pt}/\mathrm{Pd})}_2{\mathrm{HgSe}}_3/{\mathrm{CrGeTe}}_3$ heterobilayers, which are very rarely found in vdW layered solids. Moreover, the ${(\mathrm{Pt}/\mathrm{Pd})}_2{\mathrm{HgSe}}_3$ enhances the ferromagnetic properties, such as Curie temperature of ${\mathrm{CrGeTe}}_3$ in comparison to its freestanding form. Interestingly, the interfacial coupling of ${(\mathrm{Pt}/\mathrm{Pd})}_2{\mathrm{HgSe}}_3$ drives the in-plane magnetic anisotropy energy of the freestanding ${\mathrm{CrGeTe}}_3$ to the out-of-plane direction. In contrast, the ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ layer switches the out-of-plane magnetic anisotropy energy of freestanding ${\mathrm{CrI}}_3$ to the in-plane direction of the sample via an interfacial hybridization. We attribute this switching of magnetic anisotropy energy in ${(\mathrm{Pt}/\mathrm{Pd})}_2{\mathrm{HgSe}}_3/{\mathrm{CrGeTe}}_3$ and ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3/{\mathrm{CrI}}_3$ heterobilayers to the dominant contribution of the spin-conserving processes ($|\mathrm{\Delta }{S}_z|=0$ and $|\mathrm{\Delta }{m}_z|=0$) and ($|\mathrm{\Delta }{S}_z|=0$ and $|\mathrm{\Delta }{m}_z|=1$) in overall magnetic anisotropy energy, respectively. Our study may foster a suitable platform in which various degrees of freedom can be gathered to realize topological spintronics and valleytronics-based applications.

We investigate theoretically the spiral-staircase Heisenberg spin-$1/2$ ladder in the presence of antiferromagnetic long-range spin interactions and a uniform magnetic field. As a special case we also consider the Kondo necklace model. If the magnetizations of the two chains forming the ladder satisfy a certain resonance condition, involving interchain couplings as perturbations, the system is in a partially gapped magnetic phase hosting excitations characterized by fractional spins, whose values can be changed by the magnetic field. We show that these fractional spin excitations can be probed via the magnetization and by spin currents in a transport setup with a spin conductance that reveals the fractionalized spin. In some special cases, the spin conductance reaches universal values. We obtain our results analytically via bosonization techniques as well as numerically via density matrix renormalization group methods and find remarkable agreement between the two approaches.

We present magnetization and muon-spin spectroscopy measurements of Néel-skyrmion host ${\mathrm{GaV}}_4{\mathrm{S}}_8$ under the application of hydrostatic pressures up to $P=2.29$ GPa. Our results suggest that the magnetic phase diagram is altered with pressure via a reduction in the crossover temperature from the cycloidal (C) to ferromagnetic-like state with increasing $P$, such that, by 2.29 GPa, the C state appears to persist down to the lowest measured temperatures. With the aid of micromagnetic simulations, we propose that the driving mechanism behind this change is a reduction in the magnetic anisotropy of the system, and suggest that this could lead to an increase in stability of the skyrmion lattice.

We have measured angular-dependent fluorescence-yield x-ray magnetic circular dichroism spectra on single crystals of the heterometallic $3d-4f$ 12-metallacrown-4 ${\mathrm{TbMn}}_4$ and ${\mathrm{DyMn}}_4$ complexes. Simulated spectra using crystal-field multiplet calculations reproduce the experimentally observed spectra. The orientation of the molecules causes linear dichroism spectra of the $4f$ absorption spectra. This natural linear dichroism shows the anisotropic charge distribution of the rare-earth $4f$ state in the tetragonal crystal field despite the small $4f$ crystal-field splitting. The magnetic moment of the molecule is dominated by the rare-earth moment revealing a considerably large contribution of orbital moment. From a sum-rule analysis of experimental and simulated x-ray magnetic circular dichroism, we determined corrected spin and orbital Dy moments at low temperature (14 K) within a magnetic field of 7 T. We find a significant angular dependence of the Dy magnetic moments, indicating the presence of fourth-order magnetic anisotropy.

The recently synthesized air-insensitive hydrogen-doped ${\mathrm{KCr}}_3{\mathrm{As}}_3$ superconductor has aroused great research interest. This material has, for the first time in the research area of the quasi-one-dimensional (quasi-1D) Cr-based superconductivity (SC), realized a tunability through charge doping, which will potentially significantly push the development of this area. Here, based on the band structure from first-principles calculations, we construct a six-band tight-binding model equipped with multiorbital Hubbard interactions and adopt the random-phase-approximation approach to study the hydrogen-doping dependence of the pairing symmetry and superconducting $T_c$. Under the rigid-band approximation, our pairing phase diagram is occupied by the triplet $p_z$-wave pairing throughout the hydrogen-doping regime $x\in (0.4,1)$ in which SC has been experimentally detected. Remarkably, the $x$ dependence of $T_c$ shows a peak at the three-dimensional–quasi-1D Lifshitz transition point, although the total density of states exhibits a dip there. The corresponding doping level is near the experimental estimation of the optimal doping level. A thorough investigation of the band structure reveals type-II van Hove singularities (VHSs) in the $\gamma$ band, which favor the formation of the triplet SC. It turns out that the $\gamma$ Fermi surface (FS) comprises two flat quasi-1D FS sheets almost parallel to the $k_z=0$ plane and six almost perpendicular tubelike FS sheets, and the type-II VHS just lies in the boundary between these two FS parts. Furthermore, the $|k_z|$ of the van Hove planes reaches the maximum near the Lifshitz-transition point, which pushes the $T_c$ of the $p_z$-wave SC to the maximum. Our results appeal for more experimental access into this intriguing superconductor.

We investigate a bosonic Josephson junction by using the path-integral formalism with relative phase and population imbalance as dynamical variables. We derive an effective only-phase action performing functional integration over the population imbalance. We then analyze the quantum effective only-phase action, which formally contains all the quantum corrections. To the second order in the derivative expansion and to the lowest order in $\hslash$, we obtain the quantum correction to the Josephson frequency of oscillation. Finally, the same quantum correction is found by adopting an alternative approach. Our predictions are a useful theoretical tool for experiments with atomic or superconducting Josephson junctions.

We study the growth of the number entropy $S_{\mathrm{N}}$ in one-dimensional number-conserving interacting systems with strong disorder, which are believed to display many-body localization. Recently a slow and small growth of $S_{\mathrm{N}}$ has been numerically reported, which, if holding at asymptotically long times in the thermodynamic limit, would imply ergodicity and therefore the absence of true localization. By numerically studying $S_{\mathrm{N}}$ in the disordered isotropic Heisenberg model we first reconfirm that, indeed, there is a small growth of $S_{\mathrm{N}}$. However, we show that such growth is fully compatible with localization. To be specific, using a simple model that accounts for expected rare resonances we can analytically predict several main features of numerically obtained $S_{\mathrm{N}}$: trivial initial growth at short times, a slow power-law growth at intermediate times, and the scaling of the saturation value of $S_{\mathrm{N}}$ with the disorder strength. Because resonances crucially depend on individual disorder realizations, the growth of $S_{\mathrm{N}}$ also heavily varies depending on the initial state, and therefore $S_{\mathrm{N}}$ and von Neumann entropy can behave rather differently.

We discuss a spin-$\frac{1}{2}$ Heisenberg antiferromagnet on the triangular lattice using the recently proposed bond-operator technique (BOT). We use the variant of the BOT which takes into account all spin degrees of freedom in the magnetic unit cell containing three spins. Apart from conventional magnons known from the spin-wave theory (SWT), there are high-energy collective excitations in the BOT which are built from high-energy excitations of the magnetic unit cell. We also obtain another high-energy quasiparticle which has no counterpart not only in the SWT but also in the harmonic approximation of the BOT. All observed elementary excitations produce visible anomalies in dynamical spin correlators. We show that quantum fluctuations considerably change properties of conventional magnons predicted by the SWT. The effect of a small easy-plane anisotropy is discussed. The anomalous spin dynamics with multiple peaks in the dynamical structure factor is explained that was observed recently experimentally in ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ and which the SWT could not describe even qualitatively.

Topological insulators (TIs) with distinct topological surface states (TSSs) have served as fertile ground to investigate spintronics and quantum information devices. Spin transport properties of ferromagnet (FM)/TI heterostructures have been revealed due to the spin-momentum-locked TSS. The role of the TSS on laser-induced ultrafast spin dynamics, however, is not well understood. Here, we find that the TSS can not only significantly accelerate the ultrafast demagnetization but can also enhance the Gilbert damping factor of $\mathrm{Fe}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ heterostructures. We conclude that the TSS enhanced ultrafast spin dynamics is attributed to the strong hybridization of Fe and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ orbitals near the Fermi level. Our findings suggest that the manipulation of ultrafast spin dynamics of a FM/TI via the TSS can open an avenue to utilize future topological spintronic devices or quantum information processing approaching femtosecond timescales.

We propose a machine learning approach to predict the shapes of the longitudinal spin fluctuation (LSF) energy landscapes for each local magnetic moment. This approach allows the inclusion of the effects of LSFs in, e.g., the simulation of a magnetic material with ab initio molecular dynamics in an effective way. This type of simulation requires knowledge of the reciprocal interaction between atoms and moments, which, in principle, would entail calculating the energy landscape of each atom at every instant in time. The machine learning approach is based on the kernel ridge regression method and developed using bcc Fe at the Curie temperature and ambient pressure as a test case. We apply the trained machine learning models in a combined atomistic spin dynamics and ab initio molecular dynamics (ASD-AIMD) simulation, where they are used to determine the sizes of the magnetic moments of every atom at each time step. In addition to running an ASD-AIMD simulation with the LSF machine learning approach for bcc Fe at the Curie temperature, we also simulate Fe at temperature and pressure comparable to the conditions at the Earth's inner solid core. The latter simulation serves as a critical test of the generality of the method and demonstrates the importance of the magnetic effects in Fe in the Earth's core despite its extreme temperature and pressure.

We propose a solid-state implementation of the Larmor clock that exploits tunnel magnetoresistance to distill information on how long itinerant spins take to traverse a barrier embedded in it. Keeping in mind that the tunneling time innately involves pristine preselection and postselection, our proposal takes into account the detrimental aspects of multiple reflections by incorporating multiple contacts, multiple current measurements, and suitably defined magnetoresistance signals. Our analysis provides a direct mapping between the magnetoresistance signals and the tunneling times and aligns well with the interpretation in terms of generalized quantum measurements and quantum weak values. By means of an engineered preselection in one of the ferromagnetic contacts, we also elucidate how one can make the measurement “weak” by minimizing the backaction, whereas keeping the tunneling time unchanged. We then analyze the resulting interpretations of the tunneling time and the measurement backaction in the presence of phase breaking effects that are intrinsic to solid-state systems. We unravel that whereas the time-keeping aspect of the Larmor clock is reasonably undeterred due to momentum and phase relaxation processes, it degrades significantly in the presence of spin dephasing. We believe that the ideas presented here also open up a fructuous solid-state platform to encompass emerging ideas in quantum technology, such as quantum weak values and its applications that are currently exclusive to quantum optics and cold atoms.

We propose a quantum-classical hybrid scheme for implementing the nonunitary Gutzwiller factor using a discrete Hubbard-Stratonovich transformation, which allows us to express the Gutzwiller factor as a linear combination of unitary operators involving only single-qubit rotations, at the cost of the sum over the auxiliary fields. To perform the sum over the auxiliary fields, we introduce two approaches that have complementary features. The first approach employs a linear-combination-of-unitaries circuit, which enables one to probabilistically prepare the Gutzwiller wave function on a quantum computer, while the second approach uses importance sampling to estimate observables stochastically, similar to a quantum Monte Carlo method in classical computation. The proposed scheme is demonstrated with numerical simulations for the half-filled Fermi-Hubbard model. Furthermore, we perform quantum simulations using a real quantum device, demonstrating that the proposed scheme can reproduce the exact ground-state energy of the two-site Fermi-Hubbard model within error bars.