The stroboscopic evolution of a time-periodically driven isolated quantum system can always be described by an effective time-independent Hamiltonian. Whether this concept can be generalized to open Floquet systems, described by a Markovian Lindblad master equation with a time-periodic generator (Lindbladian), remains an open question. By using a two-level system as a model, we explicitly show the existence of two well-defined parameter regions. In one region the stroboscopic evolution can be described by a Markovian master equation with a time-independent Floquet Lindbladian. In the other it cannot; but here the one-cycle evolution operator can be reproduced with an effective non-Markovian master equation that is homogeneous but nonlocal in time. Interestingly, we find that the boundary between the phases depends on when during the period the evolution is stroboscopically monitored. This reveals the nontrivial role played by the micromotion in the dynamics of open Floquet systems.

We propose and model a method to excite a large number of coherent magnons with high momentum in optical cavities. This is achieved by two counterpropagating optical modes that are detuned by the frequency of a selected magnon, similar to stimulated Raman scattering. In submillimeter-size yttrium iron garnet spheres, a milliwatt laser input power generates ${10}^6–{10}^8$ coherent magnons. The large magnon population enhances Brillouin light scattering, a probe suitable to access their quantum properties.

Spintronics devices utilizing a magnetic domain-wall motion have attracted increasing attention, and ferrimagnetic materials with almost-compensated magnetic moments are highly required to realize the fast magnetic domain-wall motion. Here, we report a key function for this purpose in the antiperovskite $\mathrm{C}{\mathrm{o}}_x\mathrm{M}{\mathrm{n}}_{4-x}\mathrm{N}$ film. We have grown $\mathrm{C}{\mathrm{o}}_x\mathrm{M}{\mathrm{n}}_{4-x}\mathrm{N}$ films with various Co/Mn ratios on $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ by molecular-beam epitaxy. High-quality growth is confirmed and a perpendicular magnetization emerges at $x=0$, 0.2, 0.5, and 0.8, whereas it turns into in plane for $x\ge 1.1$. The saturation magnetization $M_S$ decreases as $x$ increases and reaches a minimum value of $15\mathrm{emu}/\mathrm{c}{\mathrm{m}}^3$ at $x=0.8$. Then, it increases with $x$ when $0.8\le x\le 3.6$ and saturates. These results indicate that $M_S$ and magnetic anisotropy of $\mathrm{C}{\mathrm{o}}_x\mathrm{M}{\mathrm{n}}_{4-x}\mathrm{N}$ films can be manipulated by the Co composition. X-ray absorption spectroscopy and magnetic circular dichroism measurements revealed that Co atoms tend to occupy the I site in the antiperovskite lattice and reasonably explains the origin of minimum $M_S$ near $x=0.8$, where a compensation of magnetic moments occurs among different atomic sites. We consider that the nearly compensated ferrimagnetic $\mathrm{C}{\mathrm{o}}_{0.8}\mathrm{M}{\mathrm{n}}_{3.2}\mathrm{N}$ is suitable for application to current-induced domain-wall motion devices.

We develop a microscopic theory of spin-lattice interactions in magnetic insulators, separating rigid-body rotations and the internal angular momentum, or spin, of the phonons, while conserving the total angular momentum. In the low-energy limit, the microscopic couplings are mapped onto experimentally accessible magnetoelastic constants. We show that the transient phonon spin contribution of the excited system can dominate over the magnon spin, leading to nontrivial Einstein-de Haas physics.

Giant Gilbert damping anisotropy is identified as a signature of strong Rashba spin-orbit coupling in a two-dimensional antiferromagnet on a honeycomb lattice. The phenomenon originates in spin-orbit-induced splitting of conduction electron subbands that strongly suppresses certain spin-flip processes. As a result, the spin-orbit interaction is shown to support an undamped nonequilibrium dynamical mode that corresponds to an ultrafast in-plane Néel vector precession and a constant perpendicular-to-the-plane magnetization. The phenomenon is illustrated on the basis of a two-dimensional $s-d$-like model. Spin-orbit torques and conductivity are also computed microscopically for this model. Unlike Gilbert damping, these quantities are shown to reveal only a weak anisotropy that is limited to the semiconductor regime corresponding to the Fermi energy staying in the close vicinity of the antiferromagnetic gap.

We report the growth and physical properties of high-quality single-crystal SnO via electrical transport, specific heat, Hall coefficients, and the high-pressure effect. Apart from polycrystalline SnO showing an insulating behavior in the whole temperature range, the in-plane resistivity ${\rho }_{\mathrm{ab}}$ of single-crystal SnO exhibits a metal-insulator transition around the characteristic temperature $T_{\mathrm{M}-\mathrm{I}}$. The anisotropic resistivity ratio ${\rho }_{\mathrm{c}}/{\rho }_{\mathrm{ab}}$ is ∼1 for $T\ge T_{\mathrm{M}-\mathrm{I}}$ and increases quickly up to ∼400 for $T<T_{\mathrm{M}-\mathrm{I}}$, which implies the enhanced anisotropic electronic structures and electronic correlations. Its multiband electronic character with dominant hole-type carriers is revealed via the Hall coefficient and the appearance of low-lying phonon models, evidenced by specific heat showing an evident peak at ∼10 K in $\left(C_{\mathrm{p}}-{\gamma }_{\mathrm{n}}T\right)/T^3$ vs $T$. The appearance of a metal-insulator phase transition in single-crystal SnO was attributed to the slight difference in the lattice parameters ratio $c/a$, the atomic coordinate of $Z\left(\mathrm{Sn}\right)$, and the chemical pressure effect. Under hydrostatic pressures generated in a cubic-anvil pressure cell, the insulating state melts at the critical pressure $P_{\mathrm{c}}\sim 3-4\mathrm{GPa}$, and the temperature exponent of resistivity $\rho \propto T^n$ in the metallic state increases gradually from $n=2$ to 3 with increasing the pressure. A domelike superconductivity is achieved in the diamond pressure cell with the pressure up to 13.5 GPa and the temperature to 80 mK, with the superconducting transition temperatures and the upper critical fields close to those of polycrystals. Several possible physical mechanisms are proposed.

In this paper we find that confining a second-order topological superconductor with a harmonic potential leads to a proliferation of Majorana corner modes. As a consequence, this results in the formation of Majorana corner flat bands which have a fundamentally different origin from that of the conventional mechanism. This is due to the fact that they arise solely from the one-dimensional gapped boundary states of the hybrid system that become gapless without the bulk gap closing under the increase of the trapping potential magnitude. The Majorana corner states are found to be robust against the strength of the harmonic trap and the transition from Majorana corner states to Majorana flat bands is merely a smooth crossover. As a harmonic trap can potentially be realized in heterostructures, this proposal paves a way to observe these Majorana corner flat bands in an experimental context.

We study the robustness of the paradigmatic kagome resonating valence bond (RVB) spin liquid and its orthogonal version, the quantum dimer model. The nonorthogonality of singlets in the RVB model and the induced finite length scale not only makes it difficult to analyze, but can also significantly affect its physics, such as how much noise resilience it exhibits. Surprisingly, we find that this is not the case: The amount of perturbations which the RVB spin liquid can tolerate is not affected by the finite correlation length, making the dimer model a viable model for studying RVB physics under perturbations. Remarkably, we find that this is a universal phenomenon protected by symmetries: First, the dominant correlations in the RVB are spinon correlations, making the state robust against doping with visons. Second, reflection symmetry stabilizes the spin liquid against doping with spinons, by forbidding mixing of the initially dominant correlations with those which lead to the breakdown of topological order.

Quantum oscillations have long been regarded as the manifestation of the Fermi surface in metals. However, they were recently observed in Kondo insulators. We examine the Kondo screening due to Landau levels in Kondo insulators. It is shown that, even for large Kondo insulating gaps, appreciable amplitudes of quantum oscillations that are consistent with experimental observations are present both in magnetization and resistivity. Specifically, we show that, due to the periodic alignment between the Landau levels in the conduction and the $f$-orbit electrons, the Kondo screening itself undergoes oscillations so that the electronic structure oscillates with the magnetic field. Our results explain main features of quantum oscillations observed in experiments. They indicate that the nonrigidity of the electronic structure results in observable quantum oscillation Kondo insulators. This effect provides a way to probe the Fermi surface geometry of insulators.

Knowledge of the phonon mean free path (MFP) holds the key to understanding the thermal properties of materials and nanostructures. Although several experiments measured the phonon MFP in bulk silicon, MFP spectra in thin membranes have not been directly measured experimentally yet. In this work, we experimentally probe the phonon MFP spectra in suspended silicon membranes. First, we measure the thermal conductivity of membranes with arrays of slits at different temperatures. Next, we develop a fully analytical procedure to extract the accumulated thermal conductivity as a function of the MFP. The measured phonon MFP in 145-nm-thick membranes with the surface roughness of 0.2 nm is shorter than that in bulk due to the scattering at the membrane boundaries. At room temperature, the phonon MFP does not exceed 400 nm. However, at 4 K, the MFP becomes longer, and some phonons can travel ballistically for up to one micrometer. These results thus shed light on the long-lasting question of the range of ballistic phonon transport at different temperatures in nanostructures based on silicon membranes.

The conductance quantization and shot noise below the first conductance plateau $G_0=2e^2/h$ are measured in a quantum point contact fabricated in a GaAs/AlGaAs tunnel-coupled double quantum well. From the conductance measurement, we observe a clear quantized conductance plateau at $0.5G_0$ and a small minimum in the transconductance at $0.7G_0$. Spectroscopic transconductance measurement reveals three maxima inside the first diamond, thus suggesting three minima in the dispersion relation for electric subbands. Shot noise measurement shows that the Fano factor behavior is consistent with this observation. We propose a model that relates these features to a wave-number directional split subband due to a strong Rashba spin-orbit interaction that is induced by the center barrier potential gradient of the double-layer sample.

Polarons are ubiquitous in many semiconductors and have been linked with conductivity and optical response of materials for photovoltaics and heterogeneous catalysis, yet it remains unclear how surface polarons influence adsorption. Here, by modeling the surface of rutile titania using density functional theory, we reveal the effect of small surface polarons on water adsorption, dissociation, and hydrogen bonding. On the one hand, the presence of such polarons significantly suppresses dissociation of water molecules that are bonded directly to polaronic sites. On the other hand, polarons facilitate water dissociation at certain nonpolaronic sites. Furthermore, polarons strengthen hydrogen bonds, which in turn affects water dissociation in hydrogen-bonded overlayer structures. This study reveals that polarons at the rutile surface have complex, multifaceted, Janus-like effects on water adsorption, dissociation, and hydrogen bonding, highlighting the importance of polarons on water structure and dynamics on such surfaces. We expect that many of the physical properties of surface polarons identified here will apply more generally to surfaces and interfaces that can host small polarons, beyond titania.

A quantum dot (QD) coupled to topological quantum wires is fertile ground to explore the physical properties of Majorana bound states (MBSs). We consider a single-level QD and a MBS, both coupled to an electron reservoir with a continuum energy spectrum. By increasing the coupling between the MBS and the metallic contact, the MBS leaks into the continuum, modifying the local spectral properties of the QD either in the noninteracting or the Coulomb blockade regimes. Surprisingly, regardless of the strength of the coupling between the MBS and the continuum, the Majorana states always appear as a bound state, as long as the QD level is resonant with the Fermi level. By applying a gate voltage in the system, we can control the QD level position while leaving the MBS pinned to the Fermi level. This provide an interesting way to explore the physical properties of this peculiar bound state in the continuum.

The Allen-Heine-Cardona theory allows us to calculate phonon-induced electron self-energies from first principles without resorting to the adiabatic approximation. However, this theory has not been able to account for the change of the electron wave function, which is crucial if interband energy differences are comparable to the phonon-induced electron self-energy as in temperature-driven topological transitions. Furthermore, for materials without inversion symmetry, even the existence of such topological transitions cannot be investigated using the Allen-Heine-Cardona theory. Here, we generalize this theory to the renormalization of both the electron energies and wave functions. Our theory can describe both the diagonal and off-diagonal components of the Debye-Waller self-energy in a simple, unified framework. For demonstration, we calculate the electron-phonon coupling contribution to the temperature-dependent band structure and hidden spin polarization of ${\mathrm{BiTlSe}}_2$ across a topological transition. These quantities can be directly measured. Our theory opens a door for studying temperature-induced topological phase transitions in materials both with and without inversion symmetry.

Recently, the notion of topological phases of matter has been extended to higher-order incarnations, supporting gapless modes on even lower dimensional boundaries, such as corners and hinges. We here identify a collection of cubic spin-$\frac{3}{2}$ fermions with biquadratic touching of Kramers degenerate valence and conduction bands as a platform to strain-engineer higher-order topological (HOT) phases: external uniaxial strain gives birth to a HOT Dirac semimetal or an insulator, depending on its sign, featuring topological hinge modes in the strain direction. The insulator in fact exhibits mixed topology, and in addition supports edge modes on orthogonal planes. These outcomes are germane when the external strain is applied along one of the $C_{4v}$ or coordinate axes, as well as $C_{3v}$ or body-diagonal, directions. Our findings place HgTe, gray-Sn, 227 pyrochlore iridates and half-Heusler compounds at the frontier of strain-engineered electronic HOT phases.

Ferromagnetic (FM) axion insulators may greatly simplify experimental explorations of the topological magnetoelectric effect, as the FM ordering can be stable in a broad range of magnetic field. Through density functional theory calculations and four-band model studies, we find that two-dimensional van der Waals FM $\mathrm{Cr}{\mathrm{I}}_3$ and $\mathrm{MnB}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_4$ overlayers induce opposite-sign exchange fields in the topological surface states of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ film when they are ferromagnetically ordered. Consequently, the $\mathrm{Cr}{\mathrm{I}}_3/\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3/\mathrm{MnB}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_4$ heterostructure is a robust FM axion insulator, according to surface-resolved Berry curvature calculations. Our work opens up opportunities for the design of materials with the topological magnetoelectric effect.

Removing valley degeneracy is a necessary condition for manipulating valley degrees of freedom and storing information in future spintronics. Magnetic proximity effect has been demonstrated to be an effective way to introduce exchange interactions, especially in the case of two-dimensional (2D) van der Waals (vdW) heterostructures. We have explored the electronic properties and the valley physics of 2D ${\mathrm{WSe}}_2/{\mathrm{CrI}}_3$ using first-principles calculations. Our results show that a valley splitting of 2 meV is achieved in ${\mathrm{WSe}}_2/{\mathrm{CrI}}_3$ heterostructures thanks to the coexistence of inversion and time-reversal symmetry breaking. This value corresponds to an effective magnetic field of ∼10 T in experiments. Moreover, we demonstrate that the valley splitting is a robust feature regardless of the stacking configuration and the thickness of ${\mathrm{CrI}}_3$. Most importantly, by reversing the magnetization in the ${\mathrm{CrI}}_3$ layer, the valley splitting and polarization at $K^{+}$ and $K^{-}$ points are completely switchable. Our findings provide fundamental insights into the magnetoelectric spin-orbit coupling based spintronics applications of 2D vdW heterostructures.

Recent experimental advances in the reconstruction of the Wigner function (WF) for electronic systems have led us to consider the possibility of employing this theoretical tool in the analysis of electron dynamics of uniaxially strained graphene. In this paper, we study the effect of strain on the WF of electrons in graphene under the interaction of a uniform magnetic field. This mechanical deformation modifies drastically the shape of the Wigner function of Landau and coherent states. The WF has a different behavior straining the material along the zigzag direction in comparison with the armchair one and favors the creation of electron coherent states. The time evolution of the WF for electron coherent states shows fluctuations between classical and quantum behavior around a closed path as time increases. The phase-space representation shows more clearly the effect of nonequidistant relative Landau levels in the time evolution of electron coherent states compared with other approaches. Our findings may be useful in establishing protocols for the realization of electron coherent states in graphene as well as a bridge between condensed matter and quantum optics.