Valley filtering processes have been explored in different graphene-based configurations and scenarios to control transport responses. Here we propose graphene multiterminal setups properly designed to obtain valley filtered currents in a broad range of energy, in addition to the possibility of controlling their directions. We explore graphene systems with extended mechanical foldlike deformations as an opportunity to enhance valley filtered transmission. The mixing between the electronic confinement effects due to a magnetic field and strain results in a selective drive of the current components in the quantum Hall regime. We adopt the mode-matching method within the Green's function formalism, allowing direct analysis of the strain effect on each valley transmission. We estimate a threshold map of confinement parameters, characterized by the magnetic, deformation, and setup lengths, to optimize valley filter transport processes and the proper switch of the valley polarized current directions.

Spin and orbital angular momentum of light plays a central role in quantum nanophotonics as well as topological electrodynamics. Here, we show that the thermal radiation from finite-size bodies comprising nonreciprocal magneto-optical materials can exert a spin torque even in global thermal equilibrium. Moving beyond the paradigm of near-field heat transfer, we calculate near-field radiative angular momentum transfer between finite-size nonreciprocal objects by combining Rytov's fluctuational electrodynamics with the theory of optical angular momentum. We prove that a single magneto-optical cubic particle in nonequilibrium with its surroundings experiences a torque in the presence of an applied magnetic field ($T$-symmetry breaking). Furthermore, even in global thermal equilibrium, two particles with misaligned gyrotropy axes experience equal-magnitude torques with opposite signs which tend to align their gyrotropy axes parallel to each other. Our results are universally applicable to semiconductors like InSb (magnetoplasmas) as well as Weyl semimetals which exhibit the anomalous Hall effect (gyrotropic) at infrared frequencies. Our work paves the way towards near-field angular momentum transfer mediated by thermal fluctuations for nanoscale devices.

$1T^{\prime }\text{−}\mathrm{Mo}{\mathrm{S}}_2$ exhibits valley-spin-polarized tilted Dirac bands in the presence of external vertical electric field and undergoes a topological phase transition between the topological insulator and band insulator around the critical value of the electric field. Within the linear response theory, we theoretically investigate the anisotropic longitudinal optical conductivities of tilted Dirac bands in both undoped and doped $1T^{\prime }\text{−}\mathrm{Mo}{\mathrm{S}}_2$, including the effects of the vertical electric field. The influence of the spin-orbit coupling gap, band tilting, and vertical electric field on the optical conductivities of tilted Dirac bands is revealed. A theoretical scheme for probing the topological phase transition in $1T^{\prime }\text{−}\mathrm{Mo}{\mathrm{S}}_2$ via exotic behaviors of longitudinal optical conductivities is proposed. The results for $1T^{\prime }\text{−}\mathrm{Mo}{\mathrm{S}}_2$ are expected to be qualitatively valid for other monolayer tilted gapped Dirac materials, such as $\alpha \text{−}\mathrm{Sn}{\mathrm{S}}_2, \mathrm{Ta}\mathrm{Co}{\mathrm{Te}}_2$, and $\mathrm{Ta}\mathrm{Ir}{\mathrm{Te}}_4$, due to the similarity in their band structures.

An atomistic effective Hamiltonian is used to compute electrocaloric (EC) effects in rare-earth-substituted ${\mathrm{BiFeO}}_3$ multiferroics. A phenomenological model is then developed to interpret these computations, with this model indicating that the EC coefficient is the sum of two terms that involve electric quantities (polarization, dielectric response), the antiferromagnetic order parameter, and the coupling between polarization and antiferromagnetic order. The first one depends on the polarization and dielectric susceptibility, has the analytical form previously demonstrated for ferroelectrics, and is thus enhanced at the ferroelectric Curie temperature. The second one explicitly involves the dielectric response, the magnetic order parameter, and a specific magnetoelectric coupling, and generates a peak of the EC response at the Néel temperature. These atomistic results and phenomenological model may be put in use to optimize EC coefficients.

The electronic spins of the nitrogen-vacancy (NV) centers in chemical-vapor-deposition (CVD) grown diamonds form ideal probes of magnetic fields and temperature, as well as promising qubits for quantum information processing. Studying and controlling the magnetic environment of NV centers in such high-purity crystals is thus essential for these applications. We demonstrate optical detection of paramagnetic species, such as hydrogen-related complexes, in a CVD-grown diamond. The resonant transfer of the NV centers' polarized electronic spins to the electronic spins of these species generates conspicuous features in the NV photoluminescence when employing magnetic field scans along the [100] crystal direction. Our results offer prospects for more detailed studies of CVD-grown processes as well as for coherent control of the spin of novel classes of hyperpolarized paramagnetic species.

We extend the notion of fragile topology to periodically driven systems. We demonstrate driving-induced fragile topology in two different models, namely, the Floquet honeycomb model and the Floquet $\pi$-flux square-lattice model. In both cases, we discover a rich phase diagram that includes Floquet fragile topological phases protected by crystalline rotation or mirror symmetries, Floquet Chern insulators, and trivial atomic phases, with distinct boundary features. Remarkably, the transitions between different phases can be feasibly achieved by simply tuning the driving amplitudes, which is a unique feature of driving-enabled topological phenomena. Moreover, corner-localized fractional charges are identified as a “smoking-gun” signal of fragile topology in our systems. Our work paves the way for studying and realizing fragile topology in Floquet systems.

The excitation of surface-localized polaronic states has recently been discussed as an additional photocatalytic channel to band gap excitation for rutile $\mathrm{Ti}{\mathrm{O}}_2$. A contribution from photoexcitation of bulk polarons could, in principle, provide a greater contribution because of their higher number and their protection from oxidation. However, determining such a contribution to the photoyield is challenging and has not been achieved thus far. Here we use two photon photoemission spectroscopy measurements to separate bulk and surface polaron photoexcitation. We find that bulk polarons are less bound by 0.2 eV compared with polarons at the surface, consistent with our results of hybrid density functional theory calculations. Because the excited state is also shifted to higher energy, bulk polarons have the same photoexcitation resonance energy as at the surface (3.6 eV) with a threshold at 3.1 eV. This is degenerate with the band gap, suggesting that bulk polarons could also provide an additional contribution to the photoyield.

Relaxation and aging rates of amorphous selenium and its undercooled melt are calculated by molecular dynamics based on an interaction model derived from density functional calculations. After a fast initial relaxation immediately following the quench, the decrease of diffusivity and the simultaneous increase of dynamic heterogeneity follow an exponential law given by defect annihilation. As observed previously in a Lennard-Jones glass, the non-Arrhenius aging of atomic volume and energy is determined by the time dependence of the diffusivity. The aging time is determined for long times by the diffusivity and diverges exponentially with decreasing temperature. Amorphous selenium differs from metallic glasses in that the breaking of covalent bonds is important for the long time decay of the intermediate self-scattering function (ISSF). Surprisingly for the temperatures investigated, aging of the relaxation times of the ISSF and the bond decay follow the aging of the diffusivity.

We study the possibility of transferring fermions from a trivial system as a particle source to an empty system but in the topological phase as a mold for casting a stable topological insulator dynamically. We show that this can be realized by a non-Hermitian unidirectional hopping, which connects a central system at topological phase and a trivial flat-band system with a periodic driving chemical potential, which scans over the valence band of the central system. The near exceptional point dynamics allows a unidirectional dynamical process: the time evolution from an initial state with a fully filled source system to approximately a stable topological insulating state. The result is demonstrated numerically by a source-assistant Qi-Wu-Zhang model and Su-Schrieffer-Heeger chain in the presence of random perturbation. Our finding reveals a classical analogy of quench dynamics in quantum matter and provides a way for topological quantum state engineering.

We investigate the creep of domain walls (DWs) in the combination of in-plane (IP) and out-of-plane magnetic fields in a series of epitaxial Cu(2 nm)/Pd(0–3 nm)/Co(0.7 nm)/Pd(3 nm) samples. Measured velocity curves, $v\left(H_x\right)$, are fitted with an extended dispersive elasticity model, which considers the dependence of both the elastic energy of the DWs and a velocity prefactor in the creep law on the IP magnetic field. The results of the calculations indicate that strong asymmetry in the $v\left(H_x\right)$ curves in the investigated system is primarily defined by the dependence of the velocity prefactor on the IP magnetic field, which may be related to a chiral damping effect. The effective energy of the Dzyaloshinskii-Moriya interaction (DMI) increases with increasing thickness of the Pd bottom layer from −0.16 ± 0.03 to $0.19\pm 0.05\mathrm{mJ}/{\mathrm{m}}^2$. We attribute the complex dependence of the effective DMI energy on the thickness of the Pd bottom layer in the epitaxial Pd/Co/Pd(111) system and the existence of the nonzero DMI in the symmetric Pd(3 nm)/Co(0.7 nm)/Pd(3 nm) samples to unequal strains in the bottom Pd/Co and top Co/Pd interfaces. The elastic strains in the interfaces varying depending on the thickness of the Pd bottom layer strongly influence the magnitude and even the sign of the contributions to the net DMI energy from each interface.

Efficient manipulation of magnetization is crucial for magnetic random-access memory (MRAM). Here we experimentally demonstrate spin-orbit-torque-driven (SOT-driven) magnetization switching of perpendicularly magnetized IrMn/CoFeB/MgO structures with the assist of a gate voltage. It is found that with the gate voltage changing from −0.6 to 0.6 V, the SOT critical switching current density $J_c$ is reduced by 59%. Furthermore, the mechanism of voltage-induced $J_c$ reduction is explored. We find that the dampinglike torque decreases with positive voltage, while the fieldlike torque shows a weak voltage dependence, which demonstrates that the voltage control of spin-orbit torque (VCSOT) effect has no positive effect on the reduction of $J_c$. In addition, a significant decrease of anisotropy energy is observed when a positive voltage is applied. These results indicate that both VCSOT effect and voltage control of magnetic anisotropy (VCMA) effect exist in voltage-gated SOT switching, while the VCMA effect plays a main role in reduction of $J_c$. This work shows low-power magnetization switching and helps to understand the mechanism of voltage-gated SOT switching.

We explore Andreev states at the interface of graphene and a superconductor for a uniform pseudomagnetic field. Near the zeroth-pseudo-Landau level, we find a topological transition as a function of applied Zeeman field, at which a gapless helical mode appears. This one-dimensional mode is protected from backscattering as long as intervalley and spin-flip scattering are suppressed. We discuss a possible experimental platform to detect this gapless mode based on strained suspended membranes on a superconductor, in which dynamical strain causes charge pumping.

We consider heterostructures obtained by stacking layers of two $s$-wave superconductors with significantly different coupling strengths in the weak- and strong-coupling regimes. The weak- and strong-coupling superconductors are chosen with similar critical temperatures for bulk systems. Using dynamical mean-field theory methods, we find a ubiquitous enhancement of the superconducting critical temperature for all the heterostructures where a single layer of one of the two superconductors is alternated with a thicker multilayer of the other. Two distinct physical regimes can be identified as a function of the thickness of the larger layer: (i) an inherently inhomogeneous superconductor characterized by the properties of the two isolated bulk superconductors where the enhancement of the critical temperature is confined to the interface and (ii) a bulk superconductor with an enhanced critical temperature extending to the whole heterostructure. We characterize the crossover between these regimes in terms of the competition between two length scales connected with the proximity effect and the pair coherence.

Current studies of cavity magnon polaritons are focused on ferromagnetic magnons for which the frequency increases with a static magnetic field. In this paper, we propose a ferromagnetic system with magnon frequency decreasing with a static magnetic field. It is achieved by antiparallel alignment between the magnetization and static magnetic field. The magnetization precession is stabilized by a large anisotropic field along the direction of magnetization. The analysis of the Polder tensor shows that the magnon modes in parallel and antiparallel alignments are analogous to those in an antiferromagnet. The strong coupling between a magnon and photon for antiparallel alignment results in an anticrossing gap in the transmission spectrum. Based on the Tavis-Cummings Hamiltonian and Bloch sphere representation, we show that the photon absorption decreases (increases) the spin angular momentum in antiparallel (parallel) alignment. The coupled Hamiltonians of harmonic oscillators are derived and have the same form for both parallel and antiparallel cases. The method developed and results presented are expected to be helpful to realize low-frequency magnon photon coupling that is similar to those in an antiferromagnet.

The sign cancellation between scattering amplitudes makes fermions different from bosons. We systematically investigate Feynman diagrams' fermionic sign structure in a representative many-fermion system—a uniform Fermi gas with Yukawa interaction. We analyze the role of the crossing symmetry and the global gauge symmetry in the fermionic sign cancellation. The symmetry arguments are then used to identify the sign-canceled groups of diagrams. Sign-structure analysis has two applications. Numerically, it leads to a cluster diagrammatic Monte Carlo algorithm for fast diagram evaluations. This algorithm is about ${10}^5$ times faster than the conventional approaches in the sixth order. Analytically, our analysis systematically reveals the relevant diagrams that dominate the dynamics.

We present an optical conductivity study of the multifold semimetal PdGa, performed in a broad spectral range ($100–\mathrm{20 000} {\mathrm{cm}}^{-1}$; 12 meV–2.5 eV) down to $T=10$ K. The conductivity at frequencies below $4000 {\mathrm{cm}}^{-1}$ is dominated by free carriers while at higher frequencies interband transitions provide the major contribution. The spectra do not demonstrate a significant temperature evolution: Only the intraband part changes as a function of temperature with the plasma frequency remaining constant. The interband contribution to the conductivity exhibits a broad peak at around $5500 {\mathrm{cm}}^{-1}$ and increases basically monotonously at frequencies above $9000 {\mathrm{cm}}^{-1}$. The band-structure-based computations reproduce these features of the interband conductivity and predict its linear-in-frequency behavior as frequency diminishes.

Twisted bilayer graphene (TBG) aligned with hexagonal boron nitride (h-BN) substrate can exhibit an anomalous Hall effect at 3/4 filling due to the spontaneous valley polarization in valley resolved moiré bands with opposite Chern number [Science 367, 900 (2020);  Science 365, 605 (2019)]. It was observed that a small DC current is able to switch the valley polarization and reverse the sign of the Hall conductance [Science 367, 900 (2020);  Science 365, 605 (2019)]. Here, we discuss the mechanism of the current switching of valley polarization near the transition temperature, where bulk dissipative transport dominates. We show that for a sample with rotational symmetry breaking, a DC current may generate an electron density difference between the two valleys (valley density difference). The current induced valley density difference in turn induces a first-order transition in the valley polarization. We emphasize that the intervalley scattering plays a central role since it is the channel for exchanging electrons between the two valleys. We further estimate the valley density difference in the TBG/h-BN system with a microscopic model and find a significant enhancement of the effect in the magic angle regime.

We analyze a correlated system in equilibrium with special emphasis on non-Hermitian topology inducing a skin effect. The pseudospectrum, computed by the real-space dynamical mean field theory, elucidates that additional pseudoeigenstates emerge for the open boundary condition in contrast to the dependence of the density of states on the boundary condition. We further discuss how the line-gap topology, another type of non-Hermitian topology, affects the pseudospectrum. Our numerical simulation clarifies that while the damping of the quasiparticles induces the nontrivial point-gap topology, it destroys the nontrivial line-gap topology. The above two effects are also reflected in the temperature dependence of the local pseudospectral weight.