Recently, symmetry-broken polaritons within low-symmetry crystals have triggered extensive research interest since they present enhanced directionality of polariton propagation for nanoscale manipulation and steering of photons. The latest discovery of hyperbolic shear polaritons (HShPs) in low-symmetry Bravais crystals provides great promise for innovating valleytronics. Herein, we theoretically demonstrate the coherent manipulation of the valley degree of freedom in a two-dimensional valleytronic material interfaced with monoclinic $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ and $\mathrm{CdW}{\mathrm{O}}_4$ crystals. Robust and wideband tunable valley interference values are achieved in the mid- to far-infrared wavelengths. By virtue of stronger shear effect in monoclinic $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$, the valley quantum interference fringes modulated by $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ crystal are more than those tuned via $\mathrm{CdW}{\mathrm{O}}_4$ crystal. After the monoclinic $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ crystal is doped by free charge carriers, the number of HShP modes gradually decreases accompanied by the blueshifts and broadening of some hyperbolic dispersion bands as the doping concentration increases. In consequence, main fringes of valley quantum interference are broadened and shift toward the short wavelengths. Additionally, the doping increases the optical losses which limit the effective propagation of shear polaritons in monoclinic crystals. Therefore, the valley quantum interference is gradually reduced to a smaller and smaller negative value range as the doping concentration increases in the monoclinic crystal. Finally, the azimuthal dispersion of the HShP propagation direction gives rise to symmetry-broken valley quantum interference patterns when tuning the azimuth and twist angles of monoclinic hybrid structures. The azimuth angles of quantum interference fringes are susceptive to the variation of lattice displacement direction induced via the doping concentration. Thus, the valley quantum interference has great potential in estimating the doping concentration and the propagation directions of HShPs in monoclinic crystals.

Achieving large tunneling electroresistance (TER) is the key to the application of two-dimensional (2D) ferroelectric tunnel junctions (FTJs) in high-density memory devices. In this work, we show that an aided layer of 2D metal materials with ultrahigh work function can lead to significantly enhanced TER effect in 2D FTJs. To implement the idea, we design a 2D van der Waals heterostructure consisting of a metallic ${\mathrm{NbS}}_2$ monolayer and a ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ ferroelectric, and propose a FTJ based on this heterostructure. By performing density functional theory and nonequilibrium Green's function method calculations, we find that the type of interfacial contact in the ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ heterostructure can be flexibly switched between Ohmic and Schottky contact by altering the ferroelectric polarization. Accordingly, the ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$-based FTJ exhibit giant TER ratio of up to ${10}^{13}\%$, and its TER ratio increases with the tunneling barrier length. These findings not only provide a feasible strategy for realizing large TER effect in 2D FTJs, but also offer promising candidates to designing nonvolatile memories.

Hole qubits in germanium quantum dots are promising candidates for coherent control and manipulation of the spin degree of freedom through electric dipole spin resonance. We theoretically study the time dynamics of a single heavy-hole qubit in a laser-driven planar germanium quantum dot confined laterally by a harmonic potential in the presence of linear and cubic Rashba spin-orbit couplings and an out-of-plane magnetic field. We obtain an approximate analytical formula of the Rabi frequency using a Schrieffer-Wolff transformation and establish a connection of our model with the ESDR results obtained for this system. For stronger beams, we employ different methods such as unitary transformation and Floquet theory to study the time evolution numerically. We observe that high radiation intensity is not suitable for the qubit rotation due to the presence of high-frequency noise superimposed on the Rabi oscillations. We display the Floquet spectrum and highlight the quasienergy levels responsible for the Rabi oscillations in the Floquet picture. We study the interplay of both the types of Rashba couplings and show that the Rabi oscillations, which are brought about by the linear Rashba coupling, vanish for typical values of the cubic Rashba coupling in this system.

We have considered theoretically nonlinear transport phenomena known as bilinear magnetoresistance (BMR) and nonlinear planar Hall effect (NPHE) within the effective model describing surface states of a 3D topological insulator. Both phenomena can occur in nonmagnetic materials with strong spin-orbit interaction and reveal a term that depends linearly on the charge current density (external electric field) and in-plane magnetic field. In earlier studies, the physical mechanism of BMR and NPHE was related to scattering on spin-momentum locking inhomogeneities or to the hexagonal warping of Dirac cones. Here, we focus on another mechanism related to scattering on impurities that inherently contain spin-orbit coupling. Using the Green's function formalism and diagramatic method, we have derived analytical results for diagonal and transverse conductivities and determined nonlinear signals. The analytical and numerical results on BMR and NPHE indicate the possibility of determining the material constants, such as the Fermi wave vector and spin-orbit coupling parameter, by simple magnetotransport measurements.

We investigate the topological phases of Weyl semimetals possessing a pair of multifold ($N$-fold) Weyl points (WPs) irradiated by an off-resonant circularly polarized light in the frame of Floquet theory. The irradiation splits each $N$-fold WP into two singlet WPs and one residual WP of $N-2$ fold in $k$ space. When the irradiation intensity increases, the two singlet WP pairs move faster and are gapped out first; therefore, a phase transition from $N$-fold to $(N-2)$-fold Weyl semimetal occurs. The pair of remaining $(N-2)$-fold WPs are annihilated later and a sequential phase transition to insulator happens. The irradiation influences the system by means of two mechanisms—the time average of the perturbation and the emission and reabsorption of photons, with the latter usually regarded as responsible for the phase transitions of matter and the former sometimes omitted due to its trivial effect. However, we find that these phase transitions are caused by the former, not by the latter, and the latter only affects how the WPs in the same pair approach each other, except for the case of $N=1$ in which both mechanisms take effect. The most common Weyl semimetal, say $N=1$, has the most special response. The irradiation can induce not only the transition from Weyl semimetal to insulator, but also the inverse transition, while the latter cannot happen in Weyl semimetals with $N\ge 2$.

In this paper, we establish an effective edge theory to characterize non-Hermitian edge-skin modes in higher dimensions. We begin by proposing a bulk projection criterion to straightforwardly identify the localized edges of skin modes. Through an exact mapping, we show that the edge-skin mode shares the same bulk-boundary correspondence and localization characteristics as the zero-energy edge states in a Hermitian semimetal under open-boundary conditions, bridging the gap between non-Hermitian edge-skin effect and Hermitian semimetals. Another key finding is the introduction of “skewness,” a term we proposed to describe the characteristic decay direction of skin mode from the localized edge into the bulk. Remarkably, we demonstrate that skewness is an intrinsic quantity of the skin mode and can be analytically determined using the corresponding cylinder-geometry bulk Hamiltonian, without requiring any boundary details. Furthermore, we reveal that, in the edge-skin effect, the spectrum exhibits anomalous spectral sensitivity to weak local disturbances, a feature that crucially distinguishes it from the corner-skin effect.

Exceptional points (EPs), which refer to degeneracies in non-Hermitian systems, have garnered significant attention in the last few years due to their potential applications in the fields of sensing, single-mode lasing, and others. Recently, the complete classification of isolated EPs based on homotopy theory has been proposed, where the topology of energy spectra around EPs can be fully characterized by the braiding group and knot topology. However, there have been few experimental observations of EPs with different knot topologies due to the need for tunable, nonlocal, and nonreciprocal couplings in the proposed non-Hermitian lattice model. Here, we report experimental observation of EPs with different types of knots/links by non-Hermitian electric circuits with voltage-tunable node couplings. Specifically, a second-order EP with trefoil knot topology and a third-order EP with $6_3^3$ link topology (according to Rolfsen's table) are achieved. Moreover, we also provide a method on the construction of higher-order Dirac EPs with nested-link topologies, and experimentally construct a sixth-order Dirac EP. Our work provides an artificial platform for exploring EPs with knot and link topologies, offering potential applications in designing EP-based electronic devices.

We investigate high-order harmonic generation (HHG) in sparsely doped semiconductors. The doping in semiconductors breaks the periodic translation invariance in space and introduces localized electronic states (LESs), which leads to two additional electron transition channels, namely, the nonvertical transitions and transitions from LESs. By involving these channels, one can intuitively understand the enhancement of the harmonic yield and the extension of the cutoff energy found in previous works. Moreover, the transition from LESs is space localized and shows self-probed behavior under strong laser fields, encoding the structure information into the HHG. To demonstrate this, we analyze the HHG process in a doped semiconductor with a pair of impurities, where the recombination process of the electron from LESs can be interpreted as a two-center interference. This imprints the internuclear separation between the impurities into the minima of the harmonic spectra. Our work reveals the underlying physical mechanisms in HHG from doped semiconductors and suggests all-optical metrology for structural and dynamic information of LESs.

The realization of the Kane-Mele model in Xene solids faces challenges due to weak spin-orbit coupling (SOC). Nevertheless, the recently discovered $X_4{Y}_2{Z}_6$ family ($X$=Pt, Pd, Ni; $Y$=Hg, Zn, Cd; and $Z$=S, Se, Te) offers a promising opportunity with larger SOC. However, the presence of centrosymmetry in this family hinders the achievement of several cross-coupling phenomena based on spintronics and valleytronics. This study explores a noncentrosymmetric version of the Kane-Mele family, $X_{4}Y{Y}^{\prime }{Z}_6$ and $X_{4}Y{Y}^{\prime }{\left(Z{Z}^{\prime }\right)}_3$, comprising over 16 experimentally accessible members. The results reveal intertwined phenomena involving topology, spin, and valley degrees of freedom, including the quantum valley/spin Hall effect, spin-valley locking, and spin-valley selective optical transitions. Additionally, Rashba coupling coexists with Ising spin splitting, enabling valley spin valve functionality and out-of-plane spontaneous electric polarization. Quantum valley Hall kink states can be achieved on the domain walls between these noncentrosymmetric monolayers due to opposite spin-valley Berry curvatures. External factors, like electric fields and strain, induce various topological phase transitions. This study lays the foundation for exploring spin-valley physics in low-dimensional topological materials with noncentrosymmetry.

The optical response of quasi-one-dimensional systems is often dominated by tightly bound excitons that significantly influence their basic electronic properties. Despite their importance for device performance, accurately predicting their excitonic effects typically requires computationally demanding many-body approaches. Here, we present a simplified model to describe the static macroscopic dielectric function, which depends only on the width of the quasi-one-dimensional system and its polarizability per unit length. We show that at certain interaction distances, the screened Coulomb potential is greater than its bare counterpart, which results from the enhanced repulsive electron-electron interactions. As a test case, we study 14 different nanoribbons, 12 of them armchair graphene nanoribbons of different families. Initially, we devised a simplified equation to estimate the exciton binding energy and extension that provides results comparable to those from the full Bethe-Salpeter equation, albeit for a specific nanoribbon family. Then, we used our proposed screening potential to solve the 1D Wannier-Mott equation, which turns out to be a broad approach that is able to predict binding energies that match quite well the ones obtained with the Bethe-Salpeter equation, irrespective of the nanoribbon family.

Wave localization is a fundamental phenomenon that appears universally in both natural materials and artificial structures and plays a crucial role in understanding the various physical properties of a system. Usually, a localized state has an exponential profile with a localization length independent of the system size. Here, we experimentally demonstrate a new class of localized states called scale-free localized states, which has an unfixed localization length scaling linearly with the system size. Using circuit lattices, we observe that a non-Hermitian defect added to a Hermitian lattice induces an extensive number of states with scale-free localization. Furthermore, we demonstrate that, in a lattice with a parity-time-symmetric non-Hermitian defect, the scale-free localization emerges because of spontaneous parity-time symmetry breaking. Our results uncover a type of localized states and extend the study of defect physics to the non-Hermitian regime.

Over the past decade, dynamical quantum phase transitions (DQPTs) have emerged as a paradigm shift in understanding nonequilibrium quantum many-body systems. However, the challenge lies in identifying order parameters that effectively characterize the associated dynamic phases. In this study we investigate the behavior of vortex singularities in the phase of the Green's function for a broad class of fermion lattice models in three dimensions after an instantaneous quench in both interacting and noninteracting systems. We find that the full set of vortices form one-dimensional dynamical objects, which we call vortex loops. We propose that the number of such vortex loops can be interpreted as a quantized order parameter that distinguishes between different nonequilibrium phases. Our results establish an explicit link between variations in the order parameter and DQPTs in the noninteracting scenario. Moreover, we show that the vortex loops are robust in the weakly interacting case, even though there is no direct relation between the Loschmidt amplitude and the Green's function. Finally, we observe that vortex loops can form complex dynamical patterns in momentum space. Our findings provide valuable insights for developing definitions of dynamical order parameters in nonequilibrium systems.

We study superconductivity in the superionic phase of the clathrate hydride ${\mathrm{Li}}_2{\mathrm{MgH}}_{16}$, where hydrogen ions diffuse among the lattice formed by lithium and magnesium ions. By employing the stochastic path-integral approach, we nonperturbatively take into account the effects of quantum diffusion and anharmonic vibrations. Our calculations reveal strong electron-ion coupling $\left[\lambda \right(0)=3.7]$ and a high superconducting transition temperature $\left(T_c\right)$ of 277 K under 260 GPa, at which the material is still superionic. $T_c$ is significantly suppressed compared with the result $T_c=473$ K obtained from the conventional approach based on the harmonic approximation. Our study, based on a first-principles approach applicable to superionic systems, indicates that the superconductivity and superionicity can coexist in ${\mathrm{Li}}_2{\mathrm{MgH}}_{16}$.

Analog computations enable information processing with negligible energy costs and massively parallel architectures, but currently are limited to process macroscale waveforms with characteristic lengths much larger than the operating wavelength ${\lambda }_0$. We explore here, in contrast, the differentiation of subwavelength waveforms by using an elastic computational metasurface. We find that the numerical aperture of metasurface governs the threshold of the characteristic length of waveforms, below which the metasurface outputs an identical differentiated pattern. Remarkably, for a subwavelength waveform below the threshold, the metasurface can locate the source because the differentiated pattern is of cylindrical wavefronts centered at the source, which can be harnessed to detect single or multiple subwavelength-scaled scatterers. The detectability reaches a deep subwavelength of $0.12{\lambda }_0$, and the localization error stays smaller than ${\lambda }_0$. Our work elucidates the physical image of subwavelength differentiations, which may promote promising applications in nondestructive testing, signal processing, and computational acoustics.

Topological phases of matter are classified based on symmetries, with nonsymmorphic symmetries like glide reflections and screw rotations being of particular importance in the classification. In contrast with extensively studied glide reflections in real space, introducing space-dependent gauge transformations can lead to momentum-space glide reflection symmetries, which may even change the fundamental domain for topological classifications, e.g., from a torus to a Klein bottle. Here, we discover a class of three-dimensional (3D) higher-order topological insulators, protected by a pair of momentum-space glide reflections. It supports gapless hinge modes, as dictated by the quadrupole moment and Wannier Hamiltonians defined on a Klein bottle manifold, and we introduce two topological invariants to characterize this phase. Our predicted topological hinge modes are experimentally verified in a 3D-printed acoustic crystal, providing direct evidence for 3D higher-order Klein bottle topological insulators. Our results not only showcase the remarkable role of momentum-space glide reflections in topological classifications but also pave the way for experimentally exploring physical effects arising from momentum-space nonsymmorphic symmetries.

Understanding the phase behavior and structural properties of salt water at high pressures is essential for understanding the dynamics and physical characteristics of icy planets. In this study, we employed high-pressure experimental and ab initio simulation techniques to investigate the impact of $\mathrm{CaC}{\mathrm{l}}_2$ on the structure of ice VII. Our findings reveal that 1.8 mol% $\mathrm{CaC}{\mathrm{l}}_2$ can be incorporated into the ice VII structure above 10 GPa. This $\mathrm{CaC}{\mathrm{l}}_2$-bearing ice VII (Cb VII) exhibits a lower O-H stretching frequency in the Raman spectra as well as a reduced volume of the unit cell compared to pure ice VII. In contrast to doping ice VII with other salts such as LiCl and NaCl that leads to an increase of the ice VII to ice X transition pressure occurring at 100–150 GPa, $\mathrm{CaC}{\mathrm{l}}_2$ doping stands out by reducing the transition pressure. It shifts the transition to a pressure of 52 GPa, which is significantly lower than the transition pressure of 80 GPa in the pure ${\mathrm{H}}_2\mathrm{O}$ ice system. This notable distinction highlights the unique influence of $\mathrm{CaC}{\mathrm{l}}_2$ on the phase behavior of water under high pressure, and we attribute these effects to the phenomenon of chemical pressure induced by $\mathrm{CaC}{\mathrm{l}}_2$ within the ice VII structure. Our study suggests that the presence of a modified ice VII phase, contaminated with salt and referred to as Cb VII, may influence the composition, structure, and evolution of planets.

Chirality as internal degree of freedom of a mesoscopic domain wall inside a quasi-one-dimensional fixture can be controlled by spin-polarized current for ferro- as well as antiferromagnetic domain walls. We show that the current density required for the chirality manipulation can be significantly reduced in the low-temperature regime where the chirality dynamics exhibits quantum effects. In this quantum regime, weak currents can excite Bloch oscillations of the domain wall angular rotation velocity, with the oscillation frequency proportional to the current, modulated by a much higher magnon-range frequency. In addition to that, the Wannier-Stark localization effects enable controlled switching between different chiral states, suppressing inertial effects characteristic for the classical regime. We also show that for recently discovered novel class of magnetic materials — altermagnets — chirality switching can be driven by the usual charge current (not spin polarized).

The high-order Hall effects, which go beyond the ordinary, unlock more possibilities of electronic transport properties and functionalities. Pioneer works focus on the manufacture of complex nanostructures with low lattice symmetry to produce them. In this paper, we theoretically show that such high-order Hall effects can alternatively be generated by curving a conducting nanomembrane which is highly tunable and also enables anisotropy. Its Hall response can be tuned from first to fourth order by simply varying the direction and magnitude of the applied magnetic field. The dominant Hall current frequency can also be altered from zero to double, or even four times that of the applied alternating electric field. This phenomenon is critically dependent on the occurrence of high-order snake orbits associated with the effective magnetic-field dipoles and quadruples induced by the curved geometry. Our results offer pathways for spatially engineering magnetotransport, current rectification, and frequency multiplication in the bent conducting nanomembrane.

We derive microscopically the Ginzburg-Landau free-energy functional for a superconductor in which the Cooper pairs are formed not only by quasiparticles from the same band, but also by quasiparticles from different bands. In the simplest case of an $s$- or $d$-wave pairing in a two-band system, the order parameter has three components describing two intraband and one interband pair condensates. The interband pairing-specific terms in the free energy bring about some qualitative features in the phase diagram, for example, time-reversal symmetry-breaking superconducting states are stabilized at low temperatures. We also show that, despite the fact that the gradient energy of the interband component turns negative at a sufficiently large band splitting, a nonuniform superconducting instability is generally suppressed due to the hybridization of the intraband and interband condensates.

We developed a method which performs the coupled adiabatic spin and lattice dynamics based on the tight-binding electronic structure model, where the intrinsic magnetic field and ionic forces are calculated from the converged self-consistent electronic structure at every time step. By doing so, this method allows us to explore limits where the physics described by a parameterized spin-lattice Hamiltonian is no longer accurate. We demonstrate how the lattice dynamics is strongly influenced by the underlying magnetic configuration, where disorder is able to induce significant lattice distortions. The presented method requires significantly less computational resources than ab initio methods, such as time-dependent density functional theory (TD-DFT). Compared to parameterized Hamiltonian-based methods, it also describes more accurately the dynamics of the coupled spin and lattice degrees of freedom, which becomes important outside of the regime of small lattice and spin fluctuations.