At ambient pressure, the $t_{2g}^5$ layered perovskite ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ is a correlated small-gap insulator. In the Mott picture, applying uniform pressure should therefore quickly close the gap; experimentally, however, the insulating state persists even under extreme pressures, suggesting that a mechanism other than Mott is at work. Yet, given the complexity of the system, it is unclear to what extent the Mott picture can be really excluded. Here, we thus reexamine the problem. We show that, surprisingly, the pressure-induced enhancement of the screened Coulomb interaction—combined with lattice distortions and spin-orbit driven $j_{\mathrm{eff}}=1/2$ orbital ordering—can hold the system close to the metal-insulator transition up to very high pressure.

We theoretically investigate the transport and shot-noise properties of a one-dimensional semiconducting nanowire with Rashba spin-orbit coupling (SOC) placed in closed proximity to a bulk $s\text{−}\mathrm{wave}$ superconductor and an altermagnet with $d\text{−}\mathrm{wave}$ symmetry. Such heterostructure with vanishing net magnetization manifests itself as an alternative route to anchor Majorana zero modes (MZMs) characterized by appropriate topological index (winding number $W$). Interestingly, this system also hosts accidental zero modes (AZMs) emerged with vanishing topological index indicating their nontopological nature. Furthermore, by incorporating three terminal setup, we explore the transport and shot-noise signatures of these zero modes. At zero temperature, we obtain zero-bias peak (ZBP) in differential conductance to be quantized with value $\left|W\right|\times 2e^2/h$ for MZMs. On the other hand, AZMs exhibit nonquantized value at zero bias. Moreover, zero-temperature shot noise manifests negative (positive) value for MZMs (AZMs) within the bulk gap. At finite temperature, shot noise exhibits negative value (negative to positive transition) concerning MZMs (AZMs). Thus, the obtained signatures clearly distinguish between the MZMs and nontopological AZMs. We extend our analysis by switching on the next-to-nearest-neighbor hopping amplitude and SOC. Our conclusion remains unaffected for this case as well. Hence, our work paves the way to differentiate between emergent MZMs and AZMs in a semiconductor/ superconductor/ altermagnet heterostructure.

We propose that a high-quality flat interface or boundary can serve as a long-range skew scatterer for charged quasiparticles in a metal. When an electric current flows parallel to the interface, the balance between clockwise and counterclockwise reflections is disrupted, leading to a net orbital magnetization. This magnetization is maximized at the interface and varies linearly in the direction perpendicular to it. We suggest that this effect can be detected using spatially resolved Kerr effect measurements at distances up to the electron phase coherence length from the interface. Unlike the orbital Hall and orbital Edelstein effects, the proposed phenomenon does not require inversion symmetry breaking in the bulk of the sample and is unrelated to Hall effect physics.

We study the instability of antiferromagnets with easy-axis anisotropy under a magnetic field, uncovering single or even multiple phase transitions at the boundary between noncollinear and collinear spin orderings. Near the phase boundary, the entanglement between the sublattice magnons diverges due to the interplay among antiferromagnetic exchange interaction, anisotropy, and magnetic field. Furthermore, our study reveals that this magnetic criticality extends to a superradiant phase transition within cavity magnonics systems. The magnon-photon interaction results in diverging cavity photon numbers and squeezing in the ground state at the transition points between spin orderings. This investigation not only elucidates the criticality of multicomponent squeezed magnons in antiferromagnets, but also proposes cavity photon measurements as a viable method for detecting magnetic phase transitions.

We studied the magnetic properties of ${\mathrm{Yb}}_2{\mathrm{SiO}}_5$ with pseudospins of $S_{\mathrm{eff}}=1/2$ using magnetic susceptibility, specific heat, and inelastic neutron scattering (INS) measurements. A broad maximum appears at $T_{\max }\sim 6$ K in the plot of the magnetic susceptibility $\left[\chi \right(T\left)\right]$ as a function of temperature ($T$), indicating a low-dimensional antiferromagnetic (AFM) spin system. The susceptibility decreases rapidly with decreasing $T$ below $T_{\max }$, suggesting a spin-singlet ground state with a spin gap. We observed clusterlike excitations for energy transfers of $\omega \sim 0.7,1.2$, and 1.9 meV in the INS measurements. The $T$ dependence of the intensity $\left[I\right(T\left)\right]$ indicates that the three excitations are magnetic ones. We determined that the spin system in ${\mathrm{Yb}}_2{\mathrm{SiO}}_5$ is the $S_{\mathrm{eff}}=1/2$ AFM tetramer with two exchange interactions of $J_1=0.74$ and $J_2=0.95$ meV. The isolated spin tetramer model can explain the excitation energies $\chi \left(T\right)$, magnetic specific heat at $T<15$ K, and magnitude of the scattering vector ($Q$) dependence of the intensity $\left[I\right(Q\left)\right]$ at 0.7 and 1.2 meV. This model, however, cannot well explain the $I\left(Q\right)$ at 1.9 meV and the $I\left(T\right)$ of the three excitations, likely because of the effects of weak intertetramer interactions.

Ferrimagnet insulator yttrium iron garnet (${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, YIG) attracts much interest in magnonics and microwave applications due to its relatively low magnetic damping and long magnon relaxation time. The magnetoelastic interaction between YIG magnetization and surface acoustic waves (SAWs) has been the basis in the development of acoustic-surface-wave isolators. However, the investigation of SAW-driven magnetic resonance in YIG is limited by the low compatibility of YIG and SAW devices. In this work, we demonstrate nonreciprocal resonant SAW absorption in an on-chip YIG-SAW device integrated by the focused-ion-beam technique. We observed two distinct SAW absorption signals attributed to the perpendicular standing spin waves (PSSW). Furthermore, the large observed nonreciprocity is compared to theoretical predictions incorporating the influence of asymmetric surface conditions on PSSW mode profiles and nonreciprocal magnon-SAW coupling. The strongly nonreciprocal SAW attenuation in YIG offers a promising opportunity for highly efficient rf signal processing technologies.

The anisotropic quantum spin-$\frac{1}{2}$ XY model on a linear chain was solved by Lieb, Schultz, and Mattis [Ann. Phys. 16, 407 (1961)] and shown to display a continuous quantum phase transition at the O(2) symmetric point separating two gapped phases with competing Ising long-range order. For the square lattice, the following is known. The two competing Ising ordered phases extend to finite temperatures, up to a boundary where a transition to the paramagnetic phase occurs, and meet at the O(2) symmetric critical line along the temperature axis that ends at a tricritical point at the Berezinskii-Kosterlitz-Thouless transition temperature where the two competing phases meet the paramagnetic phase. We show that the first-order zero-temperature (quantum) phase transition that separates the competing phases as a function of the anisotropy parameter is smoothed by thermal fluctuations into deconfined classical criticality.

The discovery of high-temperature superconductivity in FeSe/${\mathrm{SrTiO}}_3$ has sparked significant interest in exploring superconducting systems with engineered interfaces. Here, using molecular beam epitaxy growth, we successfully fabricate FeSe/$\mathrm{Pb}{\mathrm{O}}_x$ heterostructures and discover superconductivities in three different monolayer FeSe-related interfaces. We observe superconducting gaps of 13–14 meV in the monolayer FeSe films grown on two different phases of $\mathrm{Pb}{\mathrm{O}}_x$. Moreover, we discover an insulating ${\mathrm{Fe}}_{10}{\mathrm{Se}}_9$ phase with an ordered √5 × √5 Se-vacancy structure. Our first-principles calculation suggests that this insulating phase originates from electronic correlation. Intriguingly, an additional monolayer FeSe film grown on insulating ${\mathrm{Fe}}_{10}{\mathrm{Se}}_9$ also exhibits superconductivity with the gap size of 5 meV. Our results suggest that the work function differences between monolayer FeSe and the substrates, which can induce band bending and charge transfer, are crucial for interfacial superconductivity.

A generalized Sellmeier model, also referred to as the Lorentz-Dirac model, has been used for the description of the dielectric function of a number of technologically important materials in the literature. This model represents the frequency-dependent dielectric function as a sum over Green functions of classical damped harmonic oscillators, much in analogy with the functional form used for the dynamic polarizability of an atom, but with one important addition, namely, a complex-valued oscillator strength in the numerator. Here, we show that this generalized functional form can be justified based on the response function of coupled damped oscillators. The encountered analogies suggest an explanation for the generally observed success of the Lorentz-Dirac model in describing the dielectric function of crystals of consummate technological significance.

We theoretically study the impact of light-driven structural changes via nonlinear phononics on the magnetic order of untwisted bilayer van der Waals materials. We consider an illustrative example of the AA-stacked bilayer honeycomb lattice and show that high-intensity light in resonance with selected phonons induces large-amplitude phonon displacements that modify the magnetic Hamiltonian of the system. We perform a group theory analysis to identify the vibrational modes of the honeycomb bilayer and the nonlinear couplings among them in the strongly driven regime. We find that the structural changes in the strongly driven regime lower the symmetry relative to the equilibrium lattice and produce changes in the magnetic interactions between the local moments. In particular, the lattice symmetry changes permit a nonzero interlayer Dzyaloshinskii-Moriya interaction that induces a magnetic state with canted local moments. Using a spin-wave analysis around the new magnetic configuration, we study the corresponding changes in the magnon spectrum and identify a protocol for engineering topological band transitions using a combination of nonlinear phononics and an external magnetic field. Our paper suggests a strategy to induce and control interlayer Dzyaloshinskii-Moriya interactions in a class of layered van der Waals materials, the effect of which is to modify the magnetic ground state, magnon dispersions, and related band geometric properties, including topological invariants.

Building on recent advances of the embedded cluster approach combined with multiconfigurational theory, this work investigates the electronic states in thorium-doped ${\mathrm{CaF}}_2$ crystals. $\mathrm{Th}\text{:}{\mathrm{CaF}}_2$ is established as a promising material for solid-state nuclear clocks, which utilize the laser-accessible isomeric state in thorium-229. By comparing simulated absorption spectra of a library of defect configurations with experimental data, we demonstrate the impact of fluorine vacancies and calcium vacancies on the $\mathrm{Th}\text{:}{\mathrm{CaF}}_2$ electronic structure. Our results indicate that fluorine-deficient sites can introduce local electronic states within the band gap, resonant with the isomer energy, potentially contributing to nonradiative decay or quenching of the Th-229 isomer. We also explore the potential of electron-nuclear-bridge mechanisms to enhance nuclear excitation or deexcitation, offering a pathway for more efficient control over the nuclear clock. This study provides key insights for optimizing the crystal environment for nuclear metrology applications and opens new avenues for further experimental and theoretical exploration of thorium-doped ionic crystals.

We theoretically investigate heavy-hole–light-hole mixing in two-dimensional hole gases (2DHG). We restrict our analysis to the zone center, appropriate for the low-density regime, which leads to a simple description, analytical results, and physical insights. We identify two different types of hole-Hamiltonian terms concerning mixing. The first type changes the direction of the pure spinors, without admixing light-hole components. It is efficient for Rabi driving the heavy-hole spin. The second type induces mixing and changes the eigenvalues of the $g$ tensor. We analyze several measures that characterize the mixing quantitatively in Ge, Si, and GaAs, namely the $g$ factor, the light-hole weight in the wave function, the off-diagonal matrix elements in the Hamiltonian, and the strength of the induced spin-orbit interaction. We identify the canonical coordinate frame associated with a generic spin-3/2 Hamiltonian with time-reversal symmetry (TRS). In this coordinate frame, the mixing is quantified by a single parameter, the mixing angle $\vartheta$. We interpret it as the canonical (coordinate-frame and Hamiltonian-basis independent) measure of the heavy-hole–light-hole mixing. All the investigated mixing measures are simple functions of $\vartheta$. As an illustration, we use our model to analyze heavy-hole spin qubit $g$-tensor, dephasing, relaxation, and Rabi frequencies, interpreting the arising effects as due to rotations of canonical frame and changes of the mixing angle $\vartheta$.

We theoretically investigate a nonlinear optical response of a planar microcavity with an embedded transition metal dicalcogenide monolayer when the energy of a biexcitonic transition is brought in resonance with the energy of a cavity mode. We demonstrate that the emission spectrum of this system strongly depends on an external pump. For small and moderate pumps, we reveal the presence of a doublet in the emission with the corresponding Rabi splitting scaling as a square root of the number of the excitations in the system. Further increase of the pump leads to the reshaping of the spectrum, which demonstrates, at weak damping, the pattern akin a Mollow triplet. An intermediate pumping regime shows a broad irregular spectrum reminiscent of a chaotic dynamics of the system.

The interplay between symmetry and topology led to the concept of symmetry-protected topological states, including all noninteracting and weakly interacting topological quantum states. Among them, recently proposed nodal line semimetal states with space-time inversion ($\mathcal{PT}$) symmetry which are classified by the Stiefel-Whitney characteristic class associated with real vector bundles and can carry a nontrivial ${\mathbb{Z}}_2$ monopole charge have attracted widespread attention. However, such three-dimensional (3D) ${\mathbb{Z}}_2$ nodal line semimetals have been poorly studied, such as how to construct them and how to predict them efficiently. We generally construct 3D ${\mathbb{Z}}_2$ nodal line semimetals by stacking of 2D $\mathcal{PT}$-symmetric Dirac semimetals via nonsymmorphic symmetries. Based on our construction scheme, numerous models can be produced. Combined with first-principles calculations, we predict abundant candidate electronic and phononic materials for ${\mathbb{Z}}_2$ nodal line semimetals, namely two kinds of electronic materials: fourteen Si and Ge structures and 108 transition metal dichalcogenides $M{X}_2$ ($M=\mathrm{Cr}$, Mo, W, $X=\mathrm{S}$, Se, Te) and one kind of phononic material: SiH. Our theoretical construction scheme can be directly applied to metamaterials and circuit systems. Our work greatly enriches the candidate materials and deepens the understanding of ${\mathbb{Z}}_2$ nodal line semimetal states.

Complex materials encompassing different phases of matter can display new light-induced states differing from those attainable under equilibrium conditions. These states can be realized when energy is injected in the material following a nonequilibrium pathway, unbalancing the unperturbed energy landscape of the material. Guided by the fact that photoemission experiments allow for detailed insights in the electronic band structure of ordered systems, here we study bulk $1T\text{−}{\mathrm{VSe}}_2$ in its metallic and charge-density-wave phase by time- and angle-resolved photoelectron spectroscopy. After near-infrared optical excitation, the system shows a net increase of the density of states in the energy range of the valence bands, in the vicinity of the Fermi level, lasting for several picoseconds. We discuss possible origins as band shifts or correlation effects on the basis of a band structure analysis. Our results uncover the possibility of altering the electronic band structure of bulk $1T\text{−}{\mathrm{VSe}}_2$ for low excitation fluences, contributing to the understanding of light-induced electronic states.

Recursion methods such as Krylov techniques map complex dynamics to an effective noninteracting problem in one dimension. For example, the operator Krylov space for Floquet dynamics can be mapped to the dynamics of an edge operator of the one-dimensional Floquet inhomogeneous transverse field Ising model (ITFIM), where the latter, after a Jordan-Wigner transformation, is a Floquet model of noninteracting Majorana fermions and the couplings correspond to Krylov angles. We present an application of this showing that a moment method exists where given an autocorrelation function, one can construct the corresponding Krylov angles and from that the corresponding Floquet ITFIM. Consequently, when no solutions for the Krylov angles are obtained, it indicates that the autocorrelation is not generated by unitary dynamics. We highlight this by studying certain special cases: stable $m$-period dynamics derived using the method of continued fractions, exponentially decaying, and power-law decaying stroboscopic dynamics. Remarkably, our examples of stable $m$-period dynamics correspond to $m$-period edge modes for the Floquet ITFIM where, deep in the chain, the couplings correspond to a critical phase. Our results pave the way to engineer Floquet systems with desired properties of edge modes and also provide examples of persistent edge modes in gapless Floquet systems.

We investigate the pairing state in a doped band insulator under a periodic driving field. We employ a correlated fermionic model on a honeycomb lattice, in which pairing glue is obtainable via repulsive interactions, and derive an effective model under circularly polarized light. We demonstrate that the effective pairing interaction for doped fermions obtained with the second-order perturbation theory is tunable by the frequency and amplitude of the driving field. We find the optimal frequency range to enhance the pairing interaction and show that the modified effective system can strengthen the two-body bound state. Our study suggests that external driving light can reinforce superconducting pair states in strongly correlated electron systems.

Using scanning tunneling microscopy observations, the $\mathrm{Si}\left(111\right)\sqrt{3}\times \sqrt{3}\text{−}\mathrm{B}$ surface was experimentally proved to be a suitable substrate for hosting high-buckled plumbene. Magnesium (Mg) adsorbed onto the grown plumbene intercalates between the Pb layer and $\mathrm{Si}\left(111\right)\sqrt{3}\times \sqrt{3}\text{−}\mathrm{B}$ substrate to form the Pb/Mg/Si(111):B atomic sandwich. Meanwhile, the Pb layer undergoes significant transformations: its original coverage of 2.0 monolayers (ML) reduces to 1.33 ML of Pb, as revealed via the density functional theory (DFT) ab initio random structure search analysis. The layer is arranged in a striped $3\times 1$ structure, which, together with the substrate, adopts the overall $3\times 3$ periodicity. DFT analysis of the band structures of the grown Pb layers revealed that they might possess unique electronic properties. However, the intrinsic properties of the layers appear to be overshadowed by the electronic properties of the real Si(111):B substrate, which underwent a strong subsurface $p\text{−}\mathrm{type}$ doping. The dominated contribution of the substrate bands to the electronic properties was confirmed in the in situ low-temperature transport measurements, which showed that the Si(111):B substrate with and without plumbene, as well as Pb/Mg/Si(111):B samples, display very similar conductivity values. The obtained results demonstrate that, while considering possible applications, the substrate hosting the prospective low-dimensional structures (e.g., two-dimensional Xenes) must meet not only the structural but also the electronic requirements.

In this work, we study the transport of vorticity on curved dynamical two-dimensional magnetic membranes. We find that topological transport can be controlled by geometrically reducing symmetries, enabling processes absent from flat magnetic systems. To this end, the vorticity 3-current is constructed, which obeys a continuity equation immune to local disturbances of the magnetic texture and spatiotemporal fluctuations of the membrane. We show how electric current can manipulate vortex transport in geometrically nontrivial magnetic systems. As an illustrative example, we propose a minimal setup that realizes an experimentally feasible energy storage device and discuss its thermodynamic efficiency in terms of a vortexoelectric counterpart of the thermoelectric figure of merit $ZT$.