Bilayers made of the high-$T_c$ cuprate superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ assembled with a twist angle close to ${45}^{\circ }$ have been recently shown to spontaneously break time-reversal symmetry $\mathcal{T}$, consistent with theoretical predictions for emergent chiral topological $d_{x^2-y^2}+i{d}_{xy}$ phase in such twisted $d$-wave superconductors. Here we use a minimal microscopic model to estimate the size of spontaneous chiral edge currents expected to occur in the $\mathcal{T}$-broken phase. In accord with previous theoretical studies of chiral $d$-wave superconductors we find small but nonvanishing edge currents whose magnitude and spatial distribution are sensitive to the type of the edge or domain wall. We nevertheless predict these to be above the detection threshold of the state-of-the-art magnetic scanning probe microscopy. In addition, by deriving a simple relation between the edge current and the electron spectral function we help elucidate the longstanding disparity between the size of edge currents in chiral $d$-wave and $p$-wave superconductors.

While polar metals are a metallic analog of ferroelectrics, magnetic polar metals can be considered as a metallic analog of multiferroics. There have been a number of attempts to integrate magnetism into a polar metal by synthesizing new materials or heterostructures. Here we use a simple yet widely used approach—epitaxial strain in the search for intrinsic magnetic polar metals. Via first-principles calculations, we study strain engineering of a ferromagnetic metallic oxide ${\mathrm{SrCoO}}_3$, whose bulk form crystallizes in a cubic structure. We find that under an experimentally feasible biaxial strain on the $ab$ plane, collective Co polar displacements are stabilized in ${\mathrm{SrCoO}}_3$. Specifically, a compressive strain stabilizes Co polar displacements along the $c$ axis, while a tensile strain stabilizes Co polar displacements along the diagonal line in the $ab$ plane. In both cases, we find an intrinsic ferromagnetic polar metallic state in ${\mathrm{SrCoO}}_3$. In addition, we also find that a sufficiently large biaxial strain ($>4\%$) can yield a ferromagnetic-to-antiferromagnetic transition in ${\mathrm{SrCoO}}_3$. Our work demonstrates that in addition to yielding emergent multiferroics, epitaxial strain is also a viable approach to inducing magnetic polar metallic states in quantum materials.

At least since the discovery of graphene and the subsequent finding of a plethora of other 2D materials, it is well anticipated that the dimensionality of a material may constitute a functional parameter. In this paper, we discuss zero-field ${}^{53}\mathrm{Cr}$ nuclear magnetic resonance (NMR) measured in the magnetically ordered state and ${}^{35}\mathrm{Cl}$ nuclear quadruple resonance (NQR) data derived in the paramagnetic state of the two-dimensional van der Waals material ${\text{CrCl}}_3$, comparing the results for a bulk single crystal and a nanocrystal. In particular, we apply these spectroscopic methods to monitor the evolution of local environments in the single crystal across the structural phase transition and compare the structural and magnetic properties of a bulk single crystal and nanocrystal sample at low temperatures. The actual structural transition is reported to be of first order, where a certain hysteresis is to be expected. However, we see that both the high- and low-temperature phases coexist in both sample types across the full temperature range (300 K–1.5 K) albeit with different phase fractions. This coexistence of phases in different sample types originates in a kinetic arrest where the arrested structural domains are related to defects and stacking faults. Such defects are to a large part found in the nanocrystal but to a smaller extent in the bulk single crystal. These frozen-in phases have further consequences: The critical exponent $\beta$, derived by fitting the ${}^{53}\mathrm{Cr}$ NMR data, is considered to denote the dimensionality of magnetic interactions. Here, the values differ considerably for both sample types. Probably, the difference in $\beta$ arises from the specific domain structure of kinetically arrested phases and, in turn, from an altered interlayer magnetic exchange mediated by magnetic moments related to frozen-in domains that are related to defects and stacking faults,with their number being much higher in the nanocrystal. These findings may, in part, explain the different magnetic properties reported for different samples as their individual defect landscape determines the kinetically arrested phase fraction in ${\mathrm{CrCl}}_3$. Hence, the structure-property relation in ${\mathrm{CrCl}}_3$ is even more complex than anticipated.

This paper presents a systematic approach to derive physical bounds for passive systems, or equivalently for positive real (PR) functions, directly in the time-domain (TD). As a generic, canonical example we explore the TD dielectric response of a passive material. We will furthermore revisit the theoretical foundation regarding the Brendel-Bormann (BB) oscillator model which is reportedly very suitable for the modeling of thin metallic films in high-speed optoelectronic devices. To this end, an important result here is to re-establish the physical realizability of the BB model by showing that it represents a passive and causal system. The theory is based on Cauer's representation of an arbitrary PR function together with associated sum rules (moments of the measure) and exploits the unilateral Laplace transform to derive rigorous bounds on the TD response of a passive system. Similar bounds have recently been reported for more general casual systems with other a priori assumptions. To this end, it is important to note here that the existence of useful sum rules and related physical bounds rely heavily on an assumption about the PR functions having a low- or high-frequency asymptotic expansion at least of odd order 1. As a particular numerical example, we consider here the electric susceptibility of gold (Au) which is commonly modeled by well established Drude or BB models. Explicit physical bounds are given as well as an efficient fast-Fourier transform -based numerical procedure to compute the TD impulse response associated with the nonrational BB model.

Two well-known Hall-like effects occur in ferromagnets: the anomalous Hall effect (AHE) and the planar Hall effect (PHE). AHE is analogous to the classical Hall effect and is characterized by the antireciprocal Onsager relation (antisymmetric conductivity matrix), whereas PHE is defined by the reciprocal Onsager relation (symmetric conductivity matrix). The distinction is fundamental, as it stems from time-reversal symmetry breaking at the microscopic scale. We examine theoretically the Hall current generated in both AHE and PHE, along with the electric power that can be transferred from the edges of the Hall bar into a load circuit. Using a variational method based on the second law of thermodynamics, we derive expressions for the distribution of the electric currents, the distribution of electric carriers, and the power efficiencies. Our results show that the distribution of the transverse Hall current is identical for both AHE and PHE (with all other parameters being equals) but the longitudinal current and the power dissipated differ at the second order in the Hall angle.

The discovery of the super-Earth bodies has increased the need to understand planet-forming materials at extremely high pressures, crucial for insights into planetary formation and interior dynamics. Recent progress in high-pressure core-electron excitation spectroscopy, such as x-ray Raman scattering (XRS) experiments, and theoretical calculations offer an opportunity to investigate the evolution of the electronic structures of various compressed materials under such extreme pressure conditions. Despite its significance in planetary and condensed matter physics, the electronic structure of MgO, a major component of super-Earths and a prototypical ionic compound, have not yet been fully studied at pressures corresponding to the deep interiors of super-Earth bodies due to the experimental challenges. Here, we present a theoretical investigation of the XRS spectra, band structures, electron localization functions (ELFs), and Bader charges of MgO at pressures up to ∼4 TPa. The XRS patterns show the pressure-driven changes in XRS spectral shape and the emergence of new peaks with different slopes (with respect to density and interatomic distances), resulting from enhanced interatomic interactions (i.e., enhanced hybridization between O $2p$ and Mg $3s\text{−}3p$ states and between adjacent O $2p$ states). The calculated results are characterized with a pressure-driven delocalization of the unoccupied O $p$ and Mg $p$ states, revealing the electronic bonding behavior of MgO under extreme compression, far beyond the current experimental limit with XRS of ∼200 GPa. The ELF analysis provides a comprehensive understanding of the factors driving the nonlinear trend of absorption edge onset in the calculated XRS patterns and the direct-to-indirect band gap transition of MgO under compression. Together with the Bader charge analysis, it also shows how MgO undergoes a pressure-induced bonding transition from ionic to mixed ionic-covalent without a structural phase transition. This transition of the bonding behavior of MgO under extreme compression contributes to our understanding of material properties under extreme conditions relevant to the interiors of super-Earths. The current result for the prototypical ionic compound under extreme densification offers insights into deepening the fundamental understanding of ionic compounds under extremely high pressures. As the extreme pressure conditions of the current simulations cannot yet be reached in experiment, our breakthrough would be useful to guide future experimental efforts aimed at uncovering metal oxides with pressure-tunable electronic properties.

Surface polaritons in an anisotropic media possess a strong dependence of the wave vector on the propagation direction, which is called the isofrequency contour. This can lead to the fact that polariton propagation is possible only in a limited range of angles in the boundary plane. Notable examples are Dyakonov surface waves at the boundary of two dielectrics and hyperbolic plasmons in a hyperbolic metamaterial. Exact closed-form solutions of the polariton dispersion equation are known only in special cases: in a weakly anisotropic medium and in an arbitrary medium for highly symmetric directions of polariton propagation. This work provides an universal exact solution in algebraic form for a surface polariton at the interface of arbitrary isotropic and uniaxial media for the case of the optic axis parallel to the boundary. As an example, it is used to analyze the shapes of isofrequency contours of surface polaritons. The work brings together previously scattered results of studies on surface polaritons of various types in uniaxial media. In addition, two cases not previously considered in the literature are analyzed in detail here. The first corresponds to the boundary of an isotropic metal and a Type I hyperbolic medium, for which the existence of a surface polariton is predicted. In the second case, “elliptic” surface polaritons at the boundary of an isotropic dielectric and an anisotropic metal-like medium are analyzed.

This work presents an alternative scheme for modulating near-field radiative heat transfer (NFRHT) within an extended planar many-body system, marking a significant stride in thermal management for micro- and nanodevices. Our approach leverages an adjustable number of interacting bodies, offering a dynamic solution to enhance or suppress NFRHT across diverse configurations. Utilizing the general Green's function approach alongside the scattering matrix method, we establish a Landauer-like formalism, providing a clear, comprehensive framework for calculating NFRHT in many-body systems. This framework adeptly accommodates variations in physical parameters such as layer thickness, separation distance, and material properties, thus facilitating a broad spectrum of many-body planar configurations. Our findings highlight the substantial potential of introducing intermediate layers between traditional two-body systems, enabling additional heat transfer channels and significantly influencing the overall radiative heat flux. By adjusting the relative positions of these layers, the system transitions seamlessly between two-body, three-body, and four-body configurations, offering versatile heat flux modulation. The introduction of artificially engineered materials as intermediate layers further amplifies this effect, showcasing a consistent heat flux enhancement across extended gap distances ($d$) and thicknesses ($t$) up to 225 nm, with the maximum amplification of $60\%$ occurring in regions where $t\approx d$. This investigation provides insights into the interactions within complex many-body systems and introduces a practical methodology for precise thermal flux control, paving the way for advancements in nanoscale energy harvesting, photonics, and sensing devices.

Accurate prediction of thermal transport properties and fundamental understanding of multiphonon interaction in strong anharmonic systems are nontrivial but challenging in condensed matter physics and materials science. Herein, we investigate the anharmonic phonon self-energy, lattice dynamic, and thermal transport in quaternary compound $\mathrm{BaA}{\mathrm{g}}_2\mathrm{SnS}{\mathrm{e}}_4$ using the self-consistent phonon (SCP) theory and the Wigner thermal transport model. At each temperature ($T$), we utilize the SCP method, which includes the bubble and loop diagrams, to calculate anharmonic phonon frequencies and obtain particle-like phonon propagate and wavelike phonon coherent thermal conductivity within the framework of the Peierls-Boltzmann transport equation. Results show that (i) the rattling-like vibrations of the disordered Ag-Ag dimers result in anharmonic non-Lorentzian–shape phonon energy power spectral for some low-energy phonons and thus these phonon modes are overdamped; (ii) anharmonic hardening effect suppresses the low-frequency phonon scattering strength via diminishing scattering phase space, offsetting the phonon population effect as $T$ increases; and (iii) strong phonon broadening leads to non-negligible coherent contribution, which contributes up to 35.7% of the total lattice thermal conductivity (${\kappa }_{\mathrm{L}}$) at 800 K, indicating partial breakdown of the conventional phonon gas model. Taking into account the phonon anharmonic renormalization and the two-channel phonon thermal transport can effectively resolve the discrepancy between theoretical and experimental values of ${\kappa }_{\mathrm{L}}$ and yield a better description, both in magnitude and $T$ dependence. Our work demonstrates the effect of phonon broadening and frequency shift induced by multiphonon interaction on ${\kappa }_{\mathrm{L}}$ and deepens the understanding of thermal transport in disordered crystals with strong anharmonicity.

The adsorption layer of 1,4-cyclohexadiene on Si(001)-(2 × 1) is one of the most studied organic/semiconductor interfaces. To this day the stable structure of the adsorption layer and its dependence on pressure and temperature are hypothetical. In this work, we have developed several lattice models of the adsorption layer of 1,4-cyclohexadiene (1,4-CHD) on Si(001)-(2 × 1), taking into account the main features of the chemisorbed layer: different types of adsorption complexes and their asymmetry, and repulsive intermolecular interactions. A unique combination of density functional theory methods and the tensor renormalization group within the framework of the developed models allowed us to reveal the equilibrium structure of 1,4-CHD monolayer on the Si(001)-(2 × 1) surface. We have found four stable phases in the most advanced model. There are two phases consisting of tetra-bound (tetra-σ) complexes only: the phase of mixed composition with tetra-bound and double-bound (di-σ) complexes, and the close-packed phase, where each $\mathrm{Si}=\mathrm{Si}$ dimer of the Si(001)-(2 × 1) surface is occupied by a di-σ complex. All the phases differ in the number of 1,4-CHD molecules per $\mathrm{Si}=\mathrm{Si}$ dimer. Repulsive intermolecular interactions along the row of $\mathrm{Si}=\mathrm{Si}$ dimers are demonstrated to determine the self-assembly of two phases. Those are the phase of the lowest density that comprises only the tetra-bound complexes and the phase of mixed composition. Repulsion between 1,4-CHD molecules adsorbed on neighboring rows of $\mathrm{Si}=\mathrm{Si}$ dimers leads to a coordinated filling of these rows. As a result, the existence region of the mixed phase consisting of the tetra-σ and di-σ complexes narrows. The transition from the phase consisting of tetra-bound complexes to the phase of mixed composition is found to occur abruptly through a first-order phase transition.

Topological quantum computation by way of braiding of Majorana fermions is not universal quantum computation. There are several attempts to make universal quantum computation by introducing some additional quantum gates or quantum states. However, there is a serious problem in embedding an $M$-qubit quantum gate in the $N$-qubit system with $N>M$. This is an inherent problem to the Majorana system, where quantum gates for logical qubits become nonlocal in terms of physical qubits in general because braiding operations preserve the fermion parity. For instance, the CZ gate could not be embedded in the three-qubit system. We overcome this embedding problem by introducing $2(N+1)$-body interactions of Majorana fermions in the $N$-qubit system. A universal set of quantum gates is constructed for $N$ logical qubits, leading to topological-nontopological hybrid universal quantum computation. It would be more robust than conventional universal quantum computation because the quantum gates generated by braiding are topologically protected.

We introduce a one-dimensional correlated-hopping model of spinless fermions in which a particle can hop between two neighboring sites only if the sites to the left and right of those two sites have different particle numbers. Using a bond-to-site mapping, this model involving four-site terms can be mapped to an assisted pair-flipping model involving only three-site terms. This model shows strong Hilbert space fragmentation. We define irreducible strings (ISs) to label the different fragments, determine the number of fragments, and the sizes of fragments corresponding to some special ISs. In some classes of fragments, the Hamiltonian can be diagonalized completely, and in others it can be seen to have a structure characteristic of models which are not fully integrable. In the largest fragment in our model, the number of states grows exponentially with the system size, but the ratio of this number to the total Hilbert space size tends to zero exponentially in the thermodynamic limit. Within this fragment, we provide numerical evidence that only a weak version of the eigenstate thermalization hypothesis (ETH) remains valid; we call this subspace-restricted ETH. To understand the out-of-equilibrium dynamics of the model, we study the infinite-temperature time-dependent autocorrelation functions starting from a random initial state; we find that these exhibit a different behavior near the boundary compared to the bulk. Finally, we propose an experimental setup to realize our correlated-hopping model.

We analyze the implementation of high-fidelity, phonon-mediated gate operations and quantum simulation schemes for spin qubits associated with silicon vacancy centers in diamond. Specifically, we show how the application of continuous dynamical decoupling techniques can substantially boost the coherence of the qubit states while increasing at the same time the variety of effective spin models that can be implemented in this way. Based on realistic models and detailed numerical simulations, we demonstrate that this decoupling technique can suppress gate errors by more than two orders of magnitude and enable gate infidelities below $\sim {10}^{-4}$ for experimentally relevant noise parameters. Therefore, when generalized to phononic lattices with arrays of implanted defect centers, this approach offers a realistic path toward moderate- and large-scale quantum devices with spins and phonons at a level of control that is competitive with other leading quantum-technology platforms.

Breaking parity ($\mathcal{P}$) symmetry in $C_6$ crystals is a common routine to implement a valley topological phase. At an interface between two crystals of opposite valley Chern numbers, the so-called valley topological edge states (VTESs) emerge, and they have been proven useful for wave transport with robustness against ${120}^{\circ }$ bending and a certain level of disorder. Here, we first revisit the bending-immune mechanism of VTESs by developing a lattice coupled mode theory to analyze coupling dynamics between VTESs, and further performing a perturbative analysis to estimate the Bragg scattering at the bending corner. Our analysis quantitatively evidences that the bending reflection is suppressed by the unique modal profiles of the VTESs—momentum hot spots localized at high-symmetric $K$ ($K^{\prime }$) points—which only concerns the existence of the valleys in the bulk band diagrams, and imposes no requirement on topology. Second, we reveal the emergence of non-Hermitian supercoupling (SC) effects in (Hermitian) valley systems: Counterflow, momentum-matched incident and transmission ESs could reach $\sim 100\%$ coupling efficiency free of corner reflection and construct an unusual superexponential field distribution. Based on two findings, a coherent understanding of bending immunity for ESs in valley systems is established, and different designs based on rhombic lattices are proposed. Additionally, we demonstrate that the emerged superexponential field profile due to the SC effects provides another way to engineer the photonic Purcell factor.

Utilizing ultrafast optical pump-probe spectroscopy, we present a comprehensive investigation on the photoexcited quasiparticle dynamics in single-crystal ${\mathrm{CeAgSb}}_2$. We elucidate the emergence of collective $c\text{−}f$ hybridization between conduction electrons and localized $f$ moments below a critical temperature $T^{*}$ ($\approx 65$ K), as indicated by the onset of an indirect gap of $\sim 9$ meV via the fluence-dependent quasiparticle relaxation and phonon renormalization processes. Our experimental data also suggest that the $c\text{−}f$ hybridization fluctuation should appear at a much higher-temperature value, i.e., $T^{†}$ ($\approx 180$ K). Below $\sim 50$ K, we further detect an additional quasiparticle relaxation channel with sub-ps timescale, which can be attributed to the crystalline electric field (CEF) excitation. These observations provide important insights into the Kondo lattice coherence and CEF effect in ${\mathrm{CeAgSb}}_2$, shedding light on its unique properties and behaviors.

The out-of-time-order correlator (OTOC) measures the propagation and scrambling of local quantum information. For the transverse field Ising model with open boundaries, the local operator ${\sigma }^x$ shows an interesting picture of the ground state OTOC where the local information alternately gets scrambled and then “unscrambled” upon reflection at the ends. Earlier discussions of OTOCs did not explain the physical processes responsible for such scrambling and unscrambling. We explicitly show that in the paramagnetic phase, the scrambling and unscrambling is due to the scattering of a pair of low-energy spin-flip excitations, and extend it to incorporate integrability-breaking interactions. In the ferromagnetic phase these are explained by the motion of a domain-wall excitation, and we find that unscrambling survives up to moderate values of interactions. Thus, in different limits of the system parameters, we provide a simple understanding of the OTOCs, including the unscrambling, in terms of the low-energy excitations like spin flips or domain walls. We mention a system where our results can be experimentally tested.

Entanglement measures have emerged as one of the versatile probes to diagnose quantum phases and their transitions. Universal features in them expand their applicability to a range of systems, including those with quenched disorder. In this Letter, we show that when the underlying lattice has percolation disorder, free fermions at a finite density show interesting entanglement properties due to massively degenerate ground states. We define and calculate appropriate entanglement measures such as typical, annealed, and quenched entanglement entropy in both one and two dimensions, showing they can capture both geometrical aspects and electronic correlations of the percolated quantum system. In particular, while typical and annealed entanglements show a volume law character directly dependent on the number of zero modes in the system, quenched entanglement is generally an area law albeit showing characteristic signatures of the classical percolation transition. Our work presents an exotic interplay between the geometrical properties of a lattice and quantum entanglement in a many-body quantum system.

Flexoelectric effects in materials can bring novel physical properties that are absent in their perfect crystal, and have a wide range of applications, such as mechanical sensors and wrinkled triboelectric nanogenerators. In this work, electronic structures and transport properties of bended $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ monolayer are investigated through first-principles calculations and nonequilibrium Green's function (NEGF). We find that two different kinds of type-II band structures can be obtained in $\tilde{P}\uparrow$ and $\tilde{P}\downarrow$ flexed $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$, which show opposite band bending. Carriers in the center of $\tilde{P}\uparrow$ and $\tilde{P}\downarrow$ flexed $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ are mainly holes and electrons, respectively, which dominate the current behavior of the $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ p-i-n (PIN) field-effect transistor (FET). The $\tilde{P}\uparrow$ PIN-FET has enhanced forward current and the rectification ratio due to the larger density of holes. Our study achieves the homogeneous junction through bended $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$, which may simplify the device procession and be used as electromechanical sensors.