Multiferroic tunnel junctions (MFTJs) based on two-dimensional van der Waals heterostructures with sharp and clean interfaces at the atomic scale are crucial for applications in nanoscale multiresistive logic memory devices. The recently discovered sliding ferroelectricity in 2D van der Waals materials has opened new avenues for ferroelectric-based devices. Here, we theoretically investigate the spin-dependent electronic transport properties of ${\mathrm{Fe}}_3\mathrm{Ge}{\mathrm{Te}}_2/\mathrm{graphene}/\mathrm{bilayer}\text{−}h\text{−}\mathrm{BN}/\mathrm{graphene}/\mathrm{Cr}{\mathrm{I}}_3$ (FGT/Gr-BBN-Gr/CrI) all-vdW MFTJs by employing the nonequilibrium Green's function combined with density functional theory. We demonstrate that such FGT/Gr-BBN-Gr/CrI MFTJs exhibit four nonvolatile resistance states associated with different staking orders of sliding ferroelectric BBN and magnetization alignment of ferromagnetic free layer $\mathrm{Cr}{\mathrm{I}}_3$, with a maximum tunnel magnetoresistance (electroresistance) ratio, i.e., TMR (TER) up to $\sim 3.36\times {10}^4\%(\sim 6.68\times {10}^3\%)$ at a specific bias voltage. Furthermore, the perfect spin filtering and remarkable negative differential resistance effects are evident in our MFTJs. We further discover that the TMR, TER, and spin-polarization ratio under an equilibrium state can be enhanced by the application of in-plane biaxial strain. This paper highlights the significant potential of sliding ferroelectric BBN-based MFTJs in nonvolatile memories, showcasing their giant tunneling resistance ratio, multiple resistance states, and excellent spin-polarized transport properties.

With first-principles calculations and theoretical models, we reveal the connection between stacking order, film thickness, and topological behaviors in layered $M_2{\mathrm{Bi}}_2{\mathrm{Te}}_5 (M=\mathrm{Ge},\mathrm{Sn},\mathrm{Pb})$ films. We find that single-layer $M_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ is a topologically trivial indirect band-gap semiconductor. The stacking order drastically tunes the topological property for bilayer $M_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$. In the most stable AB stacking order, bilayer ${\mathrm{Pb}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ is a quantum spin Hall (QSH) insulator with a large band gap of 111.6 meV, accessible for experimental observation. Upon changing the stacking order, one can achieve topological phase transitions between nontrivial and trivial states due to the distinct interlayer coupling. Peculiarly, the multilayer ${\mathrm{Pb}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ film exhibits topological oscillation effect as the film thickness increases from one to six layers. A simplified model Hamiltonian with nearest-neighbor interlayer coupling has been proposed to understand this oscillation. In this paper, we provide a material platform for the realization of controllable topological states, which might stimulate potential applications of topological effects.

Majorana zero modes (MZMs) emerge as edge states in topological superconductors and are promising for topological quantum computation, but their detection has so far been elusive. Here we show that non-Hermiticity can be used to obtain dramatically more robust MZMs. The enhanced properties appear as a result of an extreme instability of exceptional points to superconductivity, such that even a vanishingly small superconducting order parameter already opens a large energy gap, produces well-localized MZMs, and leads to strong superconducting pair correlations. Our work thus illustrates the large potential of enhancing electronic ordering, here in the form of topological superconductivity, using non-Hermitian exceptional points.

Two-dimensional multiferroic materials are highly sought after due to their huge potential for applications in nanoelectronic and spintronic devices. Here, we predict, based on first-principle calculations, a single-phase triferroic where three ferroic orders—ferromagnetism, ferroelectricity, and ferroelasticity—coexist simultaneously in the hole doped ${\mathrm{GdCl}}_2$ monolayer (a ferromagnetic semiconductor). This is achieved by substituting 1/3rd of the ${\mathrm{Gd}}^{2+}$ ions with ${\mathrm{Eu}}^{2+}$ in the hexagonal structure of the ${\mathrm{GdCl}}_2$ monolayer. The resulting metallic state undergoes a bond-centered charge ordering driving a distortion in the hexagonal structure, making it semiconducting again and ferroelastic. Further, the lattice distortion accompanied by a breaking of the lattice centrosymmetry renders a noncentrosymmetric charge distribution, which makes the monolayer ferroelectric, at the same time. The two ferroic orders, ferroelectricity and ferroelasticity, present in the Eu-substituted ${\mathrm{GdCl}}_2$ monolayer are found to be strongly coupled, making it a promising candidate for device applications. The Eu-substituted monolayer remains a ferromagnetic semiconductor with a large $4f$ magnetic moment just like the parent monolayer and possesses an even higher (out-of-plane) magnetic anisotropy energy than its pristine counterpart as desired for two-dimensional magnets to have high transition temperature.

We propose a highly scalable method to compute the statistics of charge transfer in driven conductors. The framework can be applied in situations of nonzero temperature, strong coupling to terminals, and in the presence of nonperiodic light-matter interactions, away from equilibrium. The approach combines the so-called mesoscopic leads formalism with full counting statistics. It results in a generalized quantum master equation that dictates the dynamics of current fluctuations and higher order moments of the probability distribution function of charge exchange. For generic time-dependent quadratic Hamiltonians, we provide closed-form expressions for computing noise in the nonperturbative regime of the parameters of the system, reservoir, or system-reservoir interactions. Having access to the full dynamics of the current and its noise, the method allows us to compute the variance of charge transfer over time in nonequilibrium configurations. The dynamics reveal that in driven systems, the average noise should be defined operationally with care over which period of time is covered.

Searching for topological insulators in solids is one of the main issues of modern condensed-matter physics since robust gapless edge or surface states of the topological insulators can be used as building blocks of next-generation devices. Enhancing spin-orbit couplings is a promising way to realize topological insulators in solids, whereas the amplitude of the spin-orbit couplings is not sufficiently large in most materials. Here, we show a way to realize a topological state characterized by the quantized Zak phase, termed the Zak insulator with spin-polarized edges in organic antiferromagnetic Mott insulators without relying on the spin-orbit coupling. The obtained Zak insulator can have a large charge gap compared to the conventional topological insulators since Coulomb interactions mainly govern the amplitude of the charge gap in the antiferromagnetic Mott insulators. Besides the mean-field analysis, we demonstrate that the Zak insulator survives against electron correlation effects by calculating the many-body Zak phase. Our finding provides an unprecedented way to realize a topological state in strongly correlated electron systems.

The nonlinear coupling between phonons, though being weak, can be manifested under high intensity laser pulse. Here we use first-principles calculations to identify the third-order nonlinear coupling between Raman and IR active phonons at $\mathrm{\Gamma }$ in antiferromagnetic ${\mathrm{MnPS}}_3$ monolayer. The couplings can be categorized into two types: type-A contains only $A_{1g}$, while type-B contains $A_{1g}$, as well as other representations of $D_{3d}$. The type-A coupling $A_{1g}\left(24\right)A_{2u}^2\left(25\right)$ turns the magnetic point group of the system from $D_{3d}\left(D_3\right)$ to $C_{3v}\left(C_3\right)$ if spin-orbit coupling is ignored, and induces a phase transition from antiferromagnet to $i$-wave altermagnet. The $E_u(29-30)$ IR mode couples simultaneously with three Raman modes, and the polarization of light can be used to control the coupling between $E_u$ and $E_g$ mode. The degenerate IR modes [e.g., $E_u(10-11)$], when resonantly driven by circular polarized light pulse, can induce phonon orbital magnetic moment. The efficiency of transfer of energy and angular momentum from circular polarized light to phonon is discussed. Our work contributes to the understanding of light-induced magnetic phase transition in antiferromagnetic monolayers.

We consider a synthetic antiferromagnetic spin-torque oscillator (STO) with anisotropic interlayer exchange coupling. This system exhibits exceptional points (EPs) in its linearized dynamics. We find the nonlinear dynamics and the dynamical phase diagram of the system both analytically and numerically. Moreover, we show that, near one of the EPs, the power of the oscillator depends extremely sensitively on the injected spin current. Our findings may be useful for designing sensitive magnetometers and for other applications of STOs.

Two-dimensional multiferroic materials have attracted wide interest due to their intriguing physical properties and strong electromagnetic coupling. Among them, ${\mathrm{NiBr}}_2$ is unique with a temperature dependent magnetic modulation vector $\mathbf{Q}$ which is linearly related to the electric polarization. In this work, we perform first-principles calculations on the ${\mathrm{NiBr}}_2$ monolayer. Density functional theory calculations with appropriate Hubbard correction reproduce the electric polarization and its linear dependence on the magnitude of $\mathbf{Q}$. The calculated polarization decreases as the Hubbard parameter $U$ increases which enlarges the electronic band gap. Our results suggest the spin-orbital coupling effect of Br ions play a crucial role in generating the polarization. Wannier function analysis proves that the major contributor to electric polarization is the Br atomic orbitals which are strongly hybridized with Ni orbitals. The atomic position relaxation has a negligible contribution to the polarization.

On the Sb layer of the kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, pair density wave states have been observed. When the high-temperature charge orderings are treated as static backgrounds, these pair density wave (PDW) states exhibit the same wave vector in the effective two-dimensional Brillouin zone. Interestingly, these PDW states break the same symmetry on the surface. Considering the presence of this nondegenerate PDW, we investigate the implications for the possible existence of a vestigial charge-$4e$ phase with a nonzero center-of-mass momentum. To distinguish between different vestigial phases, we propose scanning tunneling microscopy experiments. We aim to provide insights into the nature of the vestigial phases and their distinct characteristics in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. This research sheds light on the interplay between PDW states, charge orderings, and superconductivity of the kagome superconductor.

We investigate the formation of paired states of bosons in an optical lattice, namely, pair superfluid (PSF) and pair supersolid (PSS) in the presence of pair hopping as well as the next-nearest-neighbor (NNN) interaction mimicking long-range forces. Both the zero- and finite-temperature phase diagrams are obtained using the cluster mean-field theory, which includes the effect of correlations systematically. We also compute the low-energy excitations, which capture the characteristic features of such paired states and their transitions. Apart from the gapless sound mode due to the PSF order, a gapped mode also appears in the PSF phase, similar to the Higgs mode of the usual atomic superfluid (ASF). The PSF to ASF transition exhibits an intriguing behavior due to the existence of a “tricritical” point, where the nature of transition changes. As a consequence of the continuous PSF-ASF transition, the gapped mode of both the phases becomes gapless at the critical point. For sufficiently strong NNN interaction strength, a PSS phase appears with coexisting pair superfluidity and stripe density order. The softening of the roton mode as a precursor of density ordering and the appearance of a low-energy gapped mode serve as robust features related to the formation of the PSS phase. We also investigate the melting of PSF and PSS phases to normal fluid at finite temperatures, particularly the melting pathway of PSS, which occurs in at least two steps due to the coexisting orders. Finally, we discuss the possibility of emulating such exotic phases in the ongoing cold atom experiments.

Coexistence of electronically distinct phases below metal-insulator (MI) transition temperature ($T_{MI}$) in correlated oxides undergoing temperature-driven MI transition has been observed in a number of systems. One of the consequences of the coexisting phases is that the metastable high-temperature metallic phase transforms into the stable insulating phase with a finite relaxation time as the temperature is lowered below $T_{MI}$. We report an extensive investigation of the phase transformation (referred to as relaxation) using resistivity as a tool where the ramp-dependent hysteresis and isothermal annealing-induced resistance relaxation were studied in films of ${\mathrm{NdNiO}}_3$ grown on three different crystalline substrates (${\mathrm{LaAlO}}_3, {\mathrm{SrTiO}}_3$, and ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$) down to 10 K, well below the metal-insulator transition temperature. The resistance relaxation experiments were complemented with Raman spectroscopy and high-resolution x-ray diffraction done down to 5 K and reciprocal space mapping (RSM). Isothermal annealing experiment done to temperatures $<T_{MI}$ shows that the average relaxation time $\langle \tau \rangle$ decreases on cooling. This can arise from a temperature-dependent barrier to relaxation, where the barrier reduces continuously on cooling down to a temperature, referred to as $T^{*}$. $T^{*}$ has been interpreted as a likely limit of supercooling so that the relaxation time $\to 0$ at this temperature. The resistance relaxation data were linked to x-ray diffraction and Raman spectroscopy data done to temperatures well below $T_{MI}$, in order to have a structural basis for the coexisting phases and their likely participation in the relaxation process. The experiments (both hysteresis and isothermal annealing) were analyzed by Monte Carlo simulation based on a minimal set of parameters, namely, a temperature $T^{*}$ and an energy scale of transformation $E^{*}$, which themselves had a temperature dependence. The parameters used in the simulation and other experimentally observed quantities like the width and height of hysteresis were found to be correlated with certain structural parameters, in particular, the residual in-plane strain and the crystallite grain size that determine the size range in these films.

Using an artificial neural-network machine learning interatomic potential, we have performed molecular dynamics simulations to study the structure and dynamics of ${\mathrm{Fe}}_{90}{\mathrm{Si}}_3{\mathrm{O}}_7$ liquid close to the Earth's liquid core conditions. The simulation results reveal that the short-range structural order (SRO) in the ${\mathrm{Fe}}_{90}{\mathrm{Si}}_3{\mathrm{O}}_7$ liquid is very strong. About 80% of the atoms are arranged in crystallinelike SRO motifs. In particular, $\sim 70\%$ of Fe-centered clusters can be classified as either hexagonal-close-packed-like or icosahedrallike SRO motifs. The SRO clusters centered on Fe, Si, or O atoms are strongly intermixed and homogenously distributed throughout the liquid. The atomic structure of the liquid and the fractions of dominant SRO clusters are not sensitive to pressure/temperature used in the simulations except that the SRO of the O-centered clusters is enhanced close to inner core pressures. The O-diffusion coefficient is about two to three times larger than the Fe and Si ions and increases more rapidly in the deeper core regions.

Mobility edges, separating localized from extended states, are known to arise in the single-particle energy spectrum of certain one-dimensional models with quasiperiodic disorder. Recently, some works claimed rather unexpectedly that mobility edges can exist even in disorder-free one-dimensional models, suggesting as an example the so-called mosaic Wannier-Stark lattice where a Stark potential is applied on every $M$ site of the lattice. Here, we present an exact spectral analysis of the mosaic Wannier-Stark Hamiltonian and prove that, strictly speaking, there are not mobility edges, separating extended and localized states. Specifically, we prove that the energy spectrum is almost pure point with all the wave functions displaying a higher than exponential localization with the exception of $(M-1)$-isolated extended states at energies around which a countably infinite number of localized states with a diverging localization size condense.

Acoustic deformation potential (ADP) quantifies carrier-acoustic phonon coupling and is essential for dissecting transport physics in thermoelectrics. Herein, we report the use of ultrafast spectroscopy of coherent acoustic phonons (CAPs) to characterize the ADP of thermoelectric materials, using ${\mathrm{Mg}}_3{\mathrm{Sb}}_2$ as an example. The photon energy-dependent amplitudes of the CAP-induced oscillatory reflectance were used to determine the ADP coupling constant, agreeing well with that from first-principles calculations. This method relies on the transient Coulombic interaction between carriers and acoustic phonons, free of influence from other scattering channels. It is shown that the method is particularly feasible for the study of thermoelectric materials, because their common features of strong phonon anharmonicity and small band gaps make the measurement insensitive to the uncertainty of carrier diffusion coefficients, ensuring its accuracy.

Here, we present magnetotransport properties of ${\mathrm{Sm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ (111) single-crystalline epitaxial thin films grown with different strengths of compressive strain. A long-range magnetic ordering of ${\mathrm{Ir}}^{4+}$ moments is confirmed around the observed metal-semimetal transition. The low-temperature (2–20 K) resistivity data signify a gapless semimetallic charge transport. The high-temperature (above the transition) metallic part shows a non-Fermi-liquid behavior with a resistivity saturation, highlighting the importance of electron correlation in the $5d$ electron system. Magnetoresistance (MR) measurements for the applying field along the [111] direction suggest that at low temperatures (below 10 K), the all-in-all-out/all-out-all-in (AIAO/AOAI) spin structure is stable enough against the applied magnetic field. However, from 10 K onward, observation of a hysteretic MR implies that applying a magnetic field causes a change in the spin structure (from AIAO to three-out-one-in) and a plastic domain deformation. The Hall resistivity data show an anomalous/spontaneous Hall effect in the AIAO/AOAI antiferromagnetic phase. We have found that an epitaxial strain changes the sign as well as the magnitude of the anomalous Hall component, possibly by the modification of the electronic band dispersion and associated Berry curvature [$\mathrm{\Omega }\left(\vec{k}\right)$]. Observation of an epitaxial strain-induced large spontaneous Hall effect ($\sim 50\mu \mathrm{\Omega }$ cm, at 7 K) identifies the presence of Weyl points and the Weyl semimetallic phase of ${\mathrm{Sm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. It highlights the role of epitaxial strain in discovering new topological phases. These findings deepen our understanding of the anomalous Hall effect in ${\mathrm{Sm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ and provide a practical tuning knob to manipulate the Berry curvature and related topological properties in a wide range of quantum materials.

We consider a system of interacting spinless fermions on a two-leg triangular ladder with $\pi /2$ magnetic flux per triangular plaquette. Microscopically, the system exhibits a U(1) symmetry corresponding to the conservation of total fermionic charge and a discrete ${\mathbb{Z}}_2$ symmetry—a product of parity transformation and chain permutation. Using bosonization, we show that, in the low-energy limit, the system is described by the quantum double-frequency sine-Gordon model. On the basis of this correspondence, a rich phase diagram of the system is obtained. It includes trivial and topological band insulators for weak interactions, separated by a Gaussian critical line, whereas at larger interactions a strongly correlated phase with spontaneously broken ${\mathbb{Z}}_2$ symmetry sets in, exhibiting a net charge imbalance and nonzero total current. At the intersection of the three phases, the system features a critical point with an emergent SU(2) symmetry. This non-Abelian symmetry, absent in the microscopic description, is realized at low energies as a combined effect of the magnetic flux, frustration, and many-body correlations. The criticality belongs to the SU(2)${}_1$ Wess-Zumino-Novikov-Witten universality class. The critical point bifurcates into two Ising critical lines that separate the band insulators from the strong-coupling symmetry broken phase. We establish an analytical connection between the low-energy description of our model around the critical bifurcation point on one hand and the Ashkin-Teller model and a weakly dimerized $XXZ$ spin-1/2 chain on the other. We complement our field-theory understanding via tensor network simulations, providing compelling quantitative evidences of all bosonization predictions. Our findings are of interest to up-to-date cold atom experiments utilizing Rydberg dressing that have already demonstrated correlated ladder dynamics.

We study the effects of electron-electron interactions on the excitonic properties and charge-density modulations in AB-stacked double-layer (DL) graphene, placed in an external gate potential $V$. The coexistence of canted antiferromagnetic order and an excitonic pairing gap has been studied with the help of the generalized Hubbard model. We calculate the chemical potential $\mu$, the average charge-density difference between the layers $\delta \overline{n}$, the antiferromagnetic gap function ${\mathrm{\Delta }}_{\mathrm{AFM}}$, and the excitonic order parameters ${\mathrm{\Delta }}_{\sigma }$ in the zero-temperature limit. We found that the excitonic pairing order parameter has a larger energy scale than the canted antiferromagnetic gap function. Charge neutrality in the DL graphene system occurs only in the absence of an external gate potential $V$. Moreover, we have shown that the values of the antiferromagnetic gap function ${\mathrm{\Delta }}_{\mathrm{AFM}}$ and excitonic order parameter ${\mathrm{\Delta }}_{\sigma }$ are always increasing at large values of the interlayer Coulomb interaction, while they are decreasing for large values of the applied gate potential $V$.