The exploration of the superconductivity in two-dimensional materials has garnered significant attention due to their promising low-power applications and fundamental scientific interest. Here, we report some stable non–van der Waals ${\mathrm{Nb}}_x{\mathrm{N}}_{x+1}$ ($x=1$–4) monolayers derived from the NbN bulk exfoliated along the [001] direction, as identified through first-principles calculations. Among these monolayers, ${\mathrm{Nb}}_2{\mathrm{N}}_3$, which crystallizes in the $P\overline{6}m2$ symmetry, stands out with an exceptional superconducting transition temperature of 77.8 K, setting a new high-$T_c$ benchmark for two-dimensional transition-metal nitrides and binary compounds. Our detailed analysis reveals that the strong superconductivity in ${\mathrm{Nb}}_2{\mathrm{N}}_3$ is driven by phonon modes dominated by N vibrations, with significant electron-phonon coupling contributions from N-$p$ and Nb-$d$ electronic states. Using the anisotropic Migdal-Eliashberg framework, we further determine the two-gap nature of the superconductivity in the ${\mathrm{Nb}}_2{\mathrm{N}}_3$ monolayer, characterized by pronounced electron-phonon coupling and anisotropic energy gaps. These results advance our understanding of superconductivity in two-dimensional transition-metal nitride and highlight their potential for nanoscale superconducting applications.

We investigate the electromagnetic wave absorption process in a coherently coupled two-component Bose-Einstein condensate model in different dimensionality at zero temperature. As the analog of phonon in the solid state physics, the elementary excitation of the Bose-Einstein condensate is described by Bogoliubov quasiparticle or bogolon for short. Due to the small magnitude of the sound velocity of the bogolon, the absorption process is prohibited by the conservation of energy and momentum. To surmount this depression, the additional degree of freedom must be considered inside of the simple Bose gas model. In this article, we develop a microscopical theory for electromagnetic wave absorption by a two-component Bose-Einstein condensate and investigate the absorption rate dependence in different dimensions. Our calculation shows the possibility of manipulating the absorption property by tuning the parameters of the condensates.

The orbital Hall effect (OHE) has several potential advantages over the spin Hall effect (SHE), the latter being well known for its many applications in spintronics. Like the spin Hall effect, the OHE occurs in nonmagnetic materials without stringent symmetry requirements, but unlike the SHE it does not rely on relatively weak spin-orbit interaction. In two-dimensional (2D) materials these advantages risk being nullified by the difficulty of turning the orbital moment away from the out-of-plane direction. Multilayered 2D materials offer a way out of this difficulty because the fluctuating in-plane component of the orbital moment, due to motion of electrons between the layers, can latch to a magnetic field. To describe this effect we have derived a semiphenomenological equation of motion for the density of orbital magnetic moment in stacked 2D materials subjected to a magnetic field. Unlike the equations of motion for the spin, these equations produce a strongly anisotropic dynamics, which is governed by an inverse effective mass tensor for which we provide a fully microscopic expression. As a first application, we combine our equation of motion with phenomenological drift-diffusion equations to formulate a theory of orbital Hanle magnetoresistance in multilayered 2D materials. This theoretical framework also offers a tool for exploring the microscopic theory of the orbital Hall effect, which remains an active area of debate.

We study transport in spin chains employing the Thouless approach based on the level sensitivity to the boundary conditions, $R$. Although spin transport in the integrable easy-axis XXZ model is diffusive, the corresponding $R$ is much closer to ballistic chains than to chaotic diffusive systems. In the case of the grand canonical ensemble, this observation can be rigorously justified, while in the case of the canonical ensemble it can be demonstrated by numerical calculations. Integrability-breaking perturbation (IBP) strongly reduces $R$, which reveals a pronounced minimum at the crossover from anomalous diffusive to normal dissipative transport. This minimum coincides with the onset of the universality of the random matrix theory. Results for various IBPs suggest a discontinuous jump of the spin conductivity in the thermodynamic limit, and moreover that its value is proportional to the strength of the IBP.

We investigate the operator entanglement of the time-evolution operator through the framework of eigenstate correlations. Focusing on strongly disordered quantum many-body systems in the many-body localized (MBL) regime, we analyze the operator entanglement across various spatiotemporal cuts, revealing the logarithmic lightcone of entanglement spreading. We demonstrate that this logarithmic lightcone arises directly from a hierarchy of energyscales and lengthscales encoded in eigenstate correlations. By characterizing the statistics of these hierarchical scales, we develop a microscopic theory for the spatiotemporal structure of entanglement spreading in MBL systems—without invoking phenomenological constructs such as $\ell$-bits. This approach reveals the fundamental connection between eigenstate correlations and the emergent entanglement structure in MBL systems.

Zirconium nitride holds promise for improving nuclear reactor components, where thermal conductivity is important, highlighting the need for a detailed understanding of its phonons. However, the phonon dispersion derived from previous inelastic neutron scattering data is in disagreement with modern theoretical calculations, pointing to the necessity for further experimental validation. Here, we use inelastic neutron scattering experiments on polycrystalline ZrN at 6, 100, 300, 400, and 600 K, and density functional theory (DFT) calculations to study the momentum-resolved scattering and the phonon density of states. Both experimental and theoretical results reveal a phonon cutoff energy near 65 meV, in stark disagreement with previous single-crystal neutron scattering measurements indicating a cutoff near 75 meV. The 65 meV cutoff energy is, however, in reasonable agreement with previous measurements on ${\mathrm{ZrN}}_{0.9}$. Consistency between our theoretical and experimental results confirms that ZrN is a wide phonon band gap system and that DFT provides a reliable description of its phonons.

Organic spintronics has gained significant interest within the scientific community due to its potential applications in spin-valve devices. However, the realization of an efficient room-temperature organic spin-valve device remains elusive, primarily due to the complex spin transport at metal-organic interfaces. Hence, the study of these metal-organic bilayer interfaces is essential for the conception of such devices. This study investigates the Co-${\mathrm{C}}_{60}$ interface and its impact on the magnetic properties of the Co layer. We examine the structural and magnetic properties of ultrathin cobalt (Co) films deposited on a fullerene $\left({\mathrm{C}}_{60}\right)$ layer to understand the origin of magnetic anisotropy in metal-organic bilayer structures. The penetration of ferromagnetic Co atoms into the ${\mathrm{C}}_{60}$ film is confirmed through x-ray reflectivity and secondary-ion-mass spectroscopy. Additionally, grazing incidence small-angle x-ray scattering and atomic force microscopy provide ample insights into the morphological properties of the $\mathrm{Co}/{\mathrm{C}}_{60}$ bilayers. Grazing incidence x-ray diffraction and grazing incidence wide-angle x-ray scattering confirm the texturing of Co in the bilayer at higher cobalt thicknesses. Angular-dependent magneto-optic Kerr effect hysteresis measurements, with varying Co layer thicknesses, reveal information about growth-induced uniaxial magnetic anisotropy. Unlike inorganic silicon substrates, where Co texturing and magnetic anisotropy are absent, magnetic anisotropy in Co films begins to develop at a thickness of 25 Å on the ${\mathrm{C}}_{60}$ layer and increases with further cobalt deposition. This texturing induces a dispersion in magnetic anisotropy, leading to an anomalous increase in coercivity and remanence along the hard axis. The anomalous behavior in coercivity and remanence variation along the nominal hard axis is explained by the two-grain Stoner-Wohlfarth model and is further corroborated by the nonuniform spatial distribution of magnetic domains observed through Kerr microscopy. Our simulations show that these magnetic anomalies arise due to the formation of local energy minima in the energy landscape near the nominal hard axis.

We study an interacting spinless quadratic band touching model that realizes a topological Mott insulating state. We quench the interaction from a value corresponding to the nematic insulator to that of the quantum anomalous Hall (QAH) ordered phase. We perform time-dependent Hartree-Fock simulations and show that after the quench the system realizes an excited Dirac semimetal state, which is, however, unstable and spontaneously evolves to a state with inhomogeneous nematic and QAH order parameters. The modulations form a stripe pattern that grows exponentially with time until the local Chern marker reaches unity. The alternating QAH order defines a domain structure with boundaries that host chiral sublattice currents.

Twisted transition metal dichalcogenides (TMDs) host bands with opposite Chern number for the two spin species and could thus be host for fractional topological insulator states. In multicomponent quantum Hall systems, where the spins have equal Chern number, the resulting topological liquid states can be well modeled by trial wave functions such as the Halperin $\left(lmn\right)$ wave functions. These wave functions are exact zero energy states of certain short-range (pseudopotential) Hamiltonians. However, we show that such a construction fails in the case where the Chern numbers of the two spins are opposite, since the electrons with opposite Chern number cannot avoid one another. This underlines the importance of suppressing the short-range interspin Coulomb repulsion in order to realize fractional topological insulators in twisted TMDs. We introduce a trial wave function for the opposite Chern number Landau levels for filling factor $\nu =\frac{1}{3}+\frac{1}{3}$ in terms of pairing of composite fermions and show that it has good overlaps with the exact diagonalization ground state when the short-range Coulomb repulsion is sufficiently softened.

Connecting topological states with $p$-orbital freedom has led to rich, insightful physics phenomena. Yet topological corner states have rarely been associated with modes of higher orbitals than $p$ orbital due to the entanglement proneness of higher orbital bands in photonics. This paper theoretically investigates higher orbital corner states in a two-dimensional (2D) Su-Schrieffer-Heeger (SSH) lattice and experimentally demonstrates octopole orbital corner states in a terahertz spoof plasmonic crystal. The orbital Hamiltonian is derived by introducing a pair of orthogonal orbitals into a 2D SSH lattice tight-binding model. Based on the expanded lattice, the presence of orbital corner states is demonstrated, which can be characterized by the generalized winding number. The counterpart of the tight-binding model in plasmonic crystals is identified, revealing eight octopole corner states. These states, comprising four even and four odd modes, are demonstrated through eigenenergy spectra and field distributions. Interestingly, the octopole corner states emerge without orbital-hopping symmetry, which differs from the $p$-orbital corner states in previous reports. Experimentally, we verify the emergence of octopole orbital corner states in the terahertz frequency range by designing spoof plasmonic crystals with a 2D SSH lattice. Through terahertz time-domain spectroscopy, these corner states are directly observed via spatial mapping of the electric field intensity. We suggest that these results shed light on the unique physics of the interplay between topological phases and higher-order orbitals and broaden the potential application opportunities of plasmonic crystals in particle trapping and biosensing.

Material systems which exhibit a volatile transition from a Mott insulating to conducting state (defined as Mott breakdown effect) under application of a small threshold current/ electric field (few tens of V/cm) are extremely promising for a plethora of potential applications, which include neuromorphic computations. While systems exhibiting such a phenomenon are rare, Mott breakdown with small threshold electric field has recently been identified in the family of spinel oxides $\left({\mathit{AB}}_2{\mathrm{O}}_4\right)$, i.e., in ${\mathrm{MgTi}}_{1.2}{\mathrm{V}}_{0.8}{\mathrm{O}}_4$ [Rahaman et al., Phys. Rev. B 103, 245145 (2021)]. While the low-temperature insulating and low resistive states in ${\mathrm{MgTi}}_{1.2}{\mathrm{V}}_{0.8}{\mathrm{O}}_4$ could not be reversibly toggled with the application of electric field/current at all temperatures, here we report the Mott breakdown effect in V-doped ${\mathrm{MnTi}}_2{\mathrm{O}}_4$ which consist of magnetic $\mathit{A}$-site ion (${\mathrm{Mn}}^{2+}$ in place of diamagnetic ${\mathrm{Mg}}^{2+}$ ions), where the Mott breakdown effect is reversible (i.e., true volatile switching between high and low resistive states with application of electric filed/voltages beyond a threshold value) and occur over a wider range of temperatures. The values for the threshold electric fields required to induce the negative differential resistance (i.e., the Mott breakdown effect) in the V-doped ${\mathrm{MnTi}}_2{\mathrm{O}}_4$ system is found to be much smaller as compared to the V-doped ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ system (for example, for 40% V-doping at Ti site, ${\mathit{E}}_{\mathrm{th}}$ at $\mathit{T}=50 \mathrm{K}$ is $\approx$ 0.1 V/cm for the ${\mathrm{MnTi}}_2{\mathrm{O}}_4$ series as compared to $\approx 40$ V/cm for the ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ series). The magnetic ground state of V-doped ${\mathrm{MnTi}}_2{\mathrm{O}}_4$ systems, which is distinct from the V-doped ${\mathrm{MgTi}}_2{\mathrm{O}}_4$, seems to play a critical role in lowering the threshold field. Key experimental results point to an electronic mechanism for the Mott breakdown, which, as suggested by first-principles calculations, likely proceeds through current-induced transfer of electrons between two Jahn-Teller active ions (namely ${\mathrm{V}}^{3+}\text{−}3{\mathit{d}}^2$ and ${\mathrm{Ti}}^{3+}\text{−}3{\mathit{d}}^1$ ions), which is found to coexist with the corresponding Jahn-Teller inactive ion pairs (i.e., ${\mathrm{V}}^{2+}\text{−}3{\mathit{d}}^3$ and ${\mathrm{Ti}}^{4+}\text{−}3{\mathit{d}}^0$ ions) in the ferrimagnetic ground state.

The bulk photovoltaic effect is a photocurrent generation from alternating electric field, which is a promising candidate for future efficient solar cell technology. It is the second-order optical current, which is the injection current or the shift current. We focus on the direct current generation. By employing a simple two-band model of the $d$-wave altermagnet coupled with the Rashba interaction, we show that the linearly polarized light can generate the injection and shift currents when the Néel vector points to an in-plane direction. The magnitude of the injection current is almost constant over a wide range of the frequency $\omega$ of the applied light provided it is smaller than a certain critical frequency ${\omega }_{\text{c}}$ and larger than the bulk gap energy ${\varepsilon }_{\text{gap}}, {\varepsilon }_{\text{gap}}<\hslash \omega <\hslash {\omega }_{\text{c}}$. Hence, the use of the injection current is quite efficient for solar cell technology because any photon whose energy is within this range can be equally utilized.

Atomistic pathways for strain-induced subsolidus martensitic transitions between Bi-I, Bi-II', Bi-II, and Bi-V were found by computing the set of the smallest atomic displacements required to transform the initial structures into the final structures. Minimal transformation cells with maximum of 12 Bi atoms are sufficient to describe the diffusionless martensitic transitions. We identified families of invariant shear planes during the martensitic transitions and calculated the reaction barriers using density functional theory. The barrier for the Bi-I/II reaction is about $73\mathrm{m}\mathrm{e}\mathrm{V}$/atom at ambient conditions and decreases with increasing pressures to $\approx 20\mathrm{m}\mathrm{e}\mathrm{V}$/atom at $5\mathrm{G}\mathrm{Pa}$. A similar decrease in the reaction barriers for reactions towards high-pressure phases was observed for the other transitions. The barriers for the reverse reactions increase with increasing pressures. The model calculations show quantitatively that the transitions will occur well below the equilibrium transition pressure when samples are strained.

Langevin dynamics simulations are performed to investigate the grain boundary internal friction (GBIF) in two-dimensional (2D) Yukawa solids under oscillating shear deformations. It is discovered that the GBIF exhibits a significant transition with the increasing misorientation angle, which can be used to distinguish the low- and high-angle grain boundaries (GBs). From the dependence of the GBIF on the shear amplitude and frequency, it is found that the internal friction of high-angle GBs exhibits typical properties of the linear anelasticity, while that of low-angle GBs does not. The variation trend of the calculated GB stiffness with the misorientation angle well agrees with that of GBIF, while the GB stiffnesses of the low- and high-angle GBs are both independent of frequency. For the transition between the low- and high-angle GBs of the misorientation angle $\theta =\pi /9$, the corresponding GB migration exhibits the normal diffusion, completely different from the ballistic motion of the GB migration of other misorientation angles.

In this paper, we present a thorough investigation of vibrational, structural, and electronic properties of perovskite-type rhombohedral ${\mathrm{Ba}}_2{\mathrm{ZnTeO}}_6$ (BZTO) under systematic application of pressure. To carry out the analysis, we have performed pressure-dependent Raman spectroscopic measurements, synchrotron x-ray diffraction, and density functional theory (DFT)-based calculations. At ambient conditions, BZTO stabilizes in $R\overline{3}m$ space group, which under pressure undergoes a structural transition to a monoclinic phase with space group $C2/m$ at around 18 GPa. In-depth Raman analysis reveals softening of a phonon mode ${\mathrm{E}}_g$ ($\sim 28{\mathrm{cm}}^{-1}$) leads to the structural phase transition. First-principles DFT calculations also indicate that the doubly degenerate soft mode associated with the in-phase ${\mathrm{TeO}}_6$ octahedral rotation drives the structure to a lower symmetry phase $C2/m$.