Two-dimensional van der Waals materials have recently been established experimentally as a highly-tunable condensed matter platform, facilitating the controlled manipulation of band structures and interactions. In several of these materials, Dirac cones are present in the low-energy regime near the Fermi level. Thus fermionic excitations emerging in these materials close to the Dirac cones have a linear dispersion relation near the Fermi surface as massless relativistic Dirac fermions. Here, we study low-energy fermionic excitations of such Dirac materials in the presence of a mass gap that may be generated by symmetry breaking. Introducing a dynamical Fermi velocity and/or time-dependent mass gap for the Dirac quasiparticles, we exhibit the emergence of an analog of cosmological fermion pair production in terms of observables such as the expected occupation number or two-point correlation functions. We find that it is necessary and sufficient for quasiparticle production that only the ratio between the mass gap and the Fermi velocity is time-dependent. In this way, we establish that highly-tunable Dirac materials can serve as analog models for cosmological spacetime geometries, in particular, for Friedmann-Lemaître-Robertson-Walker expanding cosmologies. We briefly discuss possibilities for experimental realization.

So-called fragile topological states of matter challenge our conventional notion of topology by lacking the robustness typically associated with topological protection, thereby displaying elusive manifestations that are difficult to harness for wave control. In this Letter, we leverage the recent discovery of fragile topological states in special classes of structural kagome lattices to document the availability of domain wall elastic wave modes that are directly traceable to fragile topology and, yet, exhibit remarkably strong signatures that support omnidirectionality. We design twisted kagome bidomains comprising two topologically distinct sublattices—one trivial and the other fragile topological—sharing a common band gap and meeting at a domain wall. The two phases are achieved via carefully engineered surface cut patterns that modify the band landscape of the underlying lattices in complementary fashions, leading to dichotomous irreps landscapes. Under these circumstances, a domain wall-bound mode emerges within the shared band gap and displays remarkable stability against domain wall orientation and introduced defects. We corroborate these findings via $\mathbf{k}·\mathbf{p}$ Hamiltonian and Jackiw-Rebbi analysis and validate them experimentally through laser vibrometry tests on a prototype endowed with water-jet-induced perforation patterns.

It is a long-thought proposal that dense light-element molecular hydrides, such as diborane (${\mathrm{B}}_2{\mathrm{H}}_6$) and methane (${\mathrm{CH}}_4$), offer an ideal platform to search for phonon-mediated superconductors. However, these hydrides are often unstable under sufficiently high pressure, e.g., ${\mathrm{B}}_2{\mathrm{H}}_6$ decomposed into BH and ${\mathrm{H}}_2$ at pressures of above 153 GPa, which are unlikely to exhibit high superconductivity. Here, we find a feasible route to stabilize these light-element molecular hydrides with high superconductivity under high pressure by high-throughput structure searches and first-principles calculations. We uncover a series of stable H-rich rare-earth ($R$) metal based boron hydrides $R{\mathrm{B}}_2{\mathrm{H}}_{10}$ with polydiborane networks. Strikingly, ${\mathrm{YB}}_2{\mathrm{H}}_{10}$ is predicted to be a high-temperature superconductor with unprecedentedly critical temperature ( $T_c$ ) of up to 93 K under 150 GPa. The present findings open a route to stabilize the unstable diborane by bringing the additional $R$ metals into the lattice under high pressure, as well as tuning the superconductivity among diborane-based hydrides and other similar dense light-element molecular hydrides.

Noble gas compounds have attracted significant research attention, mainly due to their intriguing chemical behavior under high-pressure conditions. In this paper, we identify a compound, $C2/m\text{−}{\mathrm{Xe}}_3{\mathrm{CO}}_2$, through a synergistic approach combining a particle-swarm optimization empowered structure search and first-principles calculations within a wide pressure range of 200–400 GPa, covering the pressure range of Earth's core. This compound features layered ${\mathrm{Xe}}_6{\mathrm{C}}_2{\mathrm{O}}_4$ sublattices, showing distinct covalent Xe-C bonds, supported by the calculated electron localization function. The presence of the covalent bonds is further corroborated by the large value of the integrated crystal orbital Hamilton population ($-4.02$ eV/pair) and the large negative Laplacian ($-4.65e/{\text{Å}}^5$). This leads to an unusual $s{p}^3$-like hybridization in carbon, involving two oxygen atoms, one carbon atom, and one xenon atom. Additionally, two-phase method molecular dynamics simulations suggest that the compound exhibits liquid-state behavior at 200 GPa and 5000 K above the geotherm of the Earth's core. This shows a potential role for the compound in the liquid phase as a reservoir for the “missing Xe” phenomenon. Our findings not only enhance the understanding of bonding behavior in noble gas compounds, but also suggest the potential presence of ${\mathrm{Xe}}_3{\mathrm{CO}}_2$ in various astronomical objects.

The topological phases of two-dimensional time-reversal symmetric insulators are classified by a ${\mathbb{Z}}_2$ topological invariant. Usually, the invariant is introduced and calculated by exploiting the way time-reversal symmetry acts in reciprocal space, hence implicitly assuming periodicity and homogeneity. Here, we introduce two space-resolved ${\mathbb{Z}}_2$ topological markers that are able to probe the local topology of the ground-state electronic structure also in the case of inhomogeneous and finite systems. The first approach leads to a generalized local spin-Chern marker, that usually remains well-defined also when the perpendicular component of the spin, $S_z$, is not conserved. The second marker is solely based on time-reversal symmetry, hence being more general. We validate our markers on the Kane-Mele model both in periodic and open boundary conditions, also in the presence of disorder and including topological/trivial heterojunctions.

We present a comprehensive numerical investigation of the cluster truncated Wigner approximation (cTWA) applied to quench dynamics in bond-disordered Heisenberg spin chains with power-law interactions. We find that cTWA yields highly accurate results over a wide parameter range. However, its accuracy hinges on a suitable choice of clusters. By using a clustering strategy inspired by the strong disorder renormalization group and real-space renormalization group (RSRG), clusters of two spins are sufficient to obtain essentially exact results in the regime of strong disorder. Surprisingly, even for rather weak disorder, e.g., in the presence of very long-range interactions, this choice of clustering outperforms a naive choice of clusters of consecutive spins. Additionally, we develop a discrete sampling scheme for the initial Wigner function, as an alternative to the originally introduced scheme based on Gaussian approximations. This sampling scheme puts cTWA on the same conceptional footing as regular discrete TWA for single spins and yields some reduction in the Monte Carlo shot noise compared to the Gaussian scheme.

The present work reports the ferroelectric (FE) element/site specific spectroscopic measurements in ${\mathrm{BaTi}}_{0.7}{}^{119}\mathrm{Sn}_{0.3}{\mathrm{O}}_3$ relaxor using ${}^{119}\mathrm{Sn}$ hyperfine interaction studies. The relaxor properties in the studied sample are confirmed from the macroscopic measurements such as temperature dependent dielectric constant measurements. The presence of quadrupole splitting (QS) below its dielectric maxima ($T_m$) is confirmed unambiguously using synchrotron radiation perturbed angular correlation experiments. Further, temperature dependent nuclear forward scattering data clearly depict the development of QS below $T_m$. Finite values of QS with temperature, below $T_m$, unambiguously indicate the development of electric field gradient around Sn/Ti sites in ${\mathrm{BaTi}}_{0.7}{}^{119}\mathrm{Sn}_{0.3}{\mathrm{O}}_3$ relaxor, emphasizing the applicability of conventional FE models in explaining relaxor properties. The phonon density of states (PDOS) obtained from nuclear inelastic scattering measurements has been found to qualitatively match the results of PDOS calculated from molecular dynamics simulations and experimental Raman modes that are considered to be characteristic of Ti site in ${\mathrm{BaTiO}}_3$.

An analytical theory is developed for predicting the nonlinear susceptibility of ionic polarization to continuous electromagnetic waves in both bulk and strained thin film ferroelectrics. Using a perturbation method for solving the nonlinear equation of motion for ionic polarization within the framework of Landau-Ginzburg-Devonshire theory, the full second-order nonlinear susceptibility tensor is derived as a function of frequency, temperature, and strain. The theory predicts the coexistence of a significantly enhanced second-order dielectric susceptibility and a relatively low dielectric loss in ${\mathrm{BaTiO}}_3$ films with a strain-stabilized monoclinic ferroelectric phase and in a strained ${\mathrm{SrTiO}}_3$ film near its temperature-driven second-order ferroelectric-to-paraelectric phase transition. In this paper, we establish a theoretical framework for predicting and exploiting nonlinear interactions between terahertz waves and ferroelectric materials and, more generally, suggest exciting opportunities to strain-engineer nonlinear dynamical properties of ferroelectrics beyond the static and quasistatic limits.

We investigate the ground state magnetic configurations of a Fibonacci chain of classical spins with nearest-neighbor ferromagnetic and monoaxial Dzyaloshinskii-Moriya exchange interactions. Our analysis reveals a diverse array of magnetic textures induced by an external magnetic field perpendicular to the Dzyaloshinskii-Moriya vector. These textures exhibit a spectrum ranging from a quasifully polarized noncollinear state under high magnetic fields, capable of maintaining metastable chiral soliton topological defects, to a variety of quasicrystalline chiral soliton lattices below a critical field $H_c$. For a range of magnetic fields below $H_c$, the ground state spin textures result from the interplay between an effective quasiperiodic potential influencing the solitons and their repulsive interactions. At lower magnetic fields, the system experiences a commensurate-incommensurate transition, signified by the appearance of discommensurations in the quasicrystalline soliton lattice. In the absence of an external magnetic field, the ground state assumes a helical configuration with a quasiperiodic pitch angle.

One promising way to enhance superconductivity is to have coexisting wide and incipient narrow bands, where the Fermi level intersecting the wide band lies just above the narrow band, by which finite-energy spin fluctuations act as glue to mediate pair scattering. As an extreme case of the narrow band dispersion, we investigate spin-fluctuation-mediated superconductivity in two-dimensional Hubbard models with an incipient flat band. For all of the systems investigated in this study, the Kagome, Lieb, and bilayer square lattices with a flat band, we find that spin-singlet pairing superconductivity is enhanced when the flat band is nearly fully filled, due to the interband pair scattering even when the flat band becomes dispersive by correlation effects. Among these models, enhancement of superconductivity is weak in the Lieb lattice, possibly because the density of states of the wide band goes to zero at the Dirac point where the flat and wide bands intersect. Also, when the electron density is smaller so that the flat band approaches half-filling, ferromagnetic spin fluctuations and spin-triplet pairing arises, which does not develop strongly compared to the case of the spin-singlet pairing for the incipient band situation.

In the realm of open quantum systems, steady states and high-harmonic generation (HHG) existing far from equilibrium have become core pillars of ultrafast science. Most solid-state research explores charge HHG with limited investigations into spin degrees of freedom. In this study, we theoretically address spin HHG in the steady state resulting from the terahertz laser-driven spin-phonon coupling in a dissipative dimerized spin-1/2 chain. Instead of directly driving spins using time-dependent magnetic fields, we employ the magnetophononic mechanism, where the laser first drives the lattice, and then the excited lattice subsequently drives the spins. We investigate the role of various model parameters for optimizing HHG. Increasing the laser amplitude amplifies spin HHG beyond the perturbative regime, enhancing both harmonic amplitudes and orders. We find that configuring the drive frequency far below the spin band yields the highest harmonic order. Additionally, we provide a theory matching the numerical results under weak spin-phonon coupling and propose an experimental procedure to probe the emission spectrum of spin HHG.

Room-temperature magnetization dynamics of multicore magnetic nanoparticles often account for intrinsic dipolar magnetism to behave as a single macrospin at low-frequency regime. Either magnetic particle imaging or hyperthermia benefits from the resulting superparamagnetism in terms of nonlinear magnetization response and relaxation losses at frequencies where the rotation of magnetic moments dominates over Brownian motion for a given sinusoidal field. For this situation, spontaneous thermal relaxation (in the absence of external fields) of each composing particle moment within the cluster is critical to define effective Néel time constant and may intersect with ferromagnetic resonance (FMR) at $\mathrm{G}\mathrm{Hz}$ range. Here, we performed broadband AC susceptometry on both immobilized single-core and multicore iron-oxide nanoparticles up to $26.5\mathrm{G}\mathrm{Hz}$ under DC bias fields. For each solid sample, we confirmed FMR frequency, where large single-core nanoparticle systems demonstrated typical resonance blueshift as DC field increased. Interestingly, high DC field induced the secondary satellite peak in the imaginary part of AC susceptibility spectra for the case of multicore nanoparticle systems. We further highlighted that a synchronous precession of the polarized macrospins under nonuniform effective fields was responsible for splitting FMR peaks at nearby microwave frequencies. Upon curve fitting of the field-dependent FMR frequency spectra, the Landau-Lifshitz-Gilbert-Kittel model later elaborates on the complex moment dynamics of multicore nanoparticle systems in correlation with distribution functions.

Calculation of observables with three-dimensional projected entangled pair states is generally hard, as it requires a contraction of complex multi-layer tensor networks. We utilize the multi-layer structure of these tensor networks to largely simplify the contraction. The proposed approach involves the usage of the layer structure both to simplify the search for the boundary projected entangled pair states and the single-layer mapping of the final corner transfer matrix renormalization group contraction. We benchmark our results on the cubic lattice Heisenberg model, reaching the bond dimension $D=7$, and find a good agreement with the previous results.

The strong impact of the strain-induced Dzyaloshinskii-Moriya interaction (SIDMI) on the magnetization dynamics of skyrmions in nanomagnetic structures is demonstrated. The effects of SIDMI are characterized by skyrmion equations (SEs) of motion and magnetoelastic (ME) equations. The study is performed on a model system of MgO/CoFe/Pt stacked on a piezoelectric substrate. The results demonstrate a major nonlinear amplification in both the first- and higher-harmonic magnitudes of the skyrmion breathing mode due to SIDMI. Remarkably, this enhancement can trigger a skyrmion collapse, enabling its deletion with ultraweak strain-induced excitations. The SIDMI effect is shown to be much more significant than the conventional ME effect. These findings open different avenues for the efficient manipulation of nanomagnetic structures through strain.

We systematically investigate the topological and localization properties of a quadrupolar insulator represented by the celebrated Benalcazar-Bernevig-Hughes model in the presence of quasiperiodic (QP) disorder instilled in its hopping amplitude. While disorder can be detrimental to the existence of the topological order in a system, we observe the emergence of a disorder-driven topological phase where the original (clean) system demonstrates trivial behavior. This phenomenon is confirmed by the re-emergence of zero-energy states in the band structure together with a nonzero bulk quadrupole moment, which in turn establishes the bulk-boundary correspondence (BBC). Furthermore, the distribution of the excess electronic charge shows a pattern that is reminiscent of the bulk quadrupole topology. To delve into the localization properties of the midband states, we compute the inverse participation and normalized participation ratios. It is observed that the in-gap states become critical (multifractal) at the point that discerns a transition from a topological localized to a trivial localized phase. Finally, we carry out a similar investigation to ascertain the effect of the QP disorder on the quadrupolar insulator when the model exhibits topological properties in the absence of disorder. Again, we note a multifractal behavior of the eigenstates in the vicinity of the transition.

We study the statistical properties of a single free quantum particle evolving coherently on a discrete lattice in $\mathit{\text{d}}$ spatial dimensions where every lattice site is additionally subject to continuous measurement of the occupation number. Our numerical results indicate that the system undergoes a measurement-induced phase transition (MiPT) for $\mathit{\text{d}}>1$ from a delocalized to a localized phase as the measurement strength $\gamma$ is increased beyond a critical value ${\gamma }_c$. In the language of surface growth, the delocalized phase corresponds to a smooth phase while the localized phase corresponds to a rough phase. We support our numerical results with perturbative renormalization group (RG) computations which are in qualitative agreement at one-loop order.

The presence of spin-polarized charge carriers in metallic magnets provides a mechanism for spin-lattice interactions mediated by electron-phonon coupling. Here, we present a theory of this mechanism used to estimate its effect on the exchange interactions in two-dimensional (2D) magnets. Starting from a square lattice model at half filling, we show that the presence of electron-phonon coupling with equilibrium phonon distribution leads to a notable suppression of exchange interactions with temperature. We then apply our approach to the prototypical 2D metallic ferromagnet, ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$, with moderate electron-phonon coupling. We find that the exchange interactions undergo a renormalization, leading to a softening of the magnon modes, and suppression of the Curie temperature by $\sim 10$%. We expect that this effect can be further enhanced in systems with strong electron-phonon coupling, as well as for the nonequilibrium distribution of phonons induced by strong laser fields or charge currents.