$({\mathrm{Ca}}_x{\mathrm{Sr}}_{1-x}{)}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ is a rare system that has been shown to display an interesting interplay between structural quantum criticality and superconductivity. A putative structural quantum critical point, which is hidden beneath a broad superconducting dome, is believed to give rise to optimized superconducting properties in $({\mathrm{Ca}}_x{\mathrm{Sr}}_{1-x}{)}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$. However, the presence of the superconducting dome itself hinders the examination of the quantum critical point through electrical transport, as the transport coefficients vanish in the superconducting state. Here, we use critical current density to explore within the superconducting dome. Our measurements reveal a large enhancement of the critical current density at the zero-temperature limit when the system is tuned towards the structural quantum critical point.

We present a combined x-ray and neutron diffraction study of the stripe-ordered superconductor ${\mathrm{La}}_{1.88}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_4$. The average crystal structure is consistent with the orthorhombic $Bmab$ space group as commonly reported in the literature. This structure, however, is not symmetry compatible with a second-order phase transition into the stripe order phase, and as we report here, numerous Bragg peaks forbidden in the $Bmab$ space group are observed. We have studied and analyzed these $Bmab$-forbidden Bragg reflections. Fitting of the diffraction intensities yields monoclinic lattice distortions that are symmetry consistent with charge stripe order.

The high-pressure structural and vibrational properties of a layered transition metal dichalcogenide $2H\text{−}{\mathrm{NbSe}}_2$ were investigated using single-crystal x-ray diffraction and Raman spectroscopy, demonstrating its structural stability up to 35 GPa. The lattice compressibility changes character from being highly anisotropic at low pressures to largely isotropic at high pressures. Concomitantly, the interatomic bonds demonstrate highly anisotropic compression behavior with the Se-Se interlayer bonds compressing by >20%, while the intramolecular Se-Se distance shows a nonmonotonic pressure dependence with a maximum at ∼12 GPa. The nearest-neighbor central force lattice vibrational model yields pressure dependencies of the interatomic forces in qualitative agreement with bond length compression, providing insight into the vibrational properties of $2H\text{−}{\mathrm{NbSe}}_2$ at high pressures.

Memory effects are a classic feature of disordered materials. With Raman-microspectroscopy we evidence a memory effect in the deformation of densified soda-lime silicate glasses, and with molecular dynamics simulations we identify the network reconfigurations it originates from. These results pave the way toward a better understanding of plastic instability, damage, and rupture properties in iono-covalent glassy materials.

Amorphous zirconia-alloyed-tantala (${\mathrm{ZrO}}_2$:${\mathrm{Ta}}_2{\mathrm{O}}_5$) is of interest as a prospective coating material to make the highly reflective mirrors used in gravitational wave detectors. We study the atomic structure of thin films of ${\mathrm{ZrO}}_2$:${\mathrm{Ta}}_2{\mathrm{O}}_5$ by making realistic computer models using force enhanced atomic refinement. The models are, by construction, in agreement with measured x-ray scattering functions. The models made with x-ray scattering data on as-deposited and 800 °C-annealed samples showed clear structural differences, particularly in their metal-metal correlation. The ratio of corner-shared to edge-shared polyhedra increased as a result of annealing. The electronic band gap and the density of states of ${\mathrm{ZrO}}_2$:${\mathrm{Ta}}_2{\mathrm{O}}_5$ are found to be similar to that of pure ${\mathrm{Ta}}_2{\mathrm{O}}_5$ and remained unchanged upon annealing. We used the Hyed-Scuseria-Ernzerhof hybrid functional to estimate the band gap of ${\mathrm{ZrO}}_2$:${\mathrm{Ta}}_2{\mathrm{O}}_5$ to be $\approx$ 4 eV, which is close to the experimental band gap of pure ${\mathrm{Ta}}_2{\mathrm{O}}_5$. The refractive indexes calculated from the models were in reasonable agreement with the experiment. Vibrational modes at high frequencies involve highly localized bond stretch modes associated with a corner-shared O atom connecting two metal-oxygen polyhedra.

Thermalizing and localized many-body quantum systems present two distinct dynamical phases of matter. Recently the fate of a localized system coupled to a thermalizing system viewed as a quantum bath received significant theoretical and experimental attention. In this work, we study a mobile impurity, representing a small quantum bath, that interacts locally with an Anderson insulator with a finite density of localized particles. Using static Hartree approximation to obtain an effective disorder strength, we formulate an analytic criterion for the perturbative stability of the localization. Next, we use an approximate dynamical Hartree method and the quasi-exact time-evolved block decimation (TEBD) algorithm to study the dynamics of the system. We find that the dynamical Hartree approach which completely ignores entanglement between the impurity and localized particles predicts the delocalization of the system. In contrast, the full numerical simulation of the unitary dynamics with TEBD suggests the stability of localization on numerically accessible timescales. Finally, using an extension of the density matrix renormalization group algorithm to excited states (DMRG-X), we approximate the highly excited eigenstates of the system. We find that the impurity remains localized in the eigenstates and entanglement is enhanced in a finite region around the position of the impurity, confirming the dynamical predictions. Dynamics and the DMRG-X results provide compelling evidence for the stability of localization.

The specific heat of phonon cavities is investigated in order to analyze the effect of phonon confinement on thermodynamic properties. The specific heat of freestanding very thin SiN membranes in the low-dimensional limit is measured down to very low temperatures (from 6 K to 50 mK). In the whole temperature range, we measured an excess specific heat orders of magnitude bigger than the typical value observed in amorphous solids. Below 1 K, a crossover in $c_p$ to a lower power law is seen, and the value of the specific heat of thinner membranes becomes larger than that of thicker ones demonstrating a significant contribution coming from the surface. We show that this high value of the specific heat cannot be explained by the sole contribution of two-dimensional phonon modes (Lamb waves). The excess specific heat, being thickness dependent, could come from tunneling two-level systems that form in low-density regions of amorphous solids located on the surfaces. We also show that the specific heat is strongly tuned by the internal stress of the membrane by orders of magnitude, giving high values, making low-stress SiN very efficient for energy storage at very low temperature.

The recent progress in the context of elastic metamaterials and modulated waveguides with digitally controllable properties has opened new pathways to overcome the limitations dictated by Hermitian Hamiltonians in mechanics. Among the possible implementations, non-Hermitian, $\mathcal{PT}$-symmetric systems with balanced gain and loss have emerged as an elegant mechanism to access novel functionalities by lifting the non-Hermitian degeneracies (exceptional points). Motivated by this, the paper deals with a non-Hermitian and $\mathcal{PT}$-symmetric elastic waveguide with complex stiffness-modulation. The strength of the stiffness-modulations, tailored in the form of a balanced gain/loss, delineates a transition from unbroken to broken $\mathcal{PT}$-symmetric phases, where distinct Bloch-wave modes coalesce into exceptional points. It is shown that, in the unbroken $\mathcal{PT}$-symmetric regime, and due to the interplay between real and imaginary components of the elasticity, the waveguide operates as a phononic filter. When the strength of the gain/loss interactions increases, the frequency gap closes and the bulk bands degenerate into an exceptional point, where the system operates as a waveguide with asymmetric scattering capabilities. The paper provides a connection between the distinct wave modes that populate the non-Hermitian degeneracies and the directional reflection/transmission capabilities. The asymmetric behavior is herein explained by combining the dispersion properties of a $\mathcal{PT}$-symmetric rod, obtained through the plane wave expansion method (PWEM), and the scattering matrix method (SMM) for a modulated slab series-connected to semi-infinite media.

We theoretically explore the possibility of stabilizing the trapless polariton Bose-Einstein condensates (pBECs). Exploiting the variational method, we solve the associated nonlinear, complex Gross-Pitaevskii equation and derive the equation of motion for the amplitude and width of the condensate. These variational results described by ordinary differential equations are rewritten to perform a linear stability analysis to generate a stability window in the repulsive domain. A set of coupled nonlinear ordinary differential equations obtained through the variational approach are then solved by numerical simulations through the fourth-order Runge-Kutta method, which are further supported by the split-step Crank-Nicholson method, thereby setting the platform for stable pBECs. In particular, we generate a window containing system parameters in the $g_1-{\gamma }_{\text{eff}}$ space within which the system can admit stable condensates. The highlight of the results is that one observes beating effects in the real time evolution of the condensates with attractive interactions much similar to multicomponent BECs, and their periodicity can be varied by manipulating linear and nonlinear loss/gain terms. For repulsive condensates, one notices the stretching of the density.

We analyze the out-of-equilibrium dynamics of a quantum particle coupled to local magnetic degrees of freedom that undergo a classical phase transition. Specifically, we consider a two-dimensional tight-binding model that interacts with a background of classical spins in thermal equilibrium, which are subject to Ising interactions and act as emergent, correlated disorder for the quantum particle. Particular attention is devoted to temperatures close to the ferromagnet-to-paramagnet transition. To capture the salient features of the classical transition, namely the effects of long-range correlations, we focus on the strong coupling limit, in which the model can be mapped onto a quantum percolation problem on spin clusters generated by the Ising model. By inspecting several dynamical probes such as energy level statistics, inverse participation ratios, and wave-packet dynamics, we provide evidence that the classical phase transition might induce a delocalization-localization transition in the quantum system at certain energies. We also identify further important features due to the presence of Ising correlations, such as the suppression of compact localized eigenstates.

We present a model of ultrasonic metafluids—acoustic metamaterials in the form of suspensions of discrete microscopic oscillators coupled to the embedding fluid. Contrary to a common assumption about metamaterials, and as already established in the field of metafluids, the metafluid concept need not be based on position periodicity or correlation of the suspended micro-oscillators, and in this case not even on ideally designed micro-oscillators. For the speculation that metafluids may one day be produced as solutions of macromolecules, it is essential that the micro-oscillators be allowed to be randomly distributed in the host fluid and generally have irregular (modal) shapes. We formulate the detailed operating principle of such a metafluid model, give explicit formulas for its effective dynamic moduli in terms of the modal structure of the micro-oscillators, and discuss basic practical issues of performance optimization in terms of their mass and size.

$\mathrm{Co}{\mathrm{Nb}}_2{\mathrm{O}}_6$ is one of the few materials that is known to approximate the one-dimensional transverse-field Ising model (1D-TFIM) near its quantum critical point. It has been inferred that ${\mathrm{Co}}^{2+}$ acts as a pseudospin $1/2$ with anisotropic exchange interactions that are largely Ising-like, enabling the realization of the TFIM. However, the behavior of $\mathrm{Co}{\mathrm{Nb}}_2{\mathrm{O}}_6$ is known to diverge from the ideal TFIM under transverse magnetic fields that are far from the quantum critical point, requiring the consideration of additional anisotropic, bond-dependent (Kitaev-like) terms in the microscopic pseudospin-$1/2$ Hamiltonian. These terms are expected to be controlled in part by single-ion physics, namely the wave function for the pseudospin-$1/2$ angular momentum doublet. Here, we present the results of both inelastic neutron scattering measurements and electron paramagnetic resonance spectroscopy on $\mathrm{Co}{\mathrm{Nb}}_2{\mathrm{O}}_6$, which elucidate the single-ion physics of ${\mathrm{Co}}^{2+}$ in $\mathrm{Co}{\mathrm{Nb}}_2{\mathrm{O}}_6$ for the first time. We find that the system is well-described by an intermediate spin-orbit coupled Hamiltonian, and the ground state is a well-isolated Kramers doublet with an anisotropic $g$ tensor. We provide the approximate wave functions for this doublet, which we expect will be useful in theoretical investigations of the anisotropic exchange interactions.

To design a phonon-mediated superconductor with transition temperature above the liquid-nitrogen temperature under ambient pressure, we align the Fermi level of compound ${\mathrm{SrB}}_3{\mathrm{C}}_3$ to a Van Hove singularity by introducing 0.4 hole/f.u., namely ${\mathrm{Rb}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{B}}_3{\mathrm{C}}_3$. Based on density functional first-principles calculations and the Wannier interpolation technique, the electronic structure, lattice dynamics, and electron-phonon coupling (EPC) of ${\mathrm{Rb}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{B}}_3{\mathrm{C}}_3$ are investigated. Both the enlarged density of states at the Fermi level and the softened phonons play important roles to enhance the EPC. By solving the anisotropic Eliashberg equations, we find that the transition temperature of ${\mathrm{Rb}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{B}}_3{\mathrm{C}}_3$ is 83 K, successfully exceeding the liquid-nitrogen temperature at ambient pressure condition. Moreover, our calculations reveal that there is a single-gap to two-gap superconductivity transition in comparison with ${\mathrm{SrB}}_3{\mathrm{C}}_3$.

Symmetry-protected topological (SPT) phases are short-range entangled phases of matter with a nonlocal order parameter which are preserved under a local symmetry group. Here, by using an unsupervised learning algorithm, namely, diffusion maps, we demonstrate that we can detect phase transitions between symmetry-broken and topologically ordered phases, and between nontrivial topological phases in different classes. In particular, we show that the phase transitions associated with these phases can be detected in different bosonic and fermionic models in one dimension. This includes the interacting Su-Schrieffer-Heeger model, the Affleck-Kennedy-Lieb-Tasaki model and its variants, and weakly interacting fermionic models. Our approach serves as an inexpensive computational method for detecting topological phase transitions associated with SPT systems which can also be applied to experimental data obtained from quantum simulators.

${\mathrm{Ti}}_2{\mathrm{O}}_3$ exhibits a unique metal-insulator transition (MIT) at ∼450 K over a wide temperature range of ∼150 K. This broad MIT accompanied by lattice deformation differs from the sharp MITs observed in most other transition-metal oxides. A longstanding issue is determining the role of electron-electron correlation in the electronic structure and MIT of ${\mathrm{Ti}}_2{\mathrm{O}}_3$. However, the lack of information about the band structure of ${\mathrm{Ti}}_2{\mathrm{O}}_3$ has hindered investigating the origin of its unusual physical properties. Here, we report the electronic band structure of “insulating” ${\mathrm{Ti}}_2{\mathrm{O}}_3$ films with slight hole doping by angle-resolved photoemission spectroscopy (ARPES). ARPES showed clear band dispersion on the surface of single-crystalline epitaxial films. The experimentally obtained band structures were compared with band-structure calculation results based on density functional theory (DFT) with generalized gradient approximation + $U$ correction. The obtained band structures are in good agreement with the DFT calculations at $U=2.2\mathrm{eV}$, suggesting that electron-electron correlation plays an important role in the electronic structure of ${\mathrm{Ti}}_2{\mathrm{O}}_3$. Furthermore, the detailed analyses with varying $U$ suggest that the origin of the characteristic MIT in ${\mathrm{Ti}}_2{\mathrm{O}}_3$ is a semimetal-semimetal or semimetal-semiconductor transition caused by changes in the Fermi surface due to lattice deformation.

We show that lattice symmetry groups without time reversal, also known as magnetic space group symmetries, protect topological invariants as well as surface states that are distinct from those of nonmagnetic topological states. We obtain, by explicit and exhaustive construction of topological crystals using lower-dimensional building blocks, the full topological classification of electronic band insulators that are magnetically ordered for each one of the 1421 magnetic space groups (MSGs) in three dimensions with significant spin-orbit coupling. Compared with previous works on classification which include the results for some of the MSGs, our unified framework systematically gives the complete classification for all MSGs. We have also computed the symmetry-based indicators (SIs) for each nontrivial class and, by doing so, establish the complete mapping from symmetry representations to topological invariants. Our work goes beyond Elcoro et al. [L. Elcoro, B. J. Wieder, Z. Song, Y. Xu, B. Bradlyn, and B. A. Bernevig, Nat. Commun. 12, 5965 (2021)] by constructing all gapped magnetic topological crystalline states, among which many cannot be diagnosed by SIs. By doing so, we have also identified all SIs that do not correspond to any gapped states but gapless Weyl semimetals.

Robust edge states propagate along the edges and corner states gather at the corners in two-dimensional (2D) first-order and second-order topological insulators, respectively. Here, we report two kinds of topological states in an extended 2D dimerized lattice with staggered flux threading. At $\frac{1}{2}$ filling, we observe isolated corner states as well as metallic near-edge states in the $\mathcal{C}=2$ Chern insulator states. At $\frac{1}{4}$ filling, we find a $\mathcal{C}=0$ topological state, where the robust edge states are well localized along the edges but bypass corners. These topological insulator states differ from both conventional Chern insulators and the usual high-order topological insulators.

We study the many-body ground states of two-component hard-core bosons in topological triangular-lattice models. Utilizing exact diagonalization and density-matrix renormalization group calculations, we demonstrate that at commensurate two-thirds filling per lattice site, a two-component bosonic integer quantum Hall (BIQH) effect emerges with the associated $\mathbf{K}=\left(\begin{array}{cc}0& 1\\ 1& 0\end{array}\right)$ matrix under strong intercomponent Hubbard repulsion. The topological nature is further elucidated by (i) a unique ground state degeneracy with a robust spectrum gap, (ii) a quantized topological Chern number matrix $\mathbf{C}={\mathbf{K}}^{-1}$, and (iii) two counterpropagating edge branches. Moreover, with increasing nearest-neighbor repulsions, the ground state undergoes a first-order transition from a BIQH liquid to a commensurate solid order.