In recent years, dynamical quantum phase transitions (DQPTs) have emerged as a useful theoretical concept to characterize nonequilibrium states of quantum matter. DQPTs are marked by singular behavior in an effective free energy $\lambda \left(t\right)$, which, however, is a global measure, making its experimental or theoretical detection challenging in general. We introduce two local measures for the detection of DQPTs with the advantage of requiring fewer resources than the full effective free energy. The first, called the real-local effective free energy ${\lambda }_M\left(t\right)$, is defined in real space and is therefore suitable for systems where locally resolved measurements are directly accessible such as in quantum-simulator experiments involving Rydberg atoms or trapped ions. We test ${\lambda }_M\left(t\right)$ in Ising chains with nearest-neighbor and power-law interactions, and find that this measure allows extraction of the universal critical behavior of DQPTs. The second measure we introduce is the momentum-local effective free energy ${\lambda }_k\left(t\right)$, which is targeted at systems where momentum-resolved quantities are more naturally accessible, such as through time-of-flight measurements in ultracold atoms. We benchmark ${\lambda }_k\left(t\right)$ for the Kitaev chain, a paradigmatic system for topological quantum matter, in the presence of weak interactions. Our introduced local measures for effective free energies can further facilitate the detection of DQPTs in modern quantum-simulator experiments.

Quantum transduction between microwave and optical frequencies is important for connecting superconducting quantum platforms within a quantum network. Ensembles of rare-earth ions are promising candidates to achieve this conversion due to their collective coherence properties at microwave and optical frequencies. Erbium ions are of particular interest because of their telecom wavelength optical transitions that are compatible with fiber communication networks and components. Here, we report the optical and electron spin properties of erbium-doped yttrium orthovanadate (${\mathrm{Er}}^{3+}:\mathrm{YV}{\mathrm{O}}_4$), including high-resolution optical spectroscopy, electron paramagnetic resonance studies, and an initial demonstration of microwave to optical conversion of classical fields. The highly absorptive optical transitions and narrow ensemble linewidths make ${\mathrm{Er}}^{3+}:\mathrm{YV}{\mathrm{O}}_4$ promising for magneto-optic quantum transduction.

We combine diamond anvil cell techniques and synchrotron-based infrared spectroscopy with a detailed symmetry analysis and lattice dynamics calculations to uncover a series of pressure-induced structural phase transitions in the hybrid improper ferroelectric ${\mathrm{Sr}}_3{\mathrm{Sn}}_2{\mathrm{O}}_7$. The microscopic character of each high-pressure phase is determined by comparing the measured spectrum with the predicted vibrational patterns of several related but distinct candidate space groups. Our analysis reveals a sequence of pressure-induced transitions from $A{2}_{1}am\leftrightarrow Pnab\leftrightarrow Acaa\leftrightarrow I4/mmm$ at room temperature. Remarkably, this space group progression matches the sequence of temperature-dependent structural transitions observed in ${\mathrm{Sr}}_3{\mathrm{Sn}}_2{\mathrm{O}}_7$ between 77 and 1000 K. Other hybrid improper ferroelectrics display a similar set of temperature transitions, suggesting that pressure and probably strain will be very effective tuning parameters for this entire class of materials.

We generalize the formalism of the dynamical vertex approximation ($\mathrm{D}\mathrm{\Gamma }\mathrm{A}$)—a diagrammatic extension of the dynamical mean-field theory (DMFT)—to treat magnetically ordered phases. To this aim, we start by concisely illustrating the many-electron formalism for performing ladder resummations of Feynman diagrams in systems with broken SU(2) symmetry associated to ferromagnetic (FM) or antiferromagnetic (AF) order. We then analyze the algorithmic simplifications introduced by taking the local approximation of the two-particle irreducible vertex functions in the Bethe-Salpeter equations, which defines the ladder implementation of $\mathrm{D}\mathrm{\Gamma }\mathrm{A}$ for magnetic systems. The relation of this assumption with the DMFT limit of large coordination-number/high dimensions is explicitly discussed. As a last step, we derive the expression for the ladder $\mathrm{D}\mathrm{\Gamma }\mathrm{A}$ self-energy in the FM- and AF-ordered phases of the Hubbard model. The physics emerging in the AF-ordered case is explicitly illustrated by means of approximated calculations based on a static mean-field input for $\mathrm{D}\mathrm{\Gamma }\mathrm{A}$ equations. The results obtained capture fundamental aspects of both metallic and insulating ground states of two-dimensional antiferromagnets, providing a reliable compass for future, more extensive applications of our approach. Possible routes to further develop diagrammatic-based treatments of magnetic phases in correlated electron systems are briefly outlined in the Conclusions.

We experimentally and theoretically investigate the nondegenerate two-photon absorption coefficient $\beta ({\omega }_1,{\omega }_2)$ in the prototypical semiconductor ZnSe. In particular, we provide a comprehensive data set on the dependence of $\beta ({\omega }_1,{\omega }_2)$ on the nondegeneracy parameter ${\omega }_1/{\omega }_2$ with the total frequency sum ${\omega }_1+{\omega }_2$ kept constant. We find a substantial increase in the two-photon absorption strength with increasing ${\omega }_1/{\omega }_2$. In addition, different crystallographic orientations and polarization configurations are investigated. The nonlinear optical response is analyzed theoretically by evaluating the multiband semiconductor Bloch equations including inter- and intraband excitations in the length gauge. The band structure and the matrix elements are taken as an eight-band $\mathit{k}·\mathit{p}$ model. The simulation results are in very good agreement with the experiment.

Flexural deformations of atomically thin membranes are governed by bending rigidity and the Gaussian modulus. In one-atom-thick graphene membranes, these two parameters need to be determined via bending-induced changes in topology and interaction between electron orbitals, going beyond existing studies on flat graphene. Herein, we employ atomic force microscopy to demonstrate that the configurational (strain) energy can successfully be evaluated based on changes in the surface geometry with subatomic resolution via three-dimensional analyses of attractive interatomic forces. A quadratic relation of adhesion energy with monolayer curvatures of rolled and unrolled graphene led to the finding that the probe tip can detect spatially varying surface potentials owing to the rehybridization effects and the change in the next-neighbor hopping caused by bending. The tip-induced local strain inside graphene was found to generate topological defects, independently of in-plane stretch. Their energetic analysis and relationship with local curvatures reveal the applicability of the Helfrich Hamiltonian and determine the bending rigidity and Gaussian modulus. Those evaluated at the hollow sites of the honeycomb lattice are consistent with the isotropic elastic attributes. The remarkably large negative Gaussian modulus, observed at a pyramidalized carbon atom located at the topmost center of the tip-induced bump, provides evidence for attractive interactions between the charge inhomogeneity owing to the topological defects and geometric potentials of the Gaussian curvature.

We survey the landscape of binary hydrides across the entire periodic table from 10 to 500 GPa using a crystal structure prediction method. Building a critical temperature ($T_c$) model, with inputs arising from density of states calculations and Gaspari-Gyorffy theory, allows us to predict which energetically competitive candidates are most promising for high-$T_c$ superconductivity. Implementing optimizations, which lead to an order of magnitude speedup for electron-phonon calculations, then allows us to perform an unprecedented number of “high-throughput” calculations of $T_c$ based on these predictions and to refine the model in an iterative manner. Converged electron-phonon calculations are performed for 121 of the best candidates from the final model. From these, we identify 36 above-100 K dynamically stable superconductors. To the best of our knowledge, superconductivity has not been previously studied in 27 of these. Of the 36, 18 exhibit superconductivity above 200 K, including structures of ${\mathrm{NaH}}_6$ (248–279 K) and ${\mathrm{CaH}}_6$ (216–253 K) at the relatively low pressure of 100 GPa.

We analyze Andreev bound states (ABSs) that form in normal sections of a Rashba nanowire that is only partially covered by a superconducting layer. These ABSs are localized close to the ends of the superconducting section and can be pinned to zero energy over a wide range of magnetic field strengths even if the nanowire is in the nontopological regime. For finite-size nanowires (typically $\lesssim 1\mu \mathrm{m}$ in current experiments), the ABS localization length is comparable to the length of the nanowire. The probability density of an ABS is therefore nonzero throughout the nanowire and differential-conductance calculations reveal a correlated zero-bias peak (ZBP) at both ends of the nanowire. When a second normal section hosts an additional ABS at the opposite end of the superconducting section, the combination of the two ABSs can mimic the closing and reopening of the bulk gap in local and nonlocal conductances accompanied by the appearance of the ZBP. These signatures are reminiscent of those expected for Majorana bound states (MBSs) but occur here in the nontopological regime. Our results demonstrate that conductance measurements of correlated ZBPs at the ends of a typical superconducting nanowire or an apparent closing and reopening of the bulk gap in the local and nonlocal conductance are not conclusive indicators for the presence of MBSs.

In atomic-layer superlattices constructed using three constituent phases, $\mathrm{CaTi}{\mathrm{O}}_3, \mathrm{SrTi}{\mathrm{O}}_3$, and $\mathrm{BaTi}{\mathrm{O}}_3$, the stacking sequence of the atomic layers is found to control the symmetry of the high-temperature dielectric response. In such a superlattice when a nanostructured asymmetric strain is programmed into the lattice via the stacking order, the natural symmetry at high temperatures is removed and a polarized sample is obtained in which the polarization increases as the temperature is lowered. In contrast to a ferroelectric characterized by a bistable ground state with two equal and opposite electronic polarizations, our experiments show evidence of asymmetric ferroelectric correlations that set in when such a sample becomes hysteretic below a temperature $T_x$, with two unequal polarization states. We further show that both the magnitude and direction of this ferroelectric asymmetry can be controlled by the engineered atomic-layer stacking order and periodicity of the superlattice.

The geometrically frustrated three-dimensional pyrochlore lattice has been long predicted to host a quantum spin liquid, an intrinsic long-range entangled state with fractionalized excitations. To date, most proposals for pyrochlore materials have focused on quantum spin ice, a U(1) quantum spin liquid whose only low energy excitations are emergent photons of Maxwell type. In this work, we explore the possibility of finding pyrochlore quantum spin liquids whose low energy theories go beyond this standard one. We give a complete classification of symmetric $\mathrm{U}\left(1\right)$ and ${\mathbb{Z}}_2$ spin liquids on the pyrochlore lattice within the projective symmetry group framework for fermionic spinons. We find 18 $\mathrm{U}\left(1\right)$ spin liquids and 28 ${\mathbb{Z}}_2$ spin liquids that preserve pyrochlore space-group symmetry while, upon further imposing time-reversal symmetry, the numbers of classes become 16 and 48, respectively. For each class, the most general symmetry-allowed spinon mean-field Hamiltonian is given. Interestingly, we find that several U(1) spin liquid classes possess an unusual gapless multi-nodal-line structure (“nodal star”) in the spinon bands, which is protected by the projective actions of the threefold rotation and screw symmetries of the pyrochlore space group. Through a simple model, we study the effect of gauge fluctuations on such a nodal star spin liquid and propose that the leading terms in the low temperature specific heat have the scaling form $C/T\sim \sqrt{T}+\sqrt{T}/lnT$, in contrast to the form $C/T\sim T^2$ of the standard U(1) pyrochlore spin liquid with gapped spinons.

We investigate the critical properties of the spin-1 honeycomb antiferromagnet ${\mathrm{BaNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$, both below and above the ordering temperature $T_{\text{N}}$ using neutron diffraction and muon spin rotation measurements. Our results characterize ${\mathrm{BaNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ as a two-dimensional (2D) antiferromagnet across the entire temperature range, displaying a series of crossovers from 2D XY to 2D XXZ and then to 2D Heisenberg behavior with increasing temperature. In particular, the extracted critical exponent of the order parameter reveals a narrow temperature regime close to $T_{\text{N}}$, in which the system behaves as a 2D XY antiferromagnet. Above $T_{\mathrm{N}}$, evidence for Berezinskii-Kosterlitz-Thouless behavior driven by vortex excitations is obtained from the scaling of the correlation length. Our experimental results are in agreement with classical and quantum Monte Carlo simulations performed using microscopic magnetic model Hamiltonians for ${\mathrm{BaNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$.

Magnons and phonons are elementary excitations of spin and lattice systems that can form a hybrid quasiparticle in the presence of strong magnetoelastic coupling. Here we use spin-, angle-, and energy-resolved photoemission spectroscopy to show that magnon-phonon hybridization plays an important role for the relaxation of the electron surface-state in Tb, opening both majority and minority spin channels in electron-phonon scattering. This is attributed to the strong $4f$ spin-orbit coupling, as evidenced by a comparison of the lifetime broadening in the occupied surface states of Gd and Tb. Both ferromagnetic metals have a comparable valence electronic structure, but the magnetocrystalline anisotropy is much stronger in Tb as compared to Gd. Consequently, in Gd electron-phonon and electron-magnon scatterings lead to spin-dependent photohole relaxation rates. Unlike in Gd, the lifetime broadening of the occupied surface state in Tb is only weakly spin dependent and the mass enhancement parameter $\lambda$ is twice the spin-averaged value of Gd. This difference in phase space is explained by the intimate coupling of phonons and magnons to form magnon polarons, which opens both minority and majority spin bands as decay channels.

We show that the observed time-reversal symmetry breaking (TRSB) of the superconducting state in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ can be understood as originating from inhomogeneous strain fields near edge dislocations of the crystal. Specifically, we argue that, without strain inhomogeneities, ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is a single-component, time-reversal symmetric superconductor, likely with $d_{x^2-y^2}$ symmetry. However, due to the strong strain inhomogeneities generated by dislocations, a slowly decaying subleading pairing state contributes to the condensate in significant portions of the sample. As it phase winds around the dislocation, time-reversal symmetry is locally broken. Global phase locking and TRSB occur at a sharp Ising transition that is not accompanied by a change of the single-particle gap and yields a very small heat capacity anomaly. Our model thus explains the puzzling absence of a measurable heat capacity anomaly at the TRSB transition in strained samples and the dilute nature of the time-reversal symmetry broken state probed by muon spin rotation experiments. We propose that plastic deformations of the material may be used to manipulate the onset of broken time-reversal symmetry.

We study systems of bosons whose low-energy excitations are located along a spherical submanifold of momentum space. We argue for the existence of gapless phases which we dub “Bose-Luttinger liquids,” which in some respects can be regarded as bosonic versions of Fermi liquids, while in other respects they exhibit striking differences. These phases have bosonic analogues of Fermi surfaces, and like Fermi liquids they possess a large number of emergent conservation laws. Unlike Fermi liquids, however, these phases lack quasiparticles, possess different RG flows, and have correlation functions controlled by a continuously varying exponent $\eta$, which characterizes the anomalous dimension of the bosonic field. We show that when $\eta >1$, these phases are stable with respect to all symmetric perturbations. These theories may be of relevance to several physical situations, including frustrated quantum magnets, rotons in superfluid He, and superconductors with finite-momentum pairing. As a concrete application, we show that coupling a Bose-Luttinger liquid to a conventional Fermi liquid produces a resistivity scaling with temperature as $T^{\eta }$. We argue that this may provide an explanation for the non-Fermi liquid resistivity observed in the paramagnetic phase of MnSi.

We study a diluted magnetic semiconductor system based on the spin of the ionized acceptor ${\mathrm{Cr}}^{+}$. We show that the negatively charged ${\mathrm{Cr}}^{+}$ ion, an excited state of the Cr in II-VI semiconductor, can be stable when inserted in a CdTe quantum dot (QD). The ${\mathrm{Cr}}^{+}$ attracts a heavy hole in the QD and form a stable hole-${\mathrm{Cr}}^{+}$ complex. Optical probing of this system reveals a ferromagnetic coupling between heavy holes and ${\mathrm{Cr}}^{+}$ spins. At low temperature, the thermalization on the ground state of the hole-${\mathrm{Cr}}^{+}$ system with parallel spins prevents the optical recombination of the excess electron on the $3d$ shell of the atom. We study the dynamics of the nanomagnet formed by the hole-${\mathrm{Cr}}^{+}$ exchange interaction. The ferromagnetic ground states with ${\mathrm{M}}_z=\pm 4$ can be controlled by resonant optical pumping and a spin relaxation time in the $20\mu \mathrm{s}$ range is obtained at $T=4.2$ K. This spin memory at zero magnetic field is limited by the interaction with phonons.

We investigate ytterbium gallium garnet ${\mathrm{Yb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ in the paramagnetic phase above the supposed magnetic transition at $T_{\lambda }\approx 54\mathrm{mK}$. Our study combines susceptibility and specific heat measurements with neutron scattering experiments and theoretical calculations. Below 500 mK, the elastic neutron response is strongly peaked in the momentum space. Along with that, the inelastic spectrum develops flat excitation modes. In magnetic field, the lowest energy branch follows a Zeeman shift in agreement with the field-dependent specific heat data. An intermediate state with spin canting away from the field direction is evidenced in small magnetic fields. In the field of 2 T, the total magnetization almost saturates and the measured excitation spectrum is well reproduced by the spin-wave calculations taking into account solely the dipole-dipole interactions. The small positive Curie-Weiss temperature derived from the susceptibility measurements is also accounted for by the dipolar spin model. Altogether, our results suggest that ${\mathrm{Yb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ is a quantum dipolar magnet.

We study electronic instabilities of a kagome metal with a Fermi energy close to saddle points at the hexagonal Brillouin zone face centers. Using parquet renormalization group, we determine the leading and subleading instabilities, finding superconducting, charge, orbital moment, and spin density waves. We then derive and use Landau theory to discuss how different primary density wave orders give rise to charge density wave modulations, as seen in the ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ family, with $A=$ K, Rb, Cs. The results provide strong constraints on the mechanism of charge ordering and how it can be further refined from existing and future experiments.

We report the results of our study of a recently synthesized honeycomb iridate ${\mathrm{Na}}_x{\mathrm{IrO}}_3$ $(0.60\le x\le 0.80)$. Single-crystal ${\mathrm{Na}}_x{\mathrm{IrO}}_3$ adopts a honeycomb lattice noticeably without distortions and stacking disorder inherently existent in its sister compound ${\mathrm{Na}}_2{\mathrm{IrO}}_3$. The oxidation state of the Ir ion is a mixed valence state resulting from a majority ${\mathrm{Ir}}^{5+}\left(5d^4\right)$ ion and a minority ${\mathrm{Ir}}^{6+}\left(5d^3\right)$ ion. ${\mathrm{Na}}_x{\mathrm{IrO}}_3$ is a Mott insulator likely with a predominant pseudospin $=1$ state. It exhibits an effective moment of $1.1{\mu }_{\mathrm{B}}/\mathrm{Ir}$ and a Curie-Weiss temperature of −19 K but with no discernible long-range order above 1 K. The physical behavior below 1 K features two prominent anomalies at $T_h=0.9\mathrm{K}$ and $T_l=0.12\mathrm{K}$ in both the heat capacity and AC magnetic susceptibility. Intermediate between $T_h$ and $T_l$ lies a pronounced temperature linearity of the heat capacity with a large slope of $77\mathrm{mJ}/\mathrm{mole}{\mathrm{K}}^2$, a feature expected for highly correlated metals but not at all for insulators. These results along with a comparison drawn with the honeycomb lattices ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and ${\left({\mathrm{Na}}_{0.2}{\mathrm{Li}}_{0.8}\right)}_2{\mathrm{IrO}}_3$ point to an exotic ground state in proximity to a possible Kitaev spin liquid.

Interaction between quasiparticles of a different nature, such as magnons and phonons in a magnetic medium, leads to the mixing of their properties and the formation of hybrid states in the areas of intersection of individual spectral branches. We recently reported the discovery of a new phenomenon mediated by the magnon-phonon interaction: the spontaneous bottleneck accumulation of magnetoelastic bosons under electromagnetic pumping of pure magnons into a ferrimagnetic yttrium iron garnet film. Here, by studying the transport properties of the accumulated magnetoelastic bosons, we reveal that such accumulation occurs in two frequency-distant groups of quasiparticles: quasiphonons and quasimagnons. They propagate with different velocities in different directions relative to the magnetization field. The theoretical model we propose qualitatively describes the double accumulation effect, and the analysis of the two-dimensional spectrum of quasiparticles in the hybridization region allows us to determine the wave vectors and frequencies of each of the groups.

The homogeneous line profile of a single molecule adsorbed on a surface was measured by high resolution laser spectroscopy. For the prototype organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) adsorbed at step sites of a thin epitaxial film of KCl on Ag(100) we measure a zero phonon line of Lorentzian shape with a full width at half maximum of $460\pm 25\mathrm{MHz}$ at a temperature of 6 K. The line is considerably narrower than those which have been recorded in fluorescence spectra of this and similar single molecules that were excited under the tip of a scanning tunneling microscope. The related time is, however, by a factor of $\sim 15$ smaller than the lifetime of PTCDA recorded in liquid He droplets, likely due to thermal dephasing. From this knowledge on the single molecule line profile, we also explain the inhomogeneous line profile of an ensemble of PTCDA molecules by a quantitative model based on surface defects.