In this work, we develop an optomechanical formalism for macroscopic quantum states in exciton-polariton systems with strong exciton-phonon interactions. We show that polariton optomechanical interactions induce dynamical backaction, resulting in dispersive and dissipative shifts in the complex vibrational response functions. Unlike conventional optomechanical systems, polariton optomechanics features high-dimensionality and phase-space confinement due to the dispersion relations of exciton polaritons. Consequently, vibrational modes exhibit effective positive or negative mass depending on the detuning parameter and are capable for the nonequilibrium vibrational Bose-Einstein condensation under the resonant conditions [Shishkov et al., Phys. Rev. Lett. 133, 186903 (2024)]. We demonstrate the potential for vibrational control of polariton condensates at room temperature.

Electrical control of magnon transport is preferred to exploit the ultrafast magnetization dynamics in antiferromagnetic materials for spintronics applications. However, the detection of long-distance magnon transport in antiferromagnets is challenging due to domain wall scattering, robustness against magnetic fields, and anisotropy dependent excitation modes. Specifically, for antiferromagnetic films with an easy-plane magnetic anisotropy, the transfer of angular momentum is non-trivial due to their linearly polarized magnon modes. In this work, magnon transport is studied in strained thin films of ${\mathrm{SrMnO}}_3$ (SMO), a strongly correlated easy-plane antiferromagnet. The strongly codependent structural and magnetic properties of SMO, in addition to the predicted strain-induced ferroelectric order parameter, can lead to potential tuning of magnon transport. Both electrically and thermally generated magnons are detected up to distances of at least $2.3\mu \mathrm{m}$. A variation of the external magnetic field strength and its direction confirms the easy-plane anisotropy and reveals other thermoelectric contributions. The phase of the electrically generated magnon signal and its distance dependence suggest the presence of antiferromagnetic domains. This work establishes the potential of SMO as an easy-plane antiferromagnetic conductor of magnon currents over long distances.

Eigenstate thermalization hypothesis (ETH) represents a breakthrough in many-body physics since it allows us to link thermalization of physical observables with the applicability of random matrix theory (RMT). Recent years were also extremely fruitful in exploring possible counterexamples to thermalization, ranging, among others, from integrability, single-particle chaos, many-body localization, many-body scars, to Hilbert-space fragmentation. In all these cases the conventional ETH is violated. However, it remains elusive how the conventional ETH breaks down when one approaches the boundaries of ergodicity, and whether the range of validity of the conventional ETH coincides with the validity of RMT-like spectral statistics. Here we bridge this gap and we introduce a scenario of the ETH breakdown in many-body quantum systems, dubbed fading ergodicity regime, which establishes a link between the conventional ETH and nonergodic behavior. We conjecture this scenario to be relevant for the description of finite many-body systems at the boundaries of ergodicity, and we provide numerical and analytical arguments for its validity in the quantum sun model of ergodicity-breaking phase transition. For the latter, we provide evidence that the breakdown of the conventional ETH is not associated with the breakdown of the RMT-like spectral statistics.

Local modifications of the photonic density of states accompanying optical cavity structures influence a wide variety of light-matter interactions between nearby quantum emitters, scatterers, and absorbers. Resonant energy transfer, whereby energy is exchanged between a material donor and an acceptor, represents one important example of an optical process that can be strongly modified by cavities. However, in many experimental settings the photonic cavity environment is fabricated on an underlying substrate, which may additionally influence the optical interactions between nearby emitters, especially when the substrate carries its own resonance structure that is commensurate with that of the cavity. In this paper, we present a theoretical approach to quantifying cavity-mediated energy transfer between a pair of neighboring quantum emitters in the presence of a resonant optical environment composed of a photonic cavity and supporting substrate. A composite system Green's function and local density of states for the cavity and substrate structure are derived fully analytically in the quasistatic limit, accounting for mode mixing and image interactions up to an arbitrary multipole order. The derived Green's function is utilized to investigate the spectral and spatial dependence of the associated photonic density of states and its impact upon the rate of energy transfer between the emitter pair. Analysis of the Purcell enhancement factor introduced by the composite optical environment is also discussed, with emphasis placed upon the additional energy transfer pathways opened by the substrate.

We have measured magnetic toque on single crystals of $\alpha \text{−}{\mathrm{RuCl}}_3$ with ordering temperature $T_N=7$ K as a function of the field angle in the $ab$ plane, focusing on temperatures between 2 and 20 K and fields from 0 to 9 T. We find a number of features, many of which can be classified by their angular periodicity. The torque signal shows an oscillation with a period of ${180}^{\circ }$ (i.e., twofold periodicity) and within the magnetically ordered zigzag phase there is a ${60}^{\circ }$ period (i.e., sixfold) sawtooth pattern, which can be explained by reorientation of the zigzag domains as the crystal rotates in the applied field. We argue that the sixfold sawtooth and the twofold sinusoidal signals arise from distinct regions of the crystal. Suppressing the zigzag order with an applied field above $\sim 8$ T at low temperature, a sixfold sinusoidal signal remains, suggesting that there is fluctuating zigzag order in the putative field-induced quantum spin-liquid state. Finally, in testing theoretical results which predict a torque response with divergent slope across $C_2$-preserving $b$ axes (B1 axis), we find no features like that predicted for Ising topological order. Instead we find a feature, similar to the theoretical prediction, at low temperatures and fields just above the zigzag phase, but only across the non-$C_2$-preserving $b$ axes (B2 axes). Interpretation of this feature is complicated by the development of other similar signatures nearby at slightly lower fields, and by the fact that this feature is enhanced by thermal cycling, which we also show causes the quality of the crystal to degrade. Additionally, we contrast the torque response of $T_N\sim 7$ and 14 K samples.

Motivated by systems where a high-temperature non-Fermi liquid gives way to low-temperature ${\mathbb{Z}}_3$ Potts nematic order, we studied a three-orbital Sachdev-Ye-Kitaev (SYK) model in the large-$N$ limit. In the single-site limit, this model exhibits a spontaneous orbital selective transition which preserves average particle-hole symmetry, with two orbitals becoming insulators while the third orbital remains a non-Fermi liquid down to zero temperature. We extend this study to lattice models of three-orbital SYK dots, exploring uniform symmetry-broken states on the triangular and cubic lattices. At high temperature, these lattice models exhibit an isotropic non-Fermi-liquid metal phase. On the three-dimensional cubic lattice, the low-temperature uniform ${\mathbb{Z}}_3$ nematic state corresponds to an orbital selective layered state which preserves particle-hole symmetry at small hopping and spontaneously breaks the particle-hole symmetry at large hopping. Over a wide range of temperatures, the transport in this layered state shows metallic in-plane resistivity but insulating out-of-plane resistivity. On the two-dimensional triangular lattice, the low-temperature state with uniform orbital order is also a correlated ${\mathbb{Z}}_3$ nematic with orbital selective transport but it remains metallic in both principal directions. We discuss a Landau theory with ${\mathbb{Z}}_3$ clock terms which captures salient features of the phase diagram and nematic order in all these models. We also present results on the approximate wave-vector-dependent orbital susceptibility of the isotropic non-Fermi-liquid states.

Among the most intriguing features of non-Hermitian (NH) systems is the ability of complex energies to form braids under parametric variation. Several braiding behaviors, including link and knot formation, have been observed in experiments on synthetic NH systems, such as looped optical fibers. The exact conditions for these phenomena remain unsettled, but existing demonstrations have involved long-range nonreciprocal hoppings, which are hard to implement on many experimental platforms. Here, we present a route to realizing complex energy braids using one-dimensional NH Aubry-André-Harper lattices. Under purely local gain and loss modulation, the eigenstates exhibit a variety of braiding behaviors, including unknots, Hopf links, trefoil knots, Solomon links, and catenanes. We show how these are created by the interplay between non-Hermiticity and the lattice's bulk states and topological edge states. The transitions between different braids are marked by changes in the global Berry phase of the NH lattice.

The melting of quasi-long-range superconductivity in two spatial dimensions occurs through the proliferation and unbinding of vortex-antivortex pairs, a phenomenon known as the Berezinskii-Kosterlitz-Thouless (BKT) transition. Although signatures of this transition have been observed in bulk measurements, these experiments are often complicated, ambiguous, and unable to resolve the rich physics of the vortex unbinding transition. Here we show that local noise magnetometry is a sensitive, noninvasive probe that can provide direct information about the scale-dependent vortex dynamics. In particular, by resolving the distance and temperature dependence of the magnetic noise, it may be possible to experimentally study the renormalization group flow equations of the vortex gas and track the onset of vortex unbinding in situ. Specifically, we predict (i) a nonmonotonic dependence of the noise on temperature and (ii) the local noise is almost independent of the sample-probe distance at the BKT transition. We also show that noise magnetometry can distinguish Gaussian superconducting order-parameter fluctuations from topological vortex fluctuations and can detect the emergence of unbound vortices. The weak distance dependence at the BKT transition can also be used to distinguish it from quasiparticle background noise. Our predictions may be within experimental reach for a number of unconventional superconductors.

By theoretically analyzing the recent temperature dependent transport data (Lu et al., arXiv:2408.10203) in pentalayer graphene, we establish that the experimentally observed transition from low-temperature quantum anomalous Hall effect (QAHE) to higher-temperature fractional quantum anomalous Hall effect (FQAHE) is a crossover phenomenon arising from the competition between interaction and disorder energy scales, with the likely zero temperature ground state being either a localized insulator or a Chern insulator with a quantized anomalous Hall effect. In particular, the intriguing suppression of FQAHE in favor of QAHE with decreasing temperature is explained as arising from the low-temperature localization of the carriers where disorder overcomes the interaction effects. We provide a detailed analysis of the data in support of the crossover scenario.

Recently it was shown that the topological entanglement entropy (TEE) of a two-dimensional gapped ground state obeys the universal inequality $\gamma \ge log\mathcal{D}$, where $\gamma$ is the TEE and $\mathcal{D}$ is the total quantum dimension of all anyon excitations, $\mathcal{D}=\sqrt{{\sum }_a{d}_a^2}$. Here we present an alternative, more direct proof of this inequality. Our proof uses only the strong subadditivity property of the von Neumann entropy together with a few physical assumptions about the ground-state density operator. Our derivation naturally generalizes to a variety of systems, including spatially inhomogeneous systems with defects and boundaries, higher-dimensional systems, and mixed states.

Quantum impurities, such as nitrogen vacancy (NV) centers in diamond, exhibit excellent quantum coherence, single-spin sensitivity, and a significant capability to be optically manipulated. These defects act as single spin sensors, allowing the detection of local magnetic fields on length scales of tens of nanometers. Their relaxation rates, affected by the magnetic noise in their vicinity, could capture information about the dynamics of a magnetic environment. In recent years, NV centers have been increasingly utilized to measure magnetic properties of ferromagnetic materials and, on a few occasions, to study antiferromagnets with uniaxial anisotropy. Both systems have the capability to propagate spin waves, whose quanta are called magnons. However, a complete theoretical description of how NV centers interact with antiferromagnetic magnons is still a topic to explore. In this work, we calculate the NV center relaxation rates considering collinear anisotropic antiferromagnetic insulators, such as ${\mathrm{MnF}}_2$ and NiO, represented as magnon systems. For an easy-axis of anisotropy $\widehat{\mathbf{z}}$ and an external magnetic field ${\mathbf{H}}_0$, we did these calculations for both ${\mathbf{H}}_0\parallel \widehat{\mathbf{z}}$ (antiferromagnetic phase) and ${\mathbf{H}}_0\perp \widehat{\mathbf{z}}$ (canted phase), finding that the relaxation rates are greater in the canted phase. Moreover, we found that NiO induces remarkably lower relaxation rates than ${\mathrm{MnF}}_2$, because of its high effective exchange field ${\mu }_0{H}_E\sim 1000\mathrm{T}$.

Excitons in semiconductors can form a variety of collective states leading to different regimes of exciton propagation. Here we theoretically demonstrate the possibility to reach the viscous hydrodynamic—liquidlike—regime of exciton propagation in two-dimensional materials, focusing on the mono- and bilayers of transition-metal dichalcogenides. This regime can be realized where the exciton-exciton collisions dominate over exciton-phonon and disorder scattering. We have derived the hydrodynamiclike set of equations describing viscous flow of interacting excitons based on the Boltzmann kinetic equation for the exciton distribution function. A comparison of various exciton propagation regimes including diffusive, viscous hydrodynamic, and superfluid regime is presented. Conditions that allow one to observe the hydrodynamic regime of exciton transport and the role of material are discussed.

Separating heat from charge in a material is an extremely challenging task since they are transported by the very same carriers, i.e., electrons or holes. In this Letter we show that such separation can reach $100\%$ efficiency in a hybrid superconducting quantum Hall setup, provided that the quantum Hall system is tuned to the integer filling factor. We present microscopic calculations for a three-terminal setup to illustrate our idea.

The self-consistent harmonic approximation (SCHA) describes atoms in solids, including quantum fluctuations and anharmonic effects, in a nonperturbative way. It computes ionic free energy variationally, constraining the atomic quantum-thermal fluctuations to be Gaussian. Consequently, the entropy is analytical; there is no need for thermodynamic integration or heavy diagonalization to include finite-temperature effects. In addition, as the probability distribution is fixed, SCHA solves all the equations with Monte Carlo integration without employing metropolis sampling of the quantum phase space. Unfortunately, the Gaussian approximation breaks down for rotational modes and tunneling effects. We show how to describe these non-Gaussian fluctuations using the quantum variational principle at finite temperatures, keeping the main advantage of SCHA: direct access to free energy. Our method, nonlinear SCHA (NLSCHA), employs an invertible nonlinear transformation to map Cartesian coordinates into an auxiliary manifold parametrized by a finite set of variables. So, we adopt a Gaussian ansatz for the density matrix in this new coordinate system. The nonlinearity of the mapping ensures that NLSCHA enlarges the SCHA variational subspace, and its invertibility conserves the information encoded in the density matrix. We evaluate the entropy in the auxiliary space, where it has a simple analytical form. As in the SCHA, the variational principle allows for optimizing free parameters to minimize free energy. Finally, we show that the NLSCHA gives direct access to the entropy of a crystal with non-Gaussian degrees of freedom.

Majorana fermions (MFs) in a topological superconductor exhibit anisotropic electromagnetic responses, called Majorana multipole responses, when MFs are degenerate under time-reversal and crystalline symmetries. In time-reversal symmetric systems, the Majorana multipole response relates to Cooper pair symmetry in the underlying superconducting material, which provides a way to identify pairing symmetries through surface-spin-sensitive measurements. Here, we extend the concept of Majorana multipole response to systems with magnetic point group symmetry that break time-reversal symmetry and clarify how the response of MFs includes information about underlying superconductors. From a topological classification of symmetry-protected MFs and an effective surface theory, we classify possible magnetic and electric responses for MFs, which manifests a direct connection to Cooper pair symmetry for a symmetry-enforced pair of MFs. Additionally, we find several time-reversal-even higher-order multipole responses, such as the quadrupole response, which are forbidden in time-reversal symmetric systems, whereby indicating breaking of time-reversal symmetry. The theory is applied to the odd-parity chiral superconductor ${\mathrm{UTe}}_2$ and the ferromagnetic superconductor UCoGe, demonstrating the appearance of a magnetic quadrupole response on a surface.

Antiferromagnetic spintronics is a cutting-edge research direction in modern solid-state physics because antiferromagnets have several important advantages over ferromagnets with respect to device applications. In particular, antiferromagnetic spin dynamics can occur at terahertz (THz) frequencies, which enables ultrafast information processing. Here, we describe theoretically the energy-efficient excitation of antiferromagnetic magnons in epitaxial NiO thin films by short acoustic pulses injected from a nonmagnetic substrate. This dynamic problem is solved by advanced micromagnetoelastic simulations, which take into account strong bidirectional coupling between spins and strains inherent in single-crystalline NiO, its peculiar magnetocrystalline anisotropy, and effective exchange interactions involved in the two-sublattice model of NiO. Our simulations show that picosecond bipolar strain pulses generated by an attached optomechanical transducer can efficiently create antiferromagnetic standing spin waves in the (001)-oriented NiO films. The wave numbers of these magnons are governed by the film thickness, which we varied from 5 to 20 nm, and their frequencies comply with the dispersion relation of antiferromagnetic spin waves in the NiO film. However, standing spin waves with considerable magnitudes evidently can be excited only by acoustic pulses, whose continuous frequency spectra have an overlap with the discrete spectrum of magnon modes allowed in the film. Remarkably, the latter shifts to higher frequencies in the presence of substrate-induced lattice strains, which is mostly caused by increasing frequencies of two antiferromagnetic resonance modes existing in NiO. Our calculations show that these frequencies rise from 240 to 870 GHz and from 1100 to 1355 GHz in the fully strained NiO film grown on MgO. Most importantly, antiferromagnetic standing waves with frequencies exceeding 1 THz can be excited in both strained and stress-free NiO films by bipolar acoustic pulses with durations $\tau \le 3$ ps. Our theoretical results significantly advance both antiferromagnetic magnonics and straintronics by clarifying conditions for the purely magnetoelastic generation of THz standing spin waves and revealing strong strain effects on the spin-wave dispersion in a collinear antiferromagnet.

In metals, low-energy effective theories are characterized by a set of coupling functions. Among them, the angle-dependent Fermi momentum specifies the size and shape of Fermi surface. Since the size of Fermi momentum grows incessantly relative to an energy scale that is lowered under the renormalization group (RG) flow, a metallic fixed point is defined only modulo a rescaling of Fermi momentum. In this paper, we discuss the physical consequences of this projective nature of fixed points for non-Fermi liquids with hot Fermi surfaces. The first is the absence of a unique dynamical critical exponent that dictates the relative scaling between energy and momentum across all low-energy observables. The second is mismatches between the scaling dimensions of couplings and their relevancy. Nonetheless, each projective fixed point is characterized by a few marginal and relevant coupling functions, and the notion of universality survives. We illustrate our findings by charting the space of projective fixed points and extracting their universal properties for the Ising-nematic quantum critical metal beyond the patch theory. To approach the interacting theory in two space dimensions from a controlled limit, we use the dimensional regularization scheme that tunes the codimension of Fermi surface as a control parameter. Near the upper critical dimension, two exactly marginal coupling functions span the space of stable projective fixed points: one specifies the shape of the Fermi surface and the other sets the angle-dependent Fermi velocity. All other coupling functions, including the Landau functions and the universal pairing interaction, are fixed by those two marginal functions. As the space dimension is lowered, vertex corrections alter the fate of the four-fermion coupling in a channel-dependent way. To the leading order, the forward scattering remains irrelevant while the pairing interaction becomes relevant near two dimensions. In two dimensions, it is expected that the universal superconducting fluctuations lower the symmetry of the non-Fermi liquid realized above the superconducting transition temperatures from the loop $U\left(1\right)$ group to a proper subgroup .

The ultrafast dynamics of $\alpha$-MnAs magnetization and atomic structure are studied by the time-resolved magneto-optical Kerr effect and time-resolved x-ray diffraction after photoexcitation. In the basal plane of the hexagonal unit cell, Mn atomic disorder increases on a timescale of 400 fs, comparable to the laser-induced demagnetization timescale and significantly faster than As disorder (700 fs). This differentiation of the Mn and As relaxation times is observed in the ferromagnetic phase but disappears in the paramagnetic phase. These results strongly support recent density-functional theory calculations predicting that the magnetostructural phase transition of MnAs is driven by a change in nature of the dominant interactions between Mn $3d$ electrons, from magnetic exchange to bonding interactions.

We perform a theoretical investigation of the spin susceptibility of unconventional superconductivity on the kagome lattice. Despite the existence of a sign-changing gap structure, which sums to zero over the Fermi surface, we show that such unconventional pairing states may exhibit a Hebel-Slichter peak in the temperature-dependent spin-lattice relaxation rate. It originates from destructive sublattice interference effects. For the same reason, unconventional pairing states on the kagome lattice tend not to exhibit a neutron resonance peak. These results supplement previous theoretical studies of the surprising robustness of sign-changing gap structures to disorder on the kagome lattice. Taken together these findings imply that unconventional superconductivity on the kagome lattice is deceptive in the sense that its properties may appear similar to conventional non-sign-changing superconductivity. These results may be of relevance to the superconducting state of the kagome superconductors ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A: \mathrm{K}, \mathrm{Rb}, \mathrm{Cs})$ and ${\mathrm{CsTi}}_3{\mathrm{Bi}}_5$.

Given the algebraic data characterizing any (2+1)-dimensional [(2+1)D] bosonic or fermionic topological order with a global symmetry group $G=\mathrm{U}\left(1\right)⋊H$, we construct a (3+1)-dimensional [(3+1)D] topologically invariant path integral in the presence of a curved background $G$ gauge field, as an exact combinatorial state sum. Specifically, the U(1) component of the $G$ gauge field can have a nontrivial second Chern class, extending previous work that was restricted to flat $G$ bundles. Our construction expresses the U(1) gauge field in terms of a Villain formulation on the triangulation, which includes a 1-form $\mathbb{R}$ gauge field and 2-form $\mathbb{Z}$ gauge field. We develop a graphical calculus for anyons interacting with “Villain symmetry defects,” associated with the 1-form and 2-form background gauge fields. This graphical calculus is used to define the (3+1)D path integral, which can describe either a bosonic or fermionic symmetry-protected topological phase. For example, we can construct the topological path integral on curved U(1) bundles for the (3+1)D fermionic topological insulator in class AII and topological superconductor in class AIII given appropriate (2+1)D fermionic symmetry fractionalization data; these then give invariants of 4-manifolds with ${\mathrm{spin}}^c$ or ${\mathrm{pin}}^c$ structures and their generalizations. The (3+1)D path integrals define anomaly indicators for the (2+1)D topological orders; in the case of Abelian (2+1)D topological orders, we derive by explicit computation all of the mixed U(1) anomaly indicator formulas proposed by Lapa and Levin. We also propose a ${\mathrm{spin}}^c$ generalization of the Gauss-Milgram sum, valid for supermodular categories.