In crystals with time-reversal symmetry but without inversion symmetry, the phonon angular momentum can be generated by the temperature gradient, and it is called the phonon thermal Edelstein effect. On the other hand, when both symmetries are broken and their product is conserved, the phonon angular momentum for any phonon modes at any wave vectors vanishes and the phonon thermal Edelstein effect does not occur. In this paper, we propose another mechanism of generation of the phonon angular momentum. We show that in such crystals the electric field generates the phonon angular momentum via the lattice distortion due to the electric field. This effect is in the same symmetry class with the magnetoelectric effect, and we call this effect the phonon rotoelectric effect. We discuss the temperature dependence of the phonon angular momentum generated by the temperature gradient and by the electric field in the high- and low-temperature limits.

We investigate the divergences appearing in the two-particle-irreducible vertex functions of many-fermion systems with attractive on-site interactions. By means of dynamical mean-field theory calculations, we determine the location of singularity lines in the phase diagram of the attractive Hubbard model at half-filling, where the local Bethe-Salpeter equations are noninvertible. We find that divergences appear both in the magnetic and in the density scattering channels. The former affect a sector of suppressed fluctuations and comply with the mapping of the physical susceptibilities of the repulsive case. At the same time, the appearance of singularities in the density channel of the attractive model demonstrates that vertex divergences can also plague the dominant scattering sectors associated with enhanced local susceptibilities. This constitutes a counterexample to previously proposed interpretations. Eventually, by exploiting the underlying physical symmetries and a spectral representation of the susceptibilities, we clarify the relation between vertex divergences and the local response of the system in different channels.

We propose a set of constraints on the ground-state wave functions of fracton phases, which provide a possible generalization of the string-net equations used to characterize topological orders in two spatial dimensions. Our constraint equations arise by exploiting a duality between certain fracton orders and quantum phases with “subsystem” symmetries, which are defined as global symmetries on lower-dimensional manifolds, and then studying the distinct ways in which the defects of a subsystem symmetry group can be consistently condensed to produce a gapped, symmetric state. We numerically solve these constraint equations in certain tractable cases to obtain the following results: in $d=3$ spatial dimensions, the solutions to these equations yield gapped fracton phases that are distinct as conventional quantum phases, along with their dual subsystem symmetry-protected topological (SSPT) states. For an appropriate choice of subsystem symmetry group, we recover known fracton phases such as Haah's code, along with new, symmetry-enriched versions of these phases, such as nonstabilizer fracton models which are distinct from both the X-cube model and the checkerboard model in the presence of global time-reversal symmetry, as well as a variety of fracton phases enriched by spatial symmetries. In $d=2$ dimensions, we find solutions that describe new weak and strong SSPT states, such as ones with both linelike subsystem symmetries and global time-reversal symmetry. In $d=1$ dimension, we show that any group cohomology solution for a symmetry-protected topological state protected by a global symmetry, along with lattice translational symmetry necessarily satisfies our consistency conditions.

We have used spherical neutron polarimetry to investigate the magnetic structure of the Mn spins in the hexagonal semimetal ${\mathrm{Mn}}_3\mathrm{Ge}$, which exhibits a large intrinsic anomalous Hall effect. Our analysis of the polarimetric data finds a strong preference for a spin structure with $E_{1g}$ symmetry relative to the $D_{6h}$ point group. We show that weak ferromagnetism is an inevitable consequence of the symmetry of the observed magnetic structure, and that sixth-order anisotropy is needed to select a unique ground state.

We present and analyze the fermionic time-evolving density matrix using orthogonal polynomials algorithm (fTEDOPA), which enables the numerically exact simulation of open quantum systems coupled to a fermionic environment. The method allows for simulating the time evolution of open quantum systems with arbitrary spectral densities at zero or finite temperatures with controllable and certified error. We demonstrate the efficacy of the method towards the simulation of quintessential fermionic open quantum systems including the resonant-level model and quantum dot coupled to an impurity and towards simulating hitherto intractable problems in quantum transport. Furthermore, we demonstrate significant efficiency gains in the computational costs by performing simulations in the Heisenberg picture. Finally, we compare different approaches for simulating finite-temperature situations and provide practical guidelines in order to identify the best strategy.

We present an interpretation of scar states and quantum revivals as weakly “broken” representations of Lie algebras spanned by a subset of eigenstates of a many-body quantum system. We show that the PXP model, describing strongly interacting Rydberg atoms, supports a “loose” embedding of multiple $\mathrm{su}\left(2\right)$ Lie algebras corresponding to distinct families of scarred eigenstates. Moreover, we demonstrate that these embeddings can be made progressively more accurate via an iterative process which results in optimal perturbations that stabilize revivals from arbitrary charge density wave product states, $|{\mathbb{Z}}_n\rangle$, including ones that show no revivals in the unperturbed PXP model. We discuss the relation between the loose embeddings of Lie algebras present in the PXP model and recent exact constructions of scarred states in related models.

Intense femtosecond laser pulses can induce dramatic changes on different properties of materials. Laser induced changes of the optical properties are particularly relevant, since they can lead to a modification of the amount of energy that the material absorbs from the laser pulse. In noble metals, changes of reflectivity upon femtosecond laser illumination are expected to be strong due to the excitation of $d$ electrons. In this work we perform measurements of the reflectivity of laser excited gold in the infrared and in the ultraviolet range, respectively. We find a remarkable dependence of the reflectivity on laser fluence, which is in turn different in both ranges of photon energy. In order to understand the behavior of the reflectivity in laser excited solids and to explain our measured reflectivity curves we develop a theoretical scheme in the framework of the two-temperature model with the electronic temperature as the key parameter. Our approach is based on all-electron calculations of the interband contribution to the reflectivity and a careful determination of the intraband, Drude-like terms and a realistic model for the space and time resolved energy transfer from a Gaussian laser pulse into the electronic system. We obtain very good agreement between experiment and theory and identify the main mechanisms for reflectivity changes as a function of laser fluence.

Layered $\alpha -{\mathrm{RuCl}}_3$ has been discussed as a proximate Kitaev spin-liquid compound. Raman and terahertz spectroscopy of magnetic excitations confirms that the low-temperature antiferromagnetic ordered phase features a broad Raman continuum, together with two magnonlike excitations at 2.7 and 3.6 meV, respectively. The continuum strength is maximized as long-range order is suppressed by an external magnetic field. The state above the field-induced quantum phase transition around 7.5 T is characterized by a gapped multiparticle continuum out of which a two-particle bound state emerges, together with a well-defined single-particle excitation at lower energy. Exact diagonalization calculations demonstrate that Kitaev and off-diagonal exchange terms in the Fleury-Loudon operator give rise to a pronounced intensity of these features in the Raman spectra. Our Rapid Communication firmly establishes the partially polarized quantum disordered character of the high-field phase.

We investigate the lattice dynamics in the spin-$\frac{1}{2}$ frustrated kagome compound herbertsmithite ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$ by a combination of infrared spectroscopy measurements and ab initio density functional theory calculations, and provide an unambiguous assignment of infrared-active lattice vibrations involving in-plane and out-of-plane atom displacements in the kagome layers. Upon cooling, nonthermal redshifts and broadening appear specifically for modes that deform the kagome layer or affect the Cu-O-Cu bond angles, thus creating pronounced modifications of the antiferromagnetic exchange coupling. Our results indicate the presence of a strong magnetoelastic coupling to the spin system. We discuss the effects of this coupling and its relation to recent experiments reporting a global symmetry reduction of the kagome lattice symmetry.

We consider the problem of many-body localization on Fock space, focusing on the essential features of the Hamiltonian which stabilize a localized phase. Any many-body Hamiltonian has a canonical representation as a disordered tight-binding model on the Fock-space graph. The underlying physics is, however, fundamentally different from that of conventional Anderson localization on high-dimensional graphs because the Fock-space graph possesses nontrivial correlations. These correlations are shown to lie at the heart of whether or not a stable many-body localized phase can be sustained in the thermodynamic limit, and a theory is presented for the conditions the correlations must satisfy for a localized phase to be stable. Our analysis is rooted in a probabilistic, self-consistent mean-field theory for the local Fock-space propagator and its associated self-energy, in which the Fock-space correlations, together with the extensive nature of the connectivity of Fock-space nodes, are key ingredients. The origins of many-body localization in typical local Hamiltonians where the correlations are strong, as well as its absence in uncorrelated random energy models, emerge as predictions from the same overarching theory. To test these, we consider three specific microscopic models, first establishing in each case the nature of the associated Fock-space correlations. Numerical exact diagonalization is then used to corroborate the theoretical predictions for the occurrence or otherwise of a stable many-body localized phase, with mutual agreement found in each case.

We report experimental and theoretical evidence that ${\mathrm{Rb}}_2{\mathrm{Cu}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$ has a nonmagnetic tetramer ground state of a two-leg ladder comprising antiferromagnetically coupled frustrated spin-$\frac{1}{2}$ chains and exhibits a Haldane spin gap of emergent spin-1 pairs. Three spin excitations split from the spin-1 triplet by a Dzyaloshinskii-Moriya interaction are identified in inelastic neutron-scattering and electron spin resonance spectra. A tiny magnetic field generates ferroelectricity without closing the spin gap, indicating a unique class of ferroelectricity induced by a vector spin chirality order.

${\mathrm{EtMe}}_3\mathrm{Sb}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$, an organic Mott insulator with nearly isotropic triangular lattice, is a candidate material for a quantum spin liquid, in which the zero-point fluctuations do not allow the spins to order. The itinerant gapless excitations inferred from the thermal transport measurements in this system have been a hotly debated issue recently. While the presence of a finite linear residual thermal conductivity, ${\kappa }_0/T\equiv \kappa /T(T\to 0)$, has been shown [M. Yamashita et al., Science 328, 1246 (2010)], recent experiments [P. Bourgeois-Hope et al., Phys. Rev. X 9, 041051 (2019); J. M. Ni et al., Phys. Rev. Lett. 123, 247204 (2019)] have reported the absence of ${\kappa }_0/T$. Here we show that the low-temperature thermal conductivity strongly depends on the cooling process of the sample. When cooling down very slowly, a sizable ${\kappa }_0/T$ is observed. In contrast, when cooling down rapidly, ${\kappa }_0/T$ vanishes and, in addition, the phonon thermal conductivity is strongly suppressed. These results suggest that possible random scatterers introduced during the cooling process are responsible for the apparent discrepancy of the thermal conductivity data in this organic system. The present results provide evidence that the true ground state of ${\mathrm{EtMe}}_3\mathrm{Sb}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ is likely to be a quantum spin liquid with itinerant gapless excitations.

Topological photonics aims to utilize topological photonic bands and corresponding edge modes to implement robust light manipulation, which can be readily achieved in the linear regime of light-matter interaction. Importantly, unlike solid-state physics, the common test bed for new ideas in topological physics, topological photonics provides an ideal platform to study wave mixing and other nonlinear interactions. These are well-known topics in classical nonlinear optics but largely unexplored in the context of topological photonics. Here, we investigate nonlinear interactions of one-way edge modes in frequency mixing processes in topological photonic crystals. We present a detailed analysis of the band topology of two-dimensional photonic crystals with hexagonal symmetry and demonstrate that nonlinear optical processes, such as second- and third-harmonic generation, can be conveniently implemented via one-way edge modes in this setup. Moreover, we demonstrate that more exotic phenomena, such as slow-light enhancement of nonlinear interactions and harmonic generation upon interaction of backward-propagating (left-handed) edge modes, can also be realized. Our work opens up new avenues towards topology-protected frequency mixing processes in photonics.

We used angle-resolved photoemission spectroscopy (ARPES) and density functional theory calculations to study the electronic properties of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, a material that was predicted to be an intrinsic antiferromagnetic (AFM) topological insulator. In striking contrast to earlier literature showing a full gap opening between two surface band manifolds on the (0001) surface, we observed a gapless Dirac surface state with a Dirac point sitting at $E_B=-280\mathrm{meV}$. Furthermore, our ARPES data revealed the existence of a second Dirac cone sitting closer to the Fermi level. Surprisingly, these surface states remain intact across the AFM transition. The presence of gapless Dirac states in this material may be caused by different ordering at the surface from the bulk or weaker magnetic coupling between the bulk and surface. Whereas the surface Dirac cones seem to be remarkably insensitive to the AFM ordering most likely due to weak coupling to magnetism, we did observe a splitting of the bulk band accompanying the AFM transition. With a moderately high ordering temperature and interesting gapless Dirac surface states, ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ provides a unique platform for studying the interplay between magnetic ordering and topology.

The ultrafast switching of magnetization in multiferroic materials by a femtosecond laser could provide various advantages in photonics and magnonics. An efficient approach to control the light-matter interaction is the modulation of ultrafast coherent magnons and phonons in the high-frequency range. Spontaneous Raman and infrared spectra reveal the excitation of magnons and optical phonons in multiferroic $\mathrm{BiFe}{\mathrm{O}}_3$ in the sub-few-terahertz range. However, coherent control of such quasiparticles has not been achieved yet. In this study, we demonstrate that linearly polarized laser pulses simultaneously excite coherent magnons [out-of-plane ($\mathrm{\Psi }$) and in-plane ($\mathrm{\Phi }$) cyclone modes] and optical phonon ($E$ mode) in $\mathrm{BiFe}{\mathrm{O}}_3$. Experimental results in conjugation with phenomenological theory, by considering three uniformly distributed magnetic domains, reveal that impulsive stimulated Raman scattering is responsible for the generation of coherent magnons and phonons in $\mathrm{BiFe}{\mathrm{O}}_3$. The observation of these terahertz magnon and optical phonon modes paves the way for the development of ultrafast magnetoelectro-optical devices.

We propose and demonstrate first steps toward schemes in which the librational mode of levitating ferromagnets can be strongly coupled to the electronic spin of nitrogen-vacancy (NV) centers in diamond. Experimentally, we levitate soft ferromagnets in a Paul trap and employ magnetic fields to attain oscillation frequencies in the hundreds of kHz range with $Q$-factors close to ${10}^4$. These librational frequencies largely exceed the decoherence rate of NV centers in typical chemical vapor deposition grown diamonds offering prospects for resolved sideband operation. We also prepare and levitate composite diamond-ferromagnet particles and demonstrate both coherent spin control of the NV centers and readout of the particle libration using the NV spin. Our results will find applications in ultrasensitive gyroscopy and bring levitating objects a step closer to spin-mechanical experiments at the quantum level.

Pyrite-structured ${\mathrm{PdSb}}_2$ with the nonsymmorphic crystalline symmetry, has long been predicted to host sixfold-degenerate exotic fermions beyond Dirac and Weyl. Although magnetotransport measurements on ${\mathrm{PdSb}}_2$ have suggested its topologically nontrivial character, the direct spectroscopic evidence still remains absent. By utilizing high-resolution angle-resolved photoemission spectroscopy, we present a systematic study on the low-energy bulk and surface electronic structure of pyrite-structured ${\mathrm{PdSb}}_2$. Through careful comparison with first-principles calculations, we verify the existence of sixfold fermions in this compound, which are formed by three doubly degenerate bands centered at the R point in the Brillouin zone. These bands exhibit parabolic dispersion close to sixfold fermion nodes, in sharp contrast to previously reported sixfold double spin-1 chiral fermions. Furthermore, our findings reveal no protected Fermi arcs in ${\mathrm{PdSb}}_2$, which is compatible with its achiral structure. Our findings suggest that pyrite-structured ${\mathrm{PdSb}}_2$ provides an ideal platform for the investigation of fermions and their potential applications.

We report a comprehensive neutron scattering study of low energy magnetic excitations in the breathing pyrochlore helimagnetic ${\text{Cu}}_2{\text{OSeO}}_3$. Fully documenting the four lowest energy magnetic modes that leave the ferrimagnetic configuration of the “strong tetrahedra” intact ($\left|\hslash \omega \right|<13$ meV), we find gapless quadratic dispersion at the $\mathrm{\Gamma }$ point for energies above 0.2 meV, two doublets separated by 1.6(2) meV at the $R$ point, and a bounded continuum at the $X$ point. Our constrained rigid spin cluster model relates these features to Dzyaloshinskii-Moriya (DM) interactions and the incommensurate helical ground state. Combining conventional spin wave theory with a spin cluster form factor accurately reproduces the measured equal time structure factor through multiple Brillouin zones. An effective spin Hamiltonian describing complex anisotropic intercluster interactions is obtained.

We derive the equilibrium phase diagram of the classical dipolar Ising antiferromagnet at the mean-field level on a geometry that mimics the two-dimensional kagome lattice. Our mean-field treatment is based on the combination of the cluster variational Bethe-Peierls formalism and the cavity method, developed in the context of the glass transition, and is complementary to the Monte Carlo simulations realized in a recent paper [Hamp et al., Phys. Rev. B 98, 144439 (2018)]. Our results confirm the nature of the low-temperature crystalline phase which is reached through a weakly first-order phase transition. Moreover, they allow us to interpret the dynamical slowing down observed in the work of Hamp et al. (referenced above) as a remnant of a spin-glass transition taking place at the mean-field level (and expected to be avoided in two dimensions).

We present an ab initio approach for the calculation of phonon self-energies and their fluctuation diagnostics, which allows us to identify the electronic processes behind phonon anomalies. Application to the transition-metal-dichalcogenide monolayer $1\mathrm{H}-{\mathrm{TaS}}_2$ reveals that coupling between the longitudinal-acoustic phonons and the electrons from an isolated low-energy metallic band is entirely responsible for phonon anomalies such as the mode softening and associated charge-density waves observed in this material. Our analysis allows us to distinguish between different mode-softening mechanisms including matrix-element effects, Fermi-surface nesting, and Van Hove scenarios. We find that matrix-element effects originating from a peculiar type of Dirac pseudospin textures control the charge-density-wave physics in $1\mathrm{H}-{\mathrm{TaS}}_2$ and similar transition metal dichalcogenides.