We study the magnetic and charge dynamical response of a Hubbard model in a two-leg ladder geometry using the density matrix renormalization group (DMRG) method and the random phase approximation within the fluctuation-exchange approximation (FLEX). Our calculations reveal that FLEX can capture the main features of the magnetic response from weak up to intermediate Hubbard repulsion for doped ladders, when compared with the numerically exact DMRG results. However, while at weak Hubbard repulsion both the spin and charge spectra can be understood in terms of weakly interacting electron-hole excitations across the Fermi surface, at intermediate coupling DMRG shows gapped spin excitations at large momentum transfer that remain gapless within the FLEX approximation. For the charge response, FLEX can only reproduce the main features of the DMRG spectra at weak coupling and high doping levels, while it shows an incoherent character away from this limit. Overall, our analysis shows that FLEX works surprisingly well for spin excitations at weak and intermediate Hubbard $U$ values even in the difficult low-dimensional geometry such as a two-leg ladder. Finally, we discuss the implications of our results for neutron scattering and resonant inelastic x-ray scattering experiments on two-leg ladder cuprate compounds.

The interplay between symmetry and magnetization directions in ferromagnetic materials provides an exotic platform to investigate the symmetry-protected topological order. In this work, we propose that unexpected ferromagnetic topological features, such as triple fermions and type-I and type-II Weyl fermions, can emerge in three phases of chromium dioxide (${\mathrm{CrO}}_2$), i.e., rutile ${\mathrm{CrO}}_2$, orthorhombic ${\mathrm{CrO}}_2$ of the ${\mathrm{CaCl}}_2$ type, and pyrite ${\mathrm{CrO}}_2$. Using first-principles calculations and topology analysis, we reveal that the breaking of the spin-rotation symmetry leads to tunable Weyl fermions that are robust against spin-orbital coupling, which can be easily realized by manipulating the magnetization direction. In particular, rutile ${\mathrm{CrO}}_2$, a well-known half-metallic ferromagnet used in magnetic recording tapes, is an ideal candidate to overcome obstacles of studying ferromagnetic topological semimetals in experiments. The topological features in different phases of ${\mathrm{CrO}}_2$ may bridge the gap between the band topology and local chemical bonding. Our findings substantially advance the experimental realization of ferromagnetic Weyl semimetals. More importantly, room-temperature ferromagnetic Weyl fermions in ${\mathrm{CrO}}_2$ may give rise to promising applications related to the topological phenomena in industry.

We study $S=1$ spin liquid states on the kagome lattice constructed by Gutzwiller-projected $p_x+i{p}_y$ superconductors. We show that the obtained spin liquids are either non-Abelian or Abelian topological phases, depending on the topology of the fermionic mean-field state. By calculating the modular matrices $S$ and $T$, we confirm that projected topological superconductors are non-Abelian chiral spin liquid (NACSL). The chiral central charge and the spin Hall conductance we obtained agree very well with the $SO{\left(3\right)}_1$ (or, equivalently, $SU{\left(2\right)}_2$) field-theory predictions. We propose a local Hamiltonian which may stabilize the NACSL. From a variational study, we observe a topological phase transition from the NACSL to the $Z_2$ Abelian spin liquid.

Exciton-polaritons under driven-dissipative conditions exhibit a condensation transition that belongs to a different universality class from that of equilibrium Bose-Einstein condensates. By numerically solving the generalized Gross-Pitaevskii equation with realistic experimental parameters, we show that one-dimensional exciton-polaritons display fine features of Kardar-Parisi-Zhang (KPZ) dynamics. Beyond the scaling exponents, we show that their phase distribution follows the Tracy-Widom form predicted for KPZ growing interfaces. We moreover evidence a crossover to the stationary Baik-Rains statistics. We finally show that these features are unaffected on a certain timescale by the presence of a smooth disorder often present in experimental setups.

Recent experimental studies of out-of-plane straining geometries of transition metal dichalchogenide (TMD) monolayers have demonstrated sufficient band-gap renormalization for device application, such as single-photon emitters. Here, a simple continuum-mechanical plate-theory approach is used to estimate the topography of TMD monolayers layered atop nanopillar arrays. From such geometries, the induced conduction-band potential and band-gap renormalization are given, demonstrating a curvature of the potential that is independent of the height of the deforming nanopillar. Additionally, with a semiclassical WKB approximation, the expected escape rate of electrons in the strain potential may be calculated as a function of the height of the deforming nanopillar. This approach is in accordance with experiment, supporting recent findings suggesting that increasing nanopillar height decreases the linewidth of the single-photon emitters observed at the tip of the pillar and predicting the shift in photon energy with nanopillar height for systems with consistent topography.

A simple scheme for avoiding nonspherical double counting in the combination of density functional theory with dynamical mean-field theory (DFT+DMFT) is developed. It is applied to total-energy calculations and structural optimization of the pnictide superconductor LaFeAsO. The results are compared to a recently proposed “exact” double-counting formulation. Both schemes bring the optimized Fe-As interatomic distance close to the experimental value. This resolves the long-standing controversy between DFT+DMFT and experiment for the structural optimization of LaFeAsO.

We propose a generic protocol to experimentally measure the quantum metric tensor, a fundamental geometric property of quantum states. Our method is based on the observation that the excitation rate of a quantum state directly relates to components of the quantum metric upon applying a proper time-periodic modulation. We discuss the applicability of this scheme to generic two-level systems, where the Hamiltonian's parameters can be externally tuned, and also to the context of Bloch bands associated with lattice systems. As an illustration, we extract the quantum metric of the multiband Hofstadter model. Moreover, we demonstrate how this method can be used to directly probe the spread functional, a quantity which sets the lower bound on the spread of Wannier functions and signals phase transitions. Our proposal offers a universal probe for quantum geometry, which could be readily applied in a wide range of physical settings, ranging from superconducting quantum circuits to ultracold atomic gases.

The electronic-structural modulations of ${\mathrm{Ir}}_{1-x}{\mathrm{Pt}}_x{\mathrm{Te}}_2$ ($0\leqq x\leqq 0.12$) have been examined by resonant elastic x-ray scattering (REXS) and resonant inelastic x-ray scattering (RIXS) techniques at both the Ir and Te edges. Charge-density-wave-like superstructures with wave vectors of $\mathbf{Q}=(1/50-1/5)$, $(1/80-1/8)$, and $(1/60-1/6)$ are observed on the same sample of ${\mathrm{IrTe}}_2$ at the lowest temperature, the patterns of which are controlled by the cooling speeds. In contrast, superstructures around $\mathbf{Q}=(1/50-1/5)$ are observed for doped samples ($0.02\leqq x\leqq 0.05$). The superstructure reflections persist to higher Pt substitution than previously assumed, demonstrating that a charge-density wave (CDW) can coexist with superconductivity. The analysis of the energy-dependent REXS and RIXS line shape reveals the importance of the Te $5p$ state rather than the Ir $5d$ states in the formation of the spatial modulation of these systems. The phase diagram reexamined in this work suggests that the CDW incommensurability may correlate with the emergence of superconducting states such as ${\mathrm{Cu}}_x{\mathrm{TiSe}}_2$ and ${\mathrm{Li}}_x{\mathrm{TaS}}_2$.

Out of equilibrium phases of matter exhibiting order in individual eigenstates, such as many-body localized spin glasses and discrete time crystals, can be characterized by inherently dynamical quantities such as spatiotemporal correlation functions. In this paper, we introduce dynamical potentials which act as generating functions for such correlations and capture eigenstate phases and order. These potentials show formal similarities to their equilibrium counterparts, namely thermodynamic potentials. We provide three representative examples: a disordered XXZ chain showing many-body localization, a disordered Ising chain exhibiting spin-glass order, and its periodically-driven cousin exhibiting time-crystalline order.

We investigate the formation of waveguides for Rydberg excitons in ${\mathrm{Cu}}_{\text{2}}\mathrm{O}$ from cylindrical stressors as alternatives to optical traps. We show that the achievable potential depths can easily reach the meV and the trap frequencies the $\mathrm{GHz}$ regimes. For Rydberg excitons, we find that it is sufficient to consider only the shift of the band gap, whereas the excitonic binding energies remain almost unchanged.

The transmission and reflection processes of THz phonons at solid interfaces are of fundamental interest and of importance to thermal conduction in nanocrystalline solids. The processes are challenging to investigate, however, because typical experiments and many computational approaches do not provide transmission coefficients resolved by phonon mode. Here, we examine the modal transmission and reflection processes of THz phonons across an amorphous Si region connected to two crystalline Si leads, a model interface for those that occur in nanocrystalline solids, using mode-resolved atomistic Green's functions. We find that the interface acts as a low-pass filter, reflecting modes of frequency greater than around 3 THz while transmitting those below this frequency, in agreement with a recent experimental report [C. Hua et al., Phys. Rev. B 95, 205423 (2017)]. Further, we find that these low frequency modes travel nearly unimpeded through the interface, maintaining their wave vectors on each side of the interface. Our work shows that even completely disordered regions may not be effective at reflecting THz phonons, with implications for efforts to alter thermal conductivity in nanocrystalline solids.

We theoretically investigate the transport properties of a quasi-one-dimensional ferromagnet-superconductor junction where the superconductor consists of mixed singlet and triplet pairings. We show that the relative orientation of the Stoner field ($\tilde{\mathbf{h}}$) in the ferromagnetic lead and the $\mathbf{d}$ vector of the superconductor acts like a on-off switch for the zero bias conductance of the device. In the regime, where triplet pairing amplitude dominates over the singlet counterpart (topological phase), a pair of Majorana zero modes appear at each end of the superconducting part of the nanowire. When $\tilde{\mathbf{h}}$ is parallel or antiparallel to the $\mathbf{d}$ vector, transport gets completely blocked due to blockage in pairing while, when $\tilde{\mathbf{h}}$ and $\mathbf{d}$ are perpendicular to each other, the zero energy two terminal differential conductance spectra exhibits sharp transition from $4e^2/h$ to $2e^2/h$ as the magnetization strength in the lead becomes larger than the chemical potential indicating the spin-selective coupling of a pair of Majorana zero modes to the lead.

We propose an efficient method to quantum mechanically treat anharmonic interactions in the atomistic nonequilibrium Green's function simulation of phonon transport. We demonstrate that the so-called lowest-order approximation, implemented through a rescaling technique and analytically continued by means of the Padé approximants, can be used to accurately model third-order anharmonic effects. Although the paper focuses on a specific self-energy, the method is applicable to a very wide class of physical interactions. We apply this approach to the simulation of anharmonic phonon transport in realistic Si and Ge nanowires with uniform or discontinuous cross sections. The effect of increasing the temperature above 300 K is also investigated. In all the considered cases, we are able to obtain a good agreement with the routinely adopted self-consistent Born approximation, at a remarkably lower computational cost. In the more complicated case of high temperatures ($\gg 300$ K), we find that the first-order Richardson extrapolation applied to the sequence of the Padé approximants $N-1/N$ results in a significant acceleration of the convergence.

The structural and magnetic properties of the square-cupola antiferromagnet $\mathrm{Sr}\left(\mathrm{TiO}\right){\mathrm{Cu}}_4{\left({\mathrm{PO}}_4\right)}_4$ are investigated via x-ray diffraction, magnetization, heat capacity, and ${}^{31}\mathrm{P}$ nuclear magnetic resonance experiments on polycrystalline samples, as well as density-functional band-structure calculations. The temperature-dependent unit-cell volume could be described well using the Debye approximation with a Debye temperature of ${\theta }_{\mathrm{D}}\simeq 550$ K. Magnetic response reveals a pronounced two-dimensionality with a magnetic long-range order below $T_{\mathrm{N}}\simeq 6.2$ K. High-field magnetization exhibits a kink at about $1/3$ of the saturation magnetization. Asymmetric ${}^{31}\mathrm{P}$ NMR spectra clearly suggest strong in-plane anisotropy in the magnetic susceptibility, as anticipated from the crystal structure. From the ${}^{31}\mathrm{P}$ NMR shift versus bulk susceptibility plot, the isotropic and axial parts of the hyperfine coupling between the ${}^{31}\mathrm{P}$ nuclei and the ${\mathrm{Cu}}^{2+}$ spins are calculated to be $A_{\mathrm{hf}}^{\mathrm{iso}}\simeq 6539$ and $A_{\mathrm{hf}}^{\mathrm{ax}}\simeq 952$ Oe/${\mu }_{\mathrm{B}}$, respectively. The low-temperature and low-field ${}^{31}\mathrm{P}$ NMR spectra indicate a commensurate antiferromagnetic ordering. The frustrated nature of the compound is inferred from the temperature-dependent ${}^{31}\mathrm{P}$ NMR spin-lattice relaxation rate and confirmed by our microscopic analysis, which reveals strong frustration of the square cupola by next-nearest-neighbor exchange couplings.

Motivated by the recent study on the quasiparticle-induced friction of solitons in superfluids, we theoretically study magnon-induced intrinsic friction of a domain wall in a one-dimensional ferromagnet. To this end, we start by obtaining the hitherto overlooked dissipative interaction of a domain wall and its quantum magnon bath to linear order in the domain-wall velocity and to quadratic order in magnon fields. An exact expression for the pertinent scattering matrix is obtained with the aid of supersymmetric quantum mechanics. We then derive the magnon-induced frictional force on a domain wall in two different frameworks: time-dependent perturbation theory in quantum mechanics and the Keldysh formalism, which yield identical results. The latter, in particular, allows us to verify the fluctuation-dissipation theorem explicitly by providing both the frictional force and the correlator of the associated stochastic Langevin force. The potential for magnons induced by a domain wall is reflectionless, and thus the resultant frictional force is non-Markovian similar to the case of solitons in superfluids. They share an intriguing connection to the Abraham-Lorentz force that is well known for its causality paradox. The dynamical responses of a domain wall are studied under a few simple circumstances, where the non-Markovian nature of the frictional force can be probed experimentally. Our work, in conjunction with the previous study on solitons in superfluids, shows that the macroscopic frictional force on solitons can serve as an effective probe of the microscopic degrees of freedom of the system.

We study the behavior of the pseudogap in overdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ by electronic Raman scattering (ERS) and angle-resolved photoemission spectroscopy (ARPES) on the same single crystals. Using both techniques we find that, unlike the superconducting gap, the pseudogap related to the antibonding band vanishes above the critical doping $p_c=0.22$. Concomitantly, we show from ARPES measurements that the Fermi surface of the antibonding band is holelike below $p_c$ and becomes electronlike above $p_c$. This reveals that the existence of the pseudogap depends on the Fermi surface topology in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$, and more generally, puts strong constraint on theories of the pseudogap phase.

We study the superconducting state of multiorbital spin-orbit coupled systems in the presence of an orbitally driven inversion asymmetry assuming that the interorbital attraction is the dominant pairing channel. Although the inversion symmetry is absent, we show that superconducting states that avoid mixing of spin-triplet and spin-singlet configurations are allowed, and remarkably, spin-triplet states that are topologically nontrivial can be stabilized in a large portion of the phase diagram. The orbital-dependent spin-triplet pairing generally leads to topological superconductivity with point nodes that are protected by a nonvanishing winding number. We demonstrate that the disclosed topological phase can exhibit Lifshitz-type transitions upon different driving mechanisms and interactions, e.g., by tuning the strength of the atomic spin-orbit and inversion asymmetry couplings or by varying the doping and the amplitude of order parameter. Such distinctive signatures of the nodal phase manifest through an extraordinary reconstruction of the low-energy excitation spectra both in the bulk and at the edge of the superconductor.

The effect of pressure on the zone-center optical phonon modes and the elastic properties of hafnium have been studied employing both experimental (Raman scattering) and theoretical (density-functional theory) approaches. An anomaly dependence of the phonon frequency of the $E_{2g}$ mode was found in the pressure range from 0 to 67 GPa. The calculated electronic structure of hafnium shows significant changes under pressure, which have a pronounced spatial anisotropy similar to the anisotropy of the observed phonon properties. The dependencies of the elastic properties on pressure, including the components of the elasticity tensor, bulk and shear modulus, Poisson ratio, and universal anisotropy index, obtained from the calculations have characteristic features indicating the occurrence of a structural phase transition.

${\mathrm{NaOsO}}_{\text{3}}$ undergoes a metal-insulator transition (MIT) at 410 K, concomitant with the onset of antiferromagnetic order. The excitation spectra have been investigated through the MIT by resonant inelastic x-ray scattering (RIXS) at the Os ${\mathrm{L}}_{\text{3}}$ edge. Low resolution ($\mathrm{\Delta }E\sim \text{300}\text{meV}$) measurements over a wide range of energies reveal that local electronic excitations do not change appreciably through the MIT. This is consistent with a picture in which structural distortions do not drive the MIT. In contrast, high resolution ($\mathrm{\Delta }E\sim \text{56}\text{meV}$) measurements show that the well-defined, low-energy magnons in the insulating state weaken and dampen upon approaching the metallic state. Concomitantly, a broad continuum of excitations develops which is well described by the magnetic fluctuations of a nearly antiferromagnetic Fermi liquid. By revealing the continuous evolution of the magnetic quasiparticle spectrum as it changes its character from itinerant to localized, our results provide unprecedented insight into the nature of the MIT in ${\mathrm{NaOsO}}_{\text{3}}$. In particular, the presence of weak correlations in the paramagnetic phase implies a degree of departure from the ideal Slater limit.