We present ${}^{77}\mathrm{Se}$-NMR measurements on single-crystalline FeSe under pressures up to 2 GPa. Based on the observation of the splitting and broadening of the NMR spectrum due to structural twin domains, we discovered that static, local nematic ordering exists well above the bulk nematic ordering temperature, $T_{\mathrm{s}}$. The static, local nematic order and the low-energy stripe-type antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation rate measurements, are both insensitive to pressure application. These NMR results provide clear evidence for the microscopic cooperation between magnetism and local nematicity in FeSe.

The mechanical force from light—radiation pressure—provides an intrinsically nonlinear interaction. Consequently, optomechanical systems near their steady state, such as the canonical optical spring, can display nonanalytic behavior as a function of external parameters. This nonanalyticity, a key feature of thermodynamic phase transitions, suggests that there could be an effective thermodynamic description of optomechanical systems. Here we explicitly define the thermodynamic limit for optomechanical systems and derive a set of sufficient constraints on the system parameters as the mechanical system grows large. As an example, we show how these constraints can be satisfied in a system with ${\mathbb{Z}}_2$ symmetry and derive a free energy, allowing us to characterize this as an equilibrium phase transition.

We investigate the demagnetization dynamics of the cycloidal and sinusoidal phases of multiferroic ${\mathrm{TbMnO}}_3$ by means of time-resolved resonant soft x-ray diffraction following excitation by an optical pump. The use of orthogonal linear x-ray polarizations provides information on the contribution from the different magnetic moment directions, which can be interpreted as signatures from multiferroic cycloidal spin order and sinusoidal spin order. Tracking these signatures in the time domain enables us to identify the transient magnetic phase created by intense photoexcitation of the electrons and subsequent heating of the spin system on a picosecond time scale. The transient phase is shown to exhibit mostly spin density wave character, as in the adiabatic case, while nevertheless retaining the wave vector of the cycloidal long-range order. Two different pump photon energies, 1.55 and 3.1 eV, lead to population of the conduction band predominantly via intersite $d-d$ or intrasite $p-d$ transitions, respectively. We find that the nature of the optical excitation does not play an important role in determining the dynamics of magnetic order melting. Further, we observe that the orbital reconstruction, which is induced by the spin ordering, disappears on a time scale comparable to that of the cycloidal order, attesting to a direct coupling between magnetic order and orbital reconstruction. Our observations are discussed in the context of recent theoretical models of demagnetization dynamics in strongly correlated systems, revealing the potential of this type of measurement as a benchmark for such theoretical studies.

We electrically probed the transient response of nuclear spins in an $n$-GaAs channel by performing Hanle signal and spin-valve signal measurements on an all-electrical spin-injection device having a half-metallic spin source of $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$. Furthermore, we simulated the Hanle and spin-valve signals by using the time evolution of nuclear-spin polarization under the presence of polarized electron spins by taking both $T_{1\mathrm{e}}$ and $T_1$ into consideration, where $T_{1\mathrm{e}}^{-1}$ is the polarization rate of nuclear spins through the transfer of angular momentum from polarized electron spins and $T_1^{-1}$ is the depolarization rate of nuclear spins through the interaction with the lattice. The simulation results reproduced our experimental results on all the nuclear-spin-related phenomena appearing in the Hanle and spin-valve signals at different measurement conditions, providing quantitative explanation for the transient response of nuclear spins in GaAs to a change in magnetic fields and an estimate of the time scales of $T_{1\mathrm{e}}$ and $T_1$. These experimental and simulated results will deepen the understanding of nuclear-spin dynamics in semiconductors.

Magnetic skyrmions are nanosized topologically protected spin textures with particlelike properties. They can form lattices perpendicular to the magnetic field, and the orientation of these skyrmion lattices with respect to the crystallographic lattice is governed by spin-orbit coupling. By performing small-angle neutron scattering measurements, we investigate the coupling between the crystallographic and skyrmion lattices in both ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ and the archetype chiral magnet MnSi. The results reveal that the orientation of the skyrmion lattice is primarily determined by the magnetic field direction with respect to the crystallographic lattice. In addition, it is also influenced by the magnetic history of the sample, which can induce metastable lattices. Kinetic measurements show that these metastable skyrmion lattices may or may not relax to their equilibrium positions under macroscopic relaxation times. Furthermore, multidomain lattices may form when two or more equivalent crystallographic directions are favored by spin-orbit coupling and oriented perpendicular to the magnetic field.

We study numerically the superconductor-insulator transition in two-dimensional inhomogeneous superconductors with gauge disorder, described by four different quantum rotor models: a gauge glass, a flux glass, a binary phase glass, and a Gaussian phase glass. The first two models describe the combined effect of geometrical disorder in the array of local superconducting islands and a uniform external magnetic field, while the last two describe the effects of random negative Josephson-junction couplings or $\pi$ junctions. Monte Carlo simulations in the path-integral representation of the models are used to determine the critical exponents and the universal conductivity at the quantum phase transition. The gauge- and flux-glass models display the same critical behavior, within the estimated numerical uncertainties. Similar agreement is found for the binary and Gaussian phase-glass models. Despite the different symmetries and disorder correlations, we find that the universal conductivity of these models is approximately the same. In particular, the ratio of this value to that of the pure model agrees with recent experiments on nanohole thin-film superconductors in a magnetic field, in the large disorder limit.

We show via tensor network methods that the Harper-Hofstadter Hamiltonian for hard-core bosons on a square geometry supports a topological phase realizing the $\nu =\frac{1}{2}$ fractional quantum Hall (FQH) effect on the lattice. We address the robustness of the ground-state degeneracy and of the energy gap, measure the many-body Chern number, and characterize the system using Green functions, showing that they decay algebraically at the edges of open geometries, indicating the presence of gapless edge modes. Moreover, we estimate the topological entanglement entropy by taking a combination of lattice bipartitions that reproduces the topological structure of the original proposals by Kitaev and Preskill [Phys. Rev. Lett. 96, 110404 (2006)] and Levin and Wen [Phys. Rev. Lett. 96, 110405 (2006)]. The numerical results show that the topological contribution is compatible with the expected value $\gamma =\frac{1}{2}$. Our results provide extensive evidence that FQH states are within reach of state-of-the-art cold-atom experiments.

Magnetite is often the subject of Mössbauer spectroscopy experiments either as a part of fundamental research of this compound or during various geological studies. However, the complicated structure of the low-temperature phase of magnetite exhibits 24 crystallographic iron sites, which presents a considerable obstruction for spectrum interpretation. In this work, we carried out ab initio calculations to obtain a complete set of hyperfine parameters of all the sites, and we used these parameters to simulate the corresponding Mössbauer spectrum. Simulation analysis suggested an approximation of the spectrum by four sextets. Parameters of these four sextets were calculated, and the approximation was shown to be appropriate. Further, the Mössbauer spectrum of a high-quality synthetic single crystal of magnetite was measured at 4 K, allowing for a comparison of the theoretical results with the experimental data. Finally, the four-sextet approximation was successfully applied to fit the measured spectrum.

Scanning tunneling microscopy visualizations of quasiparticle interference (QPI) enable powerful insights into the $k$-space properties of superconducting, topological, Rashba, and other exotic electronic phases, but their reliance on impurities acting as scattering centers is rarely scrutinized. Here, we investigate QPI at the vacuum-cleaved (001) surface of the Dirac semimetal ZrSiS. We find that interference patterns around impurities located on the Zr and S lattice sites appear very different, and can be ascribed to selective scattering of different subsets of the predominantly Zr $4d$-derived band structure, namely, the $m=0$ and $\pm 1$ components. We show that the selectivity of scattering channels requires an explanation beyond the different bands' orbital characteristics and their respective charge density distributions over Zr and S lattice sites. Importantly, this result shows that the usual assumption of generic scattering centers allowing observations of quasiparticle interference to shed light indiscriminately and isotropically upon the $q$ space of scattering events does not hold, and that the scope and interpretation of QPI observations can therefore be be strongly contingent on the material defect chemistry. This finding promises to spur new investigations into the quasiparticle scattering process itself, to inform future interpretations of quasiparticle interference observations, and ultimately to aid the understanding and engineering of quantum electronic transport properties.

We study the $U\left(1\right)$ quantum spin liquid on the pyrochlore spin ice systems. For the non-Kramers doublets such as ${\mathrm{Pr}}^{3+}$ and ${\mathrm{Tb}}^{3+}$, we point out that the inelastic neutron scattering result not only detects the low-energy gauge photon, but also contains the continuum of the “magnetic monopole” excitations. Unlike spinons, these “magnetic monopoles” are purely of quantum origin and have no classical analog. We further point out that the “magnetic monopole” experiences a background dual “$\pi$” flux due to the spin-1/2 nature of the local moment when the “monopole” hops on the dual diamond lattice. We then predict that the “monopole” continuum has an enhanced spectral periodicity with a folded Brillouin zone. This prediction can be examined among the existing data on non-Kramers doublet spin liquid candidate materials like ${\mathrm{Pr}}_2{\mathrm{TM}}_2{\mathrm{O}}_7$ and ${\mathrm{Tb}}_2{\mathrm{TM}}_2{\mathrm{O}}_7$ (with TM = transition metal). The application to the Kramers doublet systems and numerical simulation is further discussed. Finally, we present a general classification of distinct symmetry enriched $U\left(1\right)$ quantum spin liquids based on the translation symmetry fractionalization patterns of “monopoles” and “spinons.”

We describe the collective hydrodynamic motion of an incommensurate charge density wave state in a clean electronic system. Our description simultaneously incorporates the effects of both pinning due to weak disorder and also phase relaxation due to proliferating dislocations. We show that the interplay between these two phenomena has important consequences for charge and momentum transport. For instance, it can lead to metal-insulator transitions. We furthermore identify signatures of fluctuating density waves in frequency and spatially resolved conductivities. Phase disordering is well known to lead to a large viscosity. We derive a precise formula for the phase relaxation rate in terms of the viscosity in the dislocation cores. We thereby determine the viscosity of the superconducting state of BSCCO from the observed melting dynamics of Abrikosov lattices and show that the result is consistent with dissipation into Bogoliubov quasiparticles.

Using highly controlled coverages of graphene on SiC(0001), we have studied the structure of the first graphene layer that grows on the SiC interface. This layer, known as the buffer layer, is semiconducting. Using x-ray reflectivity and x-ray standing wave analysis, we have performed a comparative study of the buffer layer structure with and without an additional monolayer graphene layer above it. We show that no more than 26% of the buffer carbon is covalently bonded to Si in the SiC interface. We also show that the top SiC bilayer is Si depleted and is likely the cause of the incommensuration previously observed in this system. When a monolayer graphene layer forms above the buffer, the buffer layer becomes less corrugated with signs of a change in the bonding geometry with the SiC interface. At the same time, the entire SiC interface becomes more disordered, presumably due to entropy associated with the higher growth temperature.

Using an optical nanoresonator, such as a nanoparticle, to concentrate and absorb light at deep subwavelength scale is of both fundamental and technological significance. To quantify absorption, one typically introduces the absorption cross section and the absorption efficiency, i.e., the ratio of the absorption cross section of a local resonator to its geometric cross section. Exploring a general way to enlarge the absorption cross section of a deep subwavelength nanoparticle and increase the absorption efficiency certainly is important and highly desired. In this work, we show that by introducing a gain medium in the structure, the absorption cross section of any deep subwavelength particle could be boosted and even approach a universal maximum value, of the order ${\lambda }^2$. Our approach for controlling the absorption using gain is general and may be useful both for the design of light harvesting materials and high-speed active nano-optics devices.

In solids, high harmonic radiation arises from the subcycle dynamics of electrons and holes under the action of an intense laser field. The strong-field regime opens new opportunities to understand and control carrier dynamics on ultrafast time scales, including the coherent dynamics of quasiparticles such as massless Dirac fermions. Here, we irradiate monolayer and few-layer graphene with intense infrared light to produce nonperturbative harmonics of the fundamental up to the seventh order. We find that the polarization dependence shows surprising agreement with gas-phase harmonics. Using a two-band model, we explore the nonlinear current due to electrons near the Dirac points, and we discuss the interplay between intraband and interband contributions to the harmonic spectrum. This interplay opens new opportunities to access ultrafast and strong-field physics of graphene.

We show that a generic single-orbital Anderson impurity model, lacking, for instance, any kind of particle-hole symmetry, can be exactly mapped without any constraint onto a resonant level model coupled to two Ising variables, which reduce to one if the hybridization is particle-hole symmetric. The mean-field solution of this model is found to be stable to unphysical spontaneous magnetization of the impurity, unlike the saddle-point solution in the standard slave-boson representation. Remarkably, the mean-field estimate of the Wilson ratio approaches the exact value $R_{\text{W}}=2$ in the Kondo regime.

The recently synthesized ${\mathrm{FeH}}_5$ at high pressure is calculated to be a superconductor with a critical temperature ($T_c$) of 51 K at 130 GPa. The crystal structure of this compound can be described as stacking bilayers of a puckered hexagonal honeycomb of H atoms sandwiched between slabs of atomic Fe and H atoms. The honeycomb layers resemble the predicted structure of a metallic and high $T_c$ Cmca phase of solid hydrogen. Electron-phonon interactions were mainly derived from coupling of the electronic bands with bending vibrations of the hydrogens in the puckered layers.

Perovskite oxide interfaces have attracted tremendous research interest for their fundamental physics and promising all-oxide electronic applications. Here, based on first-principles calculations, we propose a surface La interstitial promoted interface insulator-metal transition in $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (110). Compared with surface oxygen vacancies, which play a determining role on the insulator-metal transition of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (001) interfaces, we find that surface La interstitials can be more experimentally realistic and accessible for manipulation and more stable in an ambient atmospheric environment. Interestingly, these surface La interstitials also induce significant spin-splitting states with a Ti $d_{yz}/d_{xz}$ character at a conducting $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (110) interface. On the other hand, for insulating $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (110) (<4 unit cells $\mathrm{LaAl}{\mathrm{O}}_3$ thickness), a distortion between La (Al) and O atoms is found at the $\mathrm{LaAl}{\mathrm{O}}_3$ side, partially compensating the polarization divergence. Our results reveal the origin of the metal-insulator transition in $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (110) heterostructures, and also shed light on the manipulation of the superior properties of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (110) for different possibilities in electronic and magnetic applications.

The possibility to effect valley splitting of an electronic current in graphene represents the essential component in the new field of valleytronics in such two-dimensional materials. Based on a symmetry analysis of the scattering matrix, we show that if the spatial distribution of multiple potential scatterers breaks mirror symmetry about the axis of incoming electrons, then a splitting of the current between two valleys is observed. This leads to the appearance of the valley Hall effect. We illustrate the effect of mirror-symmetry breaking in a minimal system of two symmetric impurities, demonstrating the splitting between valleys via the differential cross sections and nonvanishing skew parameter. We further discuss the role that these effects may play in transport experiments.

We bring together optical spectroscopy and first-principles calculations to reveal the electronic properties of the chiral ferromagnet ${\mathrm{Fe}}_{1/3}{\mathrm{TaS}}_2$. Signatures of chirality are superimposed upon a complex free-carrier response that emanates from both Ta and Fe bands. These include a honeycomb charge density pattern in the Fe layer and a hole $\to$ electron pocket crossover at the K point, low-energy excitations between spin split bands that cross the Fermi surface, and clustered rather than well-separated on-site and charge-transfer excitations. These findings advance the understanding of intercalation and symmetry breaking on the fundamental excitations in metallic chalcogenides.

Magnetic interactions are widely believed to play a crucial role in the microscopic mechanism leading to high critical temperature superconductivity. It is therefore important to study the signatures of pairing in the magnetic excitation spectrum of simple models known to show unconventional superconducting tendencies. Using the density matrix renormalization group technique, we calculate the dynamical spin structure factor $S(\mathbf{k},\omega )$ of a generalized $t-U-J$ Hubbard model away from half filling in a two-leg ladder geometry. The addition of $J$ enhances pairing tendencies. We analyze quantitatively the signatures of pairing in the magnetic excitation spectra. We found that the superconducting pair-correlation strength, that can be estimated independently from ground state properties, is closely correlated with the integrated low-energy magnetic spectral weight in the vicinity of $(\pi ,\pi )$. In this wave-vector region, robust spin incommensurate features develop with increasing doping. The branch of the spectrum with rung direction wave vector $k_{\mathrm{rung}}=0$ does not change substantially with doping where pairing dominates and thus plays a minor role. We discuss the implications of our results for neutron scattering experiments, where the spin excitation dynamics of hole-doped quasi-one-dimensional magnetic materials can be measured and also address implications for recent resonant inelastic x-ray scattering experiments.