Interacting fermions on the half-filled honeycomb lattice with short-range repulsions have been suggested to host a variety of interesting many-body ground states, e.g., a topological Mott insulator. A number of recent studies of the spinless case in terms of exact diagonalization, the infinite density matrix renormalization group, and the functional renormalization group, however, indicate a suppression of the topological Mott insulating phase in the whole range of interaction parameters. Here, we complement the previous studies by investigating the quantum many-body instabilities of the physically relevant case of spin-1/2 fermions with onsite, nearest-neighbor, and second-nearest-neighbor repulsion. To this end, we employ the multipatch functional renormalization group for correlated fermions with refined momentum resolution observing the emergence of an antiferromagnetic spin-density wave and a charge-density wave for dominating onsite and nearest-neighbor repulsions, respectively. For dominating second-nearest neighbor interaction our results favor an ordering tendency towards a charge-modulated ground state over the topological Mott insulating state. The latter evades a stabilization as the leading instability by the additional onsite interaction.

The ground-state subspace of a topological phase of matter forms a representation of the mapping class group of the space on which the state is defined. We show that elements of the mapping class group of a surface of genus $g$ can be obtained through a sequence of topological charge projections along at least three mutually intersecting noncontractible cycles. We demonstrate this both through the algebraic theory of anyons and also through an analysis of the topology of the space-time manifold. We combine this result with two observations: (i) that surfaces of genus $g$ can be effectively simulated in planar geometries by using bilayer, or doubled, versions of the topological phase of interest, and inducing the appropriate types of gapped boundaries; and (ii) that the required topological charge projections can be implemented as adiabatic unitary transformations by locally tuning microscopic parameters of the system, such as the energy gap. These observations suggest a possible path towards effectively implementing modular transformations in physical systems. In particular, they also show how the Ising $\otimes \overline{\text{Ising}}$ state, in the presence of disconnected gapped boundaries, can support universal topological quantum computation.

We present a detailed investigation of the exciton and trion dynamics in naturally doped ${\mathrm{MoSe}}_2$ and ${\mathrm{WSe}}_2$ single atomic layers as a function of temperature in the range 10–300 K under above band-gap laser excitation. By combining time-integrated and time-resolved photoluminescence (PL) spectroscopy, we show the importance of exciton and trion localization in both materials at low temperatures. We also reveal the transition to delocalized exciton complexes at higher temperatures where the exciton and trion thermal energy exceeds the typical localization energy. This is accompanied by strong changes in PL including suppression of the trion PL and decrease of the trion PL lifetime, as well as significant changes for neutral excitons in the temperature dependence of the PL intensity and the appearance of a pronounced slow PL decay component. In ${\mathrm{MoSe}}_2$ and ${\mathrm{WSe}}_2$ studied here, the temperatures where such strong changes occur are observed at around 100 and 200 K, respectively, in agreement with their inhomogeneous PL linewidth of 8 and 20 meV at $T\approx 10\text{K}$. The observed behavior is a result of a complex interplay between influences of the specific energy ordering of bright and dark excitons in ${\mathrm{MoSe}}_2$ and ${\mathrm{WSe}}_2$, sample doping, trion, and exciton localization and various temperature-dependent nonradiative processes.

Strongly scattering elastic media, such as consolidated granular materials, respond to ultrasonic pulse excitations with a long response signal with peculiar properties. The portion of the signal at late times, termed coda, is due to multiple scattering. It contains information about the elastic properties of the material, and it has been proven to be very sensitive to small variations in the modulus. Here we propose a technique based on a nonlinear analysis of the coda of a signal, which might be applied to quantify the nonlinear elastic response in consolidated granular media exhibiting a hysteretic elastic behavior. The method proposed allows for an intrinsic definition of the reference signal which is normally needed for applying coda-based methods.

The magnetic ground state of ${\mathrm{Sr}}_{3}A{\mathrm{RuO}}_6$, with $A=(\mathrm{Li},\mathrm{Na})$, is studied using neutron diffraction, resonant x-ray scattering, and laboratory characterization measurements of high-quality crystals. Combining these results allows us to observe the onset of long-range magnetic order and distinguish the symmetrically allowed magnetic models, identifying in-plane antiferromagnetic moments and a small ferromagnetic component along the $c$ axis. While the existence of magnetic domains masks the particular in-plane direction of the moments, it has been possible to elucidate the ground state using symmetry considerations. We find that due to the lack of local anisotropy, antisymmetric exchange interactions control the magnetic order, first through structural distortions that couple to in-plane antiferromagnetic moments and second through a high-order magnetoelastic coupling that lifts the degeneracy of the in-plane moments. The symmetry considerations used to rationalize the magnetic ground state are very general and will apply to many systems in this family, such as ${\mathrm{Ca}}_{3}A{\mathrm{RuO}}_6$, with $A=(\mathrm{Li},\mathrm{Na})$, and ${\mathrm{Ca}}_3{\mathrm{LiOsO}}_6$ whose magnetic ground states are still not completely understood.

We study the effect of a pure spin current on the Kondo singlet in a diluted magnetic alloy using nonlocal lateral spin valve structures with highly spin polarized ${\mathrm{Co}}_2\mathrm{FeSi}$ electrodes. Temperature dependence of the nonlocal spin signals shows a sharp reduction with decreasing temperature, followed by a plateau corresponding to the low temperature Fermi liquid regime below the Kondo temperature ($T_{\mathrm{K}}$). The spin diffusion length of the Kondo alloy is found to increase with increasing spin accumulation. The results are in agreement with the intuitive description that the Kondo singlet cannot survive any more in sufficiently large spin accumulation even below $T_{\mathrm{K}}$.

We study a one-dimensional XXZ spin chain in a random field on the metallic side of the many-body localization transition by level statistics. For a fixed interaction, and intermediate disorder below the many-body localization transition, we find that, asymptotically, the number variance grows faster than linear with a disorder-dependent exponent. This is consistent with the existence of an anomalous Thouless energy in the spectrum. In noninteracting disordered metals, this is an energy scale related to the typical time for a particle to diffuse across the sample. In the interacting case, it seems related to a more intricate anomalous diffusion process. This interpretation is not fully consistent with recent claims that for intermediate disorder, level statistics are described by a plasma model with power-law decaying interactions whose number variance grows slower than linear. As disorder is further increased, still on the metallic side, the Thouless energy is gradually washed out. In the range of sizes we can explore, level statistics are scale invariant and approach Poisson statistics at the many-body localization transition. Slightly below the many-body localization transition, spectral correlations, well described by critical statistics, are quantitatively similar to those of a high-dimensional, noninteracting, disordered conductor at the Anderson transition.

We investigate the transport properties of Chern insulators following a quantum quench between topological and nontopological phases. Recent works have shown that this yields an excited state for which the Chern number is preserved under unitary evolution. However, this does not imply the preservation of other physical observables, as we stressed in our previous work. Here we provide an analysis of the Hall response following a quantum quench in an isolated system, with explicit results for the Haldane model. We show that the Hall conductance is no longer related to the Chern number in the postquench state, in agreement with previous work. We also examine the dynamics of the edge currents in finite-size systems with open boundary conditions along one direction. We show that the late-time behavior is captured by a Generalized Gibbs Ensemble, after multiple traversals of the sample. We discuss the effects of generic open boundary conditions and confinement potentials.

Lattice modes are intrinsic to periodic structures and they can be easily tuned and controlled by changing the lattice constant of the structural array. Previous studies have revealed the excitation of sharp absorption resonances due to lattice mode coupling with the plasmonic resonances. Here, we report an experimental observation of a lattice-induced transparency (LIT) by coupling the first-order lattice mode (FOLM) to the structural resonance of a terahertz asymmetric split ring resonator. The observed sharp transparency is a result of the destructive interference between the bright mode and the FOLM assisted dark mode. As the FOLM is swept across the metamaterial resonance, the transparency band undergoes a large change in its bandwidth and resonance position. We propose a three-oscillator model to explain the underlying coupling mechanism in LIT system that shows good agreement with the observed results. Besides controlling the transparency behavior, LIT also shows a huge enhancement in its $Q$ factor and exhibits a high group delay of 28 ps with an enhanced group index of $4.5\times {10}^4$, which could be pivotal in ultrasensitive sensing and slow-light device applications.

The electron-doped high-transition-temperature $\left(T_c\right)$ iron-based pnictide superconductor ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{H}}_x$ has a unique phase diagram: Superconducting double domes are sandwiched by antiferromagnetic phases at ambient pressure and they turn into a single dome with a maximum $T_c$ that exceeds 45 K at a pressure of 3.0 GPa. We studied whether spin fluctuations are involved in increasing $T_c$ under a pressure of 3.0 GPa by using the ${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) technique. The ${}^{75}\mathrm{As}$-NMR results for the powder samples show that $T_c$ increases up to 48 K without the influence of spin fluctuations. This fact indicates that spin fluctuations are not involved in raising $T_c$, which implies that other factors, such as orbital degrees of freedom, may be important for achieving a high $T_c$ of almost 50 K.

The Mott insulator $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{Ag}}_2{\left(\mathrm{CN}\right)}_3$ forms a highly-frustrated triangular lattice of $S=1/2$ dimers with a possible quantum-spin-liquid state. Our experimental and numerical studies reveal the emergence of a slight charge imbalance between crystallographically inequivalent sites, relaxor dielectric response, and hopping dc transport. In a broader perspective we conclude that the universal properties of strongly-correlated charge-transfer salts with spin liquid state are an anion-supported valence band and cyanide-induced quasidegenerate electronic configurations in the relaxed state. The generic low-energy excitations are caused by charged domain walls rather than by fluctuating electric dipoles. They give rise to glassy dynamics characteristic of dimerized Mott insulators, including the sibling compound $\kappa -(\text{BEDT}-\text{TTF}){}_2{\mathrm{Cu}}_2\left(\mathrm{CN}\right){}_3$.

A hexagonal structure of solid molecular hydrogen with $P{6}_{1}22$ symmetry is calculated to be more stable below about 200 GPa than the monoclinic $C2/c$ structure identified previously as the best candidate for phase III. We find that the effects of nuclear quantum and thermal vibrations play a central role in the stabilization of $P{6}_{1}22$. The $P{6}_{1}22$ and $C2/c$ structures are very similar and their Raman and infrared data are in good agreement with experiment. However, our calculations show that the hexagonal $P{6}_{1}22$ structure provides better agreement with the available x-ray diffraction data than the $C2/c$ structure at pressures below about 200 GPa. We suggest that two phase-III-like structures may be formed at high pressures: hexagonal $P{6}_{1}22$ below about 200 GPa and monoclinic $C2/c$ at higher pressures.

The pressure effect on the tunneling mode and vibrational spectra of hydrogen in $\alpha -{\text{MnH}}_{0.07}$ has been studied by inelastic neutron scattering. Applying hydrostatic pressure of up to 30 kbar is shown to shift both the hydrogen optical modes and the tunneling peak to higher energies. First-principles calculations show that the potential for hydrogen in $\alpha$-Mn becomes overall steeper with increasing pressure. At the same time, the barrier height and its extent in the direction of tunneling decrease and the calculations predict significant changes of the dynamics of hydrogen in $\alpha$-Mn at 100 kbar, when the estimated tunneling splitting of the hydrogen ground state exceeds the barrier height.

In recent years, universal scaling has gained renewed attention in the study of magnetocaloric materials. It has been applied to a wide variety of pure elements and compounds, ranging from rare-earth-based materials to transition metal alloys, from bulk crystalline samples to nanoparticles. It is therefore necessary to quantify the limits within which the scaling laws would remain applicable for magnetocaloric research. For this purpose, a threefold approach has been followed: (a) the magnetocaloric responses of a set of materials with Curie temperatures ranging from 46 to 336 K have been modeled with a mean-field Brillouin model, (b) experimental data for Gd has been analyzed, and (c) a 3D-Ising model—which is beyond the mean-field approximation—has been studied. In this way, we can demonstrate that the conclusions extracted in this work are model-independent. It is found that universal scaling remains applicable up to applied fields, which provide a magnetic energy to the system up to 8% of the thermal energy at the Curie temperature. In this range, the predicted deviations from scaling laws remain below the experimental error margin of carefully performed experiments. Therefore, for materials whose Curie temperature is close to room temperature, scaling laws at the Curie temperature would be applicable for the magnetic field range available at conventional magnetism laboratories ($\sim 10$ T), well above the fields which are usually available for magnetocaloric devices.

Three members of the perovskite family $R\mathrm{F}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{r}}_{0.5}{\mathrm{O}}_3$ ($R=\mathrm{Lu},\mathrm{Yb}$, and Tm) have been synthesized and characterized. A systematic study of the crystal and magnetic structures was performed by neutron powder diffraction combined with magnetization measurements. All these compounds crystallize in a Pbnm orthorhombic unit cell and they are already antiferromagnetic at room temperature. The study of the magnetic structure vs temperature showed the occurrence of a progressive spin reorientation from ${\mathrm{\Gamma }}_4^{\text{TM}}$ to ${\mathrm{\Gamma }}_2^{\text{TM}}$ for the transition metal sublattice, and in the Tm-based sample, a long-range magnetic order of the $\mathrm{T}{\mathrm{m}}^{3+}$ sublattice was found $\left({\mathrm{\Gamma }}_8^R\right)$. These results are in excellent agreement with the magnetic susceptibility measurements. No spin reorientation is observed in the Lu-based sample for which a magnetization reversal at a compensation temperature $T_{\mathrm{comp}}=225\mathrm{K}$ was detected. A clear magnetostrictive effect was observed in the samples with $R=\mathrm{Yb}$ and Tm associated with a negative thermal expansion and was assigned to a magnetoelastic effect produced by repulsion between the magnetic moments of neighboring transition metal ions.

Antiferromagnetic (AFM) materials, with robust rigidity to magnetic field perturbations and ultrafast spin dynamics, show great advantages in information storage and have developed into a fast-emerging field of AFM spintronics. However, a direct characterization of spin alignments in AFM films has been challenging, and their manipulation by lattice distortion and magnetic proximity is inevitably accompanied by “ferromagnetic” features within the AFM matrix. Here we resolve the $G$-type AFM structure of $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ and find that the interfacial AFM structure could be modulated intrinsically from in plane to out of plane with a canted angle of ${60}^{\circ }$ by the charge transfer and orbital reconstruction in $\mathrm{SrCo}{\mathrm{O}}_{2.5}/\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ heterostructures both experimentally and theoretically. Such an interfacial AFM reconfiguration caused by electronic reconstruction does not cause the ferromagnetic feature and changes the magnetization switching process of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ from in plane to perpendicular to the plane, in turn. Our study not only reveals the coupling between charge, orbital, and AFM structure, but also provides a unique approach to manipulating AFM structure.

We investigate the zero-temperature superfluid to insulator transitions in a diluted two-dimensional quantum rotor model with particle-hole symmetry. We map the Hamiltonian onto a classical $(2+1)$-dimensional $XY$ model with columnar disorder which we analyze by means of large-scale Monte Carlo simulations. For dilutions below the lattice percolation threshold, the system undergoes a generic superfluid-Mott glass transition. In contrast to other quantum phase transitions in disordered systems, its critical behavior is of conventional power-law type with universal (dilution-independent) critical exponents $z=1.52\left(3\right), \nu =1.16\left(5\right), \beta /\nu =0.48\left(2\right), \gamma /\nu =2.52\left(4\right)$, and $\eta =-0.52\left(4\right)$. These values agree with and improve upon earlier Monte Carlo results [Phys. Rev. Lett. 92, 015703 (2004)] while (partially) excluding other findings in the literature. As a further test of universality, we also consider a soft-spin version of the classical Hamiltonian. In addition, we study the percolation quantum phase transition across the lattice percolation threshold; its critical behavior is governed by the lattice percolation exponents in agreement with recent theoretical predictions. We relate our results to a general classification of phase transitions in disordered systems, and we briefly discuss experiments.

$\left(\mathrm{L}{\mathrm{i}}_{0.8}\mathrm{F}{\mathrm{e}}_{0.2}\right)\mathrm{OHFeSe}$ is a newly discovered intercalated iron-selenide superconductor with a $T_c$ above 40 K, which is much higher than the $T_c$ of bulk FeSe (8 K). Here we report a systematic study of $\left(\mathrm{L}{\mathrm{i}}_{0.8}\mathrm{F}{\mathrm{e}}_{0.2}\right)\mathrm{OHFeSe}$ by low temperature scanning tunneling microscopy (STM). We observed two kinds of surface terminations, namely FeSe and $\left(\mathrm{L}{\mathrm{i}}_{0.8}\mathrm{F}{\mathrm{e}}_{0.2}\right)\mathrm{OH}$ surfaces. On the FeSe surface, the superconducting state is fully gapped with double coherence peaks, and a vortex core state with split peaks near $E_F$ is observed. Through quasiparticle interference (QPI) measurements, we clearly observed intra- and interpocket scatterings in between the electron pockets at the $M$ point, as well as some evidence of scattering that connects $\mathrm{\Gamma }$ and $M$ points. Upon applying the magnetic field, the QPI intensity of all the scattering channels are found to behave similarly. Furthermore, we studied impurity effects on the superconductivity by investigating intentionally introduced impurities and intrinsic defects. We observed that magnetic impurities such as Cr adatoms can induce in-gap states and suppress superconductivity. However, nonmagnetic impurities such as Zn adatoms do not induce visible in-gap states. Meanwhile, we show that Zn adatoms can induce in-gap states in thick FeSe films, which is believed to have an $s_{\pm }$-wave pairing symmetry. Our experimental results suggest it is likely that $\left(\mathrm{L}{\mathrm{i}}_{0.8}\mathrm{F}{\mathrm{e}}_{0.2}\right)\mathrm{OHFeSe}$ is a plain $s$-wave superconductor, whose order parameter has the same sign on all Fermi surface sections.