Crystallographic and morphologic features of quadruple and double superstructures in $\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{S}{\mathrm{m}}_x\mathrm{Fe}{\mathrm{O}}_3$ with $0\le x\le 0.40$ were investigated by x-ray powder diffraction and transmission electron microscopy. The presence of a quadruple superstructure accompanied by antiferroic ionic displacements was found near the morphotropic phase boundary between the $R3c$ phase ($0<x<0.10$) and the Pnma phase ($0.25\le x\le 0.40$). The quadruple superlattice phase coexists with a ferroelectric $R3c$ phase at $x=0.15$. These superstructures were found to originate from the freezing of multiple phonon modes related to transverse wave ionic displacements. These findings provide further insight towards understanding the complexity of rare-earth-doped $\mathrm{BiFe}{\mathrm{O}}_3$.

We consider the random-field $\mathrm{O}(N$) spin model with long-range exchange interactions which decay with distance $r$ between spins as $r^{-d-\sigma }$ and/or random fields which correlate with distance $r$ as $r^{-d+\rho }$, and reexamine the critical phenomena near the lower critical dimension by use of the perturbative functional renormalization group. We compute the analytic fixed points in the one-loop beta functions, and study their stability. We also calculate the critical exponents at the analytical fixed points. We show that the analytic fixed point which governs the phase transition in the system with the long-range correlations of random fields can be destabilized by the nonanalytic perturbation in both cases where the exchange interactions between spins are short ranged and long ranged. For the system with the long-range exchange interactions and uncorrelated random fields, we show that the $d\to d-\sigma$ dimensional reduction at the leading order of the $d-2\sigma$ expansion holds only for $N>2(4+3\sqrt{3})\simeq 18.3923\dots$. Our investigation into the system with the long-range exchange interactions and uncorrelated random fields also gives the value of the boundary between critical behaviors in systems with long-range and short-range exchange interactions, which is identical to that predicted by Sak [Phys. Rev. B 8, 281 (1973)]. For the system with the long-range exchange interactions and the long-range correlated random fields, we show that the $d\to d-\sigma -\rho$ dimensional reduction does not hold within the present framework, as far as $N$ is finite.

The time dependence of the response of the quantum spin liquid with fermionic excitations to parametric pumping has been calculated exactly for the special case of periodic pumping, the steplike periodic field, without using the resonance approximation, and for any value of the pumping magnitude. In the closed regime each mode (related to each eigenstate of the system) oscillates with time. The oscillations persist about the steady-state values, which are determined by the parameters of the pumping. The magnitude of the oscillations is finite for any value of the magnitude of the pumping. Those features drastically differ from the well-known resonance parametric pumping of magnons in magnetically ordered system, where the number of resonance magnons grows exponentially in time. The interference of infinitely many modes generically yields only decaying-in-time modulated oscillations of the total number of quasiparticles per mode even in the closed regime. For the fermionic system only one mode is in resonance, while others are out of resonance. It is totally different from the bosonic behavior of magnons under parametric pumping, where all modes can exist in resonance. The inclusion of the linear relaxation in the open regime for that quantum spin liquid produces the decay of oscillations to the initial state.

We present a quantitative experimental investigation of the scalar chiral magnetic order with in ${\mathrm{Nd}}_3{\mathrm{Sb}}_3{\mathrm{Mg}}_2{\mathrm{O}}_{14}$. Static magnetization reveals a net ferromagnetic ground state, and inelastic neutron scattering from the hyperfine coupled nuclear spin reveals a local ordered moment of 1.76(6) ${\mu }_B$, just 61(2)% of the saturated moment size. The experiments exclude static disorder as the source of the reduced moment. A 38(1)$\mu \mathrm{eV}$ gap in the magnetic excitation spectrum inferred from heat capacity rules out thermal fluctuations and suggests a multipolar explanation for the moment reduction. We compare ${\mathrm{Nd}}_3{\mathrm{Sb}}_3{\mathrm{Mg}}_2{\mathrm{O}}_{14}$ to Nd pyrochlores and show that ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ is in a spin fragmented state using nuclear Schottky heat capacity.

We performed terahertz magneto-optical spectroscopy of FeSe thin film to elucidate the charge carrier dynamics. The measured diagonal (longitudinal) and off-diagonal (Hall) conductivity spectra are well reproduced by a two-carrier Drude model, from which the carrier densities, scattering times, and effective masses of electron and hole carriers are determined in a wide range of temperature. The hole density decreases below the structural transition temperature while electron density increases, which is attributed to the band-structure modification in the electronic nematic phase. The scattering time of the hole carrier becomes substantially longer than that of the electron at lower temperature, which accounts for the increase of the positive dc Hall coefficient at low temperature.

We have carried out angle-integrated photoemission spectroscopy (AIPES) and angle-resolved photoemission spectroscopy (ARPES) experiments using soft x rays on single crystals of ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ grown by the Bridgman method to investigate their electronic structures. The AIPES results showed that the Eu ions in ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ are in a divalent state and a nearly trivalent state, respectively, in accordance with the previously reported magnetic properties. The three-dimensional band structures and shapes of the Fermi surfaces of ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ were studied by ARPES measurements. We found that the band structures near the Fermi level and Fermi surfaces of ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$ are very different from each other and are well reproduced by the band structure calculations based on density-functional theory for ${\mathrm{SrCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{YCu}}_2{\mathrm{Si}}_2$. This suggests that a charge transfer from the localized $4f$ states into the valence bands is responsible for the difference in the electronic states between ${\mathrm{EuCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{EuCu}}_2{\mathrm{Si}}_2$.

A ${}^{59}\mathrm{Co}$ nuclear magnetic resonance (NMR) measurement was performed on the single-crystalline ferromagnetic (FM) superconductor UCoGe under a pressure of 1.09 GPa, where the FM state is suppressed and superconductivity occurs in the paramagnetic (PM) state, to study the superconducting (SC) state in the PM state. ${}^{59}\mathrm{Co}$-NMR spectra became broader but hardly shifted across the SC transition temperature. The Knight-shift change determined from fitting the spectral peak with a Gaussian was much smaller than the spin part of the Knight shift; this is in good agreement with the spin-triplet pairing suggested from the large upper critical field. The spectrum broadening in the SC state cannot be attributed to the SC diamagnetic effect but is related to the properties of spin-triplet superconductivity. The origins of the broadening are discussed herein.

We develop a generalized Gross-Pitaevskii approach to the driven-dissipative dynamics of intersubband polaritons in patterned planar microcavities where the cavity mode is strongly coupled to the intersubband transitions in doped quantum wells. Substantial differences with respect to the case of interband excitonic polaritons are highlighted, in particular, the non-Markovian features of the radiative decay. We show that the method is able to reproduce well the experimental observations, as validated on the linear reflection properties of the cavity. The theoretical framework is then applied in the nonlinear regime to study optical parametric oscillation processes for intersubband polaritons. Our findings open interesting perspectives in view of novel coherent laser sources operating in the mid and far infrared.

Electronic current densities can reach extreme values in highly conducting nanostructures where constrictions limit current. For bias voltages on the 1 V scale, the highly nonequilibrium situation can influence the electronic density between atoms, leading to significant interatomic forces of the order of 1 nN. An easy interpretation of the nonequilibrium forces is currently not available, to our knowledge. In this work, we present an ab initio study based on density functional theory of bias-induced atomic forces in gated graphene nanoconstrictions consisting of junctions between graphene electrodes and graphene nanoribbons in the presence of current. We find that current-induced bond forces and bond charges are correlated, while bond forces are not simply correlated to bond currents. We discuss, in particular, how the forces are related to induced charges and the electrostatic potential profile (voltage drop) across the junctions. For long current-carrying junctions we may separate the junction into a part with a voltage drop, and a part without voltage drop. The latter situation can be compared to a nanoribbon in the presence of current using an ideal ballistic velocity-dependent occupation function. This shows how the combination of voltage drop and current give rise to the strongest current-induced forces in nanostructures.

Quantum Hall edge states have some characteristic features that can prove useful to measure and control solid state qubits. For example, their high voltage to current ratio and their dissipationless nature can be exploited to manufacture low-loss microwave transmission lines and resonators with a characteristic impedance of the order of the quantum of resistance $h/e^2\sim 25\mathrm{k}\mathrm{\Omega }$. The high value of the impedance guarantees that the voltage per photon is high, and for this reason, high-impedance resonators can be exploited to obtain larger values of coupling to systems with a small charge dipole, e.g., spin qubits. In this paper, we provide a microscopic analysis of the physics of quantum Hall effect devices capacitively coupled to external electrodes. The electrical current in these devices is carried by edge magnetoplasmonic excitations and by using a semiclassical model, valid for a wide range of quantum Hall materials, we discuss the spatial profile of the electromagnetic field in a variety of situations of interest. Also, we perform a numerical analysis to estimate the lifetime of these excitations and, from the numerics, we extrapolate a simple fitting formula which quantifies the $Q$ factor in quantum Hall resonators. We then explore the possibility of reaching the strong photon-qubit coupling regime, where the strength of the interaction is higher than the losses in the system. We compute the Coulomb coupling strength between the edge magnetoplasmons and singlet-triplet qubits, and we obtain values of the coupling parameter in the order of 100 MHz; comparing these values to the estimated attenuation in the resonator, we find that for realistic qubit designs the coupling can indeed be strong.

The newly designed $A{\mathrm{MnBi}}_2$ ($A=\text{alkaline}$ as well as rare-earth atom) materials based on quasi-two-dimensional bismuth layers provide a suitable playground to address the physics of Dirac fermions, in connection with magnetism and structural changes. Here, we perform an optical investigation as a function of temperature of ${\mathrm{Ca}}_{1-x}{\mathrm{Na}}_x{\mathrm{MnBi}}_2$, which reveals a vestigial linear frequency-dependent behavior of the optical conductivity in the midinfrared, ascribed to electronic interband transitions involving Dirac bands. Furthermore, we uncover optical signatures for a partial gapping of the Fermi surface, for energy scales up to 0.2 eV, at the onset of the spin reorientation transition which also manifests as an anomaly in the dc transport data. This may reveal the inclination towards a Fermi surface instability in topological materials, possibly related to a density-wave order.

We extend density functional perturbation theory for lattice dynamics with fully relativistic ultrasoft pseudopotentials to magnetic materials. Our approach is based on the application of the time-reversal operator to the Sternheimer linear system and to its self-consistent solutions. Moreover, we discuss how to include in the formalism the symmetry operations of the magnetic point group which require the time-reversal operator. We validate our implementation by comparison with the frozen phonon method in fcc Ni and in a monatomic ferromagnetic Pt wire.

We study the nature of orbital and magnetic order in the layered perovskite ${\mathrm{K}}_2{\mathrm{CuF}}_4$, and compare to the case of the infinite-layer system ${\mathrm{KCuF}}_3$. To this end, we augment the local-density approximation $+$ dynamical mean-field theory technique with linear-response functions. We explain orbital and magnetic order, and their evolution with increasing pressure. We show that both the tetragonal (${\varepsilon }_{\mathrm{T}}$) and the Jahn-Teller (${\varepsilon }_{\mathrm{JT}}$) crystal-field splitting play a key role. We find that surprisingly, unlike in ${\mathrm{KCuF}}_3,{\varepsilon }_{\mathrm{T}}$ is comparable to, or even larger than, ${\varepsilon }_{\mathrm{JT}}$; in addition, ${\varepsilon }_{\mathrm{T}}$ is mostly determined by the layered structure itself and by the compression of the K cage, rather than by the deformations of the ${\mathrm{CuF}}_6$ octahedra. Next, we study the nature of orbital order. We calculate the superexchange transition temperature, finding $T_{\mathrm{KK}}\sim 300\mathrm{K}$, a value close to the one for ${\mathrm{KCuF}}_3$. Thus, in ${\mathrm{K}}_2{\mathrm{CuF}}_4$ as in ${\mathrm{KCuF}}_3,T_{\mathrm{KK}}$ is too small to explain the existence of orbital order up to the melting temperature. We show, however, that in the case of the layered perovskite, an additional superexchange mechanism is at work. It is an orbital Zeeman term, ${\widehat{h}}_{\mathrm{KK}}$, and it is active also above $T_{\mathrm{KK}}$. We show that due to ${\widehat{h}}_{\mathrm{KK}}$, phases with different types of ordering can coexist at temperatures below $T_{\mathrm{KK}}$. Similar effects are likely to play a role in other layered correlated systems.

Quantum spin liquids are long-range entangled states of matter with emergent gauge fields and fractionalized excitations. While candidate materials, such as the Kitaev honeycomb ruthenate $\alpha \text{−}{\mathrm{RuCl}}_3$, show magnetic order at low temperatures $T$, here we demonstrate numerically a dynamical crossover from magnonlike behavior at low $T$ and frequencies $\omega$ to long-lived fractionalized fermionic quasiparticles at higher $T$ and $\omega$. This crossover is akin to the presence of spinon continua in quasi-1D spin chains. It is further shown to go hand in hand with persistent typicality down to very low $T$. This aspect, which has also been observed in the spin-1/2 kagome Heisenberg antiferromagnet, is a signature of proximate spin liquidity and emergent gauge degrees of freedom more generally, and can be the basis for the numerical study of many finite-$T$ properties of putative spin liquids.

We combine time-resolved pump-probe magneto-optical Kerr effect and photoelectron spectroscopy experiments supported by theoretical analysis to determine the relaxation dynamics of delocalized electrons in half-metallic ferromagnetic manganite $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Mn}{\mathrm{O}}_3$. We observe that the half-metallic character of $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Mn}{\mathrm{O}}_3$ determines the timescale of both the electronic phase transition and the quenching of magnetization, revealing a quantum isolation of the spin system in double-exchange ferromagnets extending up to hundreds of picoseconds. We demonstrate the use of time-resolved hard x-ray photoelectron spectroscopy as a unique tool to single out the evolution of strongly correlated electronic states across a second-order phase transition in a complex material.

Charge density response is responsible for the excited-state properties of lead iodide perovskites and is related to both the light absorption properties as well as subsequent electronic and lattice relaxation in the system, important for the working conditions of the material in solar cell applications. Here we investigate the nature of the excited state and its relation to pathways for electronic and lattice relaxations by performing time-dependent density-functional theory (TDDFT). Charge density response upon photoexcitation close to the band edge and deeper into the absorption spectra are investigated for three lead perovskite compounds with different $A$-site monovalent cations ${\mathrm{CsPbI}}_3,\mathrm{C}{\mathrm{H}}_2{\left(\mathrm{N}{\mathrm{H}}_2\right)}_2\mathrm{Pb}{\mathrm{I}}_3\left(\mathrm{FAPb}{\mathrm{I}}_3\right)$, and $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3\left(\mathrm{MAPb}{\mathrm{I}}_3\right)$. The carrier cooling mechanism is analyzed and shows that the initial force acting on the nuclei follows the symmetry of the ground-state electronic structure upon photoexcitation with a force parallel to the polarization of the incoming light. This effect is investigated for the three different compounds and shows an initial force for induced ionic movement that depends on both the underlying symmetry of the inorganic lattice as well as on the type and orientation of the organic cation. The excess energy after thermalization under blue-light illumination is large enough for overcoming the activation energy for iodide migration and can thus trigger vacancy formation. Iodide vacancies are seen to be dipole-field compensated by the organic cation, with a shielding of the local field, and thus form an explanation for the defect tolerance found in these systems under photovoltaic operation. A partial charge transfer from the inorganic cage to the monovalent organic cation is predicted with TDDFT calculations for blue- and UV-light illumination with a population of antibinding orbitals in the N–H bond in both ${\mathrm{CH}}_3{\mathrm{NH}}_3$ (MA) and ${\mathrm{CH}}_2{\left({\mathrm{NH}}_2\right)}_2$ (FA), where the implication for this is discussed in terms of the intrinsic photostability of organic cation containing lead perovskites. The results show the importance of a fundamental understanding of the excited-state properties of perovskite material to reveal the underlying mechanism for the defect tolerance and thus high photovoltaic performance when using organic dipolar cations as well as a rationale for using mixed halide perovskites to decrease the halide migration, effect of vacancy formation, and stability issues under blue- and UV-light illumination.

The pressure effect on crystal structures, lattice vibrations, and electrical transport properties of $\mathrm{Pd}{\mathrm{S}}_2$ and $\mathrm{NiS}{\mathrm{e}}_2$ were studied under high pressures of up to ∼50 GPa using Raman spectroscopy and electrical resistivity measurements. $\mathrm{Pd}{\mathrm{S}}_2$ undergoes semiconductor-to-metal transition at a pressure of ∼7 GPa without a structural phase transition. Structural transition to the pyrite-type phase occurs at a pressure of ∼16 GPa. Superconductivity emerges in the pyrite-type phase of $\mathrm{Pd}{\mathrm{S}}_2$ with a well-defined dome-shaped dependence of critical temperature of superconductivity on pressure, with a maximum value of 8.0 K at 37.4 GPa. Although this behavior is similar to that in isostructural $\mathrm{PdS}{\mathrm{e}}_2$, correlation with anomalous anion bond softening at critical temperature is not observed in $\mathrm{Pd}{\mathrm{S}}_2$ in contrast to $\mathrm{PdS}{\mathrm{e}}_2$. Conversely, the anion bond instability is observed by the softening of the $\mathrm{S}{{\mathrm{e}}_2}^{2-}$ internal vibrational Raman mode in pyrite $\mathrm{NiS}{\mathrm{e}}_2$ under pressure. Despite the persisting structural instability due to destabilization of $\mathrm{S}{{\mathrm{e}}_2}^{2-}$ dimers, $\mathrm{NiS}{\mathrm{e}}_2$ remains a normal nonsuperconducting metal.

Planar topological superconductors with power-law-decaying pairing display different kinds of topological phase transitions where quasiparticles dubbed nonlocal-massive Dirac fermions emerge. These exotic particles form through long-range interactions between distant Majorana modes at the boundary of the system. We show how these propagating-massive Dirac fermions neither mix with bulk states nor Anderson-localize up to large amounts of static disorder despite having finite energy. Analyzing the density of states (DOS) and the band spectrum of the long-range topological superconductor, we identify the formation of an edge gap and a surprising double-peak structure in the DOS which can be linked to a twisting of energy bands with nontrivial topology. Our findings are amenable to experimental verification in the near-future using atom arrays on conventional superconductors, planar Josephson junctions on two-dimensional electron gases, and Floquet driving of topological superconductors.