The description of nonequilibrium states of solids in a simplified manner is a challenge in the field of ultrafast dynamics. Here, the phonon thermalization in solids through the three-phonon scatterings is investigated by solving the Boltzmann transport equation (BTE). The numerical solution of the BTE shows that the transverse acoustic and longitudinal acoustic (LA) phonon temperatures are not well defined during the relaxation, indicating the breakdown of the quasiequilibrium approximation. The development of hot and cold phonons and the backward energy flow from low to high energy phonons are observed in the initial and final stage of the relaxation, respectively. A minimal model is presented to relate the latter with the power-law decay of the LA phonon energy.

We present a detailed study on the magnetic structure and magnetoelectric properties of several double perovskites $R_2\mathrm{CoMn}{\mathrm{O}}_6$ ($R=\mathrm{Ho}$, Tm, Yb, and Lu). All of these samples show an almost perfect $(\sim 94\%)$ ordering of $\mathrm{C}{\mathrm{o}}^{2+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$ cations in the unit cell. Our research reveals that the magnetic ground state strongly depends on the $R$ size. For samples with larger $R$ (Ho and Tm), the ground state is formed by a ferromagnetic order ($F$ type) of $\mathrm{C}{\mathrm{o}}^{2+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$ moments, while $R$ either remains mainly disordered (Ho) or is coupled antiferromagnetically (Tm) to the Co/Mn sublattice. For samples with smaller $R$ (Yb or Lu), competitive interactions lead to the formation of an $E$-type arrangement for the $\mathrm{C}{\mathrm{o}}^{2+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$ moments with a large amount of extended defects such as stacking faults. The $\mathrm{Y}{\mathrm{b}}^{3+}$ is partly ordered at very low temperature. The latter samples undergo a metamagnetic transition from the $E$ into the $F$ type, which is coupled to a negative magnetodielectric effect. Actually, the real part of dielectric permittivity shows an anomaly at the magnetic transition for the samples exhibiting an $E$-type order. This anomaly is absent in samples with $F$-type order, and, accordingly, it vanishes coupled to the metamagnetic transition for $R=\mathrm{Yb}$ or Lu samples. At room temperature, the huge values of the dielectric constant reveal the presence of Maxwell-Wagner depletion layers. Pyroelectric measurements reveal a high polarization at low temperature, but the onset of pyroelectric current is neither correlated to the kind of magnetic ordering nor to the magnetic transition. Our study identified the pyroelectric current as thermally stimulated depolarization current and electric-field polarization curves show a linear behavior at low temperature. Therefore, no clear ferroelectric transition occurs in these compounds.

The correct identification of topological magnetic objects in experiments is an important issue. In the present paper we report on the characterization of metastable skyrmionic spin structures with various topological charges ($Q=-3,-2,-1,0,+1,+2$) in the $\left({\mathrm{Pt}}_{1-x}{\mathrm{Ir}}_x\right)\mathrm{Fe}/\mathrm{Pd}\left(111\right)$ ultrathin magnetic film by performing spin-polarized scanning tunneling microscopy (SP-STM) calculations. We find that an out-of-plane magnetized tip already results in distinguished SP-STM contrasts for the different skyrmionic structures corresponding to their symmetries. Our paper also establishes an understanding of the relationship between in-plane SP-STM contrasts and skyrmionic topologies through an investigation of the variation of the in-plane angle between the spins along the perimeter of the structures, which can be characterized by the local vorticity or linear density of the winding number. For spin structures exhibiting a uniform sign of the local vorticity throughout the whole skyrmionic area, we demonstrate that (i) $\left|Q\right|$ can be determined from a single SP-STM image taken by any in-plane magnetized tip and (ii) an in-plane tip magnetization rotation provides the sign of $Q$ independently of the sign of the effective spin polarization in the tunnel junction. We also discuss cases where the local vorticity is changing sign. Finally, by increasing the Ir content of the PtIr overlayer, we find an appearing secondary outer ring in-plane SP-STM contrast that is indicative of attractive skyrmions or antiskyrmions.

Lattice translation symmetry gives rise to a large class of “weak” topological insulators (TIs), characterized by translation-protected gapless surface states and dislocation bound states. In this work we show that space group symmetries lead to constraints on the weak topological indices that define these phases. In particular, we show that screw rotation symmetry enforces the Hall conductivity in planes perpendicular to the screw axis to be quantized in multiples of the screw rank, which generally applies to interacting systems. We further show that certain 3D weak indices associated with quantum spin Hall effects (class AII) are forbidden by the Bravais lattice and by glide or even-fold screw symmetries. These results put strong constraints on weak TI candidates in the experimental and numerical search for topological materials, based on the crystal structure alone.

The exciton spin dynamics are investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The magnetic field induced circular polarization of photoluminescence $P_c$ is studied as function of the magnetic field strength and direction as well as sample temperature. The observed nonmonotonic behavior of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. To interpret the experiment, we have developed a kinetic master equation model which accounts for the dynamics of the spin states in this exciton quartet, radiative and nonradiative recombination processes, and redistribution of excitons between these states as result of spin relaxation. The model offers quantitative agreement with experiment and allows us to evaluate, for the studied structure, the heavy-hole $g$ factor, $g_{hh}=+3.5$, and the spin relaxation times of electron, ${\tau }_{se}=33\mu \mathrm{s}$, and hole, ${\tau }_{sh}=3\mu \mathrm{s}$, bound in the exciton.

We investigate photoluminescence (PL) spectra and dynamics of clean silicon photonic crystal nanocavities at 10 K. A sharp emission peak due to the nanocavity mode has the largest intensity when the energy of the nanocavity mode is equal to the emission energy of the electron-hole droplets (EHDs). Time-resolved PL spectroscopy indicates that the PL lifetime of the EHD is reduced to as short as 1.2 ns by the nanocavity mode. A careful analysis of the lifetimes indicates that the radiative recombination rate for EHD is enhanced by a factor of larger than 5 by the Purcell effect.

$8\text{−}Pmmn$ borophene is a polymorph of borophene which hosts an anisotropic tilted Dirac cone. Using a low-energy effective Hamiltonian for borophene, we analytically calculate the density-density response function of borophene and study its anisotropic plasmon dispersion and screening properties. We find that the anisotropic plasmon mode in $8\text{−}Pmmn$ borophene remains undamped for higher energies along the mirror symmetry direction. The Friedel oscillation in borophene is found to decay as $r^{-3}$ in the large-$r$ limit, with both the amplitude as well as the periodicity of the Friedel oscillations being anisotropic.

We study structural and electronic properties of graphene grown on silicone carbide (SiC) substrate using a scanning tunneling microscope, spot-profile-analysis low-energy electron diffraction, and angle-resolved photoemission spectroscopy. We find several new replicas of Dirac cones in the Brillouin zone. Their locations can be understood in terms of a combination of basis vectors linked to SiC 6 $\times$ 6 and graphene $6\sqrt{3}\times 6\sqrt{3}$ reconstruction. Therefore, these new features originate from the moiré caused by the lattice mismatch between SiC and graphene. More specifically, Dirac cone replicas are caused by underlying weak modulation of the ionic potential by the substrate that is then experienced by the electrons in the graphene. We also demonstrate that this effect is equally strong in single- and trilayer graphene; therefore, the additional Dirac cones are intrinsic features rather than the result of photoelectron diffraction. These new features in the electronic structure are very important for the interpretation of recent transport measurements and can assist in tuning the properties of graphene for practical applications.

Much excitement has been generated recently by the experimental observation of the chiral anomaly in condensed matter physics. This manifests as strong negative longitudinal magnetoresistance and has so far been clearly observed in ${\mathrm{Na}}_3\mathrm{Bi}, {\mathrm{ZrTe}}_5$, and GdPtBi. In this Rapid Communication, we point out that the chiral anomaly must lead to another effect in topological metals, the giant planar Hall effect (GPHE), which is the appearance of a large transverse voltage when the in-plane magnetic field is not aligned with the current. Moreover, we demonstrate that the GPHE is closely related to the angular narrowing of the negative longitudinal magnetoresistance signal, observed experimentally.

Experiment and theory indicate that ${\mathrm{UPt}}_3$ is a topological superconductor in an odd-parity state, based in part from the temperature independence of the NMR Knight shift. However, quasiparticle spin-flip scattering near a surface, where the Knight shift is measured, might be responsible. We use polarized neutron scattering to measure the bulk susceptibility with $H\parallel c$, finding consistency with the Knight shift but inconsistency with theory for this field orientation. We infer that neither spin susceptibility nor a Knight shift are a reliable indication of odd parity.

We report on electrically and nonresonant optically induced switching in the bistable regime of an electrically pumped polariton laser. Electrical switching effects can be observed by adding controlled noise in the electrical pump of the system. Noise is expected to influence the hysteresis characteristics of a bistable device and determines its application robustness. We find that the hysteresis width decreases symmetrically with a linear dependency until we observe a quenching of the bistability at a certain noise level and the output of the system becomes monostable. Furthermore, we explore the possibility to switch between the two bistable branches by a nonresonant optical pulse. Our experimental findings can be described by a set of rate equations modeling the population dynamics with additional noise terms.

The total and radiative spontaneous emission (SE) rates of a quantum emitter (QE) are enhanced several orders of magnitudes in the presence of a ${\mathrm{MoS}}_2$ nanodisk, compared to their free space value, by exciting localized exciton polariton modes. The quantum efficiency can reach values above $24\%$, at emission energies where the dipole resonance is excited, for small values of the material damping parameters. The radiative emission rate is enhanced 565 times compared to its free-space value. Two set of sharp resonances of the SE rate emerge, which are connected with the excitation of the exciton energies of the ${\mathrm{MoS}}_2$ nanodisk. When the emission energy is in the halo, between the first two peaks of the normalized SE rate, the quantum efficiency has its largest value of $53\%$, where the radiative emission is enhanced 4.3 times compared to the free-space value. The high values for the quantum efficiency are sustained over distances up to $40\text{nm}$, where the radiative spontaneous emission can compete with the nonradiative emission.

X-ray diffraction studies to $\sim 90\text{GPa}$ at room temperature show that $\mathrm{F}{\mathrm{e}}_2\mathrm{Ti}{\mathrm{O}}_4$ ferrous inverse spinel undergoes the following sequence of structural transitions: $\mathrm{cubic}\left(Fd\overline{3}m\right)\stackrel{\sim 8\text{GPa}}{\to }\mathrm{tetragonal}\left(I{4}_1/amd\right)\stackrel{\sim 16\text{GPa}}{\to }\mathrm{orthorhombic}\left(Cmcm\right)\stackrel{\sim 55\text{GPa}}{\to }\mathrm{orthorhombic}\left(Pmma\right),$ at the indicated onset transition pressures. Within the Cmcm phase, site-specific spin crossover is initiated and involves only highly distorted octahedral sites constituting $\sim 25\%$ of all Fe locations. This is manifest as a steeper volume decrease of $\mathrm{\Delta }V/V_0\sim 3.5\%$ beyond $\sim 40\text{GPa}$ and an emergent diamagnetic component discerned in ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy at variable cryogenic temperatures. A subsequent $Cmcm\to Pmma$ Fe/Ti disorder-order reconfiguration is facilitated at sixfold coordinated (octahedral) sites. The rest of the high-spin Fe in sixfold and eightfold coordinated sites ($\sim 75\%$ abundance) in the Pmma phase exhibit average saturation internal magnetic fields of $H_{hf}\sim 42\text{T}$ to $\sim 90\text{GPa}$, typical of spin-only (orbitally quenched) Fermi-contact values. By contrast, average $H_{hf}\sim 20\text{T}$ values, signifying unquenched orbital moments, occur below the $40–45\text{GPa}$ spin-crossover initiation regime in the Cmcm phase. Therefore, site-specific spin crossover invokes a cooperative lattice response and polyhedral distortions at the rest of the high-spin Fe sites, translating to $3d$ level (sub-band) changes and consequential orbital moment quenching. Near $\sim 90\text{GPa}, \mathrm{F}{\mathrm{e}}_2\mathrm{Ti}{\mathrm{O}}_4$ is a partially spin-converted chemically ordered Pmma post-spinel having a persistent charge gap of $\sim 100\text{meV}$. Despite structural symmetry changes, partial spin crossover and lattice compressibility, resulting in a $\sim 33\%$ total reduction in unit-cell volume and corresponding $3d$ bandwidth broadening, strong electron correlations persist at high densification.

The implementation and control of room-temperature ferromagnetism (RTFM) by adding magnetic atoms to a semiconductor's lattice has been one of the most important problems in solid-state physics in the last decade. Herein we report on the mechanism that allows RTFM to be tuned by the inclusion of nonmagnetic aluminum in nickel ferrite. This material, ${\mathrm{NiFe}}_{2-x}{\mathrm{Al}}_x{\mathrm{O}}_4$ ($x=0,0.5,1.5$), has already shown much promise for magnetic semiconductor technologies, and we are able to add to its versatility technological viability with our results. The site occupancies and valencies of Fe atoms (${\mathrm{Fe}}^{3+}{\mathrm{T}}_d, {\mathrm{Fe}}^{2+}{\mathrm{O}}_h$, and ${\mathrm{Fe}}^{3+}{\mathrm{O}}_h$) can be methodically controlled by including aluminum. Using the fact that aluminum strongly prefers a 3+ octahedral environment, we can selectively fill iron sites with aluminum atoms, and hence specifically tune the magnetic contributions for each of the iron sites, and therefore the bulk material as well. Interestingly, the influence of the aluminum is weak on the electronic structure, allowing one to retain the desirable electronic properties while achieving desirable magnetic properties.

We study the dissipative decay of states with a doubly occupied site in a two-electron Hubbard model, known as doublons. For the environment, we consider charge and current noise, which are modeled as a bosonic heat bath that couples to the on-site energies and the tunnel couplings, respectively. It turns out that the dissipative decay depends qualitatively on the type of environment, as for charge noise, the lifetime grows with the electron-electron interaction. For current noise, by contrast, doublons become increasingly unstable with larger interaction. Numerical studies within a Bloch-Redfield approach are complemented by analytical estimates for the decay rates. For typical quantum dot parameters, we predict doublon lifetimes up to 50 ns.

A nonequilibrium statistical domain nucleation model of polarization dynamics in less understood antiferroelectric systems is introduced. Predictions of the model have been successfully tested experimentally using an antiferroelectric $\mathrm{P}{\mathrm{b}}_{0.99}\mathrm{N}{\mathrm{b}}_{0.02}{\left[{\left(\mathrm{Z}{\mathrm{r}}_{0.57}\mathrm{S}{\mathrm{n}}_{0.43}\right)}_{0.94}\mathrm{T}{\mathrm{i}}_{0.06}\right]}_{0.98}{\mathrm{O}}_3$ polycrystalline ceramic. We determined the activation energy of the domain nucleation process for this particular antiferroelectric sample to be $W_b=1.07\mathrm{eV}$ and the critical volume of the polar nucleus $V^{*}=98\times {10}^{-27}{\mathrm{m}}^3$, which corresponds to a linear length scale of 2.86 nm.

Neutron-diffraction and magnetic susceptibility studies of polycrystalline ${\mathrm{SrCr}}_2{\mathrm{As}}_2$ reveal that this compound is an itinerant G-type antiferromagnet below the Néel temperature $T_{\mathrm{N}}$ = 590(5) K with the Cr magnetic moments aligned along the tetragonal $c$ axis. The system remains tetragonal to the lowest measured temperature ($\sim 12$ K). The lattice parameter ratio $c/a$ and the magnetic moment saturate at about the same temperature below $\sim 200$ K, indicating a possible magnetoelastic coupling. The ordered moment $\mu =1.9\left(1\right){\mu }_{\mathrm{B}/\mathrm{Cr}}$, measured at $T=12$ K, is significantly reduced compared to its localized value ($4{\mu }_{\mathrm{B}/\mathrm{Cr}}$) due to the itinerant character brought about by hybridization between the Cr $3d$ and As $4p$ orbitals.

We study the formation of dipolar excitons and their superfluidity in a phosphorene double layer. The analytical expressions for the single dipolar exciton energy spectrum and wave function are obtained. It is predicted that a weakly interacting gas of dipolar excitons in a double layer of black phosphorus exhibits superfluidity due to the dipole-dipole repulsion between the dipolar excitons. In calculations are employed the Keldysh and Coulomb potentials for the interaction between the charge carriers to analyze the influence of the screening effects on the studied phenomena. It is shown that the critical velocity of superfluidity, the spectrum of collective excitations, concentrations of the superfluid and normal component, and mean-field critical temperature for superfluidity are anisotropic and demonstrate the dependence on the direction of motion of dipolar excitons. The critical temperature for superfluidity increases if the exciton concentration and the interlayer separation increase. It is shown that the dipolar exciton binding energy and mean-field critical temperature for superfluidity are sensitive to the electron and hole effective masses. The proposed experiment to observe a directional superfluidity of excitons is addressed.

We show that the counter-rotating terms of the dispersive qubit-cavity Rabi model can produce relatively large and nonmonotonic Bloch-Siegert shifts in the cavity frequency as the system is driven through a quantum-to-classical transition. Using a weak microwave probe tone, we demonstrate experimentally this effect by monitoring the resonance frequency of a microwave cavity coupled to a transmon and driven by a microwave field with varying power. In the weakly driven regime (quantum phase), the Bloch-Siegert shift appears as a small constant frequency shift, while for a strong drive (classical phase) it presents an oscillatory behavior as a function of the number of photons in the cavity. The experimental results are in agreement with numerical simulations based on the quasienergy spectrum.

Ferrimagnetic ${\mathrm{Mn}}_2{\mathrm{Fe}}_x\mathrm{Ga}(0.26\le x\le 1.12)$ thin films have been characterized by x-ray diffraction, magnetometry, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and Mössbauer spectroscopy with the aim of determining the structure and site-specific magnetism of this tetragonal, $\mathrm{D}{0}_{22}$-structure Heusler compound. High-quality epitaxial films with low root-mean-square surface roughness ($\sim 0.6$ nm) are grown by magnetron cosputtering. The tetragonal distortion induces strong perpendicular magnetic anisotropy along the $c$ axis with a typical coercive field ${\mu }_{0}H\sim 0.8\mathrm{T}$ and an anisotropy field ranging from 6 to 8 T. On increasing the Fe content $x$, substantial uniaxial anisotropy, $K_{\mathrm{u}}\ge 1.0{\mathrm{MJ}\mathrm{m}}^{-3}$, can be maintained over the full $x$ range, while the magnetization of the compound is reduced from 400 to $280{\mathrm{kA}\mathrm{m}}^{-1}$. The total magnetization is almost entirely given by the sum of the spin moments originating from the ferrimagnetic Mn and Fe sublattices, with the latter being coupled ferromagnetically to one of the former. The orbital magnetic moments are practically quenched and have negligible contributions to the magnetization. The films with $x=0.73$ exhibit an anomalous Hall angle of $2.5\%$ and a Fermi-level spin polarization above $51\%$, as measured by point contact Andreev reflection. The Fe-substituted ${\mathrm{Mn}}_2\mathrm{Ga}$ films are tunable with a unique combination of high anisotropy, low magnetization, appreciable spin polarization, and low surface roughness, making them strong candidates for thermally stable spin-transfer-torque switching nanomagnets with lateral dimensions down to 10 nm.