${\mathrm{Co}}^{2+}$ ions in an octahedral crystal field stabilize a $j_{\mathrm{eff}}=1/2$ ground state with an orbital degree of freedom and have been recently put forward for realizing Kitaev interactions, a prediction we have tested by investigating spin dynamics in two cobalt honeycomb lattice compounds, ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ and ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$, using inelastic neutron scattering. We used linear spin wave theory to show that the magnetic spectra can be reproduced with a spin Hamiltonian including a dominant Kitaev nearest-neighbor interaction, weaker Heisenberg interactions up to the third neighbor, and bond-dependent off-diagonal exchange interactions. Beyond the Kitaev interaction that alone would induce a quantum spin liquid state, the presence of these additional couplings is responsible for the zigzag-type long-range magnetic ordering observed at low temperature in both compounds. These results provide evidence for the realization of Kitaev-type coupling in cobalt-based materials, despite hosting a weaker spin-orbit coupling than their $4d$ and $5d$ counterparts.

Magnetic susceptibility, magnetization, and specific-heat measurements under both in-plane and out-of-plane field on ${\mathrm{K}}_2{\mathrm{Co}}_2{\left(\mathrm{Se}{\mathrm{O}}_3\right)}_3$ single crystals are reported. The Co dimers in this compound, which are made from face-sharing effective spin-1/2 $\mathrm{Co}{\mathrm{O}}_6$ octahedra, sit on the corners of a geometrically frustrating triangular planar lattice. No long-range magnetic ordering is found down to 0.35 K, which, together with the large negative Curie-Weiss temperature obtained from fitting the temperature-dependent magnetic susceptibility, indicates that the system is magnetically frustrated. Analysis of the field and orientation dependence of the magnetic and thermodynamic properties suggests the presence of an easy-axis anisotropy. The magnetic fluctuations present due to geometric frustration are more resistant to an in-plane field than a perpendicular-to-plane field.

We study the dynamics of correlation functions of a class of $d$-dimensional integrable models coupled linearly to a fermionic or bosonic bath in the presence of a periodic drive with a square pulse protocol. It is well known that in the absence of the bath, these models exhibit a dynamical phase transition; all correlators decay to their steady-state values as $n_0^{-(d+2)/2}\left[n_0^{-d/2}\right]$ above [below] a critical frequency ${\omega }_c$, where $n_0$ is the number of drive cycles. We find that the presence of a linearly coupled fermionic bath which maintains integrability of the system preserves this transition and leads to additional ones that have no analogs in closed quantum systems. We provide a semianalytic expression for the evolution operator for this system and use it to provide a phase diagram showing the different dynamical regimes as a function of the system-bath coupling strength and the bath parameters. In contrast, when such models are coupled to a bosonic bath which breaks integrability of the model, we find exponential decay of the correlators to their steady state. Our numerical analysis shows that this exponential decay sets in above a critical number of drive cycles $n_c$ which depends on the system-bath coupling strength and the amplitude of perturbation. Below $n_c$, the system retains the power-law behavior identical to that for the closed integrable models and the dynamical transition survives. We discuss the applicability of our results for interacting fermion systems and discuss experiments which can test our theory.

We investigate the phononic in-plane longitudinal low-temperature thermal conductivity ${\kappa }_{ab}$ of the Kitaev quantum magnet $\alpha \text{−}\mathrm{Ru}{\mathrm{Cl}}_3$ for large in-plane magnetic fields up to 33 T. Our data reveal, for fields larger than the critical field $B_c\approx 8\mathrm{T}$, at which the magnetic order is suppressed, a dramatic increase of ${\kappa }_{ab}$ at all temperatures investigated. The analysis of our data shows that the phonons are not only strongly scattered by a magnetic mode at relatively large energy which scales roughly linearly with the magnetic field, but also by a small-energy mode which emerges near $B_c$ with a square-root-like field dependence. While the former is in striking agreement with recent spin-wave theory (SWT) results of the magnetic excitation spectrum at the $\mathrm{\Gamma }$ point, the energy of the latter is too small to be compatible with the SWT-expected magnon gap at the $M$ point, despite the matching field dependence. Therefore, an alternative scenario based on phonon scattering off the thermal excitation of random-singlet states is proposed.

We introduce and solve a two-band model of spinless fermions with $p_x$-wave pairing on a square lattice. The model reduces to the well-known extended Harper-Hofstadter model with half-flux quanta per plaquette and weakly coupled Kitaev chains in two respective limits. We show that its phase diagram contains a topologically nontrivial weak pairing phase as well as a trivial strong pairing phase as the ratio of the pairing amplitude and hopping is tuned. Introducing periodic driving to the model, we observe a cascade of Floquet phases with well defined quasienergy gaps and featuring chiral Majorana edge modes at the zero or $\pi$ gap or both. Dynamical topological invariants are obtained to characterize each phase and to explain the emergence of edge modes in the anomalous phase where all the quasienergy bands have zero Chern number. The analytical solution is achieved by exploiting a generalized mirror symmetry of the model, so that the effective Hamiltonian is decomposed into that of spin-$1/2$ in magnetic field, and the loop unitary operator becomes spin rotations. We further show the dynamical invariants manifest as the Hopf linking numbers.

A theory of electron spin-flip Raman scattering (SFRS) is presented that describes the Raman spectral signals shifted by both one and two times the electron Zeeman energy under nearly resonant excitation of the heavy hole excitons in semiconductor nanoplatelets. We analyze the spin structure of photoexcited intermediate states, derive compound matrix elements of the spin-flip scattering, and obtain polarization properties of the one- and two-electron SFRS common for all the intermediate states. We show that, in the resonant scattering process under consideration, the complexes “exciton plus localized resident electrons” play the role of main intermediate states rather than tightly bound trion states. It is demonstrated that, in addition to the direct photoexcitation (and similar photorecombination) channel, there is another indirect channel contributing to the SFRS process. In the indirect channel, the photohole forms the exciton state with the resident electron removed from the localization site while the photoelectron becomes localized on this site. The theoretical results are compared with recent experimental findings for ensembles of CdSe nanoplatelets.

The carrier generation in insulators subjected to strong electric fields is characterized by the Landau-Zener formula for the tunneling probability with a nonperturbative exponent. Despite its long history with diverse applications and extensions, study of nonequilibrium steady states and associated current response in the presence of the generated carriers has been mainly limited to numerical simulations so far. Here, we develop a framework to calculate the nonequilibrium Green's function of generic insulating systems under a DC electric field, in the presence of a fermionic reservoir. Using asymptotic expansion techniques, we derive a semiquantitative formula for the Green's function with nonperturbative contribution. This formalism enables us to calculate dissipative current response of the nonequilibrium steady state, which turns out to be not simply characterized by the intraband current proportional to the tunneling probability. We also apply the present formalism to noncentrosymmetric insulators, and propose nonreciprocal charge and spin transport peculiar to tunneling electrons.

In perovskite-type compounds, the interplay of the cooperative Jahn-Teller effect, electronic correlations, and orbital degrees of freedom leads to intriguing properties. ${\mathrm{NaCrF}}_3$ is a newly synthesized Jahn-Teller active fluoroperovskite where the ${\mathrm{CrF}}_6{}^{4-}$ octahedrons are considerably distorted. Based on the first-principles calculation, we analyze its electronic structure and magnetic properties. Our numerical results show that the ${\mathrm{Cr}}^{2+}$ ions adopt the high-spin $t_{2g\uparrow }^3{e}_{g\uparrow }^1$ configuration and the material exhibits $G$-type orbital ordering. We also estimate the magnetic exchange couplings and find that the in-plane and interplanar nearest-neighbor interactions are ferromagnetic and antiferromagnetic, respectively. The ground state of this material is $A$-type antiferromagnetic, in agreement with the experiments. Our results give a complete explanation of its electronic structure and magnetic and orbital order, and help to further comprehend the behaviors of Jahn-Teller active perovskite-type fluoride.

Motivated by experiments on the organic compound ${\left(\text{Per}\right)}_2\left[\text{Pt}{\left(\text{mnt}\right)}_2\right]$, we study the ground state of the one-dimensional Kondo lattice model at quarter filling with the density matrix renormalization group method. We show a coupled dimer and bond-order-wave (BOW) state in the weak-coupling regime for the localized spins and itinerant electrons, respectively. The quantum phase transitions for the dimer and the BOW orders occur at the same critical coupling parameter $J_c$, with the opening of a charge gap. The emergence of the combination of dimer and BOW order agrees with the experimental findings of the simultaneous Peierls and spin-Peierls transitions at low temperatures, which provides a theoretical understanding of such a phase transition. We also show that the localized spins in this insulating state have quasi-long-ranged spin correlations with collinear configurations, which resemble the classical dimer order in the absence of a magnetic order.

The electron-phonon interaction controls the intrinsic mobility of charges in metal halide perovskites, and determines the rate at which carriers lose energy. Here, the carrier mobility and cooling dynamics were directly examined using a combination of ultrafast transient absorption spectroscopy and optical pump, THz probe spectroscopy, in perovskites with different lead and tin content, and for a range of carrier densities. Significantly, the carrier mobility in the “hot phonon bottleneck” regime, where the LO phonon bath keeps carriers warm, was found to be similar to the mobility of cold carriers. A model was developed that provides a quantitative description of the experimental carrier cooling dynamics, including electron-phonon coupling, phonon-phonon coupling and the Auger mechanism. In the Pb and Sn alloy the duration of the hot carrier regime was extended as a result of the slower decay of optical phonons. The findings offer an intuitive link between macroscopic properties and the underlying microscopic energy transfer processes, and suggest new routes to control the carrier cooling process in metal halide perovskites to optimize optoelectronic devices.

The dominant role of strong electron-electron interactions in driving two-dimensional metal-insulator transitions has long been debated, but its clear experimental demonstration is still not available. Here, we examine the finite-temperature transport behavior of few-layered ${\mathrm{MoS}}_2$ material in the vicinity of the density-driven metal-insulator transition, revealing previously overlooked universal features characteristic of strongly correlated electron systems. Our scaling analysis, based on the Wigner-Mott theoretical viewpoint, conclusively demonstrates that the transition is driven by strong electron-electron interactions and not disorder, in striking resemblance to what is seen in other Mott systems. Our results provide compelling evidence that transition-metal dichalcogenides provide an ideal testing ground, and should open an exciting avenue for the study of strong correlation physics.

${\text{GaTa}}_4{\text{Se}}_8$ belongs to the lacunar spinel family and there have been intensive studies on its novel properties, such as its possible Mott-insulator state and superconductivity under pressure. However, its crystal structure and phase transition are still not well known. In this work, we investigated three different crystal structures, proposed in experiments, using first-principles calculations. For the cubic phase with space group $F\overline{4}3m$, its phonon spectra have three soft modes in the whole Brillouin zone, indicating the strong dynamical instability. The second one is the trigonal phase with space group $R3m$, which has been proposed based on Raman spectra under high pressure. This phase can be deduced from the soft phonon modes at $\mathrm{\Gamma }$ of the cubic phase and it is dynamically stable according to its phonon spectra. The third one is the tetragonal phase with space group $P\overline{4}{2}_{1}m$, which is also stable according to its phonon spectra and may be the low-temperature phase from x-ray diffraction. Within local density approximation calculations, the cubic and trigonal phases are metals, while the tetragonal phase is a band insulator consistent with the insulating feature in experiments. Our results suggest the possibility of the non-Mott state of ${\text{GaTa}}_4{\text{Se}}_8$ at low temperature and ambient pressure as a result of lattice distortion. On the other hand, the electronic structure of the trigonal phase can be viewed as a single-band Hubbard model. The Mott-insulator state has been obtained within dynamical mean-field theory calculation when the interaction parameter $U$ is larger than 0.40 eV vs a bandwidth of 0.25 eV. We hope these findings would be helpful in solving the long-standing problem of the ambiguity in the structural phase of ${\text{GaTa}}_4{\text{Se}}_8$.

Acousticlike excitations in topologically disordered media at mesocale/nanoscale present anomalous features with respect to the Debye's theory. The so-called Rayleigh scattering manifests in a strong increase of the attenuation of the acousticlike excitations and a softening of the phase velocity with respect to its continuum limit value. Mean field models developed in the random media theory framework can successfully predict the occurrence, at the proper length scale, of Rayleigh scattering. The overall attenuation in the Rayleigh region is, however, underestimated. In the framework of random media theory we developed an analytical model, which permits a quantitative description of the acousticlike excitations in topological glasses in the whole first pseudo-Brillouin zone. The underestimation of the Rayleigh scattering is avoided and, importantly, the model allows to account also for the polarization properties of the acousticlike excitations. In a three-dimensional medium an acoustic wave is characterized by its phase velocity, intensity, and polarization. Rayleigh scattering emphasizes how the topological disorder affects the first two properties. The topological disorder is, however, expected to influence also the third one. In common with the Rayleigh scattering, hallmarks possibly related to the mixing of polarizations have been traced in different classes of amorphous solids at nanoscale. The quantitative theoretical approach developed permits to demonstrate how the mixing of polarizations generates a distinctive feature in the dynamic structure factor of amorphous solids. The modeling capability of the proposed mean field theory is tested on glassy 1-octyl-3-methylimidazolium chloride, whose spatial distribution of the elastic moduli is well assessed and can be experimentally characterized. Contrast between theoretical and experimental features for the selected glass reveals an excellent agreement. The mean field approach we present retains a certain degree of generality and can be possibly extended to different stochastic media or different wave fields.

Bloch lines (BLs) play a key role in determining the static and dynamic behavior of chiral domain walls and skyrmions in multilayer films with a significant Dzyaloshinskii-Moriya interaction (DMI). Here, using in situ Lorentz phase microscopy, we reveal a type of spin texture termed a type-II domain-wall pair (DWP), stabilized in DMI multilayer thin films with BLs. For a type-II DWP, the Bloch components of each complementary domain wall are parallel, implying opposite chirality. We find that type-II DWPs preferentially form through bifurcation of a type-I DWP and require the formation of at least two BLs. We demonstrate a distinct phase jump associated with type-II DWPs, and further we reveal the role they play in the formation of mixed-character skyrmions where the Bloch component can be of either left- or right-handed chirality.

Metallic hydrogen is expected to exhibit remarkable physics. However, the pressures at which it is expected to be stable are extremely high relative to current experimental capabilities (static conditions). For practical (and terrestrial) significance, a key question is therefore whether it is metastable. In this work, this possibility is investigated, using first-principles calculations. Particular attention is given to the atomic body-centered tetragonal structures, predicted as being representative of the lowest-pressure stable metallic phase. The results show that, of these, the Cs-IV structure is metastable upon decompression, from molecular dissociation (expected to occur just below 500 GPa) to approximately 250 GPa. Results for structure prediction for metallic phases at lower pressures are also presented. Together, these results suggest that below 200 GPa, metallic hydrogen has no region of stability. All of the approximations used in the calculations are considered, and they are not expected to qualitatively affect the results. Atomic metallic hydrogen is therefore expected to have a (pressure) region of metastability, but the results strongly suggest that the metallic phase (more generally) is not so to zero pressure. These results are expected to provide important information for experiments.

Relaxor ferroelectrics are of great scientific and technological significance as they exhibit large and unusual responses to external stimuli. Their hallmark features are broadness and frequency dispersion of the peak in the temperature dependence of the dielectric constant. Both are believed to originate from the dynamics of polar nanoregions. We apply first-principles-based molecular dynamics to resolve the gigahertz electric response of dynamically poled $\mathrm{Ba}({\mathrm{Ti}}_{1-x},{\mathrm{Zr}}_x){\mathrm{O}}_3$ ferroelectric relaxors that remained overlooked so far. We find that (i) the hallmark relaxor features continue to persist even in the dynamically poled structures, but do not necessarily originate from the polar nanoregions dynamics; (ii) dynamically poled samples exhibit polarization aging which leads to the frequency dependence of both remnant polarization and the Curie temperature; (iii) “dynamical” nature of the latter naturally explains the frequency dependence of the dielectric susceptibility maximum in dynamically poled $\mathrm{Ba}({\mathrm{Ti}}_{1-x},{\mathrm{Zr}}_x){\mathrm{O}}_3$; and (iv) incorporation of the polarization aging into dielectric susceptibility expression explains its enhancing contribution in an intuitive way.

We study the transport properties and the spectral statistics of a one-dimensional closed quantum system of interacting spinless fermions in a quasiperiodic potential which produces a single-particle mobility edge in the absence of interaction. For such systems, it has been shown that the many-body eigenstates can be of three different kinds: extended and eigenstate thermalization hypothesis (ETH) obeying (thermal), localized and ETH violating (many body localized), and extended and ETH violating (nonergodic extended). Here we investigate the nonergodic extended phase from the point of view of level spacing statistics and charge transport. We calculate the dc conductivity and the low-frequency conductivity $\sigma \left(\omega \right)$ and show that both are consistent with subdiffusive transport. This is contrasted with diffusive transport in the thermal phase and blocked transport in the MBL phase.

CrSb is a layered antiferromagnet (AFM) with perpendicular magnetic anisotropy, a high Néel temperature, and large spin-orbit coupling (SOC), which makes it interesting for AFM spintronic applications. To elucidate the various mechanisms of Néel vector control, the effects of strain, band filling, and electric field on the magnetic anisotropy energy (MAE) of bulk and thin-film CrSb are determined and analyzed using density functional theory. The MAE of the bulk crystal is large (1.2 meV per unit cell). Due to the significant ionic nature of the Cr-Sb bond, finite slabs are strongly affected by end termination. Truncation of the bulk crystal to a thin film with one surface terminated with Cr and the other surface terminated with Sb breaks inversion symmetry, creates a large charge dipole and average electric field across the film, and breaks spin degeneracy, such that the thin film becomes a ferrimagnet. The MAE is reduced such that its sign can be switched with realistic strain, and the large SOC gives rise to an intrinsic voltage controlled magnetic anisotropy. A slab terminated on both faces with Cr remains a compensated AFM, but with the compensation occurring nonlocally between mirror-symmetric Cr pairs. In-plane alignment of the moments is preferred, the magnitude of the MAE remains large, similar to that of the bulk, and it is relatively insensitive to filling.

In this work, we discuss the nature of Dzyaloshinskii-Moriya interaction (DMI) in transition metal heterostructures. We first derive the expression of DMI in the small-spatial-gradient limit using Keldysh formalism. This derivation provides us with a Green's function formula that is well adapted to tight-binding Hamiltonians. With this tool, we first uncover the role of orbital mixing: Using both a toy model and a realistic multiorbital Hamiltonian representing transition metal heterostructures, we show that symmetry breaking enables the onset of interfacial orbital momentum that is at the origin of the DMI. We then investigate the contribution of the different layers to the DMI and reveal that it can expand over several nonmagnetic metal layers depending on the Fermi energy, thereby revealing the complex orbital texture of the band structure. Finally, we examine the thickness dependence of DMI on both ferromagnetic and nonmagnetic metal thicknesses and we find that whereas the former remains very weak, the latter can be substantial.