We show that the $t_{2g}^2$ perovskite ${\mathrm{LaVO}}_3$, in its orthorhombic phase, is a rare case of a system hosting an orbital-ordering Kugel'-Khomskii phase transition rather than being controlled by the Coulomb-enhanced crystal-field splitting. We find that, as a consequence of this, the magnetic transition is close to (and even above) the superexchange driven orbital-ordering transition, whereas typically magnetism arises at much lower temperatures than orbital ordering. Our results support the experimental scenario of orbital ordering and $G$-type spin correlations just above the monoclinic-to-orthorhombic structural change. To explore the effects of crystal-field splitting and filling, we compare to ${\mathrm{YVO}}_3$ and $t_{2g}^1$ titanates. In all these materials the crystal field is sufficiently large to suppress the Kugel'-Khomskii phase transition.

Given a critical quantum spin chain with a microscopic Lie group symmetry, corresponding, e.g., to $U\left(1\right)$ or $SU\left(2\right)$ spin isotropy, we numerically investigate the emergence of Kac-Moody symmetry at low energies and long distances. In that regime, one such critical quantum spin chain is described by a conformal field theory where the usual Virasoro algebra associated with conformal invariance is augmented with a Kac-Moody algebra associated with conserved currents. Specifically, we first propose a method to construct lattice operators corresponding to the Kac-Moody generators. We then numerically show that, when projected onto low-energy states of the quantum spin chain, these operators indeed approximately fulfill the Kac-Moody algebra. The lattice version of the Kac-Moody generators allows us to compute the so-called level constant and to organize the low-energy eigenstates of the lattice Hamiltonian into Kac-Moody towers. We illustrate the proposal with the XXZ model and the Heisenberg model with a next-to-nearest-neighbor coupling.

Historically, the transfer of ideas and concepts between optics and condensed-matter physics has been flourishing. Inspired by shaping the scattering of electromagnetic waves by dielectric nanoparticle clusters through the interference of excited multipole modes, we address shaping the scattering of matter waves by quantum dot clusters embedded in a host semiconductor through the interference of excited partial waves. We theoretically demonstrate nearly complete suppression of backward, forward, and both backward and forward scattering, i.e., three versions of the generalized Kerker scattering of matter waves. We envisage that thermoelectric properties of semiconductors can be improved by incorporation of quantum dot clusters exhibiting ultradirectional forward scattering: Such clusters act as phonon-blocking/electron-transmitting inclusions.

Optical and transport properties of doped monolayer semiconductors are dominated by trions, which are three-particle compounds formed by two electrons and one hole or vice versa. In this work, we investigate the trion-phonon interaction on a microscopic footing and apply our model to the exemplary case of a molybdenum diselenide (${\mathrm{MoSe}}_2$) monolayer. We determine the trion series of states and their internal quantum structure by solving the trion Schrödinger equation. Transforming the system into a trion basis and solving equations of motion, including the trion-phonon interaction within the second-order Born-Markov approximation, provides a microscopic access to the trion dynamics. In particular, we investigate trion propagation and compute the diffusion coefficient and mobility. In the low density limit, we find that trions propagate less efficiently than excitons and electrons due to their stronger coupling with phonons and their larger mass. For increasing densities, we predict a drastic enhancement of diffusion caused by the build-up of a large pressure by the degenerate trion gas, which is a direct consequence of the fermionic character of trions. Our work provides microscopic insights into the trion-phonon interaction and its impact on trion transport in atomically thin semiconductors.

A theoretical derivation of two-photon absorption (2PA) from semiconductors, based on the length gauge analysis and the electron density operator, is formulated; the intraband ${\mathit{r}}_i$ part and the interband ${\mathit{r}}_e$ part of the position operator $\mathit{r}$ are properly accounted for. Within the independent-particle approximation, the nonlinear third-order susceptibility tensor ${\chi }^{\mathrm{a}\mathrm{b}\mathrm{c}\mathrm{d}}(-\omega ;-\omega ,-\omega ,\omega )$ and the two-photon absorption coefficient are calculated, including the scissors correction needed to correct the well-known underestimation of the local-density-approximation band gap. Using time-reversal symmetry, it is shown that the expression for ${\chi }^{\mathrm{a}\mathrm{b}\mathrm{c}\mathrm{d}}(-\omega ;-\omega ,-\omega ,\omega )$ is finite at $\omega =0$, avoiding nonphysical divergences presented in previous calculations when $\omega \to 0$. Ab initio band structure calculations using different pseudopotential schemes that include spin-orbit coupling are used to calculate the 2PA for several semiconductors, and the calculations are compared with available experimental results for photon energies that are below the optical band gap.

We theoretically investigate exciton-polaritons in a two-dimensional (2D) semiconductor heterostructure, where a static magnetic field is applied perpendicular to the plane. To explore the interplay between the magnetic field and strong light-matter coupling, we employ a fully microscopic theory that explicitly incorporates electrons, holes, and photons in a semiconductor microcavity. Furthermore, we exploit a mapping between the 2D harmonic oscillator and the 2D hydrogen atom that allows us to efficiently solve the problem numerically for the entire Rydberg series as well as for the ground-state exciton. In contrast to previous approaches, we can readily obtain the real-space exciton wave functions and we show how they shrink in size with the increasing magnetic field, which mirrors their increasing interaction energy and oscillator strength. We compare our theory with recent experiments on exciton-polaritons in GaAs heterostructures in an external magnetic field and we find excellent agreement with the measured polariton energies. Crucially, we are able to capture the observed light-induced changes to the exciton in the regime of very strong light-matter coupling where a perturbative coupled oscillator description breaks down. Our work can guide future experimental efforts to engineer and control Rydberg excitons and exciton-polaritons in a range of 2D materials.

We study the dielectric response of few layered crystals of various transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN). We showed that the out-of-plane polarizability of a multilayer crystal (which characterizes response to the external displacement field) scales linearly with the number of layers, ${\alpha }_{zz}^{NL}=N{\alpha }_{zz}^{1L}$, independently of the stacking configuration in the film. We also established additivity of ferroelectric polarizations of consecutive interfaces in case when such interfaces have broken inversion symmetry. Then we used the obtained data of monolayer ${\alpha }_{zz}^{1L}$ to calculate the values of the dielectric susceptibilities for semiconductor TMDs and hBN bulk crystals.

While it is established that the pinch point scattering pattern in spin ice arises from an emergent Coulomb phase associated with a magnetic moment that is divergence free, more complex Hamiltonians can introduce a divergence-full part. If these two parts remain decoupled, they give rise to the coexistence of distinct features. Here, we show that the moment in ${\mathrm{Nd}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$ forms a static long-range ordered ground state, a flat, gapped pinch point excitation, and dispersive excitations. These results confirm recent theories which predict that the dispersive modes, which arise from the divergence-full moment, host a pinch point pattern of their own, observed experimentally as “half-moons.”

We consider the temperature-induced melting of a Wigner solid in one-dimensional (1D) and two-dimensional (2D) lattices of electrons interacting via the long-range Coulomb interaction in the presence of strong disorder arising from charged impurities in the system. The system simulates semiconductor-based 2D electron layers where Wigner crystallization is often claimed to be observed experimentally. Using exact diagonalization and utilizing the inverse participation ratio as well as conductance to distinguish between the localized insulating solid phase and the extended metallic liquid phase, we find that the effective melting temperature may be strongly enhanced by disorder since the disordered crystal typically could be in a localized glassy state incorporating the combined nonperturbative physics of both Anderson localization and Wigner crystallization. This disorder-induced enhancement of the melting temperature may explain why experiments often manage to observe insulating disorder-pinned Wigner solids in spite of the experimental temperature being decisively far above the theoretical melting temperature of the pristine Wigner crystal phase in many cases.

The newly synthesized ${\mathrm{Tm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ iridate is investigated using neutron scattering techniques. Powder neutron diffraction patterns confirm that ${\mathrm{Tm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ crystallizes in a cubic structure of pyrochlore type, consistent with the rest of the rare-earth $A_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ series, and the crystal structure is preserved down to 0.2 K. The thermal expansion of the crystal lattice is well described by a Debye model of lattice vibrations. A weak magnetic signal consistent with the so-called all-in-all-out magnetic order (AIAO) is traced at low temperature; however, it cannot serve as unambiguous evidence of the AIAO structure in the compound. Nondispersive magnetic excitations in energy-momentum space are observed in inelastic neutron scattering spectra of ${\mathrm{Tm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. The crystal-field scheme and parameters of the thulium iridate are refined and discussed with respect to the specific heat and magnetization data. The results are supported by mean-field calculations.

We have carried out a detailed high pressure investigation using x-ray diffraction, Raman spectroscopy, and low temperature resistivity measurements on 1T ${\mathrm{ZrSe}}_2$ having an excess of 3 at.% Zr. A careful x-ray diffraction data analysis shows that the sample undergoes a gradual structural transition to a monoclinic structure, with a mixed phase in the pressure range of 5.9 GPa to 14.8 GPa. We observe a minimum in the $c/a$ ratio as well as in the full width at half maximum of the $A_{1g}$ Raman mode at the same pressure in the parent 1T phase, which indicates an electronic phase transition. The sample shows a metallic characteristic in its low temperature resistivity data at ambient pressure, which persist until about 5.1 GPa and can be related to the presence of slight excess Zr. At and above 7.3 GPa, the sample shows a metal to semiconductor transition with the opening of a very small band gap, which increases with pressure. The low temperature resistivity data show an upturn, which flattens with the increase in pressure. The phenomenological analysis of the low temperature resistivity data indicates the presence of the Kondo effect in the sample, which may be due to the excess Zr.

We analyze the thermal fluctuations of magnetization textures in two stray field coupled elements, forming mesospins. To this end, the energy landscape associated with the thermal dynamics of the textures is mapped out, and asymmetric energy barriers are identified. These barriers are modified by changing the gap that separates the mesospins. Moreover, the coupling between the edges leads to an anisotropy in the curvature of the energy surface at the metastable minima. This yields a dynamic mode splitting of the edge modes and affects the attempt switching frequencies. Thus, we elucidate the mechanism with which the magnons in the thermal bath generate the stochastic fluctuations of the magnetization at the edges.

Understanding the extreme sensitivity of the eigenvalues of non-Hermitian Hamiltonians to the boundary conditions is of great importance when analyzing non-Hermitian systems, as it appears generically and is intimately connected to the skin effect and the breakdown of the conventional bulk boundary correspondence. Here we describe a method to find the eigenvalues of one-dimensional one-band models with arbitrary boundary conditions. We use this method on several systems to find analytical expressions for the eigenvalues, which give us conditions on the parameter values in the system for when we can expect the spectrum to be insensitive to a change in boundary conditions. By stacking one-dimensional chains, we use the derived results to find corresponding conditions for insensitivity for some two-dimensional systems with periodic boundary conditions in one direction. This would be hard by using other methods to detect skin effect, such as the winding of the determinant of the Bloch Hamiltonian. Finally, we use these results to make predictions about the (dis)appearance of the skin effect in purely two-dimensional systems with open boundary conditions in both directions.

We study the transverse electrical conductivity and the corresponding magnetic noise of a two-dimensional U(1) spin liquid state with a spinon Fermi surface. We show that in the quasistatic regime these responses have the same wave-vector dependence as that of a metal but are reduced by a dimensionless prefactor controlled by the ratio of orbital diamagnetic susceptibilities of the spinons and chargons, correcting previous work. We estimate that this quasistatic regime is comfortably accessed by the typical nitrogen vacancy (NV) center splittings of a few gigahertz and estimate that the expected $T_1$ times for an NV center placed above candidate materials, such as organic dmit and ET salts and monolayer $1T\text{−}{\mathrm{TaS}}_2/{\mathrm{Se}}_2$, would range from several tens to a few hundred milliseconds.

We present a comparative study of light-induced topological phases in two-dimensional massive Dirac models with and without quadratic corrections, which depicts different inversion symmetry-breaking Dirac materials. We find that a light has two effects on the Floquet band topology: it not only inverts the massive bands near the Dirac points but also introduces pseudospin-orbital coupling around the resonant points of light-matter interaction. Depending on the presence or absence of the quadratic corrections, the interplay of these two effects of light results in versatile topological phases with dissipationless valley polarized edge currents. In particular, without quadratic correction as exemplified by gapped graphene, a circularly polarized light induces a pair of in-gap edge states at each valley, which propagate on the same direction at a fixed boundary. A linearly polarized light also induces a pair of in-gap edge states, while propagating on the opposite direction at a fixed boundary. When including the quadratic correction as exemplified by monolayer transition-metal dichalcogenides, a circularly polarized light can induce two pairs of in-gap edge states located at a single valley, while a linearly polarized light does not induce any in-gap edge state. These edge currents as controlled by switching on/off the applied light and its polarization nature suggest an optical way to engineer valleytronic devices.

A kagome magnet usually hosts nontrivial electronic or magnetic states, drawing great interest in condensed matter physics. In this paper, we report a systematic study on the transport properties of the kagome magnet ${\mathrm{TmMn}}_6{\mathrm{Sn}}_6$. The prominent topological Hall effect (THE) has been observed in a wide temperature region spanning over several magnetic phases and exhibiting strong temperature and field dependence. This phenomenon, due to nonzero spin chirality, indicates the possible appearance of nontrivial magnetic states accompanying by strong fluctuations. The planar applied field drives the planar Hall effect and anisotropic magnetoresistivity, exhibiting sharp disconnections in angular-dependent planar resistivity violating the empirical law. By using an effective field, we identify a magnetic transition separating the planar anisotropic magnetoresistivity into two groups belonging to various magnetic states. We extended the empirical formula to scale the field- and temperature-dependent planar magnetoresistivity and provide an understanding of the planar transport behaviors with the crossover between various magnetic states. Our results shed light on the transport effects in the presence of multiple nontrivial magnetic states for a kagome lattice with complicated magnetic structures.

Motivated by the $\mathrm{\Omega }$-spectrum proposal of unique gapped ground states by Kitaev, we study adiabatic cycles in gapped quantum spin systems from various perspectives. We give a few exactly solvable models in one and two spatial dimensions and discuss how nontrivial adiabatic cycles are detected. For one spatial dimension, we study the adiabatic cycle in detail with the matrix product state and show that the symmetry charge can act on the space of matrices without changing the physical states, which leads to nontrivial loops with symmetry charges. For generic spatial dimensions, based on the Bockstein isomorphism $H^d(G,U\left(1\right))\cong H^{d+1}(G,\mathbb{Z})$, we study a group cohomology model of the adiabatic cycle that pumps a symmetry-protected topological phase on the boundary by one period. It is shown that the spatial texture of the adiabatic Hamiltonian traps a symmetry-protected topological phase in one dimension lower.

In an outstanding experimental advance in the field of two-dimensional nanomaterials, cuprous iodide (CuI) and silver iodide (AgI) monolayers have been grown via a novel graphene encapsulation synthesis approach [K. Mustonen et al., Adv. Mater. 34, 2106922 (2022)]. Inspired by this accomplishment, we conduct first-principles calculations to investigate the elastic, phonon, and electron thermal transport, electronic, and optical properties of the non-Janus CuI and AgI and Janus ${\mathrm{Cu}}_2\mathrm{BrI}$ and ${\mathrm{Ag}}_2\mathrm{BrI}$ monolayers. Electronic and excitonic optical properties are elaborately studied using the many-body perturbation theory on the basis of $GW$ approximation. Our results indicate that these novel systems are stable but with soft elastic modulus and ultralow lattice thermal conductivity. It is also shown that the studied monolayers are wide-gap semiconductors with exciton binding energies close to 1 eV. The spin-orbit induced band splitting of Janus monolayers are increased more than $100\%$ under a uniaxial strain of $3\%$, and for non-Janus monolayers, a noticeable increase is observed under a perpendicular electric field. Thermoelectric efficiency of silver-based monolayers is higher than 1.2, making them promising candidates for next-generation thermoelectric devices. The presented first-principles results provide a deep understanding of the stability, thermal transport, and tunable optoelectronic properties of CuI, AgI, ${\mathrm{Cu}}_2\mathrm{BrI}$, and ${\mathrm{Ag}}_2\mathrm{BrI}$ monolayers, which can serve as a guide for the oncoming studies.