We report successive electric-polarization $\left(P\right)$ switches in the $S=1/2$ quantum magnet ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ by application of a magnetic field along the $b$ axis, where a 1/2-magnetization plateau is seen at 5.4–11.6 T. Polarization reversal appears at $\sim 5$ T from $-P\parallel b$ to $+P\parallel b$, leading to an irreversible magnetoelectric history involved with a memory effect, while the polarization flop in fields of 12–17 T is identified by a transition from the $P\parallel b$ to $P\parallel ac$ plane, different from those reported previously. These intriguing magnetoelectric phenomena are owing to the unique nature of the skew-chain-like magnetic structure of ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ and can be understood by a change in symmetry of the magnetic order in applied fields. The emergent ferroelectricities deviating from the half-plateau state may arise from magnon Bose-Einstein condensation in this quantum magnet.

In (Ga,Mn)As, a model dilute magnetic semiconductor, the electric and magnetic properties are strongly influenced by the lattice sites occupied by the Mn atoms. In particular, the highest Curie temperatures are achieved upon thermal annealing in a narrow temperature window around $200{}^{\circ }\mathrm{C}$, by promoting the diffusion of interstitial Mn towards the surface. In this work, we determined the thermal stability of both interstitial and substitutional Mn in ferromagnetic (Ga,Mn)As thin films, using the emission channeling technique. At a higher Mn concentration, the temperatures at which substitutional and interstitial Mn become mobile not only decrease, but also become closer to each other. These findings advance our understanding of self-compensation in (Ga,Mn)As by showing that the strong dependence of the Curie temperature on annealing temperature around $200{}^{\circ }\mathrm{C}$ is a consequence of balance between diffusion of interstitial Mn and segregation of substitutional Mn.

Magnetization and neutron scattering measurements have been carried out on an antiferromagnet ${\mathrm{Ba}}_2{\mathrm{CoSi}}_2{\mathrm{O}}_7$. The observed magnetic excitation is almost dispersionless, and the neutron intensity is only modulated along the [101] direction. The dispersionless magnetic excitation suggest that the ${\mathrm{Ba}}_2{\mathrm{CoSi}}_2{\mathrm{O}}_7$ system is a quasi-one-dimensional antiferromagnet. Classical spin-wave theory for a one-dimensional antiferromagnet can explain the dispersionless spin excitation. The magnetic structure determined by the measurement of the neutron powder diffraction is consistent with no observation of the multiferroic property in this system.

To gain a better understanding of the interplay between frustrated long-range interactions and zero-temperature quantum fluctuations, we investigate the ground-state phase diagram of the transverse-field Ising model with algebraically decaying long-range Ising interactions on quasi-one-dimensional infinite-cylinder triangular lattices. Technically, we apply various approaches including low- and high-field series expansions. For the classical long-range Ising model, we investigate cylinders with an arbitrary even circumference. We show the occurrence of gapped stripe-ordered phases emerging out of the infinitely degenerate nearest-neighbor Ising ground-state space on the two-dimensional triangular lattice. Further, while cylinders with circumferences 6, 10, 14, etc., are always in the same stripe phase for any decay exponent of the long-range Ising interaction, the family of cylinders with circumferences 4, 8, 12, etc., displays a phase transition between two different types of stripe structures. For the full long-range transverse-field Ising model, we concentrate on cylinders with circumference four and six. The ground-state phase diagram consists of several quantum phases in both cases including an $x$-polarized phase, stripe-ordered phases, and clock-ordered phases which emerge from an order-by-disorder scenario already present in the nearest-neighbor model. In addition, the generic presence of a potential intermediate gapless phase with algebraic correlations and associated Kosterlitz-Thouless transitions is discussed for both cylinders.

We present combined spin model and first-principles electronic-structure calculations to study the weak ferromagnetism in bulk ${\mathrm{Mn}}_{3}Z (Z=\mathrm{Sn},\mathrm{Ge},\mathrm{Ga})$ compounds. The spin-model parameters were determined from a spin-cluster expansion technique based on the relativistic disordered local moment formalism implemented in the screened Korringa–Kohn–Rostoker method. We describe the magnetic ground state of the system within a three-sublattice model and investigate the formation of the weak ferromagnetic states in terms of the relevant model parameters. First, we give a group-theoretical argument how the point-group symmetry of the lattice leads to the formation of weak ferromagnetic states. Then we study the ground states of the classical spin model and derive analytical expressions for the weak ferromagnetic distortions by recovering the main results of the group-theoretical analysis. As a third approach, we obtain the weak ferromagnetic ground states from self-consistent density-functional calculations and compare our results with previous first-principles calculations and with available experimental data. In particular, we demonstrate that the orbital moments follow a decomposition predicted by group theory. For a deeper understanding of the formation of weak ferromagnetism, we selectively trace the effect of the spin-orbit coupling at the Mn and $Z$ sites. In addition, for the case of ${\mathrm{Mn}}_3\mathrm{Ga}$, we gain information on the role of the induced moment of Ga from constrained local density-functional calculations.

Using low-temperature scanning laser microscopy (LTSLM), we have studied the cavity resonance behavior of a rectangular $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{CaC}{\mathrm{u}}_2{\mathrm{O}}_{8+\delta }$ mesa THz device containing $N=640$ stacked intrinsic Josephson junctions. Our results show that this microscopy technique is an effective means of mapping electromagnetic resonances in this type of device, and we present a detailed analysis of the mechanisms underlying contrast formation in scanning laser microscopy. The LTSLM images reveal that the THz excitation of the stacked junctions contains more than one mode with components along both the long and short axes of the mesa. Thermoluminescent mapping of the same device shows that the mesa temperature is uniform demonstrating that the features seen in LTSLM are electromagnetic in origin, not thermal. Our results also imply that for the purposes of maximizing the THz emission power from $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{CaC}{\mathrm{u}}_2{\mathrm{O}}_{8+\delta }$ mesas, it is important to design devices that are capable of being excited in a single, purely transverse mode.

We report the first-principles study on the H-intercalated Cr-based superconductor ${\mathrm{KCr}}_3{\mathrm{As}}_3{\mathrm{H}}_x$. Our results show a paramagnetic ground state for ${\mathrm{KCr}}_3{\mathrm{As}}_3\mathrm{H}$. The electronic structure consists of two quasi-one-dimensional (Q1D) Fermi surfaces and one three-dimensional Fermi surface which are mainly contributed by Cr-$d_{z^2}, d_{x^2-y^2}$, and $d_{xy}$ orbitals. The bare electron susceptibility shows a $\mathrm{\Gamma }$-centered imaginary peak, indicating possible ferromagnetic spin fluctuations. Upon moderate hole doping, the system undergoes a Lifshitz transition, which may enhance the Q1D feature of the system. The Bader charge analysis and electron localization functions reveal a strong bonding nature of hydrogen in ${\mathrm{KCr}}_3{\mathrm{As}}_3\mathrm{H}$, which results in a nontrivial electron doping in ${\mathrm{KCr}}_3{\mathrm{As}}_3\mathrm{H}$.

The one-dimensional configuration coordinate model (1D-CCM) is widely used for the analysis of photoluminescence in molecules and doped solids, and relies on a linear combination of the equilibrium nuclear configurations of ground and excited states. It delivers an estimation of the energy barrier at which ground and excited state curves cross, semiclassically linked to nonradiative transition rate and thermal quenching. To assess its predictive power for the latter properties, we propose a new optimized configuration path (OCP) method in which the ground-state and excited-state forces are mixed instead of their configurations. We also define another one-parameter model thanks to a double energy parabola hypothesis (DEPH). We compare the OCP method and the DEPH reference with the 1D-CCM for three paradigmatic $4f-5d$ phosphors ${\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$:Ce, ${\mathrm{Lu}}_2{\mathrm{SiO}}_5$:Ce, and ${\mathrm{YAlO}}_3$:Ce. We find that the OCP and DEPH methods yield similar results with geometries that have significantly lower ground-state energies than the 1D-CCM for the same $4f-5d$ energy difference. However, the OCP method suffers from the appearance of multiple local minima, rendering the clear determination of the optimal geometry very difficult in practice. Still the OCP method allows one to quantify the deviations from the 1D-CCM, therefore increasing confidence in the lower bound obtained from the DEPH for the $4f-5d$ crossing barrier, and its comparison with the energy of the autoionization thermal quenching mechanism. We expect the OCP approach to be applicable to other luminescent materials or molecules.

Using angle-resolved photoemission spectroscopy (ARPES) and resonant ARPES, we report evidence of strong anisotropic conduction-$f$ electron mixing ($c\text{−}f$ mixing) in CeBi by observing a largely expanded Ce $5d$ pocket at low temperature, with no change in the Bi $6p$ bands. The anisotropic Fermi surface (FS) expansion is accompanied by a pronounced spectral weight transfer from the local $4f^0$ peak of Ce (corresponding to $\mathrm{C}{\mathrm{e}}^{3+}$) to the itinerant conduction bands near the Fermi level. Careful analysis suggests that the observed large FS change (with a volume expansion of the electron pocket up to 40%) can most naturally be explained by a small valence change $\left(\sim 1\%\right)$ of Ce, which coexists with a very weak Kondo screening. Our work therefore provides evidence for a FS change driven by real charge fluctuations deep in the Kondo limit, which is highly dependent on the orbital character and momentum and is made possible by the low carrier density.

A characteristic feature of magnetic impurities in superconductors is the existence of a spin- and parity-changing quantum phase transition (known as “0-$\pi$” transition) which has been observed in scanning tunneling microscopy (STM) and quantum transport experiments. Using the Abelian bosonization technique, here we analyze the ground-state properties and the quantum phase diagram of an artificial “Shiba impurity” in a one-dimensional superconductor. Within the bosonization framework, the ground-state properties are determined by simple solitonlike solutions of the classical equations of motion of the bosonic fields, whose topological charge is related to the spin and fermion-parity quantum numbers. Interestingly, the same theoretical framework can be used in the case of a bulk superconductor, where previous results can be rederived in an elegant fashion. Our results indicate that the quantum phase diagram of a magnetic impurity in the superconductor can be strongly affected by geometrical and dimensional effects. Exploiting this fact, in the one-dimensional case we propose an experimental superconducting setup in which a novel parity-preserving, spin-changing “0-0” transition is predicted.

The quantized thermal Hall effect is an important probe for detecting chiral topological order and revealing the nature of chiral gapless edge states. The standard Kubo formula approach for the thermal Hall conductance ${\kappa }_{xy}$ based on the linear-response theory faces difficulties in practical application due to the lack of a reliable numerical method for calculating dynamical quantities in microscopic models at finite temperature. In this work, we propose an approach for calculating ${\kappa }_{xy}$ in two-dimensional lattice models displaying chiral topological order. Our approach targets the edge current localized at the boundary, which involves only thermal averages of local operators in equilibrium, thus drastically lowering the barrier for the calculation of ${\kappa }_{xy}$. We use the chiral $p$-wave superconductor (with and without disorder) and the Hofstadter model as benchmark examples to illustrate several sources of finite-size effects, and we suggest the infinite (or sufficiently long) strip as the best geometry for carrying out numerical simulations.

We report on high-pressure $p\le 45$ GPa resistivity measurements on the perovskite-related mixed-valent compound ${\mathrm{CsAuBr}}_3$. The compounds high-pressure resistivity can be classified into three regions: For low pressures ($p<10$ GPa) an insulator to metal transition is observed; between $p=10$ GPa and 14 GPa the room temperature resistivity goes through a minimum and increases again; and above $p=14$ GPa a semiconducting state is observed. From this pressure up to the highest pressure of $p=45$ GPa reached in this experiment, the room-temperature resistivity remains nearly constant. We find an extremely large resistivity reduction between ambient pressure and 10 GPa by more than six orders of magnitude. This decrease is among the largest reported changes in the resistivity for this narrow pressure regime. We show—by an analysis of the electronic band structure evolution of this material—that the large change in resistivity under pressure in not caused by a crossing of the bands at the Fermi level. We find that it instead stems from two bands that are pinned at the Fermi level and that are moving toward one another as a consequence of the mixed-valent to single-valent transition. This mechanism appears to be especially effective for the rapid buildup of the density of states at the Fermi level.

Metallic oxide ${\mathrm{SrVO}}_3$ represents a prototype system for the study of the mechanism behind thickness-induced metal-to-insulator transition (MIT) or crossover in thin films due to its simple cubic symmetry with one electron in the $3d$ state in the bulk. Here we report a deviation of chemical composition and distortion of lattice structure existing in the initial 3 unit cells of ${\mathrm{SrVO}}_3$ films grown on ${\mathrm{SrTiO}}_3$ (001) from its bulk form, which shows a direct correlation to the thickness-dependent MIT. In situ photoemission and scanning tunneling spectroscopy indicate a MIT at the critical thickness of ∼3 unit cells (u.c.), which coincides with the formation of a $(\surd 2\times \surd 2)R{45}^{\circ }$ surface reconstruction. However, atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy show depletion of Sr, and a change of V valence, thus implying the existence of a significant amount of oxygen vacancies in the 3 u.c. of ${\mathrm{SrVO}}_3$ near the interface. Transport and magnetotransport measurements further reveal that disorder, rather than electron correlations, is likely to be the main cause for the MIT in the ${\mathrm{SrVO}}_3$ ultrathin films.

We extend the methodology for calculating Auger recombination from first principles to include spin-orbit interaction, which allows us to study materials in which the spin-orbit splitting and the band gap are of similar magnitude. We use this methodology to compute the direct and indirect Auger coefficients in InAs and related alloys. The direct process involves only Coulomb interaction, while the indirect process is mediated by phonons. We show how the spin-orbit split-off states allow for efficient direct Auger recombination, greatly enhancing the excitation rate of Auger holes due to a near resonance between the spin-orbit splitting and the band gap of the material. We find that the direct hole-hole-electron Auger recombination coefficient decays exponentially with increasing band gap, while the electron-electron-hole Auger recombination coefficient decays faster than exponentially. Additionally, we modeled the phonon-assisted process in InAs and show that the direct Auger process is dominant.

The strong Coulomb forces in monolayer transition metal dichalcogenides ensure that optical excitation of band electrons gives rise to Wannier-Mott excitonic states, each of which can be conceptualized as a composite of a wave packet corresponding to center-of-mass motion and an orbital state corresponding to the motion of the electron and hole about the center-of-mass. Here we show that at low temperature in monolayer ${\mathrm{MoS}}_2$, given quasilocalized excitons and consequently a significant interexciton spacing, the excitons undergo dipole-dipole interaction and annihilate one another in a manner analogous to Auger recombination. To design our model, we assume that each exciton is localized in a region whose length is on the same scale as the excitonic diameter, thus causing the exciton to behave in a fermionlike manner, while the distance between neighboring excitons is much larger than the exciton diameter. We construct the orbital ladder operators for each exciton and apply Fermi's golden rule to derive the overall recombination rate as a function of exciton density.

The magnetotransport properties of disordered ferromagnetic kagome layers are investigated numerically. We show that a large domain-wall magnetoresistance or negative magnetoresistance can be realized in kagome layered materials (e.g., ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$, ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, and ${\mathrm{Mn}}_3\mathrm{Sn}$), which show the quantum anomalous Hall effect. The kagome layers show a strong magnetic anisotropy and a large magnetoresistance depending on their magnetic texture. These domain-wall magnetoresistances are expected to be robust against disorder and observed irrespective of the domain-wall thickness, in contrast to conventional domain-wall magnetoresistance in ferromagnetic metals.

We report $\beta$-detected NMR of ion-implanted ${}^8\mathrm{Li}$ in a single crystal and thin film of the strongly correlated metal ${\mathrm{LaNiO}}_3$. Spin-lattice relaxation measurements reveal two distinct local environments, both metallic as evident from $T$-linear Korringa $1/T_1$ below $200\mathrm{K}$ with slopes comparable to other metals. A small approximately temperature-independent Knight shift of $\sim 74\mathrm{ppm}$ is observed, yielding a normalized Korringa product characteristic of substantial antiferromagnetic correlations. We find no evidence for a magnetic transition from 4 to $310\mathrm{K}$. The similarity of these features in the two very different samples indicates that they are intrinsic and unrelated to dilute oxygen vacancies. We attribute the two environments to two distinct but similar crystallographic ${}^8\mathrm{Li}$ sites and not to any form of phase inhomogeneity, but this is inconsistent with the conventional rhombohedral structure of ${\mathrm{LaNiO}}_3$, and also cannot be simply explained by the common alternative orthorhombic or monoclinic distortions.

We present a generalized gradient approximation kinetic energy functional family with a simple rational form and nonempirical parameter derived from the semiclassical energy expansion of neutral atoms. The family unifies the Pauli-enhancement-factor-based semilocal noninteracting kinetic energy functionals (known as Pauli functionals) that achieve good, balanced accuracy with respect to metallic and semiconductor systems. We show that these functionals' performance can be understood in terms of the small-$s$ expansion, where $s$ is the reduced density gradient. We derive Pauli functionals parameters from the large-$Z$ kinetic energy limits of neutral atoms, which prevents overfitting to bulk systems. These results spotlight the current state of the art for semilocal kinetic energy functionals. For the next generation of functionals, including more constraints and variables, these results would allow to fix second-order coefficients nonempirically and concentrate on exploring next-order terms in the small-$s$ expansion.