Current-driven domain wall motion is investigated experimentally in in-plane magnetized (Ga,Mn)As tracks. The wall dynamics is found to differ in two important ways with respect to perpendicularly magnetized (Ga,Mn)As: the wall mobilities are up to ten times higher and the walls move in the same direction as the hole current. We demonstrate that these observations cannot be explained by spin-orbit field torques (Rashba and Dresselhaus types) but are consistent with nonadiabatic spin transfer torque enhanced by the strong spin-orbit coupling of (Ga,Mn)As. This mechanism opens the way to domain wall motion driven by bulk rather than interfacial spin-orbit coupling as in ultrathin ferromagnet/heavy metal multilayers.

At high pressure, Mg is expected to transform to the body-centered-cubic (BCC) phase. We use density functional theory to explore the structure of $\langle 111\rangle$-type dislocation cores in BCC Mg as a function of pressure. As the pressure is reduced from the region of absolute stability for the BCC phase, the dislocation cores spread. When dislocation cores overlap the displacements of columns of atoms resemble the nanodisturbances observed in TiNb alloys known as gum metal. As the pressure is lowered further, these regions transform into the hexagonal close-packed phase. The ideal tensile strength of BCC Mg is also computed as a function of pressure. Despite its low shear modulus, BCC Mg is predicted to be intrinsically brittle at absolute zero.

FeRh and its first-order phase transition can open new routes for magnetic hybrid materials and devices under the assumption that it can be exploited in ultra-thin-film structures. Motivated by experimental measurements showing an unexpected increase in the phase transition temperature with decreasing thickness of FeRh on top of MgO, we develop a computational model to investigate strain effects of FeRh in such magnetic structures. Our theoretical results show that the presence of the MgO interface results in a strain that changes the magnetic configuration which drives the anomalous behavior.

Using Wannier functions to represent the density functional results we calculate the hybridization corrections to the orbital momentum operator in the Os $5d$ shell of the Mott insulators ${\mathrm{Ba}}_2{\mathrm{NaOsO}}_6$ and ${\mathrm{Ba}}_2{\mathrm{YOsO}}_6$. The $g$-factors are obtained by evaluating the spin and orbital momentum operators in the atomic ground states of the Os ion. While the hybridization corrections play a minor role in the $d^3$ ion of ${\mathrm{Ba}}_2{\mathrm{YOsO}}_6$ with a dominant spin moment, they are instrumental for the observation of the nonzero $g$-factor of the $d^1$ ions of ${\mathrm{Ba}}_2{\mathrm{NaOsO}}_6$. In addition, we analyze the exchange interactions in ${\mathrm{Ba}}_2{\mathrm{YOsO}}_6$ and find them to be consistent with the reported magnetic structure.

We perform self-consistent studies of two-dimensional (2D) $s$-wave topological superconductivity (TSC) with Rashba spin-orbit coupling and Zeeman field by solving the Bogoliubov-de Gennes equations. In particular, we examine the effects of a nonmagnetic impurity in detail and show that the nature of the spin-polarized midgap bound state varies significantly depending on the material parameters. Most notably, a nonmagnetic impurity in a 2D $s$-wave topological superconductor can act like a magnetic impurity in a conventional $s$-wave superconductor, leading to phase transitions of the ground state as the impurity potential is varied. Furthermore, by solving for the spin-dependent Hartree potential self-consistently along with the superconducting order parameter, we demonstrate that topological charge density waves can coexist with TSC at half filling just as in a conventional $s$-wave superconductor.

We derive an effective $Z_2$ gauge theory to describe the quantum kagome ice (QKI) state that has been observed by Carrasquilla et al. [Nat. Commun. 6, 7421 (2015)] in Monte Carlo studies of the $S=\frac{1}{2}$ kagome XYZ model in a Zeeman field. The numerical results on QKI are consistent with, but do not confirm or rule out, the hypothesis that it is a $Z_2$ spin liquid. Our effective theory allows us to explore this hypothesis and make a striking prediction for future numerical studies, namely, that symmetry-protected vison zero modes arise at lattice disclination defects, leading to a Curie defect term in the spin susceptibility, and a characteristic $(N_{\mathrm{dis}}-1)ln2$ contribution to the entropy, where $N_{\mathrm{dis}}$ is the number of disclinations. Only the $Z_2$ Ising symmetry is required to protect the vison zero modes. This is remarkable because a unitary $Z_2$ symmetry cannot be responsible for symmetry-protected degeneracies of local degrees of freedom. We also discuss other signatures of symmetry fractionalization in the $Z_2$ spin liquid, and phase transitions out of the $Z_2$ spin liquid to nearby ordered phases.

The quantum phase transition between two clean, noninteracting topologically distinct gapped states in three dimensions is governed by a massless Dirac fermion fixed point, irrespective of the underlying symmetry class, and this constitutes a remarkably simple example of superuniversality. For a sufficiently weak disorder strength, we show that the massless Dirac fixed point is at the heart of the robustness of superuniversality. We establish this by considering both perturbative and nonperturbative effects of disorder. The superuniversality breaks down at a critical strength of disorder, beyond which the topologically distinct localized phases become separated by a delocalized diffusive phase. In the global phase diagram, the disorder controlled fixed point where superuniversality is lost, serves as a multicritical point, where the delocalized diffusive and two topologically distinct localized phases meet and the nature of the localization-delocalization transition depends on the underlying symmetry class. Based on these features, we construct the global phase diagrams of noninteracting, dirty topological systems in three dimensions. We also establish a similar structure of the phase diagram and the superuniversality for weak disorder in higher spatial dimensions. By noting that $1/r^2$ power-law correlated disorder acts as a marginal perturbation for massless Dirac fermions in any spatial dimension $d$, we have established a general renormalization group framework for addressing disorder driven critical phenomena for fixed spatial dimension $d>2$.

We present a first-principles framework to investigate the electron scattering channels and transport properties for polar materials by combining the exact solution of the linearized electron-phonon (e-ph) Boltzmann transport equation in its integral-differential form associated with the e-ph coupling matrices obtained from the polar Wannier interpolation scheme. No ad hoc parameter is required throughout this calculation, and GaAs, a well-studied polar material, is used as an example to demonstrate this method. In this work, the long-range and short-range contributions as well as the intravalley and intervalley transitions in the e-ph interactions (EPIs) have been quantitatively addressed. Promoted by such mode-by-mode analysis, we find that in GaAs, the piezoelectric scattering is comparable to deformation-potential scattering for electron scatterings by acoustic phonons in EPI even at room temperature, and it makes a significant contribution to mobility. Furthermore, we achieved good agreement with experimental data for the mobility, and we identified that electrons with mean free paths between 130 and 210 nm provide the dominant contribution to the electron transport at 300 K. Such information provides a deeper understanding of the electron transport in GaAs, and the presented framework can be readily applied to other polar materials.

Electronic and topological properties of $\mathrm{Mo}{\mathrm{S}}_2$ monolayers endowed with $3d$ transition metal (TM) adatoms (V-Fe) are explored by using ab initio methods and $k·p$ models. Without the consideration of the Hubbard $U$ interaction, the V, Cr, and Fe adatoms tend to locate on the top of the Mo atoms, while the most stable site for the Mn atom is at the hollow position of the Mo-S hexagon. After the Hubbard $U$ is applied, the most stable sites of all the systems become the top of the Mo atoms. Chern insulators without band inversion are achieved in these systems. The V and Fe adsorption systems are the best candidates to produce the topological states. The $k·p$ model calculations indicate that these topological states are determined by the TM magnetism, the $C_{3v}$ crystal field from the $\mathrm{Mo}{\mathrm{S}}_2$ substrate, and the TM atomic spin-orbit coupling (SOC). The special two-meron pseudospin texture is found to contribute to the topology. The apparent difference between the Berry curvatures for the V and Fe adsorption systems is also explored. Our results widen the understanding of the Chern insulators and are helpful for the applications of the $\mathrm{Mo}{\mathrm{S}}_2$ monolayers in the future electronics and spintronics.

We analyze the nature of a Mott metal-insulator transition in multiorbital systems using dynamical mean-field theory. The auxiliary multiorbital quantum impurity problem is solved using continuous-time quantum Monte Carlo and the rotationally invariant slave-boson (RISB) mean-field approximation. We focus our analysis on the Kanamori Hamiltonian and find that there are two markedly different regimes determined by the nature of the lowest-energy excitations of the atomic Hamiltonian. The RISB results at $T\to 0$ suggest the following rule of thumb for the order of the transition at zero temperature: a second-order transition is to be expected if the lowest-lying excitations of the atomic Hamiltonian are charge excitations, while the transition tends to be first order if the lowest-lying excitations are in the same charge sector as the atomic ground state. At finite temperatures, the transition is first order and its strength, as measured, e.g., by the jump in the quasiparticle weight at the transition, is stronger in the parameter regime where the RISB method predicts a first-order transition at zero temperature. Interestingly, these results seem to apply to a wide variety of models and parameter regimes.

Strong electronic interactions and spin-orbit coupling can be conducive for realizing novel broken symmetry phases supporting quasiparticles with nontrivial band topology. 227 pyrochlore iridates provide a suitable material platform for studying such emergent phenomena where both topology and competing orders play important roles. In contrast to the most members of this material class, which are thought to display “all-in–all-out” (AIAO) type magnetically ordered low-temperature insulating ground states, ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ remains metallic while exhibiting “spin-ice” (SI) correlations at low temperatures. Additionally, this is the only 227 iridate compound, which exhibits a large anomalous Hall effect (AHE) along the [1,1,1] direction below 1.5 K, without possessing any measurable magnetic moment. By focusing on the normal state of 227 iridates, described by a parabolic semimetal with quadratic band touching, we use renormalization group analysis, mean-field theory, and phenomenological Landau theory as three complementary methods to construct a global phase diagram in the presence of generic local interactions among itinerant electrons of Ir ions. While the global phase diagram supports several competing multipolar orders, motivated by the phenomenology of 227 iridates we particularly emphasize the competition between AIAO and SI orders and how it can cause a mixed phase with “three-in–one-out” (3I1O) spin configurations. In terms of topological properties of Weyl quasiparticles of the 3I1O state, we provide an explanation for the magnitude and the direction of the observed AHE in ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. We propose a strain-induced enhancement of the onset temperature for AHE in thin films of ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ and additional experiments for studying competing orders in the vicinity of the metal-insulator transition. In addition to providing a theory for competing orders and magnetic properties of ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, the theoretical framework developed in this work should also be useful for a better understanding of competing multipolar orders in other correlated materials.

We study how the non-Fermi-liquid nature of the overscreened multichannel Kondo impurity model affects the response to a BCS pairing term that, in the absence of the impurity, opens a gap $\mathrm{\Delta }$. We find that the low-energy spectrum in the limit $\mathrm{\Delta }\to 0$ actually does not correspond to the spectrum strictly at $\mathrm{\Delta }=0$. In particular, in the two-channel Kondo model, the $\mathrm{\Delta }\to 0$ ground state is an orbitally degenerate spin singlet, while it is an orbital singlet with a residual spin degeneracy at $\mathrm{\Delta }=0$. In addition, there are fractionalized spin-1/2 subgap excitations whose energy in units of $\mathrm{\Delta }$ tends toward a finite and universal value when $\mathrm{\Delta }\to 0$, as if the universality of the anomalous power-law exponents that characterize the overscreened Kondo effect turned into universal energy ratios when the scale invariance is broken by $\mathrm{\Delta }\ne 0$. This intriguing phenomenon can be explained by the renormalization flow toward the overscreened fixed point and the gap cutting off the orthogonality catastrophe singularities. We also find other non-Fermi-liquid features at finite $\mathrm{\Delta }$: the local density of states lacks coherence peaks, the states in the continuum above the gap are unconventional, and the boundary entropy is a nonmonotonic function of temperature. The persistent subgap excitations are characteristic of the non-Fermi-liquid fixed point of the model, and thus depend on the impurity spin and the number of screening channels.

Recent progress in the field of molecular electronics has revealed the fundamental importance of the coupling between the electronic degrees of freedom and specific vibrational modes. Considering the examples of a molecular dimer and a carbon nanotube double quantum dot, we here theoretically investigate transport through a two-site system that is strongly coupled to a single vibrational mode. Using a quantum master equation approach, we demonstrate that, depending on the relative positions of the two dots, electron-phonon interactions can lead to negative differential conductance and suppression of the current through the system. We also discuss the experimental relevance of the presented results and possible implementations of the studied system.

While first-principles density functional theory (DFT)-based models have been effective in capturing the physics of ferroelectric phase transitions in ${\mathrm{BaTiO}}_3, {\mathrm{PbTiO}}_3$, and ${\mathrm{KNbO}}_3$, quantitative estimates of the transition temperatures ($T_C$) suffer from errors that are believed to originate from the errors in estimating lattice constants obtained within the local density approximation (LDA) and generalized gradient approximation (GGA) of DFT. The recently developed strongly constrained and appropriately normed (SCAN) meta-GGA functional has been shown to be quite accurate in the estimation of lattice constants. Here, we present a quantitative analysis of the estimates of ferroelectric ground-state properties of eight perovskite oxides and transition temperatures of ${\mathrm{BaTiO}}_3, {\mathrm{PbTiO}}_3$, and ${\mathrm{KNbO}}_3$ obtained with molecular dynamics simulations using an effective Hamiltonian derived from the SCAN meta-GGA-based DFT. Relative to LDA, we find an improvement in the estimates of $T_C$, which arises from the changes in the calculated strain-phonon, anharmonic coupling constants, and strength of ferroelectric instabilities, i.e., frequencies of the soft modes. We also assess the errors in $T_C$ originating from approximately integrating out the high-energy phonons during construction of the model Hamiltonian through estimates of the effects of fourth-order couplings between the soft mode and higher-energy modes of ${\mathrm{BaTiO}}_3, {\mathrm{PbTiO}}_3$, and ${\mathrm{KNbO}}_3$. We find that inclusion of these anharmonic couplings results in deeper double-well energy functions of ferroelectric distortions and further improvement in the estimates of transition temperatures. Consistently improved estimates of lattice constants and transition temperatures with the SCAN meta-GGA calculations augur well for their use in simulations of superlattices or heterostructures of perovskite oxides, in which the effects of lattice matching are critical.

We analyze the topological deformations of the ground state manifold of a quantum spin-1/2 in a magnetic field $\mathit{H}=H(sin\theta cos\varphi ,sin\theta sin\varphi ,cos\theta )$ induced by a coupling to an ohmic quantum dissipative environment at zero temperature. From Bethe ansatz results and a variational approach, we confirm that the Chern number associated with the geometry of the reduced spin ground state manifold is preserved in the delocalized phase for $\alpha <1$. We report a divergence of the Berry curvature at ${\alpha }_c=1$ for magnetic fields aligned along the equator $\theta =\pi /2$. This divergence is caused by the complete quenching of the transverse magnetic field by the bath associated with a gap closing that occurs at the localization Kosterlitz-Thouless quantum phase transition in this model. Recent experiments in quantum circuits have engineered nonequilibrium protocols to access topological properties from a measurement of a dynamical Chern number defined via the out-of-equilibrium spin expectation values. Applying a numerically exact stochastic Schrödinger approach we find that, for a fixed field sweep velocity $\theta \left(t\right)=vt$, the bath induces a crossover from (quasi)adiabatic to nonadiabatic dynamical behavior when the spin bath coupling $\alpha$ increases. We also investigate the particular regime $H/{\omega }_c\ll v/H\ll 1$ with large bath cutoff frequency ${\omega }_c$, where the dynamical Chern number vanishes already at $\alpha =1/2$. In this regime, the mapping to an interacting resonance level model enables us to analytically describe the behavior of the dynamical Chern number in the vicinity of $\alpha =1/2$. We further provide an intuitive physical explanation of the bath-induced breakdown of adiabaticity in analogy to the Faraday effect in electromagnetism. We demonstrate that the driving of the spin leads to the production of a large number of bosonic excitations in the bath, which strongly affect the spin dynamics. Finally, we quantify the spin-bath entanglement and formulate an analogy with an effective model at thermal equilibrium.

We took ${}^{31}\text{P}$ NMR measurements of mainly paramagnetic phase ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2$ ($0\le x\le 0.5$) to reveal the itinerant-electron metamagnetic transition, and of its magnetically ordered phase ($0.7\le x\le 1$), and characterized their spin fluctuations by estimating the spin fluctuation parameter $T_0$ corresponding to the width of the spin fluctuation in the spectrum in frequency space. ${\mathrm{SrCo}}_2{\mathrm{P}}_2$ has a quasi-two-dimensional uncollapsed tetragonal (ucT) cell without interlayer P-P bonds, whereas ${\mathrm{CaCo}}_2{\mathrm{P}}_2$ has a three-dimensional collapsed tetragonal (cT) cell with P-P bonds. The $ab$-in-plane component of $T_0$ is much larger than the out-of-plane component in ${\mathrm{SrCo}}_2{\mathrm{P}}_2$. As $x$ increases from 0 to 0.5, the in-plane component of $T_0$ decreases proportionally with the metamagnetic transition field. In the antiferromagnetic cT phase ($0.7\le x\le 1$), $T_0$ is constant and spin fluctuations show an isotropic character in contrast to their behavior in the paramagnetic ucT phase ($0\le x\le 0.5$). These results indicate that the in-plane spin fluctuations due to the quasi-two-dimensional crystal structure play a significant role in the metamagnetic transition of this system.

We investigate the effects of Dzyaloshinsky-Moriya (DM) interactions on the frustrated $J_1-J_2$ kagome-Heisenberg model using the pseudofermion functional renormalization group (PFFRG) technique. In order to treat the off-diagonal nature of DM interactions, we develop an extended PFFRG scheme. We benchmark this approach in parameter regimes that have previously been studied with other methods and find good agreement of the magnetic phase diagram. Particularly, finite DM interactions are found to stabilize all types of noncollinear magnetic orders of the $J_1-J_2$ Heisenberg model ($\mathbf{q}=0, \sqrt{3}\times \sqrt{3}$, and cuboc orders) and shrink the extents of magnetically disordered phases. We discuss our results in the light of the mineral herbertsmithite which has been experimentally predicted to host a quantum spin liquid at low temperatures. Our PFFRG data indicate that this material lies in close proximity to a quantum critical point. In parts of the experimentally relevant parameter regime for herbertsmithite, the spin-correlation profile is found to be in good qualitative agreement with recent inelastic-neutron-scattering data.