We report that current-induced spin-orbit torques (SOTs) in heavy-metal/ferromagnetic-metal bilayers are strongly altered by the oxidation of the ferromagnetic layer near the interface. We measured damping-like (DL) and field-like (FL) SOTs for Pt/Co and $\mathrm{Pt}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ (Pt/Py) films using spin-torque ferromagnetic resonance. In the Pt/Co film, we found that the oxidation of the Co layer near the interface enhances both DL and FL SOTs in spite of the insulating nature of the ${\mathrm{CoO}}_x$ layer. The enhancement of the SOTs disappears by inserting a thin Ti layer at the $\mathrm{Pt}/{\mathrm{CoO}}_x$ interface, indicating that the dominant source of the SOTs in the $\mathrm{Pt}/{\mathrm{CoO}}_x/\mathrm{Co}$ film is the spin-orbit coupling at the $\mathrm{Pt}/{\mathrm{CoO}}_x$ interface. In contrast to the $\mathrm{Pt}/{\mathrm{CoO}}_x/\mathrm{Co}$ film, the SOTs in the $\mathrm{Pt}/{\mathrm{PyO}}_x/\mathrm{Py}$ film are dominated by the bulk spin-orbit coupling. Our result shows that the interfacial oxidation of the Pt/Py film suppresses the DL-SOT and reverses the sign of the FL-SOT. The change of the SOTs can be attributed to the change of the real and imaginary parts of the spin mixing conductance induced by the insertion of the insulating ${\mathrm{PyO}}_x$ layer. These results show that the interfacial oxidation provides an effective way to manipulate the strength and sign of the SOTs.

We study the critical behavior of a generalized icosahedral model on the simple cubic lattice. The field variable of the icosahedral model might take 1 of 12 vectors of unit length which are given by the normalized vertices of the icosahedron as value. Similar to the Blume-Capel model, where in addition to $-1$ and 1, as in the Ising model, the spin might take the value 0, we add in the generalized model (0,0,0) as allowed value. There is a parameter $D$ that controls the density of these voids. For a certain range of $D$, the model undergoes a second-order phase transition. On the critical line, $\text{O}\left(3\right)$ symmetry emerges. Furthermore, we demonstrate that within this range, similar to the Blume-Capel model on the simple cubic lattice, there is a value of $D$, where leading corrections to scaling vanish. We perform Monte Carlo simulations for lattices of a linear size up to $L=400$ by using a hybrid of local Metropolis and cluster updates. The motivation to study this particular model is mainly of technical nature. Less memory and CPU time are needed than for a model with $\text{O}\left(3\right)$ symmetry at the microscopic level. As the result of a finite-size scaling analysis we obtain $\nu =0.71164\left(10\right)$, $\eta =0.03784\left(5\right)$, and $\omega =0.759\left(2\right)$ for the critical exponents of the three-dimensional Heisenberg universality class. The estimate of the irrelevant renormalization group eigenvalue that is related with the breaking the $\text{O}\left(3\right)$ symmetry is $y_{\mathrm{ico}}=-2.19\left(2\right)$.

Several recent studies have shown that SCAN, a functional belonging to the meta-generalized gradient approximation (MGGA) family, leads to significantly overestimated magnetic moments in itinerant ferromagnetic metals. However, this behavior is not inherent to the MGGA level of approximation since TPSS, for instance, does not lead to such severe overestimations. In order to provide a broader view of the accuracy of MGGA functionals for magnetism, we extend the assessment to more functionals but also to antiferromagnetic solids. The results show that to describe magnetism there is overall no real advantage in using a MGGA functional compared to GGAs. For both types of approximation, an improvement in ferromagnetic metals is necessarily accompanied by a deterioration (underestimation) in antiferromagnetic insulators, and vice versa. We also provide some analysis in order to understand in more detail the relation between the mathematical form of the functionals and the results.

Compounds with two-dimensional (2D) layers of magnetic ions weakly connected by van der Waals bonding offer routes to enhance quantum behavior, stimulating both fundamental and applied interest. ${\mathrm{CrPS}}_4$ is one such magnetic van der Waals material, however, it has undergone only limited investigation. Here we present a comprehensive series of neutron scattering measurements to determine the magnetic structure and exchange interactions. The observed magnetic excitations allow a high degree of constraint on the model parameters not normally associated with measurements on a powder sample. The results demonstrate the 2D nature of the magnetic interactions, while also revealing the importance of interactions along 1D chains within the layers. The subtle role of competing interactions is observed, which manifest in a nontrivial magnetic transition and a tunable magnetic structure in a small applied magnetic field through a spin-flop transition. Our results on the bulk compound provide insights that can be applied to an understanding of the behavior of reduced layer ${\mathrm{CrPS}}_4$.

We report on the magnon spectrum of Kitaev-Heisenberg magnets and extract the parameters to model a two-dimensional ${\text{CrI}}_3$. Our minimal spin Hamiltonian includes a contribution stemming from a Heisenberg, isotropic exchange, and a contribution arising from a Kitaev interaction, anisotropic and frustrated exchange. Our calculations reveal a gap that opens at the $K$ and $K^{\prime }$ points and the topological nature of the magnons which lead to the thermal Hall effect. Furthermore, we calculate the magnon spectrum of nanoribbons illustrating the corresponding edge states.

The self-energy of a moving vortex is shown to decrease with increasing velocity. The interaction energy of two parallel slowly moving vortices differs from the static case by a small term $\propto v^2$; the “slow” motion is defined as having the velocity $v<v_c=c^2/4\pi \sigma \lambda$, where $\sigma \left(T\right)$ is the conductivity of normal excitations and $\lambda \left(T\right)$ is London penetration depth. For higher velocities, $v>v_c\left(T\right)$, the interaction energy of two vortices situated along the velocity direction is enhanced and in the perpendicular direction is suppressed compared to the static case.

Nanomolecular assemblies of ${\mathrm{C}}_{60}$ can be synthesized to enclose dipolar molecules. The low-temperature states of such endofullerenes are described by quantum mechanical rotors, which are candidates for quantum information devices with higher-dimensional local Hilbert spaces. The experimental exploration of endofullerene arrays comes at a time when machine learning techniques are rapidly being adopted to characterize, verify, and reconstruct quantum states from measurement data. In this paper, we develop a strategy for reconstructing the ground state of chains of dipolar rotors using restricted Boltzmann machines (RBMs) adapted to train on data from higher-dimensional Hilbert spaces. We demonstrate accurate generation of energy expectation values from an RBM trained on data in the free-rotor eigenstate basis and explore the learning resources required for various chain lengths and dipolar interaction strengths. Finally, we show evidence for fundamental limitations in the accuracy achievable by RBMs due to the difficulty in imposing symmetries in the sampling procedure. We discuss possible avenues to overcome this limitation in the future, including the further development of autoregressive models such as recurrent neural networks for the purposes of quantum state reconstruction.

Deviations of low-energy electronic structurse of iron-based superconductors from density-functional-theory predictions have been parametrized in terms of band- and orbital-dependent mass renormalizations and energy shifts. The former have typically been described in terms of a local self-energy within the framework of dynamical mean field theory, while the latter appears to require nonlocal effects due to interband scattering. By calculating the renormalized band structure in both random phase approximation (RPA) and the two-particle self-consistent approximation (TPSC), we show that correlations in pnictide systems like LaFeAsO and LiFeAs can be described rather well by a nonlocal self-energy. In particular, Fermi pocket shrinkage as seen in experiments occurs due to repulsive interband finite-energy scattering. For the canonical iron chalcogenide system FeSe in its bulk tetragonal phase, the situation is, however, more complex since even including momentum-dependent band renormalizations cannot explain experimental findings. We propose that the nearest-neighbor Coulomb interaction may play an important role in band-structure renormalization in FeSe. We further compare our evaluations of nonlocal quasiparticle scattering lifetime within RPA and TPSC with experimental data for LiFeAs.

The electronic properties in a solid depend on the specific form of the wave functions that represent the electronic states in the Brillouin zone. Since the discovery of topological insulators, much attention has been paid to the restrictions that the symmetry imposes on the electronic band structures. In this work we apply two different approaches to characterize all types of bands in a solid by the analysis of the symmetry eingenvalues: the induction procedure and the Smith decomposition method. The symmetry eigenvalues or irreducible representation (irreps) of any electronic band in a given space group can be expressed as the superposition of the eigenvalues of a relatively small number of building units (the basic bands). These basic bands in all the space groups are obtained following a group-subgroup chain starting from $P1$. Once the whole set of basic bands are known in a space group, all other types of bands (trivial, strong topological, or fragile topological) can be easily derived. In particular, we confirm previous calculations of the fragile root bands in all the space groups. Furthermore, we define an automorphism group of equivalences of the electronic bands which allows to define minimum subsets of, for instance, independent basic or fragile root bands.

We report angle-resolved photoemission spectroscopy and first-principles numerical calculations for the band-structure evolution of the $3d$ heavy-fermion compound $\mathrm{CaC}{\mathrm{u}}_3\mathrm{R}{\mathrm{u}}_4{\mathrm{O}}_{12}$. Below $T\sim 200$ K, we observed an emergent hybridization gap between the Cu 3d-electron-like band and the Ru 4d-hole-like band and the resulting flat band features near the Fermi level centered around the Brillouin-zone corner. Our results confirm the non-Kondo nature of the $\mathrm{CaC}{\mathrm{u}}_3\mathrm{R}{\mathrm{u}}_4{\mathrm{O}}_{12}$, in which the $\mathrm{Cu}3d_{\mathrm{xy}}$ electrons are less correlated and not in the Kondo limit. Comparison between theory and experiment also suggests that other mechanisms may be needed to give a full quantitative explanation of the peculiar properties in this material.

We apply a stochastic version of an optimally tuned range-separated hybrid functional to provide insight on the electronic properties of P- and B-doped Si nanocrystals of experimentally relevant sizes. We show that we can use the range-separation parameter for undoped systems to calculate accurate results for dopant activation energies. We apply this strategy for tuning functionals to study doped nanocrystals up to 2.5 nm in diameter at the hybrid functional level. In this confinement regime, the P and B dopants have large activation energies and have strongly localized states that lie deep within the energy gaps. Structural relaxation plays a greater role for B-substituted dopants and contributes to the increase in activation energy when the B dopant is near the nanocrystal surface.

The origin of orbital order in correlated transition-metal compounds is strongly debated. For the paradigmatic $e_g$ systems ${\mathrm{KCuF}}_3$ and ${\mathrm{LaMnO}}_3$, it has been shown that the electronic Kugel'-Khomskii mechanism alone is not sufficient to drive the orbital-ordering transition up to the high temperatures at which it is experimentally observed. In the case of $t_{2g}$ compounds, however, the role played by the superexchange interaction remains unclear. Here we investigate this question for two representative systems, the $3d{t}_{2g}^1$ Mott insulators ${\mathrm{LaTiO}}_3$ and ${\mathrm{YTiO}}_3$. We show that the Kugel'-Khomskii superexchange transition temperature $T_{\mathrm{KK}}$ is unexpectedly large, comparable to the value for the $e_g^3$ fluoride ${\mathrm{KCuF}}_3$. By deriving the general form of the orbital superexchange Hamiltonian for the $t_{2g}^1$ configuration, we show that the ${\mathrm{GdFeO}}_3$-type distortion plays a key part in enhancing $T_{\mathrm{KK}}$ to about 300 K. Still, orbital ordering above 300 K can be ascribed only to the presence of a static crystal-field splitting.

Analytic continuation of imaginary time or frequency data to the real axis is a crucial step in extracting dynamical properties from quantum Monte Carlo simulations. The average spectrum method provides an elegant solution by integrating over all nonnegative spectra weighted by how well they fit the data. In a recent paper, we found that discretizing the functional integral, as in Feynman's path-integrals, does not have a well-defined continuum limit. Instead, the limit depends on the discretization grid whose choice may strongly bias the results. In this paper, we demonstrate that sampling the grid points, instead of keeping them fixed, also changes the functional integral limit and rather helps to overcome the bias considerably. We provide an efficient algorithm for doing the sampling and show how the density of the grid points acts now as a default model with a significantly reduced biasing effect. The remaining bias depends mainly on the width of the grid density, so we go one step further and average also over densities of different widths. For a certain class of densities, including Gaussian and exponential ones, this width averaging can be done analytically, eliminating the need to specify this parameter without introducing any computational overhead.

Closed-form exact expressions are derived for the macroscopic permittivity tensor of a parallel metal-dielectric sandwichlike composite microstructure in the presence of an externally applied static in-plane magnetic field. An exact closed-form expression for a magnetic-field- and structure-dependent resonance frequency of the multisurface plasmon is obtained. The optical properties of this plasmon are compared with those of the surface plasmons localized around cylindrical holes made in the metallic layers. All analytical results are verified by numerical simulations.

We demonstrate here electrical control of the sign of the circularly polarized emission from the negatively charged trion, going from co- to contrapolarized with respect to the circular polarization of the laser, using a GaAs/AlAs quantum dot (QD) embedded in a field effect structure. The voltage range over which the trion is negatively (contra) circularly polarized is shown to be dependent on the laser excitation energy within the $P$-shell resonance. The negative polarization never exceeds $\sim -15\%$, in stark contrast to measurements on InAs/GaAs QDs reported by M. E. Ware et al. [Phys. Rev. Lett. 95, 177403 (2005).] in which a negative polarization reaching $-95\%$ was observed. This result is shown to be a consequence of the low-symmetry confinement potential of these GaAs/AlAs QD, which are fabricated by partial infilling of asymmetric droplet-etched nanoholes. This low QD symmetry also leads to optical activity of the dark spin configuration of the triplet state, which we measure experimentally by photoluminescence excitation spectroscopy. A simple, semiquantitative model explaining both the optical activity of the dark spin configuration and the maximum degree of negative polarization is presented.

Electromigration, due to its technological and scientific significance, has been a subject of extensive studies for many years. We present evidence of electromigration in dielectric materials, namely $C$-plane sapphire, obtained from direct experimental observation of an atomic step-bunching instability driven by electromigration. We further expand upon our previously reported findings of electromigration induced step-bunching transformation of a metal surface. The only system where electromigration driven step bunching has been observed and comprehensively investigated is the low index surfaces of silicon. In this study we show that electromigration driven SB can be induced on a variety of crystallographic surfaces, including metals and insulating oxides, and may be more prevalent than previously thought. Electric fields were applied at high temperature to W(110) and $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\left(0001\right)$ crystals whereupon their surface reordered to a morphology closely resembling that of Si(111) with atomic steps bunched by electromigration. This suggests that the mechanism of step bunching on the W(110), $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\left(0001\right)$, and Si(111) can be fundamentally the same. Annealing W(110) offcut in the [001] direction with an up-step current produced a morphology with the bunch edges composed of zigzag segments meeting at a right angle.

In some topological insulators such as monolayers of ${\mathrm{WTe}}_2$ or graphene with spin-orbit coupling, band inversion originates from chemical bonding and space group symmetry, in contrast to materials such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, where the band inversion derives from relativistic effects in the atoms. In the former, band inversion is susceptible to changes of the chemical environment, e.g., by defects, while the latter are less affected by defects due to the larger energy scale associated with atomic relativistic effects. Motivated by recent experiments, we study the effect of Te vacancies and Te adatoms on the electronic properties of ${\mathrm{WTe}}_2$. We find that the Te vacancies have a formation energy of 2.21 eV, while the formation energy of the Te adatoms is much lower with 0.72 eV. The vacancies strongly influence the band structure and we present evidence that band inversion is already reversed at the nominal composition of ${\mathrm{WTe}}_{1.97}$. In contrast, we show that the adatoms do not change the electronic structure in the vicinity of the Fermi level and thus the topological properties.

We have performed high-resolution angle-resolved photoemission spectroscopy of the ternary pnictide CaAuAs, which is predicted to be a three-dimensional topological Dirac semimetal (TDS). By accurately determining the bulk-band structure, we have revealed the coexistence of three-dimensional and quasi-two-dimensional Fermi surfaces with dominant hole carriers. The band structure around the Brillouin-zone center is characterized by an energy overlap between the hole and electron pockets, in excellent agreement with first-principles band-structure calculations. This indicates the occurrence of bulk-band inversion, supporting the TDS state in CaAuAs. Because of the high tunability in the chemical composition besides the TDS nature, CaAuAs provides a precious opportunity for investigating the quantum phase transition from TDS to other exotic topological phases.