Terahertz-field-induced carrier generation processes were investigated in Dirac electron systems, single-crystalline bismuth antimony alloy thin films ($\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{S}{\mathrm{b}}_x$; $0\le x\le 0.16$). This investigation was performed by precisely tuning, via the substituent ratio $x$, the band structure of the films from that associated with a semimetal to that characteristic of a narrow-gap semiconductor. Terahertz-field-induced absorption was clearly observed within a few picoseconds after the terahertz pump-pulse illumination of $\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{S}{\mathrm{b}}_x$ semimetal and semiconductor samples. The field-strength dependence of the induced absorption was compared with the calculated Zener tunneling probability in the Dirac-like band dispersion. Through this comparison, the mechanism of the induced absorption was attributed to the carrier generation via the terahertz-field-induced Zener tunneling. The tunneling occurred in subpicosecond timescales even at room temperature, demonstrating that $\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{S}{\mathrm{b}}_x$ thin films are promising for future high-speed electronics and the investigation of universal ultrafast tunneling dynamics.

Spin dynamics of the staircase kagome lattice ${\mathrm{PbCu}}_3{\mathrm{TeO}}_7$ was investigated by static magnetic susceptibility $\chi \left(T\right)$, high-field magnetization $M\left(H\right)$, and muon spin rotation/relaxation $\left(\mu \mathrm{SR}\right)$ measurements. $\chi \left(T\right)$ displays a sub-Curie power-law behavior $\chi \left(T\right)\sim T^{-\alpha }$ with changing exponent from $\alpha =0.5$ to 0.3 crossing $T^{*}=150$ K. We identify a spin-flop transition at ${\mu }_{0}H=8.5$ T and another field-induced anomaly at ${\mu }_0{H}^{*}=30$ T in $M\left(H\right)$. Our $\mu \mathrm{SR}$ results unveil that the magnetically inequivalent Cu(1) and Cu(2) spins develop site-specific magnetic structures through the successive magnetic transitions at $T_{\mathrm{N}1}=36$ K, $T_{\mathrm{N}2}=25$ K, and $T_{\mathrm{N}3}=17$ K. The magnetic anomalies of ${\mathrm{PbCu}}_3{\mathrm{TeO}}_7$ suggest that the Cu(1) and Cu(2) subsystems feature a largely distinct magnetism due to the presence of a dozen competing exchange interactions having largely different energy scales and thus the magnetism of ${\mathrm{PbCu}}_3{\mathrm{TeO}}_7$ is far more complex than the staircase kagome lattice.

Ab initio calculation of intrinsic spin Hall conductivity (SHC) generally requires a strict convergence criterion and a dense $k$-point mesh to sample the Brillouin zone, making its convergence challenging and time consuming. Here we present a scheme for efficiently and accurately calculating SHC based on the maximally localized Wannier function (MLWF). The quantities needed by the Kubo formula of SHC are derived in the space of the MLWF and it is shown that only the Hamiltonian, the overlap, and the spin operator matrices are required from the initial ab initio calculation. The computation of these matrices and the interpolation of the Kubo formula on a dense $k$-point mesh can be easily achieved. We validate our results by prototypical calculations on fcc Pt and GaAs, which demonstrate that the Wannier interpolation approach is of high accuracy and efficiency. Calculations of $\alpha \text{−}\mathrm{Ta}$ and $\beta \text{−}\mathrm{Ta}$ show that SHC of $\beta \text{−}\mathrm{Ta}$ is 2.7 times that of $\alpha \text{−}\mathrm{Ta}$, while both have the opposite sign relative to fcc Pt and are an order of magnitude smaller than Pt. The calculated spin Hall angle of $-0.156$ is quite consistent with previous experiments on $\beta \text{−}\mathrm{Ta}$, further suggesting intrinsic contribution may dominate in $\beta \text{−}\mathrm{Ta}$. Our approach could facilitate large-scale SHC calculations and may benefit the discovery of materials with high intrinsic SHC.

We develop a framework to analyze the topological properties of one-dimensional systems with charge conservation and tendency towards topological superconducting order. In particular, we consider models with $N$ flavors of fermions and ${\left({\mathbb{Z}}_2\right)}^N$ symmetry, associated with the conservation of the fermionic parity of each flavor. For $N=1$, and with no other symmetry other than charge conservation, we recover the result that there is no distinct topological phase with exponentially localized zero modes. For $N>1$, however, we show that the ends of the system can host low-energy, exponentially-localized modes. To illustrate these ideas, we focus on lattice models with $SO\left(N\right)$ symmetric interactions and study the phase transition between the trivial and the topological gapless phases using bosonization and a weak-coupling renormalization group analysis. As a concrete example, we study in detail the case of $N=3$. In this case, the topologically nontrivial superconducting phase corresponds to a gapless analog of the Haldane phase in spin-1 chains. In this phase, although the bulk hosts gapless modes, corresponding to composite fermionic excitations with an enlarged Fermi surface, the ends host spin-$1/2$ degrees of freedom which are exponentially localized and protected by the spin gap in the bulk. We obtain the full phase diagram of the model using density matrix renormalization group calculations. Within this model, we identify the self-dual line studied by Andrei and Destri [Nucl. Phys. B 231, 445 (1984)] as a first-order transition line between the gapless Haldane phase and a trivial gapless phase. This allows us to identify the propagating spin-$1/2$ kinks in the Andrei-Destri model as the topological end modes at the domain walls between the two phases.

We have performed a detailed x-ray diffraction structural study of Zr under pressure and unambiguously identify the existence of a first-order isostructural bcc-to-bcc phase transition near 58 GPa. First-principles quantum molecular dynamics calculations support the existence of this phase transition, in excellent agreement with experimental results, triggered by anharmonic effects. Our results highlight the potential ubiquity of anharmonically driven isostructural transitions within the periodic table under pressure and call for follow-up experimental and theoretical studies.

We show that an individual adatom on a magnetic surface can exhibit a noncollinear spin density. Using density functional theory we study Co and Ir adatoms on a Mn monolayer on the W(110) surface which possesses a noncollinear canted spin structure. Due to hybridization with the nearest and next-nearest Mn atoms of the monolayer the spin direction of the underlying substrate is encoded into the orbitals of the adatom. This explains recent scanning tunneling microscopy experiments showing a spin sensitive shape asymmetry of adatoms [Serrate et al., Phys. Rev. B 93, 125424 (2016)] which confirms the intra-atomic noncollinear magnetism of the adatoms.

Thermoelectric effects, measured by the Seebeck coefficients, refer to the phenomena in which a temperature difference or gradient imposed across a thermoelectric material induces an electrical potential difference or gradient, and vice versa, enabling the direct conversion of thermal and electric energies. All existing first-principles calculations of Seebeck coefficients have been based on the Boltzmann kinetic transport theory. In this work, we present a fundamentally different method for the first-principles calculations of Seebeck coefficients without using any assumptions of the electron-scattering mechanism, being in contrast to the traditional theory by Cutler and Mott that shows the dependence of the Seebeck coefficient on the scattering mechanisms. It is shown that the Seebeck coefficient is a well-defined thermodynamic quantity that can be determined from the change in the chemical potential of electrons induced by the temperature change and thus can be computed solely based on the electronic density of states through first-principles calculations at different temperatures. The proposed approach is demonstrated using the prototype PbTe and SnSe thermoelectric materials.

We present a general scheme to accurately determine melting properties of materials from ab initio free energies. This scheme does not require prior fitting of system-specific interatomic potentials and is straightforward to implement. For the solid phase, ionic entropies are determined from the phonon quasiparticle spectra (PQS), which fully account for lattice anharmonicity in the thermodynamic limit. The resulting free energies are nearly identical (within 10 meV/atom) to those from the computationally more demanding thermodynamic integration (TI) approach. For the liquid phase, PQS are not directly applicable and free energies are determined via TI using the Weeks-Chandler-Andersen (WCA) gas as the reference system. The WCA is a simple, short-range, purely repulsive potential with established equation of states. As such, it is an ideal reference for ab initio TI of liquids. We apply this scheme to determine melting properties of hexagonal close-packed (hcp) iron at the Earth's inner core boundary $(P=330$ GPa), a subject of great significance in Earth sciences. The important influences of system size and pseudopotentials are carefully analyzed. The results (melting temperature equals $6170\pm 200$ K, latent heat $56\pm 2$ kJ/mol, Clapeyron slope $8.1\pm 0.2$ K/GPa) are consistent with experiments as well as previous calculations.

The energy-momentum tensor of a ferromagnet derived according to the standard prescription of Noether's theorem has a major flaw: the term originating from the spin Berry phase is gauge dependent. As a consequence, some physical quantities computed from the tensor show unphysical behavior. For example, the presence of a spin-polarized current does not affect the energy of the domain wall in the commonly accepted gauge, which implies, incorrectly, the absence of the adiabatic spin torque. In other gauges, the spin torque shows unphysical glitches occurring when the plane of magnetization crosses the Dirac string associated with a magnetic monopole in spin space. We derive a gauge-invariant energy-momentum tensor that is free from these artifacts but requires the addition of an extra spatial dimension, with the ferromagnet living on its boundary. It can be obtained most directly from the Wess-Zumino action for spins, which relies on the same extra dimension.

The interplay between lattice topology, frustration, and spin quantum number, $s$, is explored for the Heisenberg antiferromagnet (HAFM) on the 11 two-dimensional Archimedean lattices (square, honeycomb, CaVO, SHD, SrCuBO, triangle, bounce, trellis, maple-leaf, star, and kagome). We show that the coupled cluster method (CCM) provides consistently accurate results when compared to the results of other approximate methods. The CCM also provides valuable information relating to the selection of ground states and we find that this depends on spin quantum number for the kagome and star lattices. Specifically, the $\sqrt{3}\times \sqrt{3}$ model state provides lower ground-state energies than those of the $q=0$ model state for the kagome and star lattices for most values of $s$. The $q=0$ model state provides lower ground-state energies only for $s=1/2$ for the kagome lattice and $s=1/2$ and $s=1$ for the star lattice. The kagome and star lattices demonstrate the least amount of magnetic ordering and the unfrustrated lattices (square, honeycomb, SHD, and CaVO) demonstrate the most magnetic ordering for all values of $s$. The SrCuBO and triangular lattices also demonstrate high levels of magnetic ordering, while the remaining lattices (bounce, maple-leaf, and trellis) tend to lie between these extremes, again for all values of $s$. These results also clearly reflect the strong increase in magnetic order with increasing spin quantum number $s$ for all lattices. The ground-state energy, $E_g/\left(NJ{s}^2\right)$, scales with $s^{-1}$ to first order, as expected from spin-wave theory, although the order parameter, $M/s$, scales with $s^{-1}$ for most of the lattices only. Self-consistent spin-wave theory calculations indicated previously that $M/s$ scales with $s^{-2/3}$ for the kagome lattice HAFM, whereas previous CCM results (replicated here also) suggested that $M/s$ scales with $s^{-1/2}$. It is probable, therefore, that different scaling for $M/s$ than with $s^{-1}$ does indeed occur for the kagome lattice. By using similar arguments, we find here also that $M/s$ scales with $s^{-1/3}$ for the star lattice and with $s^{-2/3}$ for the SrCuBO lattice.

In ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, correlations, magnetism, and spin-orbit coupling compete on similar energy scales, creating a new context to study metal-insulator transitions (MIT). We use here angle-resolved photoemission to investigate the MIT as a function of hole and electron doping in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, obtained respectively by Ir/Rh and Sr/La substitutions. We show that there is a clear reduction as a function of doping of the gap between a lower and upper band on both sides of the Fermi level, from 0.2 to 0.05 eV. Although these two bands have a counterpart in band structure calculations, they are characterized by a very different degree of coherence. The upper band exhibits clear quasiparticle peaks, while the lower band is very broad and loses weight as a function of doping. Moreover, their ARPES spectral weights obey different periodicities, reinforcing the idea of their different nature. We argue that a very similar situation occurs in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and conclude that the physics of the two families is essentially the same.

Hydrodynamic and viscous effects in electronic liquids are at the focus of much current research. Most intriguing is perhaps the nondissipative Hall viscosity, which, due to its symmetry-protected topological nature, can help identify complex topological orders. In this work we study the effects of viscosity in general, and Hall viscosity in particular, on the dispersion relation of edge magnetoplasmons in two-dimensional electronic systems. Using an extension of the standard Wiener-Hopf technique, we derive a general solution to the problem, accounting for the long-range Coulomb potential. Among other features we find that on the edge viscosity affects the dispersion already to leading order in the wave vector, making edge modes better suited for its measurement compared to their bulk counterparts.

Helium films show various quantum phases that undergo quantum phase transitions by changing coverage $n$. We found anomalous elastic phenomena in bosonic ${}^4\mathrm{He}$ and fermionic ${}^3\mathrm{He}$ films adsorbed on a glass substrate. The films stiffen under ac strain at low temperature with an excess dissipation. The onset temperature of the stiffening decreases to 0 K as $n$ approaches a critical coverage $n_{\mathrm{c}}$. The elastic anomaly is explained by thermal activation of helium atoms from the localized to extended states with a distributed energy gap. We determine the energy band structure of helium films from elasticity. The ground states of ${}^4\mathrm{He}$ and ${}^3\mathrm{He}$ at $n<n_{\mathrm{c}}$, which are possibly a sort of Mott insulator or Mott glass, are identically gapped and compressible.

We introduce the concept of self-energy dispersion as an error bound on local theories and apply it to the two-dimensional Hubbard model on the square lattice at half filling. Since the self-energy has no single-particle analog and is not directly measurable in experiments, its general behavior as a function of momentum is an open question. In this paper, we benchmark the momentum dependence with the two-particle self-consistent approach together with analytical and numerical considerations and we show that through the addition of a local single-particle potential to the Hubbard model the self-energy can be flattened, such that it is essentially described by only a frequency-dependent term. We use this observation to motivate that local theories, such as the dynamical mean-field theory, should be expected to give very accurate results in the presence of a potential of this kind. This is especially interesting in the context of topology, where such a term is present in many popular models, e.g., the Haldane and Kane-Mele models. Finally, we propose a simple energy argument as an estimator for the crossover from nonlocal to local self-energies, which can be computed even by local theories such as dynamical mean-field theory.

Transition metal dichalcogenides (TMDs), whether in bulk or in monolayer form, exhibit a rich variety of charge-density-wave (CDW) phases and stronger periodic lattice distortions. While the actual role of nesting has been under debate, it is well understood that the microscopic interaction responsible for the CDWs is the electron-phonon coupling. The case of $1T-{\mathrm{TiSe}}_2$ is, however, unique in this family in that the normal state above the critical temperature $T_{\mathrm{CDW}}$ is characterized by a small quasiparticle band gap as measured by angle-resolved photoemission spectroscopy, so that no nesting-derived enhancement of the susceptibility is present. It has therefore been argued that the mechanism responsible for this CDW should be different and that this material realizes the excitonic insulator phase proposed by W. Kohn. On the other hand, it has also been suggested that the whole phase diagram can be explained by a sufficiently strong electron-phonon coupling. In this paper, in order to estimate how close this material is to the pure excitonic insulator instability, we quantify the strength of electron-hole interactions by computing the exciton band structure at the level of hybrid density functional theory, focusing on the monolayer. We find that in a certain range of parameters the indirect gap at $q_{\mathrm{CDW}}$ is significantly reduced by excitonic effects. We also stress the important role of the spin-orbit coupling in significantly reducing the band gap. We discuss the consequences of those results regarding the debate on the physical mechanism responsible for this CDW. Based on the dependence of the calculated exciton binding energies as a function of the mixing parameter of hybrid density functional theory, we conjecture that a necessary condition for a pure excitonic insulator is that its noninteracting electronic structure is metallic.

We report measurements of the Hall effect in SmN thin films, spanning from ambient temperature to well inside the ferromagnetic state. The carrier concentrations of $0.8–4\times {10}^{21}$ ${\mathrm{cm}}^{-3}$ show little temperature variation, signaling that at least one electron from the dominant nitrogen-vacancy dopant is unbound. The anomalous Hall effect in the ferromagnetic phase has a sign consistent with the known spin magnetic moment in SmN that is antiparallel to the magnetic field, in turn relating to an orbital dominance of the net magnetization. The AHE magnitude strongly supports a band structure with a heavy-fermion $4f$ conduction band lying below the $5d$ band.

The effect of anisotropy in the energy spectrum on the binding energy and structural properties of excitons, trions, and biexcitons is investigated. To this end we employ the stochastic variational method with a correlated Gaussian basis. We present results for the binding energy of different excitonic complexes in black phosphorus (bP) and ${\mathrm{TiS}}_3$ and compare them with recent results in the literature when available, for which we find good agreement. The binding energies of excitonic complexes in bP are larger than those in ${\mathrm{TiS}}_3$. We calculate the different average interparticle distances in bP and ${\mathrm{TiS}}_3$ and show that excitonic complexes in bP are strongly anisotropic whereas in ${\mathrm{TiS}}_3$ they are almost isotropic, even though the constituent particles have an anisotropic energy spectrum. This is also confirmed by the correlation functions.