Hidden-order phase transition in the heavy-fermion superconductor ${\mathrm{URu}}_2{\mathrm{Si}}_2$ exhibits the mean-field-like anomaly in temperature dependence of heat capacity. Motivated by this observation, here we explore the impact of the complex order parameter fluctuations on the thermodynamic properties of the hidden-order phase. Specifically, we employ the mean-field theory for the hidden order which describes the hidden-order parameter by an average of the hexadecapole operator. We compute the Gaussian fluctuation corrections to the mean-field theory equations including both the fluctuations due to “hidden order” as well as antiferromagnetic order parameters. We find that the Gaussian fluctuations lead to the smearing of the second-order transition rendering it to become the first-order one. The strength of the first-order transition is weakly dependent on the strength of underlying antiferromagnetic exchange interactions.

The search for room-temperature magnetocaloric materials for refrigeration has led to investigations of double perovskites. In particular, a puzzle has appeared in the ${\mathrm{La}}_2{\mathrm{MnNiO}}_6$, ${\mathrm{La}}_2{\mathrm{MnCoO}}_6$, and ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$ family of compounds. They share the same crystal structure, but while ${\mathrm{La}}_2{\mathrm{MnNiO}}_6$ and ${\mathrm{La}}_2{\mathrm{MnCoO}}_6$ are ferromagnets below room temperature, ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$, contrary to simple expectations, is a ferrimagnet. To solve this puzzle, we use density functional theory calculations to investigate the electronic structure and magnetic exchange interactions of the ordered double perovskites. Our study reveals the critical role played by local electron-electron interaction in the Fe-$d$ orbital to promote the ${\mathrm{Fe}}^{3+}$ valence state with half-filled $d$ shell over ${\mathrm{Fe}}^{2+}$ and to establish a ferrimagnetic ground state for ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$. The importance of Hund's coupling and Jahn-Teller distortion on the ${\mathrm{Mn}}^{4+}$ ion is also pointed out. Exchange constants are extracted by comparing different magnetically ordered states. Mean field and classical Monte Carlo calculations on the resulting model give trends in $T_C$ that are in agreement with experiments on this family of materials.

Transition between the topologically trivial and nontrivial phase of ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ alloy is driven by the increasing content $x$ of Sn, or by the hydrostatic pressure for $x<0.3$. We show that a sharp border between these two topologies exists in the virtual crystal approximation only. In more realistic models, the special quasirandom structure method and the supercell method (with averaging over various atomic configurations), the transitions are broadened. We find a surprisingly large interval of alloy composition, $0.3<x<0.5$, in which the energy gap is practically vanishing. A similar strong broadening is also obtained for transitions driven by hydrostatic pressure. Analysis of the band structure shows that the alloy broadening originates in splittings of the energy bands caused by the different chemical nature of Pb and Sn, and by the decreased crystal symmetry due to spatial disorder. Based on our results of ab initio and tight-binding calculations for ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ we discuss different criteria of discrimination between trivial and nontrivial topology of the band structure of alloys.

Spin-orbit interaction is an important vehicle for spin relaxation. At finite temperature lattice vibrations modulate the spin-orbit interaction and thus generate a mechanism for spin-phonon coupling, which needs to be incorporated in any quantitative analysis of spin transport. Starting from a density functional theory ab initio electronic structure, we calculate spin-phonon matrix elements over the basis of maximally localized Wannier functions. Such coupling terms form an effective Hamiltonian to be used to extract thermodynamic quantities, within a multiscale approach particularly suitable for organic crystals. The symmetry of the various matrix elements is analyzed by using the $\mathrm{\Gamma }$-point phonon modes of a one-dimensional chain of Pb atoms. Then the method is employed to extract the spin-phonon coupling of solid durene, a high-mobility crystal organic semiconductor. Owing to the small masses of carbon and hydrogen spin orbit is weak in durene and so is the spin-phonon coupling. Most importantly, we demonstrate that the largest contribution to the spin-phonon interaction originates from Holstein-like phonons, namely, from internal molecular vibrations.

We predict that a single oscillation of a strong optical pulse can significantly populate the surface conduction band of a three-dimensional topological insulator, ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Both linearly- and circularly-polarized pulses generate chiral textures of interference fringes of population in the surface Brillouin zones. These fringes constitute a self-referenced electron hologram carrying information on the topology of the surface Bloch bands, in particular, on the effect of the warping term of the low-energy Hamiltonian. These electron-interference phenomena are in sharp contrast to graphene where there are no chiral textures for a linearly-polarized pulse and no interference fringes for circularly-polarized pulse. These predicted reciprocal space electron-population textures can be measured experimentally by time resolved angle resolved photoelectron spectroscopy (TR-ARPES) to gain direct access to non-Abelian Berry curvature at topological insulator surfaces.

We examine spin-dependent displacement of a single electron, resulting in separation and relocation of the electron wave-function components, and thus charge parts, corresponding to opposite spins. This separation is induced by a pulse of an electric field which generates varying Rashba type spin-orbit coupling. This mechanism is next implemented in a nanodevice based on a gated quantum dot defined within a quantum nanowire. The electric-field pulse is generated by ultrafast changes of voltages, of the order of several hundred mV, applied to nearby gates. The device is modeled realistically with appropriate material parameters and voltages applied to the gates, yielding an accurate confinement potential and Rashba coupling. At the end, we propose a spin-to-charge conversion device, which with an additional charge detector will allow for electron-spin state measurement.

We study theoretically the problem of electrons moving on a two-dimensional square lattice characterized by nearest-neighbor hopping terms of constant amplitude but random sign. The original motivation came from the discovery that this “bond-sign” disorder can be present in square lattices of epitaxially connected PbSe nanocrystals, which have been recently synthesized using colloidal routes. We investigate how this type of disorder tends to localize the electronic wave-functions and modifies the electronic structure. This is done via the calculation of the density-of-states, the participation ratio and the localization length. We show that, when the relative fraction $p$ of negative signs increases from 0% to 50%, the effect of the disorder on the wave functions saturates at a constant level when $p$ reaches values above $\sim 25\%$. This behavior reveals that the true disorder experienced by the electrons is not the nominal disorder defined by $p$ but a smaller part of it, which is irreducible due to frustrations. The amount of true disorder can be obtained by successive local gauge transformations as developed in the past to study models of spin glasses. In the thermodynamic limit, this irreducible gauge-invariant disorder induces localization of all electronic states, except at the center of the band where our calculations suggest that zero-energy states have a critical behavior. The particle-hole symmetry, which characterizes these disordered systems plays a crucial role in this behavior, as already found in lattices with random hopping or random magnetic flux, for example. In the case of lattices of PbSe nanocrystals, the effects of the bond-sign disorder are found to be weaker than those of more conventional types of disorder.

Magnetic order and the underlying magnetic model of the multiferroic pyroxene ${\mathrm{NaFeGe}}_2{\mathrm{O}}_6$ are systematically investigated by neutron powder diffraction, thermodynamic measurements, density-functional band-structure calculations, and Monte Carlo simulations. Upon cooling, ${\mathrm{NaFeGe}}_2{\mathrm{O}}_6$ first reveals one-dimensional spin-spin correlations in the paramagnetic state below about 50 K, uncovered by magnetic diffuse scattering. The sinusoidal spin-density wave with spins along the $a$ direction sets in at 13 K, followed by the cycloidal configuration with spins lying in the $\left(ac\right)$ plane below 11.6 K. Microscopically, the strongest magnetic coupling runs along the structural chains, $J_1\simeq 12\mathrm{K}$, which is likely related to the one-dimensional spin-spin correlations. The interchain couplings $J_2\simeq 3.8\mathrm{K}$ and $J_3\simeq 2.1\mathrm{K}$ are energetically well balanced and compete, thus giving rise to the incommensurate order, in sharp contrast to other transition-metal pyroxenes, in which one type of the interchain couplings prevails. The magnetic model of ${\mathrm{NaFeGe}}_2{\mathrm{O}}_6$ is further completed by the weak single-ion anisotropy along the $a$ direction. Our results resolve the earlier controversies regarding the magnetic order in ${\mathrm{NaFeGe}}_2{\mathrm{O}}_6$ and establish relevant symmetries of the magnetic structures. These results, combined with symmetry analysis, enable us to identify the possible mechanisms of the magnetoelectric coupling in this compound. We also elucidate microscopic conditions for the formation of incommensurate magnetic order in pyroxenes.

We theoretically investigate the ground-state spin texture and spin transport properties of triangular rings with on-site spins $S_q=\frac{1}{2}(q=1–3)$. In the limit of strong antiferromagnetic exchange coupling and weak spin-orbit coupling, we find it is possible to prepare a noncollinear degenerate ground state with a zero magnetic moment and a nonzero toroidal moment $\mathbf{\tau }=g{\mu }_B{\sum }_q{\mathbf{r}}_q\times {\mathbf{S}}_q$, aligned along the $C_3$-symmetry axis. These pure toroidal states can be prepared: (i) within the fourfold degenerate spin-frustrated ground state even without any spin-orbit coupling; (ii) within the ground Kramers doublet resulting from weak spin-orbit splitting of the fourfold degenerate frustrated manifold via Dzyaloshinskii-Moriya antiferromagnetic exchange coupling. We also investigate the relationship between toroidal states and chiral spin states, characterized by the eigenvalues of the scalar spin chirality operator $\widehat{\chi }=\frac{4}{\sqrt{3}}{\widehat{\mathbf{S}}}_1·{\widehat{\mathbf{S}}}_2\times {\widehat{\mathbf{S}}}_3$, and find that, since $[\widehat{\mathbf{\tau }},\widehat{\chi }]\ne 0$, it is not possible to prepare states that are both toroidal and chiral simultaneously. Finally, by setting up a quantum transport model in the Coulomb blockade regime, we find that a spin current injected through a spin-polarized source electrode into the triangle is partially reversed upon scattering with the molecular toroidal states. This spin-switching effect is, in fact, a signature of molecular spin-transfer torque, which can be harnessed to modify the nonequilibrium populations of the $+\tau$- and $-\tau$-toroidal states, thus, to induce a net toroidal magnetization in the device using a spin current.

In this paper the changes of magnetic properties with increasing disorder in the exchange-enhanced Pauli paramagnet ${\mathrm{YCo}}_2$ are discussed. The structural disorder is initially introduced by rapid quenching, while further changes on a micro- or nanoscale are caused by a high-pressure torsion (HPT). Values of the magnetic moment determined for the plastically deformed ribbons reach $0.10{\mu }_{\mathrm{B}}/\mathrm{Co}$ atom (for a sample subjected to the deformation at a pressure of 4 GPa) and $0.25{\mu }_{\mathrm{B}}/\mathrm{Co}$ (6 GPa) at 2 K. The magnetic moment arises not only from the surface of nanocrystals but also from volume. Ab initio calculations explained the influence of chemical disorder and different types of structural defects on the electronic structure and magnetic properties of ${\mathrm{YCo}}_2$-based Laves phases. The calculated magnetic ground states are in qualitative agreement with experimental results for all considered structures with point defects.

Damping mechanisms in magnetic systems determine the lifetime, diffusion, and transport properties of magnons, domain walls, magnetic vortices, and skyrmions. Based on the phenomenological Landau-Lifshitz-Gilbert equation, here the effective damping parameter in noncollinear magnetic systems is determined describing the linewidth in resonance experiments or the decay parameter in time-resolved measurements. It is shown how the effective damping can be calculated from the elliptic polarization of magnons, arising due to the noncollinear spin arrangement. It is concluded that the effective damping is larger than the Gilbert damping, and it may significantly differ between excitation modes. Numerical results for the effective damping are presented for the localized magnons in isolated skyrmions, with parameters based on the Pd/Fe/Ir(111) model–type system.

We uncover a novel mechanism for inducing a gapful phase in interacting many-body quantum chains. The mechanism is nonperturbative, being triggered only in the presence of both strong interactions and strong aperiodic (disordered) modulation. In the context of the critical antiferromagnetic spin-1/2 XXZ chain, we identify an emerging dimerization, which removes the system from criticality and stabilizes the novel phase. This mechanism is shown to be quite general in strongly interacting quantum chains in the presence of strongly modulated quasiperiodic disorder, which is, surprisingly, perturbatively irrelevant. Finally, we also characterize the associated quantum phase transition via the corresponding critical exponents and thermodynamic properties.

We have developed a simple but general analytical theory that elucidates the mechanism of spin-wave generation from interfaces between ferromagnetic media pumped by a uniform microwave magnetic field. Our calculations show that, provided there is a finite coupling between the two media, the amplitude of the emitted spin waves depends linearly on the difference between their magnetic susceptibilities. The theory is successfully applied to interpret qualitatively three recent experimental studies in which such a spin-wave emission was observed. Furthermore, we describe how our approach can be extended to several more complicated spin-wave excitation schemes employing electric, elastic, and optical stimuli.

We show that when there is a sudden transition from a small to a large Fermi surface at a field-induced quantum critical point—similar to what may have been observed in some heavy-fermion compounds—an additional term has to be taken into account in the Luttinger-Friedel sum rule. We calculate this additional term for a local model which has a field-induced quantum critical point (QCP) and show that it changes abruptly at the transition, such that it satisfies a generalized Luttinger-Friedel sum rule on each side of the transition, and characterizes the two Fermi-liquid phases separated by the QCP as a discrete (topological) index.

Phase diagram of a Hubbard model for Majorana fermions on the honeycomb lattice is explored using a combination of field theory, renormalization group, and mean-field arguments, as well as exact numerical diagonalization. Unlike the previously studied versions of the model we find that even weak interactions break symmetries and lead to interesting topological phases. We establish two topologically nontrivial phases at weak coupling, one gapped with chiral edge modes and the other gapless with antichiral edge modes. At strong coupling a mapping onto a novel frustrated spin-$\frac{1}{2}$ model suggests a highly entangled spin liquid ground state.

We investigate theoretically the spectrum of a graphenelike sample (honeycomb lattice) subjected to a perpendicular magnetic field and irradiated by circularly polarized light. This system is studied using the Floquet formalism, and the resulting Hofstadter spectrum is analyzed for different regimes of the driving frequency. For lower frequencies, resonances of various copies of the spectrum lead to intricate formations of topological gaps. In the Landau-level regime, new winglike gaps emerge upon reducing the driving frequency, thus revealing the possibility of dynamically tuning the formation of the Hofstadter butterfly. In this regime, an effective model may be analytically derived, which allows us to retrace the energy levels that exhibit avoided crossings and ultimately lead to gap structures with a winglike shape. At high frequencies, we find that gaps open for various fluxes at $E=0$, and upon increasing the amplitude of the driving, gaps also close and reopen at other energies. The topological invariants of these gaps are calculated and the resulting spectrum is elucidated. We suggest opportunities for experimental realization and discuss similarities with Landau-level structures in nondriven systems.

We present a finite temperature generalization of the COHSEX approximation to the quasiparticle electron self-energy, with explicit comparisons of the static and dynamic screened exchange (SEX) and Coulomb-hole (COH) contributions for the homogeneous electron gas. Consistent with the zero-temperature behavior, we show that the static SEX approximation agrees well with the fully dynamic SEX at all temperatures and densities. In contrast, dynamic corrections to the COH approximation are always significant, especially for high energy unoccupied states. Notably the COHSEX quasiparticle self-energy is complex valued at finite temperature even at the Fermi level.

We discuss the $SU\left(3\right)/\left[U\right(1)\times U(1\left)\right]$ nonlinear sigma model in 1+1D and, more broadly, its linearized counterparts. Such theories can be expressed as $U\left(1\right)\times U\left(1\right)$ gauge theories and therefore allow for two topological $\theta$ angles. These models provide a field theoretic description of the $SU\left(3\right)$ chains. We show that, for particular values of $\theta$ angles, a global symmetry group of such systems has a 't Hooft anomaly, which manifests itself as an inability to gauge the global symmetry group. By applying anomaly matching, the ground-state properties can be severely constrained. The anomaly matching is an avatar of the Lieb-Schultz-Mattis (LSM) theorem for the spin chain from which the field theory descends, and it forbids a trivially gapped ground state for particular $\theta$ angles. We generalize the statement of the LSM theorem and show that 't Hooft anomalies persist even under perturbations which break the spin-symmetry down to the discrete subgroup ${\mathbb{Z}}_3\times {\mathbb{Z}}_3\subset SU\left(3\right)/{\mathbb{Z}}_3$. In addition, the model can further be constrained by applying global inconsistency matching, which indicates the presence of a phase transition between different regions of $\theta$ angles. We use these constraints to give possible scenarios of the phase diagram. We also argue that at the special points of the phase diagram the anomalies are matched by the $SU\left(3\right)$ Wess-Zumino-Witten model. We generalize the discussion to the $SU\left(N\right)/U{\left(1\right)}^{N-1}$ nonlinear sigma models as well as the 't Hooft anomaly of the $SU\left(N\right)$ Wess-Zumino-Witten model, and show that they match. Finally, the $(2+1)$-dimensional extension is considered briefly, and we show that it has various 't Hooft anomalies leading to nontrivial consequences.

Although the quantum anomalous Hall (QAH) effect has been experimentally observed in several magnetically doped topological insulators, up to now, it only survives at a very low temperature. More suitable candidate QAH insulators that can work at high temperature are much desired. Here, we propose an experimentally feasible way to realize a robust QAH insulator: atomically dispersed transition metals (e.g., W) on a two-dimensional porous metal halide normal insulator (e.g., ${\mathrm{InI}}_3$), which has been developed as a state-of-the-art chemical technology broadly adopted for homogeneous catalysis. Based on the first-principles calculations, we predict that the atomic W embedded in an ${\mathrm{InI}}_3$ monolayer forms an intrinsic ferromagnetic QAH insulator, which exhibits robust uniform out-of-plane ferromagnetic order up to $\sim 160\mathrm{K}$ and a topologically nontrivial band gap of 56 meV with a nonzero Chern number $\left(\left|C\right|=2\right)$. We also study its magneto-optical Kerr effect and collective plasma excitation modes, which may help for further experimental verifications and measurement of interesting physical features of Dirac-like electronic dispersion. Our results introduce a feasible method to obtain the QAH effect, which may motivate intensive experimental interest in this field.