In layer-structured $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$, strong crystal electric field and spin-orbit coupling result in an intriguing spin-orbit entangled $J_{\mathrm{eff}}=1/2$ state, resembling the spin $S=1/2$ state of high-temperature superconducting cuprates. Our study unravels the intricate relationship between the lattice and the pseudospin interactions using uniaxial strain. Applied along the [1 1 0] direction, a compressive strain does not induce any effect. In contrast, the strain along [1 0 0] triggers a dramatic detwinning of the magnetic domains bringing the system to a single domain at around 0.04% strain. The strain driven detwinning rate is temperature independent showing that it does not exhibit a spontaneous orthorhombic lattice distortion driven by the pseudo-Jahn-Teller effect.

In this work, we train neural networks to predict transport properties of a quasi-one-dimensional tight-binding model with disordered on-site energies. In the case where the on-site energies of all sites in the scattering region pick up values randomly and independently from a given interval (model I), it is found that the performance of the neural network depends strongly on the system size. For a small system size, the neural network can predict accurately the conductance at the Fermi energy. With an increase in the system size, the mean absolute error ($\mathrm{MAE}$) increases. This tendency persists under an increase in the number of training samples and the number of neurons in the neural network. The physical mechanism behind this phenomenon can be ascribed to the universal conductance fluctuation. To reduce the effective dimension of features fed into the neural network, we consider the case of substitutional doping where the impurities have a fixed on-site energy, random position distribution, and variable concentration (model II). It is found that even for a relatively large system size, the trained neural network in model II performs much better in predicting the conductance at the Fermi energy. When the prediction target is changed to the average conductance under a finite bias (average value of 10 conductances at energies near the Fermi energy), the $\mathrm{MAE}$ can be reduced by almost a half. The small fluctuation of the average conductance reduces further the prediction difficulty of the neural network. The trained neural network can predict quickly and accurately the disorder-averaged transport properties for model II.

In the present study, the interplay among interaction, topology, quasiperiodicity, and non-Hermiticity is studied. The hard-core bosons model on a one-dimensional lattice with asymmetry hoppings and quasiperiodic onsite potentials is selected. This model, which preserves time-reversal symmetry (TRS), will exhibit three types of phase transition: Real-complex transition of eigenenergies, topological phase transition, and many-body localization (MBL) phase transition. For the real-complex transition, it is found that the imaginary parts of the eigenenergies are always suppressed by the MBL. Moreover, by calculating the winding number, a topological phase transition can be revealed with the increase of potential amplitude, and we find that the behavior is quite different from the single-particle systems. Based on our numerical results, we conjecture that these three types of phase transition occur at the same point in the thermodynamic limit, and the MBL transition of quasiperiodic system and disordered system should belong to different universality classes. Finally, we demonstrate that these phase transitions can profoundly affect the dynamics of the non-Hermitian many-body system.

We evaluate the qualitative and quantitative accuracy of various flavors of density functionals with and without accounting for dispersion corrections. Our test system is nickel in the form of bulk, surfaces, and nanoparticles for which we compute structural properties, bulk cohesive energies, surface energies, and work functions and compare to experimental data. We find that the inclusion of any dispersion, either by an a posteriori correction or by a self-consistent treatment by explicitly computing the nonlocal correlation contribution to the total energy, has a significant effect on the calculated properties and improves the quantitative comparison to experiments. Besides the quantitative agreement, we also investigate qualitative features by comparing Wulff shapes of metal nanoparticles as obtained using the different density functionals. We find that all tested functionals predict similar Wulff shapes for nickel nanoparticles but still have some small differences. These results show that the relative energies calculated using the semilocal GGA and meta-GGA functionals, with and without dispersion, are quite similar. Our findings can also be generalized to other systems when rationalized in terms of the computed reduced density gradients. We find that the distribution of reduced density gradients in a material is correlated to the steepness of the exchange enhancement factor and propose that this information can be used as a quantitative guide when it comes to picking the most appropriate density functional for specific target systems as well as when it comes to extrapolating DFT data to predict experiments.

We compare two crystallographic phases of the low-dimensional ${\mathrm{WP}}_2$ to better understand features of electron-electron and electron-phonon interactions in topological systems. The topological $\beta$ phase, a Weyl semimetal with a giant magnetoresistance, shows a larger intensity of electronic Raman scattering compared to the topologically trivial $\alpha$ phase. This intensity collapses for $T<T^{*}=20\mathrm{K}$, which evidences a crossover in the topological phase from marginal quasiparticles to a coherent low-temperature regime. In contrast, the topologically trivial $\alpha$ phase shows more pronounced signatures of electron-phonon interaction, i.e., an enhanced phonon linewidth and deviations from conventional anharmonicity in an intermediate-temperature regime. These effects provide evidence for an interesting interplay of electronic correlations and electron-phonon coupling. Interband and intraband electronic fluctuations can be distinguish by the energy range of electronic Raman scattering and their temperature dependence. Furthermore, we demonstrate a decisive dependency on symmetry that is critical to understand their interplay.

A series of full Heusler alloys, ${\text{Fe}}_2{\text{V}}_{1-x}{\text{W}}_x\text{Al}$, $0\le x\le 0.2$, was prepared and characterized, and relevant physical properties to account for the thermoelectric performance were studied in a wide temperature range. Additionally, off-stoichiometric samples with similar compositions have been included, and a 10% improvement of the thermoelectric figure of merit was obtained. The V/W substitution causes (i) a change of the main carrier type, from holes to electrons as evidenced from Seebeck and Hall measurements, and (ii) a substantial reduction of the lattice thermal conductivity due to a creation of lattice disorder by means of a distinct different mass and metallic radius upon the V/W substitution. Moreover $ZT$ values above 0.2 have been obtained. A microscopic understanding of the experimental data observed is revealed from ab initio calculations of the electronic and phononic structure. This series of alloys constitutes the basis for thin film systems, which have recently been found to exhibit $ZT$ values beyond those reported so far in the literature.

We investigate the condition for the photoinduced enhancement of an excitonic order in a two-orbital Hubbard model, which has been theoretically proposed in our previous work [Phys. Rev. B 97, 115105 (2018)], and analyze it from the viewpoint of the Rabi oscillation. Within the mean-field approximation, we simulate real-time dynamics of an excitonic insulator with a direct gap, where the pair condensation in the initial state is of BEC nature and the photoexcitation is introduced by electric dipole transitions. We first discuss that in the atomic limit our model is reduced to a two-level system that undergoes the Rabi oscillation, so that for single cycle pulses physical quantities after the photoirradiation are essentially determined by the ratio of the Rabi frequency to the pump-light frequency. Then, it is shown that this picture holds even in the case of nonzero transfer integrals where each one-particle state exhibits the Rabi oscillation leading to the enhancement of the excitonic order. We demonstrate that effects of electron-phonon interactions do not alter the results qualitatively. We also examine many-body dynamics by the exact diagonalization method on small clusters, which strongly suggests that our mechanism for the enhancement of the excitonic order survives even when quantum fluctuations are taken into account.

While electrostatic confinement in single-layer graphene and AA-stacked bilayer graphene is precluded by Klein tunneling and the gapless energy spectrum, we theoretically show that a circular domain wall that separates domains of single-layer graphene and AA-stacked bilayer graphene can provide bound states. Solving the Dirac-Weyl equation in the presence of a global mass potential and a local electrostatic potential, we obtain the energy spectrum of these states and the corresponding radial probability densities. Depending on the mass potential profile, regular bound states can exist inside the quantum dot and topological bound states at the domain wall. Controlling the electrostatic potential inside the quantum dot enables the simultaneous presence of both types of states. We find that the number of nodes of the radial wave function of the regular bound states inside the quantum dot is equal to the radial quantum number. The energy spectra of the bound states display anticrossings, reflecting coupling of electron- and holelike states.

Quantum anomalies offer a useful guide for the exploration of transport phenomena in topological semimetals. In this work, we introduce a model describing a semimetal in four spatial dimensions, whose nodal points act like tensor monopoles in momentum space. This system is shown to exhibit monopole-to-monopole phase transitions, as signaled by a change in the value of the topological Dixmier-Douady invariant as well as by the associated surface states on its boundary. We use this model to reveal an intriguing “4D parity magnetic effect,” which stems from a parity-type anomaly. In this effect, topological currents are induced upon time modulating the separation between the fictitious monopoles in the presence of a magnetic perturbation. Besides its theoretical implications in both condensed matter and quantum field theory, the peculiar four-dimensional (4D) magnetic effect revealed by our model could be measured by simulating higher-dimensional semimetals in synthetic matter.

Microwave absorption of a two-dimensional electron system (2DES) with a disk-shaped metallic gate has been investigated. We have found that relativistic plasmons are excited in the system when the gate is electrically connected to the 2DES. As a result, the 2DES is no longer electrically neutral during the plasma oscillation. This property significantly distinguishes “charged” relativistic plasmons from all other types of quasineutral plasma excitations. We have shown that the anomalously weak damping of relativistic plasmons is attributed to the hybridization of the plasmon with the photon mode of the external circuit. We have developed a theoretical basis for the relativistic plasmon phenomenon. The experimental data provide conclusive evidence in support of the theory.

For a given quantum impurity model, Wilson's numerical renormalization group (NRG) naturally defines a NRG Hamiltonian whose exact eigenstates and eigenenergies are obtainable. We give exact expressions for the free energy, static, as well as dynamical quantities of the NRG Hamiltonian. The dynamical spectral function from this approach contains full excitations including intra- and intershell excitations. For the spin-boson model, we compare the spectral function obtained from the present method and the full density matrix (FDM) method, showing that while both guarantee rigorous sum rule, the full excitation approach avoids the causality problem of FDM method.

Surface arcs (SAs) or Fermi arcs connecting pairs of bulk Weyl points with opposite chiralities are the signatures of Weyl semimetals in angle-resolved photoemission spectroscopy (ARPES) studies. The nontrivial topology of the bulk band structure guarantees the existence of these exotic Fermi arcs with connectivity that is strongly dependent on the surface. It has been theoretically proposed and experimentally confirmed that Fermi arcs at opposite surfaces can complete an unusual closed cyclotron orbit called a Weyl orbit, which leads to various intriguing transport properties. In this paper, a systematic ARPES study on opposite terminations (001) of type-II Weyl semimetal ${\mathrm{NbIrTe}}_4$ reveals different Fermi arc connections which result in a unique closed intersurface Fermi arc loop configurations (combining both projections of SAs) containing two pairs of Weyl points. In particular, the top surface ARPES data and corresponding ab initio calculation suggests that a topological Lifshitz transition occurs by tuning the chemical potential. SA rewiring on the top surface opens the intersurface arc loop at the Weyl node energy level into an open line, challenging the close-orbit description and leading to an unexplored scenario. Our results demonstrate the intrinsic alteration of Fermi arc connections and propose ${\mathrm{NbIrTe}}_4$ as a potential platform to examine Fermi-arc related phenomenon.

The ground-state electronic structure and magnetic behaviors of curium dioxide ($\mathrm{Cm}{\mathrm{O}}_2$) are controversial. In general, the formal valence of Cm ions in $\mathrm{Cm}{\mathrm{O}}_2$ should be tetravalent. The tetravalent state implies a $5f^{6.0}$ electronic configuration and a nonmagnetic ground state. However, it is in sharp contrast to the large magnetic moment measured by painstaking experiments. In order to clarify this contradiction, we tried to study the ground-state electronic structure of $\mathrm{Cm}{\mathrm{O}}_2$ by means of a combination of density functional theory and dynamical mean-field theory. We find that $\mathrm{Cm}{\mathrm{O}}_2$ is a wide-gap charge transfer insulator with a strong $5f$ valence state fluctuation. It indeed belongs to a mixed-valence compound. The predominant electronic configurations for Cm ions are $5f^{6.0}$ and $5f^{7.0}$. The resulting magnetic moment agrees quite well with the experimental value. Therefore, the magnetic puzzle in $\mathrm{Cm}{\mathrm{O}}_2$ can be appropriately explained by the mixed-valence scenario.

A quite general finite-size chain of fermions with $N$ internal degrees of freedom (flavors) and $O\left(N\right)$ symmetry is considered. In the case of the free boundary condition, we prove that the ground state in the invariant sector having exactly $m$ flavors with an odd particle number is represented by a single rank-$m$ antisymmetric multiplet. For the even-length chains, its particle-hole quantum number (if it's a good one) is given by the parity of the $m$. For the odd-length chains, the particle-hole symmetry leads to the twofold degeneracy among the conjugate multiplets. Similar statements are proven for the $O\left(N\right)$ mixed-spin chains in antisymmetric representations. The results are extended to the long-range interacting fermions and (partially) to the translation invariant chains.

We use the density functional theory and lattice dynamics calculations to investigate the properties of potassium superoxide ${\mathrm{KO}}_2$ in which spin, orbital, and lattice degrees of freedom are interrelated and determine the low-temperature phase. After calculating phonon dispersion relations in the high-temperature tetragonal $I4/mmm$ structure, we identify a soft phonon mode leading to the monoclinic $C2/c$ symmetry and optimize the crystal geometry resulting from this mode. Thus we reveal a displacive character of the structural transition with the group-subgroup relation between the tetragonal and monoclinic phases. We compare the electronic structure of ${\mathrm{KO}}_2$ with antiferromagnetic spin order in the tetragonal and monoclinic phases. We emphasize that realistic treatment of the electronic structure requires including the local Coulomb interaction $U$ in the valence orbitals of the ${\mathrm{O}}_2^{-}$ ions. The presence of the “Hubbard” $U$ leads to the gap opening at the Fermi energy in the tetragonal structure without orbital order but with weak spin-orbit interaction. We remark that the gap opening in the tetragonal phase could also be obtained when the orbital order is initiated in the calculations with a realistic value of $U$. Finally, we show that the local Coulomb interactions and the finite lattice distortion, which together lead to the orbital order via the Jahn-Teller effect, are responsible for the enhanced insulating gap in the monoclinic structure.

Transition-metal dichalcogenides are a class of widely studied two-dimensional layered materials which exist in various polymorphs. The $1T^{\prime }$ phase of $\mathrm{Mo}{\mathrm{Te}}_2$ is of prime importance as it has been reported to show quantum spin-Hall (QSH) behavior with a fairly large band gap of ∼60 meV, in contrast to most QSH materials known. It is noteworthy that though the monolayer $1T^{\prime }\text{−}\mathrm{Mo}{\mathrm{Te}}_2$ was initially predicted to show the QSH behavior, recent theoretical studies claim that the few-layered counterparts also exhibit higher-order topological behavior. Besides, $1T^{\prime }\text{−}\mathrm{Mo}{\mathrm{Te}}_2$ also undergoes a hysteretic phase transition to the $T_d$ phase (which is a type-II Weyl semimetal) by breaking the inversion symmetry of the crystal. While the phase transition between these two topological phases is of utmost importance, its study has been mostly restricted to bulk single-crystal flakes, thereby not sufficiently exploring the effect of dimensionality. We have studied the phase transition in $1T^{\prime }\text{−}\mathrm{Mo}{\mathrm{Te}}_2$ as a function of flake thickness. Altough our Raman studies show a suppression of the phase transition in the thin (thickness $<10\mathrm{nm}$) flakes [similar to the report by He et al., Phys. Rev. B 97, 041410 (2018)], we have experimentally demonstrated the possibility of stabilizing the desired phase ($1T^{\prime }$ or $T_{\mathrm{d}}$) at room temperature by charge doping. Further, we have observed clear signatures of electron-phonon coupling in $\mathrm{Mo}{\mathrm{Te}}_2$, which evolves as a function of flake thickness and charge doping.

Many-body localization (MBL) has been widely investigated for both fermions and bosons, it is, however, much less explored for anyons. Here we numerically calculate several physical characteristics related to MBL of a one-dimensional disordered anyon-Hubbard model in both localized and delocalized regions. We figure out a logarithmically slow growth of the half-chain entanglement entropy and an area-law rather than volume-law obedience for the highly excited eigenstates in the MBL phase. The adjacent energy level gap-ratio parameter is calculated and is found to exhibit a Poisson-like probability distribution in the deep MBL phase. By studying a hybridization parameter, we reveal an intriguing effect that the statistics can induce localization-delocalization transition. Several physical quantities, such as the half-chain entanglement, the adjacent energy level gap-ratio parameter, the long-time limit of the particle imbalance, and the critical disorder strength, are shown to be nonmonotonically dependent on the anyon statistical angle. Furthermore, a feasible scheme based on the spectroscopy of energy levels is proposed for the experimental observation of these statistically related properties.

We present a combined polarized neutron and x-ray scattering study on two enantiopure langasite single crystals aimed at the determination of their absolute structural and magnetic chiralities and the coupling between them. Our respective data sets unambiguously reveal two samples of opposite structural chirality, where the magnetic handedness is pinned by the structural one. Simple energy considerations of the magnetic exchange and single-ion anisotropy parameters reveal that it is not the Dzyaloshinskii-Moriya interaction but the local single-ion anisotropy on a triangular plaquette which plays a key role in stabilizing one of the two magnetic helices.

We report a Raman scattering study of $\mathrm{Y}\mathrm{Fe}{\mathrm{O}}_3$ orthoferrite. Polarized Raman measurements in wide temperature range and first-principles calculations provided the assignment of the observed phonon modes to vibrational symmetries and atomic displacements. We did not observe previously reported anomalies in the behavior of phonon frequencies near the temperature of the antiferromagnetic transition. Temperature behavior of frequencies and linewidths of most phonons may be described by known expressions for cubic and quartic anharmonicity, which implies a significant contribution of phonon-phonon interaction. At the same time, several phonons exhibit asymmetric profiles and anomalies in the behavior of their widths, which indicates an interaction with a continuum of some excitations. The most striking evidence of this is the observation of the coupling of the $B_{2g}$ phonon at 640 ${\mathrm{cm}}^{-1}$ with the two-magnon continuum, expressed in anomalous phonon self-energies in the vicinity of $T_N$. The appearance of a broad continuum of low-frequency excitations when approaching $T_N$, which is possibly induced by the damping of spin excitations due to interaction with the lattice, has been revealed. This interaction can lead to additional contributions to the renormalization of phonon self-energies, formally described by formulas for anharmonic effects. Phonon scattering induced by defects has been identified. The temperature behavior of resonant second-order phonon and two-magnon scattering was studied in detail.