The quasi-one-dimensional antiferromagnetic Ising-like compound ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ has been shown to be describable by the Tomonaga-Luttinger liquid theory in its gapless phase induced by a magnetic field applied along the Ising axis. Above 3.9 T, this leads to an exotic field-induced low-temperature magnetic order, made of a longitudinal incommensurate spin-density wave, stabilized by weak interchain interactions. By single-crystal neutron diffraction we explore the destabilization of this phase at a higher magnetic field. We evidence a transition at around 8.5 T towards a more conventional magnetic structure with antiferromagnetic components in the plane perpendicular to the magnetic field. The phase diagram boundaries and the nature of this second field-induced phase are discussed with respect to previous results obtained by means of nuclear magnetic resonance and electron spin resonance, and in the framework of the simple model based on the Tomonaga-Luttinger liquid theory, which obviously has to be refined in this complex system.

The evolution of the filamentary 3-Kelvin (3-K) superconducting phase at the interface between ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and Ru-metal inclusions is discussed for Pb-Ru-${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ contacts. Using the Ginzburg-Landau model, the influence of proximity-induced superconductivity in Ru on the topology of the 3-K phase is analyzed. Because the $s$-wave order parameter in Ru favors a 3-K state of trivial topology, the onset temperature of the phase with a nontrivial topology, which is compatible with the bulk phase of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, is essentially reduced to the bulk transition temperature. Because the topology of the superconducting state in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is crucial for the Josephson effect through Pb-Ru-${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ contacts, this model qualitatively reproduces the experimental observation of the anomalous temperature dependence on the critical current.

We study the fractional Josephson effect in a particle-number-conserving system consisting of a quasi-one-dimensional superconductor coupled to a nanowire or an edge carrying $e/m$ fractional charge excitations with $m$ being an odd integer. We show that, due to the topological ground-state degeneracy in the system, the periodicity of the supercurrent on magnetic flux through the superconducting loop is nontrivial, which provides a possibility to detect topological phases of matter by the dc supercurrent measurement. Using a microscopic model for the nanowire and quasi-one-dimensional superconductor, we derived an effective low-energy theory for the system which takes into account effects of quantum phase fluctuations. We discuss the stability of the fractional Josephson effect with respect to the quantum phase slips in a mesoscopic superconducting ring with a finite charging energy.

The superconducting properties of a recently discovered high-$T_{\mathrm{c}}$ superconductor, Sr/ammonia-intercalated FeSe, have been measured using pulsed magnetic fields down to 4.2 K and muon spin spectroscopy down to 1.5 K. This compound exhibits intrinsic disorder resulting from random stacking of the FeSe layers along the $c$ axis that is not present in other intercalates of the same family. This arises because the coordination requirements of the intercalated Sr and ammonia moieties imply that the interlayer stacking (along $c$) involves a translation of either $\mathbf{a}/2$ or $\mathbf{b}/2$ that locally breaks tetragonal symmetry. The result of this stacking arrangement is that the Fe ions in this compound describe a body-centered-tetragonal lattice in contrast to the primitive arrangement of Fe ions described in all other Fe-based superconductors. In pulsed magnetic fields, the upper critical field $H_{\text{c2}}$ was found to increase on cooling with an upward curvature that is commonly seen in type-II superconductors of a multiband nature. Fitting the data to a two-band model and extrapolation to absolute zero gave a maximum upper critical field ${\mu }_0{H}_{\mathrm{c}2}\left(0\right)$ of $33\left(2\right)\text{T}$. A clear superconducting transition with a diamagnetic shift was also observed in transverse-field muon measurements at $T_{\text{c}}\approx 36.3\left(2\right)\text{K}$. These results demonstrate that robust superconductivity in these intercalated FeSe systems does not rely on perfect structural coherence along the $c$ axis.

We report on de Haas–van Alphen (dHvA) and band-structure studies of the iridium-pnictide superconductor ${\mathrm{BaIr}}_2{\mathrm{P}}_2$ ($T_c=2.1$ K). The observed dHvA frequencies can be well understood by our band-structure calculations with two bands crossing the Fermi energy leading to a strongly corrugated Fermi-surface cylinder around the X point and a highly evolved, multiconnected Fermi surface extending over the whole Brillouin zone. The experimental effective masses are found to be considerably larger than the calculated band masses suggesting strong many-body interactions. Nevertheless, $T_c$ remains only moderate in ${\mathrm{BaIr}}_2{\mathrm{P}}_2$ contrary to isostructural iron pnictides which probably is related to the largely different Fermi-surface topologies in these materials.

We experimentally investigate the dielectric response of the low-dimensional gapped quantum magnet ${\mathrm{Cu}}_2{\mathrm{Cl}}_4·{\mathrm{H}}_8{\mathrm{C}}_4{\mathrm{SO}}_2$ near a magnetic-field-induced quantum critical point, which separates the quantum-disordered and helimagnetic ground states. The observed magnetocapacitive effect originates from an improper ferroelectric nature of the transition, which itself is perhaps one of the best known realizations of Bose-Einstein condensation of magnons. Despite that, we find that the magnetocapacitive effect associated with the transition exhibits huge and very unusual anharmonicities.

Despite the long history, so far there is no general theoretical framework for calculating the acoustic emission spectrum accompanying any plastic deformation. We set up a discrete wave equation with plastic strain rate as a source term and include the Rayleigh-dissipation function to represent dissipation accompanying acoustic emission. We devise a method of bridging the widely separated time scales of plastic deformation and elastic degrees of freedom. While this equation is applicable to any type of plastic deformation, it should be supplemented by evolution equations for the dislocation microstructure for calculating the plastic strain rate. The efficacy of the framework is illustrated by considering three distinct cases of plastic deformation. The first one is the acoustic emission during a typical continuous yield exhibiting a smooth stress-strain curve. We first construct an appropriate set of evolution equations for two types of dislocation densities and then show that the shape of the model stress-strain curve and accompanying acoustic emission spectrum match very well with experimental results. The second and the third are the more complex cases of the Portevin-Le Chatelier bands and the Lüders band. These two cases are dealt with in the context of the Ananthakrishna model since the model predicts the three types of the Portevin-Le Chatelier bands and also Lüders-like bands. Our results show that for the type-C bands where the serration amplitude is large, the acoustic emission spectrum consists of well-separated bursts of acoustic emission. At higher strain rates of hopping type-B bands, the burst-type acoustic emission spectrum tends to overlap, forming a nearly continuous background with some sharp acoustic emission bursts. The latter can be identified with the nucleation of new bands. The acoustic emission spectrum associated with the continuously propagating type-A band is continuous. These predictions are consistent with experimental results. More importantly, our study shows that the low-amplitude continuous acoustic emission spectrum seen in both the type-B and type-A band regimes is directly correlated to small-amplitude serrations induced by propagating bands. The acoustic emission spectrum of the Lüders-like band matches with recent experiments as well. In all of these cases, acoustic emission signals are burstlike, reflecting the intermittent character of dislocation-mediated plastic flow.

I extend the analytical arguments of Metlitski and Grover (arXiv:1112.5166) to compute the entanglement spectrum and entanglement entropy of coplanar antiferromagnets with $SO\left(3\right)$ order parameter symmetry. The low-energy states in the entanglement spectrum exhibit the tower-of-states structure, as is expected for systems that undergo spontaneous symmetry breaking. My results are consistent with numerical results on the triangular and Kagomé lattice.

We investigate a type of magnetoelectric effect in rare-earth iron garnets related to the presence of domain walls of the iron subsystem. The lack of space inversion in the dodecahedral rare-earth ion environments allows ferroelectric ordering in these materials. Electric polarization is absent in single-domain samples due to the antiferroelectricity of the electric-dipole structure. We show that the magnetic domain walls of the iron subsystem generate an effective inhomogeneous magnetic field arising from the rare-earth iron exchange interaction, which leads to emergence of electric polarization. A hallmark of this effect is that magnetoelectricity is independent of the type of magnetic domain walls. In contrast to the accepted concepts, we argue that the electric polarization in rare-earth iron garnets also occurs in the vicinity of Bloch magnetic walls. Magnetoelectric properties are inherent to rare-earth iron garnets in the presence of magnetic domain structure in any temperature range. Rare-earth iron garnets refer to multiferroics with $d$ and $f$ interacting subsystems.

The submicrosecond dynamics of thermomagnetic avalanches in superconducting films with nonconducting holes (antidots) is considered. When such an avalanche reaches a hole, it is quickly filled with magnetic flux, and often its rim becomes unstable and a second avalanche is nucleated. In this work the time- and space-resolved behavior of such cascading avalanche behavior is determined using numerical simulations. Results are presented for films with holes of different shape. It is found that holes with sharp corners are those that most frequently create secondary avalanches, and they tend to nucleate in corners. Magneto-optical imaging of Nb films patterned with the same set of holes strongly supports the numerical results.

In the hole-doped cuprate superconductors, the superconducting transition temperature $T_c$ exhibits a domelike feature against the doping rate. By contrast, recent experiments reveal that $T_c$ in the electron-doped systems monotonically increases as the doping is reduced, at least up to a very small doping rate. Here we show that this asymmetry is reproduced by performing a two-particle self-consistent analysis for the three-band model of the ${\mathrm{CuO}}_2$ plane. This is explained as a combined effect of the intrinsic electron-hole asymmetry in systems comprising $\mathrm{Cu}3d$ and $\mathrm{O}2p$ orbitals and the band-filling-dependent vertex correction.

We propose to calculate spectral functions of quantum impurity models using the time evolving block decimation (TEBD) for matrix product states. The resolution of the spectral function is improved by a so-called linear prediction approach. We apply the method as an impurity solver within the dynamical mean-field theory (DMFT) for the single- and two-band Hubbard model on the Bethe lattice. For the single-band model, we observe sharp features at the inner edges of the Hubbard bands. A finite-size scaling shows that they remain present in the thermodynamic limit. We analyze the real time-dependence of the double occupation after adding a single electron and observe oscillations at the same energy as the sharp feature in the Hubbard band, indicating a long-lived coherent superposition of states that correspond to the Kondo peak and the side peaks. For a two-band Hubbard model, we observe an even richer structure in the Hubbard bands, which cannot be related to a multiplet structure of the impurity, in addition to sharp excitations at the band edges of a type similar to the single-band case.

Projected entangled-pair states (PEPS) provide a framework for the construction of models where a single tensor gives rise to both Hamiltonian and ground state wave function on the same footing. A key problem is to characterize the behavior which emerges in the system in terms of the properties of the tensor, and thus of the Hamiltonian. In this paper, we consider PEPS models with ${\mathbb{Z}}_2$ on-site symmetry and study the occurrence of long-range order and spontaneous symmetry breaking. We show how long-range order is connected to a degeneracy in the spectrum of the PEPS transfer operator, and how the latter gives rise to spontaneous symmetry breaking under perturbations. We provide a succinct characterization of the symmetry-broken states in terms of the PEPS tensor, and find that using the symmetry-broken states we can derive a local entanglement Hamiltonian, thereby restoring locality of the entanglement Hamiltonian for all gapped phases.

Insulating $\mathrm{FeGa}_3$ poses peculiar puzzles beyond the occurrence of an electronic gap in an intermetallic compound. This Fe-based material has a very distinctive structural characteristic with the Fe atoms occurring in dimers. The insulating gap can be described comparably well in either the weakly correlated limit or the strongly correlated limit within density functional theory viewpoints, where the latter corresponds to singlet formation on the ${\mathrm{Fe}}_2$ dimers. Though most of the calculated occupied Wannier functions are an admixture of Fe $3d$ and Ga $4s$ or $4p$ states, there is a single bonding-type Wannier function per spin centered on each ${\mathrm{Fe}}_2$ dimer. Density functional theory methods have been applied to follow the evolution of the magnetic properties and electronic spectrum with doping, where unusual behavior is observed experimentally. Both electron and hole doping are considered, by Ge and Zn on the Ga site, and by Co and Mn on the Fe site, the latter introducing direct disturbance of the ${\mathrm{Fe}}_2$ dimer. Results from weakly and strongly correlated pictures are compared. Regardless of the method, magnetism including itinerant phases appears readily with doping. The correlated picture suggests that in the low doping limit Mn (for Fe) produces an in-gap hole state, while Co (for Fe) introduces a localized electronic gap state.

We study the quench dynamics of an Anderson impurity model using the configuration interaction (CI) method. In particular, we focus on the relaxation behavior of the impurity occupation. The system is found to behave very differently in the weak-coupling and strong-coupling regimes. In the weak-coupling regime, the impurity occupation relaxes to a time-independent constant quickly after only a few oscillations. In the strong-coupling regime, the impurity occupation develops a fast oscillation, with a much slower relaxation. We show that it is the multipeak structure in the many-body energy spectrum that separates these two regimes. The characteristic behavior, including the power-law decay and the period of oscillation, can also be related to certain features in the many-body energy spectrum. The respective advantages of several impurity solvers are discussed, and the convergence of different CI truncation schemes is provided.

We report electrical measurements of the current-induced spin polarization of the surface current in topological insulator devices where contributions from bulk and surface conduction can be disentangled by electrical gating. The devices use a ferromagnetic tunnel junction (permalloy/$\mathrm{Al}{}_2{\mathrm{O}}_3$) as a spin detector on a back-gated ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ channel. We observe hysteretic voltage signals as the magnetization of the detector ferromagnet is switched parallel or antiparallel to the spin polarization of the surface current. The amplitude of the detected voltage change is linearly proportional to the applied dc bias current in the ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ channel. As the chemical potential is tuned from the bulk bands into the surface state band, we observe an enhancement of the spin-dependent voltages up to 300% within the range of the electrostatic gating. Using a simple model, we extract the spin polarization near charge neutrality (i.e., the Dirac point).

The two-dimensional electron system (2DES) in $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (LAO/STO) heterostructures, showing superconductivity below 200 mK at ambient pressure, displays incipient localization under hydrostatic pressure $p$. Even small $p$ significantly affects the transport properties for $10\mathrm{K}<T<100\mathrm{K}$ where impurity scattering dominates. The distinct $T$ dependence of impurity scattering results in a minimum of the sheet resistance $R_{\mathrm{s}}\left(T\right)$, which has been often attributed to Kondo scattering. We show that the strong suppression of the dielectric permittivity $\varepsilon \left(p\right)$ of STO enhances impurity scattering via the loss of screening and leads to incipient localization of charge carriers with increasing hydrostatic pressure. For $T<10\mathrm{K}$ negative logarithmic corrections to the conductance are attributed to electron-electron interaction. Weak localization and antilocalization, if present, are dominated by the pivotal role of impurity scattering by, e.g., oxygen vacancies, for the transport properties of the 2DES in LAO/STO heterostructures. This interpretation is favorited by exploiting the strong dependence of the dielectric permittivity of STO on pressure. Measurements of the magnetoresistance corroborate this interpretation.

We study theoretically the 2D HgTe/CdTe quantum well topological insulator illuminated by circularly polarized light with frequencies higher than the difference between the equilibrium Fermi level and the bottom of the conduction band (THz range). We show that electron-hole asymmetry results in spin-dependent electric dipole transitions between edge and bulk states, and we predict an occurrence of a circular photocurrent. If the edge state is tunnel-coupled to a conductor, then the photocurrent can be detected by measuring an electromotive force in the conductor, which is proportional to the photocurrent.

The adsorption of MgO molecules on a Fe(001) surface was studied using density functional theory and projector augmented-wave methods. The energetically most favored configurations for different adsorption sites considered were identified. The most preferable adsorption geometry is when the MgO molecules are parallel to the surface, with Mg in the interstitial site and O in the on-top site of the Fe atom. During the adsorption of subsequent MgO molecules in this geometry, a sharp, nonoxidized interface is formed between the MgO adlayer and the Fe(001) surface. The adsorption of MgO perpendicular to the surface, with oxygen incorporated in the topmost Fe layer, is less probable, but it may lead to the formation of the FeO layer when stabilized with an excess of oxygen atoms. Structural, electronic, and magnetic properties of both interface types were examined for the MgO coverage from 1/9 to 1 monolayer (ML). Electronic and magnetic properties are sensitive to the MgO coverage. For lower coverage of MgO, clear hybridization between the Fe $3d$ and O $2p$ states is shown. The average magnetic moment of the surface Fe atoms is reduced with coverage, achieving $2.78{\mu }_{\mathrm{B}}$ for 1 ML of MgO.

In general, the stability of a band crossing point indicates the presence of a quantized topological number associated with it. In particular, the recent discovery of three-dimensional Dirac semimetals in ${\mathrm{Na}}_3\mathrm{Bi}$ and ${\mathrm{Cd}}_3{\mathrm{As}}_2$ demonstrates that a Dirac point with fourfold degeneracy can be stable as long as certain crystalline symmetries are supplemented in addition to the time-reversal and inversion symmetries. However, the topological charges associated with ${\mathrm{Na}}_3\mathrm{Bi}$ and ${\mathrm{Cd}}_3{\mathrm{As}}_2$ are not clarified yet. In this work, we identify the topological charge of three-dimensional Dirac points. It is found that although the simultaneous presence of the time-reversal and inversion symmetries forces the net chiral charge to vanish, a Dirac point can carry another quantized topological charge when an additional rotation symmetry is considered. Two different classes of Dirac semimetals are identified depending on the nature of the rotation symmetries. First, the conventional symmorphic rotational symmetry which commutes with the inversion gives rise to the class I Dirac semimetals having a pair of Dirac points on the rotation axes. Since the topological charges of each pair of Dirac points have the opposite sign, a pair creation or a pair annihilation is required to change the number of Dirac points in the momentum space. On the other hand, the class II Dirac semimetals possess a single isolated Dirac point at a time-reversal invariant momentum, which is protected by a screw rotation. The nonsymmorphic nature of screw rotations allows the anticommutation relation between the rotation and inversion symmetries, which enables to circumvent the doubling of the number of Dirac points and create a single Dirac point at the Brillouin zone boundary.