An important contribution to the thermoelectric and spin-caloric transport properties in magnetic materials at elevated temperatures is the formation of a spin-disordered state due to local moment fluctuations. This effect has not been largely investigated so far. We focus on various magnetic nanostructures of CrTe in the form of thin layers or nanowires embedded in ZnTe matrix, motivated by the miniaturization of spintronics devices and by recent suggestions that magnetic nanostructures can lead to extraordinary thermoelectric effects due to quantum confinement. The electronic structure of the studied systems is calculated within the multiple scattering screened Korringa-Kohn-Rostoker Green function (KKR-GF) framework. The Monte Carlo method is used to simulate the magnetization in the temperature induced spin disorder. The transport properties are evaluated from the transmission probability obtained using the Baranger-Stone approach within the KKR-GF framework. We find qualitative and quantitative changes in the thermoelectric and spin-caloric coefficients when spin disorder is included in the calculation. Furthermore, we show that substitutional impurities in CrTe nanowires could considerably enhance the Seebeck coefficient and the thermoelectric figure of merit.

A phenomenological description of the phase diagram of the ferromagnet superconductor URhGe is presented. In the frame of Landau phenomenological theory it was found that the phase transition between anisotropic ferromagnetic and paramagnetic states under a strong enough magnetic field perpendicular to the direction of easy magnetization changes from the second- to the first-order type. It is shown that magnetic susceptibility corresponding to longitudinal magnetic fluctuations strongly increases in the vicinity of the first-order transition stimulating reentrance of the superconducting state. The reentrant superconductivity observed near the first-order transition line at temperatures about twice lower than the tricritical point temperature exists both in ferromagnet and in paramagnet states. The critical temperature of transition to the superconducting state falls down at the intersection with the line of ferromagnet-paramagnet phase transition.

We have measured the current-voltage characteristics of a Josephson junction with tunable Josephson energy $E_J$ embedded in an inductive environment provided by a chain of SQUIDs. Such an environment induces localization of the charge on the junction, which results in an enhancement of the zero-bias resistance of the circuit. We explain this result quantitatively in terms of the Bloch band dynamics of the localized charge. This dynamics is governed by charge diffusion in the lowest Bloch band of the Josephson junction as well as by Landau-Zener transitions out of the lowest band into the higher bands. In addition, the frequencies corresponding to the self-resonant modes of the SQUID array exceed the Josephson energy $E_J$ of the tunable junction, which results in a renormalization of $E_J$, and, as a consequence, an increase of the effective bandwidth of the lowest Bloch band.

A novel stable crystal structure is discovered in a variety of $AB{O}_3,AB{F}_3$, and $A_2{O}_3$ compounds, via the use of first principles. This novel structure appears under hydrostatic pressure, and can be considered to be a post-post-perovskite phase. It provides a successful solution to experimental puzzles in important systems and is characterized by one-dimensional channels linked by a group of two via edge-sharing oxygen/fluorine octahedra. Such organization automatically results in anisotropic elastic properties and new magnetic arrangements. Depending on the system of choice, this post-post-perovskite structure also possesses electronic band gaps ranging from zero to $\simeq 10\mathrm{eV}$ being direct or indirect in nature.

The use of a special quasirandom structure (SQS) is a rational and efficient way to approximate random alloys. A wide variety of physical properties of metallic and semiconductor random alloys have been successfully estimated by a combination of an SQS and density functional theory calculation. Here, we investigate the application of an SQS to the ionic multicomponent systems with configurations of heterovalent ions, including point-charge lattices, ${\mathrm{MgAl}}_2{\mathrm{O}}_4$ and ${\mathrm{ZnSnP}}_2$. It is found that the physical properties do not converge with the supercell size of the SQS. This is ascribed to the fact that the correlation functions of long-range clusters larger than the period of the supercell are not optimized in the SQS. However, we demonstrate that the physical properties of the perfectly disordered structure can be estimated by linear extrapolation using the inverse of the supercell size.

The magnetic properties of $R_{1-x}{\text{Y}}_x{\text{B}}_4$ ($R=\mathrm{Tb}$ and Dy) single crystals were investigated. As the concentration of Y increased, the antiferromagnetic transition temperature was observed to systematically decrease, and an exotic weak ferromagnetic transition was also observed at the antiferromagnetic transition temperature. The weak ferromagnetism occurred in the magnetic easy plane, (001) for ${\text{Tb}}_{1-x}{\text{Y}}_x{\text{B}}_4$, and along the magnetic easy axis, [001] for ${\text{Dy}}_{1-x}{\text{Y}}_x{\text{B}}_4$. For ${\text{Tb}}_{1-x}{\text{Y}}_x{\text{B}}_4$, the saturated ferromagnetic signal at $T=2$ K was also anisotropic, even in the (001) plane; that is, there was a local maximum along the principal axis and a local minimum along the [110] axis (equivalent to those along Tb dimers), where the parent compound, ${\text{TbB}}_4$, was characterized as a magnetically frustrated dimer in a Shastry-Sutherland lattice. In addition, the saturated signal exhibited a strong dependence on the Y concentration, i.e., the signal reached a maximum at $x\simeq 0.35$, indicating that the emergence of the weak ferromagnetism is not due to an individual atomic effect but rather to a collective correlation effect. This is an experimental observation of the development of weak ferromagnetism in a magnetically frustrated system induced by replacing a magnetic element with a nonmagnetic element.

We study the interplay between the Kitaev and Ising interactions on a ladder geometry. We show that the ground state of the Kitaev ladder is a symmetry-protected topological (SPT) phase, which is protected by a ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry. The nature of the SPT phase is confirmed by degeneracy of the entanglement spectrum. Nonlocal order parameters that indirectly measure phase factors (inequivalent projective representations of the symmetries) of the ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry explicitly show the protection of the SPT phase under the ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry. We derive the effective theory to describe the topological phase transition on the ladder geometry, which is given by a transverse field Ising model with/without next-nearest-neighbor coupling based on the primary Ising configurations. The ladder has three phases, namely, the Kitaev SPT, symmetry-broken ferro/antiferromagnetic order, and classical spin liquid. The nonzero quantum critical point and its corresponding central charge are provided by the effective theory, which are in full agreement with the numerical results, i.e., the divergence of entanglement entropy at the critical point and change of the entanglement spectrum degeneracy. The central charge of the critical points are either $c=1$ or $c=2$, with the magnetization and correlation exponents being 1/4 and 1/2, respectively. The transition from the classical spin-liquid phase of the frustrated Ising ladder to the Kitaev SPT phase is mediated by a floating phase, which shows strong finite entanglement scaling.

We consider the Hall conductivity due to the motion of a vortex in a lattice model of a clean superconductor, using a combination of general arguments, unrestricted Hartree-Fock calculations, and exact diagonalization. In the weak-coupling limit, $k_F\xi \gg 1$, the sign of the Hall response of the superconducting state is the same as that of the normal (nonsuperconducting) state. For intermediate and strong coupling ($k_F\xi \sim 1$), however, we find that the sign of the Hall response in the superconducting state can be opposite to that of the normal state. In addition, we find that the sign reversal of the Hall response is correlated with a discontinuous change in the density profile at the vortex core. Implications for experiments in the cuprate superconductors are discussed.

The Anderson impurity model with a density of states $\rho \left(\varepsilon \right)\propto {\left|\varepsilon \right|}^r$ containing a power-law pseudogap centered on the Fermi energy ($\varepsilon =0$) features for $0<r<1$ a Kondo-destruction quantum critical point (QCP) separating Kondo-screened and local-moment phases. The observation of mixed valency in quantum critical $\beta \text{−}{\mathrm{YbAlB}}_4$ has prompted study of this model away from particle-hole symmetry. The critical spin response associated with all Kondo destruction QCPs has been shown to be accompanied, for $r=0.6$ and noninteger occupation of the impurity site, by a divergence of the local charge susceptibility on both sides of the QCP. In this work, we use the numerical renormalization-group method to characterize the Kondo-destruction charge response using five critical exponents, which are found to assume nontrivial values only for $0.55\lesssim r<1$. For $0<r\lesssim 0.55$, by contrast, the local charge susceptibility shows no divergence at the QCP, but rather exhibits nonanalytic corrections to a regular leading behavior. Both the charge critical exponents and the previously obtained spin critical exponents satisfy a set of scaling relations derived from an ansatz for the free energy near the QCP. These critical exponents can all be expressed in terms of just two underlying exponents: the correlation-length exponent $\nu \left(r\right)$ and the gap exponent $\Delta \left(r\right)$. The ansatz predicts a divergent local charge susceptibility for $\nu <2$, which coincides closely with the observed range $0.55\lesssim r<1$. Many of these results are argued to generalize to interacting QCPs that have been found in other quantum impurity models.

A theory is developed for the density and temperature-dependent carrier conductivity in doped three-dimensional (3D) Dirac materials focusing on resistive scattering from screened Coulomb disorder due to random charged impurities (e.g., dopant ions and unintentional background impurities). The theory applies both in the undoped intrinsic (“high-temperature,” $T\gg T_F)$ and the doped extrinsic (“low-temperature,” $T\ll T_F)$ limit with analytical scaling properties for the carrier conductivity obtained in both regimes, where $T_F$ is the Fermi temperature corresponding to the doped free carrier density (electrons or holes). The scaling properties describing how the conductivity depends on the density and temperature can be used to establish the Dirac nature of 3D systems through transport measurements. We also consider the temperature-dependent conductivity limited by the acoustic phonon scattering in 3D Dirac materials. In addition, we theoretically calculate and compare the single-particle relaxation time ${\tau }_s$, defining the quantum level broadening, and the transport scattering time ${\tau }_t$, defining the conductivity, in the presence of screened charged impurity scattering. A critical quantitative analysis of the ${\tau }_t/{\tau }_s$ results for 3D Dirac materials in the presence of long-range screened Coulomb disorder is provided.

The electronic structure of CaO films of 10–60 monolayer thickness grown on Mo(001) has been investigated with synchrotron-mediated x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Upon annealing or reducing the thickness of the film, a rigid shift of the CaO bands to lower energy is revealed. This evolution is explained with a temperature-induced diffusion of Mo ions from the metal substrate to the oxide and their accumulation in the interface region of the film. The Mo substitutes divalent Ca species in the rocksalt lattice and is able to release electrons to the system. The subsequent changes in the Mo oxidation state have been followed with high-resolution XPS measurements. While near-interface Mo transfers extra electrons back to the substrate, generating an interface dipole that gives rise to the observed band shift, near-surface species are able to exchange electrons with adsorbates bound to the oxide surface. For example, exposure of ${\mathrm{O}}_2$ results in the formation of superoxo species on the oxide surface, as revealed from STM measurements. Mo interdiffusion is therefore responsible for the pronounced donor character of the initially inert oxide, and largely modifies its adsorption and reactivity behavior.

We analyze the fundamental process of crystallization of silicon nanoclusters by means of molecular dynamics simulations, complemented by magnetron-sputter inert gas condensation, which was used to synthesize polycrystalline silicon nanoclusters with good size control. We utilize two well-established Si interatomic potentials: the Stillinger-Weber and the Tersoff III. Both the simulations and experiments show that upon cooling down by an Ar gas thermal bath, initially liquid, free-standing Si nanocluster can grow multiple crystal nuclei, which drive their transition into polycrystalline solid nanoclusters. The simulations allow detailed analysis of the mechanism, and show that the crystallization temperature is size-dependent and that the probability of crystalline phase nucleation depends on the highest temperature the cluster reaches during the initial condensation and the cooling rate after it.

The structure and resistive properties of buried ${\mathrm{SrVO}}_3$ layers between two insulating ${\mathrm{LaVO}}_3$ layers are investigated by varying the thickness of the ${\mathrm{SrVO}}_3$ layers between 3 and 35 monolayers. The thickest ${\mathrm{SrVO}}_3$ layer shows a bulklike metallic behavior, while in the thinnest ${\mathrm{SrVO}}_3$ layer, a weak localization regime is observed below 100 K manifesting a logarithmic temperature dependence. Angular-dependent magnetoresistance measurements indicate a cylindric shape of the Fermi surface, and therefore a two-dimensional transport in the thinnest buried ${\mathrm{SrVO}}_3$ layer. The modification of the charge carrier properties by the reduced thickness of the ${\mathrm{SrVO}}_3$ layer are furthermore underlined by the appearance of a relatively strong positive magnetoresistance under a magnetic field perpendicular to the sample surface. The present study therefore highlights a way to synthesize oxide electrodes with reduced dimension for future oxide electronics application.

Motivated by the itinerant band structure of high-$T_c$ iron pnictides, which exhibit four Dirac cones in the bulk, we demonstrate the prospect of pnictides with transition elements to be topological insulators in two dimensions. In this report, we explore interaction-induced topological phases, in contrast to the spin-orbit-coupling interaction, as the crucial mechanism for tuning Dirac metals into $Z_2$-topological insulators protected by time reversal and mirror symmetries. We find spontaneous orbital currents generated through nearest-neighbor interorbital Coulomb interaction in the $t_{2g}$ manifold of the $d$ orbitals. When spin degrees of freedom are incorporated, spontaneous orbital currents lead to two stable topological phases of the ground state. The first topological insulator is an anomalous orbital Hall phase, characterized by an even Chern number, while the second topological insulator is realized by protected mirror symmetries with a $Z_2$ index.

In the recently synthesized ${\mathrm{Li}}_x{\left({\mathrm{NH}}_2\right)}_y{\left({\mathrm{NH}}_3\right)}_z{\mathrm{Fe}}_2{\mathrm{Se}}_2$ family of iron chalcogenides, a molecular spacer consisting of lithium ions, lithium amide, and ammonia separates the layers of FeSe. It has been shown that upon variation of the chemical composition of the spacer layer, superconducting transition temperatures can reach $T_c\sim 44\mathrm{K}$, but the relative importance of the layer separation and effective doping to the $T_c$ enhancement is currently unclear. Using state of the art band structure unfolding techniques, we construct eight-orbital models from ab initio density functional theory calculations for these materials. Within an RPA spin-fluctuation approach, we show that the electron doping enhances the superconducting pairing, which is of $s_{\pm }$ symmetry and explain the experimentally observed limit to $T_c$ in the molecular spacer intercalated FeSe class of materials.

In an attempt to destabilize the magnetic state of the heavy lanthanide Dy, extreme pressures were applied in an electrical resistivity measurement to 157 GPa over the temperature range 1.3–295 K. The magnetic ordering temperature $T_{\text{o}}$ and spin-disorder resistance $R_{\text{sd}}$ of Dy, as well as the superconducting pair-breaking effect $\Delta T_{\text{c}}$ in Y(1 at.% Dy), are found to track each other in a highly nonmonotonic fashion as a function of pressure. Above 73 GPa, the critical pressure for a 6% volume collapse in Dy, all three quantities increase sharply ($d{T}_{\text{o}}/dP\simeq 5.3$ K/GPa), $T_{\text{o}}$ appearing to rise above ambient temperature for $P>107$ GPa. In contrast, $T_{\text{o}}$ and $\Delta T_{\text{c}}$ for Gd and Y(0.5 at.% Gd), respectively, show no such sharp increase with pressure ($d{T}_{\text{o}}/dP\simeq 0.73$ K/GPa). Taken together, these results suggest that extreme pressure transports Dy into an unconventional magnetic state with an anomalously high magnetic ordering temperature.

Short-range resonating valence-bond states in an orbitally degenerate magnet on a honeycomb lattice are studied. A quantum-dimer model is derived from the Hamiltonian which represents the superexchange interaction and the dynamical Jahn-Teller (JT) effect. We introduce two local units termed “spin-orbital singlet dimer,” where two spins in a nearest-neighbor bond form a singlet state associated with an orbital polarization along the bond, and “local JT singlet,” where an orbital polarization is quenched due to the dynamical JT effect. A derived quantum-dimer model consists of the hopping of the spin-orbital singlet dimers and the JT singlets, and the chemical potential of the JT singlets. We analyze the model by the mean-field approximation, and find that a characteristic phase, termed “JT liquid phase,” where both the spin-orbital singlet dimers and the JT singlets move quantum mechanically, is realized. Possible scenarios for the recently observed non-magnetic-ordered state in ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ are discussed.

A procedure to obtain single-electron wave functions within the tight-binding formalism is proposed. It is based on linear combinations of Slater-type orbitals whose screening coefficients are extracted from the optical matrix elements of the tight-binding Hamiltonian. Bloch functions obtained for zinc-blende semiconductors in the extended-basis $spd{s}^{*}$ tight-binding model demonstrate very good agreement with first-principles wave functions. We apply this method to the calculation of the electron-hole exchange interaction, and obtain the dispersion of excitonic fine structure in bulk GaAs. Beyond semiconductor nanostructures, this work is a fundamental step toward modeling many-body effects from post-processing single-particle wave functions within the tight-binding theory.

The charge photogeneration process in organic molecules is investigated by a quantum heat engine model, in which two molecules are modeled by a two-spin system sandwiched between two bosonic baths. The two baths represent the high-temperature photon emission source and the low-temperature phonon environment, respectively. We utilize the time-dependent density matrix renormalization group algorithm to investigate the quantum dynamics of the model. It is found that the transient energy current flowing through the two molecules exhibits two stages. In the first stage the energy current is of a coherent feature and represents the ultrafast delocalization of the charge-transfer state, and in the second stage a steady incoherent current is established. The power conversion efficiency is significantly high and may reach the maximum value of $93\%$ with optimized model parameters. The long-lived quantum entanglement between the two spins is found to be primarily responsible for the hyperefficiency.