A major challenge to the chiral $p$-wave hypothesis for the pairing symmetry of the unconventional superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is the null result of sensitive scanning magnetometry experiments designed to detect the expected spontaneous charge currents. Motivated by junction tunneling conductance measurements, which indicate the quenching of superconductivity at the surfaces of even high-purity samples, we examine the spontaneous currents in a chiral $p$-wave superconductor near a normal metal/superconductor interface using the lattice Bogoliubov-de Gennes equations and Ginzburg-Landau theory, and find that the edge current is suppressed by more than an order of magnitude compared to previous estimates. These calculations demonstrate that interface details can have a quantitatively meaningful effect on the expectations for magnetometry experiments.

It is generally believed that spin-orbit coupling (SOC) follows $Z^4$ (atomic number) dependence and becomes significant only in heavy elements. Consequently, SOC in $3d$ transition metals should be negligible given their small $Z$. Using dynamic spin pumping of ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$-based structures, we uncover a systematic evolution of the spin Hall angle (${\theta }_{\mathrm{SH}}$) with $d$-orbital filling in a series of $3d$ metals, reminiscent of behavior observed in $5d$ metals. In particular, Cr and Ni show very large ${\theta }_{\mathrm{SH}}$ (half of that for Pt), indicating that $d$-orbital filling rather than $Z$ plays a dominant role in the spin Hall effect (SHE) in $3d$ metals. This result enriches our understanding of SHE and broadens the scope of materials available for exploring the rich phenomena enabled by SOC as well as presenting a guidepost for testing theoretical models of spin-orbit coupling in transition metals.

The properties of materials at high temperatures are often determined by complex thermodynamic mechanisms. One of the most prominent examples is the stabilization of tetragonal and cubic zirconia, which we investigate using density functional theory. The results show that the minimum energy path for the tetragonal-to-cubic phase transformation differs significantly from the paths discussed in the literature so far. This provides insight into the properties of compositions codoped with yttria and titania, an approach that has recently been proposed for the design of thermal barrier coatings.

We investigate the classical Heisenberg and planar $\left(XY\right)$ spin models on the windmill lattice. The windmill lattice is formed out of two widely occurring lattice geometries: a triangular lattice is coupled to its dual honeycomb lattice. Using a combination of iterative minimization, heat-bath Monte Carlo simulations, and analytical calculations, we determine the complete ground-state phase diagram of both models and find the exact energies of the phases. The phase diagram shows a rich phenomenology due to competing interactions and hosts, in addition to collinear and various coplanar phases, also intricate noncoplanar phases. We briefly outline different paths to an experimental realization of these spin models. Our extensive study provides a starting point for the investigation of quantum and thermal fluctuation effects.

Highly symmetric magnetic environments have been suggested to stabilize the magnetic information stored in magnetic adatoms on a surface. Utilized as memory devices such systems are subjected to electron tunneling and external magnetic fields. We analyze theoretically how such perturbations affect the switching probability of a single quantum spin for two characteristic symmetries encountered in recent experiments and suggest a third one that exhibits robust protection against surface-induced spin flips. Further we illuminate how the switching of an adatom spin exhibits characteristic behavior with respect to low energy excitations from which the symmetry of the system can be inferred.

By employing a variational approach, the density matrix renormalization group (DMRG), the exact diagonalization, and symmetry and mean-field analyses, the ground-state properties of the two-bath spin-boson model with simultaneous diagonal and off-diagonal coupling are systematically studied in the sub-Ohmic regime. A quantum phase transition from a doubly degenerate “localized phase” to the other doubly degenerate “delocalized phase” is uncovered. Via the multi-${\mathrm{D}}_1$ Ansatz as the variational wave function, transition points are determined accurately, consistent with the results from DMRG and exact diagonalization. An effective spatial dimension $d_{\mathrm{eff}}=2.37\left(6\right)$ is then estimated, which is found to be compatible with the mean-field prediction. Furthermore, the quantum phase transition is inferred to be of first order for the baths described by a continuous spectral density function. In the single-mode case, however, the transition is softened.

Machine learning methods are applied to finding the Green's function of the Anderson impurity model, a basic model system of quantum many-body condensed-matter physics. Different methods of parametrizing the Green's function are investigated; a representation in terms of Legendre polynomials is found to be superior due to its limited number of coefficients and its applicability to state of the art methods of solution. The dependence of the errors on the size of the training set is determined. The results indicate that a machine learning approach to dynamical mean-field theory may be feasible.

We present a computational methodology to evaluate the conductivity tensors of “large-scale” systems in a magnetic field based on the time-dependent wave-packet diffusion method. As demonstrations, we first apply the method to the two-dimensional square lattice model with static disorder and confirm appropriate magnetic-field dependence of conductivities from weak to strong magnetic-field regimes. Furthermore, we extend the method to apply to realistic systems and evaluate the influence of dynamical disorder on the Hall effects of organic semiconductors, taking microscopic molecular vibrations into account.

Short-range order in strongly disordered structures plays an important role in their heat conduction property. Using numerical and analytical methods, we show that short-range spatial correlation (with a correlation length of ${\Lambda }_m$) in the mass distribution of the one-dimensional (1D) alloylike random binary lattice leads to a dramatic enhancement of the high-frequency phonon transmittance but also increases the low-frequency phonon opacity. High-frequency semiextended states are formed while low-frequency modes become more localized. This results in ballistic heat conduction at finite lengths but also paradoxically higher thermal resistance that scales as $\sqrt{{\Lambda }_m}$ in the $L\to \infty$ limit. We identify an emergent crossover length ($L_c$) below which the onset of thermal transparency appears. The crossover length is linearly dependent on but is two orders of magnitude larger than ${\Lambda }_m$. Our results suggest that the phonon transmittance spectrum and heat conduction in a disordered 1D lattice can be controlled via statistical clustering of the constituent component atoms into domains. They also imply that the detection of ballistic heat conduction in disordered 1D structures may be a signature of the intrinsic mass correlation at a much smaller length scale.

Strong nonmagnetic impurities on the surface of three-dimensional topological insulators (TIs) generate localized resonance peaks close to the Dirac point. We show that this results in a strongly reduced critical Coulomb interaction strength to reach a magnetic surface state, following a Stoner-like criterion. Thus even weakly interacting TIs host a finite (local) magnetization around strong nonmagnetic impurities. The local magnetization gives rise to a global energy gap, linearly dependent on the maximum value of the magnetization but decreasing with reduced impurity concentration.

We study the quantum theory of a Fermi surface coupled to a gapless boson scalar in $D=4-\epsilon$ space-time dimensions as a simple model for non-Fermi liquids (NFL) near a quantum phase transition. Our analysis takes into account the full backreaction from Landau damping of the boson, and obtains an RG flow that proceeds through three distinct stages. Above the scale of Landau damping, the Fermi velocity flows to zero, while the coupling evolves according to its classical dimension. Once damping becomes important, its backreaction leads to a crossover regime where dynamic and static damping effects compete and the fermion self-energy does not respect scaling. Below this crossover and having tuned the boson to criticality, the theory flows to a $z=3$ scalar interacting with an NFL. We finally analyze the IR phases of the theory with arbitrary number of flavors $N_c$. When $N_c$ is small, the superconducting dome covers the NFL behavior; strikingly, for moderately large $N_c$, we find that NFL effects become important first, before the onset of superconductivity. A generic prediction of the theory is that the Fermi velocity and quasiparticle residue vanish with a power law ${\omega }^{\epsilon }$ as the fixed point is approached. These features may be useful for understanding some of the phenomenology of high-$T_c$ materials in a systematic $\epsilon$ expansion.

The physics of the strongly interacting Hubbard chain (with $t/U\ll 1$) at finite temperatures undergoes a crossover to a spin-incoherent regime when the temperature is very small relative to the Fermi energy, but larger than the characteristic spin energy scale. This crossover can be understood by means of Ogata and Shiba's factorized wave function, where charge and spin are totally decoupled, and assuming that the charge remains in the ground state, while the spin is thermally excited and at an effective “spin temperature.” We use the time-dependent density matrix renormalization group method to calculate the dynamical contributions of the spin, to reconstruct the single-particle spectral function of the electrons. The crossover is characterized by a redistribution of spectral weight both in frequency and momentum, with an apparent shift by $k_F$ of the minimum of the dispersion.

The theory for the onset of spin density wave order in a metal in two dimensions flows to strong coupling, with strong interactions not only at the “hot spots,” but on the entire Fermi surface. We advocate the computation of dc transport in a regime where there is rapid relaxation to local equilibrium around the Fermi surface by processes which conserve total momentum. The dc resistivity is then controlled by weaker perturbations which do not conserve momentum. We consider variations in the local position of the quantum-critical point, induced by long-wavelength disorder, and find a contribution to the resistivity which is linear in temperature (up to logarithmic corrections) at low temperature. Scattering of fermions between hot spots, by short-wavelength disorder, leads to a residual resistivity and a correction which is linear in temperature.

We consider theoretically the dynamics of electric currents optically injected in carbon nanotubes. Although the plasma oscillations are not seen in these systems, the main effect on the carrier's motion is due to strongly nonuniform space-charge Coulomb forces produced by time-dependent separation of injected electron and hole densities. We calculate the evolution of the dipole moment characterizing the time- and coordinate-dependent charge-density distributions and analyze different regimes of the dynamics. The developed time-dependent dipole moment leads to a dipole radiation in the THz frequency range for typical parameters of injected currents.

We investigate the subgap bulk transport through short and wide superconducting hybrid structures based on HgTe quantum wells (QWs). We show that the differential conductance of a normal-metal–insulator–superconductor (NIS) proximity structure behaves in a qualitatively different way with respect to the topological phase of the HgTe QW. We compare the differential conductance for the NIS structure within the wave-matching method based on the Bogoliubov–de Gennes equation and the matrix method based on the normal-state scattering matrix and find that the two models agree for highly doped N and S contacts. We also show that the effect of a possible Rashba spin-orbit interaction on the differential conductance can be significant for weakly doped N and S contacts. Our findings should be important in samples with a large aspect ratio where bulk contributions in transport are dominant.

Vacancy diffusion and clustering processes in body-centered-cubic (bcc) Fe are studied using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo method with on-the-fly catalog building capabilities. For monovacancies and divacancies, k-ART recovers previously published results while clustering in a 50-vacancy simulation box agrees with experimental estimates. Applying k-ART to the study of clustering pathways for systems containing from one to six vacancies, we find a rich set of diffusion mechanisms. In particular, we show that the path followed to reach a hexavacancy cluster influences greatly the associated mean-square displacement. Aggregation in a 50-vacancy box also shows a notable dispersion in relaxation time associated with effective barriers varying from 0.84 to 1.1 eV depending on the exact pathway selected. We isolate the effects of long-range elastic interactions between defects by comparing to simulations where those effects are deliberately suppressed. This allows us to demonstrate that in bcc Fe, suppressing long-range interactions mainly influences kinetics in the first 0.3 ms, slowing down quick energy release cascades seen more frequently in full simulations, whereas long-term behavior and final state are not significantly affected.

We studied angle-dependent magnetoresistance in $\mathrm{Pt}/{\mathrm{Fe}}_3{\mathrm{O}}_4$ bilayers at room temperature. Based on field-dependent and angle-dependent measurements, we show that spin Hall magnetoresistance (SMR) exists in the $\mathrm{Pt}/{\mathrm{Fe}}_3{\mathrm{O}}_4$ system at room temperature, although the $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ layer is not a good insulator. The SMR contribution in $\mathrm{Pt}/{\mathrm{Fe}}_3{\mathrm{O}}_4$ can be clearly identified for thin enough $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ films. Moreover, the SMR in $\mathrm{Pt}/{\mathrm{Fe}}_3{\mathrm{O}}_4$ does not rely on the current direction, which is different from the anisotropic magnetoresistance in single-crystalline $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ films.

We investigate possible quantum ground states as well as the classical limit of a frustrated $J_1\text{−}{J}_2$ Heisenberg model on the three-dimensional (3D) hyperhoneycomb lattice. Our study is inspired by the recent discovery of $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$, where ${\mathrm{Ir}}^{4+}$ ions form a 3D network with each lattice site being connected to three nearest neighbors. We focus on the influence of magnetic frustration caused by the second-nearest-neighbor spin interactions. Such interactions are likely to be significant due to the large extent of $5d$ orbitals in iridates or other $5d$ transition metal oxides. In the classical limit, the ground state manifold is given by line degeneracies of the spiral magnetic-order wave vectors when $J_2/J_1\gtrsim 0.17$ while the collinear stripy order is included in the degenerate manifold when $J_2/J_1=0.5$. Quantum order-by-disorder effects are studied using both the semiclassical $1/S$ expansion in the spin-wave theory and the Schwinger-boson approach. In general, certain coplanar spiral orders are chosen from the classical degenerate manifold for a large fraction of the phase diagram. Nonetheless quantum fluctuations favor the collinear stripy order over the spiral orders in an extended parameter region around $J_2/J_1=0.5$, despite the spin-rotation invariance of the underlying Hamiltonian. This is in contrast to the emergence of stripy order in the Heisenberg-Kitaev model studied earlier on the same lattice, where the Kitaev-type Ising interactions are important for stabilizing the stripy order. As quantum fluctuations become stronger, $U\left(1\right)$ and $Z_2$ quantum spin liquid phases are shown to arise via quantum disordering of the Néel, stripy, and spiral magnetically ordered phases. The effects of magnetic anisotropy and their relevance to future experiments are also discussed.

We have investigated the possible evolution of an intrinsic stable ferromagnetic moment in oxygen deficient undoped and magnetically doped ${\mathrm{HfO}}_{2-x}$ thin films grown by reactive molecular beam epitaxy. Neither oxygen vacancies nor substituted Ni in combination with such vacancies results in an observable magnetic moment for a broad range of oxygen vacancy concentrations. By combining integral and element specific magnetization measurements, we show that a fluctuating deposition rate of the magnetic dopant induces extrinsic ferromagnetism by promoting the formation of metallic clusters. We suggest the element specific measurement of an induced magnetic moment at the nonmagnetic site as a proof of intrinsic ferromagnetism in diluted magnetic semiconductors.