A full implementation of the ab initio density functional plus dynamical mean field theory (DFT+DMFT) formalism to perform total energy calculations and structural relaxations is proposed and implemented. The method is applied to the structural and metal-insulator transitions of the rare earth nickelate perovskites as a function of rare earth ion, pressure, and temperature. In contrast to previous DFT and DFT+$U$ theories, the present method accounts for the experimentally observed structure of ${\mathrm{LaNiO}}_3$ and the insulating nature of the other perovskites, and quantitatively reproduces the metal-insulator and structural phase diagram in the plane of pressure and rare earth element. The temperature dependence of the energetics of the phase transformation indicates that the thermal transition is driven by phonon entropy effects.

The anisotropy of friction on graphitic surfaces is investigated by a combined friction force microscopy and modeling study. Friction vectors deviate up to ${15}^{\circ }$ from pulling directions. The strongest deviations are found for pulling directions which lie almost along one zigzag direction of the honeycomb structure, the preferred sliding direction on graphite surfaces and epitaxial graphene grown on SiC(0001). Atomic stick-slip events along and across molecular rows determine direction and magnitude of friction. Simulation and modeling reveal the role of temperature and of the two-dimensional character of the surface potential for the friction anisotropy.

Ferroelectric nanostructures are important for a variety of applications in electronic and electro-optical devices, including nonvolatile memories and thin-film capacitors. These applications involve stability and switching of polarization using external stimuli, such as electric fields. We present a theoretical model describing how the shape of a nanoparticle affects its polarization in the absence of screening charges, and quantify the electron-optical phase shift for detecting ferroelectric signals with phase-sensitive techniques in a transmission electron microscope. We provide an example phase shift computation for a uniformly polarized prolate ellipsoid with varying aspect ratio in the absence of screening charges.

We study the zero-temperature ($T=0$) ground-state (GS) properties of a frustrated spin-half $J_1^{\mathit{X}XZ}$–$J_2^{\mathit{X}XZ}$ model on the honeycomb lattice with nearest-neighbor and next-nearest-neighbor interactions with exchange couplings $J_1>0$ and $J_2\equiv \kappa J_1>0$, respectively, using the coupled cluster method. Both interactions are of the anisotropic XXZ type. We present the $T=0$ GS phase diagram of the model in the ranges $0\le \Delta \le 1$ of the spin-space anisotropy parameter and $0\le \kappa \le 1$ of the frustration parameter. A possible quantum spin-liquid region is identified.

We show that the topologically trivial zero bias peak (ZBP) emerging in semiconductor Majorana wires due to soft confinement exhibits correlated splitting oscillations as a function of the applied Zeeman field, similar to the correlated splitting of the Majorana ZBP. Also, we find that the presence of a strong impurity can effectively cut the wire in two and destroy the correlated splitting in both the trivial and the Majorana regimes. We identify a strong nonlocal effect that operates only in the topologically trivial regime and demonstrate that the dependence of the ZBP on the confining gate potential at the opposite end in Majorana wires with two normal metal end contacts represents a powerful tool for discriminating between topologically trivial and nontrivial ZBPs.

Using an explicit one-dimensional model, we provide direct evidence that the one-dimensional topological phases from the AIII and BDI symmetry classes follow a $\mathbb{Z}$-classification, even in the strong disorder regime when the Fermi level is embedded in a dense localized spectrum. The main tool for our analysis is the winding number $\nu$ in the noncommutative formulation introduced in I. Mondragon-Shem, J. Song, T. L. Hughes, and E. Prodan, arXiv:1311.5233. For both classes, by varying the parameters of the model and/or the disorder strength, a cascade of sharp topological transitions $\nu =0\to \nu =1\to \nu =2$ is generated, in the regime where the insulating gap is completely filled with the localized spectrum. We demonstrate that each topological transition is accompanied by an Anderson localization-delocalization transition. Furthermore, to explicitly rule out a ${\mathbb{Z}}_2$ classification, a topological transition between $\nu =0$ and 2 is generated. These two phases are also found to be separated by an Anderson localization-delocalization transition, hence proving their distinct identity.

Single electron pumping based on the topological property of Majorana fermions (MFs), which specify the parity of electron number, is proposed. The setup consists of a quantum dot (QD) and two NOT gates each with four nanotopological superconductors (TSs) connected by constriction junctions and an additional vortex located in the loop of TSs. Operation is performed by gate voltages at constriction junctions. The condition of QD energy for sequential electron pumping is derived in terms of interactions among TSs, indicating that the QD energy may lie outside the energies of electrodes, which is unique as compared with other existing pumping mechanisms. The essence of the present topological single electron pumping lies in the nonlocal property of MFs, which makes crossing between quasiparticle levels possible and electrically controllable. The operation of the NOT gate is demonstrated, explicitly solving the time-dependent Bogoliubov-de Gennes equation for a microscopic model.

We study the stability of gap-closing (Weyl or Dirac) points in the three-dimensional Brillouin zone of semimetals using Clifford algebras and their representation theory. We show that a pair of Weyl points with ${\mathbb{Z}}_2$ topological charge are stable in a semimetal with time-reversal and reflection symmetries when the square of the product of the two symmetry transformations equals minus identity. We present toy models of ${\mathbb{Z}}_2$ Weyl semimetals which have surface modes forming helical Fermi arcs. We also show that Dirac points with ${\mathbb{Z}}_2$ topological charge are stable in a semimetal with time-reversal, inversion, and SU(2) spin rotation symmetries when the square of the product of time-reversal and inversion equals plus identity. Furthermore, we briefly discuss the topological stability of point nodes in superconductors using Clifford algebras.

Optimized measurements for the susceptibility, self-energy, as well as three-leg and four-leg vertex functions are introduced for the continuous-time hybridization-expansion quantum Monte Carlo solver for the impurity model in the presence of a retarded interaction. The self-energy and vertex functions are computed from impurity averages which involve time integrals over the retarded interaction. They can be evaluated efficiently within the segment representation. These quantities are computed within dynamical mean-field theory in the presence of plasmonic screening. In the antiadiabatic regime, the self-energy is strongly renormalized but retains features of the low energy scale set by the screened interaction. An explicit expression for its high-frequency behavior is provided. Across the screening driven and interaction driven metal-insulator transitions, the vertex functions are found to exhibit similar structural changes, which are hence identified as generic features of the Mott transition.

Charge-density-wave systems have a static modulation of the electronic charge at low temperatures when they enter an ordered state. While they have been studied for decades in equilibrium, it is only recently that they have been examined in nonequilibrium with time-resolved studies in solids, or in cold-atom systems on optical lattices. Here, we present the exact solution for the nonequilibrium response of electrons (in the simplest model for a charge density wave) when the system is placed under a strong dc electric field. This allows us to examine the formation of driven Bloch oscillations and how the presence of a current modifies the nonequilibrium density of states.

Inverse spin Hall effect (ISHE) allows the conversion of pure spin current into charge current in nonmagnetic materials due to spin-orbit interaction (SOI). In ferromagnetic materials (FMs), SOI is known to contribute to anomalous Hall effect, anisotropic magnetoresistance, and other spin-dependent transport phenomena. However, SOI in FM has been ignored in ISHE studies in spintronic devices, and the possibility of “self-induced ISHE” in FM has never been explored until now. In this paper, we demonstrate the experimental verification of ISHE in FM. We found that the spin-pumping-induced spin current in permalloy (Py) film generates a transverse electromotive force (EMF) in the film itself, which results from the coupling of spin current and SOI in Py. The control experiments ruled out spin rectification effect and anomalous Nernst effect as the origin of the EMF.

The pseudogap phase occurring in cuprate and organic superconductors is analyzed based on the dynamical cluster approximation approach to the Hubbard model. In this method a cluster embedded in a self-consistent bath is studied. With increasing Coulomb repulsion, $U$, the antinodal point [$\mathbf{k}=(\pi ,0)$] displays a gradual suppression of spectral density of states around the Fermi energy which is not observed at the nodal point [$\mathbf{k}=(\pi /2,\pi /2)$]. The opening of the antinodal pseudogap is found to be related to the internal structure of the cluster and the much weaker bath-cluster couplings at the antinodal than nodal point. The role played by internal cluster correlations is elucidated from a simple four-level model. For small $U$, the cluster levels form Kondo singlets with their baths leading to a peak in the spectral density. As $U$ is increased a localized state is formed localizing the electrons in the cluster. If this cluster localized state is nondegenerate, the Kondo effect is destroyed and a pseudogap opens up in the spectra at the antinodal point. The pseudogap can be understood in terms of destructive interference between different paths for electrons hopping between the cluster and the bath. However, electrons at the nodal points remain in Kondo states up to larger $U$ since they are more strongly coupled to the bath. The strong correlation between the $(\pi ,0)$ and the $(0,\pi )$ cluster levels in the localized state leads to a large correlation energy gain, which is important for localizing electrons and opening up a pseudogap at the antinodal point. Such a scenario is in contrast with two independent Mott transitions found in two-band systems with different bandwidths in which the localized cluster electron does not correlate strongly with any other cluster electron for intermediate $U$. The important intracluster sector correlations are associated with the resonating valence bond character of the cluster ground state containing $d$-wave singlet pairs. The low-energy excitations determining the pseudogap have suppressed $d$-wave pairing, indicating that the pseudogap can be related to breaking very short-range $d$-wave pairs. Geometrical frustration on the anisotropic triangular lattice relevant to $\kappa$-(BEDT-TTF)${}_{2}X$ leads to a switch in the character of the ground state of the cluster at intermediate hopping ratios $t^{\prime }/t\sim 0.7$. Electron doping of the frustrated square lattice destroys the pseudogap, in agreement with photoemission experiments on cuprates, due to a larger Schrieffer-Wolff exchange coupling, $J_K$, and a stronger cluster-bath coupling for the antinodal point.

We have studied the concentration-dependent near-Fermi-level valence-band electronic structure of ${\mathrm{Sm}}_{0.1}{\mathrm{Ca}}_{0.9-x}{\mathrm{Sr}}_x\mathrm{Mn}{\mathrm{O}}_3$ ($x=0$, 0.1, 0.3, and 0.6) using high-resolution ultraviolet photoelectron spectroscopy (HRUPS) across the metal insulator transition. At low temperatures (50 and 100 K), a transformation from pseudogap type behavior ($x=0$ and 0.1) to insulating behavior ($x=0.3$ and 0.6) is observed with an increase in Sr content. While at the high temperatures, the metalliclike density of states appears up to $x=0.3$ and then vanishes at $x=0.6$. The temperature-dependent spectra reveal a changeover from pseudogap to metalliclike states for $x=0$ above its magnetic cluster-glass ordering temperature (110 K). In the case of $x=0.1$, the temperature-dependent change in the density of states is quite different from that of $x=0$ due to the weaker cluster-glass component and exhibits an interesting spectral weight transfer in the high-temperature paramagnetic phase. These findings would immensely help in understanding the puzzling charge transport scenario in ${\mathrm{Sm}}_{0.1}{\mathrm{Ca}}_{0.9-x}{\mathrm{Sr}}_x\mathrm{Mn}{\mathrm{O}}_3$ from a microscopic point of view.

We demonstrate that electron transfer induced by fast ion impact can be used as an imaging technique of two-dimensional materials. Applied to a keV proton beam passing through a graphene surface, it is shown that coherent single-electron capture gives a sub-ångström-scale spatial resolution image of the electronic structure of a single sheet. This imaging scheme is shown to be particularly effective, resolving missing atoms (vacancies) in the lattice, in a narrow projectile 5–10-keV energy region, where the capture probability exhibits a minimum at the center of the hexagonal cells. This geometry-dependent phenomenon is caused by the coupling dynamic between the initial state and a multi-electron entangled one-hole state and is therefore highly sample selective.

First-principles computations are performed to investigate various properties of two different kinds of nanofilms, namely those made by hydrofluorinating graphene and those formed by hydrofluorinated BN sheets. Monolayers and bilayers are considered for these two different kinds of nanofilms. Calculations show that all these systems adopt $s{p}^3$ bonds between carbon atoms or between B and N atoms due to the bindings of these atoms with H and F ions. Moreover, some of these nanofilms, especially the boron nitride-based ones, possess a spontaneous electrical polarization and piezoelectric coefficients that can be larger than those of typical ferroelectric polymers. Analysis of Wannier functions further reveals the microscopic origins of these significant polar and piezoelectric properties. In particular, it points out the importance of the orbitals of the F anion on these properties.

First-principles calculations are performed to investigate several low-enthalpy phases of the prototype ${\text{PbZrO}}_3$ antiferroelectric material under hydrostatic pressure. They are the orthorhombic $Pbam$ and $Pnma$ states, the monoclinic $P{2}_1/c$ and rhombohedral $R3c$ phases, and possess different ionic degrees of freedom (e.g., antipolar motions, oxygen octahedral tilting, and polarization). Several anomalous features are discovered, including a change in pressure behavior of the polarization in the $R3c$ state, which renders such ferroelectric phase the ground state of ${\text{PbZrO}}_3$ (PZO) (along with the nearly degenerate $P{2}_1/c$ phase) for pressure ranging between $\simeq 30$ and $\simeq 85$ GPa. Another example is the strong first-order isostructural transition exhibited by $Pbam$ at 70 GPa, which results in the reentrance of this phase as the ground state of PZO for pressure above 85 GPa. Another more continuouslike isostructural transition is also predicted for the $Pnma$ phase at $\simeq 50$ GPa. Experiments are conducted to confirm some of these predictions.

We present a large deviations theory of the spin-spin correlation functions in the random field Ising model on the Bethe lattice, both at finite and zero temperature. Rare events of atypically correlated variables are particularly important at the critical point: The phase transition is driven by a few pairs of strongly correlated spins, while the majority remains basically uncorrelated. At the zero-temperature critical point the number of spin pairs correlated over a distance $\ell$ is shown to be no longer exponential, but only linear in the spins separation.

We present a lattice nonperturbative renormalization group (NPRG) approach to quantum $XY$ spin models by using a mapping onto hardcore bosons. The NPRG takes as initial condition of the renormalization group flow the (local) limit of decoupled sites, allowing us to take into account the hardcore constraint exactly. The initial condition of the flow is equivalent to the large $S$ classical results of the corresponding spin system. Furthermore, the hardcore constraint is conserved along the RG flow, and we can describe both local and long-distance fluctuations in a nontrivial way. We discuss a simple approximation scheme, and solve the corresponding flow equations. We compute both the zero-temperature thermodynamics and the finite temperature phase diagram on the square and cubic lattices. The NPRG allows us to recover the correct critical physics at finite temperature in two and three dimensions. The results compare well with numerical simulations.

We present a comprehensive study of the current-induced spin-orbit torques in perpendicularly magnetized Ta/CoFeB/MgO layers. The samples were annealed in steps up to 300 °C and characterized using x-ray-absorption spectroscopy, transmission electron microscopy, resistivity, and Hall effect measurements. By performing adiabatic harmonic Hall voltage measurements, we show that the transverse (fieldlike) and longitudinal (antidampinglike) spin-orbit torques are composed of constant and magnetization-dependent contributions, both of which vary strongly with annealing. Such variations correlate with changes of the saturation magnetization and magnetic anisotropy and are assigned to chemical and structural modifications of the layers. The relative variation of the constant and anisotropic torque terms as a function of annealing temperature is opposite for the fieldlike and antidamping torques. Measurements of the switching probability using sub-μs current pulses show that the critical current increases with the magnetic anisotropy of the layers, whereas the switching efficiency, measured as the ratio of magnetic anisotropy energy and pulse energy, decreases. The optimal annealing temperature to achieve maximum magnetic anisotropy, saturation magnetization, and switching efficiency is determined to be between 240 and 270 °C.

We present an extensive experimental and theoretical study of the proximity effect in InAs nanowires connected to superconducting electrodes. We fabricate and investigate devices with suspended gate-controlled nanowires and nonsuspended nanowires, with a broad range of lengths and normal-state resistances. We analyze the main features of the current-voltage characteristics: the Josephson current, excess current, and subgap current as functions of length, temperature, magnetic field, and gate voltage, and compare them with theory. The Josephson critical current for a short-length device, $L=30$ nm, exhibits a record high magnitude of 800 nA at low temperature that comes close to the theoretically expected value. The critical current in all other devices is typically reduced compared to the theoretical values. The excess current is consistent with the normal resistance data and agrees well with the theory. The subgap current shows a large number of structures; some of them are identified as subharmonic gap structures generated by multiple Andreev reflection. The other structures, detected in both suspended and nonsuspended devices, have the form of voltage steps at voltages that are independent of either the superconducting gap or length of the wire. By varying the gate voltage in suspended devices, we are able to observe a crossover from typical tunneling transport at large negative gate voltage, with suppressed subgap current and negative excess current, to pronounced proximity junction behavior at large positive gate voltage, with enhanced Josephson current and subgap conductance as well as a large positive excess current.