For finite systems, the real part of the conductivity is usually decomposed as the sum of a zero frequency delta peak and a finite frequency regular part. In studies with periodic boundary conditions, the Drude weight, i.e., the weight of the zero frequency delta peak, is found to be nonzero for integrable systems, even at very high temperatures, whereas it vanishes for generic (nonintegrable) systems. Paradoxically, for systems with open boundary conditions, it can be shown that the coefficient of the zero frequency delta peak is identically zero for any finite system, regardless of its integrability. In order for the Drude weight to be a thermodynamically meaningful quantity, both kinds of boundary conditions should produce the same answer in the thermodynamic limit. We shed light on these issues by using analytical and numerical methods.

We present an ab initio formalism for the calculation of transport properties in compositionally disordered systems within the framework of the Korringa-Kohn-Rostoker nonlocal coherent potential approximation. Our formalism is based on the single-particle Kubo-Greenwood linear response and provides a natural means of incorporating the effects of short-range order upon the transport properties. We demonstrate the efficacy of the formalism by examining the effects of short-range order and clustering upon the transport properties of disordered AgPd and CuZn alloys.

We study the intrinsic spin Hall conductivity (SHC) in various $5d$ transition metals (Ta, W, Re, Os, Ir, Pt, and Au) and $4d$ transition metals (Nb, Mo, Tc, Ru, Rh, Pd, and Ag) based on the Naval Research Laboratory tight-binding model, which enables us to perform quantitatively reliable analysis. In each metal, the obtained intrinsic SHC is independent of resistivity in the low resistive regime $(\rho <50\mu \Omega \mathrm{cm})$ whereas it decreases in proportion to ${\rho }^{-2}$ in the high resistive regime. In the low resistive regime, the SHC takes a large positive value in Pt and Pd, both of which have approximately nine $d$ electrons per ion $(n_d=9)$. On the other hand, the SHC takes a large negative value in Ta, Nb, W, and Mo, where $n_d<5$. In transition metals, a conduction electron acquires the trajectory-dependent phase factor that originates from the atomic wave function. This phase factor, which is reminiscent of the Aharonov–Bohm phase, is the origin of the SHC in paramagnetic metals and that of the anomalous Hall conductivity in ferromagnetic metals. Furthermore, each transition metal shows huge and positive $d$-orbital Hall conductivity (OHC), independent of the strength of the spin-orbit interaction. Since the OHC is much larger than the SHC, it will be possible to realize an orbitronics device made of transition metals.

The magnetic properties of ferromagnetic ${\mathrm{Ga}}_{0.93}{\mathrm{Mn}}_{0.07}\mathrm{As}$ thin films have been investigated by $X$-band ferromagnetic resonance (FMR) spectroscopy as a function of hole concentration. The hole concentration $\left[p\right]$ was varied from ${10}^{21}\text{to}<{10}^{18}{\mathrm{cm}}^{-3}$ by hydrogen passivation. We determined the limit for the onset of ferromagnetism as $\left[p\right]\approx 1\times {10}^{19}{\mathrm{cm}}^{-3}$, the critical temperatures as a function of hole concentration, and the easy and hard axes for magnetization as a function of temperature and hole concentration. The temperature and hole concentration dependences of the magnetocrystalline anisotropy constants were determined. The dominant constant $K_{2\perp }$ changes for $T=4\mathrm{K}$ from $-8\times {10}^4\text{to}+1\times {10}^4\mathrm{erg}∕{\mathrm{cm}}^3$ with decreasing the hole concentration. The anisotropy coefficient ${\kappa }_2$ varies with the reduced magnetization with an expected quadratic power law. The effective $g$ factor is different from the paramagnetic value of ${\mathrm{Mn}}^{2+}$ of $g=2.00$, due to the hole contribution; it changes from 1.90 to 1.97 with decreasing the hole concentration. In addition to the resonance positions, the FMR linewidth and intensity were analyzed. The FMR linewidth varies with the inverse of the hole concentration. Free energy surfaces are calculated for the simulation of magnetization switching.

We have analyzed by using scanning tunneling microscopy (STM) thin films made of few (three to five) graphene layers grown on the C terminated face of $6H\text{−}\mathrm{Si}\mathrm{C}$ in order to identify the nature of the azimuthal disorder reported in this material. We observe superstructures which are interpreted as Moiré patterns due to a misorientation angle between consecutive layers. The presence of stacking faults is expected to lead to electronic properties reminiscent of single layer graphene even for multilayer samples. Our results indicate that this apparent electronic decoupling of the layers can show up in STM data.

We report the synthesis and characterization of well-defined CoPt clusters with a mean diameter of 3 nm, produced in ultrahigh vacuum conditions following a physical route. Samples made of diluted layers of CoPt clusters embedded in amorphous carbon have been studied by transmission electron microscopy. High-resolution observations have revealed the appearance of $L{1}_0$ chemical order upon annealing, even for clusters with a 2 nm diameter, without cluster coalescence. The magnetic properties of both chemically disordered and ordered CoPt clusters embedded in amorphous carbon have then been measured by x-ray magnetic circular dichroism and superconducting quantum interference device magnetometry. Despite a striking change of the Co magnetic moment, the magnetic anisotropy of chemically ordered nanoparticles increases, with respect to the chemically disordered $A1$ phase, in much lower proportions than what is observed for the bulk.

The superfluid-insulator transition of bosons is strongly modified by the presence of fermions. Through an imaginary-time path-integral approach, we derive the self-consistent mean-field transition line, and account for both the static and dynamic screening effects of the fermions. We find that an effect akin to the fermionic orthogonality catastrophe, arising from the fermionic screening fluctuations, suppresses superfluidity. We analyze this effect for various mixture parameters and temperatures, and consider possible signatures of the orthogonality-catastrophe effect in other measurables of the mixture.

We present a comprehensive numerical study of a microscopic model of the fractional quantum Hall system at filling fraction $\nu =5∕2$, based on the disk geometry. Our model includes Coulomb interaction and a semirealistic confining potential. We also mix in a three-body interaction in some cases to help elucidate the physics. We obtain a phase diagram, discuss the conditions under which the ground state can be described by the Moore-Read state, and study its competition with neighboring stripe phases. We also study quasihole excitations and edge excitations in the Moore-Read-like state. From the evolution of the edge spectrum, we obtain the velocities of the charge and neutral edge modes, which turn out to be very different. This separation of velocities is a source of decoherence for a non-Abelian quasihole and/or quasiparticle (with charge $\pm e∕4$) when propagating at the edge; using numbers obtained from a specific set of parameters, we estimate the decoherence length to be around $4\mu \mathrm{m}$. This sets an upper bound for the separation of the two point contacts in a double point-contact interferometer, designed to detect the non-Abelian nature of such quasiparticles. We also find a state that is a potential candidate for the recently proposed anti-Pfaffian state. We find the speculated anti-Pfaffian state is favored in weak confinement (smooth edge), while the Moore-Read Pfaffian state is favored in strong confinement (sharp edge).

After reviewing the concept of vison excitations in ${\mathbb{Z}}_2$ dimer liquids, we study the liquid-crystal transition of the quantum dimer model on the triangular lattice by means of a semiclassical spin-wave approximation to the dispersion of visons in the context of a “soft-dimer” version of the model. This approach captures some important qualitative features of the transition: continuous nature of the transition, linear dispersion at the critical point, and $\sqrt{12}\times \sqrt{12}$ symmetry-breaking pattern. In a second part, we present a variational calculation of the vison dispersion relation at the Rokhsar-Kivelson (RK) point, which reproduces the qualitative shape of the dispersion relation and the order of magnitude of the gap. This approach provides a simple but reliable approximation of the vison wave functions at the RK point.

We apply algorithms based on Lieb-Robinson bounds to simulate time-dependent and thermal quantities in quantum systems. For time-dependent systems, we modify a previous mapping to quantum circuits to significantly reduce the computer resources required. This modification is based on a principle of “observing” the system outside the light cone. We apply this method to study spin relaxation in systems started out of equilibrium with initial conditions that give rise to a very rapid entanglement growth. We also show that it is possible to approximate time evolution under a local Hamiltonian by a quantum circuit whose light cone naturally matches the Lieb-Robinson velocity. Asymptotically, these modified methods allow a doubling of the system size that one can obtain as compared to a direct simulation. We then consider a different problem of thermal properties of disordered spin chains and use quantum belief propagation to average over different configurations. We test this algorithm on one-dimensional systems with mixed ferromagnetic and antiferromagnetic bonds, where we can compare to quantum Monte Carlo, and then we apply it to the study of disordered, frustrated spin systems.

Mott insulators with a half-filled band of electrons on the triangular lattice have been recently studied in a variety of organic compounds. All of these compounds undergo transitions to metallic and/or superconducting states under moderate hydrostatic pressure. We describe the Mott insulator using its hypothetical proximity to a $Z_2$ spin liquid of bosonic spinons. This spin liquid has quantum phase transitions to descendant confining states with Néel or valence bond solid order, and the insulator can be on either side of one of these transitions. We present a theory of fermionic charged excitations in these states and describe the route to metallic states with Fermi surfaces. We argue that an excitonic condensate can form near this insulator-metal transition due to the formation of charge neutral pairs of charge $+e$ and charge $-e$ fermions. This condensate breaks the lattice space group symmetry, and we propose its onset as an explanation of a low temperature anomaly in $\kappa \text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{C}\mathrm{N}\right)}_3$. We also describe the separate BCS instability of the metallic states to the pairing of like-charge fermions and the onset of superconductivity.

Graphene nanodisk is a graphene derivative with a closed edge. The trigonal zigzag nanodisk with size $N$ contains a zero-energy sector with the $\text{SU}\left(N\right)$ symmetry. We investigate electron-electron interaction effects within the zero-energy sector. We explicitly derive the direct and exchange interactions, which explicitly break the $\text{SU}\left(N\right)$ symmetry. Then, by regarding a nanodisk as a quantum dot with an internal degree of freedom, we analyze the nanodisk-lead system consisting of a nanodisk and two leads. By employing the standard Green’s function method, we reveal Coulomb blockade effects in this system. As the chemical potential increases, a peculiar series of plateaux and dips develops in the occupation number and a peculiar series of Coulomb blockade peaks develops in the conductance, which reflect the energy spectrum of a nanodisk with a broken $\text{SU}\left(N\right)$ symmetry. It is argued that dips emerge in the occupation number due to a Coulomb correlation effect and that tiny Coulomb blockade peaks emerge due to a tunneling effect between the nanodisk and leads.

Using a fully three-dimensional numerical treatment of the Ginzburg–Landau (GL) theory, we study superconducting and magnetic properties of submicron superconducting spheres. Systematic analysis is performed for different radii of the sphere and different constituent materials, i.e., with different GL parameter $\kappa$. The distribution of the magnetic field in and around the sample is calculated, and its repercussions on magnetometry measurements are discussed. Furthermore, we demonstrate a unique feature in the magnetization curves that can distinguish multi- from giant-vortex states.

We present experiments in which the interaction of a single gold nanoparticle with glass substrates or with another gold particle can be tuned by in situ control of their separations using scanning probe technology. We record the plasmon resonances of the coupled systems as a function of the polarization of the incident field and the particle position. The distinct spectral changes of the scattered light from the particle pair are in good agreement with the outcome of finite-difference time-domain calculations. We believe that our experimental technique holds promise for the investigation of the van der Waals–Casimir-type interactions between nanoscopic neutral bodies.

Changes in the mobility induced by an extended application of a gate bias are reported in polythiophene thin film transistors. The gate-induced decrease in mobility is unrelated to the previously studied threshold shift effect that arises from trapped holes. The mobility decrease has a strong temperature dependence, is irreversible by low temperature annealing, and depends on the number of times the gate is switched as well as the duration of the gate bias. The effect exhibits both as an increase in series resistance and as a change in bulk mobility. The thermal activation of the process suggests an energy barrier to a new physical structure which is too stable to reverse. A possible mechanism for structural changes is proposed, based on the electrostrictive effect.

We propose a viable approach to artificial half-metallic antiferromagnets (HM-AFMs) with no spontaneous magnetization (or fully compensated ferrimagnets) through introduction of compositional periodicity into a tetrahedrally coordinated magnetic compound, which consists of alternating monolayers of transition-metal chalcogenides: $TX/MX$ ($X=\text{S}$, Se, and Te). By using a full-potential augmented plane-wave plus local-orbitals method within density-functional theory, we find two promising series of monolayer superlattices (MSLs), $\text{Cr}X/\text{Fe}X$ and $\text{V}X/\text{Co}X$, where two constituent magnetic ions in a supercell have antialigned local moments that exactly cancel by virtue of the integer filling of one spin channel. All of these MSLs with [001] orientation are predicted to be bulk HM-AFMs, whereas CrS/FeS and CrSe/FeSe are the only systems that show half-metallicity among MSLs with [111] orientation. Spin-orbit coupling of spin-polarized conduction electrons can be neglected and never destroy the half-metallicity. The relation between bonding nature and magnetic interaction is discussed in terms of the “ghost-bond-orbital model”; the coexisting covalency in $T\text{−}X$ bonds and ionicity in $M\text{−}X$ bonds would stabilize and enhance the ferromagnetic double exchange and antiferromagnetic superexchange interactions within each local bond, respectively, realizing the half-metallic antiferromagnetism of the $TX/MX$ systems.

We present scanning tunneling microscopy and spectroscopy measurements at 100 mK in the superconducting material $2H\text{−}{\text{NbSe}}_2$ that show well defined features in the superconducting density of states, with changes that follow atomic periodicity. Our experiment demonstrates that the intrinsic superconducting density of states can show atomic size modulations, which reflect the reciprocal space structure of the superconducting gap. In particular, we obtain that the superconducting gap of $2H\text{−}{\text{NbSe}}_2$ has sixfold modulated components at 0.75 and 1.2 mV. Moreover, we also find related atomic size modulations inside vortices, demonstrating that the much discussed star shape vortex structure produced by localized states inside the vortex cores has a superposed atomic size modulation, which has hitherto been undetected. The tip substrate interaction in an anisotropic superconductor has been calculated, giving position dependent changes related to the observed gap anisotropy.

In this paper, I first analytically calculate the magnetic-field and sheet-current distributions generated in an infinite stack of thin superconducting strips of thickness $d$, width $2a⪢d$, and arbitrary separation $D$ when adjacent strips carry net current of magnitude $I$ in opposite directions. Each strip is assumed to have uniform critical current density $J_c$, critical sheet-current density $K_c=J_{c}d$, and critical current $I_c=2a{K}_c$, and the distribution of the current density within each strip is assumed to obey critical-state theory. I then derive expressions for the ac losses due to magnetic-flux penetration both from the strip edges and from the top and bottom of each strip and I express the results in terms of integrals involving the perpendicular and parallel components of the magnetic field. After numerically evaluating the ac losses for typical dimensions, I present analytic expressions from which the losses can be estimated.

A coupled cluster expansion was used to direct an ab initio search for stable ordered structures on the cubic fluorite lattice across the ${\text{ZrO}}_2{\text{-Y}}_2{\text{O}}_3$ composition range. The energies of 453 arrangements of (Zr, Y) cations and (O, vacancy) anions on the fluorite lattice were calculated by using density functional theory (DFT) in the generalized gradient approximation. These DFT energies were used to construct a coupled cluster expansion that allowed the search for possible $T=0\text{K}$ ground-state structures through $\sim {10}^5$ cation/anion configurations. Our approach correctly identifies the experimentally observed compound ${\text{Zr}}_3{\text{Y}}_4{\text{O}}_{12}$ and also predicts an as-yet-unobserved ordered structure with ${\text{Zr}}_4{\text{Y}}_2{\text{O}}_{11}$ stoichiometry. In the latter structure, vacancies are at second nearest neighbors from yttrium and every Zr has seven oxygen first nearest neighbors. The Zr-O bond lengths and oxygen coordination around Zr in the ${\text{Zr}}_4{\text{Y}}_2{\text{O}}_{11}$ ground state are nearly the same as those in monoclinic ${\text{ZrO}}_2$. We also predict structures at ${\text{Zr}}_5{\text{Y}}_2{\text{O}}_{13}$ and ${\text{Zr}}_6{\text{Y}}_2{\text{O}}_{15}$ that are stable with respect to cubic ${\text{ZrO}}_2$ but metastable with respect to monoclinic ${\text{ZrO}}_2$.

We consider the propagation of a classical electromagnetic wave through a transmission line, formed by identical superconducting charge qubits inside a superconducting resonator. Since the qubits can be in a coherent superposition of quantum states, we show that such a system demonstrates interesting effects, such as a “breathing” photonic crystal with an oscillating band gap and a “quantum Archimedean screw” that transports, at an arbitrary controlled velocity, Josephson plasma waves through a transmission line. The key ingredient of these effects is that the optical properties of the Josephson transmission line are controlled by the quantum coherent state of the qubits.