The nanocrystallization kinetics of Fe${}_{90}$Zr${}_7$B${}_3$ is investigated by in situ nuclear forward scattering of synchrotron radiation upon isothermal annealing. The nucleation and growth processes are accessed separately for the nanograins, their interfaces, and the residual amorphous matrix by monitoring the time evolution of the corresponding hyperfine parameters. This approach discloses the structural transformations taking place in this class of technologically important materials.

The band structure of graphene exhibits van Hove singularities (VHSs) at dopings $x=\pm 1/8$ away from the Dirac point. Near the VHS, interactions effects, enhanced due to the large density of states, can give rise to various many-body phases. We study the competition between many-body instabilities in graphene using the functional renormalization group. We predict a rich phase diagram, which, depending on band structure as well as the range and scale of Coulomb interactions, contains a $d+id$-wave superconducting (SC) phase, or a spin-density-wave phase at the VHS. The $d+id$ state is expected to exhibit quantized charge and spin Hall response, as well as Majorana modes bound to vortices. Nearby the VHS, we find singlet $d+id$-wave and triplet $f$-wave SC phases.

Polariton fluids have revealed huge potentialities in order to investigate the properties of bosonic fluids at the quantum scale. Among those properties, the opportunity to create dark as well as bright solitons has been demonstrated recently. In the present experiments, we image the formation dynamics of oblique dark solitons. They nucleate in the wake of an engineered attractive potential that perturbs the polariton quantum fluid. Thanks to time and phase measurements, we assess quantitatively the formation process. The formation velocity is observed to increase with increasing distance between the flow injection point and the obstacle which modulates the density distribution of the polariton fluid. We propose an explanation in terms of the increased resistance to the flow and of the conditions for the convective instability of dark solitons. By using an iterative solution of the generalized Gross-Pitaevskii equation, we are able to reproduce qualitatively our experimental results.

We have studied the surface of pure and oxidized Pt${}_3$Zr(0001) by scanning tunneling microscopy (STM), Auger electron microscopy, and density functional theory (DFT). The well-annealed alloy surface shows perfect long-range chemical order. Occasional domain boundaries are probably caused by nonstoichiometry. Pt${}_3$Zr exhibits ABAC stacking along [0001]; only the A-terminated surfaces are seen by STM, in agreement with DFT results showing a lower surface energy for the A termination. DFT further predicts a stronger inward relaxation of the surface Zr than for Pt, in spite of the larger atomic size of Zr. A closed ZrO${}_2$ film is obtained by oxidation in ${10}^{-7}$ mbar O${}_2$ at 400 ${}^{\circ }$C and post-annealing at $\approx 800{}^{\circ }$C. The oxide consists of an O-Zr-O trilayer, equivalent to a (111) trilayer of the fluorite structure of cubic ZrO${}_2$, but contracted laterally. The oxide forms a $(\sqrt{19}\times \sqrt{19})R{23}^{\circ }$ superstructure. The first monolayer of the substrate consists of Pt and contracts, similar to the metastable reconstruction of pure Pt(111). DFT calculations show that the oxide trilayer binds rather weakly to the substrate. In spite of the O-terminated oxide, bonding to the substrate mainly occurs via the Zr atoms in the oxide, which strongly buckle down toward the Pt substrate atoms if near a Pt position. According to DFT, the oxide has a band gap; STM indicates that the conduction band minimum lies $\approx 2.3$ eV above $E_F$.

In granular superconductors, individual grains can contain bound Cooper pairs while the system as a whole is strongly insulating. In such cases the conductivity is determined by hopping of single electrons or electron pairs between localized states in individual grains. Here we examine a model of hopping conductivity in such an insulating granular superconductor, where disorder is assumed to be provided by random charges embedded in the insulating gaps between grains. We use computer simulations to calculate the single-electron and electron-pair density of states at different values of the superconducting gap $\Delta$, and we identify “triptych” symmetries and scaling relations between them. At a particular critical value of $\Delta$, one can define an effective charge $\sqrt{2}e$ that characterizes the density of states and the hopping transport. We discuss the implications of our results for magnetoresistance and tunneling experiments.

We have experimentally determined the spin-dependent Seebeck coefficient of permalloy (Ni${}_{80}$Fe${}_{20}$) and cobalt (Co) using nanopillar spin valve devices, a stack of two ferromagnetic layers separated by a nonmagnetic layer. The devices were specifically designed to separate heat-related effects from charge-related effects. A heat current, with no accompanying charge current, through the nanopillar spin valve leads to a thermovoltage proportional to the spin-dependent Seebeck coefficient $S_S=S_{\uparrow }-S_{\downarrow }$ of the ferromagnet, where $S_{\uparrow }$ and $S_{\downarrow }$ are the Seebeck coefficient for spin-up and spin-down electrons. By using a three-dimensional finite-element model based on spin-dependent thermoelectric theory, whose input material parameters were measured in separate devices, we were able to accurately determine a spin-dependent Seebeck coefficient of $-$1.8 $\mu$V K${}^{-1}$ and $-$4.5 $\mu$V K${}^{-1}$ for cobalt and permalloy, respectively, corresponding to a Seebeck coefficient polarization $P_S=S_S/S_F$ of 0.08 and 0.25, where $S_F$ is the Seebeck coefficient of the ferromagnet. The results are in agreement with earlier theoretical work in Co/Cu multilayers and spin-dependent Seebeck and spin-dependent Peltier measurements in Ni${}_{80}$Fe${}_{20}$/Cu spin valve structures.

Light emission from a scanning tunneling microscope is investigated from Ag(111) surfaces using tips with a clean metallic apex and a molecule-covered shaft. The measurements are conducted such that no molecules are located in the tunneling gap. Nevertheless, spectra of the emission exhibit absorption and emission features of the tetraphenyl-H${}_2$-porphyrine molecules used. The experimental data show that molecules in the vicinity of the tunneling gap are excited through tip-induced plasmons and their radiative decay. The excitation of plasmons is due to inelastic tunneling and does not involve an external light source.

We exploit a prescription to observe directly the physical properties of the thermodynamic limit in a continuously applied field in one-dimensional quantum finite lattice systems. By systematically scaling down the energy of the Hamiltonian of the open system from center toward both ends, one could adopt the edge sites with a negligibly small energy scale as the grand canonical small particle bath, and equilibrium states with noninteger arbitrary conserved numbers, e.g., electron numbers or $s_z$, are realized in the main part of the system. This will enable the evaluation of response functions under a continuously varying external field in a small lattice without any fine-tuning or scaling of parameters while keeping the standard numerical accuracy. Demonstrations are given on quantum spin systems and on a Hubbard model by the density-matrix renormalization group.

We performed nuclear-magnetic-resonance measurements on itinerant-electron metamagnet UCoAl to investigate the critical behavior of the magnetism near a metamagnetic (MM) critical endpoint (CEP). We derived $c$-axis magnetization $M_c$ and its fluctuation $S_c$ from the measurements of Knight shift and nuclear spin-lattice relaxation rate $1/T_1$ as a function of the $c$-axis external field ($H_c$) and temperature ($T$). We developed contour plots of $M_c$ and $S_c$ on the $H_c$-$T$ phase diagram, and observed the strong divergence of $S_c$ at the CEP. The critical exponents of $M_c$ and $S_c$ near the CEP are estimated and found to be close to the universal properties of a three-dimensional Ising model. We indicate that the critical phenomena at the itinerant-electron MM CEP in UCoAl have a common feature as a gas–liquid transition.

We present a fractionalized metallic phase which is indistinguishable from the Fermi liquid in conductivity and thermodynamics, but is sharply distinct in one-electron properties, such as the electron spectral function. We dub this phase the “orthogonal metal.” The orthogonal metal and the transition to it from the Fermi liquid are naturally described using a slave-particle representation wherein the electron is expressed as a product of a fermion and a slave Ising spin. We emphasize that when the slave spins are disordered, the result is not a Mott insulator (as erroneously assumed in the prior literature), but rather the orthogonal metal. We construct prototypical ground-state wave functions for the orthogonal metal by modifying the Jastrow factor of Slater-Jastrow wave functions that describe ordinary Fermi liquids. We further demonstrate that the transition from the Fermi liquid to the orthogonal metal can, in some circumstances, provide a simple example of a continuous destruction of a Fermi surface with a critical Fermi surface appearing right at the critical point. We present exactly soluble models that realize an orthogonal metal phase, and the phase transition to the Fermi liquid. These models thus provide valuable solvable examples for phase transitions associated with the death of a Fermi surface.

We solve the Anderson localization problem on a two-leg ladder by the Fokker-Planck equation approach. The solution is exact in the weak disorder limit at a fixed interchain coupling. The study is motivated by progress in investigating the hybrid particles such as cavity polaritons. This application corresponds to parametrically different intrachain hopping integrals (a “fast” chain coupled to a “slow” chain). We show that the canonical Dorokhov-Mello-Pereyra-Kumar (DMPK) equation is insufficient for this problem. Indeed, the angular variables describing the eigenvectors of the transmission matrix enter into an extended DMPK equation in a nontrivial way, being entangled with the two transmission eigenvalues. This extended DMPK equation is solved analytically and the two Lyapunov exponents are obtained as functions of the parameters of the disordered ladder. The main result of the paper is that near the resonance energy, where the dispersion curves of the two decoupled and disorder-free chains intersect, the localization properties of the ladder are dominated by those of the slow chain. Away from the resonance they are dominated by the fast chain: a local excitation on the slow chain may travel a distance of the order of the localization length of the fast chain.

We perform highly accurate density matrix renormalization group (DMRG) simulations to investigate the ground-state properties of the spin-$1/2$ antiferromagnetic square lattice Heisenberg $J_1$-$J_2$ model. Based on studies of numerous long cylinders with circumferences of up to 14 lattice spacings, we obtain strong evidence for a topological quantum spin liquid state in the region $0.41\le J_2/J_1\le 0.62$, separating conventional Néel and striped antiferromagnetic states for smaller and larger $J_2/J_1$, respectively. The quantum spin liquid is characterized numerically by the absence of magnetic or valence bond solid order, and nonzero singlet and triplet energy gaps. Furthermore, we positively identify its topological nature by measuring a nonzero topological entanglement entropy $\gamma =0.70\pm 0.02$, extremely close to $\gamma =\ln \left(2\right)\approx 0.69$ (expected for a $Z_2$ quantum spin liquid) and a nontrivial finite size dimerization effect depending upon the parity of the circumference of the cylinder. We also point out that a valence bond solid, and indeed any discrete symmetry breaking state, would be expected to show a constant correction to the entanglement entropy of opposite sign to the topological entanglement entropy.

We evaluate quantum corrections to conductivity in an electrically gated thin film of a three-dimensional topological insulator. We derive approximate analytical expressions for the low-field magnetoresistance as a function of bulk doping and bulk-surface tunneling rate. Our results reveal parameter regimes for both weak localization and weak antilocalization, and include diffusive Weyl semimetals as a special case.

We analyze electronic and phononic quantum transport in zigzag or chiral graphene nanoribbons (GNRs) perforated with an array of nanopores. Since local charge current profiles in these GNRs are peaked around their edges, drilling nanopores in their interior does not affect edge charge currents while drastically reducing the phonon heat current in sufficiently long wires. The combination of these two effects can yield highly efficient thermoelectric devices with maximum figure of merit $ZT\simeq 5$, at both liquid nitrogen and room temperature, achieved in $\sim$1 $\mu$m long zigzag GNRs with nanopores of variable diameter and spacing between them. Our analysis is based on the nonequilibrium Green's function formalism combined with the $\pi$-orbital tight-binding Hamiltonian including up to third-nearest-neighbor hopping for an electronic subsystem, or with an empirical fifth-nearest-neighbor force-constant (5NNFC) model for a phononic subsystem. Additionally, we demonstrate that different empirical FC models typically overestimate the phonon conductance when compared to first-principles results.

The relaxation rate in Nb-doped strontium titanate involving different scattering channels is investigated theoretically. It is demonstrated that the total relaxation rate in SrTi${}_{1-x}$Nb${}_x$O${}_3$ is provided mainly by two mechanisms. The Baber electron-electron scattering with participation of both Coulomb and phonon-mediated electron-electron interactions provides the $T^2$ dependence of the relaxation rate. The scattering on the potential landscape caused by impurities is responsible for the residual relaxation rate at low temperatures. A good agreement with experiment is achieved accounting for all phonon branches in strontium titanate, both the optical and acoustic phonons. It is shown that the effective electron-electron interaction can be attractive in strontium titanate, and provides superconductivity at low temperatures and Fermi-liquid response in a wide range of temperatures. Thus our microscopic model supports the notion that superconductivity and Fermi-liquid properties of $n$-type SrTiO${}_3$ have a common origin.

We consider the effect of elasticity on torsional oscillator experiments to investigate the possible supersolidity of helium. We show that in most, and possibly all torsional oscillators there is a significant and hitherto unconsidered contribution to the torsional period from the helium elasticity. This effect needs to be carefully considered before it can be concluded that a supersolid component is indeed present.

The properties of molecular films are determined by the geometric structure of the first layers near the interface. These are in contact with the substrate and feel the effect of the interfacial bonding, which particularly, for metal substrates, can be substantial. For the model system 3,4,9,10-perylenetetracarboxylic dianhydride on Ag(110), the geometric structure of the first monolayer can be modified by preparation parameters. This leads to significant differences in the electronic structure of the first layer. Here, we show that, by combining angle-resolved photoelectron spectroscopy with low-energy electron diffraction, we cannot only determine the electronic structure of the interfacial layer and the unit cell of the adsorbate superstructure, but also the arrangement of the molecules in the unit cell. Moreover, in bilayer films, we can distinguish the first from the second layer and, thus, study the formation of the second layer and its influence on the buried interface.

Artificial multiferroic systems, in which novel properties can emerge from elastic coupling between piezoelectric and magnetostrictive phases, are a promising route to obtain significant room-temperature magnetoelectric coupling at the nanoscale. In this work, we have used element-specific soft x-ray photoemission electron microscopy to spatially resolve the effects of ferroelectric BaTiO${}_3$ on the magnetic properties of ferromagnetic layers of CoFe${}_2$O${}_4$ and NiFe${}_2$O${}_4$. Resulting from the large magnetoelastic anisotropy present in these artificial multiferroic systems, a modification of the orientation and symmetry of the magnetic easy axes of the ferromagnetic film upon changing the underlying domain structure of the ferroelectric has been observed. This opens the possibility to strain-imprint magnetic states in these systems.

We study current carrying helical edge states in a two-dimensional topological insulator coupled to an environment of localized spins, i.e., a spin bath. The localized spins mediate elastic spin-flip scattering between the helical edge states, and we show how this induces a spin-bath magnetization for a finite current through the edge states. The magnetization appears near the boundaries of the topological insulator, while the bulk remains unmagnetized, and it reaches its maximal value in the high bias regime. Furthermore, the helical edge states remain ballistic in steady state, if no additional spin-flip mechanisms for the localized spins are present. However, we demonstrate that if such mechanisms are allowed, then these will induce a finite current decrease from the ballistic value.

The nonlinear interaction between two mechanical resonances of the same freely suspended carbon nanotube resonator is studied. We find that, in the Coulomb-blockade regime, the nonlinear modal interaction is dominated by single-electron-tunneling processes and that the mode-coupling parameter can be tuned with the gate voltage, allowing both mode-softening and mode-stiffening behaviors. This is in striking contrast to tension-induced mode coupling in strings where the coupling parameter is positive and gives rise to a stiffening of the mode. The strength of the mode coupling in carbon nanotubes in the Coulomb-blockade regime is observed to be 6 orders of magnitude larger than the mechanical-mode coupling in micromechanical resonators.