An extended investigation of the electronic phase diagram of FeSe${}_{1-x}$ up to pressures of $p\simeq 2.4$ GPa by means of ac and dc magnetization, zero-field muon spin rotation (ZF $\mu$SR), and neutron diffraction is presented. ZF $\mu$SR indicates that at pressures $p\ge 0.8$ GPa static magnetic order occurs in FeSe${}_{1-x}$ and occupies the full sample volume for $p\gtrsim 1.2$ GPa. ac magnetization measurements reveal that the superconducting volume fraction stays close to 100% up to the highest pressure investigated. In addition, above $p\ge 1.2$ GPa both the superconducting transition temperature $T_{\mathrm{c}}$ and the magnetic ordering temperature $T_{\mathrm{N}}$ increase simultaneously, and both superconductivity and magnetism are stabilized with increasing pressure. Calculations indicate only one possible muon stopping site in FeSe${}_{1-x}$, located on the line connecting the Se atoms along the $c$ direction. Different magnetic structures are proposed and checked by combining the muon stopping calculations with a symmetry analysis, leading to a similar structure as in the LaFeAsO family of Fe-based superconductors. Furthermore, it is shown that the magnetic moment is pressure dependent and with a rather small value of $\mu \approx 0.2{\mu }_B$ at $p\simeq 2.4$ GPa.

The time-dependent numerical renormalization-group approach (TD-NRG), originally devised for tracking the real-time dynamics of quantum-impurity systems following a single quantum quench, is extended to multiple switching events. This generalization of the TD-NRG encompasses the possibility of periodic switching, allowing for coherent control of strongly correlated systems by an external time-dependent field. To this end, we have embedded the TD-NRG in a hybrid framework that combines the outstanding capabilities of the numerical renormalization group to systematically construct the effective low-energy Hamiltonian of the system with the prowess of complementary approaches for calculating the real-time dynamics derived from this Hamiltonian. We demonstrate the power of our approach by hybridizing the TD-NRG with the Chebyshev expansion technique in order to investigate periodic switching in the interacting resonant-level model. Although the interacting model shares the same low-energy fixed point as its noninteracting counterpart, we surprisingly find the gradual emergence of damped oscillations as the interaction strength is increased. Focusing on a single quantum quench and using a strong-coupling analysis, we reveal the origin of these interaction-induced oscillations and provide an analytical estimate for their frequency. The latter agrees well with the numerical results.

Single crystalline vanadium dioxide (VO${}_2$) nanobeams offer an ideal material basis for exploring the widely observed insulator–metal transition in strongly correlated materials. Here, we investigate nonequilibrium carrier dynamics and electronic structure in single crystalline VO${}_2$ nanobeam devices using scanning photocurrent microscopy in the vicinity of their phase transition. We extracted a Schottky barrier height of $\sim$0.3 eV between the metal and the insulator phases of VO${}_2$, providing direct evidence of the nearly symmetric band gap opening upon phase transition. We also observed unusually long photocurrent decay lengths in the insulator phase, indicating unexpectedly long minority carrier lifetimes on the order of microseconds, consistent with the nature of carrier recombination between two d-subbands of VO${}_2$.

We study four different models of Chern insulators in the presence of strong electronic repulsion at partial fillings. We observe that all cases exhibit a Laughlin-like phase at filling fraction $1/3$. We provide evidence of such a strongly correlated topological phase by studying both the energy and the entanglement spectra. In order to identify the key ingredients of the emergence of Laughlin physics in these systems, we show how they are affected when tuning the band structure. We also address the question of the relevance of the Berry curvature flatness in this problem. Using three-body interactions, we show that some models can also host a topological phase reminiscent of the $\nu =1/2$ Pfaffian Moore-Read state. Additionally, we identify the structures indicating cluster correlations in the entanglement spectra.

The authors report the use of a radio frequency (rf) silicon nanowire mechanical oscillator as a low-temperature nuclear magnetic resonance force sensor to detect the statistical polarization of ${}^1$H spins in polystyrene. To couple the ${}^1$H spins to the nanowire oscillator, a magnetic resonance force detection protocol was developed that utilizes a nanoscale current–carrying wire to produce large time-dependent magnetic field gradients as well as the rf magnetic field. Under operating conditions, the nanowire experienced negligible surface-induced dissipation and exhibited an ultralow force noise near the thermal limit of the oscillator.

Highly accurate results for the homogeneous electron gas (HEG) have only been achieved to date within a diffusion Monte Carlo (DMC) framework. Here, we introduce a recently developed stochastic technique, full configuration interaction quantum Monte Carlo (FCIQMC), which samples the exact wave function expanded in plane-wave Slater determinants. Despite the introduction of a basis-set incompleteness error, we obtain a finite-basis energy, which is significantly and variationally lower than any previously published work for the 54-electron HEG at $r_s$ $=$ 0.5 a.u., in a Hilbert space of ${10}^{108}$ Slater determinants. At this value of $r_s$, as well as of 1.0 a.u., we remove the remaining basis-set incompleteness error by extrapolation, yielding results comparable to state-of-the-art DMC backflow energies. In doing so, we demonstrate that it is possible to yield highly accurate results with the FCIQMC method in periodic systems.

The width and shape of the density of states (DOS) are key parameters to describe the charge transport of organic semiconductors. Here we extract the DOS using scanning Kelvin probe microscopy on a self-assembled monolayer field effect transistor (SAMFET). The semiconductor is only a single monolayer which has allowed extraction of the DOS over a wide energy range, pushing the methodology to its fundamental limit. The measured DOS consists of an exponential distribution of deep states with additional localized states on top. The charge transport has been calculated in a generic variable range-hopping model that allows any DOS as input. We show that with the experimentally extracted DOS an excellent agreement between measured and calculated transfer curves is obtained. This shows that detailed knowledge of the density of states is a prerequisite to consistently describe the transfer characteristics of organic field effect transistors.

The possibility of finding multistability in the density and current of an interacting nanoscale junction coupled to semi-infinite leads is studied at various levels of approximation. The system is driven out of equilibrium by an external bias and the nonequilibrium properties are determined by real-time propagation using both time-dependent density functional theory (TDDFT) and many-body perturbation theory (MBPT). In TDDFT the exchange-correlation effects are described within a recently proposed adiabatic local density approximation (ALDA). In MBPT the electron-electron interaction is incorporated in a many-body self-energy which is then approximated at the Hartree-Fock (HF), second-Born, and $GW$ level. Assuming the existence of a steady state and solving directly the steady-state equations we find multiple solutions in the HF approximation and within the ALDA. In these cases we investigate whether and how these solutions can be reached through time evolution and how to reversibly switch between them. We further show that for the same cases the inclusion of dynamical correlation effects suppresses bistability.

We analyze the dynamics of a single-level quantum dot with Coulomb interaction, weakly tunnel coupled to an electronic reservoir, after it has been brought out of equilibrium, e.g., by a step-pulse potential. We investigate the exponential decay toward the equilibrium state, which is governed by three time scales. In addition to the charge and spin relaxation time there is a third time scale which is independent of the level position and the Coulomb interaction. This time scale emerges in the time evolution of physical quantities sensitive to two-particle processes.

In order to understand the material dependence of $T_c$ within the single-layered cuprates, we study a two-orbital model that considers both $d_{x^2-y^2}$ and $d_{z^2}$ orbitals. We reveal that a hybridization of $d_{z^2}$ on the Fermi surface substantially affects $T_c$ in the cuprates, where the energy difference $\Delta E$ between the $d_{x^2-y2}$ and the $d_{z^2}$ orbitals is identified to be the key parameter that governs both the hybridization and the shape of the Fermi surface. A smaller $\Delta E$ tends to suppress $T_c$ through a larger hybridization, whose effect supersedes the effect of diamond-shaped (better-nested) Fermi surface. The mechanism of the suppression of $d$-wave superconductivity due to $d_{z^2}$ orbital mixture is clarified from the viewpoint of the ingredients involved in the Eliashberg equation, that is, the Green's functions and the form of the pairing interaction described in the orbital representation. The conclusion remains qualitatively the same if we take a three-orbital model that incorporates the Cu $4s$ orbital explicitly, where the $4s$ orbital is shown to have an important effect of making the Fermi surface rounded. We have then identified the origin of the material and lattice-structure dependence of $\Delta E$, which is shown to be determined by the energy difference $\Delta E_d$ between the two Cu $3d$ orbitals (primarily governed by the apical oxygen height) and the energy difference $\Delta E_p$ between the in-plane and apical oxygens (primarily governed by the interlayer separation $d$).

We present a first-principles scheme that allows the orbital magnetization of a magnetic crystal to be evaluated accurately and efficiently even in the presence of complex Fermi surfaces. Starting from an initial electronic-structure calculation with a coarse ab initio $\mathbf{k}$-point mesh, maximally localized Wannier functions are constructed and used to interpolate the necessary $\mathbf{k}$-space quantities on a fine mesh, in parallel to a previously developed formalism for the anomalous Hall conductivity [X. Wang, J. Yates, I. Souza, and D. Vanderbilt, Phys. Rev. B 74, 195118 (2006)]. We formulate our new approach in a manifestly gauge-invariant manner, expressing the orbital magnetization in terms of traces over matrices in Wannier space. Since only a few (e.g., of the order of 20) Wannier functions are typically needed to describe the occupied and partially occupied bands, these Wannier matrices are small, which makes the interpolation itself very efficient. The method has been used to calculate the orbital magnetization of bcc Fe, hcp Co, and fcc Ni. Unlike an approximate calculation based on integrating orbital currents inside atomic spheres, our results nicely reproduce the experimentally measured ordering of the orbital magnetization in these three materials.

We present a full potential, all electron augmented plane wave (APW) implementation of first-principles calculations of NMR chemical shifts. In order to obtain the induced current we follow a perturbation approach [Pickard and Mauri, Phys. Rev. B 63, 245101 (2001)] and extended the common APW + local orbital (LO) basis by several LOs at higher energies. The calculated all-electron current is represented in traditional APW manner as Fourier series in the interstitial region and with a spherical harmonics representation inside the nonoverlapping atomic spheres. The current is integrated using a “pseudocharge” technique. The implementation is validated by comparison of the computed chemical shifts with some “exact” results for spherical atoms and for a set of solids and molecules with available published data.

The vortex states on optimally doped Ba${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$ and underdoped Ba${}_{0.77}$K${}_{0.23}$Fe${}_2$As${}_2$ single crystals are imaged by magnetic force microscopy at various magnetic fields below 100$$Oe. Local triangular vortex clusters are observed in optimally doped samples. The vortices are more ordered than those in Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$, and the calculated pinning force per unit length is about 1 order of magnitude weaker than that in optimally Co-doped 122 at the same magnetic field, indicating that the Co doping at the Fe sites induces stronger pinning. The proportion of six-neighbored vortices to the total amount increases quickly with increasing magnetic field, and the estimated value reaches 100$\%$ at several tesla. Vortex chains are also found in some local regions, which enhance the pinning force as well as the critical current density. Lines of vortex chains are observed in underdoped samples, and they may have originated from the strong pinning near the twin boundaries arising from the structural transition.

The high-field region of the magnetic phase diagram of MnSi is probed by magnetization, resistivity, and magnetoresistance measurements carried out in the temperature range 1.8–300 K for magnetic fields up to 8 T. It is shown that the phase boundary between the paramagnetic (PM) phase and the spin-polarized (SP) phase has no positive slope as was suggested previously, and appears to be practically vertical at the transition temperature $T_c\sim$ 30 K. We argue that the broad maxima of the resistivity and magnetization derivative, which develop in the range $T>T_c$, are determined by the specific form of functional dependences of these quantities in the PM phase and do not correspond to any “diffuse” SP-PM transition. A universal relation between magnetoresistance and magnetization $\Delta \rho /\rho =-a_0{M}^2$ found in the PM phase of MnSi is shown to hold in a wide temperature and magnetic field range where magnetoresistance varies by more than two orders of magnitude. The analysis of the transport and magnetic resonance data favors the explanation of the magnetic properties of MnSi by Heisenberg-type localized magnetic moments rather than within the magnetism of itinerant electrons. A low-temperature anomaly at $T\sim$ 15 K corresponding to the features of magnetoresistance and $g$ factor is reported.

In long Josephson junctions with multiple discontinuities of the Josephson phase, fractional vortex molecules are spontaneously formed. At each discontinuity point a fractional Josephson vortex carrying a magnetic flux $\left|\Phi \right|<{\Phi }_0$, where ${\Phi }_0\approx 2.07\times {10}^{-15}$ Wb is the magnetic flux quantum, is pinned. Each vortex has an oscillatory eigenmode with a frequency that depends on $\Phi /{\Phi }_0$ and lies inside the plasma gap. We experimentally investigate the dependence of the eigenfrequencies of a two-vortex molecule on the distance between the vortices, on their topological charge $\wp =2\pi \Phi /{\Phi }_0$, and on the bias current $\gamma$ applied to the Josephson junction. We find that, with decreasing distance between vortices, a splitting of the eigenfrequencies occurs that corresponds to the emergence of collective oscillatory modes of both vortices. We use a resonant microwave spectroscopy technique and find good agreement between experimental results and theoretical predictions.

We study the shot noise (nonequilibrium current fluctuation) associated with dynamic nuclear polarization in a nonequilibrium quantum wire (QW) fabricated in a two-dimensional electron gas. We observe that the spin-polarized conductance quantization of the QW in the integer quantum Hall regime collapses when the QW is voltage biased to be driven to nonequilibrium. By measuring the shot noise, we prove that the spin polarization of electrons in the QW is reduced to $\sim$$0.7$ instead of unity as a result of electron-nuclear spin-flip scattering. The result is supported by Knight shift measurements of the QW using resistively detected NMR.

We report an unusual temperature dependence in the magnetoresistance of a weakly interacting high mobility 2D electron gas (2DEG) under a parallel magnetic field and in the current configuration $\mathit{I}\perp \mathit{B}$. While the linear temperature dependence below 10 K and the exponential temperature dependence above 40 K agree with the existing theory of electron-phonon scattering, a field-induced resistivity saturation behavior characterized by an almost complete suppression of the temperature dependence is observed from approximately 20 to 40 K, which is in sharp contrast to the phenomenology observed in the configuration $\mathit{I}\parallel \mathit{B}$. Possible origins of this intermediate temperature phenomenon are discussed.

An ab initio equation of motion method is introduced to calculate the temperature-dependent mean-square vibrational amplitudes ${\sigma }^2$ which appear in the Debye-Waller factors in x-ray absorption, x-ray scattering, and related spectra. The approach avoids explicit calculations of phonon modes, and is based instead on calculations of the displacement-displacement time correlation function from ab initio density functional theory molecular dynamics simulations. The method also yields the vibrational density of states and thermal quantities such as the lattice free energy. Illustrations of the method are presented for a number of systems and compared with other methods and experiment.

General scaling arguments, and the behavior of the thermal entropy density, are shown to lead to an infrared metric holographically representing a compressible state with hidden Fermi surfaces. This metric is characterized by a general dynamic critical exponent, $z$, and a specific hyperscaling violation exponent, $\theta$. The same metric exhibits a logarithmic violation of the area law of entanglement entropy, as shown recently by Ogawa et al. [e-print arXiv:1111.1023 (unpublished)]. We study the dependence of the entanglement entropy on the shape of the entangling region(s), on the total charge density, on temperature, and on the presence of additional visible Fermi surfaces of gauge-neutral fermions; for the latter computations, we realize the needed metric in an Einstein-Maxwell-dilaton theory. All our results support the proposal that the holographic theory describes a metallic state with hidden Fermi surfaces of fermions carrying gauge charges of deconfined gauge fields.

Boron nitride (BN) sheets were exfoliated, and proof of the presence of single and double layers was obtained via electron diffraction and plasmon electron energy loss spectroscopy. A plasmon structure unique to mono- and bi-layer BN was established, and was accompanied by WIEN2K DFT calculations. The latter reproduced plasmon energies and general plasmon structure well; however, the detailed shape of the π-plasmon of the calculated pure BN spectra shows discrepancies with the experimental data. The theoretical models were then modified to include impurity atoms. Both oxygen and carbon impurities were considered, as well as different structures, including singular oxygen atoms and oxygen next to carbon atoms. These various configurations were obtained by analyzing atomic-resolution high-angle annular dark field (HAADF) images. Using these modified models, a π-plasmon structure close to the experimentally observed structure could be simulated.