Numerical simulations and experiments on nanostructures out of equilibrium usually exhibit strong finite size and finite measuring time $t_{\mathrm{m}}$ effects. We discuss how these affect the determination of the full counting statistics for a general quantum impurity problem. We find that, while there are many methods available to improve upon finite-size effects, any real-time simulation or experiment will still be subject to finite-time effects: In short size matters, but time is limiting. We show that the leading correction to the cumulant generating function (CGF) at zero temperature for single-channel quantum impurity problems is proportional to $ln{t}_{\mathrm{m}}$, where the constant of proportionality is universally related to the steady state CGF itself for non-interacting systems; universal in this context means independent of details of the quench procedure, i.e., independent of the switching on of both voltage and counting field. We give detailed numerical evidence for the case of the self-dual interacting resonant level model that this relation survives the addition of interactions. This allows the extrapolation of finite measuring time in our numerics to the long-time limit, in excellent agreement with Bethe-ansatz results.

We present an ab initio study of the electronic response function of sodium in its five known metallic phases from 0 to 180 GPa at room temperature. The considered formalism is based on a interpolation scheme within time-dependent density functional theory that uses maximally localized Wannier functions, providing an accurate sampling of the reciprocal space. Besides showing an excellent agreement with inelastic x-ray scattering experiments [Phys. Rev. Lett. 107, 086402 (2011), Proc. Natl. Acad. Sci. USA 108, 20434 (2011)], our calculations reveal that the drastic decrease of the optical reflectivity measured in the high pressure phases oP8 and tI19 [Proc. Natl. Acad. Sci. USA 106, 6525 (2009)] is associated with a new low-energy plasmon arising from collective interband excitations. Additionally, our calculations predict the existence of an anisotropic interband plasmon in the stability pressure range of fcc Na (65 to 105 GPa).

The effective on-site Coulomb interaction (Hubbard $U$) is an important parameter for studying strongly correlated systems. While $U$ is determined empirically by fitting to bulk values, its value for a cluster with a finite number of atoms remains uncertain. Here, we choose Gd${}_{13}$ as a prototypical example of a strongly correlated cluster. Contrary to the well-known results in transition-metal clusters where magnetic moments of clusters are larger than their bulk, in Gd${}_{13}$ cluster the magnetic moment is smaller than its bulk value. Using density functional theory and the linear response approach, we determine $U$ self-consistently for the cluster and apply it to explain the anomalous magnetic properties of Gd${}_{13}$. We demonstrate that the interaction between core and shell atoms of the Gd${}_{13}$ cluster strongly depends on the Hubbard $U$. For $U=0$ eV magnetism is governed by a direct f-f electron interaction between core and shell atoms, while for $U=5.5$ eV it is the indirect Ruderman-Kittel-Kasuya-Yosida interaction that prevails. We also demonstrate that the noncollinear spin arrangement of each atom in the cluster strongly depends on the Hubbard $U$. Monte Carlo calculations further confirm that magnetic moments decrease with temperature, thus addressing a long-standing disagreement in experimental results.

We study the ground-state properties of a frustrated two-species mixture of hard-core bosons on a triangular lattice, as a function of tunable amplitudes for tunneling and interactions. By combining three different methods, a self-consistent cluster mean-field, exact diagonalization, and effective theories, we unravel a very rich and complex phase diagram. More specifically, we discuss the existence of three original mixture supersolids: (i) a commensurate with frozen densities and supersolidity in spin degrees of freedom, in a regime of strong interspecies interactions; and (ii) when this interaction is weaker, two mutually competing incommensurate supersolids. Finally, we show how these phases can be stabilized by a quantum fluctuation enhancement of peculiar insulating parent states.

Using high-resolution angle-resolved photoemission spectroscopy, we elucidate the Rashba splitting of $\Delta k_F=0.003$ ${\AA }^{-1}$ near the Fermi level ($E_F$) in the Shockley surface state of Cu(110) at the $\overline{\mathrm{Y}}$ point of the surface Brillouin zone. The observed energy-band dispersion exhibits a kink structure at $\sim -20$ meV, which is a clear indication of band renormalization caused by an electron-phonon interaction. The electron-phonon coupling parameter is found to be ${\lambda }_{ep}=0.17\pm 0.02$ based on the experimentally obtained real part of the self-energy. First-principle calculations yield ${\lambda }_{ep}=0.160$ and $\Delta k_F=0.004$ ${\AA }^{-1}$ at $E_F$, which are fully consistent with the experimental results. In addition, the contributions of the electron-electron and electron-phonon interactions to the linewidth of the surface state at the $\overline{\mathrm{Y}}$ point are experimentally determined to be ${\Gamma }_{ee}\sim 9$ meV and ${\Gamma }_{ep}\sim 7$ meV, respectively. We demonstrate that the Rashba splitting must be resolved by photoemission line-shape analysis for an accurate determination of the electron self-energy and coupling parameters.

Experiments on spin torque oscillators commonly observe multimode signals. Recent theoretical works have ascribed the multimode signal generation to coupling between energy-separated spin wave modes. Here, we analyze in detail the dynamics generated by such mode coupling. We show analytically that the mode-hopping dynamics broaden the generation linewidth and makes it generally well described by a Voigt line shape. Furthermore, we show that the mode-hopping contribution to the linewidth can dominate, in which case it provides a direct measure of the mode-hopping rate. Due to the thermal drive of mode-hopping events, the mode-hopping rate also provides information on the energy barrier separating modes and temperature-dependent linewidth broadening. Our results are in good agreement with experiments, revealing the physical mechanism behind the linewidth broadening in multimode spin torque oscillators.

We study the flow of the renormalized model parameters obtained from a sequence of simple transformations of the 1D Anderson model with long-range hierarchical hopping. Combining numerical results with a perturbative approach for the flow equations, we identify three qualitatively different regimes at weak disorder. For a sufficiently fast decay of the hopping energy, the Cauchy distribution is the only stable fixed point of the flow equations, whereas for sufficiently slowly decaying hopping energy the renormalized parameters flow to a $\delta$-peak fixed-point distribution. In an intermediate range of the hopping decay, both fixed-point distributions are stable and the stationary solution is determined by the initial configuration of the random parameters. We present results for the critical decay of the hopping energy separating the different regimes.

The linear and nonlinear ac susceptibility measurements of Yb-pyrochlores, Yb${}_2{X}_2$O${}_7$ ($X=\text{Sn}$, Ti, and Ge), show transitions with a ferromagnetic nature at 0.13 and 0.25 K for Yb${}_2$Sn${}_2$O${}_7$ and Yb${}_2$Ti${}_2$O${}_7$, respectively, and an antiferromagnetic ordering at 0.62 K for Yb${}_2$Ge${}_2$O${}_7$. These systematical results (i) provided information about the nature of the unconventional magnetic ground state in Yb${}_2$Ti${}_2$O${}_7$; (ii) realized a distinct antiferromagnetic ordering state in Yb${}_2$Ge${}_2$O${}_7$; and (iii) demonstrated that the application of chemical pressure through the series of Yb-pyrochlores can efficiently perturb the fragile quantum spin fluctuations of the Yb${}^{3+}$ ions and lead to very different magnetic ground states.

We theoretically explore possible $\pi$-state Josephson junctions made from graphene-like two-dimensional materials (G) with the honeycomb lattice structure. It is shown that the valley polarization in the G sheet could lead to a 0-$\pi$ state transition of the Josephson junction because of the valley-singlet Cooper pairs acquiring a nonzero momentum. When the valley-mixing scattering exists in the interfaces of the junction due to lattice mismatch, an odd-frequency valley-triplet supercurrent flows in the system even though the G sheet is fully valley polarized, and the supercurrent is characterized by a rapid atomic-scale oscillation with a periodicity of three lattice constants.

We analyze the transformation from insulator to metal induced by thermal fluctuations within the Falicov-Kimball model. Using the dynamic mean field theory (DMFT) formalism on the Bethe lattice we find rigorously the temperature dependent density of states (DOS) at half filling in the limit of high dimensions. At zero temperature ($T=0$) the system is ordered to form the checkerboard pattern and the DOS has the gap $\Delta$ at the Fermi level ${\varepsilon }_F=0$, which is proportional to the interaction constant $U$. With an increase of $T$ the DOS evolves in various ways that depend on $U$. For $U>U_{cr}$ the gap persists for any $T$ (then $\Delta >0$), so the system is always an insulator. However, if $U<U_{cr}$, two additional subbands develop inside the gap. They become wider with increasing $T$ and at a certain $U$-dependent temperature $T_{\mathrm{MI}}$ they join with each other at ${\varepsilon }_F$. Since above $T_{\mathrm{MI}}$ the DOS is positive at ${\varepsilon }_F$, we interpret $T_{\mathrm{MI}}$ as the transformation temperature from insulator to metal. It appears that $T_{\mathrm{MI}}$ approaches the order-disorder phase transition temperature $T_{\mathrm{O}\text{−}\mathrm{DO}}$ when $U$ is close to 0 or $U_{cr}$, but $T_{\mathrm{MI}}$ is substantially lower than $T_{\mathrm{O}\text{−}\mathrm{DO}}$ for intermediate values of $U$. Moreover, using an analytical formula we show that $T_{\mathrm{MI}}=0$ at $U=\sqrt{2}<U_{cr}$, so we prove that the quantum critical point exists for the ordered metal at $(T=0,U=\sqrt{2})$. Having calculated the temperature dependent DOS we study thermodynamic properties of the system starting from its free energy $F$. Then we find how the order parameter $d$ and the gap $\Delta$ change with $T$ and we construct the phase diagram in the variables $T$ and $U$, where we display regions of stability of four different phases: ordered insulator, ordered metal, disordered insulator, and disordered metal. Finally, we use a low temperature expansion to demonstrate the existence of a nonzero DOS at a characteristic value of $U$ on a general bipartite lattice.

We investigate the stability of O, Ti, and Sr vacancies in SrTiO${}_3$ and their interactions with hydrogen impurities. Based on density functional calculations with a hybrid functional, we analyze formation energies, binding energies, and H-related vibrational modes. We find that interstitial hydrogen (H${}_i^{+}$) and substitutional hydrogen on an oxygen site (H${}_{\mathrm{O}}$) both act as shallow donors and are likely to contribute to unintentional $n$-type conductivity. Hydrogen can also bind to Ti vacancies in the form of ($V_{\mathrm{Ti}}$-H)${}^{-3}$ and ($V_{\mathrm{Ti}}$-2H)${}^{-2}$ complexes. Sr vacancies can form ($V_{\mathrm{Sr}}$-H)${}^{-}$ or accommodate an H${}_2$ molecule in the form of ($V_{\mathrm{Sr}}$-H${}_2$)${}^{-2}$ complex. The latter provides an explanation for the “hidden” hydrogen recently observed in annealing experiments [M. C. Tarun and M. D. McCluskey, J. Appl. Phys. 109, 063706 (2011)].

Formation and excitation energies as well charge transition levels are determined for the substitutional nitrogen (N${}_{\mathrm{s}}$), the vacancy (V), and related point defects (NV, NVH, N${}_2$, N${}_2$V, and V${}_2$) by screened nonlocal hybrid density functional supercell plane wave calculations in bulk diamond. In addition, the activation energy for V and NV diffusion is calculated. We find good agreement between theory and experiment for the previously well-established data and predict missing ones. Based on the calculated properties of these defects, the formation of the negatively charged NV center is studied, because it is a prominent candidate for application in quantum information processing and for nanosensors. Our results indicate that NV defects are predominantly created directly by irradiation, while simultaneously produced vacancies will form V${}_2$ pairs during postirradiation annealing. Divacancies may pin the Fermi level, making the NV defects neutral.

We have investigated the superconducting (SC)-gap anisotropy for several Ba-doped KFe${}_2$As${}_2$ samples using laser-based angle-resolved photoemission spectroscopy. We show that the SC-gap anisotropy and node positions drastically change with a small amount of Ba doping. Our results totally exclude a possibility of $d$-wave symmetry and strongly suggest that both spin and orbital fluctuations are important for the pairing interaction in the Ba-doped K122.

Although it is well known that dilute species often significantly modify the properties of solids in which they are hosted, a basic question remains open: How critical are their local properties in determining the macroscopic ones of the host material? Here we address this issue by taking N in dilute III-V-N alloys as a paradigmatic case and propose an original approach based on synergic progress in x-ray spectroscopies, density functional theory simulations, and hydrogen exposure as a tool to modify local bonding. In this way we are able to clarify how local properties operate by identifying the coupling mechanism between specific atomic orbitals responsible for the changes of both deep and band edge states. The success of our approach indicates a new way to connect local and macroscopic properties.

The non-centro-symmetric semiconductor BiTeI exhibits two distinct surface terminations that support spin-split Rashba surface states. Their ambipolarity can be exploited for creating spin-polarized $p$-$n$ junctions at the boundaries between domains with different surface terminations. We use scanning tunneling microscopy (STM) and spectroscopy (STS) to locate such junctions and investigate their atomic and electronic properties. The Te- and I-terminated surfaces are identified owing to their distinct chemical reactivity and an apparent height mismatch of electronic origin. The Rashba surface states are revealed in the STS spectra by the onset of a van Hove singularity at the band edge. Eventually, an electronic depletion is found on interfacial Te atoms, consistent with the formation of a space-charge area in typical $p$-$n$ junctions.

We consider the production of dynamic nuclear spin polarization (DNP) in a two-electron double quantum dot, in which the electronic levels are repeatedly swept through a singlet-triplet avoided crossing. Our analysis helps to elucidate the intriguing interplay between electron-nuclear hyperfine coupling, electronic spin-orbit coupling, and electron and nuclear Larmor precession in an externally applied magnetic field in guiding the production of DNP. In particular, we specifically address the roles of multiple nuclear spin species. By treating the nuclear spin dynamics semiclassically, we identify two contributions to the DNP production rate, a “geometric contribution” and a “dynamic contribution,” which depend in very different ways on control parameters such as the sweep rate and holding time near the level crossing. We find that the dynamical contribution dominates the DNP when the system is held near the singlet-triplet avoided crossing for a time on the order of the nuclear Larmor period. Detailed numerical calculations provide a physical picture for understanding the oscillations observed by Foletti et al. in arXiv:0801.3613.

We propose a Landau theory for phase change materials (PCMs), which describes stress-induced amorphization in vacancy-free, ordered PCMs as a condensation of defects in analogy to equilibrium gas-liquid transitions. Three dimensionless parameters suffice to deduce a highly accurate equation of state for both phases from it. The reference data for our model alloy Ge${}_x$Sb${}_{1-x}$ are produced from molecular dynamics simulations and synchrotron x-ray diffraction. Raman spectroscopy is used to estimate the density of tetrahedrally coordinated germanium atoms, which we relate to the order parameter. All methods provide consistent support for the reversibility of the transition.

The spin Hall effect (SHE), induced by spin-orbit interaction in nonmagnetic materials, is one of the promising phenomena for conversion between charge and spin currents in spintronic devices. The spin Hall (SH) angle is the characteristic parameter of this conversion. We have performed experiments of the conversion from spin into charge currents by the SHE in lateral spin valve structures. We present experimental results on the extrinsic SHEs induced by doping nonmagnetic metals, Cu or Ag, with impurities having a large spin-orbit coupling, Bi or Pb, as well as results on the intrinsic SHE of Au. The SH angle induced by Bi in Cu or Ag is negative and particularly large for Bi in Cu, 10 times larger than the intrinsic SH angle in Au. We also observed a large SH angle for CuPb, but the SHE signal disappeared in a few days. Such an aging effect could be related to a fast mobility of Pb in Cu and has not been observed in CuBi alloys.

We present a fast deterministic approach to the ptychographic reconstruction of the transmission function of a specimen at atomic resolution. The method is demonstrated using a data set obtained from a cerium dioxide nanoparticle using an aberration-corrected electron microscope and is compared to established approaches to ptychography. The method is based on the solution of an overdetermined set of linear equations, and is computationally efficient and robust to measurement noise. The set of linear equations is efficiently solved using the conjugate gradient least squares method implemented using fast Fourier transforms.

We describe an application of variational Monte Carlo to two-dimensional fermionic systems within the recently developed tensor-network string-bond state ansatz. We use a combination of variational Monte Carlo and stochastic optimization to optimize the matrix-product state matrices representing the ground state. We present results for a two-dimensional spinless fermion model including nearest-neighbor Coulomb interactions and determine using finite-size scaling the phase boundary between charge-ordered insulating and metallic phases. This approach can treat frustrated systems and be easily extended to fermion models with spin.