Systematic and Causal Corrections to the Coherent Potential Approximation

The Dynamical Cluster Approximation (DCA) is modified to include disorder. The DCA incorporates non-local corrections to local approximations such as the Coherent Potential Approximation (CPA) by mapping the lattice problem with disorder, and in the thermodynamic limit, to a self-consistently embedded finite-sized cluster problem. It satisfies all of the characteristics of a successful cluster approximation. It is causal, preserves the point-group and translational symmetry of the original lattice, recovers the CPA when the cluster size equals one, and becomes exact as $N_c\to\infty$. We use the DCA to study the Anderson model with binary diagonal disorder. It restores sharp features and band tailing in the density of states which reflect correlations in the local environment of each site. While the DCA does not describe the localization transition, it does describe precursor effects of localization.Comment: 11 pages, LaTeX, and 11 PS figures, to appear in Phys. Rev. B. Revised version with typos corrected and references adde

Zero-bias anomalies and boson-assisted tunneling through quantum dots

We study resonant tunneling through a quantum dot with one degenerate level in the presence of a strong Coulomb repulsion and a bosonic environment. Using a real-time approach we calculate the spectral density and the nonlinear current within a conserving approximation. The spectral density shows a multiplet of Kondo peaks split by the transport voltage and boson frequencies. As a consequence we find a zero-bias anomaly in the differential conductance which can show a local maximum or minimum depending on the level position. The results are compared with recent experiments.Comment: 4 pages, revtex, 5 postscript figures, submitted to Phys. Rev. Let

Charge correlations in polaron hopping through molecules

In many organic molecules the strong coupling of excess charges to vibrational modes leads to the formation of polarons, i.e., a localized state of a charge carrier and a molecular deformation. Incoherent hopping of polarons along the molecule is the dominant mechanism of transport at room temperature. We study the far-from-equilibrium situation where, due to the applied bias, the induced number of charge carriers on the molecule is high enough such that charge correlations become relevant. We develop a diagrammatic theory that exactly accounts for all many-particle correlations functions for incoherent transport through a finite system. We compute the transport properties of short sequences of DNA by expanding the diagrammatic theory up to second order in the hopping parameters. The correlations qualitatively modify the I-V characteristics as compared to those approaches where correlations are dealt with in a mean-field type approximation only.Comment: 13 pages, 10 figures, submitted to Phys. Rev.

Nanotubes from Lanthanide-Based Misfit-Layered Compounds: Understanding the Growth, Thermodynamic, and Kinetic Stability Limits

Gaining insights into the kinetics and the thermodynamic limits of nanostructures in high-temperature reactions is crucial for controlling their unique morphology, phase, and structure. Nanotubes from lanthanide-based misfit-layered compounds (MLCs) have been known for more than a decade and were successfully produced mostly via a chemical vapor transport protocol. The MLC nanotubes show diverse structural arrangements and lattice disorders, which could have a salient impact on their properties. Though their structure and charge transfer properties are reasonably well understood, a lack of information on their thermodynamic and kinetic stability limits their scalable synthesis and their applicability in modern technologies. In this study, the growth, thermodynamic stability, and decomposition kinetics of lanthanide-based misfit nanotubes of two model compounds, i.e., (LaS)1.14TaS2 and (SmS)1.19TaS2 are elucidated in detail. The nanotubes were carefully analyzed via atomic resolution electron microscopy imaging and synchrotron-based X-ray and electron diffraction techniques, and the information on their morphology, phase, and structures was deduced. The key insights gained would help to establish the parameters to explore their physio-chemical properties further. Furthermore, this study sheds light on the complex issue of the high-temperature stability of nanotubes and nanostructures in general

Non-equilibrium electronic transport and interaction in short metallic nanobridges

We have observed interaction effects in the differential conductance $G$ of short, disordered metal bridges in a well-controlled non-equilibrium situation, where the distribution function has a double Fermi step. A logarithmic scaling law is found both for the temperature and for the voltage dependence of $G$ in all samples. The absence of magnetic field dependence and the low dimensionality of our samples allow us to distinguish between several possible interaction effects, proposed recently in nanoscopic samples. The universal scaling curve is explained quantitatively by the theory of electron-electron interaction in diffusive metals, adapted to the present case, where the sample size is smaller than the thermal diffusion length.Comment: Published version, 6 Pages, 6 postscript figures, 1 tabl

Microwave Conductivity due to Scattering from Extended Linear Defects in d-Wave Superconductors

Recent microwave conductivity measurements of detwinned, high-purity, slightly overdoped YBa$_{2}$Cu$_{3}$O$_{6.993}$ crystals reveal a linear temperature dependence and a near-Drude lineshape for temperatures between 1 and 20 K and frequencies ranging from 1 to 75 GHz. Prior theoretical work has shown that simple models of scattering by point defects (impurities) in d-wave superconductors are inconsistent with these results. It has therefore been suggested that scattering by extended defects such as twin boundary remnants, left over from the detwinning process, may also be important. We calculate the self-energy and microwave conductivity in the self-consistent Born approximation (including vertex corrections) for a d-wave superconductor in the presence of scattering from extended linear defects. We find that in the experimentally relevant limit ($\Omega, 1/\tau \ll T \ll \Delta_{0}$), the resulting microwave conductivity has a linear temperature dependence and a near-Drude frequency dependence that agrees well with experiment.Comment: 13 pages, 7 figure

YS-TaS2 and YxLa1–xS-TaS2 (0 ≤ x ≤ 1) nanotubes: A family of misfit layeredcompounds

We present the analysis of a family of nanotubes (NTs) based on the quaternary misfit layered compound (MLC) YxLa1–xS-TaS2. The NTs were successfully synthesized within the whole range of possible compositions via the chemical vapor transport technique. In-depth analysis of the NTs using electron microscopy and spectroscopy proves the in-phase (partial) substitution of La by Y in the (La,Y)S subsystem and reveals structural changes compared to the previously reported LaS-TaS2 MLC-NTs. The observed structure can be linked to the slightly different lattice parameters of LaS and YS. Raman spectroscopy and infrared transmission measurements reveal the tunability of the plasmonic and vibrational properties. Density-functional theory calculations showed that the YxLa1–xS-TaS2 MLCs are stable in all compositions. Moreover, the calculations indicated that substitution of La by Sc atoms is electronically not favorable, which explains our failed attempt to synthesize these MLC and NTs thereof.A.E. acknowledges the support by Act 211 Government of the Russian Federation, Contract No. 02.A03.21.0006. The support of the Israel Science Foundation (Grant No. 7130970101), Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging, and the Perlman Family Foundation and the Kimmel Center for Nanoscale Science (Grant No. 43535000350000) is greatly acknowledged. R.A. gratefully acknowledges the support from the Spanish Ministry of Economy and Competitiveness (MINECO) through Project Grant MAT2016-79776-P (AEI/FEDER, UE) and from the European Union H2020 program “ESTEEM3” (823717). S.H. acknowledges funding by the German Research Foundation (HE 7675/1-1). I.P. is the incumbent of the Sharon Zuckerman Research Fellow Chair.Peer reviewe

A Quantum Monte Carlo algorithm for non-local corrections to the Dynamical Mean-Field Approximation

We present the algorithmic details of the dynamical cluster approximation (DCA), with a quantum Monte Carlo (QMC) method used to solve the effective cluster problem. The DCA is a fully-causal approach which systematically restores non-local correlations to the dynamical mean field approximation (DMFA) while preserving the lattice symmetries. The DCA becomes exact for an infinite cluster size, while reducing to the DMFA for a cluster size of unity. We present a generalization of the Hirsch-Fye QMC algorithm for the solution of the embedded cluster problem. We use the two-dimensional Hubbard model to illustrate the performance of the DCA technique. At half-filling, we show that the DCA drives the spurious finite-temperature antiferromagnetic transition found in the DMFA slowly towards zero temperature as the cluster size increases, in conformity with the Mermin-Wagner theorem. Moreover, we find that there is a finite temperature metal to insulator transition which persists into the weak-coupling regime. This suggests that the magnetism of the model is Heisenberg like for all non-zero interactions. Away from half-filling, we find that the sign problem that arises in QMC simulations is significantly less severe in the context of DCA. Hence, we were able to obtain good statistics for small clusters. For these clusters, the DCA results show evidence of non-Fermi liquid behavior and superconductivity near half-filling.Comment: 25 pages, 15 figure