Many-body Green's function theory for electron-phonon interactions: ground state properties of the Holstein dimer

We study ground-state properties of a two-site, two-electron Holstein model describing two molecules coupled indirectly via electron-phonon interaction by using both exact diagonalization and self-consistent diagrammatic many-body perturbation theory. The Hartree and self-consistent Born approximations used in the present work are studied at different levels of self-consistency. The governing equations are shown to exhibit multiple solutions when the electron-phonon interaction is sufficiently strong whereas at smaller interactions only a single solution is found. The additional solutions at larger electron-phonon couplings correspond to symmetry-broken states with inhomogeneous electron densities. A comparison to exact results indicates that this symmetry breaking is strongly correlated with the formation of a bipolaron state in which the two electrons prefer to reside on the same molecule. The results further show that the Hartree and partially self-consistent Born solutions obtained by enforcing symmetry do not compare well with exact energetics, while the fully self-consistent Born approximation improves the qualitative and quantitative agreement with exact results in the same symmetric case. This together with a presented natural occupation number analysis supports the conclusion that the fully self-consistent approximation describes partially the bipolaron crossover. These results contribute to better understanding how these approximations cope with the strong localizing effect of the electron-phonon interaction.Comment: 9 figures, corrected typo

Fully Ab initio Simulations of Tip Enhanced Raman Scattering Reveal Active Role of Substrate on High-Resolution Images

Tip-enhanced Raman scattering (TERS) has emerged as a powerful tool to obtain subnanometer spatial resolution fingerprints of atomic motion. Theoretical calculations that can simulate the Raman scattering process and provide an unambiguous interpretation of TERS images often rely on crude approximations of the local electric field. In this work, we present a novel and fully ab initio method to compute TERS images by combining Time Dependent Density Functional Theory (TD-DFT) and Density Functional Perturbation Theory (DFPT) to calculate Raman cross sections with realistic local fields. We present TERS results on the benzene and the TCNE molecule, the latter adsorbed at Ag(110). We demonstrate that chemical effects on adsorbed molecules, often ignored in TERS simulations, dramatically change TERS images. This calls for the inclusion of chemical effects for predictive theory-experiment comparisons and understanding of molecular motion at the nanoscale

Compiler verification meets cross-language linking via data abstraction

Many real programs are written in multiple different programming languages, and supporting this pattern creates challenges for formal compiler verification. We describe our Coq verification of a compiler for a high-level language, such that the compiler correctness theorem allows us to derive partial-correctness Hoare-logic theorems for programs built by linking the assembly code output by our compiler and assembly code produced by other means. Our compiler supports such tricky features as storable cross-language function pointers, without giving up the usual benefits of being able to verify different compiler phases (including, in our case, two classic optimizations) independently. The key technical innovation is a mixed operational and axiomatic semantics for the source language, with a built-in notion of abstract data types, such that compiled code interfaces with other languages only through axiomatically specified methods that mutate encapsulated private data, represented in whatever formats are most natural for those languages.National Science Foundation (U.S.) (Grant CCF-1253229)United States. Defense Advanced Research Projects Agency (Agreement FA8750-12-2-0293)United States. Dept. of Energy. Office of Science (Award DE-SC0008923

Networking - A Statistical Physics Perspective

Efficient networking has a substantial economic and societal impact in a broad range of areas including transportation systems, wired and wireless communications and a range of Internet applications. As transportation and communication networks become increasingly more complex, the ever increasing demand for congestion control, higher traffic capacity, quality of service, robustness and reduced energy consumption require new tools and methods to meet these conflicting requirements. The new methodology should serve for gaining better understanding of the properties of networking systems at the macroscopic level, as well as for the development of new principled optimization and management algorithms at the microscopic level. Methods of statistical physics seem best placed to provide new approaches as they have been developed specifically to deal with non-linear large scale systems. This paper aims at presenting an overview of tools and methods that have been developed within the statistical physics community and that can be readily applied to address the emerging problems in networking. These include diffusion processes, methods from disordered systems and polymer physics, probabilistic inference, which have direct relevance to network routing, file and frequency distribution, the exploration of network structures and vulnerability, and various other practical networking applications.Comment: (Review article) 71 pages, 14 figure

Effects of disorder on two coupled Hubbard chains at half-filling

We investigate the effects of quenched disorder on two chain Hubbard models at half-filling by using bosonization and renormalization group methods. It is found that the sufficiently strong forward scattering due to impurities and the random gauge field, which is generated by impurity backward scattering, destroy the charge gaps as well as the spin gaps. Random backward scattering due to impurities then drives the resulting massless phase to the Anderson localization phase. For intermediate strength of random forward scattering, however, the spin gaps still survive, and only one of the charge gaps is collapsed. In this parameter region, one of the charge degrees of freedom is in the Anderson localized state, while the other one is still in the massive state.Comment: 10 pages, RevTex, 3 eps figure

Spin-Rotation Symmetry Breaking in the Superconducting State of CuxBi2Se3

Spontaneous symmetry breaking is an important concept for understanding physics ranging from the elementary particles to states of matter. For example, the superconducting state breaks global gauge symmetry, and unconventional superconductors can break additional symmetries. In particular, spin rotational symmetry is expected to be broken in spin-triplet superconductors. However, experimental evidence for such symmetry breaking has not been conclusively obtained so far in any candidate compounds. Here, by 77Se nuclear magnetic resonance measurements, we show that spin rotation symmetry is spontaneously broken in the hexagonal plane of the electron-doped topological insulator Cu0.3Bi2Se3 below the superconducting transition temperature Tc=3.4 K. Our results not only establish spin-triplet superconductivity in this compound, but may also serve to lay a foundation for the research of topological superconductivity

Weak-Coupling Approach to Hole-Doped S=1 Haldane Systems

As a weak-coupling analogue of hole-doped $S=1$ Haldane systems, we study two models for coupled chains via Hund coupling; coupled Hubbard chains, and a Hubbard chain coupled with an $S=1/2$ Heisenberg chain. The fixed point properties of these models are investigated by using bosonization and renormalization group methods. The effect of randomness on these fixed points is also studied. It is found that the presence of the disorder parameter inherent in the Haldane state in the former model suppresses the Anderson localization for weak randomness, and stabilizes the Tomonaga-Luttinger liquid state with the spin gap.Comment: 4 pages, RevTex, 1 postscript figure (uuencoded and compressed), to appear in Phys. Rev.