Two-mode entanglement in two-component Bose-Einstein condensates

We study the generation of two-mode entanglement in a two-component Bose-Einstein condensate trapped in a double-well potential. By applying the Holstein-Primakoff transformation, we show that the problem is exactly solvable as long as the number of excitations due to atom-atom interactions remains low. In particular, the condensate constitutes a symmetric Gaussian system, thereby enabling its entanglement of formation to be measured directly by the fluctuations in the quadratures of the two constituent components [Giedke {\it et al.}, Phys. Rev. Lett. {\bf 91}, 107901 (2003)]. We discover that significant two-mode squeezing occurs in the condensate if the interspecies interaction is sufficiently strong, which leads to strong entanglement between the two components.Comment: 22 pages, 4 figure

Spin-gap phase in nearly-half-filled one-dimensional conductors coupled with phonons

Asymptotic properties of nearly-half-filled one-dimensional conductors coupled with phonons are studied through a renormalization group method. Due to spin-charge coupling via electron-phonon interaction, the spin correlation varies with filling as well as the charge correlation. Depending on the relation between cut-off energy scales of the Umklapp process and of the electron-phonon interaction, various phases appear. We found a metallic phase with a spin gap and a dominant charge- density-wave correlation near half filling between a gapless density-wave phase (like in the doped repulsive Hubbard model) and a superconductor phase with a spin gap. The spin gap is produced by phonon-assisted backward scatterings which are interfered with the Umklapp process constructively or destructively depending on the character of electron-phonon coupling.Comment: 14 pages, revtex, replaced 5 ps figures, published in PR

Introduction to Chiral Perturbation Theory

A brief introduction to chiral perturbation theory, the effective field theory of quantum chromodynamics at low energies, is given.Comment: 26 pages, 11 figures. Lectures given at the summer school ISSSMB 2006 in Akyaka, Turkey, September 200

Polaron Effective Mass, Band Distortion, and Self-Trapping in the Holstein Molecular Crystal Model

We present polaron effective masses and selected polaron band structures of the Holstein molecular crystal model in 1-D as computed by the Global-Local variational method over a wide range of parameters. These results are augmented and supported by leading orders of both weak- and strong-coupling perturbation theory. The description of the polaron effective mass and polaron band distortion that emerges from this work is comprehensive, spanning weak, intermediate, and strong electron-phonon coupling, and non-adiabatic, weakly adiabatic, and strongly adiabatic regimes. Using the effective mass as the primary criterion, the self-trapping transition is precisely defined and located. Using related band-shape criteria at the Brillouin zone edge, the onset of band narrowing is also precisely defined and located. These two lines divide the polaron parameter space into three regimes of distinct polaron structure, essentially constituting a polaron phase diagram. Though the self-trapping transition is thusly shown to be a broad and smooth phenomenon at finite parameter values, consistency with notion of self-trapping as a critical phenomenon in the adiabatic limit is demonstrated. Generalizations to higher dimensions are considered, and resolutions of apparent conflicts with well-known expectations of adiabatic theory are suggested.Comment: 28 pages, 15 figure

Chiral Lagrangians

An overview of the field of Chiral Lagrangians is given. This includes Chiral Perturbation Theory and resummations to extend it to higher energies, applications to the muon anomalous magnetic moment, $\epsilon^\prime/\epsilon$ and others.Comment: Invited talk at the XX International Symposium on Lepton and Photon Interactions at High Energies 23rd-28th July 2001, Rome Italy, 15 pages, uses ws-p10x7.cls Changes: 2 references added, numbers in g-2 hadronic changed slightl

Low temperature resistivity in a nearly half-metallic ferromagnet

We consider electron transport in a nearly half-metallic ferromagnet, in which the minority spin electrons close to the band edge at the Fermi energy are Anderson-localized due to disorder. For the case of spin-flip scattering of the conduction electrons due to the absorption and emission of magnons, the Boltzmann equation is exactly soluble to the linear order. From this solution we calculate the temperature dependence of the resistivity due to single magnon processes at sufficiently low temperature, namely $k_BT\ll D/L^2$, where $L$ is the Anderson localization length and $D$ is the magnon stiffness. And depending on the details of the minority spin density of states at the Fermi level, we find a $T^{1.5}$ or $T^{2}$ scaling behavior for resistivity. Relevance to the doped perovskite manganite systems is discussed

Density-Matrix Algorithm for Phonon Hilbert Space Reduction in the Numerical Diagonalization of Quantum Many-Body Systems

Combining density-matrix and Lanczos algorithms we propose a new optimized phonon approach for finite-cluster diagonalizations of interacting electron-phonon systems. To illustrate the efficiency and reliability of our method, we investigate the problem of bipolaron band formation in the extended Holstein Hubbard model.Comment: 14 pages, 6 figures, Workshop on High Performance Computing in Science and Engineering, Stuttgart 200

Excitation Spectrum of the Holstein Model

In this paper the polaron problem for the Holstein model is studied in the weak coupling limit. We use second order perturbation theory to construct renormalized electron and phonons. Eigenstates of the Hamiltonian are labelled and the excitation spectrum is constructed.Comment: 4 pages, revtex, 1 figures, more stuff at http://www.mpipks-dresden.mpg.de/~robin/robin.htm