Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D32: Many Body and Strongly Correlated Systems |
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Sponsoring Units: DCOMP Chair: Barry Schneider, NSF Room: LACC 507 |
Monday, March 21, 2005 2:30PM - 2:42PM |
D32.00001: Conserving Gapless Mean-Field Theroy for Bose-Einstein Condensates Takafumi Kita A new conserving gapless mean-field theory for Bose-Einstein condensates is constructed based on a Luttinger-Ward thermodynamic functional. It is applied to a weakly interacting uniform gas with the density $n$ and the $s$-wave scattering length $a$ to clarify its basic thermodynamic properties. It is found that the condensation here occurs as a first-order transition. The shift of the transition temperature from the ideal-gas result $T_{0}$ is positive and given to the leading order by $\Delta T_c \!=\! 2.33a n^{1/3}T_0$, in agreement with a couple of preceding estimates. The theory is expected to form a new theoretical basis for the trapped Bose-Einstein condensates at finite temperatures. [Preview Abstract] |
Monday, March 21, 2005 2:42PM - 2:54PM |
D32.00002: Dynamical spin susceptibility of a spin-polarized two-dimensional electron liquid with Rashba spin-orbit interaction Giovanni Vignale, Ruben Portugal It has recently been shown$^1$ that a macroscopic equation of motion for the magnetization of an inhomogeneous electron liquid can be derived from the dynamical spin susceptibility of the latter. This equation of motion includes dissipation due to the coupling between the macroscopic magnetization and more microscopic degrees of freedom. However, a calculation based on the homogeneous spin-polarized electron liquid without spin-orbit coupling fails to produce a realistic model of dissipation, since the dissipative terms vanish at low temperature and/or for global rotations of the magnetization. As a first step towards a realistic theory of dissipation we present a mode- coupling calculation of the dynamical spin susceptibility of a spin- polarized two-dimensional electron liquid in the presence of a spin-orbit interaction of the Rashba form. The equation of motion for the magnetization derived from this spin susceptibility contain a dissipative term of the form that is usually described by the Gilbert damping constant. This and other features of the resulting spin dynamics will be examined in detail.\\ 1. Z. Qian and G. Vignale, PRL 88, 056404 (2002) [Preview Abstract] |
Monday, March 21, 2005 2:54PM - 3:06PM |
D32.00003: Excitonic Mode and Negative Compressibility in Electron Liquids Yasutami Takada A highly self-consistent theory maintaining the exact functional relations between the electron self-energy and the vertex part[1] is employed to calculate the dielectric function $\varepsilon({\bf q},\omega)$ very accurately in the homogeneous electron gas in two and three dimensions for the range of densities covering the transition region where $\kappa$ the compressibility of the system changes from positive to negative. The calculated $\varepsilon({\bf q},\omega)$ exhibits a sharp peak-like structure in its imaginary part as a function of $\omega$, indicating the existence of a mode sustained by the attractive electron-hole multiple scattering (the excitonic effect). The eigen energy of the mode becomes soft in proportion to $1/\kappa$ with decreasing the density to make $\kappa$ diverge. With further decreasing the density to make $\kappa$ go beyond the divergent point into a negative value, this mode turns into a pole of $\varepsilon ({\bf q},\omega)$ on the positive imaginary-$\omega$ axis, implying the spontaneous excitation of this mode. This feature can be understood as the microscopic origin to bring about the negative dielectric function occurring in this system at sufficiently low densities. [1] YT, PRL {\bf 87}, 226402 (2001); YT and H. Yasuhara, PRL {\bf 89}, 216402 (2002). [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D32.00004: The Hartree-Fock ground state of high-density jellium Shiwei Zhang, David Ceperley The usual textbook paramagnetic solution is not the Hartree-Fock (HF) ground state of the uniform electron gas (jellium). Overhauser\footnote{A. W. Overhauser, Phys. Rev. 128, 1437 (1962).} showed that, for example, a spin-density-wave state would have lower energy, even at the high density limit. We obtain numerically the unrestricted Hartree-Fock (uHF) ground state of 3-D jellium, using a projection technique starting from random initial states. Supercells with around 1000 electrons are studied, with a range of $r_s$ values at high densities. We study the energetics of the uHF ground state, and discuss its real- and ${\mathbf k}$-space structures. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D32.00005: Variational path-integral treatment of a translation invariant many-polaron system Fons Brosens, Sergei N. Klimin, Jozef T. Devreese The ground-state properties of a translation invariant $N$-polaron system are theoretically investigated for an arbitrary electron-phonon coupling strength, using a variational principle for path integrals for identical particles. A rigorous upper bound for the ground state energy is found as a function of the number of spin up and spin down polarons, taking the electron-electron interaction and the Fermi statistics into account. For sufficiently high values of the electron-phonon coupling constant and of the ratio of the static and high-frequency dielectric constants $1/\eta=\varepsilon_{0}/\varepsilon _{\infty}$, the system of $N$ interacting polarons can form a stable multipolaron ground state. When this state is formed, the total spin of the system takes its minimal possible value. For a stable multipolaron state, the addition energy reveals peaks corresponding to closed shells. This feature of the addition energy, as well as the total spin as a function of the number of electrons, might be resolved experimentally using, e.g., capacity and magnetization measurements. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D32.00006: Surface Kondo Impurities in the Slave-Boson Approach Enrique Anda, Edson Vernek, Nancy Sandler, Sergio Ulloa Transport properties of magnetic impurities on surfaces have captured a great deal of attention lately. Atom manipulation and topographic imaging techniques using scanning tunneling microscope have confirmed some theoretical predictions on Kondo physics and at the same time revealed other interesting behavior in these systems. For example, experiments have reported unexpectedly high Kondo temperatures for multi-impurity and molecular structures on metallic surfaces. Motivated by these experimental results we apply slave boson techniques for finite Coulomb interaction (finite $U$) to study the transport properties of magnetic impurities on a metallic surface in the Kondo regime. We report here on our studies of the role of fluctuations on the slave boson number for the case of one impurity on metallic surfaces. We compare our results to other theoretical approaches and to experimental results. \newline \newline Supported by CAPES-Brazil and NSF-IMC and NSF-NIRT. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D32.00007: DMRG Analysis for Arbitrary Densities Fatih Dogan, Frank Marsiglio DMRG(Density-Matrix-Renormalization-Group) method provides an accurate analysis of 1D systems that otherwise cannot be solved exactly. The method has now been applied to many spin and fermion systems. However, it was restricted to magical densities, single particle or quarter or half filling. The main reason for this restriction is the use of Grand-Canonical-Ensemble to set the number of fermions. Here, we present our results of a new approach that uses canonical restrictions to obtain arbitrary densities. We applied this method, first to the Attractive Hubbard Model and then the Dynamic Hubbard Model [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D32.00008: Self-consistent projection operator approach to momentum dependent excitation spectra Yoshiro Kakehashi, Peter Fulde Description of the momentum-dependent excitation spectrum with high resolution in momentum and energy at low temperature is a current issue in the theory of strongly correlated electrons. We present such an effective medium approach on the basis of the projection operator technique to the retarded Green function. The momentum dependent self-energy is self-consistently calculated from nonlocal memory function via Fourier transformation. The latters are calculated by means of the incremental cluster expansion using the cluster memory functions embedded in a medium. The medium is self-consistently determined by an extended CPA condition. Our theory has no problem with momentum and energy resolutions and can systematically describe long-range intersite correlations. We demonstrate these advantages by calculating the excitation spectra of the 3D Hubbard model on the simple cubic lattice. We clarify the reduction of the quasiparticle weight, the relaxation of the quasiparticle band width, and the formation of the Mott-Hubbard band due to nonlocal correlations. [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D32.00009: A fermionic quantum Monte-Carlo algorithm without the sign problem Congjun Wu, Shou-Cheng Zhang Quantum Monte-Carlo (QMC) simulations involving fermions have the notorious sign problem. The only known case without this problem involves a fermion determinant which can be factorized into two real parts with the same sign. Recently, a new fermion QMC algorithm has been discovered in which the fermion determinant may not necessarily factorizable, but can instead be expressed as a product of complex conjugate pairs of eigenvalues, thus eliminating the sign problem for a much wider class of models. In this paper, we present general conditions for the applicability of this new algorithm and show that it is deeply related to the time reversal symmetry of the fermion matrix. We apply this algorithm to many models of strongly correlated systems, including models with purely repulsive interactions, and study their novel phases, for all doping levels and lattice geometries. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D32.00010: The correlation density matrix: new tool for analyzing exact diagonalizations Christopher L. Henley, Siew-Ann Cheong When a lattice model of strongly interacting spins or fermions is studied numerically, it may be unclear {\em a priori} what kind of correlation will be dominant -- especially in cases where exotic order or disorder has been suggested (e.g. Kagom\'e spin-1/2 antiferromagnet, or doped square-lattice Hubbard model). To resolve this, consider two small clusters of a few sites $A$ and $B$, offset by a vector $\bf r$. Let $\hat{\rho}_{AB}$ be the many-body density matrix for the disconnected cluster $A\cup B$, constructed from the whole system's ground state wavefunction by tracing out all sites except those in $A$ or $B$, with $\hat{\rho}_A$ and $\hat{\rho}_B$ similarly defined. Then all possible correlations between $A$ and $B$ are contained in the ``correlation density matrix'' $\hat{\rho}_{\rm corr}(\bf r) \equiv \hat{\rho}_{AB} -\hat{\rho}_A \otimes \hat{\rho}_B$. Using singular-value decomposition we can write $\hat{\rho}_{\rm corr} = \sum _i \lambda_i \hat{\Phi}_i(A) \hat{\Phi}'_i(B)$, where $\hat{\Phi}_i$ and $\hat{\Phi}'_i$ are normalized operators on the respective clusters; the terms represent different correlation functions, which are naturally ordered by the magnitudes $|\lambda_i|$. This permits a systematic, unbiased numerical method to identify the important correlations, given the ground state wavefunction. The procedure will be tested on ladders of spinless fermions with infinite nearest-neighbor repulsion, which are expected to have Luttinger liquid behavior. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D32.00011: The electronic structure of $\zeta$-BEDT-TTF$\cdot$PF$_6$; a comparison of self-consistent field and Hubbard model analyses N. A. W. Holzwarth, Ping Tang, Freddie R. Salsbury, Jr. The electronic structure of the organic charge-transfer salt $\zeta$-BEDT-TTF$\cdot$PF$_6$ was studied using both density functional and Hartree-Fock theories. The results were in qualitative disagreement with experimental conductivity and optical measurements for the material\footnote{H.-L. Liu and co-workers,{\em{Chem. Mater.}} {\bf{9}}, 1865 (1997) } which suggested a Mott insulator behavior. We examine the Hubbard model for this system to understand both the Mott insulating behavior and the failure of the self-consistent field methods to model it. Using a Hubbard dimer model, we find that an important component of the analysis is the inclusion of more valence-bond than ionic character in the ground state wavefunction. The Hartree-Fock treatment includes too much ionic character which destabilizes the result. A spin density wave treatment reduces the ionic character, but also contaminates the wavefunction with terms having the incorrect total spin. We have also shown that the one-electron density and therefore the density functional approach in its usual form is nearly blind to the valence-bond content of the wavefunction. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D32.00012: New perspectives in transition metal compounds' design from first-principles calculations Matteo Cococcioni Transition-metal (TM) compounds are involved in many important physical processes and chemical reactions (such as minerals in the earth's mantle, Li ion batteries' cathodes, spintronics, catalysis on surfaces, chemical reactions on biomolecules). Unfortunately, standard DFT approaches are often not quantitatively accurate for these materials, due to the poor representation of electronic correlation. We address several of these problems by establishing a parameter-free approach to the GGA+U method. The Hubbard U is defined and obtained through a fully self-consistent linear-response approach. An accurate description of TM compounds under different conditions is obtained; examples are given for a) ground state and structure of FeO under pressure, b) voltages, stability and conduction properties of cathode materials such as olivine, layered and spinel TM compounds, and c) valence state and coordination for solvated aqua iron ions. [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D32.00013: Proximity of Antiferromagnetism and d-wave superconductivity in $\kappa$-BEDT-TTF: A V-CPT study Peyman Sahebsara, David Senechal The organic conductors $\kappa$-(BEDT-TTF)$_2$X (X=Cu(NCS)$_2$, Cu[N(CN)$_2$]Br and Cu[N(CN)$_2$]Cl) have attracted attention because the superconducting phase exists in the vicinity of an antiferromagntic insulating phase [1]. The salts with the first two substitutions are superconductors with critical temperature near 10K. The third one is a commensurate insulator, which transits to a superconducting phase at ambient pressure. We use the 2D, half-filled Hubbard model on a triangular lattice to describe these compounds. The ground state antiferromagnetism and superconductivity is investigated using variational cluster perturbation theory (V-CPT) [2-3]. The on-site repulsion $U$ and the diagonal hopping $t'$ are varied and a phase diagram of the $\kappa$-family of BEDT is obtained. [1] H. Kino and H. Fukuyama, J. Phys. Soc. Jpn. 65 (1996) 2158. [2] M. Potthoff, M. Aichorn and C. Dahnken, Phys. Rev. Lett. 91 (2003) 206402. [3] D. Senechal and A.-M. S. Tremblay, cond-mat/0410162 [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D32.00014: Atomistic simulations in nanostructures composed of tens of millions of atoms Marek Korkusinski, Gerhard Klimeck, Haiying Xu, Sebastien Goasguen, Faisal Saied, Seungwon Lee Strain in self-assembled quantum dots (QDs) is a long-range phenomenon, and its realistic determination requires a large computational domain. To find it for a dome-shaped InAs QD with diameter of 19.2 nm, the nanoelectronic modeling tool NEMO-3D uses the atomistic VFF Keating model with domain sizes of up to 64 million atoms. Interatomic distance changes thus obtained are used to influence the sp3d5s* tight-binding electronic Hamiltonian defined on a subdomain containing up to 21 million atoms (matrix size of order of 4x10$^{8})$. Targeted eigenstates with correct symmetry are found reliably even in such large systems. NEMO-3D is used to analyze the dependence of the QD states on the size of the strain domain and the boundary conditions. The energies of a deeply embedded QD depend dramatically on the strain domain size. For QDs buried under a thin capping layer, on the other hand, the existence of a free surface at the top of the sample allows for an effective relaxation of atoms, and the penetration of strain into the barrier is small. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D32.00015: Density wave packets of ultracold atoms in an optical lattice with the adaptive t-DMRG Corinna Kollath, Ulrich Schollw\"ock, Jan von Delft, Wilhelm Zwerger We investigate the propagation of wave packets in systems of ultracold atoms in an optical lattice with the help of the adaptive time-dependent density-matrix renormalization-group method (adaptive t-DMRG). We discuss the dependence of the velocity, in particular of the sound velocity, on the interaction strength and the height of the perturbation in a quasiexact calculation. [Preview Abstract] |
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