Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A6: Correlated Electrons |
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Sponsoring Units: DCOMP Chair: Barry Schneider, NSF Room: LACC 502A |
Monday, March 21, 2005 8:00AM - 8:36AM |
A6.00001: Localization-delocalization transitions in strongly correlated electron systems Invited Speaker: Correlations can give rise to localization of charge and spin of electrons. A methodology will be presented which allows to describe localized states and also localization- delocalization transitions as a function of pressure or doping. The method is the self-interaction- corrected local-spin-density approximation (SIC-LSDA) to density functional theory. The SIC- LSDA can differentiate between localized and itinerant electrons. Results of calculations for 4f, 5f and 3d compounds will illustrate the method. [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 9:12AM |
A6.00002: Magnetic to valence-bond-solid transition in an S=1/2 XY model with ring-exchange Invited Speaker: Within the Landau-Ginzburg-Wilson framework, phase transitions between two ordered phases with different symmetries are generically of first order, or there is a region of coexistence of the two phases. However, It has recently been argued [1] that there is a generic class of continuous order-order {\it quantum phase transitions}, where the critical point is characterized by deconfined spinon degrees of freedom. Evidence of such a transition, between a magnetic (or superfluid in a bosonic representation) and a valence-bond-solid (VBS) phase had previously been observed in large-scale quantum Monte Carlo simulations [2] of a 2D XY model which in addition to the standard nearest-neighbor exchange J contains a four-particle exchange of strength K. The VBS phase in this model is not favored by the J and K interactions individually (the K-only model has an Ising-like antigerromagnetic ground state), but emerges out of competition between the two terms. Here I will discuss recent efforts [3] to characterize the magnetic-VBS transition in more detail (extracting the critical exponents) and comparing the behavior with predictions of the deconfined quantum-criticality scenario. \vskip2mm [1] T. Senthil, A. Vishwanath, L. Balents, S. Sachdev, and M. P. A. Fisher, Science {\bf 303}, 1490 (2004).\hfill\break [2] A. W. Sandvik, S. Daul, R. R. P. Singh, and D. J. Scalapino, Phys. Rev. Lett. {\bf 89}, 247201 (2002).\hfill\break [3] A. W. Sandvik, R. G. Melko, and D. J. Scalapino (work in progress). [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:48AM |
A6.00003: Fast methods for evaluating molecular electron correlation energies Invited Speaker: Some of the issues associated with developing fast methods for the wavefunction- based description of electron correlation will be re-examined in this talk, after a brief introductory overview of standard methods for treating electron correlation in molecules. The first main topic is the description of so-called dynamic correlation effects that are closely related to atomic correlations. Local correlation methods that describe two and three-body correlations at reduced computational cost, while still ensuring continuous potential energy surfaces will be described and their performance assessed in terms of accuracy and computational cost. The second main topic is describing strong correlations associated with near-degeneracies, such as occur in diradicaloid molecules and transition metal compounds. Simplified coupled cluster methods appropriate for such problems will be described, along with examples of their application to several molecules believed to have significant diradical character. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:24AM |
A6.00004: Correlation effects in the compressed rare earth metals Invited Speaker: A number of the trivalent rare earth metals (Ce, Pr, Gd, and Dy) are known to undergo electron-correlation driven phase transitions under pressure that are characterized by unusually large volume changes (5--15{\%}). These ``volume collapse'' transitions demarcate regimes of different behavior with high-symmetry structures and local moments on the low-pressure (strongly correlated) side versus low-symmetry structures and screened moments on the high-pressure (more weakly correlated) side. Interestingly, Nd reaches this high-pressure regime without undergoing a significant collapse. This talk describes calculations using the local density approximation combined with dynamical mean field theory (LDA+DMFT) for Ce [1], Pr, and Nd, which seek insight into this behavior. Results for an assumed fcc structure suggest that the interesting correlation effects are pushed to higher pressures from Ce to Pr to Nd, and must compete there against successively stiffer underlying equations of state, which may contribute to the absence of the collapse in Nd. LDA estimates of the structure dependence of the energy appear to be smaller effects after these correlation contributions. Spin orbit plays an interesting role in that the lower Hubbard band remains exclusively j=5/2 under compression, whereas the quasiparticle weight is of mixed j=5/2, 7/2 character so that the pressure-induced transfer of spectral weight to the Fermi level effectively quenches the spin orbit. Collaborations with K. Held and R.T. Scalettar are gratefully acknowledged. [1] K. Held, A.K. McMahan, and R.T. Scalettar, Phys. Rev. Lett. \textbf{87}, 276404 (2001); A.K. McMahan, K. Held, and R.T. Scalettar, Phys. Rev. B 67, 075108 (2003). [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 11:00AM |
A6.00005: The Dynamical Cluster Approximation Invited Speaker: The DCA is a general approach for the theory of correlated and disordered lattice systems which maps the lattice onto a self consistently embedded periodic cluster. When the cluster size is one the mean field solution is recovered (DMFT or CPA), and as the cluster size increases non-local corrections are systematically incorporated. A variety of methods may be used to solve the cluster problem. The DCA has been used to study a variety of systems, including the cuprates, dilute magnetic semiconductors and carbon nanotubes. The DCA formalism, comparison with other cluster methods, and these applications will be reviewed. [Preview Abstract] |
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