Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session X7: Understanding Strongly Correlated Materials with Dynamical Mean Field Theory Methods |
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Sponsoring Units: DCOMP Chair: Matthias Troyer, Theoretische Physik, ETH Zurich Room: Morial Convention Center RO5 |
Friday, March 14, 2008 8:00AM - 8:36AM |
X7.00001: Mott Transition, Antiferromagnetism, and d-wave Superconductivity in Two-Dimensional Organic Conductors and in Cuprates Using Cluster Dynamical Mean Field Theory Invited Speaker: The main features of the phase diagram of high-temperature superconducting cuprates are a Mott insulator at half-filling, a pseudogap at finite doping and, in the ground state, the competition between antiferromagnetism, d-wave superconductivity and possibly other inhomogeneous phases. In the layered organics of the $\kappa$-BEDT-X family, it is pressure instead of doping that is varied but the competing phases and the Mott insulating behavior are similar to the cuprates. Approaches that claim to explain d-wave superconductivity in the cuprates must also explain this phenomenon in all other related classes of compounds described by the Hubbard model. Using Cluster Dynamical Mean-Field theory, we show that the main features of both phase diagrams, cuprates and organics, can be understood from the one-band model with hopping parameters taken from band structure and interaction of the order of the bandwidth. We emphasize the case of the organics, studying the Mott transition, antiferromagnetism and superconductivity on the anisotropic triangular lattice. The Mott transition in the normal phase can be continuous or first order depending on the value of the frustrating hopping $t'/t$. A $d$-wave superconducting phase appears between an antiferromagnetic insulator and a metal for $t^{\prime}/t=0.3-0.7 $, or between a nonmagnetic Mott insulator (spin liquid) and a metal for $t^{\prime }/t\geq 0.8$, in agreement with experiments on layered organic conductors including $\kappa $-(ET)$_{2}$Cu$_{2}$(CN)$_{3}$. These phases are separated by a strong first order transition. The phase diagram gives much insight into the mechanism for d-wave superconductivity and on the question of the glue. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 9:12AM |
X7.00002: Continuous time quantum Monte Carlo (CTQMC): a fast algorithm to solve the DMFT equations Invited Speaker: Dynamical mean field calculations involve the repeated numerical solution of an impurity problem, which is the time critical step in the self-consistency loop. The performance and flexibility of available impurity solvers therefore defines what type of problems can be treated within dynamical mean field theory. Over the past few years, significant progress has been achieved with the development of so-called continuous- time quantum Monte Carlo methods. These algorithms are based on a diagrammatic expansion of the partition function in either the interactions or hybridizations, and the stochastic sampling of appropriate collections of diagrams. I will explain the key ideas behind this powerful and versatile approach, with a particular emphasis on the expansion in hybridization. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:48AM |
X7.00003: Nodal/Antinodal Dichotomy and the Two Gaps of a Superconducting Doped Mott Insulator Invited Speaker: Using Cellular Dynamical Mean Field Theory, implemented with exact diagonalization as impurity solver, we study the superconducting state of the hole-doped two-dimensional Hubbard model. We mainly focus on qualitative aspects which characterize the approach to the Mott transition. We will show that our formalism leads to a natural decomposition of the photoemission energy-gap into two components. A first gap, stemming from the anomalous self-energy, dominates near the nodes and decreases with decreasing doping. The second gap has an additional contribution from the normal self-energy, inherited from the normal-state pseudo-gap. It is dominant near the antinodes and increases as the Mott insulating phase is approached. This behavior of the one-particle gap is relevant in the light of recent experimental studies reporting the presence of two different energy scales in the nodal and antinodal regions of high-temperature superconductors. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:24AM |
X7.00004: DCA study of magnetic mediated superconductivity in the Hubbard model Invited Speaker: The Dynamical Cluster Approximation (DCA) with quantum Monte Carlo as a cluster solver is used to study pairing in the two dimensional Hubbard model. The DCA adds non-local corrections to dynamical mean field theory by mapping the lattice onto a self-consistently embedded periodic cluster. The qualitative features of the cuprate phase diagram are captured by the DCA with a 2x2 cluster, which provides a mean field solution of the model. With increasing cluster size, the results are found to converge and display a finite d-wave transition temperature, establishing the presence of superconductivity in the model. A decomposition of the pairing interaction into its cross channels reveals that pairing is mediated by S=1 spin fluctuations. A simple renormalized spin fluctuation model is found to capture many of the properties of pairing and the spectra, including the high-energy kink waterfall structure and the structure of the leading order parameter. However, it fails to capture realistic features including long-ranged hopping, phonons and the pseudogap. Phonons, in particular, are found to enhance the paring interaction by enhancing antiferromagnetism. Despite this, superconductivity is suppressed by local (Holstein, Breathing and Buckling) phonon modes through the formation of polarons which dramatically reduce the particle mobility. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 11:00AM |
X7.00005: DMFT calculations of materials properties using the continuous time QMC method Invited Speaker: The combination of DFT with DMFT has proven to be an instrumental method for describing realistic strongly correlated electron systems. In essence, DMFT treats the strongly correlated electrons near the Fermi surface while DFT treats the electrons which are less correlated. DMFT effectively maps the intractable lattice many-body problem onto a tractable impurity many-body problem. The DMFT impurity problem must be solved using numerical methods or approximate analytical methods, and this is the bottleneck of the entire procedure. Continuous time QMC has recently emerged as a dominant method to solve the DMFT impurity problem. We present applications of DFT+DMFT(CTQMC-atomic-limit) to the cobaltates and Pu. In Pu, a variety of physical properties are calculated such as the Photoemission spectra, magnetic susceptibility, and the heat capacity. These physical properties are probed as a function of temperature and volume, and compared with experimental measurements. Additionally, we demonstrate the effect of the full on-site exchange interaction on the physical observables. In the cobaltates, the Fermi surface and heat capacity are calculated for Na$_{0.3}$CoO$_2$. We demonstrate that the topology of the Fermi surface depends sensitively upon the bare Hamiltonian. It is shown that consistent agreement with heat capacity measurements and ARPES experiments can only be achieved if the $e_g'$ satellite pockets are not present at the Fermi surface. [Preview Abstract] |
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