Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N2: Spin and Charge in Mott Systems |
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Sponsoring Units: DCMP Chair: Dennis Newns, IBM Room: LACC 151 |
Wednesday, March 23, 2005 8:00AM - 8:36AM |
N2.00001: Cluster Dynamical Mean Field Analysis of the Mott transition Invited Speaker: I will present recent results on the evolution from an anomalous metallic phase to a Mott insulator within the two dimensional Hubbard model, using a cluster extension of dynamical mean field theory. In particular, the density-driven Mott metal-insulator transition is approached in a non-uniform way in different regions of the momentum space. This gives rise to a breakup of the Fermi surface and to the formation of hot and cold regions, whose position depends on the hole or electron like nature of the carriers in the system. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 9:12AM |
N2.00002: Orbital-selective Mott transitions in the degenerate Hubbard model Invited Speaker: Strongly correlated electron systems with multi-orbital bands possess various unusual properties. Here we address how the multi-orbital nature affects the Mott transition and under which conditions consecutive orbital-selective Mott transitions may occur upon growing electron correlations. We study a model of an extended two-orbital Hubbard model (including onsite intra-orbital repulsion $U$, onsite inter-orbital repulsion $U'$ and Hund coupling $J_H$ with $ U=U'+2J_H $) with distinct hopping matrix elements for the two orbitals. By combining dynamical mean-field theory with exact diagonalization, the stability of itinerant quasi-particle states in each band is examined. There is a single Mott transition, simultaneously for both orbitals, in the absence of the Hund coupling, when the electron interaction is gradually increased. Once the Hund coupling is introduced, the Mott transitions splits into two, such that an intermediate region appears where the system consists of localized electronic degrees of freedom and of itinerant, though strongly renormalized, electrons. We also discuss the finite-temperature properties by means of Quantum Monte Carlo simulation. This allows us to elucidate the behavior of spin and orbital fluctuations in the vicinity of the Mott transition by analyzing the spin, charge and orbital susceptibilities as well as the one-particle spectral function. Moreover we study the effect of hybridization between the orbitals which leads to some essential modifications of the properties compared to the case of non-hybridized orbitals. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:48AM |
N2.00003: Effects of Particle-Hole Asymmetry on the Mott-Hubbard Metal-Insulator Transition Invited Speaker: The Mott-Hubbard metal-insulator transition (MIT) is one of the most important problems in correlated electron systems. In the past decade, much progress has been made on examining a particle-hole symmetric form of the transition in the Hubbard model with dynamical mean field theory (DMFT) where it was found that the electronic self energy develops a pole at the transition. However, since most real materials are not at half filling one would like to examine the particle-hole asymmetric MIT. Here we analyze this problem using Falicov-Kimball (or simplified Hubbard) model. It is believed to describe correlated electron behavior and MIT in materials that can be fit into a binary alloy picture. Unlike the Hubbard model, which has a metal-insulator transition only at half filling, the Falicov-Kimball model exhibits a MIT for asymmetric particle densities. An example of the system that fits this picture is Ta$_x$N, which exhibits the MIT away from half filling at $x=0.6$. We find that away from half filling a number of features change when the noninteracting density of states has a finite bandwidth. First, we compare the nature of Mott-Hubbard transition at zero temperature in the Falicov-Kimball model for the lattices with finite and infinite bandwidths within the DMFT. We derive simple formulas for the critical interaction strength $U$ for both the development of a pole in the self energy and for the opening of a gap in the single-particle density of states. While the critical $U$ values are the same at half filling on both lattices, and for arbitrary filling on infinite bandwidth lattice, they are different for the particle-hole asymmetric cases on the finite bandwidth lattice. We discuss what role the development of the pole has on the physical properties of the MIT and the consequences these results have for the MIT in real materials. As an illustration we calculate a number of thermal transport properties and show how they are influenced by the bandwidth and the MIT for different fillings. [1] D.O.Demchenko, A.V.Joura, J.K.Freericks, Phys.Rev.Lett. {\bf 92}, 216401 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:24AM |
N2.00004: Ferromagnetism in vanadium oxide nanotubes spin-tuned by electron/hole doping Invited Speaker: An intrinsic feature of Mott insulators is that doping, i.e., changing the particle density, can dramatically affect their properties. I will report on our discovery of charge-doping controlled ferromagnetism at room temperature in self-assembled vanadium oxide nanotubes. By adding either electrons or holes, the initially spin-frustrated nanotubes develop a nearly identical nonlinear ferromagnetic spin response, demonstrating a novel unexpected electron-hole complementarity in the nanotube structures. The underlying picture is that, on doping, the Fermi level is swept through the Mott gap in this multiband strongly correlated system, removing the frustration responsible for the spin-gap. Itinerant carriers under spin control are produced in one vanadium Hubbard band which strongly interacts with other more localized vanadium spins. These findings show a path to new spin-aligned nanoscale building blocks, where the Fermi level sweep can be accomplished by applied voltage. [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 11:00AM |
N2.00005: Prominent Metal Phase Quasi-Particle Peak and High Temperature Mott-Hubbard Gap Filling in Photoemission Spectra of (V$_{1-x}$Cr$_x$)$_2$O$_3$ Invited Speaker: J.W. Allen Two new findings [1,2] have been made in photoemission spectra of the paradigm Mott-Hubbard transition system (V$_{1-x}$Cr$_x$)$_2$O$_3$. First [1], in the paramagnetic metal phase of V$_2$O$_3$ there is a prominent quasi-particle peak at the Fermi energy, of amplitude larger than the rest of the V 3d spectrum by a factor approaching two. The peak is qualitatively much like that found in spectral calculations that combine dynamic mean field theory (DMFT) and band theory in the local density approximation (LDA), but shows important differences quantitatively. Second [2], in the paramagnetic insulating phase of (V$_{0.972}$Cr$_{0.028}$)$_2$O$_3$, spectra taken in ultra high vacuum up to the unusually high temperature (T) of 800K reveal a property unique to the Mott-Hubbard insulator. With increasing T the MH gap is filled by spectral weight transfer, in qualitative agreement with high-T LDA+DMFT theory. [1] S.-K. Mo {\em et al}, Phys. Rev. Lett. {\bf 90} 186403 (2003). [2] S.-K. Mo {\em et al}, Phys. Rev. Lett. {\bf 93} 076404 (2004). [Preview Abstract] |
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