Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H38: Metal-Insulator Phase Transitions - Theory II |
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Sponsoring Units: DCMP Chair: Steven Kivelson, UCLA Room: Concention Center 513 |
Tuesday, March 22, 2005 8:00AM - 8:12AM |
H38.00001: Universal Aspects of Coulomb Frustrated Phase Separation Reza Jamei, Steven Kivelson, Boris Spivak We study the consequences of Coulomb interactions on a system undergoing a putative first order phase transition. In two dimensions (2D), near the critical density, the system is universally unstable to the formation of new intermediate phases, which we call ``electronic microemulsion phases,'' which consist of an intermediate scale mixture of regions of the two competing phases. A corollary is that there can be no direct transition as a function of density from a 2D Wigner crystal to a uniform electron liquid. In 3D, if the strength of the Coulomb interactions exceeds a critical value, no phase separation occurs, while for weaker Coulomb strength, electronic microemulsions are inevitable. This tendency is considerably more pronounced in anisotropic (quasi 2D or quasi 1D) systems, where a devil's staircase of transitions is possible. [Preview Abstract] |
Tuesday, March 22, 2005 8:12AM - 8:24AM |
H38.00002: Phases and Phase Transitions in the Two-Dimensional Ionic Hubbard Model Tyler Bryant, Rajiv R. P. Singh We study the two-dimensional Ionic Hubbard model at half filling on a square lattice using linked cluster expansions [1]. The model consists of a Hubbard model with alternating site energies. In one dimension, it is known that there is an intermediate spontaneously dimerized phase separating the band insulator phase from the Mott insulator phase [2,3]. We calculate the ground state energy, local moment, spin-spin correlations, and dimer-dimer correlations to 12th order, starting from the band insulator phase. Using series extrapolation techniques the phase diagram of the model and the nature of the phase transitions is studied.\\\\ \noindent[1] M.P. Gelfand, R.R.P. Singh, Adv. Phys. {\bf 49}, 93 (2000)\\ \noindent[2] C.D. Batista and A.A. Aligia, Phys. Rev. Lett. {\bf 92}, 246405 (2004)\\ \noindent[3] S.R. Manmana, V. Meden, R.M. Noack, K. Sch\" onhammer, Phys. Rev. B {\bf 70}, 155115 (2004) [Preview Abstract] |
Tuesday, March 22, 2005 8:24AM - 8:36AM |
H38.00003: Metal and insulator coexistence region in disordered correlated systems M.C.O. Aguiar, V. Dobrosavljevic, E. Abrahams, G. Kotliar In this work we present the phase diagram for the Mott transition in the presence of disorder at finite temperature. We show that the temperature end point of the metal and insulator coexistence region decreases and that the coexistence region moves towards larger interaction potential when disorder increases. Our results also indicate that the width of the coexistence region at zero temperature and the temperature critical end point scale in the same way with disorder. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H38.00004: Analytic properties of insulating solution of half-filled Hubbard model Alexander Joura, Denis Demchenko, Jim Freericks We discuss analytic properties of insulating solution of half-filled Hubbard model on the hypercubic lattice within DMFT framework. We use an approximation where Green’s function is obtained as an average of two magnetic Hartree-Fock solutions. Under that assumption we obtain relations for the Green’s function, self-energy and quasi-particle scattering time in two limiting cases, around zero frequency and at large frequencies, and a critical value of interaction $U_c$ leading to metal-insulator transition. We compare our results for $U_c$ with other approaches, including numerical renormalization group and iterated perturbation theory (IPT). We also discuss a possibility of extending our method to IPT approximation, which is widely used in studying half-filled Hubbard model in infinite dimensions. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H38.00005: Phase diagram of (TMTSF)2PF6 revisited Lev Gor'kov, Pavel Grigoriev, Pavel Krotkov The antiferromagnetic spin-density wave (SDW) as function of pressure reveals an appearance of a quantum critical point embedded in a low-temperature superconducting (SC) phase. This diagram is commonly interpreted in terms of the tight- binding Q1D model with an antinesting term $t'_b(p_{\perp})$ dependence on which simulates changes in electronic spectrum with pressure. The model predicts two SDW phases with different SDW vectors [1] and second-order transitions between SDW and metallic states. Recent experiments have shown that the transition is actually of the first order and, what is even more surprising, the phase adjacent to the metallic phase bears the character of a periodic domain structure (both above and inside the SC state) [2]. The revision of the problem has led us first to the conclusion that the claim of [1] concerning two SDW vectors is incorrect, and second, that the model itself has some flaws. Being slightly modified the model may lead to a more complicated behavior of SDW near the critical point. Physical mechanisms for the periodic domain formation are discussed. [1] Y.Hasegawa and H.Fukuyama, J. Phys. Soc. Jap. 55, 3978 (1986); [2] T.Vuletic et al., Eur. Phys. J. B 25, 319 (2002); I.J.Lee et al., Phys. Rev. Lett. 88, 207002 (2002) [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H38.00006: Mean field theory of charge-density wave state in magnetic field Pavel Grigoriev, Dmitrij Lyubshin We develop a mean field theory of charge-density wave (CDW) state in magnetic field and study properties of this state below the transition temperature. We show that the CDW state with shifted wave vector in high magnetic field (CDWx phase) has a double harmonic modulation on the most part of the phase diagram. At perfect nesting the single harmonic CDW state with shifted wave vector exists only in a very narrow region near the triple point. We show that the transition from CDW0 to CDWx state below the critical temperature is accompanied by a jump of the CDW order parameter and of the CDW wave vector rather than by their continuous increase. This implies a first order transition between these CDW states and explains a strong hysteresis accompanying this transition. The similarities between CDW in high magnetic field and nonuniform LOFF superconducting phase are pointed out. Our investigation provides a theoretical description for recent experiments on organic metal $\alpha$-(BEDT-TTF)$_2$KHg(SCN)$_4$ and other compounds. In particular, we explain the higher value of the kink transition field and provide the calculation of the phase diagram in the case of perfect nesting. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H38.00007: Instabilities in Strongly Interacting Electron Liquids Sankar Das Sarma, Ying Zhang We show that the low-density strongly interacting electron liquid, interacting via the long-range Coulomb interaction, could develop two types of instabilities that fundamentally change the magnetic and dispersion properties of the ground state of the system. As the electron density decreases, both the 2D and 3D paramagnetic electron liquids first experience a magnetic instability which may be of either Bloch or Stoner type. These instabilities suggest a first or second order quantum phase transition into a ferromagnetic Fermi liquid. As the electron density further decreases, both 2D and 3D electron liquids develop a dispersion instability at a critical density associated with the approximate flattening of the quasiparticle energy dispersion. At the critical density the quasiparticle effective mass diverges at the Fermi surface, but the signature of this Fermi surface instability manifests itself away from the Fermi momentum at higher densities. For densities below the critical density the system is unstable since the quasiparticle velocity becomes negative. We show that one physical mechanism underlying the dispersion instability is the emission of soft plasmons by the quasiparticles. We discuss the implications of the magnetic and dispersion instabilities for experiments at low electron. This work is supported by by the US-ONR and NSF. [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H38.00008: Electron-phonon interaction close to a Mott transition Giorgio Sangiovanni, Massimo Capone, Claudio Castellani, Marco Grilli The effect of Holstein electron-phonon interaction on a Hubbard model close to a Mott-Hubbard transition at half-filling is investigated by means of Dynamical Mean-Field Theory. We observe a reduction of the effective mass that we interpret in terms of a reduced effective repulsion. When the repulsion is rescaled to take into account this effect, the quasiparticle low-energy features are unaffected by the electron-phonon interaction. Phonon features are only observed within the high-energy Hubbard bands. The lack of electron-phonon fingerprints in the quasiparticle physics can be explained interpreting the quasiparticle motion in terms of rare fast processes. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H38.00009: The doped spin 3/2 half filled Mott insulator Stellan Ostlund, T. Hans Hansson, Anders Karlhede We develop an exact generalized Bogoliubov transformation for the spin $ 3/2$ Hubbard model with large anti-Hunds rule coupling near half filling. Since the transformation is unitary, we can employ standard approximate mean field theory methods in the full Hilbert space to analyze the doped Mott insulator, in contrast to a conventional approach based on truncated Hilbert spaces complemented with hard core constraints. The ground state at exactly half filling is an insulating (Mott) singlet, and according to our analysis a non-Fermi liquid order parameter $ \Delta $ usually associated with extended s-wave superconductivity, will appear self-consistently as soon as a finite density $ n $ of holes is introduced. The non-Fermi liquid behavior is a consequence of the nonlinear nature of the unitary transformation mapping the Mott singlet state to a Fock vacuum which introduces anomalous terms such as $\Delta n$ in the effective Hamiltonian. Our analysis uses an approach that generalizes readily to multi-band Hubbard models and could provide a mechanism whereby a non-Fermi liquid order parameter proportional to density develops in Mott insulators with locally entangled ground states. For more complicated systems, such an order parameter could coexist naturally with a variety of other order parameters. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H38.00010: Effects of short-ranged interactions in random hopping models on 2D bipartite lattices Matthew S. Foster, Andreas W.W. Ludwig Understanding the combined effects of disorder and interactions in electronic systems has been an important problem for many years. These questions have attracted renewed attention in the context of a possible metal-insulator transition in the disordered and interacting 2D electron gas. Here we study the effects of generic short-ranged interactions on a special class of systems: these are tight-binding models of spinless fermions subject to random hopping disorder on 2D bipartite lattices. It is known that in the absence of interactions, these disordered systems are special in that they do not localize in 2D, but possess extended states and a finite conductivity at zero energy, as well as a strongly divergent low energy density of states.[1] Using a perturbative 1-loop renormalization group analysis we show that the same mechanism responsible for the divergence of the density of states leads to an instability in which the interactions are driven strongly relevant by the disorder. [1] R. Gade and F. Wegner, Nucl. Phys. B 360, 213 (1991); R. Gade, Nucl. Phys. B 398, 499 (1993); see also e.g. O. Motrunich et al., Phys. Rev. B65, 064206 (2002). [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H38.00011: Metal-Insulator transition in 2D: Non-perturbative results Alexander Punnoose, Alexander Finkelstein In recent years, systematic experimental studies of dilute two dimensional electron systems have revived the fundamental question: can a metal-insulator transition occur in two dimensions? We demonstrate how the metal-insulator transition may occur in a very low density system with strong electron-electron interactions. Renormalization group equations beyond the lowest order in the disorder have been obtained using a controlled large-N approximation scheme. Our results indicate that the interacting electron gas undergoes a metal to insulator transition when the disorder is increased. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H38.00012: Phase diagram of the one dimensional 1/2-filled Hubbard-Holstein model R. T. Clay, R. P. Hardikar We present a detailed numerical study of the Hubbard-Holstein model in one dimension at half filling, including full finite-frequency quantum phonons within the numerically exact Stochastic Series Expansion quantum Monte Carlo method. At half filling, the effects of the electron-phonon and electron-electron interactions compete, with the Holstein phonon coupling acting as an effective negative Hubbard onsite interaction $U$ that promotes on-site electron pairs and a Peierls charge-density wave state. Most previous work on this model has assumed that only Peierls or $U>0$ Mott phases are possible. However, there has been speculation that a third metallic phase exists between the Peierls and Mott phases, with possible superconducting correlations. We present results confirming the intermediate metallic phase, and show that the Luttinger liquid correlation exponent $K_\rho>1$ in this region, indicating dominant superconducting correlations. We further present the full phase diagram as a function of onsite Hubbard $U$, phonon coupling constant, and phonon frequency. [Preview Abstract] |
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