Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W38: Correlated Electrons: Lattice Models |
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Sponsoring Units: DCMP Chair: G Fernando, University of Connecticut Room: F149 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W38.00001: Spin rotational symmetry breaking by orbital current patterns in two-leg Cu-O Hubbard ladders Piotr Chudzinski, Marc Gabay, Thierry Giamarchi In the weak-coupling limit, we study, as a function of doping, two-leg ladders with a unit cell containing both Cu and O atoms. For purely repulsive interactions, using bosonization and a novel RG scheme, we find that in a broad region of the phase diagram, the ground state consists of a pattern of orbital currents (OCP) defined on the top of an incommensurate density wave. The internal symmetry of the OCP is specific for the ladder structure, different than the ones suggested up to now for 2D cuprates. We focus on this OCP and look for measurable signals of its existence: we compute magnetic fields induced within the ladder and we check what kind of changes in the phase diagram one may expect due to SU(2) spin-rotational symmetry breaking. We also investigate a single impurity problem (incl. OCP): we discuss if Kondo physics is at play, and make qualitative predictions about the nature of impurity backscattering. This enables us to show the influence of SU(2) symmetry breaking on conductivity. We estimate the value of gap opened due to the OCP, give analytic expressions for correlation functions and discuss magnetic properties of a new phase. [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W38.00002: Mott transition in multi-orbital Hubbard models for iron pnictides: a slave-rotor mean-field study Rong Yu, Qimiao Si One key question is whether the iron pnictides contain sufficiently strong electron correlations. Towards this end, it would be important to establish how Mott transition may in principle happen in models appropriate for the iron pnictides, i.e., models with even number of electrons partially occupying multiple orbitals. Here we study multi-orbital Hubbard models using a slave-rotor mean-field theory. We show that a Mott insulating phase does exist. We determine the critical values of the Coulomb interactions for several choices of band parametrization. We also discuss the role of Hund's rule coupling in the Mott transition. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W38.00003: Momentum-selective metal-insulator transition in the two-dimensional Hubbard model: An 8-site dynamical cluster approximation study Emanuel Gull, Philipp Werner, Olivier Parcollet, Andrew J. Millis The dynamical cluster approximation with eight momentum cells reveals that the paramagnetic phase of the Hubbard model exhibits a pseudogap at intermediate coupling strengths and doping. We show that within this approximation the pseudogap arises because the metal-insulator transition is multistage and momentum-sector specific with Fermi- liquid metal and fully gapped insulator phases separated by an intermediate phase, in which some regions of the Brillouin zone are gapped while others sustain gapless quasiparticles. For reasonable second-neighbor hopping the pseudogap occurs for hole but not electron doping. The doping dependence of the gap is determined and results are presented for spectra. Comparison to dynamical mean-field studies on smaller clusters is made. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W38.00004: Competing interactions and symmetry breaking in the Hubbard-Holstein model Johannes Bauer Competing interactions are often responsible for intriguing phase diagrams in correlated electron systems. Here we analyze the competition of instantaneous short range Coulomb interaction $U$ with the retarded electron-electron interaction induced by an electron-phonon coupling $g$ as described by the Hubbard-Holstein model. The ground state phase diagram of this model in the limit of infinite dimensions at half filling is established. The study is based on dynamical mean field theory combined with the numerical renormalization group. Depending on $U$, $g$, and the phonon frequency $\omega_0$, the ground state is antiferromagnetically (AFM) or charge ordered (CO) [1]. The transition between the states is found to occur when the electron-electron coupling strength $U$ and the induced interaction $\lambda$ due to electron-phonon coupling approximately coincide. The transition is continuous for small couplings and large phonon frequencies $\omega_0$ and becomes discontinuous for large couplings and small values of $\omega_0$. We present results for the static and dynamic electronic and bosonic properties near the transition. We also comment on the behavior of the model away from half filling. \\[4pt] [1] J. Bauer, cond-mat/0907.3751 (2009). [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W38.00005: Momentum distribution of the hard-core extended Bose-Hubbard model in a frustrated two-leg ladder Jong-Geun Shin, In-Ho Jeon, Min-Chul Cha Interacting hard-core bosons in a two-leg ladder frustrated in phase are studied by a Lanczos method. The ground state properties are investigated through the momentum distribution. At half-filling, the superfluid-insulator transition occurs even in the presence of the frustration as the interaction strength between neighboring sites is tuned. For a weak frustration, a peculiar signature in the momentum distributions appears, indicating an asymmetry between the two legs. Some distinct features for different frustrations and fillings are also discussed [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W38.00006: Spectral function of the ionic Hubbard model (IHM) Sinan Bulut, Bill Atkinson Using two-pole approximations, which are based on the equation of motion method, we calculate the excitation spectrum of the one dimensional IHM. To be specific, we use the composite operator method and the Roth-approximation. Though very simple in nature, these approximations capture the physics of the IHM qualitatively at least. As is predicted by several other numerical and/or theoretical studies, a {\it bond-order} (BO) {\it phase} is given by these approximate methods. In the BO phase, atoms in the system are dimerized leading to a gap in the excitation spectrum. We find that the BO phase flattens both low and high-energy bands. When the BO phase is suppressed, however, the system can be driven from the band-insulating phase to the metal one by electron-electron repulsions, which is somewhat counter-intuitive. Additionally, two-pole approximations generate a reasonably good DOS spectrum of this model when compared with exact numerical results for small systems. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W38.00007: Examining an Exactly Solvable First-Order Transition in a Modified Hubbard Model Pawel Pisarski, R.J. Gooding A previous study (PRB {\bf 63}, 035014 (2000)) of the success of the different diagrammatic theories in reproducing the physics of the Hubbard model led to the introduction of a modified Hubbard interaction, the latter of which is given by $U~\sum_i \big(({\hat n}_{i,\uparrow} - \langle ({\hat n}_ {i,\uparrow} \rangle)~(({\hat n}_{i,\downarrow} - \langle ({\hat n}_{i,\downarrow} \rangle)\big)$. We show that the Hubbard model with this modified interaction leads to an exact phase diagram corresponding to a first-order phase transition, with a thermodynamic potential of the same form as that found for the familiar van der Waals equation of state. Then we show that the fully self-consistent T-Matrix Approximation for the repulsive Hubbard interaction accurately tracks the low electron density regime, and that {\it two} self-consistent solutions are found, corresponding to both the stable and metastable phases of this model. This is in contract to the minimally self-consistent T-Matrix theory that was shown to be successful for the attractive model (PRB {\bf 71}, 155111 (2005)). [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W38.00008: Study of the anisotropic two-orbital Hubbard model using dynamical cluster approximation Hunpyo Lee, Yu-Zhong Zhang, Harald O. Jeschke, Roser Valenti, Hartmut Monien We employ a combination of dynamical cluster approximation with a continuous-time quantum Monte Carlo impurity solver to investigate the properties of a two-orbital Hubbard model with two different band widths on the square lattice. As a function of cluster size $N_c$, we study the influence of short-range spatial fluctuations on the nature of the metal-insulator transition and on the occurence of an orbital-selective Mott transition (OSMT). We observe that for $N_c=2$ the OSMT is absent, and instead, at low temperatures a band insulator state is observed for both orbitals. For $N_c=4$, cooperation and competition between spatial fluctuations and orbitals of different bandwidths allow us to distinguish five different phases including an OSMT phase. Based on our results, we discuss the nature of the gap opening. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W38.00009: Doping induced Mott transition in the two dimensional Hubbard model Giovanni Sordi, A.-M.S. Tremblay The description of the Mott transition by single-site dynamical mean-field theory is exact in infinite dimensions but, in two dimensions, substantial deviations from those results have been found for the interaction driven transition [1]. In addition, the experimentally relevant transition for layered systems such as the high-$T_c$ cuprates is doping driven. We thus study this transition in the two dimensional Hubbard model on the square lattice using cluster dynamical mean-field theory with continuous-time quantum Monte Carlo in the hybridization expansion [2]. We find that the Mott transition is strongly influenced by the inclusion of short-range antiferromagnetic correlations. Doping of the Mott insulating state occurs gradually in the different momentum sectors, as found in previous studies [3], but in addition we find a first order transition between an incoherent metal and an insulator or between two incoherent metals, depending on interaction strength. Short range spin correlations create a pseudogap in a doping range that increases with interaction. [1] H. Park et al., PRL 101, 186403 (2008) [2] K. Haule, PRB 75, 155113 (2007) [3] E. Gull et al., arXiv:0909.1795 (2009) [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W38.00010: Real-time dynamics of particle-hole excitations in Mott insulator-metal junctions Luis Dias da Silva, Khaled Al-Hassanieh, Adrian Feiguin, Fernando Reboredo, Elbio Dagotto Charge excitations in Mott insulators (MIs) are distinct from their band-insulator counterparts and can provide a mechanism for energy harvesting in solar cells based on strongly correlated materials. In this work [1], we study the real-time dynamics of a holon-doublon pair in a 1D Hubbard model (a prototypical example of a MI) connected to metallic leads using the time-dependent density matrix renormalization group (tdDMRG) method. Doublons and holons scatter off the MI-metal boundaries on opposite sides, leading to an effective charge transfer into the leads. This charge transfer is strongly affected by the electron-electron correlations in the MI and is nonzero even in the case of charge balance between the leads, in contrast to the case of a band insulator-metal junction. Moreover, the propagation of holon-doublon excitations dynamically changes the spin-spin correlations within the MI, introducing time-dependent phase shifts in the spin structure factor. \\[4pt] [1] L. Dias da Silva et al., arXiv:0911.2141. See also K.A. Al-Hassanieh et al., PRL {\bf 100} 166403 (2008). [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W38.00011: Numerical study of polaron problem in the adiabatic limit Frank Marsiglio, Zhou Li, Cindy Blois, Devin Baillie We study the polaron problem in a one dimensional chain and on a two dimensional square lattice. The models we have used are the Holstein model and the Su-Schrieffer-Heeger (SSH) model. By a variational procedure based on the Lanczos method, we are able to examine the polaron problem in the limit when the mass of the ion is very large, i.e. close to the adiabatic limit. It is known that for the Holstein model there is no phase transition [1] for any nonzero phonon energy. It is also known that for the one dimensional Holstein or SSH model there will be long range order [2] (e.g. dimerization) in the adiabatic limit at half-filling. It is then interesting to study the long range order on a two dimensional square lattice in and away from the adiabatic limit. Moreover, recent progress for the single polaron near an impurity (disorder) [3] make it an interesting problem for studying bond length disorder which can change the hopping energy in a specific direction in the Holstein model. Reference: [1] H. Lowen, Phys.Rev.B 37, 8661 (1988) [2] J.E.Hirsch and E. Frandkin, Phys. Rev. Lett. 49, 402 (1982) [3]A.S.Mishchenko et.al Phys.Rev.B 79(2009) 180301(R) [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W38.00012: Solving the dynamical mean field theory impurity problem with equations of motion and decoupling Qingguo Feng, Yu-Zhong Zhang, Harald O. Jeschke We address the need for efficient and reliable solutions to the effective Anderson impurity model in the context of dynamical mean field theory by developing a decoupling scheme for the single particle Greens function for finite interaction strength $U$. We solve the resulting closed system of integral equations by a method that combines iteration with genetic algorithms. The method is promising as the single band results for the Hubbard model compare well to numerically exact methods for insulating and metallic states. The possibility to extend the method to many orbitals is discussed. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W38.00013: The Paired-Electron Crystal in the Two-Dimensional Frustrated Quarter-Filled Band R. T. Clay, S. Dayal, H. Li, S. Mazumdar The competition between antiferromagnetic (AFM) and spin-singlet ground states within quantum spin models and the $\frac{1}{2}$-filled band Hubbard model has received intense scrutiny. Using exact diagonalization and path integral renormalization group (PIRG) calculations we demonstrate a frustration-induced transition from Neel AFM to spin-singlet in the interacting $\frac{1}{4}$-filled band on an anisotropic triangular lattice. While the AFM state has equal charge densities 0.5 on all sites, the spin-singlet state is a paired-electron crystal (PEC), with pairs of charge-rich sites separated by pairs of charge-poor sites. The PEC provides a natural description of the spin-gapped state proximate to superconductivity (SC) in many organic charge-transfer solids. Our theory explains the semiconducting behavior of Na$_x$CoO$_2$ at x=0.5, and also applies to spinels isoelectronic with superconducting LiTi$_2$O$_4$ and CuRh$_2$S$_4$. We discuss recent experimental results in the light of our theory. Pressure-induced SC in these correlated-electron systems is likely a transition from the $\frac{1}{4}$-filled band valence bond solid to a valence bond liquid. [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W38.00014: Sodium doped TiOCl as a realization of a multiband ionic Hubbard model Harald O. Jeschke, Yu-Zhong Zhang, Kateryna Foyevtsova, Martin U. Schmidt, Roser Valenti Using first principles molecular dynamics simulations, we investigate the effect of Na intercalation in the layered Mott insulator TiOCl. In agreement with recent photoemission experiments, we find that the system remains insulating for all studied Na concentrations. Our calculations also explain the evolution of the spectral weight upon Na doping. This is possible due to a prediction of the Na-doped superlattice structures. In the search for alternatives to metallize TiOCl, we also carried out simulations of substitutions of O by F, Cl by S, or Ti by V (or Sc), respectively, but all resulting structures turn out to be insulating. We propose a description in terms of a multiorbital ionic Hubbard model in a quasi-two-dimensional lattice and discuss the nature of the insulating state under doping. Finally, a different route for metallizing TiOCl by doping is proposed. [Preview Abstract] |
Thursday, March 18, 2010 2:03PM - 2:15PM |
W38.00015: Effects of local moments on a Mott transition Rajesh Narayanan, Serge Florens, Priyanka Mohan, Janani Chander, Tribikram Gupta In this work, we study the influence of interacting (long-ranged) local moments on a Mott-transition. We show that at low temperatures even in the presence of these local moments the Mott-transition remains first order. However, at higher temperatures the Mott tricritical point is depressed. We also show that the transitions lines are bent due to the effects of these local moment fluctuations. Finally, we study the behavior of various thermodynamic observables as we scan the various parts of the phase diagram. These results were obtained by using a Hubbard- Heisenberg model with a local Coulomb repulsion and infinite ranged spin interactions. The model is solved by allying dynamical mean field theory equations with the slave- rotor technique. [Preview Abstract] |
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