Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W11: Low-dimensional Systems; Theory |
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Sponsoring Units: DCMP Chair: Kirill Shtengel, University of California, Riverside Room: Colorado Convention Center Korbel 1F |
Thursday, March 8, 2007 2:30PM - 2:42PM |
W11.00001: Pseudogap in a Magnetic Field: A Case of Quantum Vortex Liquid in a Corelated d-wave Superconductor Zlatko Tesanovic Recent experiments by the Princeton group provide a clear indication of enhanced superconducting fluctuations in the pseudogap state of underdoped cuprates. In addition to the giant Nernst effect, which testifies to the thermal vortex excitations, strong diamagnetic and ``vortex liquid-type'' responses at very low temperatures point to predominance of {\em quantum} phase fluctuations throughout the pseudogap regime. This is of much importance: while {\em all} superconductors exhibit thermal phase fluctuations, the quantum phase disorder -- leading to a non-superconducting ground state despite robust local pairing correlations -- is decidedly outside the reach of any BCS-like, weak-coupling style theory. I will introduce general aspects of the theory of quantum disordered, strongly correlated d-wave superconductors and will use the theory to sketch the phenomenology of quantum vortex-liquid in heavily underdoped cuprates. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W11.00002: Hidden charge 2e boson in doped Mott insulators philip phillips, Robert Leigh, Ting-Pong choy We construct the low energy theory of a doped Mott insulator, such as the high-temperature superconductors, by explicitly integrating over the degrees of freedom far away from the chemical potential. For either hole or electron doping, integration of the high energy scale generates a charge 2e bosonic field that accounts for dynamical spectral weight transfer across the Mott gap. The bosonic field represents non-projective physics as its vanishing leads to the standard projected result, namely the $t-J$ model. A mean-field solution reveals that the free energy acquires a lower value in the presence of the bosonic field than when it is absent. Moreover, we find that at mean-field boson condensation defeats d-wave superconducting order in the vicinity of half-filling. The relationship between boson condensation and local-pairing in the pseudogap phase of the cuprates is explored. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W11.00003: Metal-Insulator Transition Revisited for Cold Atoms in Non-Abelian Gauge Potentials Indubala Satija, Daniel Dakin, Charles Clark We discuss the possibility of realizing metal-insulator transitions with ultracold atoms in two-dimensional optical lattices in the presence of artificial gauge potentials. Such transitions have been extensively studied for magnetic fields corresponding to Abelian gauges; they occur when the magnetic flux penetrating the lattice plaquette is an irrational multiple of the magnetic flux quantum. Here we present the first study of these transitions for non-Abelian $U(2)$ gauge fields. In contrast to the Abelian case, the spectrum and localization transition in the non-Abelian case is strongly influenced by atomic momenta. In addition to determining the localization boundary, the momentum fragments the spectrum. Other key characteristics of the non-Abelian case include the absence of localization for certain states and satellite fringes around the Bragg peaks in the momentum distribution and an interesting possibility that the transition can be tuned by the atomic momenta. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W11.00004: Anomalous superfluidity in two dimensional spin-1 systems Shailesh Chandrasekharan We study thermodynamics of a strongly coupled lattice gauge system in $2+1$ dimensions. The partition function of our model can be written elegantly as a statistical mechanics of dimers and loops. The model is invariant under an $SO(3) \times U(1)$ symmetry which makes it interesting in describing phase transitions in spin-1 systems. At low temperatures, we find evidence for superfluidity in the $U(1)$ symmetry sector while the $SO(3)$ symmetry remains unbroken. The finite temperature phase transition belongs to the Kosterlitz-Thouless universality class, but the superfluid density jump $\rho(T_c)$ at the critical temperature Tc is anomalously higher than the normal value of $2Tc/\pi$. We show that by adding an $SO(3)$ symmetry breaking term to the model, one can obtain a variety of superfluid density jumps, including the normal jump and four times the normal jump that arises in the presence of half vortices. We believe the presence of spin causes the anomalous superfluid behavior seen. Our results may be of interest to researchers studying superfluidity in spin-1 systems. Our work is published in PRL 97, 182001 (2006). [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W11.00005: Superconductivity in a two-orbital Hubbard model on a square lattice with Hund's rule coupling Katsunori Kubo It has been recognized that orbital degree of freedom plays important roles in determination of physical properties, in particular magnetism, of several materials. In recent years, effects of orbital degree of freedom on superconductivity have also been discussed. In this study, we investigate possible superconducting states of a two-orbital Hubbard model on a square lattice by applying fluctuation exchange approximation. We can classify superconducting states by spin states, orbital states, and representations of tetragonal symmetry. In particular, we focus on superconducting states peculiar to multi-orbital systems, such as a spin-triplet orbital-singlet s-wave state. We show that the Hund's rule coupling stabilizes such an exotic state. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W11.00006: Phase separation and electron pairing in repulsive Hubbard clusters Armen Kocharian, Gayanath Fernando, Tun Wang, Kalum Palandage, Jim Davenport The exact numerical diagonalization in ensemble of small Hubbard clusters reveal pairing fluctuations, Bose condensation and charge-spin separation in the ground state and finite temperatures [1,2]. The phase diagram off half filling strongly suggests existence of subsequent transitions from electron pairing into unsaturated ferromagnetic and saturated ferromagnetic Mott- Hubbard like insulators driven by electron repulsion. Rigorous criteria for occurrence of corresponding quantum critical points and crossover temperatures are formulated. The phase diagram for ${2\times 4}$-site clusters illustrates how these features are scaled with cluster size. The phase separation and electron pairing monitored by magnetic field and electron doping surprisingly resemble phase diagrams in family of doped high T$_c$ cuprates. [1] Phys. Rev. B 74, 024511, [2] (2006) cond-mat/0608579 (2006) [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W11.00007: Ferromagnetism in the one-band Hubbard Model on a triangular lattice Shi-quan Su, Zhong-bing Huang, Rui Fan, Hai-qing Lin We investigated numerically the existence of ferromagnetic phase in the one-band Hubbard model on a triangular lattice. By studying the spin susceptibilty, we found the model exhibits ferromagnetic properties when the density of electrons is low. Auxillary Field Quantum Monte Carlo (AFQMC) and Constrained Path Monte Carlo (CPMC) data are used to present the system behaviors including spin susceptibility, pair correlation, when the parameters of the model change. We found that these behaviors are related to the ferromagnetism of the model. These results can be viewed as evidences to support a route to metallic ferromagnetism in the one-band Hubbard models. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W11.00008: Novel Electronic Properties of the Hubbard Model on a Frustrated Triangular Lattice Bumsoo Kyung We study novel electronic properties of the Hubbard model on a triangular lattice using the cellular dynamical mean-field theory. The interplay of strong geometric frustration and electron correlations causes a Mott transition at the Hubbard interaction $U/t=10.5$ and an unusual suppression of low energy spin excitations. Doping of a triangular Mott insulator leads to a quasiparticle peak (no pseudogap) at the Fermi surface and to an unexpected increase of low energy spin excitations, in stark contrast to the unfrustrated square lattice case. The present results give much insight into strongly frustrated electronic systems. A few predictions are made. cond-mat/0608202 [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W11.00009: Luttinger Liquid Kink Trinanjan Datta, Erica W. Carlson, Jiangping Hu We consider a spin rotation invariant Luttinger Liquid at finite temperature and show the existence of a kink in the effective dispersion, as determined by the frequency-dependent peak in the momentum distribution curve (MDC). The MDC is defined by considering the single hole spectral function $A^<(\vec{k},\omega)$ as a function of $\vec{k}$ at a fixed frequency $\omega$. When the charge velocity is greater than the spin velocity, $v_c > v_s$, the high frequency dispersion is linear in $\vec{k}$ and follows $v_c$, while the low frequency dispersion (which is also linear in $\vec{k}$) follows some average of the two velocities. The energy scale of the crossover between the two velocities defines a kink, $E_{\rm kink}$. Since the Luttinger Liquid is quantum critical, $E_{\rm kink}$ scales with the temperature. The kink energy is also affected by the interaction strength, and the strength of the kink is controlled by the ratio of the spin and charge velocities. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W11.00010: Heisenberg spin chains with four-spin couplings Alexios Klironomos, Julia Meyer, Toshiya Hikihara, Konstantin Matveev We obtain and analyze the phase diagram of the zigzag Heisenberg spin chain including four-spin interactions arising from ring exchange processes. We perform exact diagonalization of chains up to 24 sites. In addition to the expected ferromagnetic, antiferromagnetic and dimer phases, the phase diagram contains a region of partial spin polarization as well as a region occupied by novel phase(s). [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W11.00011: Phase diagram of the one dimensional Hubbard-Holstein Model at 1/2 and 1/4 filling Rahul Hardikar, Torsten Clay We present a detailed study of the phase diagram of the Hubbard Holstein model at 1/2 filling and 1/4 filling, including finite-frequency quantum phonons within the numerically exact Stochastic Series Expansion quantum Monte Carlo method. In one dimension at 1/2 filling, the electron-phonon (e-ph) coupling gives a Peierls charge density wave, while Hubbard onsite $U$ promotes antiferromagnetic correlations and a Mott insulating state. Our previous study revealed a third Intermediate phase when the electron-electron and e-ph interaction are closely balanced. We show here from direct calculations of charge and spin susceptibilities that (i) as the e-ph coupling strength is increased first a spin gap transition and then the Peierls transition occurs, (ii) transitions between Mott/Intermediate and Intermediate/Peierls states are of the Kosterlitz-Thouless type, (iii) for large $U$, the two transitions merge into a single fist order transition. Our data is consistent with a renormalization of the Luttinger Liquid exponent $K_\rho$, which gives a slightly larger intermediate region as determined from susceptibilities than in previous calculations of $K_\rho$. At 1/4 filling we find a very similar phase diagram. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W11.00012: Deformation of SU(4) singlet spin-orbital state due to Hund's rule coupling Hiroaki Onishi, Takashi Hotta It has been widely recognized that the interplay of spin and orbital degrees of freedom plays a crucial role in the emergence of novel magnetism in strongly correlated systems. In this context, a one-dimensional spin-orbital model with the highest SU(4) symmetry has been one of the subjects of much interests from a theoretical viewpoint, and the critical behavior of the SU(4) singlet ground state has been clarified. However, in a more realistic situation, the Hund's rule coupling should break the SU(4) symmetry. In the present work, by exploiting a density-matrix renormalization group method, we investigate a one-dimensional spin-orbital model in which the SU(4) symmetry is broken down to SU(2)$_{\rm spin}$$\times$U(1)$_{\rm orbital}$ due to the Hund's rule coupling ($J$). At $J=0$, spin and orbital correlations coincide with each other with a peak at $q=\pi/2$, indicating the SU(4) singlet state with a four-site periodicity. On the other hand, with increasing $J$, the peak position of orbital correlation changes to $q=\pi$, while that of spin correlation remains at $q=\pi/2$. We will discuss in detail how the SU(4) singlet state is deformed by the Hund's rule coupling. [Preview Abstract] |
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