Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W38: Critical and Inhomogeneous Fermionic Systems |
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Sponsoring Units: DCMP Chair: A. Vishwanath, MIT Room: LACC 513 |
Thursday, March 24, 2005 2:30PM - 2:42PM |
W38.00001: Deconfined critical theories R. Shankar, Ganpathy Murthy We pursue the paradigm advocated by Senthil {\em et al } and discuss a model in two-dimensions that exhibits deconfinement at criticality. There are also differences between this model and their paradigm. For example, in our model the transition line is typically first order, with a second order end point. We gratefully acknowledge grants DMR 0311761 (GM) and DMR 0354517 (RS). \\[4pt] T.Senthil {\em at al}, Phys. Rev. {\bf B 70}, 144407, (2004). [Preview Abstract] |
Thursday, March 24, 2005 2:42PM - 2:54PM |
W38.00002: Breakdown of the Perturbative Renormalization Group at Certain Quantum Critical Points Joerg Rollbuehler, D. Belitz, T.R. Kirkpatrick We show that a controlled loop expansion for critical exponents can break down if multiple time scales are present at a quantum critical point. We present a mechanism that leads to a divergence of coefficients in a controlled $\epsilon$-expansion. This can invalidate results obtained from a finite-order perturbative renormalization group treatment. The mechanism is explained in terms of dangerous irrelevant variables. As a physically relevant example we discuss the quantum ferromagnetic transition in disordered metals. [Preview Abstract] |
Thursday, March 24, 2005 2:54PM - 3:06PM |
W38.00003: Quantum phase transitions of magnetic rotons Joerg Schmalian, Misha Turlakov Due to weak spin-orbit coupling, the magnetic excitations of an itinerant ferromagnet become magnetic rotons, excitations with degenerate minima on a hypersphere at finite wavevector. Using self-consistent Hartree and renormalization group calculations, we study weak fluctuation-driven first-order quantum phase transitions, a quantum tricritical point controlled by anisotropy and the non-Fermi liquid behavior associated with the large phase volume of magnetic rotons. We propose that magnetic rotons are essential for the description of the anomalous high-pressure behavior of the itinerant helical ferromagnet MnSi. [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:18PM |
W38.00004: Monte Carlo study of a 2D diluted XY model with annealed disorder: stripes versus phase separation Daniel Valdez-Balderas, David Stroud Using Monte Carlo simulations, we study the effect of introducing annealed disorder and a second nearest neighbor repulsion between spins on a two-dimensional site-diluted XY model. At low temperatures we observe that spins rearrange themselves in either blobs (phase separation) or stripes, depending on the strength of the second nearest neighbor repulsion. This rearrangement of spins results in a change on the low-temperature helicity modulus of systems with annealed disorder when compared to that of systems with quenched disorder. We have verified the relation between the zero temperature helicity of an XY model and the conductance of an associated resistor network. [Preview Abstract] |
Thursday, March 24, 2005 3:18PM - 3:30PM |
W38.00005: Universal scaling of conductivity at the superfluid-insulator quantum phase transition from numerical simulations Jurij Smakov, Erik Sorensen We address the question of universal conductivity scaling at the insulator-superfluid quantum phase transition by studying the Bose-Hubbard model numerically, using stochastic series expansion (SSE) technique, at two different quantum critical points. With SSE we are able to directly measure the current-current correlation function (related to the conductivity) as a function of Matsubara frequencies. By doing proper data analysis we demonstrate how the universal scaling behavior of conductivity (and the breakdown thereof) may be observed in the numerical simulations. We also attempt to analytically continue the imaginary-frequency data using MaxEnt and stochastic analytic continuation methods. [Preview Abstract] |
Thursday, March 24, 2005 3:30PM - 3:42PM |
W38.00006: Exact Stripe, Checkerboard and Droplet Ground States in Two Dimensions Zsolt Gulacsi, Miklos Gulacsi Exact static non-degenerated stripe and checkerboard ground states are obtained in a broad concentration range below quarter filling in two dimensions, in the frame of a two-band type model containing Hubbard type of interactions. The random droplet states also present in the degenerated ground state, are eliminated in exact terms by extension terms of different physical origin such as dimerization, periodic charge displacements, density waves or distorsion lines. Since stripes and checkerboards are observed in a broad spectrum of materials, we were primarly interested in ground states which exhibit these inhomogeneities and less in the properties of the homogenous phases in which they appear. The procedure itself is based on a positive semidefinite decomposition of the Hamiltonian, which allows the deduction of exact ground states for non-integrable quantum mechanical many-body systems, being previously successfully applied even in 3D (Zs. Gulacsi, D. Vollhardt, Phys. Rev. Lett. {\bf 91},186401,(2003)), or disordered and interacting systems in 2D (Zs. Gulacsi, Phys. Rev. {\bf B69},054204,(2004)). [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W38.00007: The stripe Phase in Ladder systems Ming-Shyang Chang, Ian Affleck, Nicolas Laflorencie The density matrix renormalization group (DMRG) results suggest stripe phases as the ground state in N-leg doped t-J model with open boundary conditions. The most unusual feature of this phase is the coexistence of $4K_{F}$ density oscillation and d-wave pairing correlation. Here we analyze this phase by a general bosonization approach, which doesn't rely on the underlying Hamiltonian. We will also discuss the comparison with numerical data. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W38.00008: Phase separation in Hubbard model Alexandru Macridin, Mark Jarrell, Thomas Pruschke, Thomas Maier We investigate the phase separation in the Hubbard model by employing the dynamical cluster approximation. We find that an enhanced compressibility in the underdoped regime is a general characteristic of the Hubbard model for the most regions of the parameter space. Moreover, a next-nearest-neighbor hopping $t'$ corresponding to electron-doped cuprates will drive a finite temperature transition into a phase separated state consisting of an undoped phase and a rich doped one. A Coulomb repulsion $U\agt W$ is a necessary condition for the transition to take place. Phase separation and charge ordering scenarios for high $T_c$ are becoming increasingly relevant in the highlight of the recent experiments which reveal that charge modulation is an intrinsic characteristic of hole-doped cuprates. We find that phase separation is more prevalent when $t'>0$, consistent with the electron-doped cuprates. [Preview Abstract] |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W38.00009: Inhomogeneous Hubbard Models: from Weak to Strong Coupling Wei-Feng Tsai, S. A. Kivelson We systematically study the ground-state phase diagram of two inhomogeneous Hubbard models -- the dimerized and checkerboard models -- in the limit in which the coupling between clusters, $t'$, is small. Using $t'$ as a small parameter, we can solve the problem for any strength of the Hubbard $U$, and so can trace the evolution of the ground state from the small $U$ (band structure) to the large $U$ (strongly correlated) limit. On both lattices, we have found that there are Fermi liquid phases with only modestly renormalizations of the effective mass as a function of $U$, and with residual repulsive interactions between the quasiparticles whose strength, likewise, varies smoothly as $U$ changes. In addition, there are also robust superconducting phases, especially on the checkerboard lattice, of various symmetries, despite the presence of only repulsive interactions in the microscopic model. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W38.00010: Reconciling Neutron and STM Results for Charge Order in Hole-Doped Cuprates Robert Markiewicz Incommensurate elastic or inelastic magnetic neutron scattering in the cuprates has often been interpreted in terms of {\it one-dimensional stripe} arrays, either static or dynamic. Recent experiments in La$_{2-x}$Ba$_x$CuO$_4$ suggest that the resonance mode phenomenon may also be stripe related[1]. On the other hand, while recent STM studies in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) and Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ have found clear evidence of charge order, it is in terms of {\it checkerboards}, not stripes[2]. Intriguingly, both stripes and checkerboards have the same approximate $4a$ periodicity. There is a second puzzling feature of the STM data: there is no sign of bilayer splitting in the Fermi surface reconstructed from the STM quasiparticle interference effect. This may be related to the smearing out of the bilayer splitting in underdoped Bi2212. I propose that both anomalies have the same origin: crossed stripes in alternate layers. Model calculations will be presented. 1: J.M. Tranquada, {\it et al.}, Nature {\bf 429}, 534 (2004). 2: T. Hanaguri, {\it et al.}, Nature {\bf 430}, 1001 (2004). [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W38.00011: Combined Magnetic Phase Diagram of Cation- and Anion- doped Lanthanum Cuprates Zheng Wu, Pei-Herng Hor We have studied doping dependences of antiferromagnetic (AF) transition temperature$ T_{N}$ in La$_{2}$CuO$_{4+\delta }$for 0 $< \quad \delta <$ 0.01. For $\delta <$ 0.005, we have observed and stabilized a weak ferromagnetic-like anomaly that appears right before $T_{N}$. The suppression of $T_{N}$ due to doped holes is identical to that in La$_{2-x}$Sr$_{x}$CuO$_{4}$ for 0$<$ x $<$ 0.01. For $\delta \quad >$ 0.005, we observed a phase separation into a $T_{N }\sim $ 250K AF phase and T$_{c}$ $\sim $ 30K superconducting phase. Comparing the magnetic phase diagrams of both cation (Sr)- and Anion (O)- doped lanthanum cuprates we conclude that the dopant effects are very important and should be included in the interpretation of any data above 1{\%} doping level. We present a unified ``intrinsic'' magnetic phase diagram of doped cuprates. [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W38.00012: Two and four particle condensation in fermion systems with multiple internal states Aseem Talukdar, Michael Ma, Fu-Chun Zhang We investigate two-particle and four-particle condensation in fermion systems with four internal states (a, b, c, d). Possible physical realizations include transition metal oxides (TMO) with a two-fold orbital degeneracy, bi-layered systems with electrons and holes, and a quadra-layer spin-polarized electron hole system. We consider a model with attraction U between a, b and between c, d; and attraction V between a, c and between b, d. For weaker V, we study quasi-particle pairing above two decoupled BCS condensates, and find both even and odd frequency pairing solutions. The former exists for all values of V while the latter exists only above a critical value. We then consider the condensation of such quasi-particle pairs and its destabilization of the underlying condensates, leading to pairing of more complex structure or possibly condensation of quartets. The phase transitions between the various two-particle condensates, and their transition into the quartet condensate close to the SU(4) limit will be discussed. [Preview Abstract] |
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W38.00013: Two-dimensional s-wave Superconductivity for Different Pairing Patterns Karan Aryanpour Various electron spectroscopies (STM, Xray) have observed electronic inhomogeneities in HTS materials in the form of stripes or checkerboard patterns. We study $2D$ s-wave superconductivity by employing the two-dimensional attractive Hubbard Hamiltonian. Different patterns for the distribution of the interacting lattice sites including random, stripes and checkerboard lead to different superconducting or insulating phases under the variation of the interaction strength and doping. The Hamiltonian for this problem can be mapped onto an effective bilinear form using the Bogolibov-de Gennes mean field approximation. Stripes and checkerboard patterns under different dopings and interaction strengths correspond to larger order parameters compared to the random pattern. For the case of the half-filled stripes, our preliminary results also indicate the destruction of superconductivity at large values of interaction due to the formation of a charge density wave (CDW) long range order. [Preview Abstract] |
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W38.00014: Checkerboard Pattern Formation in a Quasiclassical Model of Superconducting Transition Metal Oxides Cynthia Olson Reichhardt, Charles Reichhardt, Alan Bishop Using numerical simulations of a quasiclassical model for holes in transition metal oxides, we analyze the thermal fluctuations of off-lattice particles that have a short-range dipolar attraction and a long-range repulsion. In an inhomogeneous particle density region, or "soft phase," filamentary patterns appear which are destroyed only at very high temperatures. The filaments act as a fluctuating template for correlated percolation in which low-energy excitations can move through the stable pattern by local rearrangements. At intermediate temperatures, dynamically averaged checkerboard states appear. We discuss possible implications for doped cuprate oxides and related materials. [1] C.J. Olson Reichhardt, C. Reichhardt, and A.R. Bishop, Phys. Rev. Lett. 92, 016801 (2004). [Preview Abstract] |
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