Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H44: Quantum Phase Transitions |
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Sponsoring Units: DCMP Chair: T. Vojta, University of Missouri, Rola Room: Baltimore Convention Center 347 |
Tuesday, March 14, 2006 11:15AM - 11:27AM |
H44.00001: Quantum Phase Transition in Hard-Core Bosons Due to Background Potentials Anand Priyadarshee, Ji-Woo Lee, Shailesh Chandrasekharan, Harold Baranger We study the zero temperature phase diagram of hard-core bosons hopping on a two dimensional lattice under the influence of three types of background potentials: (1) staggered, (2) uniform, and (3) random (on-site disorder). Using the directed-loop quantum Monte Carlo algorithm on large square lattices, we examine the susceptibility, superfluid density, compressibility, and particle-particle correlation length. For all three types of potentials, the system undergoes a quantum phase transition from a superfluid phase at small potential to a normal phase when the applied potential is large. For a staggered or uniform potential, the transition is to an insulating phase; as expected, the staggered case shows XY universality, while the uniform case belongs to the mean field universality class with dynamic exponent z=2. In contrast, the disorder driven transition is clearly different from either of these. We find a transtion to a phase with non-zero compressibility with critical exponents $\nu \sim 1$, $\beta \sim 0.6$ and $z \sim 1.4$ [Preview Abstract] |
Tuesday, March 14, 2006 11:27AM - 11:39AM |
H44.00002: Thermal Transport at the Superfluid--Insulator Transition Miraculous Bhaseen, Andrew Green, Shivaji Sondhi We investigate the finite temperature thermoelectric response in the vicinity of the Superfluid--Insulator quantum phase transition. We present results for the Nernst coefficient in the disorder free Bose--Hubbard model. [Preview Abstract] |
Tuesday, March 14, 2006 11:39AM - 11:51AM |
H44.00003: The quasi-particle gap in a disordered boson Hubbard model in two dimensions JI-Woo Lee, Min-Chul Cha We investigate the behavior of the quasi-particle energy gap near quantum phase transitions in a two-dimensional disordered boson Hubbard model at a commensurate filling. Via Monte Carlo simulations of ensembles with fixed numbers of particles, we observe the behavior of the gap as a function of the tuning parameter for various strength of diagonal disorder. For weak disorder, we find that gapped Mott insulating phase is sustained up to the transition point and disappears only in a superfluid, strongly supporting a direct Mott-insulator-to-superfluid transition. Bose glass behavior, insulating with vanishing gap, appears only when the strength of disorder is bigger than a critical value. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H44.00004: Current noise near to the 2D superconductor-insulator quantum critical point Andrew G. Green, Joel E. Moore, Ashvin Vishwanath, Shivaji L. Sondhi We consider current fluctuations near to the two-dimensional superconductor-insulator transition described by a quantum XY model. This model displays metallic conductivity at criticality. We consider the system both in thermal equilibrium and when a large electric field drives it far from thermodynamic equilibrium. As the strength of the electric field is increased, we find a crossover from thermal, Johnson-Nyquist noise (whose form is demanded by the fluctuation dissipation relation) to a high-field non-linear ``shot noise'' or Schwinger regime, where the current noise is proportional to $\sqrt{E}$ . Comparison with noise in diffusive electronic systems and the possible relevance of using noise measurements in experiments on S-I systems will be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H44.00005: Quantum Monte Carlo studies of charged monolayer Bose fluids Efstratios Manousakis, Keola Wierschem Computational studies of phase separation due to competing forces are implemented on monolayers of charged bosons above a smooth substrate (effectively a two dimensional Bose fluid). Hard-core bosons with van der Waals attraction are modeled with a Lennard-Jones potential, and to this is added a Coulomb repulsion of variable strength. The long range interparticle repulsion induces the Bose fluid to phase separate into ordered clumps of equilibrium liquid and low density gas, instead of a single domain of equilibrium liquid surrounded by its vapors. The ``clumps'' form bubbles or stripes (or a combination of these states), depending on the particle density as well as the repulsion strength. A Bose fluid with weak van der Waals attraction and large quantum fluctuations will become superfluid at low temperatures (as is the case with helium). The possibility of superfluidity in the microscopically phase separated state is also investigated. Path integral Monte Carlo is employed to include the effects of quantum fluctuations and particle permutations. [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:27PM |
H44.00006: Phases, RK Points and Possible Deconfined Transitions in a Model of Bosons on the Honeycomb Lattice Ashvin Vishwanath, Cenke Xu, Joel Moore We consider a model of hard-core bosons (or S=1/2 spins) on the sites of the honeycomb lattice with an interaction that favors exactly three bosons per hexagon. A rich phase diagram of insulating states is obtained, which includes a solvable point of the Rokhsar-Kivelson type where the ground-state wavefunction is related to a constrained version of the three-color model. On introducing charge fluctuations a superfluid phase obtains. We study transitions between the different insulating phases and between the superfluid and insulating states utilizing a duality due to Motrunich. The unusual aspect of these transitions is that when continuous, they lead to 1/3 charged fractional excitations and an emergent U(1)xU(1) gauge structure in the vicinity of the critical point. The phase diagram of this model makes it a promising candidate for numerically studying `deconfined' quantum criticality using Quantum Monte Carlo techiniques. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H44.00007: Comparison of the Superconductor to Insulator Transition in Nano-Perforated and Conventional Homogeneous Films M.D. Stewart, Jr., James M. Valles, Jr., Aijun Yin, J.M. Xu Near a critical resistance of 6.5 kOhms a quantum Superconductor to Insulator Transition (SIT) occurs in homogeneous ultra-thin films. We have measured this transition at dilution refrigerator temperatures in conventional Bi/Sb films as well as Bi/Sb films that are perforated by a regular array of holes. The perforations are separated by an amount ($\sim $100nm) that is much less than the penetration depth but on the same scale as the coherence length. We will compare these transitions to determine whether the macroscopic normal state sheet resistance (measured over many perforations) or the microscopic (characteristic of the links between holes) normal state sheet resistance determines whether a film is on the insulating or superconducting side of the transition. We will discuss how the results provide insight into the relative influences of vortex and quasiparticle fermionic degrees of freedom on the SIT. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H44.00008: Continuous Phase Transition of the Fully Frustrated 3D XY Model with a Magnetic Field in the [111] Direction Kwangmoo Kim, David Stroud We study the fully frustrated three-dimensional XY model on a simple cubic lattice. This model describes a 3D array of superconducting grains in an applied magnetic field ${\bf H}=(\Phi_{0}/a^{2})(1/2,1/2,1/2)$. Using standard Metropolis Monte Carlo simulations with periodic boundary conditions, we obtain the internal energy $U$, the specific heat $C_{V}$, and the helicity modulus $\gamma$ of our system. Our results support the conclusion that our system has a continuous phase transition between two liquid-like phases. Disorder in the low-temperature phase is suggested by the behavior of the vortex density-density correlation function at a very low temperature, $T=0.01J/k_{\mathrm{B}}$. By contrast, previous results for ${\bf H}=(\Phi_{0}/a^{2})(1/3,1/3,1/3)$ indicate a first-order phase transition. Mean-field theory suggests a possible explanation for the liquid-like low-temperature phase: there are four degenerate unstable modes at the mean-field transition temperature $T_{c}^{\mathrm{MF}}$. We also use finite-size scaling and two renormalization group methods to determine the critical exponents $\alpha$, $v$, and $\nu$ for $C_{V}$, $\gamma$, and the correlation length $\xi$. We compare our values of these critical exponents with those for other phase transitions. [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H44.00009: Noise and Phase Transitions Zhi Chen, Clare C. Yu Noise is present in many physical systems and is often viewed as a nuisance. Yet it can also be a probe of microscopic fluctuations. There have been indications recently that the noise in the resistivity increases in the vicinity of the metal-insulator transition. But what are the characteristics of the noise associated with well-understood first and second order phase transitions? It is well known that critical fluctuations are associated with second order phase transitions, but do these fluctuations lead to enhanced noise? We have addressed these questions using Monte Carlo simulations to study the noise in the 2D Ising model which undergoes a second order phase transition, and in the 5-state Potts model which undergoes a first order phase transition. We monitor these systems as the temperature drops below the critical temperature. At each temperature, after equilibration is established, we obtain the time series of quantities characterizing the properties of the system, i.e., the energy and magnetization per site. We apply different methods, such as the noise power spectrum, the Detrended Fluctuation Analysis (DFA) and the second spectrum of the noise, to analyze the fluctuations in these quantities. [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H44.00010: Percolation quantum phase transitions in diluted magnets Thomas Vojta, Joerg Schmalian We show that the interplay of geometric criticality and quantum fluctuations leads to a novel universality class for the percolation quantum phase transition in diluted magnets. All critical exponents involving dynamical correlations are different from the classical percolation values, but in two dimensions they can nonetheless be determined exactly. We develop a complete scaling theory of this transition, and we relate it to recent experiments in La$_{2}$Cu$_{1-p}$(Zn,Mg)$_{p}$O$_{4}$. Our results are also relevant for disordered interacting boson systems. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H44.00011: Competing order in a 3D antiferromaget Kevin Beach It has long been believed that systems of interacting spins can support, in addition to the usual collinear N\'{e}el state, a variety of paramagnetic ground states with resonating or static valence bond order. Confirmation of their existence in candidate models has been complicated by the fact that the frustrating interactions that might support these exotic phases are generally sign problematic and not amenable to exact numerical simulation. Recent advances in projector valence bond Monte Carlo [A. W. Sandvik, Phys.\ Rev.\ Lett.\ \textbf{95}, 207203 (2005)], however, have expanded the limited class of models that can be simulated. This algorithm is compatible with a class of SU(2) invariant interactions that suppress antiferromagnetism. In particular, a continuous transition (a candidate for a deconfined quantum-critical point) between an antiferromagnetic and columnar bond phase in 2D can be engineered by tuning the strength of a four-spin interaction. We consider a generalization of this interaction in a 3D antiferromagnet, where it is suspected that the N\'{e}el and bond-ordered phases are separated not by a single quantum critical point but by an extended spin liquid phase. New developments for the valence bond basis allow us to calculate higher-order spin correlations, Binder cumulants, and the spin stiffness. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H44.00012: N\'eel and disordered phases of coupled Heisenberg chains Samuel Moukouri We use the recently proposed two-step density-matrix renormalization group method to study the effects of frustration in Heisenberg models with S=$\frac{1}{2}$ to S=4 in a two- dimensional spatially anisotropic lattice. We find that the system is made of nearly disconnected chains at the maximally frustrated point, $J_d/J_{\perp}=0.5$, i.e., the transverse spin-spin correlations decay exponentially. This leads to the following consequences: (i) all half-integer spins systems are gapless, behaving like a sliding Luttinger liquid; (ii) for integer spins, there is an intermediate disordered phase with a spin gap, with the width of the disordered state is roughly proportional to the 1D Haldane gap. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H44.00013: Relevance of Disorder to Critical Behavior of Antiferromagnets Omid Nohadani, Stefan Wessel, Stephan Haas We study the magnetic-field-induced antiferromagnetic order in cubic dimer systems with bond disorder. The critical exponents, in absence of randomness, were reported to be mean-field-like in 3D. Using stochastic series expansion quantum Monte Carlo simulations at ultra-low temperatures, we investigate the relevance of disorder to the critical behavior in the vicinity of a quantum critical point. Furthermore, we demonstrate that in the presence of bond disorder, a new Bose-glass phase separates the dimer spin liquid regime from the antiferromagnetically ordered phase. Since most of the experimentally probed compounds reveal traces of disorder, our results are significant for quantum phase transition studies. [Preview Abstract] |
Tuesday, March 14, 2006 1:51PM - 2:03PM |
H44.00014: Quantum disordered phase in bond-diluted two-dimensional Heisenberg antiferromagnets Rong Yu, Tommaso Roscilde, Stephan Haas We investigate quantum phase transitions in the spin-$1/2$ Heisenberg antiferromagnet on square lattices with \emph{inhomogeneous} bond dilution. It is shown that quantum fluctuations can be continuously tuned by inhomogeneous bond dilution, eventually leading to the destruction of long-range magnetic order on the percolating cluster. We find two multicritical points at which the magnetic transition separates from the percolation transition, taking a quantum nature. Beyond these multicritical points a quantum-disordered phase appears, characterized by an infinite percolating cluster with short ranged antiferromagnetic order. In this phase, the low-temperature uniform susceptibility diverges algebraically with non-universal exponents. This is a signature that the novel quantum-disordered phase is a \emph{quantum Griffiths phase}, as also directly confirmed by the statistical distribution of local gaps. This study thus presents evidence of a genuine quantum Griffiths phenomenon in a two-dimensional Heisenberg antiferromagnet. [Preview Abstract] |
Tuesday, March 14, 2006 2:03PM - 2:15PM |
H44.00015: Universal adiabatic dynamics in the vicinity of a quantum critical point Anatoli Polkovnikov I will discuss temporal behavior of a quantum system under a slow external perturbation, which drives the system across a second order quantum phase transition. Despite the conventional adiabaticity conditions are always violated near the critical point, the number of created excitations still goes to zero in the limit of infinitesimally slow variation of the tuning parameter. It scales with the adiabaticity parameter as a power related to the critical exponents $z$ and $\nu$ characterizing the phase transition. I will support general arguments by direct calculations for the Boson Hubbard and the transverse field Ising models. [Preview Abstract] |
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