Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B11: Exotic Quantum Phases and Transitions |
Hide Abstracts |
Sponsoring Units: DMP Chair: Shailesh Chandrasekharan, Duke University Room: Colorado Convention Center Korbel 1F |
Monday, March 5, 2007 11:15AM - 11:27AM |
B11.00001: Spinon Deconfinement at the Quantum Critical Point of $2+1$ D Antiferromagnets Zaira Nazario, David I. Santiago The natural spin 1 excitations of $2+1$ D antiferromagnets are made of constituent confined quarks of spin 1/2, spinons. The quantum paramagnetic phase possesses quantum tunneling events or instantons, which confine the spinons. There have been recent suggestions of new critical points where spinons are deconfined. Instanton events which cause the spinon confinement disappear at the deconfined critical point because the massless spinons screen them effectively and because instanton tunneling becomes infinitely costly. We point out that this happens irrespective of the intrinsic spin of the antiferromagnet. Hence spinons are deconfined irrespective of microscopic spin. Berry phase terms relevant to the paramagnetic phase make the confinement length scale diverge more strongly for half-integer spins, next strongest for odd integer spins, and weakest for even integer spins. There is an emergent photon at the deconfined critical point, but the ``semimetallic'' nature of critical spinons screens such photon making it irrelevant to long distance physics and the deconfined spinons behave as strictly free particles. A unique prediction critical free spinons at the quantum critical point is an anomalous exponent $\eta$ for the susceptibility exactly equal to one. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B11.00002: Experimental Consequences of O(3) Deconfined Criticality in 2+1 D Antiferromagnets David I. Santiago, Zaira Nazario The paramagnetic phase of 2+1 D antiferromagnets can be described in terms of electrodynamics of charged, massive bosonic spinons interacting through an emergent compact U(1) gauge field. Spinons in the paramagnet are confined due to the presence of nontrivial tunneling effects, instantons which provide a long range interaction between the gauge fields and the charges that gaps the gauge fields and provides a linear potential for the charges. The instantons responsible for spinon confinement in the paramagnetic phase vanish at the quantum critical point. Therefore, spinons are deconfined at criticality. We have recently obtained the effective theory that describes the universal physics of these deconfined critical points. From the deconfined critical theory, we calculate the critical Neel field propagator and find a critical exponent eta=1. We also obtain measurable effects and quantities that follow from the prediction eta=1 and serve as characterization of O(3) deconfined criticality. Those are the inelastic and elastic neutron scattering response, Nuclear Magnetic Resonance (NMR) response, magnetic field response and the specific heat. All of these response functions serve to define the O(3) deconfined universality class. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B11.00003: Emergent supersymmetry at a critical point of a lattice model Sung-Sik Lee We present a two dimensional lattice model which exhibits an emergent space-time supersymmetry at a critical point. The lattice model consists of spinless fermion on the honeycomb lattice and boson on the triangular lattice which is dual to the honeycomb lattice. It will be shown that there is only one relevant perturbation at the supersymmetric critical point and the critical theory becomes the 2+1 dimensional N=2 Wess-Zumino theory with two copies of chiral multiplets. Exact values of scaling dimensions can be obtained due to the emergent superconformal symmetry although the critical theory is the interacting theory. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B11.00004: Critical exponents in a transition between an AFM and a valence bond crystal Samuel Moukouri, Kenneth Graham We use the two-step density-matrix renormalization group method to extract the critical exponents $\beta$ and $\nu$ in the transition from a N\'eel $Q=(\pi,\pi)$ phase to a magnetically disordered phase with a spin gap. We find that the exponent $\beta$ computed from the magnetic side of the transition is consistent with that of the classical Heisenberg model, but not the exponent $z\nu$ computed from the disordered side. We also show the contrast between integer and half-integer spin cases. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B11.00005: Self-optimized resonating-valence-bond trial wavefunctions Kevin Beach, Anders Sandvik The spin singlet ground state of a quantum antiferromagnet can be expanded in the overcomplete basis of valence bond states.~[1] To first approximation, the weight associated with each configuration is factorizable into a product of individual bond amplitudes. For nonfrustrated antiferromagnets with local interactions, mean field calculations indicate that the amplitudes are generically powerlaw in the bond length with exponent d+1, where d is the dimension of the lattice. Such states can be employed as the initial trial state for a valence bond projector calculation of the exact ground state.~[2] Moreover, the amplitudes can be determined self- consistently by measuring the statistics of the bonds appear in the projected state and feeding this information back into the trial state. It is also possible to build some of the neglected bond-bond correlations into the trial state itself. The next level of approximation is to factorize the weights in terms of amplitudes that depend on the lengths and orientations of two valence bonds. Again, these amplitudes can be self-optimized in a simulation by matching them to the bond-- bond correlations of the projected state. \newline [1] K.\ S.\ D.\ Beach and A.\ W.\ Sandvik, Nucl.\ Phys.\ B \textbf{750}, 142 (2006).\newline [2] A.\ W.\ Sandvik, Phys.\ Rev.\ Lett.\ \textbf{95}, 207203 (2005). [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B11.00006: Simulating finite-momentum states of quantum spin systems in the valence bond basis Anders Sandvik, Kevin Beach Quantum spin systems such as the Heisenberg model can be simulated numerically in the valence bond basis, as an alternative to the standard basis of eigenstates of the $S^z_i$ operators [1]. One advantage of this approach is that also the triplet sector can be studied based on the configurations generated in the singlet sector [1,2]. This way an improved estimator for the singlet-triplet gap can be constructed. Here we show that also finite-momentum triplet states can be studied [in practice for $q$ close to $0$ or $\mathbf{\pi}$ due to a phase problem], thus allowing us to calculate the triplet dispersion $E(q)$. Matrix elements $\langle T(q)|S^z_q|0\rangle$ are also accessible. These matrix elements give directly the magnon weight in the dynamic structure factor $S(q,\omega)$. We also discuss how deconfined spinon excitations can be detected in this approach. \hskip10.5cm [1] A. W. Sandvik, Phys. Rev. Lett. \textbf{95}, 207203 (2005).\hfill\break [2] K. S. D. Beach and A. W. Sandvik, Nucl. Phys. B \textbf{750}, 142 (2006). [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B11.00007: Spin-liquid phase in a spin-1/2 quantum magnet on the kagome lattice Sergei Isakov, Yong Baek Kim, Arun Paramekanti We study a model of hard-core bosons with short-range repulsive interactions at half filling on the kagome lattice. This model is equivalent to an easy-axis spin-$1/2$ quantum model with no special conservation laws. Using quantum Monte Carlo numerics, we find that this model exhibits a continuous superfluid-insulator quantum phase transition, with exponents $z=1$ and $\nu=0.67(5)$. We show unambiguously that the insulator is a Z$_2$ fractionalized spin liquid phase with short-ranged density and bond correlations, topological order, and exponentially decaying spatial vison correlations. In addition, we map out the finite temperature phase diagram. A Kosterlitz-Thouless finite temperature superfluid-insulator transition becomes strongly first order as the strength of the repulsive interactions increases. This is consistent with the zero temperature transition to the fractionalized phase. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B11.00008: First-order phase transition in a gauge theory of $S=1/2$ quantum antiferromagnets in the deep easy-plane limit Asle Sudbo, Steinar Kragset, Eivind Smorgrav, Joakim Hove, Flavio Nogueira We perform large-scale Monte Carlo simulations on an effective gauge theory for a deep easy-plane antiferromagnet, including a Berry phase term that projects out the $S=1/2$ sector. Without a Berry phase term, the model exhibits a phase transition in the $3DXY$ universality class associated with proliferation of gauge-charge neutral $U(1)$ vortices. The instantons that eliminate the phase transition in the gauge-charged sector are suppressed by the Berry phases. The result is a {\it first-order} phase transition. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B11.00009: Non-Abelian Anyon Superconductivity Waheb Bishara, Chetan Nayak Non-Abelian Anyons are proposed to exist in certain spin models and in Quantuam Hall systems at certain filling fractions. In this work we studied the ground state of dynamical $SU(2)$ level $\kappa$ Chern Simons non-abelian anyons at finite density and no external magnetic field. We find that in the large $\kappa$ limit the topological interaction induces a pairing instability and the ground state is a superconductor with $\it{d+id}$ gap symmetry. We also develop a picture of pairing for the special value $\kappa=2$ and argue that the ground state is a superfluid of pairs for all values of $\kappa$. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B11.00010: Strongly correlated fermions on frustrated lattices Frank Pollmann, Kirill Shtengel, Joseph Betouras, Erich Runge, Peter Fulde Systems with frustrated interactions are generally characterized by a high density of low--lying excitations which leads to a high susceptibility and thus interesting physical effects. We study a novel class of strongly correlated fermions on frustrated lattices which allows for excitations which carry fractional charges [1]. For a systematic study, we firstly consider a model of spinless fermions on a geometrically frustrated planar pyrochlore (checkerboard) lattice. An effective Hamiltonian is derived for the strongly correlated limit which describes the low--lying excitations. We solve the fermionic sign problem for the latter Hamiltonian and thus make it possible to apply quantum Monte Carlo methods [3]. The ground state is shown to be charged ordered and fractional charges are linearly confined. Secondly, we consider a model of spinful fermions on the kagome lattice and study the interplay between charge -- and spin -- degrees of freedom. [1] P.~Fulde, K.~Penc, and N.~Shannon, Annalen der Physik {\bf 11}, 892 (2002) [2] E.~Runge and P.~Fulde, Phys. Rev. B {\bf 70}, 245113 (2004) [3] F.~Pollmann, J.~J.~Betouras, K.~Shtengel, and P.~Fulde, Phys. Rev. Lett. \textbf{97}, 170407 (2006) [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B11.00011: Spinless Fermions on a Checkerboard Lattice Kirill Shtengel, Frank Pollmann, Joseph Betouras, Peter Fulde We present a study of the low-energy physics of a spinless fermionic model on a checkerboard lattice at half-filling. The bosonic version of this model has been recently studied and found to have several unusual features. Fermionic models tend to be more interesting: the inherent sign problem resulting from the fermionic statistics makes them notoriously difficult to handle. The low-energy physics of the model can be described by a fermionic quantum loop model on the square lattice. We found a non-local transformation that can, in certain cases, cure the sign problem. We also identified a large class of fluctuationless states specific to the fermionic models -- a result hinting at a possible explanation of the extended ground-state entropy recently found in a few other fermionic models. Finally, we looked at the so-called Rokhsar-Kivelson quantum critical point, where we found the exact ground state(s) as well as studied the low-lying excitations. This allowed us to make several educated guesses about the phase diagram for the model in question. [1] F.~Pollmann, J.~J.~Betouras, K.~Shtengel, and P.~Fulde, Phys. Rev. Lett. \textbf{97}, 170407 (2006) [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B11.00012: Spinless charges on the triangular lattices in the strong repulsion limit: possibility of a new charge ordered liquid Nobuo Furukawa, Chisa Hotta We propose a new type of charge ordered liquid state in the spinless fermion system on a triangular lattice under strong inter-site Coulomb interactions, $V$. In the absence of fermion hoppings, the ground state is disordered due to geometrical frustration. Introduction of hopping terms lifts the degeneracy and drives the system to a metalic state with possible partial charge orders, which we call a ``pinball liquid''. There, a gapless charge liquid component moves around a possible long range ordered Wigner crystal solid component. This liquid state is dominant over wide range of filling, $n=1/3 \sim 2/3$. When an anisotropy in $V$ exceeds its critical value at half-filling $n=1/2$, an metal-insulator transition accompanied by another charge order with a different periodicity is induced. Relevance to the organic conductors $\theta$-ET$_2$X which show novel nonlinear transport properties is discussed.\\ REFERENCES:\\ \qquad cond-mat/0605045, cond-mat/0607181, cond-mat/0607717. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B11.00013: Local density of states in electronic nematic phase Hyeonjin Doh, Hae-Young Kee We study spatial patterns of local density of states in electronic nematic phase in the presence of a non-magnetic impurity. Since the Fourier transform of the spatial pattern represents the symmetry of an electronic structure of a system, the local density of state can be a direct probe for the isotropic-nematic phase transition. In this work, we show local density of states near the nematic-isotropic phase transition tuned by a magnetic field, and discuss its application to the bilayer ruthenate, Sr$_3$Ru$_2$O$_7$. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700