Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U31: Supersolids Theory |
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Sponsoring Units: DCMP Chair: Stratos Manousakis, Florida State University Room: Colorado Convention Center 401 |
Thursday, March 8, 2007 8:00AM - 8:12AM |
U31.00001: Thermodynamic instability of a vacancy gas in solid Helium Lode Pollet, Massimo Boninsegni, Anatoly Kuklov, Nikolay Prokof'ev, Boris Svistunov, Matthias Troyer The supersolid phase of matter, characterized by non-dissipative flow in a crystal, has been elusive for some 35 years. The recent discovery of a non-classical moment of inertia in solid $^4$He by Kim and Chan has provided the first piece of experimental evidence, although its interpretation in terms of supersolidity of the ideal crystal phase remains controversial. Using Quantum Monte Carlo methods, we investigate the long-standing question of vacany-induced superflow. We find that a uniform gas of vacancies is thermodynamically unstable against separation into two phases, an insulating, vacancy-free crystal and a liquid. We investigate the thermodynamics of other defects, such as edge dislocations. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U31.00002: Superfluidity of Grain Boundaries in Solid $^4$He Nikolay Prokofiev, Lode Pollet, Massimo Boninsegni, Anatoly Kuklov, Boris Svistunov, Matthias Troyer Superfluid grain boundaries (GB) were proposed as a plausible scenario of the effect discovered by Kim and Chan, and have now been experimentally observed (at the melting point) by Sasaki, Ishiguro, Caupin, Maris, and Balibar [Science {\bf 313}, 1098 (2006)]. We report results of large-scale quantum Monte Carlo simulations (up to $\approx 14000$ atoms) of GB in $^4$He crystals at the melting pressure, as well as deep inside the solid phase. We find that generically GB are superfluid, with transition temperatures about $\sim$ 1 K--0.5 K (we estimate the maximum $T_c$ at about $1.50(5)$~K). At the melting point, the density of particles in the GB is slightly higher than that of a crystal and the GB width is about 3 interatomic spacings. We also observe insulating grain boundaries which typically occur at special orientations of the two crystallites. By simulating GB in direct contact with the superfluid liquid and observing that the system of two liquid-solid and one solid-solid interfaces is mechanically stable we prove that the phenomenon of GB-superfluidity is not related to the close vicinity of the melting line. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U31.00003: Supersolid is Dirty Jiansheng Wu, Philip Phillips A microscopic model for the supersolid phase in $^4$He is given. On the grain boundary, the motion of atoms is well described by a disordered Bose-Hubbard model. We argue that the clean system is a commensurate Mott insulator but in the presence of disorder, a supersolid state obtains. At work is the disorder-induced closing of the Mott gap. We find that the transition temperature to the supersolid state is an increasing function of disorder as is seen experimentally. In addition, we are able to explain the saturation of the superfluid fraction below a characteristic temperature and the pressure dependence of the superfluid fraction. Finally, we also find that a glassy Bose metal phase (BM) is possible and possesses a period shift, though it lacks superflow. This latter observation is useful in explaining the period shift without superflow in hydrogen. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U31.00004: Defects and Impurities in Solid $^4$He Keola Wierschem, Efstratios Manousakis We perform path integral Monte Carlo studies of defects and impurities in solid $^4$He near the low temperature melting transition. The worm algorithm, recently developed for continuum systems, is used to study off-diagonal properties such as the one-body density matrix (OBDM). While this quantity approaches zero exponentially with increasing particle displacement for the ``pure'' solid, interstitial defects and $^3$He impurities appear to enhance and/or stabilize the OBDM at long distances. Thus, imperfections in solid helium may lead to the formation of a condensate. These calculations are repeated for two-dimensional solid helium, and compared with results from lattice boson models. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U31.00005: Understanding supersolids Miklos Gulacsi, Andre Stoffel We model the newly discovered supersolid phase of ${}^4$He by a hard-core bosonic quantum lattice model in 3 dimension including nearest and next-nearest neighbor interactions. As hard-core Boson exhibit the same algebra as spin-1/2 operators there exists a one-to-one correspondence to the anisotropic Heisenberg model in an external field. To solve this Heisenberg model we used the Tyablikov Green's function technique and in order to obtain a closed set of equations we used a cumulant decoupling scheme. The obtained Green's functions have been used to study the properties of the system. Here, we are particularly interested in the normal-solid (NS) and supersolid (SS) phases as well as the corresponding phase transition. It was long proposed that vacancies and defects may play a crucial role in the formation of the supersolid phase. Hence we studied the incommensurability which is a measure of the net fraction of vacancies. For the NS phase we re-obtained the well-known thermal activation theory. However, the incommensurability in the SS displays a rather different behavior, which also suggests that the NS to SS transition is a commensurate-incommensurate transition. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U31.00006: Field Induced Supersolid Phase in Spin-One Heisenberg Models Pinaki Sengupta, Cristian Batista We use quantum Monte Carlo methods to demonstrate that the quantum phase diagram of the S=1 Heisenberg model with uniaxial anisotropy contains an extended supersolid phase. We also show that this Hamiltonian is a particular case of a more general and ubiquitous model that describes the low energy spectrum of a class of {\it isotropic} and {\it frustrated} spin systems. This crucial result provides the required guidance for finding experimental realizations of a spin supersolid state. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U31.00007: A striped supersolid phase in bosons on the triangular lattice Roger Melko Using large-scale quantum Monte Carlo simulations, we explore the ground-state phase diagram of bosons hopping on a triangular lattice with nearest (V) and next-nearest (V') neighbor repulsive interactions. In the limit where V=0 but V' is large, we find an example of an unusual striped supersolid state that is stable at 1/2-filling. We discuss the peculiar properties of this phase, as well as the phase transitions out of it into the neighboring superfluid and Mott phases. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U31.00008: Theory of Small Para-Hydrogen Clusters: Magic Numbers and Superfluid Sizes Saad Khairallah, Mikhail Sevryuk, David Ceperley, Peter Toennies We apply the Path Integral Monte Carlo method to study the low temperature structures and superfluidity of para\-hydrogen clusters with up to 40 molecules. We find an enhanced stability at certain `` magic '' cluster sizes and sharp jumps in superfluidity for specific cluster sizes ($N>26$). Superfluidity is largely localized on the surface and coexists with clusters with solid- like core. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U31.00009: Computer simulations of helium-solvated ions: solid-like versus liquid-like defect structures Stefano Paolini, Francesco Ancilotto, Flavio Toigo The local order around several alkali (Li$^+$ and Na$^+$) and alkali-earth (Be$^+$, Mg$^+$ and Ca$^+$) ions in $^4$He clusters has been studied using ground-state path integral Monte Carlo simulations. We apply a criterion based on multipole dynamical correlations to discriminate between solid-like versus liquid-like behavior of the He solvent surrounding the impurity-ion. In agreement with existing experimental measurements in bulk helium, our findings suggest that Be$^+$ produces a solid-(``snowball'')-like structure, similarly to alkali ions and in contrast to heavier alkali-earth ones, for which a liquid-like environment is predicted. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U31.00010: The BCS-BEC crossover in density in GaAs heterostructures for bilayers of electrons and holes with mismatched densities David Neilson, Pierbiagio Pieri, Giancarlo Strinati We have investigated excitonic superfluidity in electron-hole bilayers in GaAs at low temperatures. We analyze the crossover from the BCS limit of overlapping pairs (high carrier density) to the BEC limit of non-overlapping tightly-bound pairs (low carrier density) by independently varying the densities of the electrons and holes. The different electron and hole effective masses in GaAs causes the phase diagram to depend strongly on the direction of the density imbalance. We can identify the crossover region between the BCS and BEC regimes in the phase diagram, and we find the richest variety of phases in this crossover region. We propose detection of a jump in the electron and hole chemical potentials across zero-density imbalance as the criterion for the occurrence of superfluidity. We make a comment on the relation of our results to density and mass imbalances in ultracold Fermi atoms. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U31.00011: ABSTRACT HAS BEEN MOVED TO S31.00014 |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U31.00012: Possible role of $^3He$ impurities in solid $^4He$ Efstratios Manousakis We use a quantum lattice gas model to describe the essential aspects of the motion $^4He$ atoms and of a $^3He$ impurity in solid $^4He$. We find that $^3He$ impurities promote $^4He$ atoms to interstitial sites and this can turn the bosonic quantum crystal into a metastable supersolid. It is suggested that $^3He$ impurity atoms, which produce the interstitial $^4He$ atoms, might have been reabsorbed by pure $^4He$ solid formed during the first stage of a multi-stage solid $^4He$ nucleation process. While we use the ``spin''-wave approximation and low dimensional lattices to illustrate some of the ideas, we argue that the conclusions drawn from these studies may be valid for the real system. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U31.00013: Magnetothermoelectric response at a superfluid--Mott insulator transition Miraculous Bhaseen, Andrew Green, Shivaji Sondhi We present recent results on the finite temperature magnetothermoelectric response in the vicinity of a superfluid--Mott insulator quantum phase transition [cond-mat/0610687]. We focus on the particle-hole symmetric transitions of the Bose--Hubbard model, and combine Lorentz invariance arguments with entropy drift and quantum Boltzmann calculations. Depending on the ratio of the applied fields, the model displays distinct regimes of behavior. We discuss how a non-vanishing thermoelectric tensor and a finite thermal conductivity are supported in this quantum critical regime. [Preview Abstract] |
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