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Fabio Mezzacapo
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ABSTRACT: We study the ground-state phase diagram of the quantum $J_1-J_2$ model on the
square lattice by means of an entangled-plaquette variational ansatz. In the
range $0\le {J_2}/{J_1} \le 1$, we find classical magnetic order of N\'eel and
collinear type, for ${J_2}/{J_1}\lesssim 0.5$, and $J_2/J_1 \gtrsim 0.6$
respectively. For intermediate values of $J_2/J_1$ the ground state is a spin
liquid (i.e., paramagnetic with no valence bond crystalline order). Our
estimates of the entanglement entropy show that such a spin liquid is
topological.
03/2012;
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ABSTRACT: We study the ground-state phase diagram of the quantum $J_1-J_2$ model on the
honeycomb lattice by means of an entangled-plaquette variational ansatz. Values
of energy and relevant order parameters are computed in the range $0\le
{J_2}/{J_1} \le 1$. The system displays classical order for
${J_2}/{J_1}\lesssim 0.2$ (N\'eel), and for $J_2/J_1 \gtrsim 0.4$ (collinear).
In the intermediate region, the ground-state is disordered. Our results show
that the reduction of the half-filled Hubbard model to the model studied here
does not yield accurate predictions.
09/2011;
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ABSTRACT: We present results of a theoretical study of structural and superfluid properties of parahydrogen (p-H(2)) clusters comprising 25, 26, and 27 molecules at low temperature. The microscopic model utilized here is based on the Silvera-Goldman pair potential. Numerical results are obtained by means of quantum Monte Carlo simulations, making use of the continuous-space worm algorithm. The clusters are superfluid in the low temperature limit, but display markedly different physical behaviors. For N = 25 and 27, superfluidity at low temperature arises as clusters melt, that is, become progressively liquid-like as a result of quantum effects. On the other hand, for N = 26, the cluster remains rigid and solid-like. We argue that the cluster (p-H(2))(26) can be regarded as a mesoscopic "supersolid". This physical picture is supported by results of simulations in which a single p-H(2) molecule in the cluster is isotopically substituted.
The Journal of Physical Chemistry A 02/2011; 115(25):6831-7. · 2.95 Impact Factor
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Fabio Mezzacapo
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ABSTRACT: We study the properties of a mobile hole in the $t-J$ model on the square
lattice by means of variational Monte Carlo simulations based on the
entangled-plaquette ansatz. Our energy estimates for small lattices reproduce
available exact results. We obtain values for the hole energy dispersion curve
on large lattices in quantitative agreement with earlier findings based on the
most reliable numerical techniques. Accurate estimates of the hole spectral
weight are provided.
12/2010;
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ABSTRACT: We present results of a comprehensive theoretical investigation of the low temperature (T) properties of clusters of para-hydrogen (p-H(2)), both pristine as well as doped with isotopic impurities (i.e., ortho-deuterium, o-D(2)). We study clusters comprising up to N = 40 molecules, by means of quantum simulations based on the continuous-space Worm algorithm. Pristine p-H(2) clusters are liquid-like and superfluid in the [Formula: see text] limit. The superfluid signal is uniform throughout these clusters; it is underlain by long cycles of permutation of molecules. Clusters with more than 22 molecules display solid-like, essentially classical behavior at temperatures down to T∼1 K; some of them are seen to turn liquid-like at sufficiently low T (quantum melting).
Journal of Physics Condensed Matter 04/2009; 21(16):164205. · 2.55 Impact Factor
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ABSTRACT: We study by quantum Monte Carlo simulations the local superfluid response of small (up to 27 molecules) parahydrogen clusters, down to temperatures as low as 0.05 K. We show that at low temperature superfluidity is not confined at the surface of the clusters, as recently claimed by Khairallah et al. [Phys. Rev. Lett. 98, 183401 (2007)10.1103/PhysRevLett.98.183401]. Rather, even clusters with a pronounced shell structure are essentially uniformly superfluid. Superfluidity occurs as a result of long exchange cycles involving all molecules.
Physical Review Letters 04/2008; 100(14):145301. · 7.37 Impact Factor
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ABSTRACT: It is shown by computer simulations that superfluid {\it para}-hydrogen clusters of more than 22 molecules can be turned insulating and "solidlike" by the replacement of as few as one or two molecules, with ones of the heavier {\it ortho}-deuterium isotope. A much smaller effect is observed with substitutional {\it ortho}-hydrogen. Substitutional {\it ortho}-deuterium molecules prevalently sit in the inner part of the cluster, whereas {\it ortho}-hydrogen impurities reside primarily in the outer shell, near the surface. Implications on the superfluidity of pure {\it para}-hydrogen clusters are discussed. Comment: 4 pages, 5 figures
08/2007;
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ABSTRACT: We present results of a theoretical study of para-hydrogen and ortho-deuterium clusters at low temperature (0.5 K < T < 3.5 K), based on Path Integral Monte Carlo simulations. Clusters of size up to N=21 para-hydrogen molecules are nearly entirely superfluid at T < 1 K. For 21 < N < 30, the superfluid response displays strong variations with N, reflecting structural changes that occur on adding or removing even a single molecule. Some clusters in this size range display quantum melting, going from solid- to liquid-like as T tends to 0. Melting is caused by quantum exchanges of molecules. The largest para-hydrogen cluster for which a significant superfluid response is observed comprises 27 molecules. Evidence of a finite superfluid response is presented for ortho-deuterium clusters of size up to 14 molecules. Magic numbers are observed, at which both types of clusters feature pronounced stability.
12/2006;
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ABSTRACT: Structural and superfluid properties of p-H2 clusters of size up to N=40 molecules, are studied at low temperature (0.5 K<or=T<or=4 K) by path integral Monte Carlo simulations. The superfluid fraction rhoS(T) displays an interesting, nonmonotonic behavior for 22<or=N<or=30. We interpret this dependence in terms of variations with N of the cluster structure. Superfluidity is observed at low T in clusters of as many as 27 molecules; in the temperature range considered here, quantum melting is observed in some clusters, which are seen to freeze at high temperature.
Physical Review Letters 08/2006; 97(4):045301. · 7.37 Impact Factor
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ABSTRACT: Structural and superfluid properties of para-Hydrogen clusters of size up to N=40 molecules, are studied at low temperature (0.5 K < T < 4 K) by Path Integral Monte Carlo simulations. The superfluid fraction displays an interesting, non-monotonic behavior for 22 < N < 30. We interpret this dependence in terms of variations with N of the cluster structure. Superfluidity is observed at low T in clusters of as many as 27 molecules; in the temperature range considered here, quantum melting is observed in some clusters, which freeze at high temperature.
03/2006;
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New Journal of Physics, v.12 (2010).
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Journal of Physics: Conference Series, v.150, Part 3 (2009).
