2,818 research outputs found
A variational path integral molecular dynamics study of a solid helium-4
金沢大学理工研究域数物科学系In the present study, a variational path integral molecular dynamics method developed by the author [Chem. Phys. Lett. 482 (2009) 165] is applied to a solid helium-4 in the ground state. The method is a molecular dynamics algorithm for a variational path integral method which can be used to generate the exact ground state numerically. The solid state is shown to successfully be realized by the method, although a poor trial wavefunction that cannot describe the solid state is used. © 2010 Elsevier B.V. All rights reserved
Hydrogen-Helium Mixtures at High Pressure
The properties of hydrogen-helium mixtures at high pressure are crucial to
address important questions about the interior of Giant planets e.g. whether
Jupiter has a rocky core and did it emerge via core accretion? Using path
integral Monte Carlo simulations, we study the properties of these mixtures as
a function of temperature, density and composition. The equation of state is
calculated and compared to chemical models. We probe the accuracy of the ideal
mixing approximation commonly used in such models. Finally, we discuss the
structure of the liquid in terms of pair correlation functions.Comment: Proceedings article of the 5th Conference on Cryocrystals and Quantum
Crystals in Wroclaw, Poland, submitted to J. Low. Temp. Phys. (2004
Leggett's bound for amorphous solids
We investigate the constraints on the superfluid fraction of an amorphous
solid following from an upper bound derived by Leggett. In order to accomplish
this, we use as input density profiles generated for amorphous solids in a
variety of different manners including by investigating Gaussian fluctuations
around classical results. These rough estimates suggest that, at least at the
level of the upper bound, there is not much difference in terms of
superfluidity between a glass and a crystal characterized by the same Lindemann
ratio. Moreover, we perform Path Integral Monte Carlo simulations of
distinguishable Helium 4 rapidly quenched from the liquid phase to very lower
temperature, at the density of the freezing transition. We find that the system
crystallizes very quickly, without any sign of intermediate glassiness. Overall
our results suggest that the experimental observations of large superfluid
fractions in Helium 4 after a rapid quench correspond to samples evolving far
from equilibrium, instead of being in a stable glass phase. Other scenarios and
comparisons to other results on the super-glass phase are also discussed.Comment: 11 pages, 5 figure
The Coupled Electron-Ion Monte Carlo Method
In these Lecture Notes we review the principles of the Coupled Electron-Ion
Monte Carlo methods and discuss some recent results on metallic hydrogen.Comment: 38 pages, 6 figures, Lecture notes for the International School of
Solid State Physics, 34th course: "Computer Simulation in Condensed Matter:
from Materials to Chemical Biology", 20 July-1 August 2005 Erice (Italy). To
appear in Lecture Notes in Physics (2006
The finite-temperature Monte Carlo method and its application to superfluid helium clusters
We review the use of the path integral Monte Carlo (PIMC) methodology to the
study of finite-size quantum clusters, with particular emphasis on recent
applications to pure and impurity-doped He clusters. We describe the principles
of PIMC, the use of the multilevel Metropolis method for sampling particle
permutations, and the methods used to accurately incorporate anisotropic
molecule-helium interactions into the path integral scheme. Applications to
spectroscopic studies of embedded atoms and molecules are summarized, with
discussion of the new concepts of local and nanoscale superfluidity that have
been generated by recent PIMC studies of the impurity-doped He clusters.Comment: P. Huang, Y. Kwon, and K. B. Whaley, in "Quantum Fluids in
Confinement", Vol. 4 of "Advances in Quantum Many-Body Theories", edited by
E. Krotscheck and J. Navarro (World Scientific, Singapore, 2002), in pres
Localized helium excitations in 4He_N-benzene clusters
We compute ground and excited state properties of small helium clusters 4He_N
containing a single benzene impurity molecule. Ground-state structures and
energies are obtained for N=1,2,3,14 from importance-sampled, rigid-body
diffusion Monte Carlo (DMC). Excited state energies due to helium vibrational
motion near the molecule surface are evaluated using the projection operator,
imaginary time spectral evolution (POITSE) method. We find excitation energies
of up to ~23 K above the ground state. These states all possess vibrational
character of helium atoms in a highly anisotropic potential due to the aromatic
molecule, and can be categorized in terms of localized and collective
vibrational modes. These results appear to provide precursors for a transition
from localized to collective helium excitations at molecular nanosubstrates of
increasing size. We discuss the implications of these results for analysis of
anomalous spectral features in recent spectroscopic studies of large aromatic
molecules in helium clusters.Comment: 15 pages, 5 figures, submitted to Phys. Rev.
Finite-temperature Wigner solid and other phases of ripplonic polarons on a helium film
Electrons on liquid helium can form different phases depending on density,
and temperature. Also the electron-ripplon coupling strength influences the
phase diagram, through the formation of so-called "ripplonic polarons", that
change how electrons are localized, and that shifts the transition between the
Wigner solid and the liquid phase. We use an all-coupling, finite-temperature
variational method to study the formation of a ripplopolaron Wigner solid on a
liquid helium film for different regimes of the electron-ripplon coupling
strength. In addition to the three known phases of the ripplopolaron system
(electron Wigner solid, polaron Wigner solid, and electron fluid), we define
and identify a fourth distinct phase, the ripplopolaron liquid. We analyse the
transitions between these four phases and calculate the corresponding phase
diagrams. This reveals a reentrant melting of the electron solid as a function
of temperature. The calculated regions of existence of the Wigner solid are in
agreement with recent experimental data.Comment: 12 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1012.4576, arXiv:0709.4140 by other author
What makes a crystal supersolid ?
For nearly half a century the supersolid phase of matter has remained
mysterious, not only eluding experimental observation, but also generating a
great deal of controversy among theorists. Recent discovery of what is
interpreted as a non-classical moment of inertia at low temperature in solid
He-4 has elicited much excitement as a possible first observation of a
supersolid phase. In the two years following the discovery, however, more
puzzles than answers have been provided to the fundamental issue of whether the
supersolid phase exists, in helium or any other naturally occurring condensed
matter system. Presently, there is no established theoretical framework to
understand the body of experimental data on He-4. Different microscopic
mechanisms that have been suggested to underlie superfluidity in a perfect
quantum crystal do not seem viable for \he4, for which a wealth of experimental
and theoretical evidence points to an insulating crystalline ground state. This
perspective addresses some of the outstanding problems with the interpretation
of recent experimental observations of the apparent superfluid response in He-4
(seen now by several groups) and discusses various scenarios alternative to the
homogeneous supersolid phase, such as superfluidity induced by extended defects
of the crystalline structure which include grain boundaries, dislocations,
anisotropic stresses, etc. Can a metastable superfluid "glassy" phase exist,
and can it be relevant to some of the experimental observations ? One of the
most interesting and unsolved fundamental questions is what interatomic
potentials, given the freedom to design one, can support an ideal supersolid
phase in continuous space, and can they be found in Nature.Comment: Perspective to appear in Advances in Physics, 25 pages, 7 figure
Metropolis Methods for Quantum Monte Carlo Simulations
Since its first description fifty years ago, the Metropolis Monte Carlo
method has been used in a variety of different ways for the simulation of
continuum quantum many-body systems. This paper will consider some of the
generalizations of the Metropolis algorithm employed in quantum Monte Carlo:
Variational Monte Carlo, dynamical methods for projector monte carlo ({\it
i.e.} diffusion Monte Carlo with rejection), multilevel sampling in path
integral Monte Carlo, the sampling of permutations, cluster methods for lattice
models, the penalty method for coupled electron-ionic systems and the Bayesian
analysis of imaginary time correlation functions.Comment: Proceedings of "Monte Carlo Methods in the Physical Sciences"
Celebrating the 50th Anniversary of the Metropolis Algorith
Correlations in Hot Dense Helium
Hot dense helium is studied with first-principles computer simulations. By
combining path integral Monte Carlo and density functional molecular dynamics,
a large temperature and density interval ranging from 1000 to 1000000 K and 0.4
to 5.4 g/cc becomes accessible to first-principles simulations and the changes
in the structure of dense hot fluids can be investigated. The focus of this
article are pair correlation functions between nuclei, between electrons, and
between electrons and nuclei. The density and temperature dependence of these
correlation functions is analyzed in order to describe the structure of the
dense fluid helium at extreme conditions.Comment: accepted for publication in Journal of Physics
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