127 research outputs found
Computer simulations of two-dimensional melting with dipole-dipole interactions
We perform molecular dynamics and Monte Carlo simulations of two-dimensional
melting with dipole-dipole interactions. Both static and dynamic behaviors are
examined. In the isotropic liquid phase, the bond orientational correlation
length 6 and susceptibility 6 are measured, and the data are fitted to the
theoretical ansatz. An algebraic decay is detected for both spatial and
temporal bond orientational correlation functions in an intermediate
temperature regime, and it provides an explicit evidence for the existence of
the hexatic phase. From the finite-size scaling analysis of the global bond
orientational order parameter, the disclination unbinding temperature Ti is
estimated. In addition, from dynamic Monte Carlo simulations of the positional
order parameter, we extract the critical exponents at the dislocation unbinding
temperature Tm. All the results are in agreement with those from experiments
and support the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory.Comment: 23 pages, 12figure
Two-component mixture of charged particles confined in a channel: melting
The melting of a binary system of charged particles confined in a {\it
quasi}-one-dimensional parabolic channel is studied through Monte Carlo
simulations. At zero temperature the particles are ordered in parallel chains.
The melting is anisotropic and different melting temperatures are obtained
according to the spatial direction, and the different types of particles
present in the system. Melting is very different for the single-, two- and
four-chain configurations. A temperature induced structural phase transition is
found between two different four chain ordered states which is absent in the
mono-disperse system. In the mixed regime, where the two types of particles are
only slightly different, melting is almost isotropic and a thermally induced
homogeneous distribution of the distinct types of charges is observed.Comment: To appear in Journal of Physics: condensed matter ; (13 pages, 12
figures
Interacting electrons in a 2D quantum dot
The exact numerical diagonalization of the Hamiltonian of a 2D circular
quantum dot is performed for 2, 3, and 4 electrons.The results are compared
with those of the perturbation theory.Our numerical results agree reasonably
well for small values of the dimensionles coupling constant \lambda=a\over a_B
where a is the dot radius and a_B is the effective Bohr radius.Exact
diagonalization results are compared with the classical predictions, and they
are found to be almost coincident for large \lambda values. PACS Numbers:
73.20.Dx, 73.61.-rComment: 12 pages, 5 postscript figure
Thermal fluctuations of vortex clusters in quasi-two-dimensional bose-einstein condensates
We study the thermal fluctuations of vortex positions in small vortex clusters in a harmonically trapped rotating Bose-Einstein condensate. It is shown that the order-disorder transition of two-shell clusters occurs via the decoupling of shells with respect to each other. The corresponding "melting" temperature depends strongly on the commensurability between numbers of vortices in shells. We show that melting can be achieved at experimentally attainable parameters and very low temperatures. Also studied is the effect of thermal fluctuations on vortices in an anisotropic trap with small quadrupole deformation. We show that thermal fluctuations lead to the decoupling of a vortex cluster from the pinning potential produced by this deformation. The decoupling temperatures are estimated and strong commensurability effects are revealed.</p
Hysteresis and re-entrant melting of a self-organized system of classical particles confined in a parabolic trap
A self-organized system composed of classical particles confined in a
two-dimensional parabolic trap and interacting through a potential with a
short-range attractive part and long-range repulsive part is studied as
function of temperature. The influence of the competition between the
short-range attractive part of the inter-particle potential and its long-range
repulsive part on the melting temperature is studied. Different behaviors of
the melting temperature are found depending on the screening length ()
and the strength () of the attractive part of the inter-particle potential.
A re-entrant behavior and a thermal induced phase transition is observed in a
small region of ()-space. A structural hysteresis effect is observed
as a function of temperature and physically understood as due to the presence
of a potential barrier between different configurations of the system.Comment: 8 pages, 6 figure
Exact broken-symmetry states and Hartree-Fock solutions for quantum dots at high magnetic fields
Wigner molecules formed at high magnetic fields in circular and elliptic
quantum dots are studied by exact diagonalization (ED) and unrestricted
Hartree-Fock (UHF) methods with multicenter basis of displaced lowest Landau
level wave functions. The broken symmetry states with semi-classical charge
density constructed from superpositions of the ED solutions are compared to the
UHF results. UHF overlooks the dependence of the few-electron wave function on
the actual relative positions of electrons localized in different charge
puddles and partially compensates for this neglect by an exaggerated separation
of charge islands which are more strongly localized than in the exact
broken-symmetry states.Comment: QD2004 proceedings under press in Physica
Wigner crystallization in the two electron quantum dot
Wigner crystallization can be induced in a quantum dot by increasing the
effective electron-electron interaction through a decrease of the electron
density or by the application of a strong magnetic field. We show that the
ground state in both cases is very similar but the energy scales are very
different and therefore also the dynamics.Comment: 4 pages, 4 figure
Molecular dynamics study of a classical two-dimensional electron system: Positional and orientational orders
Molecular dynamics simulation is used to investigate the crystallization of a
classical two-dimensional electron system, in which electrons interact with the
Coulomb repulsion. From the positional and the orientational correlation
functions, we have found an indication that the solid phase has a
quasi-long-range (power-law correlated) positional order and a long-range
orientational order. This implies that the long-range 1/r system shares the
absence of the true long-range crystalline order at finite temperatures with
short-range ones for which Mermin's theorem applies. We also discuss the
existence of the ``hexatic'' phase predicted by the
Kosterlitz-Thouless-Halperin-Nelson-Young theory.Comment: 6 pages, 4 figures; contributed to EP2DS-13; revised; to be published
in Physica
Local Symmetries and Order-Disorder Transitions in Small Macroscopic Wigner Islands
The influence of local order on the disordering scenario of small Wigner
islands is discussed. A first disordering step is put in evidence by the time
correlation functions and is linked to individual excitations resulting in
configuration transitions, which are very sensitive to the local symmetries.
This is followed by two other transitions, corresponding to orthoradial and
radial diffusion, for which both individual and collective excitations play a
significant role. Finally, we show that, contrary to large systems, the focus
that is commonly made on collective excitations for such small systems through
the Lindemann criterion has to be made carefully in order to clearly identify
the relative contributions in the whole disordering process.Comment: 14 pages, 10 figure
Structural Transitions in a Classical Two-Dimensional Molecule System
The ground state of a classical two-dimensional (2D) system with finite
number of charged particles, trapped by two positive impurities charges
localized at a distance (zo) from the 2D plane and separated from each other by
a distance xp are obtained. The impurities are allowed to carry more than one
positive charge. This classical system can form a 2D-like classical molecule
that exhibits structural transitions and spontaneous symmetry breaking as a
function of the separation between the positive charges before it transforms
into two independent 2D-like classical atoms. We also observe structural
transitions as a function of the dielectric constant of the substrate which
supports the charged particles, in addition to broken symmetry states and
unbinding of particles.Comment: 9 pages, 7 figure
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