626 research outputs found
Reduced density-matrix functional theory in quantum Hall systems
We apply reduced density-matrix functional theory to the parabolically
confined quantum Hall droplet in the spin-frozen strong magnetic field regime.
One-body reduced density matrix functional method performs remarkably well in
obtaining ground states, energies, and observables derivable from the one-body
reduced density matrix for a wide range of system sizes. At the strongly
correlated regime, the results go well beyond what can be obtained with the
density functional theory. However, some of the detailed properties of the
system, such as the edge Green's function, are not produced correctly unless we
use the much heavier two-body reduced density matrix method.Comment: 13 pages, 7 figure
Effects of thickness in quantum dots at strong magnetic fields
We study the effects of thickness on the ground states of two-dimensional
quantum dots in high magnetic fields. To be specific, we assume the thickness
to be small so that only the lowest state in the corresponding direction is
occupied, but which however leads to a modification of the effective
interaction between the electrons. We find the ground state phase diagram and
demonstrate the emergence of new phases as the thickness is accounted for.
Finally, the wave functional form and vortex structure of different phases is
analyzed.Comment: 5 pages, 4 figure
Variational Monte Carlo for Interacting Electrons in Quantum Dots
We use a variational Monte Carlo algorithm to solve the electronic structure
of two-dimensional semiconductor quantum dots in external magnetic field. We
present accurate many-body wave functions for the system in various magnetic
field regimes. We show the importance of symmetry, and demonstrate how it can
be used to simplify the variational wave functions. We present in detail the
algorithm for efficient wave function optimization. We also present a Monte
Carlo -based diagonalization technique to solve the quantum dot problem in the
strong magnetic field limit where the system is of a multiconfiguration nature.Comment: 34 pages, proceedings of the 1st International Meeting on Advances in
Computational Many-Body Physics, to appear in Journal of Low Temperature
Physics (vol. 140, nos. 3/4
Half-Integer Filling Factor States in Quantum Dots
Emergence of half-integer filling factor states, such as nu=5/2 and 7/2, is
found in quantum dots by using numerical many-electron methods. These states
have interesting similarities and differences with their counterstates found in
the two-dimensional electron gas. The nu=1/2 states in quantum dots are shown
to have high overlaps with the composite fermion states. The lower overlap of
the Pfaffian state indicates that electrons might not be paired in quantum dot
geometry. The predicted nu=5/2 state has high spin polarization which may have
impact on the spin transport through quantum dot devices.Comment: 4 pages, accepted to Phys. Rev. Let
Quantum Hall droplet laterally coupled to a quantum ring
We study a two-dimensional cylindrically-symmetric electron droplet separated
from a surrounding electron ring by a tunable barrier using the exact
diagonalization method. The magnetic field is assumed strong so that the
electrons become spin-polarized and reside on the lowest Fock-Darwin band. We
calculate the ground state phase diagram for 6 electrons. At weak coupling, the
phase diagram exhibits a clear diamond structure due to the blockade caused by
the angular momentum difference between the two systems. We find separate
excitations of the droplet and the ring as well as the transfer of charge
between the two parts of the system. At strong coupling, interactions destroy
the coherent structure of the phase diagram, while individual phases are still
heavily affected by the potential barrier.Comment: 7 pages, 7 figure
Modelling line emission of deuterated H_3^+ from prestellar cores
Context: The depletion of heavy elements in cold cores of interstellar
molecular clouds can lead to a situation where deuterated forms of H_3^+ are
the most useful spectroscopic probes of the physical conditions.
Aims: The aim is to predict the observability of the rotational lines of
H_2D^+ and D_2H^+ from prestellar cores.
Methods: Recently derived rate coefficients for the H_3^+ + H_2 isotopic
system were applied to the "complete depletion" reaction scheme to calculate
abundance profiles in hydrostatic core models. The ground-state lines of
H_2D^+(o) (372 GHz) and D_2H^+(p) (692 GHz) arising from these cores were
simulated. The excitation of the rotational levels of these molecules was
approximated by using the state-to-state coefficients for collisions with H_2.
We also predicted line profiles from cores with a power-law density
distribution advocated in some previous studies.
Results: The new rate coefficients introduce some changes to the complete
depletion model, but do not alter the general tendencies. One of the
modifications with respect to the previous results is the increase of the D_3^+
abundance at the cost of other isotopologues. Furthermore, the present model
predicts a lower H_2D^+ (o/p) ratio, and a slightly higher D_2H^+ (p/o) ratio
in very cold, dense cores, as compared with previous modelling results. These
nuclear spin ratios affect the detectability of the submm lines of H_2D^+(o)
and D_2H^+(p). The previously detected H_2D^+ and D_2H^+ lines towards the core
I16293E, and the H_2D^+ line observed towards Oph D can be reproduced using the
present excitation model and the physical models suggested in the original
papers.Comment: 10 pages, 11 Figures; ver2: updated some of the Figures, added some
references, added an entry to acknowledgement
Rectangular quantum dots in high magnetic fields
We use density-functional methods to study the effects of an external
magnetic field on two-dimensional quantum dots with a rectangular hard-wall
confining potential. The increasing magnetic field leads to spin polarization
and formation of a highly inhomogeneous maximum-density droplet at the
predicted magnetic field strength. At higher fields, we find an oscillating
behavior in the electron density and in the magnetization of the dot. We
identify a rich variety of phenomena behind the periodicity and analyze the
complicated many-electron dynamics, which is shown to be highly dependent on
the shape of the quantum dot.Comment: 6 pages, 6 figures, submitted to Phys. Rev.
Search for grain growth towards the center of L1544
In dense and cold molecular clouds dust grains are surrounded by thick icy
mantles. It is however not clear if dust growth and coagulation take place
before the switch-on of a protostar. This is an important issue, as the
presence of large grains may affect the chemical structure of dense cloud
cores, including the dynamically important ionization fraction, and the future
evolution of solids in protoplanetary disks. To study this further, we focus on
L1544, one of the most centrally concentrated pre-stellar cores on the verge of
star formation, and with a well-known physical structure. We observed L1544 at
1.2 and 2 mm using NIKA, a new receiver at the IRAM 30 m telescope, and we used
data from the Herschel Space Observatory archive. We find no evidence of grain
growth towards the center of L1544 at the available angular resolution.
Therefore, we conclude that single dish observations do not allow us to
investigate grain growth toward the pre-stellar core L1544 and high sensitivity
interferometer observations are needed. We predict that dust grains can grow to
200 m in size toward the central ~300 au of L1544. This will imply a dust
opacity change by a factor of ~2.5 at 1.2 mm, which can be detected using the
Atacama Large Millimeter and submillimeter Array (ALMA) at different
wavelengths and with an angular resolution of 2".Comment: 12 pages, 14 figures. Accepted for publication in A&
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