4,659 research outputs found
The maximum density droplet to lower density droplet transition in quantum dots
We show that, Landau level mixing in two-dimensional quantum dot wave
functions can be taken into account very effectively by multiplying the exact
lowest Landau level wave functions by a Jastrow factor which is optimized by
variance minimization. The comparison between exact diagonalization and fixed
phase diffusion Monte Carlo results suggests that the phase of the many-body
wave functions are not affected much by Landau level mixing. We apply these
wave functions to study the transition from the maximum density droplet state
(incipient integer quantum Hall state with angular momentum L=N(N-1)/2) to
lower density droplet states (L>N(N-1)/2).Comment: 8 pages, 5 figures, accepted for publication in Phys. Rev.
Langevin dynamics in crossed magnetic and electric fields: Hall and diamagnetic fluctuations
Based on the classical Langevin equation, we have re-visited the problem of
orbital motion of a charged particle in two dimensions for a normal magnetic
field crossed with or without an in-plane electric bias. We are led to two
interesting fluctuation effects: First, we obtain not only a longitudinal
"work-fluctuation" relation as expected for a barotropic type system, but also
a transverse work-fluctuation relation perpendicular to the electric bias. This
"Hall fluctuation" involves the product of the electric and the magnetic
fields. And second, for the case of harmonic confinement without bias, the
calculated probability density for the orbital magnetic moment gives non-zero
even moments, not derivable as field derivatives of the classical free energy.Comment: 4 pages, 2 figures, revised versio
The Exchange Gate in Solid State Spin Quantum Computation: The Applicability of the Heisenberg Model
Solid state quantum computing proposals rely on adiabatic operations of the
exchange gate among localized spins in nanostructures. We study corrections to
the Heisenberg interaction between lateral semiconductor quantum dots in an
external magnetic field. Using exact diagonalization we obtain the regime of
validity of the adiabatic approximation. We also find qualitative corrections
to the Heisenberg model at high magnetic fields and in looped arrays of spins.
Looped geometries of localized spins generate flux dependent, multi-spin terms
which go beyond the basic Heisenberg model.Comment: 13 pages, 8 figure
Composite Fermions in Quantum Dots
We demonstrate the formation of composite fermions in two-dimensional quantum
dots under high magnetic fields. The composite fermion interpretation provides
a simple way to understand several qualitative and quantitative features of the
numerical results obtained earlier in exact diagonalization studies. In
particular, the ground states are recognized as compactly filled quasi-Landau
levels of composite fermions.Comment: Revtex. Postscript files of figures are appended the tex
Real-Space Imaging of Alternate Localization and Extension of Quasi Two-Dimensional Electronic States at Graphite Surfaces in Magnetic Fields
We measured the local density of states (LDOS) of a quasi two-dimensional
(2D) electron system near point defects on a surface of highly oriented
pyrolytic graphite (HOPG) with scanning tunneling microscopy and spectroscopy.
Differential tunnel conductance images taken at very low temperatures and in
high magnetic fields show a clear contrast between localized and extended
spatial distributions of the LDOS at the valley and peak energies of the Landau
level spectrum, respectively. The localized electronic state has a single
circular distribution around the defects with a radius comparable to the
magnetic length. The localized LDOS is in good agreement with a spatial
distribution of a calculated wave function for a single electron in 2D in a
Coulomb potential in magnetic fields.Comment: 4 pages, 4 figure
Dynamical Diffraction Theory for Wave Packet Propagation in Deformed Crystals
We develop a theory for the trajectory of an x ray in the presence of a
crystal deformation. A set of equations of motion for an x-ray wave packet
including the dynamical diffraction is derived, taking into account the Berry
phase as a correction to geometrical optics. The trajectory of the wave packet
has a shift of the center position due to a crystal deformation. Remarkably, in
the vicinity of the Bragg condition, the shift is enhanced by a factor (: frequency of an x ray, : gap frequency
induced by the Bragg reflection). Comparison with the conventional dynamical
diffraction theory is also made.Comment: 4 pages, 2 figures. Title change
Shifting with
Precision measurements at the resonance agree well with the standard
model. However, there is still a hint of a discrepancy, not so much in by
itself (which has received a great deal of attention in the past several years)
but in the forward-backward asymmetry together with . The two
are of course correlated. We explore the possibilty that these and other
effects are due to the mixing of and with one or more heavy quarks.Comment: 11 pages, 1 Figure, LaTex fil
Three-electron anisotropic quantum dots in variable magnetic fields: exact results for excitation spectra, spin structures, and entanglement
Exact-diagonalization calculations for N=3 electrons in anisotropic quantum
dots, covering a broad range of confinement anisotropies and strength of
inter-electron repulsion, are presented for zero and low magnetic fields. The
excitation spectra are analyzed as a function of the strength of the magnetic
field and for increasing quantum-dot anisotropy. Analysis of the intrinsic
structure of the many-body wave functions through spin-resolved two-point
correlations reveals that the electrons tend to localize forming Wigner
molecules. For certain ranges of dot parameters (mainly at strong anisotropy),
the Wigner molecules acquire a linear geometry, and the associated wave
functions with a spin projection S_z=1/2 are similar to the representative
class of strongly entangled states referred to as W-states. For other ranges of
parameters (mainly at intermediate anisotropy), the Wigner molecules exhibit a
more complex structure consisting of two mirror isosceles triangles. This
latter structure can be viewed as an embryonic unit of a zig-zag Wigner crystal
in quantum wires. The degree of entanglement in three-electron quantum dots can
be quantified through the use of the von Neumann entropy.Comment: To appear in Physical Review B. REVTEX4. 13 pages with 16 color
figures. To download a copy with higher-quality figures, go to publication
#78 in http://www.prism.gatech.edu/~ph274cy
Chirality in Quantum Computation with Spin Cluster Qubits
We study corrections to the Heisenberg interaction between several lateral,
single-electron quantum dots. We show, using exact diagonalization, that
three-body chiral terms couple triangular configurations to external sources of
flux rather strongly. The chiral corrections impact single qubit encodings
utilizing loops of three or more Heisenberg coupled quantum dots.Comment: 5 pages, 2 figure
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