1,196 research outputs found
Structural properties of electrons in quantum dots in high magnetic fields: Crystalline character of cusp states and excitation spectra
The crystalline or liquid character of the downward cusp states in N-electron
parabolic quantum dots (QD's) at high magnetic fields is investigated using
conditional probability distributions obtained from exact diagonalization.
These states are of crystalline character for fractional fillings covering both
low and high values, unlike the liquid Jastrow-Laughlin wave functions, but in
remarkable agreement with the rotating-Wigner-molecule ones [Phys. Rev. B 66,
115315 (2002)]. The crystalline arrangement consists of concentric polygonal
rings that rotate independently of each other, with the electrons on each ring
rotating coherently. We show that the rotation stabilizes the Wigner molecule
relative to the static one defined by the broken-symmetry
unrestricted-Hartree-Fock solution. We discuss the non-rigid behavior of the
rotating Wigner molecule and pertinent features of the excitation spectrum,
including the occurrence of a gap between the ground and first excited states
that underlies the incompressibility of the system. This leads us to conjecture
that the rotating crystal (and not the static one) remains the relevant ground
state for low fractional fillings even at the thermodynamic limit.Comment: Published version. Typos corrected. REVTEX4. 10 pages with 8
postscript figures (5 in color). For related papers, see
http://www.prism.gatech.edu/~ph274cy
Strongly correlated wave functions for artificial atoms and molecules
A method for constructing semianalytical strongly correlated wave functions
for single and molecular quantum dots is presented. It employs a two-step
approach of symmetry breaking at the Hartree-Fock level and of subsequent
restoration of total spin and angular momentum symmetries via Projection
Techniques. Illustrative applications are presented for the case of a
two-electron helium-like single quantum dot and a hydrogen-like quantum dot
molecule.Comment: 9 pages. Revtex with 2 GIF and 1 EPS figures. Published version with
extensive clarifications. A version of the manuscript with high quality
figures incorporated in the text is available at
http://calcite.physics.gatech.edu/~costas/qdhelproj.html For related papers,
see http://www.prism.gatech.edu/~ph274c
The role of structural evolution on the quantum conductance behavior of gold nanowires during stretching
Gold nanowires generated by mechanical stretching have been shown to adopt
only three kinds of configurations where their atomic arrangements adjust such
that either the [100], [111] or [110] zone axes lie parallel to the elongation
direction. We have analyzed the relationship between structural rearrangements
and electronic transport behavior during the elongation of Au nanowires for
each of the three possibilities. We have used two independent experiments to
tackle this problem, high resolution transmission high resolution electron
microscopy to observe the atomic structure and a mechanically controlled break
junction to measure the transport properties. We have estimated the conductance
of nanowires using a theoretical method based on the extended H\"uckel theory
that takes into account the atom species and their positions. Aided by these
calculations, we have consistently connected both sets of experimental results
and modeled the evolution process of gold nanowires whose conductance lies
within the first and third conductance quanta. We have also presented evidence
that carbon acts as a contaminant, lowering the conductance of one-atom-thick
wires.Comment: 10 page
Premelting of Thin Wires
Recent work has raised considerable interest on the nature of thin metallic
wires. We have investigated the melting behavior of thin cylindrical Pb wires
with the axis along a (110) direction, using molecular dynamics and a
well-tested many-body potential. We find that---in analogy with cluster
melting---the melting temperature of a wire with radius is lower
than that of a bulk solid, , by . Surface melting
effects, with formation of a thin skin of highly diffusive atoms at the wire
surface, is observed. The diffusivity is lower where the wire surface has a
flat, local (111) orientation, and higher at (110) and (100) rounded areas. The
possible relevance to recent results on non-rupturing thin necks between an STM
tip and a warm surface is addressed.Comment: 10 pages, 4 postscript figures are appended, RevTeX, SISSA Ref.
131/94/CM/S
Free-electron Model for Mesoscopic Force Fluctuations in Nanowires
When two metal electrodes are separated, a nanometer sized wire (nanowire) is
formed just before the contact breaks. The electrical conduction measured
during this retraction process shows signs of quantized conductance in units of
G_0=2e^2/h. Recent experiments show that the force acting on the wire during
separation fluctuates, which has been interpreted as being due to atomic
rearrangements. In this report we use a simple free electron model, for two
simple geometries, and show that the electronic contribution to the force
fluctuations is comparable to the experimentally found values, about 2 nN.Comment: 4 pages, 3 figures, reference correcte
Energetics, forces, and quantized conductance in jellium modeled metallic nanowires
Energetics and quantized conductance in jellium modeled nanowires are
investigated using the local density functional based shell correction method,
extending our previous study of uniform in shape wires [C. Yannouleas and U.
Landman, J. Phys. Chem. B 101, 5780 (1997)] to wires containing a variable
shaped constricted region. The energetics of the wire (sodium) as a function of
the length of the volume conserving, adiabatically shaped constriction leads to
formation of self selecting magic wire configurations. The variations in the
energy result in oscillations in the force required to elongate the wire and
are directly correlated with the stepwise variations of the conductance of the
nanowire in units of 2e^2/h. The oscillatory patterns in the energetics and
forces, and the correlated stepwise variation in the conductance are shown,
numerically and through a semiclassical analysis, to be dominated by the
quantized spectrum of the transverse states at the narrowmost part of the
constriction in the wire.Comment: Latex/Revtex, 11 pages with 5 Postscript figure
MIRA: a Multiphysics Approach to Designing a Fusion Power Plant
Fusion systems codes (SCs) are deployed to produce the baseline of the European fusion power reactor (DEMO) within its conceptual design. A DEMO baseline is mostly defined by a radial/vertical reactor sketch and major reactor parameters, such as fusion and net electric power, magnetic fields, and plasma burn time. A baseline shall also meet a set of prescribed reactor requirements, constraints, and architectural features. According to the conceptual design workflow implemented within the EU-DEMO programme, the output from the SC is transferred to the detailed physics and engineering design codes. Presently-available fusion SCs rely on rather basic physics and engineering models (mostly at zero or one-dimensional level). The design codes, instead, are very detailed but run on much longer computing times. To fill the gap between systems and design codes, the multi-fidelity systems/design tool modular integrated reactor analysis (MIRA)—has been recently developed. MIRA incorporates the physics and the engineering insights of the utmost domains of tokamak reactors and relies on a higher spatial resolution, spanning from 1D up to 3D modelling frames. The MIRA approach has been applied to the DEMO 2017 baseline, generated by the EU reference SC PROCESS and used as input to MIRA. In the paper, the architectural and mathematical insights of the MIRA package are described, along with an EU-DEMO 2017 baseline analysis
Quantum Conductance in Semimetallic Bismuth Nanocontacts
Electronic transport properties of bismuth nanocontacts are analyzed by means
of a low temperature scanning tunneling microscope. The subquantum steps
observed in the conductance versus elongation curves give evidence of atomic
rearrangements in the contact. The underlying quantum nature of the conductance
reveals itself through peaks in the conductance histograms. The shape of the
conductance curves at 77 K is well described by a simple gliding mechanism for
the contact evolution during elongation. The strikingly different behaviour at
4 K suggests a charge carrier transition from light to heavy ones as the
contact cross section becomes sufficiently small.Comment: 5 pages including 4 figures. Accepted for publication in Phys. Rev.
Let
Structure and Magnetism of Neutral and Anionic Palladium Clusters
The properties of neutral and anionic Pd_N clusters were investigated with
spin-density-functional calculations. The ground state structures are
three-dimensional for N>3 and they are magnetic with a spin-triplet for 2<=N<=7
and a spin nonet for N=13 neutral clusters. Structural- and spin-isomers were
determined and an anomalous increase of the magnetic moment with temperature is
predicted for a Pd_7 ensemble. Vertical electron detachment and ionization
energies were calculated and the former agree well with measured values for
anionic Pd_N clusters.Comment: 5 pages, 3 figures, fig. 2 in color, accepted to Phys. Rev. Lett.
(2001
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