162 research outputs found
Nonlinear screening in two-dimensional electron gases
We have performed self-consistent calculations of the nonlinear screening of
a point charge Z in a two-dimensional electron gas using a density functional
theory method. We find that the screened potential for a Z=1 charge supports a
bound state even in the high density limit where one might expect perturbation
theory to apply. To explain this behaviour, we prove a theorem to show that the
results of linear response theory are in fact correct even though bound states
exist.Comment: 4 pages, 4 figure
Dissipative dynamics of an extended magnetic nanostructure: Spin necklace in a metallic environment
We study theoretically the dynamics of an ``xxz'' spin necklace coupled to a
conduction electron sea, a model system for a nanostructure in a dissipative
environment. We extract the long-time behavior via a mapping to a multichannel
Coulomb gas problem followed by a scaling analysis. The strong quantum
fluctuations of the necklace cause a nontrivial dependence of couplings on
system size which we extract via an analysis involving the ``boundary condition
changing operator'', and confirm via a detailed numerical evaluation of one
case.Comment: 4 pages, 4 figure
Magnetic properties of Quantum Corrals from first principles calculations
We present calculations for electronic and magnetic properties of surface
states confined by a circular quantum corral built of magnetic adatoms (Fe) on
a Cu(111) surface. We show the oscillations of charge and magnetization
densities within the corral and the possibility of the appearance of
spin--polarized states. In order to classify the peaks in the calculated
density of states with orbital quantum numbers we analyzed the problem in terms
of a simple quantum mechanical circular well model. This model is also used to
estimate the behaviour of the magnetization and energy with respect to the
radius of the circular corral. The calculations are performed fully
relativistically using the embedding technique within the
Korringa-Kohn-Rostoker method.Comment: 14 pages, 9 figures, submitted to J. Phys. Cond. Matt. special issue
on 'Theory and Simulation of Nanostructures
Theory of a Scanning Tunneling Microscope with a Two-Protrusion Tip
We consider a scanning tunneling microscope (STM) such that tunneling occurs
through two atomically sharp protrusions on its tip. When the two protrusions
are separated by at least several atomic spacings, the differential conductance
of this STM depends on the electronic transport in the sample between the
protrusions. Furthermore two-protrusion tips commonly occur during STM tip
preparation. We explore possible applications to probing dynamical impurity
potentials on a metallic surface and local transport in an anisotropic
superconductor.Comment: revtex, 11 pages, 6 figures upon reques
The Anderson prescription for surfaces and impurities
We test the Anderson prescription [1], a BCS formalism for describing
superconductivity in inhomogeneous systems, and compare results with those
obtained from the Bogoliubov-de Gennes formalism, using the attractive Hubbard
model with surfaces and nonmagnetic impurities. The Anderson approach captures
the essential features of the spatial variation of the gap parameter and
electron density around a surface or an impurity over a wide range of
parameters. It breaks down, however, in the strong-coupling regime for a weak
impurity potential.
[1] P. W. Anderson, J. Phys. Chem. Solids 11, 26 (1959).Comment: 4 pages, 4 figure
Sensitivity of quantum chaotic wavefunction intensities to changes in external perturbations
We examine the sensitivity of wavefunction intensities in chaotic quantum
systems to small changes in an arbitrary external perturbation. A universal
scaling is proposed for all three Dyson ensembles and a novel theoretical
approach is used to determine exact expressions for systems which violate
T-invariance. Analytical results are compared with numerical simulations of
tight-binding Anderson Hamiltonians.Comment: 7 pages, RevTeX 3.0, 3 Postscript figure
Self-interference of a single Bose-Einstein condensate due to boundary effects
A simple model wavefunction, consisting of a linear combination of two
free-particle Gaussians, describes many of the observed features seen in the
interactions of two isolated Bose-Einstein condensates as they expand, overlap,
and interfere. We show that a simple extension of this idea can be used to
predict the qualitative time-development of a single expanding BEC condensate
produced near an infinite wall boundary, giving similar interference phenomena.
We also briefly discuss other possible time-dependent behaviors of single BEC
condensates in restricted geometries,such as wave packet revivals.Comment: 8 pages, no figures, to appear in Physica Script
Classical trajectories in quantum transport at the band center of bipartite lattices with or without vacancies
Here we report on several anomalies in quantum transport at the band center
of a bipartite lattice with vacancies that are surely due to its chiral
symmetry, namely: no weak localization effect shows up, and, when leads have a
single channel the transmission is either one or zero. We propose that these
are a consequence of both the chiral symmetry and the large number of states at
the band center. The probability amplitude associated to the eigenstate that
gives unit transmission ressembles a classical trajectory both with or without
vacancies. The large number of states allows to build up trajectories that
elude the blocking vacancies explaining the absence of weak localization.Comment: 5 pages, 5 figure
Influence of diffraction on the spectrum and wavefunctions of an open system
In this paper, we demonstrate the existence and significance of diffractive
orbits in an open microwave billiard, both experimentally and theoretically.
Orbits that diffract off of a sharp edge of the system are found to have a
strong influence on the transmission spectrum of the system, especially in the
regime where there are no stable classical orbits. On resonance, the
wavefunctions are influenced by both classical and diffractive orbits. Off
resonance, the wavefunctions are determined by the constructive interference of
multiple transient, nonperiodic orbits. Experimental, numerical, and
semiclassical results are presented.Comment: 27 pages, 29 figures, and 3 tables. Submitted to Physical Review E. A
copy with higher resolution figures is available at
http://monsoon.harvard.edu/~hersch/papers.htm
Single-Atom Gating of Quantum State Superpositions
The ultimate miniaturization of electronic devices will likely require local
and coherent control of single electronic wavefunctions. Wavefunctions exist
within both physical real space and an abstract state space with a simple
geometric interpretation: this state space--or Hilbert space--is spanned by
mutually orthogonal state vectors corresponding to the quantized degrees of
freedom of the real-space system. Measurement of superpositions is akin to
accessing the direction of a vector in Hilbert space, determining an angle of
rotation equivalent to quantum phase. Here we show that an individual atom
inside a designed quantum corral can control this angle, producing arbitrary
coherent superpositions of spatial quantum states. Using scanning tunnelling
microscopy and nanostructures assembled atom-by-atom we demonstrate how single
spins and quantum mirages can be harnessed to image the superposition of two
electronic states. We also present a straightforward method to determine the
atom path enacting phase rotations between any desired state vectors. A single
atom thus becomes a real space handle for an abstract Hilbert space, providing
a simple technique for coherent quantum state manipulation at the spatial limit
of condensed matter.Comment: Published online 6 April 2008 in Nature Physics; 17 page manuscript
(including 4 figures) + 3 page supplement (including 2 figures);
supplementary movies available at http://mota.stanford.ed
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