647 research outputs found
Chaos in Quantum Dots: Dynamical Modulation of Coulomb Blockade Peak Heights
The electrostatic energy of an additional electron on a conducting grain
blocks the flow of current through the grain, an effect known as the Coulomb
blockade. Current can flow only if two charge states of the grain have the same
energy; in this case the conductance has a peak. In a small grain with
quantized electron states, referred to as a quantum dot, the magnitude of the
conductance peak is directly related to the magnitude of the wavefunction near
the contacts to the dot. Since dots are generally irregular in shape, the
dynamics of the electrons is chaotic, and the characteristics of Coulomb
blockade peaks reflects those of wavefunctions in chaotic systems. Previously,
a statistical theory for the peaks was derived by assuming these wavefunctions
to be completely random. Here we show that the specific internal dynamics of
the dot, even though it is chaotic, modulates the peaks: because all systems
have short-time features, chaos is not equivalent to randomness. Semiclassical
results are derived for both chaotic and integrable dots, which are
surprisingly similar, and compared to numerical calculations. We argue that
this modulation, though unappreciated, has already been seen in experiments.Comment: 4 pages, 3 postscript figs included (2 color), uses epsf.st
Targeting the choroid plexus-CSF-brain nexus using peptides identified by phage display.
Drug delivery to the central nervous system requires the use of specific portals to enable drug entry into the brain and, as such, there is a growing need to identify processes that can enable drug transfer across both blood-brain and blood-cerebrospinal fluid barriers. Phage display is a powerful combinatorial technique that identifies specific peptides that can confer new activities to inactive particles. Identification of these peptides is directly dependent on the specific screening strategies used for their selection and retrieval. This chapter describes three selection strategies, which can be used to identify peptides that target the choroid plexus (CP) directly or for drug translocation across the CP and into cerebrospinal fluid
Targeting choroid plexus epithelia and ventricular ependyma for drug delivery to the central nervous system
Background: Because the choroid plexus (CP) is uniquely suited to control the composition of cerebrospinal fluid (CSF), there may be therapeutic benefits to increasing the levels of biologically active proteins in CSF to modulate central nervous system (CNS) functions. To this end, we sought to identify peptides capable of ligand-mediated targeting to CP epithelial cells reasoning that they could be exploited to deliver drugs, biotherapeutics and genes to the CNS.Methods: A peptide library displayed on M13 bacteriophage was screened for ligands capable of internalizing into CP epithelial cells by incubating phage with CP explants for 2 hours at 37C and recovering particles with targeting capacity.Results: Three peptides, identified after four rounds of screening, were analyzed for specific and dose dependant binding and internalization. Binding was deemed specific because internalization was prevented by co-incubation with cognate synthetic peptides. Furthermore, after i.c.v. injection into rat brains, each peptide was found to target phage to epithelial cells in CP and to ependyma lining the ventricles.Conclusion: These data demonstrate that ligand-mediated targeting can be used as a strategy for drug delivery to the central nervous system and opens the possibility of using the choroid plexus as a portal of entry into the brain
Wigner Crystallization in a Quasi-3D Electronic System
When a strong magnetic field is applied perpendicularly (along z) to a sheet
confining electrons to two dimensions (x-y), highly correlated states emerge as
a result of the interplay between electron-electron interactions, confinement
and disorder. These so-called fractional quantum Hall (FQH) liquids form a
series of states which ultimately give way to a periodic electron solid that
crystallizes at high magnetic fields. This quantum phase of electrons has been
identified previously as a disorder-pinned two-dimensional Wigner crystal with
broken translational symmetry in the x-y plane. Here, we report our discovery
of a new insulating quantum phase of electrons when a very high magnetic field,
up to 45T, is applied in a geometry parallel (y-direction) to the
two-dimensional electron sheet. Our data point towards this new quantum phase
being an electron solid in a "quasi-3D" configuration induced by orbital
coupling with the parallel field
Towards an Explanation of the Mesoscopic Double-Slit Experiment: a new model for charging of a Quantum Dot
For a quantum dot (QD) in the intermediate regime between integrable and
fully chaotic, the widths of single-particle levels naturally differ by orders
of magnitude. In particular, the width of one strongly coupled level may be
larger than the spacing between other, very narrow, levels. In this case many
consecutive Coulomb blockade peaks are due to occupation of the same broad
level. Between the peaks the electron jumps from this level to one of the
narrow levels and the transmission through the dot at the next resonance
essentially repeats that at the previous one. This offers a natural explanation
to the recently observed behavior of the transmission phase in an
interferometer with a QD.Comment: 4 pages, 2 figures, Journal versio
Semiclassical Theory of Coulomb Blockade Peak Heights in Chaotic Quantum Dots
We develop a semiclassical theory of Coulomb blockade peak heights in chaotic
quantum dots. Using Berry's conjecture, we calculate the peak height
distributions and the correlation functions. We demonstrate that the
corrections to the corresponding results of the standard statistical theory are
non-universal and can be expressed in terms of the classical periodic orbits of
the dot that are well coupled to the leads. The main effect is an oscillatory
dependence of the peak heights on any parameter which is varied; it is
substantial for both symmetric and asymmetric lead placement. Surprisingly,
these dynamical effects do not influence the full distribution of peak heights,
but are clearly seen in the correlation function or power spectrum. For
non-zero temperature, the correlation function obtained theoretically is in
good agreement with that measured experimentally.Comment: 5 color eps figure
The Addition Spectrum and Koopmans' Theorem for Disordered Quantum Dots
We investigate the addition spectrum of disordered quantum dots containing
spinless interacting fermions using the self-consistent Hartree-Fock
approximation. We concentrate on the regime r_s >~1, with finite dimensionless
conductance g. We find that in this approximation the peak spacing fluctuations
do not scale with the mean single particle level spacing for either Coulomb or
nearest neighbour interactions when r_s >~1. We also show that Koopmans'
approximation to the addition spectrum can lead to errors that are of order the
mean level spacing or larger, both in the mean addition spectrum peak spacings,
and in the peak spacing fluctuations.Comment: 35 pages including 22 figures (eps
Density Modulations and Addition Spectra of Interacting Electrons in Disordered Quantum Dots
We analyse the ground state of spinless fermions on a lattice in a weakly
disordered potential, interacting via a nearest neighbour interaction, by
applying the self-consistent Hartree-Fock approximation. We find that charge
density modulations emerge progressively when r_s >1, even away from
half-filling, with only short-range density correlations. Classical geometry
dependent "magic numbers" can show up in the addition spectrum which are
remarkably robust against quantum fluctuations and disorder averaging.Comment: 4 pages, 3 eps figure
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