49 research outputs found
Decoherence in Nearly-Isolated Quantum Dots
Decoherence in nearly-isolated GaAs quantum dots is investigated using the
change in average Coulomb blockade peak height upon breaking time-reversal
symmetry. The normalized change in average peak height approaches the predicted
universal value of 1/4 at temperatures well below the single-particle level
spacing, but is greatly suppressed for temperature greater than the level
spacing, suggesting that inelastic scattering or other dephasing mechanisms
dominate in this regime.Comment: Significant revisions to include comparison to theory. Related papers
available at http://marcuslab.harvard.ed
Distributions of the Conductance and its Parametric Derivatives in Quantum Dots
Full distributions of conductance through quantum dots with single-mode leads
are reported for both broken and unbroken time-reversal symmetry. Distributions
are nongaussian and agree well with random matrix theory calculations that
account for a finite dephasing time, , once broadening due to finite
temperature is also included. Full distributions of the derivatives of
conductance with respect to gate voltage are also investigated.Comment: 4 pages (REVTeX), 4 eps figure
Spin Degeneracy and Conductance Fluctuations in Open Quantum Dots
The dependence of mesoscopic conductance fluctuations on parallel magnetic
field is used as a probe of spin degeneracy in open GaAs quantum dots. The
variance of fluctuations at high parallel field is reduced from the low-field
variance (with broken time-reversal symmetry) by factors ranging from roughly
two in a 1 square-micron dot at low temperature, to four or greater in 8
square-micron dots. The factor of two is expected for simple Zeeman splitting
of spin degenerate channels. A possible explanation for the unexpected larger
factors in terms of field-dependent spin orbit scattering is proposed.Comment: Includes new reference to related theoretical work, cond-mat/0010064.
Other minor changes. Related papers at http://marcuslab.harvard.ed
Weak localization in disordered systems at the ballistic limit
The weak localization (WL) contribution to the two-level correlation function
is calculated for two-dimensional disordered conductors. Our analysis extends
to the nondiffusive (ballistic) regime, where the elastic mean path is of order
of the size of the system. In this regime the structure factor (the Fourier
transform of the two-point correlator) exhibits a singular behavior consisting
of dips superimposed on a smooth positive background. The strongest dips appear
at periods of the periodic orbits of the underlying clean system. Somewhat
weaker singularities appear at times which are sums of periods of two such
orbits. The results elucidate various aspects of the weak localization physics
of ballistic chaotic systems.Comment: 13 pages, 13 figure
Quantum chaos in a deformable billiard: Applications to quantum dots
We perform a detailed numerical study of energy-level and wavefunction
statistics of a deformable quantum billiard focusing on properties relevant to
semiconductor quantum dots. We consider the family of Robnik billiards
generated by simple conformal maps of the unit disk; the shape of this family
of billiards may be varied continuously at fixed area by tuning the parameters
of the map. The classical dynamics of these billiards is well-understood and
this allows us to study the quantum properties of subfamilies which span the
transition from integrability to chaos as well as families at approximately
constant degree of chaoticity (Kolmogorov entropy). In the regime of hard chaos
we find that the statistical properties of interest are well-described by
random-matrix theory and completely insensitive to the particular shape of the
dot. However in the nearly-integrable regime non-universal behavior is found.
Specifically, the level-width distribution is well-described by the predicted
distribution both in the presence and absence of magnetic flux when
the system is fully chaotic; however it departs substantially from this
behavior in the mixed regime. The chaotic behavior corroborates the previously
predicted behavior of the peak-height distribution for deformed quantum dots.
We also investigate the energy-level correlation functions which are found to
agree well with the behavior calculated for quasi-zero-dimensional disordered
systems.Comment: 25 pages (revtex 3.0). 16 figures are available by mail or fax upon
request at [email protected]
Statistics of Coulomb Blockade Peak Spacings
Distributions of Coulomb blockade peak spacing are reported for large
ensembles of both unbroken (magnetic field B = 0) and broken (B 0) time
reversal symmetry in GaAs quantum dots. Both distributions are symmetric and
roughly gaussian with a width ~ 2-6% of the average spacing, with broad,
non-gaussian tails. The distribution is systematically wider at B = 0 by a
factor of ~ 1.2 +- 0.1. No even-odd spacing correlations or bimodal structure
in the spacing distribution is found, suggesting an absence of spin-degeneracy.
There is no observed correlation between peak spacing and peak height.Comment: To appear in PRL; 13 pages, one table, 3 figures; pdf available at
http://www-leland.stanford.edu/group/MarcusLab/papers/Patel_peakspacing.pd
Random-Matrix Theory of Quantum Transport
This is a comprehensive review of the random-matrix approach to the theory of
phase-coherent conduction in mesocopic systems. The theory is applied to a
variety of physical phenomena in quantum dots and disordered wires, including
universal conductance fluctuations, weak localization, Coulomb blockade,
sub-Poissonian shot noise, reflectionless tunneling into a superconductor, and
giant conductance oscillations in a Josephson junction.Comment: 85 pages including 52 figures, to be published in Rev.Mod.Phy
Tgf-β1/cd105 signaling controls vascular network formation within growth factor sequestering hyaluronic acid hydrogels
Cell-based strategies for the treatment of ischemic diseases are at the forefront of tissue engineering and regenerative medicine. Cell therapies purportedly can play a key role in the neovascularization of ischemic tissue; however, low survival and poor cell engraftment with the host vasculature following implantation limits their potential to treat ischemic diseases. To overcome these limitations, we previously developed a growth factor sequestering hyaluronic acid (HyA)-based hydrogel that enhanced transplanted mouse cardiosphere-derived cell survival and formation of vasculature that anastomosed with host vessels. In this work, we examined the mechanism by which HyA hydrogels presenting transforming growth factor beta-1 (TGF-beta 1) promoted proliferation of more clinically relevant human cardiospherederived cells (hCDC), and their formation of vascular-like networks in vitro. We observed hCDC proliferation and enhanced formation of vascular-like networks occurred in the presence of TGF-beta 1. Furthermore, production of nitric oxide (NO), VEGF, and a host of angiogenic factors were increased in the presence of TGF-beta 1. This response was dependent on the co-activity of CD105 (Endoglin) with the TGF-beta R2 receptor, demonstrating its role in the process of angiogenic differentiation and vascular organization of hCDC. These results demonstrated that hCDC form vascular-like networks in vitro, and that the induction of vascular networks by hCDC within growth factor sequestering HyA hydrogels was mediated by TGF-beta 1/CD105 signaling