19,496 research outputs found
Tetragonal states from epitaxial strain on metal films
The tetragonal states produced by isotropic pseudomorphic epitaxial strain in
the (001) plane on a tetragonal phase of a crystal are calculated for V, Ti,
Rb, Li, K, Sr from first-principles electronic theory. It is shown that each
metal has two tetragonal phases corresponding to minima of the total energy
with respect to tetragonal deformations, hence are equilibrium phases, and that
the equilibrium phases are separated by a region of inherent instability. The
equilibrium phase for any strained tetragonal state can thus be uniquely
identified. Lattice constants and relative energies of the two phases and the
saddle point between them are tabulated, as well as the tetragonal elastic
constants of each phase.Comment: 6 pages, 4 figures, appeared in Phys. Rev. B 57, 1971 (1998). Other
related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
Fast Single-Charge Sensing with an rf Quantum Point Contact
We report high-bandwidth charge sensing measurements using a GaAs quantum
point contact embedded in a radio frequency impedance matching circuit
(rf-QPC). With the rf-QPC biased near pinch-off where it is most sensitive to
charge, we demonstrate a conductance sensitivity of 5x10^(-6) e^(2)/h Hz^(-1/2)
with a bandwidth of 8 MHz. Single-shot readout of a proximal few-electron
double quantum dot is investigated in a mode where the rf-QPC back-action is
rapidly switched.Comment: related papers available at http://marcuslab.harvard.ed
Bayesian DNA copy number analysis
BACKGROUND: Some diseases, like tumors, can be related to chromosomal aberrations, leading to
changes of DNA copy number. The copy number of an aberrant genome can be represented as a
piecewise constant function, since it can exhibit regions of deletions or gains. Instead, in a healthy
cell the copy number is two because we inherit one copy of each chromosome from each our
parents.
Bayesian Piecewise Constant Regression (BPCR) is a Bayesian regression method for data that are
noisy observations of a piecewise constant function. The method estimates the unknown segment
number, the endpoints of the segments and the value of the segment levels of the underlying
piecewise constant function. The Bayesian Regression Curve (BRC) estimates the same data with
a smoothing curve. However, in the original formulation, some estimators failed to properly
determine the corresponding parameters. For example, the boundary estimator did not take into
account the dependency among the boundaries and succeeded in estimating more than one
breakpoint at the same position, losing segments.
RESULTS: We derived an improved version of the BPCR (called mBPCR) and BRC, changing the
segment number estimator and the boundary estimator to enhance the fitting procedure. We also
proposed an alternative estimator of the variance of the segment levels, which is useful in case of
data with high noise. Using artificial data, we compared the original and the modified version of
BPCR and BRC with other regression methods, showing that our improved version of BPCR
generally outperformed all the others. Similar results were also observed on real data.
CONCLUSION: We propose an improved method for DNA copy number estimation, mBPCR, which
performed very well compared to previously published algorithms. In particular, mBPCR was more
powerful in the detection of the true position of the breakpoints and of small aberrations in very
noisy data. Hence, from a biological point of view, our method can be very useful, for example, to
find targets of genomic aberrations in clinical cancer samples
Coulomb-Modified Fano Resonance in a One-Lead Quantum Dot
We investigate a tunable Fano interferometer consisting of a quantum dot
coupled via tunneling to a one-dimensional channel. In addition to Fano
resonance, the channel shows strong Coulomb response to the dot, with a single
electron modulating channel conductance by factors of up to 100. Where these
effects coexist, lineshapes with up to four extrema are found. A model of
Coulomb-modified Fano resonance is developed and gives excellent agreement with
experiment.Comment: related papers available at http://marcuslab.harvard.ed
Effective g-factor in Majorana Wires
We use the effective g-factor of subgap states, g*, in hybrid InAs nanowires
with an epitaxial Al shell to investigate how the superconducting density of
states is distributed between the semiconductor core and the metallic shell. We
find a step-like reduction of g* and improved hard gap with reduced carrier
density in the nanowire, controlled by gate voltage. These observations are
relevant for Majorana devices, which require tunable carrier density and g*
exceeding the g-factor of the proximitizing superconductor. Additionally, we
observe the closing and reopening of a gap in the subgap spectrum coincident
with the appearance of a zero-bias conductance peak
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