3,460 research outputs found
First-principles study of the Young's modulus of Si <001> nanowires
We report the results of first-principles density functional theory
calculations of the Young's modulus and other mechanical properties of
hydrogen-passivated Si nanowires. The nanowires are taken to have
predominantly {100} surfaces, with small {110} facets. The Young's modulus, the
equilibrium length and the residual stress of a series of prismatic wires are
found to have a size dependence that scales like the surface area to volume
ratio for all but the smallest wires. We analyze the physical origin of the
size dependence, and compare the results to two existing models.Comment: 5 pages, 3 figure
Hole Spin Coherence in a Ge/Si Heterostructure Nanowire
Relaxation and dephasing of hole spins are measured in a gate-defined Ge/Si
nanowire double quantum dot using a fast pulsed-gate method and dispersive
readout. An inhomogeneous dephasing time
exceeds corresponding measurements in III-V semiconductors by more than an
order of magnitude, as expected for predominately nuclear-spin-free materials.
Dephasing is observed to be exponential in time, indicating the presence of a
broadband noise source, rather than Gaussian, previously seen in systems with
nuclear-spin-dominated dephasing.Comment: 15 pages, 4 figure
Antilocalization of Coulomb Blockade in a Ge-Si Nanowire
The distribution of Coulomb blockade peak heights as a function of magnetic
field is investigated experimentally in a Ge-Si nanowire quantum dot. Strong
spin-orbit coupling in this hole-gas system leads to antilocalization of
Coulomb blockade peaks, consistent with theory. In particular, the peak height
distribution has its maximum away from zero at zero magnetic field, with an
average that decreases with increasing field. Magnetoconductance in the
open-wire regime places a bound on the spin-orbit length ( < 20 nm),
consistent with values extracted in the Coulomb blockade regime ( < 25
nm).Comment: Supplementary Information available at http://bit.ly/19pMpd
Statistical inference of the generation probability of T-cell receptors from sequence repertoires
Stochastic rearrangement of germline DNA by VDJ recombination is at the
origin of immune system diversity. This process is implemented via a series of
stochastic molecular events involving gene choices and random nucleotide
insertions between, and deletions from, genes. We use large sequence
repertoires of the variable CDR3 region of human CD4+ T-cell receptor beta
chains to infer the statistical properties of these basic biochemical events.
Since any given CDR3 sequence can be produced in multiple ways, the probability
distribution of hidden recombination events cannot be inferred directly from
the observed sequences; we therefore develop a maximum likelihood inference
method to achieve this end. To separate the properties of the molecular
rearrangement mechanism from the effects of selection, we focus on
non-productive CDR3 sequences in T-cell DNA. We infer the joint distribution of
the various generative events that occur when a new T-cell receptor gene is
created. We find a rich picture of correlation (and absence thereof), providing
insight into the molecular mechanisms involved. The generative event statistics
are consistent between individuals, suggesting a universal biochemical process.
Our distribution predicts the generation probability of any specific CDR3
sequence by the primitive recombination process, allowing us to quantify the
potential diversity of the T-cell repertoire and to understand why some
sequences are shared between individuals. We argue that the use of formal
statistical inference methods, of the kind presented in this paper, will be
essential for quantitative understanding of the generation and evolution of
diversity in the adaptive immune system.Comment: 20 pages, including Appendi
Atomic layer deposition of ZnS nanotubes
We report on growth of high-aspect-ratio () zinc sulfide
nanotubes with variable, precisely tunable, wall thicknesses and tube diameters
into highly ordered pores of anodic alumina templates by atomic layer
deposition (ALD) at temperatures as low as 75 C. Various
characterization techniques are employed to gain information on the
composition, morphology, and crystal structure of the synthesized samples.
Besides practical applications, the ALD-grown tubes could be envisaged as model
systems for the study of a certain class of size-dependent quantum and
classical phenomena.Comment: 1 LaTeX source file, 8 eps figures, and the manuscript in PDF forma
Magnetoconductance oscillations in quasiballistic multimode nanowires
We calculate the conductance of quasi-one-dimensional nanowires with
electronic states confined to a surface charge layer, in the presence of a
uniform magnetic field. Two-terminal magnetoconductance (MC) between two leads
deposited on the nanowire via tunnel barriers is dominated by density-of-states
(DOS) singularities, when the leads are well apart. There is also a mesoscopic
correction due to a higher-order coherent tunneling between the leads for small
lead separation. The corresponding MC structure depends on the interference
between electron propagation via different channels connecting the leads, which
in the simplest case, for the magnetic field along the wire axis, can be
crudely characterized by relative winding numbers of paths enclosing the
magnetic flux. In general, the MC oscillations are aperiodic, due to the Zeeman
splitting, field misalignment with the wire axis, and a finite extent of
electron distribution across the wire cross section, and are affected by
spin-orbit coupling. The quantum-interference MC traces contain a wealth of
information about the electronic structure of multichannel wires, which would
be complimentary to the DOS measurements. We propose a four-terminal
configuration to enhance the relative contribution of the higher-order
tunneling processes and apply our results to realistic InAs nanowires carrying
several quantum channels in the surface charge-accumulation layer.Comment: 11 pages, 8 figure
Imaging a 1-electron InAs quantum dot in an InAs/InP nanowire
Nanowire heterostructures define high-quality few-electron quantum dots for
nanoelectronics, spintronics and quantum information processing. We use a
cooled scanning probe microscope (SPM) to image and control an InAs quantum dot
in an InAs/InP nanowire, using the tip as a movable gate. Images of dot
conductance vs. tip position at T = 4.2 K show concentric rings as electrons
are added, starting with the first electron. The SPM can locate a dot along a
nanowire and individually tune its charge, abilities that will be very useful
for the control of coupled nanowire dots
First-principles calculation of mechanical properties of Si <001> nanowires and comparison to nanomechanical theory
We report the results of first-principles density functional theory
calculations of the Young's modulus and other mechanical properties of
hydrogen-passivated Si nanowires. The nanowires are taken to have
predominantly {100} surfaces, with small {110} facets according to the Wulff
shape. The Young's modulus, the equilibrium length and the constrained residual
stress of a series of prismatic beams of differing sizes are found to have size
dependences that scale like the surface area to volume ratio for all but the
smallest beam. The results are compared with a continuum model and the results
of classical atomistic calculations based on an empirical potential. We
attribute the size dependence to specific physical structures and interactions.
In particular, the hydrogen interactions on the surface and the charge density
variations within the beam are quantified and used both to parameterize the
continuum model and to account for the discrepancies between the two models and
the first-principles results.Comment: 14 pages, 10 figure
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