122 research outputs found
Decay of nuclear hyperpolarization in silicon microparticles
We investigate the low-field relaxation of nuclear hyperpolarization in
undoped and highly doped silicon microparticles at room temperature following
removal from high field. For nominally undoped particles, two relaxation time
scales are identified for ambient fields above 0.2 mT. The slower, T_1s, is
roughly independent of ambient field; the faster, T_1f, decreases with
increasing ambient field. A model in which nuclear spin relaxation occurs at
the particle surface via a two-electron mechanism is shown to be in good
agreement with the experimental data, particularly the field-independence of
T_1s. For boron-doped particles, a single relaxation time scale is observed.
This suggests that for doped particles, mobile carriers and bulk ionized
acceptor sites, rather than paramagnetic surface states, are the dominant
relaxation mechanisms. Relaxation times for the undoped particles are not
affected by tumbling in a liquid solution.Comment: related papers at http://marcuslab.harvard.ed
Spin transport in mesoscopic rings with inhomogeneous spin-orbit coupling
We revisit the problem of electron transport through mesoscopic rings with
spin-orbit (SO) interaction. In the well-known path-integral approach, the
scattering states for a quasi-1D ring with quasi-1D leads can be expressed in
terms of spinless electrons subject to a fictitious magnetic flux. We show that
spin-dependent quantum-interference effects in small rings are strongest for
spatially inhomogeneous SO interactions, in which case spin currents can be
controlled by a small external magnetic field. Mesoscopic spin Hall effects in
four-terminal rings can also be understood in terms of the fictitious magnetic
flux.Comment: 5 pages, 2 figure
Spin Depolarization in Quantum Wires Polarized Spontaneously in a Zero Magnetic Field
The conditions for a spontaneous spin polarization in a quantum wire
positioned in a zero magnetic field are analyzed under weak population of
one-dimensional subbands that gives rise to the efficient quenching of the
kinetic energy by the exchange energy of carriers. The critical linear
concentration of carriers above which the quasi one-dimensional gas undergoes a
complete spin depolarization is determined by the Hartree-Fock approximation.
The dependence of the critical linear concentration on the concentration of
carriers is defined to reveal the interplay of the spin depolarization with the
evolution of the 0.7 (2e2/h) feature in the quantum conductance staircase from
the e2/h to 3/2 (e2/h) values. This dependence is used to study the effect of
the hole concentration on the 0.7 (2e2/h) feature in the quantum conductance
staircase of the quantum wire prepared inside the p-type silicon quantum well
using the split-gate technique. The 1D channel is demonstrated to be
spin-polarized at the linear concentration of holes lower than the critical
linear concentration, because the 0.7 (2e2/h) feature is close to the value of
0.5 (2e2/h) that indicates the spin degeneracy lifting for the first step of
the quantum conductance staircase. The 0.7 (2e2/h) feature is found to take
however its normal magnitude when the linear concentration of holes attains the
critical value corresponding to the spin depolarization. The variations in the
height of the 0.7 (2e2/h) feature observed in the hole quantum conductance
staircase that is revealed by the p-type silicon quantum wire seem to be
related to the evidences of the quantum conductance staircase obtained by
varying the concentration of electrons in the 1D channel prepared inside the
GaAs-AlGaAs heterojunction.Comment: 27 pages, 5 figure
Spin-dependent transport in p+-CdBxF2-x - n-CdF2 planar structures
The CV measurements and tunneling spectroscopy are used to study the
ballistic transport of the spin-polarized holes by varying the value of the
Rashba spin-orbit interaction (SOI) in the p-type quantum well prepared on the
surface of the n-CdF2 bulk crystal. The findings of the hole conductance
oscillations in the plane of the p-type quantum well that are due to the
variations of the Rashba SOI are shown to be evidence of the spin transistor
effect, with the amplitude of the oscillations close to e2/h.Comment: 5 pages, 6 figure
Superconductivity in Silicon Nanostructures
We present the findings of the superconductivity in the silicon
nanostructures prepared by short time diffusion of boron after preliminary
oxidation of the n-type Si (100) surface. These Si-based nanostructures
represent the p-type high mobility silicon quantum well (Si-QW) confined by the
delta - barriers heavily doped with boron. The ESR studies show that the delta
- barriers appear to consist of the trigonal dipole centers, B(+)-B(-), which
are caused by the negative-U reconstruction of the shallow boron acceptors,
2B(0)=>B(+)-B(-). The temperature and magnetic field dependencies of the
resistance, thermo-emf, specific heat and magnetic susceptibility demonstrate
that the high temperature superconductivity observed seems to result from the
transfer of the small hole bipolarons through these negative-U dipole centers
of boron at the Si-QW - delta - barrier interfaces. The value of the
superconductor energy gap obtained is in a good agreement with the data derived
from the oscillations of the conductance in normal state and of the
zero-resistance supercurrent in superconductor state as a function of the bias
voltage. These oscillations appear to be correlated by on- and off-resonance
tuning the two-dimensional subbands of holes with the Fermi energy in the
superconductor delta - barriers. Finally, the proximity effect in the S- Si-QW
-S structure is revealed by the findings of the multiple Andreev reflection
(MAR) processes and the quantization of the supercurrent
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