1,138 research outputs found
Hyperfine Interactions and Spin Transport in Ferromagnet-Semiconductor Heterostructures
Measurements and modeling of electron spin transport and dynamics are used to
characterize hyperfine interactions in Fe/GaAs devices with -GaAs channels.
Ga and As nuclei are polarized by electrically injected electron spins, and the
nuclear polarization is detected indirectly through the depolarization of
electron spins in the hyperfine field. The dependence of the electron spin
signal on injector bias and applied field direction is modeled by a coupled
drift-diffusion equation, including effective fields from both the electronic
and nuclear polarizations. This approach is used to determine the electron spin
polarization independently of the assumptions made in standard transport
measurements. The extreme sensitivity of the electron spin dynamics to the
nuclear spin polarization also facilitates the electrical detection of nuclear
magnetic resonance.Comment: Submitted to Phys. Rev.
Homogeneous links, Seifert surfaces, digraphs and the reduced Alexander polynomial
We give a geometric proof of the following result of Juhasz. \emph{Let
be the leading coefficient of the Alexander polynomial of an alternating knot
. If then has a unique minimal genus Seifert surface.} In
doing so, we are able to generalise the result, replacing `minimal genus' with
`incompressible' and `alternating' with `homogeneous'. We also examine the
implications of our proof for alternating links in general.Comment: 37 pages, 28 figures; v2 Main results generalised from alternating
links to homogeneous links. Title change
Onset of Superfluidity in 4He Films Adsorbed on Disordered Substrates
We have studied 4He films adsorbed in two porous glasses, aerogel and Vycor,
using high precision torsional oscillator and DC calorimetry techniques. Our
investigation focused on the onset of superfluidity at low temperatures as the
4He coverage is increased. Torsional oscillator measurements of the 4He-aerogel
system were used to determine the superfluid density of films with transition
temperatures as low as 20 mK. Heat capacity measurements of the 4He-Vycor
system probed the excitation spectrum of both non-superfluid and superfluid
films for temperatures down to 10 mK. Both sets of measurements suggest that
the critical coverage for the onset of superfluidity corresponds to a mobility
edge in the chemical potential, so that the onset transition is the bosonic
analog of a superconductor-insulator transition. The superfluid density
measurements, however, are not in agreement with the scaling theory of an onset
transition from a gapless, Bose glass phase to a superfluid. The heat capacity
measurements show that the non-superfluid phase is better characterized as an
insulator with a gap.Comment: 15 pages (RevTex), 21 figures (postscript
Images and nonlocal vortex pinning in thin superfluid films
For thin films of superfluid adsorbed on a disordered substrate, we derive
the equation of motion for a vortex in the presence of a random potential
within a mean field (Hartree) description of the condensate. The compressible
nature of the condensate leads to an effective pinning potential experienced by
the vortex which is nonlocal, with a long range tail that smoothes out the
random potential coupling the condensate to the substrate. We interpret this
nonlocality in terms of images, and relate the effective potential governing
the dynamics to the pinning energy arising from the expectation value of the
Hamiltonian with respect to the vortex wavefunction.Comment: 19 pages, revtex, to appear Phys. Rev.
Electron Spin Dynamics and Hyperfine Interactions in Fe/Al_0.1Ga_0.9As/GaAs Spin Injection Heterostructures
We have studied hyperfine interactions between spin-polarized electrons and
lattice nuclei in Al_0.1Ga_0.9As/GaAs quantum well (QW) heterostructures. The
spin-polarized electrons are electrically injected into the semiconductor
heterostructure from a metallic ferromagnet across a Schottky tunnel barrier.
The spin-polarized electron current dynamically polarizes the nuclei in the QW,
and the polarized nuclei in turn alter the electron spin dynamics. The
steady-state electron spin is detected via the circular polarization of the
emitted electroluminescence. The nuclear polarization and electron spin
dynamics are accurately modeled using the formalism of optical orientation in
GaAs. The nuclear spin polarization in the QW is found to depend strongly on
the electron spin polarization in the QW, but only weakly on the electron
density in the QW. We are able to observe nuclear magnetic resonance (NMR) at
low applied magnetic fields on the order of a few hundred Oe by electrically
modulating the spin injected into the QW. The electrically driven NMR
demonstrates explicitly the existence of a Knight field felt by the nuclei due
to the electron spin.Comment: 19 Figures - submitted to PR
Measurement of the half-life of the T= mirror decay of Ne and its implication on physics beyond the standard model
The superallowed mixed mirror decay
of Ne to F is excellently suited for high precision studies of
the weak interaction. However, there is some disagreement on the value of the
half-life. In a new measurement we have determined this quantity to be
= s, which differs
from the previous world average by 3 standard deviations. The impact of this
measurement on limits for physics beyond the standard model such as the
presence of tensor currents is discussed.Comment: 5 pages, 3 figures, 1 tabl
Phase diagram of a Disordered Boson Hubbard Model in Two Dimensions
We study the zero-temperature phase transition of a two-dimensional
disordered boson Hubbard model. The phase diagram of this model is constructed
in terms of the disorder strength and the chemical potential. Via quantum Monte
Carlo simulations, we find a multicritical line separating the weak-disorder
regime, where a random potential is irrelevant, from the strong-disorder
regime. In the weak-disorder regime, the Mott-insulator-to-superfluid
transition occurs, while, in the strong-disorder regime, the
Bose-glass-to-superfluid transition occurs. On the multicritical line, the
insulator-to-superfluid transition has the dynamical critical exponent and the correlation length critical exponent ,
that are different from the values for the transitions off the line. We suggest
that the proliferation of the particle-hole pairs screens out the weak disorder
effects.Comment: 4 pages, 4 figures, to be published in PR
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