6,506 research outputs found
Comment on "Clock Shift in High Field Magnetic Resonance of Atomic Hydrogen"
In this Comment, we reanalyze the experiments on the collision frequency
shift of the b-c and a-d hyperfine transitions in three-dimensional atomic
hydrogen in the presence of, respectively, a and b-state atoms. Accurate
consideration of the symmetry of the spatial and spin part of the diatomic
wavefunction yields the difference a_T-a_S=0.30(5) \AA between the triplet and
singlet s-wave scattering lengths of hydrogen atoms. This corrects the
factor-of two error of the commented work [Phys. Rev. Lett. 101, 263003
(2008)].Comment: 1 pag
Metallic and insulating behaviour of the two-dimensional electron gas on a vicinal surface of Si MOSFETs
The resistance R of the 2DEG on the vicinal Si surface shows an unusual
behaviour, which is very different from that in the (100) Si MOSFET where an
unconventional metal to insulator transition has been reported. The crossover
from the insulator with dR/dT0 occurs at a low
resistance of R_{\Box}^c \sim 0.04h/e^2. At the low-temperature transition,
which we attribute to the existence of a narrow impurity band at the interface,
a distinct hysteresis in the resistance is detected. At higher temperatures,
another change in the sign of dR/dT is seen and related to the crossover from
the degenerate to non-degenerate 2DEG.Comment: 4 pages, 4 figure
Thermal compression of two-dimensional atomic hydrogen to quantum degeneracy
We describe experiments where 2D atomic hydrogen gas is compressed thermally
at a small "cold spot" on the surface of superfluid helium and detected
directly with electron-spin resonance. We reach surface densities up to 5e12
1/cm^2 at temperatures of approximately 100 mK corresponding to the maximum 2D
phase-space density of about 1.5. By independent measurements of the surface
density and its decay rate we make the first direct determination of the
three-body recombination rate constant and get the value of 2e-25 cm^4/s for
its upper bound, which is an order of magnitude smaller than previously
reported experimental results.Comment: 4 pages, 4 postscript figures, bibliography (.bbl) file, submitted to
PR
Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain
The interaction of coherent magnetization rotation with a system of two-level
impurities is studied. Two different, but not contradictory mechanisms, the
`slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system
of integro-differential equations for the magnetization. In the case that the
impurity relaxation rate is much greater than the magnetization precession
frequency, these equations can be written in the form of the Landau-Lifshitz
equation with damping. Thus the damping parameter can be directly calculated
from these microscopic impurity relaxation processes
Hyperfine frequency shift in two-dimensional atomic hydrogen
We propose the explanation of a surprisingly small hyperfine frequency shift
in the two-dimensional (2D) atomic hydrogen bound to the surface of superfluid
helium below 0.1 K. Owing to the symmetry considerations, the microwave-induced
triplet-singlet transitions of atomic pairs in the fully spin-polarized sample
are forbidden. The apparent nonzero shift is associated with the
density-dependent wall shift of the hyperfine constant and the pressure shift
due to the presence of H atoms in the hyperfine state not involved in the
observed transition. The interaction of adsorbed atoms with one
another effectively decreases the binding energy and, consequently, the wall
shift by the amount proportional to their density. The pressure shift of the
resonance comes from the fact that the impurity -state atoms
interact differently with the initial -state and final -state atoms and
is also linear in density. The net effect of the two contributions, both
specific for 2D hydrogen, is comparable with the experimental observation. To
our knowledge, this is the first mentioning of the density-dependent wall
shift. We also show that the difference between the triplet and singlet
scattering lengths of H atoms, pm, is exactly twice smaller
than the value reported by Ahokas {\it et al.}, Phys. Rev. Lett. {\bf101},
263003 (2008).Comment: 4 pages, no figure
Shot Noise in Mesoscopic Transport Through Localised States
We show that shot noise can be used for studies of hopping and resonant
tunnelling between localised electron states. In hopping via several states,
shot noise is seen to be suppressed compared with its classical Poisson value
( is the average current) and the suppression depends on the
distribution of the barriers between the localised states. In resonant
tunnelling through a single impurity an enhancement of shot noise is observed.
It has been established, both theoretically and experimentally, that a
considerable increase of noise occurs due to Coulomb interaction between two
resonant tunnelling channels.Comment: 7 pages, 5 figures; Proceedings of the 10th Conference on Hopping and
Related Phenomena (Trieste 2003); requires Wiley style files (included
Adsorption and two-body recombination of atomic hydrogen on He-He mixture films
We present the first systematic measurement of the binding energy of
hydrogen atoms to the surface of saturated He-He mixture films.
is found to decrease almost linearly from 1.14(1) K down to 0.39(1) K, when the
population of the ground surface state of He grows from zero to
cm, yielding the value K cm
for the mean-field parameter of H-He interaction in 2D. The experiments
were carried out with overall He concentrations ranging from 0.1 ppm to 5 %
as well as with commercial and isotopically purified He at temperatures
70...400 mK. Measuring by ESR the rate constants and for
second-order recombination of hydrogen atoms in hyperfine states and we
find the ratio to be independent of the He content and to
grow with temperature.Comment: 4 pages, 4 figures, all zipped in a sigle file. Submitted to Phys.
Rev. Let
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