6,991 research outputs found
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
Cold Collision Frequency Shift in Two-Dimensional Atomic Hydrogen
We report a measurement of the cold collision frequency shift in atomic
hydrogen gas adsorbed on the surface of superfluid 4He at T<=90 mK. Using
two-photon electron and nuclear magnetic resonance in 4.6 T field we separate
the resonance line shifts due to the dipolar and exchange interactions, both
proportional to surface density sigma. We find the clock shift Delta v_c =
-1.0(1)x10^-7 Hz cm^-2 x sigma, which is about 100 times smaller than the value
predicted by the mean field theory and known scattering lengths in the 3D case.Comment: 4 pages, 3 figure
Bound states of three and four resonantly interacting particles
We present an exact diagrammatic approach for the problem of dimer-dimer
scattering in 3D for dimers being a resonant bound state of two fermions in a
spin-singlet state, with corresponding scattering length . Applying this
approach to the calculation of the dimer-dimer scattering length , we
recover exactly the already known result . We use the developed
approach to obtain new results in 2D for fermions as well as for bosons.
Namely, we calculate bound state energies for three and four
resonantly interacting bosons in 2D. For the case of resonant interaction
between fermions and bosons we calculate exactly bound state energies of the
following complexes: two bosons plus one fermion , two bosons plus two
fermions , and three bosons plus one fermion
.Comment: 10 pages, 9 figure
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
Resistivity and 1/f Noise in Non-Metallic Phase Separated Manganites
A simple model is proposed to calculate resistivity, magnetoresistance, and
noise spectrum in non-metallic phase-separated manganites containing small
metallic droplets (magnetic polarons). The system is taken to be far from the
percolation transition into a metallic state. It is assumed that the charge
transfer occurs due to electron tunneling from one droplet to another through
the insulating medium. As a result of this tunneling, the droplets acquire or
lose extra electrons forming metastable two-electron and empty states. In the
framework of this model, explicit expressions for dc conductivity and noise
power of the system are derived. It is shown that the noise spectrum has 1/f
form in the low-frequency range.Comment: 6 pages, 1 fugure include
Interaction of surface acoustic waves with a two-dimensional electron gas in the presence of spin splitting of the Landau bands
The absorption and variation of the velocity of a surface acoustic wave of
frequency = 30 MHz interacting with two-dimensional electrons are
investigated in GaAs/AlGaAs heterostructures with an electron density at =1.5 - 4.2 K in magnetic fields up to 7 T.
Characteristic features associated with spin splitting of the Landau level are
observed. The effective g factor and the width of the spin-split Landau bands
are determined: and =0.6 meV. The greater width of the
orbital-split Landau bands (2 meV) relative to the spin-split bands is
attributed to different shielding of the random fluctuation potential of
charged impurities by 2D electrons. The mechanisms of the nonlinearities
manifested in the dependence of the absorption and the velocity increment of
the SAW on the SAW power in the presence of spin splitting of the Landau levels
are investigated.Comment: Revtex 5 pages + 5 EPS Figures, v.2 - minor corrections in text and
pic
Decoherence due to three-body loss and its effect on the state of a Bose-Einstein condensate
A Born-Markov master equation is used to investigate the decoherence of the
state of a macroscopically occupied mode of a cold atom trap due to three-body
loss. In the large number limit only coherent states remain pure for times
longer than the decoherence time: the time it takes for just three atoms to be
lost from the trap. For large numbers of atoms (N>10^4) the decoherence time is
found to be much faster than the phase collapse time caused by intra-trap
atomic collisions
A semi-classical field method for the equilibrium Bose gas and application to thermal vortices in two dimensions
We develop a semi-classical field method for the study of the weakly
interacting Bose gas at finite temperature, which, contrarily to the usual
classical field model, does not suffer from an ultraviolet cut-off dependence.
We apply the method to the study of thermal vortices in spatially homogeneous,
two-dimensional systems. We present numerical results for the vortex density
and the vortex pair distribution function. Insight in the physics of the system
is obtained by comparing the numerical results with the predictions of simple
analytical models. In particular, we calculate the activation energy required
to form a vortex pair at low temperature.Comment: 19 page
Thermalization of an impurity cloud in a Bose-Einstein condensate
We study the thermalization dynamics of an impurity cloud inside a
Bose-Einstein condensate at finite temperature, introducing a suitable
Boltzmann equation. Some values of the temperature and of the initial impurity
energy are considered. We find that, below the Landau critical velocity, the
macroscopic population of the initial impurity state reduces its depletion
rate. For sufficiently high velocities the opposite effect occurs. For
appropriate parameters the collisions cool the condensate. The maximum cooling
per impurity atom is obtained with multiple collisions.Comment: 4 pages 6 figure
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