203 research outputs found
Spin waves in the B-phase of superfluid ³He in confined cylindrical geometry
We describe experiments on superfluid ³He in a cylinder of 1 mm in diameter. This geometry causes the preferred
orientation of the n-vector in the superfluid B-phase to be locally different, resulting in a curved configuration
across the sample. Exclusive to our experiment is the observation that we succeeded in obtaining a texture
which is metastable and unchanged in our pressure and temperature ranges, most likely because the experiment
is performed at low pressures and low magnetic fields. As this texture can be considered as a potential for spin
waves, we had the unique opportunity to study spin waves for several pressures in exactly the same texture. Our
geometry causes this texture potential to be nearly quadratic, allowing an analytic solution of the theory which
can be compared to our experimental results. As predicted we find the intensities of all spin wave modes more or
less equal. Increasing the pressure shows a gradual increase in the number of spin wave modes in our cell. Finally
we were able to cause a transition from the metastable to the predicted stable texture, concluding unexpectedly
that the metastable texture is realized if the growing (or cooling) speed is sufficiently slow
Space Charge Limited Transport and Time of Flight Measurements in Tetracene Single Crystals: a Comparative Study
We report on a systematic study of electronic transport in tetracene single
crystals by means of space charge limited current spectroscopy and time of
flight measurements. Both - and time of flight measurements show that the
room-temperature effective hole-mobility reaches values close to
cm/Vs and show that, within a range of temperatures, the mobility increases
with decreasing temperature. The experimental results further allow the
characterization of different aspects of the tetracene crystals. In particular,
the effects of both deep and shallow traps are clearly visible and can be used
to estimate their densities and characteristic energies. The results presented
in this paper show that the combination of - measurements and time of
flight spectroscopy is very effective in characterizing several different
aspects of electronic transport through organic crystals.Comment: Accepted by J. Appl. Phys.; tentatively scheduled for publication in
the January 15, 2004 issue; minor revisions compared to previous cond-mat
versio
Growth of ³He crystals at different magnetic fields
The experiments on ³He crystal growth are carried out in magnetic field up 9 T. The data were analyzed
and compared with the results found at zero magnetic field. It was found that the interface and the crystal lattice
couple weakly in the presence of an external magnetic field, and we could set an upper limit of the step
energy of the , and facets at different magnetic fields
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
Melting process and interface instability of highly magnetized solid 3He: Role of the magnetization gradient
Theoretical PhysicsQuantum Matter and Optic
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
Wannier-function description of the electronic polarization and infrared absorption of high-pressure hydrogen
We have constructed maximally-localized Wannier functions for prototype
structures of solid molecular hydrogen under pressure, starting from LDA and
tight-binding Bloch wave functions. Each occupied Wannier function can be
associated with two paired protons, defining a ``Wannier molecule''. The sum of
the dipole moments of these ``molecules'' always gives the correct macroscopic
polarization, even under strong compression, when the overlap between nearby
Wannier functions becomes significant. We find that at megabar pressures the
contributions to the dipoles arising from the overlapping tails of the Wannier
functions is very large. The strong vibron infrared absorption experimentally
observed in phase III, above ~ 150 GPa, is analyzed in terms of the
vibron-induced fluctuations of the Wannier dipoles. We decompose these
fluctuations into ``static'' and ``dynamical'' contributions, and find that at
such high densities the latter term, which increases much more steeply with
pressure, is dominant.Comment: 17 pages, two-column style with 14 postscript figures embedded. Uses
REVTEX and epsf macro
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