44 research outputs found
Melting curve of He: no sign of the supersolid transition down to 10 mK
We have measured the melting curve of He in the temperature range from 10
to 400 mK with the accuracy of about 0.5 bar. Crystals of different
quality show the expected -dependence in the range from 80 to 400 mK
without any sign of the supersolid transition, and the coefficient is in
excellent agreement with available data on the sound velocity in liquid He
and on the Debye temperature of solid He. Below 80 mK we have observed a
small deviation from -dependence which however cannot be attributed to the
supersolid transition because instead of decrease the entropy of the solid
rather remains constant, about Comment: 4 pages, 2 figures, published in Physical Review Letter
Observation of a New Surface State on 4He Crystal Interfaces
The equilibrium shape of hcp 4He crystals has been studied at temperatures 0.05≤T≤0.7 K by means of a high-precision optical interferometer. We find that the profile of the interfacial boundary, close to an almost horizontal c-facet, has a well-defined slope discontinuity separating two angular regions with different behavior of the surface stiffness α̃. For surfaces tilted by an angle φ≲100 μrad with respect to the c-facet, we obtain α̃=κ/φ, contrary to the linear angular dependence predicted by current theories. Our results on four large crystals yield κ=(11±3)×10 exp −4T erg/ cm exp 2 K.Peer reviewe
Facet Growth of 4He Crystals at mK Temperatures
We have investigated growth of c facets in good quality helium crystals with screw dislocation densities 0–20 cm exp −2 along the c axis. Three distinct regimes of growth were observed. One of them can be explained by spiral growth provided that kinetic energy of moving steps and their tendency to localization at large driving forces are taken into account. In the absence of screw dislocations we find burstlike growth unless the speed is less than 0.5 nm/s, in which case anomalous, intrinsic growth of facets is detected.Peer reviewe
Effect of Nuclear Quadrupole Interaction on the Relaxation in Amorphous Solids
Recently it has been experimentally demonstrated that certain glasses display
an unexpected magnetic field dependence of the dielectric constant. In
particular, the echo technique experiments have shown that the echo amplitude
depends on the magnetic field. The analysis of these experiments results in the
conclusion that the effect seems to be related to the nuclear degrees of
freedom of tunneling systems. The interactions of a nuclear quadrupole
electrical moment with the crystal field and of a nuclear magnetic moment with
magnetic field transform the two-level tunneling systems inherent in amorphous
dielectrics into many-level tunneling systems. The fact that these features
show up at temperatures , where the properties of amorphous materials
are governed by the long-range interaction between tunneling systems,
suggests that this interaction is responsible for the magnetic field dependent
relaxation. We have developed a theory of many-body relaxation in an ensemble
of interacting many-level tunneling systems and show that the relaxation rate
is controlled by the magnetic field. The results obtained correlate with the
available experimental data. Our approach strongly supports the idea that the
nuclear quadrupole interaction is just the key for understanding the unusual
behavior of glasses in a magnetic field.Comment: 18 pages, 9 figure
Light interference in a hybrid-aligned nematic layer with nonordered surface disclination lines
The propagation of a laser beam through a hybrid-aligned nematic layer with a surface disclination line has been
investigated. A model of the light interference has been developed to consider the scattering by the structural
inhomogeneities. The analytical expression that includes the factor characterizing an exponential decrease in
the light scattering has been obtained. The dependence of the intensity of light transmitted through the layer
on the magnetic field has been measured. The dependence has been accompanied by the interference oscillations.
The theoretical expression is consistent with the experiment, which confirms the correctness of the model
concepts
Polar anchoring energy and tilt angle measured by magneto-optical technique in nematic doped with ionic surfactant
The surface anchoring of a nematic doped with the ionic surfactant has been investigated and
compared with the one in the undoped sample. The director tilt angle at the substrates coated
with the orienting polymer film has been determined by the null method in a rotating magnetic
field. The Frederiks transition in a magnetic field has been chosen as a convenient technique to
measure the polar anchoring energy Wθ. The temperature dependences of anchoring energy have
been obtained for the various nematic cells. The Wθ values for nematic doped with the ionic
surfactant are less than for the undoped one. The factors affecting the measurement accuracy have
been discussed. The accuracy is higher for the thinner nematic layers and weaker anchoring
energy