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Isochoric thermal conductivity of solid nitrogen
The isochoric thermal conductivity of solid nitrogen has been investigated on
four samples of different densities in the temperature interval from 20 K to
the onset of melting. In alfa-N2 the isochoric thermal conductivity exhibits a
dependence weaker than 1/T; in beta-N2 it increases slightly with temperature.
The experimental results are discussed within a model in which the heat is
transported by low-frequency phonons or by "diffusive" modes above the mobility
boundary. The growth of the thermal conductivity in beta-N2 is attributed to
the decreasing "rotational" component of the total thermal resistance, which
occurs as the rotational correlations between the neighboring molecules become
weaker.Comment: Postscript 12 pages, 3 figures, 1 table. To be published in 200
Sliding of Electron Crystal of Finite Size on the Surface of Superfluid He-4 Confined in a Microchannel
We present a new study of the nonlinear transport of a two-dimensional
electron crystal on the surface of liquid helium confined in a 10
micrometer-wide channel in which the effective length of the crystal can be
varied from 10 to 215 micrometers. At low driving voltages, the moving electron
crystal is strongly coupled to deformation of the liquid surface arising from
resonant excitation of surface capillary waves, ripplons, while at higher
driving voltages the crystal decouples from the deformation. We find strong
dependence of the decoupling threshold of the driving electric field acting on
the electrons, on the size of the crystal. In particular, the threshold
electric field significantly decreases when the length of the crystal becomes
shorter than 25 micrometers. We explain this effect as arising from weakening
of surface deformations due to radiative loss of resonantly-excited ripplons
from an electron crystal of finite size, and we account for the observed effect
using an instructive analytical model.Comment: 5 figure
Causal signal transmission by quantum fields. IV: The causal Wick theorem
Wick's theorem in the Schwinger-Perel-Keldysh closed-time-loop formalism is
written in a form where the place of contractions is taken by the linear
response function of the field. This result demonstrates that the physical
information supplied by Wick's theorem for operators is propagation of the free
field in space and time.Comment: Final version, to appear in Phys Rev
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