62 research outputs found
Simultaneous Measurements of the Torsional Oscillator Anomaly and Thermal Conductivity in Solid 4He
In these torsional oscillator experiments the samples of solid He were
characterized by measuring their thermal conducitvity. Polycrystalline samples
of helium of either high isotopic purity or natural concentration of He
were grown in an annular container by the blocked-capillary method and
investigated before and after annealing. No correlation has been found between
the magnitude of the low-temperature shift of the torsional oscillator
frequency and the amount of crystalline defects as measured by the thermal
conductivity. In samples with the natural He concentration a substantial
excess thermal conductivity over the usual dependence was observed below
120 mK.Comment: 4 pages, 3 figure
On the low-temperature anomalies in the thermal conductivity of plastically deformed crystals due to phonon-kink scattering
Previous experimental studies of the thermal conductivity of plastically
deformed lead crystals in the superconducting state have shown strong anomalies
in the thermal conductivity. Similar effects were also found for the thermal
conductivity of bent samples. Until now, a theoretical
explanation for these results was missing. In this paper we will introduce the
process of phonon-kink scattering and show that it qualitatively explains the
anomalies that experiments had found.Comment: 3 pages, follow-up paper to appear soo
Nanotube-based source of charges for experiments with solid helium at low temperatures
Methods of preparation of the field-emission sources of charges from carbon nanotubes suitable for study of injected
charges in solid helium at low temperatures T < 1 K are presented. The sources have been prepared by arc
discharge deposition of nanotubes onto a flat copper substrate or by mechanical rubbing of nanotubes into porous
metal surface. The test study of the voltage-current characteristics of a diode cell with the nanotube source in superfluid
He II have shown that at voltages above 120 V one can observe a relatively large current I ≥ 10⁻¹³ A of negative
charges in liquid helium. The field and temperature dependences of positive and negative currents in solid ⁴He
were studied in samples grown by the blocked capillary technique. Usage of the nanotube based source of injected
charges had permitted us for the first time to observe motion of the positive charges in solid helium at temperatures
below 0.1 К. The current-voltage dependence could be described by a power law I ~ Uα, with the value of the exponent α >> 2, much higher than what one would expect for the regime of space charge limited currents
Observation of acoustic turbulence in a system of nonlinear second sound waves in superfluid 4He.
We discuss the results of recent studies of acoustic turbulence in a system of nonlinear second sound waves in a high-quality resonator filled with superfluid 4He. It was found that, when the driving amplitude was sufficiently increased, a steady-state direct wave cascade is formed involving a flux of energy towards high frequencies. The wave amplitude distribution follows a power law over a wide range of frequencies. Development of a decay instability at high driving amplitudes results in the formation of subharmonics of the driving frequency, and to a backflow of energy towards the low-frequency spectral domain, in addition to the direct cascade
Supersolid Helium at High Pressure
We have measured the pressure dependence of the supersolid fraction by a
torsional oscillator technique. Superflow is found from 25.6 bar up to 136.9
bar. The supersolid fraction in the low temperature limit increases from 0.6 %
at 25.6 bar near the melting boundary up to a maximum of 1.5% near 55 bar
before showing a monotonic decrease with pressure extrapolating to zero near
170 bar.Comment: 4 pages, 4 figure
Decay of capillary turbulence on the surface of a semiquantum liquid
We study the free decay of capillary turbulence on the charged surface of liquid hydrogen. Contrary to expectations based on the existing self-similar theory of nonstationary wave turbulent processes in ideal liquid, we find that decay begins from the high frequency end of the spectral range, while most of the energy remains localized at low frequencies. We show that finite damping of the waves changes qualitatively the character of the turbulent decay. Numerical calculations based on this idea agree well with the experimental data
Decay of the turbulent cascade of capillary waves on the charged surface of liquid hyrdrogen.
We study the free decay of capillary turbulence on the charged surface of liquid hydrogen. We find that the decay begins from the high frequency spectral domains of the surface oscillations and is of a quasi-adiabatic character. The characteristic relaxation time of the whole turbulent cascade is close to the viscous damping time for capillary waves of frequency equal to the driving frequency
Experiments on wave turbulence : the evolution and growth of second sound acoustic turbulence in superfluid 4He confirm self-similarity.
We report our experiments on the formation of second sound acoustic turbulence in superfluid 4He. The initial growth in spectral amplitude follows power laws that steepen rapidly with increasing harmonic number n, corresponding to a propagating front in frequency space. The lower growth exponents agree well with analytic predictions and numerical modeling. The observed increase in the formation delay with n validates the concept of selfsimilarity in the growth of wave turbulence
Observation of an Inverse Energy Cascade in Developed Acoustic Turbulence in Superfluid Helium
We report observation of an inverse energy cascade in second sound acoustic
turbulence in He II. Its onset occurs above a critical driving energy and it is
accompanied by giant waves that constitute an acoustic analogue of the rogue
waves that occasionally appear on the surface of the ocean. The theory of the
phenomenon is developed and shown to be in good agreement with the experiments.Comment: 4 pages, 5 figures. The final version just prior to publicatio
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