V. B. Efimov

Phonon scattering in diamond films

Abstract We have studied the thermal conductivity κ of free standing diamond films with different quality and sizes of the crystallines in bulk. The measurements were performed in the temperature range 10–350xa0K by steady-state heat flux technique. We have estimated the value and temperature dependence of the phonon mean free path lp using the simple gas kinetic equation for thermal conductivity: κ≈1/3 l p C p v. The values of heat capacity Cp and mean sound velocity v are known from literature for massive crystals. At high (room) temperatures the mean free path of phonons is limited by the phonon–phonon Umklapp interaction and phonon scattering on point defect in bulk. Both these processes lead to increasing of lp(T) with cooling the sample but they have different temperature dependencies. So from lp(T) dependence one can judge the quality (purity) of the sample. At low temperatures the phonon-boundary scattering dominates and lp is limited by the crystalline sizes. The mean free path of phonons in the best of samples has reached the value of a few microns at temperatures 20–70xa0K, which agrees with the middle sizes of crystallites estimated from X-ray measurements and from microphotos. At lower temperatures the value of lp is increasing again. It can be attributed to the phonon penetration through the boundaries between crystallines. The temperature dependence of λ in this process follows l(T)∼T−(1−2).

Structural transitions in ice samples at low temperatures and pressures

The structure of ice samples formed in the decay of a water impurity gel at temperatures above 4 K and atmospheric pressure has been examined. The X-ray diffraction analysis indicates that three phases coexist in the initial sample at temperatures of 85–110 K. These phases are amorphous ice occupying up to 30% of the sample volume, cubic-phase ice Ic metastable at low pressures (∼60%), and normal hexagonal ice Ih (≤6%). The characteristic sizes of crystals of the cubic and hexagonal phases are about 6 and 30 nm, respectively. The amorphous phase at annealing above 110 K is gradually transformed to the crystalline phase both cubic and hexagonal. This transition is accompanied by two processes, including a fast increase in the sizes of cubicphase nanocrystals and the partial transition of the cubic phase Ic to the hexagonal one Ih. Hexagonal ice Ih prevails in the bulk of the sample above 200 K.

Propagation of short nonlinear second-sound pulses through He-II in one- and three-dimensional geometry

The results of an experimental study of the evolution of the shape of nonlinear second-sound pulses in superfluid He-II are reported. The pulses propagate in the bulk (3D geometry) and along a cryoacoustic waveguide filled with liquid helium (quasi-1D geometry) at temperatures corresponding to the negative, positive, or zero nonlinearity coefficient. A strong dependence of the shape of the propagating pulse on the dimensionality of the wave was observed. The finite size of the heater (generator of a sound) affects the profile of a short 3D pulse even at distances many times greater than the heater size, which restricts the minimal width of the excited pulse. The experimental data are compared with the results of numerical simulations.

Thermal conductivity of crystalline fullerite C60 in the simple cubic phase

The behavior of the thermal conductivity k(T) of bulk faceted fullerite C60 crystals is investigated at temperatures T=8–220 K. The samples are prepared by the gas-transport method from pure C60, containing less than 0.01% impurities. It is found that as the temperature decreases, the thermal conductivity of the crystal increases, reaches a maximum at T=15–20 K, and drops by a factor of ∼2, proportional to the change in the specific heat, on cooling to 8 K. The effective phonon mean free path λp, estimated from the thermal conductivity and known from the published values of the specific heat of fullerite, is comparable to the lattice constant of the crystal λp∼d=1.4 nm at temperatures T>200 K and reaches values λp∼50d at T<15 K, i.e., the maximum phonon ranges are limited by scattering on defects in the volume of the sample in the simple cubic phase. In the range T=25−75 K the observed temperature dependence k(T) can be described by the expression k(T)∼exp(Θ/bT), characteristic for the behavior of the thermal conductivity of perfect nonconducting crystals at temperatures below the Debye temperature Θ (Θ=80 K in fullerite), where umklapp phonon-phonon scattering processes predominate in the volume of the sample.

Turbulence of second sound waves in superfluid He II

We communicate the results of numerical studies of acoustic turbulence in a system of slightly dissipating, nonlinear second sound waves in superfluid He II. It is shown that at sufficiently high amplitude of the external driving force a power-like energy distribution over frequency is formed in the system of second sound waves. This distribution is attributed to formation of the acoustic turbulence regime in the system. The interval of frequencies in which the distribution has a power-like form is expanded to high frequencies with increasing amplitude of the driving force. The energy distribution inside this interval is close to Eω∼ω2. It is shown that the distribution of energy Eω depends on the value of the nonlinearity coefficient of the second sound but does not depend on the sign of the coefficient, i.e., the coherent structures (shock waves) do not contribute to the statistical properties of the turbulent state.

Thermal conductivity of anisotropic HTS crystals YBa2Cu3O7−x and Bi2Sr2CaCu2O8+y

The results of measurements of thermal conductivity κ and resistivity ρ of some superconducting crystals of YBa2Cu3O7−x and Bi2Sr2CaCu2O8+y in the ab plane (κab and ρab) and along the c axis (κc and ρc) are presented. The magnitudes and temperature dependences of the thermal conductivity of Y- and Bi-crystals are found to be close and to vary less significantly than the resistance of samples of the same composition. In contrast to the predictions of the electron heat transport theory in HTS cuprate crystals, the relative height of the peaks in κab(T)/κab(Tc) curves becomes equal to two in highly anisotropic Bi-samples, and the peak is displaced toward lower temperatures Tm<0.4Tc with increasing value of this ratio, as well as in Y-crystals which have a lower anisotropy. All these facts indicate that in theoretical calculations of the thermal conductivity of layered HTS crystals in the ab plane at temperatures below Tc, the possibility of an increase in the electron and the phonon components of thermal con...

Helium impurity nanocluster gels in superfluid helium

Rapid cooling of a helium impurity gas mixture to the superfluid helium temperatures allows us to prepare an impurity-helium condensate with jelly-like structure (impurity gel) in superfluid He-II. It is clear that the properties of these gels should be substantially different from those of a bulk impurity substance, constituting a new class of soft matter — quantum gel, where superfluid He-II filling the nanopores between the randomly connected impurity nanoclusters serves as the dispersion medium of the gel.

Heat transport in fullerite samples

Abstract The absolute value of the coefficient of thermal conductivity k(T) of fullerite samples and its temperature dependence strongly depend on the conditions of the sample preparation. In our experiments we have studied the transport properties of perfect single crystals and of pure C60 powder compacted under different conditions. The compacted samples were pressurized at low temperature (only compacted samples) or polymerized at high pressure and high temperatures.

Nonlinear second sound in He-II under pressure

The dependence of the nonlinearity coefficient α for the roton second sound in superfluid He-II on pressure P is studied for the first time. It is found that as the value of P increases from saturated vapor pressure to 25 atm, the temperature Tα at which the coefficient α reverses its sign decreases from 1.88 to 1.58 K, i.e., there exists a wide temperature interval below Tλ in He-II at all pressures, in which the nonlinearity coefficient α is negative, and a temperature discontinuity (shock wave) is formed at the trailing edge of the second sound compression wave.

First-sound rarefaction and compression waves in superfluid He-II

The evolution of the form of first-sound waves, excited in superfluid He-II by a pulsed heater, with increasing power Q of the perturbing heat pulse is investigated. In liquid compressed to 13.3 atm, a first-sound rarefaction wave (wave of heating) is observed, which transforms into a compression wave and then into a compression shock wave as Q increases, i.e., the change in the conditions of heat transfer at a solid-He-II interface can be judged according to the change in the form of the sound wave. It follows from our measurements that in He-II compression waves are excited at pressures P⩾1 atm primarily as a result of the thermal expansion of a normal He-I liquid layer arising at the He-II-heater interface for power Q above a critical level.