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The possibility has been investigated for measuring the viscosity of concentrated superfluid mixtures of 3He in 4He in the temperature range 0.4−2.2 K using commercial quartz tuning forks with a resonance frequency of ≈ 32 kHz. It was found that the temperature dependence of the viscosity coefficient in a mixture with a concentration of 5% of 3He in 4He is in good qualitative agreement with the data obtained using other experimental techniques in the entire temperature range of the research. The same dependence of a mixture with 3He concentration of 15% agrees with the previously obtained data only at temperature T > 1.4 K, and at lower temperatures a difference appears that grows with decreasing temperature. The experimental temperature dependences of the viscosity coefficient obtained by different methods are compared with those calculated within the framework of the kinetic theory for the quasiparticles in superfluid 3He−4He solutions, developed by Landau, Khalatnikov, and Zharkov for the temperature range T > 0.6 K and by Baym, Saam, and Ebner for lower temperature.

We have studied the resonance curves of a quartz tuning fork of the fundamental frequency 32 kHz immersed in superfluid 4He and mixtures 3He–4He with 3He concentration of 5 and 15% in the temperature range of 0.35–2.5 K at saturated vapor pressure. Two types of experiments have been carried out, with a tuning fork both not covered by a bulb and coated by a bulb, i.e., in the restricted geometry. In both cases, the velocity-force dependences for the tuning fork showed a linear damping force at low peak velocities and extra drag due to the appearance of vortex lines accompanying the transition to turbulence under increasing peak velocity. These dependencies are mainly determined by the density of the normal helium component. There is a pronounced difference between superfluid 4He and mixtures of 3He in 4He, where 3He impurity particles provide a constant temperature-independent contribution to the normal component of the mixture. The extra contribution to the damping force, so-called “turbulent drag force”, decreases with concentration increase at the same peak velocity of the tuning fork that can be explained by the extension of the range of laminar flow with an increase in the concentration of 3He. We found that the drag coefficient in superfluid 4He and mixtures 3He–4He reaches a plateau at different peak velocities and different exciting forces and explained this fact by different conditions for vortex formation, depending on the different thickness of the near-wall viscous layer. The comparison between the data obtained in restricted and unrestricted geometries shows that there is an excessive dissipation of the tuning fork motion associated with the emission of the first sound wave in unrestricted geometry.

The state of the problem of cavitation in superfluid helium and the content of previous works on this topic are analyzed. The transition between turbulent flow and cavitation under oscillations of a quartz tuning fork in a normal and superfluid isotopic mixture containing 15% of 3He in the temperature range of 0.5–3 K is studied experimentally. The temperature dependences of the critical velocity for the cavitation onset and the critical transition excitation force are obtained. We compare the results of our measurements with the data obtained by other groups. The results of measurements with tuning forks in restricted and unrestricted geometries suggest that the presence of acoustic emission facilitates the conditions for the onset of cavitation and indirectly confirms the role of vortex lines. A comparison of the temperature dependences of the critical velocity of cavitation onset in pure 4He and a concentrated 3He–4He mixture did not give an unambiguous answer to the reason for the sharp increase of this velocity, at the transition to the superfluid state, and the contributions of both thermal effects and the 3He impurities to the velocity increase.

A detailed description of the standard measuring techniques and original technical solutions used by the authors while studying the recently discovered phenomenon of electric response in the second sound wave. The most successful amplifier circuits used for precision low-temperature measurements are presented, as well as a sensitivity analysis of the different measuring systems. The main goal of the study is to substantiate the reliability of the electrical activity effect occurring in a resonator with He II when a second sound wave is excited. It is shown that the observed electric response signal is not associated with external interference or internal pick-ups, and is also not the result of either direct acoustoelectric conversion or thermal contact potentials. An analysis and comparison of new experimental data obtained by two scientific groups is also presented.

The signal of electric response in superfluid helium was recently observed at the excitation of a second sound standing wave in an acoustic resonator (Rybalko in Low Temp Phys 30:994, 2004). It has been suspected that similar resonance signals may be observed also in a first sound wave; however, earlier experiments showed the absence of any electric response induced by the pressure wave. Here, we report the first experimental evidence of electrical activity that arises at the fundamental frequency of the first sound resonance. It is found that the signal of electric response agrees fairly well with the fundamental frequency of the first sound resonance. The absence of the signal of interest in the original experiment is discussed.

We report an experimental investigation of a flow of the superfluid phase of 4He–He II—thermally induced by a fountain pump through vertical channels of square cross-section with ends blocked by sintered silver superleaks and its decay. We confirm the existence of a weakly temperature dependent critical velocity vcrI of order 1 cm/s, which does not scale with the channel size and is therefore an intrinsic property of the self-sustained vortex tangle of vortex line density L, measured by second sound attenuation. In addition to the previously reported turbulent A-state characterized by L1/2=γ(T)(v−vcrI), where v is the mean superflow velocity through the channel, we have discovered a new B-state characterized by L=β(v−vcrII), where β seems only weakly temperature dependent. It poses an important question why, at higher flow velocities, the quadratic generation mechanism, so well established in thermal counterflow, ceases to work. We offer a phenomenological model assuming that in the B-state the coarse-grained superflow profile matches the classical parabolic profile, with a finite, temperature dependent slip velocity vcrII of order few cm/s and that a confined viscous normal fluid flow of toroidal form is induced inside the channel due to the mutual friction force. When the fountain pump is switched off, after an initial decay, a confined quasi-viscous flow of a single-component fluid with an effective kinematic viscosity νeff(T) establishes, giving rise to the observed exponential decay. The values of νeff(T), calculated using our model from the measured decay times, are in agreement with those deduced from other experiments on decaying He II turbulence.

A quartz fork vibrating at high amplitude is used to study cavitation in He I and He II along the saturated vapor pressure (SVP) curve and at slightly elevated pressures. Cavitation is observed as a breakdown of the resonance response at critical velocity when slowly sweeping the frequency of the drive across the resonance and confirmed by visual observation of a bubble occurring in He II in the space between the prongs of the fork. On decreasing the temperature from 4.2 K along the SVP curve the critical velocity slowly increases from about 0.4 m/s to 1 m/s, until a steep increase up to about 2 m/s occurs within about 20 mK just below the superfluid transition. We discuss our results, including the measured dependence of the critical velocity versus overpressure at fixed bath temperature.

We report an experimental investigation of the electric response of superfluid helium. Our results confirm the presence of electric potential that appears at the relative oscillatory motion of normal fluid and superfluid components in helium generated by the heater. The resonance of the electric potential was observed in the first four harmonics. A suggested method for the detection of the electric response allows the required resonance peak to be distinguished from spurious signals. Our results are in qualitative agreement with the data published by previous researchers. The reasons for the discrepancy in the measured values of the potential difference are discussed.

The temperature dependence of the specific heat of the paramagnetic and ferromagnetic compounds PrNi 5− x Cu x ( x ≤1.2) has been studied. It was found that the lowest singlet levels of the Pr ions in the crystal field of the compounds approach each other as Cu substitutes for Ni. The lowering of the crystal field symmetry of the quasibinary compounds PrNi 5− x Cu x as compared to the terminal binary Van Vleck paramagnets is mainly responsible for their ferromagnetic ordering.

Studies of the temperature dependence of the specific heat C in the temperature range 4.2–16 K and of the magnetization polytherms σH(T) in the ZFC and FC regimes for H=(1–25)×102 Oe in the temperature range 4.2 K

A compact, top-loading 3He evaporation refrigerator is built for doing heat-capacity studies on various samples at temperatures below 1 K. It uses adsorption pumping and utilizes a portable helium Dewar of the STG-40 type as the cryostat. A calorimetric cell is built which permits investigation of the heat capacity of samples with masses of less than one gram. The lowest measurement temperature is 0.32 K. The dependence of the temperature of the 3He evaporation chamber on the power delivered to it is established. The refrigerator can operate continuously for two hours at a heat load of 50 μW.

An experimental apparatus has been designed to analyze by visualization cryogenic flows of liquid (4)He and consequently address unresolved problems of quantum turbulence. The newly implemented flow visualization setup is described and its specific features discussed. Thermal counterflow experiments have been performed and the motion of solid hydrogen and deuterium tracers studied by using the particle tracking velocimetry technique in order to probe the system capabilities. It is shown that the obtained results are consistent with the two-fluid model describing the behavior of superfluid (4)He. A number of technical and fundamental issues, such as particles' aggregation, role of rotating particles in counterflow and evidence of non-Gaussian distribution of tracers' velocities, are also discussed. The apparatus appears to be well-suited to the task of analyzing cryogenic flows and potentially capable of obtaining new results stimulating further understanding of the underlying physics.

We report a new thermally generated steady-state flow of He II and its decay. An upward superflow through vertical channels of a square cross section with ends blocked by sintered silver superleaks is induced by a fountain pump. On increasing the heat power Q[over ] a vortex-free superflow and, above an intrinsic critical velocity of about 1.5 cm/s, two distinctly different turbulent steady-state flows characterized by the vortex line density L proportional, variantQ[over ];{2} and L proportional, variantQ[over ] are observed using the second sound attenuation technique. When Q[over ] is switched off, after a fast initial decay an exponential decay law is observed. A simple phenomenological model is introduced, capturing the observed features.

A profound change occurs in the stability of quantized vortices in externally applied flow of superfluid 3He-B at temperatures ~ 0.6 Tc, owing to the rapidly decreasing damping in vortex motion with decreasing temperature. At low damping an evolving vortex may become unstable and generate a new independent vortex loop. This single-vortex instability is the generic precursor to turbulence. We investigate the instability with non-invasive NMR measurements on a rotating cylindrical sample in the intermediate temperature regime (0.3 - 0.6) Tc. From comparisons with numerical calculations we interpret that the instability occurs at the container wall, when the vortex end moves along the wall in applied flow., revised & extended version. Journal of Low Temperature Physics, accepted (2008)

The temperature dependence of the effective kinematic viscosity of turbulent He II, nu(eff)(T) , is deduced from second sound attenuation data using the late stage of decay of thermally induced counterflow He II turbulence in two channels of square cross section. It is shown to qualitatively agree with the published data for nu(eff)(T) calculated based on experiments on decaying-grid-generated He II turbulence [Niemela, J. Low Temp. Phys. 138, 537 (2005)]. Corrections to these data due to the "sine squared" law that describes attenuation of the second sound wave propagating along an arbitrary direction with respect to the direction of the core of a quantized vortex in turbulent He II are discussed and applied.

We report an experimental investigation of He II forced flow over a cylindrical obstacle by a flow visualization technique that uses micron scale solid hydrogen particles. It was found in the range of Reynolds numbers studied, 1500 < Red < 11 500, that the flow pattern has a tendency to form a wake with a dynamic separation point similar to that in classical fluids. Furthermore, we did not confirm the existence of large-scale turbulent structures both upstream and downstream of the cylinder as have been observed in He II thermal counterflow across the cylinder at the same range of Reynolds numbers.

We report experimental investigations of He II turbulence and its decay, generated by pure superflow through a channel of square crossection. Both ends of the channel are covered by sintered‐silver superleaks, which prevent any net normal fluid flow through the channel. The steady‐state turbulence is generated by applying a power to the heater immersed in He II in a volume adjacent to one of the superleaks having an outlet above the helium bath level in the glass cryostat. When the power is switched off, the superflow through the channel and the vortex line density in the middle of the channel decay. The steady‐state vortex line density and the form of its decay is continuously measured by second sound attenuation and analyzed within available theoretical models.

We report the experimentally investigated evolution of thermal counterflow across a cylinder in He II by using a modified particle tracking visualization technique with solidified hydrogen particles as tracers. It was found, for a relatively small velocity of the normal fluid, that the flow is similar to laminar flow over cylinder in classical fluids. Increasing the flow velocity leads to the appearance of a pair of normal fluid eddies downstream the cylinder. We confirm the existence of large scale eddies both upstream and downstream of the cylinder at the highest velocities achieved. In addition to the previously reported turbulent state, we confirm the existence of a state where more than one pair of eddies coexist downstream from the cylinder.

The motion of solid hydrogen particles in He II thermal counterflow has been measured using the particle tracking velocimetry technique. The tracers can move with the velocity comparable to the velocity of the normal fluid. However, below some transition velocity, the particles can be trapped on individual vortex lines and towed by the vortex tangle in the direction of superfluid flow. Above this transition velocity, the Stokes drag with the normal fluid dislodges the trapped particles from vortex cores and such particles move in the same direction as the normal fluid, albeit with lower average velocity. We discuss the measurement of the transition velocity and the mechanism by which tracers may be trapped by or dislodged from quantized vortices.