Haruo Kojima

Probing porous media with first and second sound. I. Dynamic permeability

The acoustic properties of first and second sound in Heu2009ii are used to probe the dynamic permeability k(ω), of porous media whose pores are large enough that the bulk properties of 4He are relevant. The measurements of phase velocity and attenuation, considered as functions of frequency and temperature, are in excellent agreement with the predictions based on a model k(ω) which involves four parameters: φ (porosity), α∞ (tortuosity), k0 (dc permeability), and Λ (dynamically connected pore size). The first three of these are measured by independent means; accurate values of Λ are extracted directly from the measurements in the high‐frequency region of the data where the observed temperature and frequency dependences of the modes are precisely those predicted on theoretical grounds. It is shown empirically that Λ is in fact closely related to the dc permeability k0 for all known data sets, thus supporting an earlier conjecture based on theoretical ideas.

Probing porous media with first and second sound. II. Acoustic properties of water‐saturated porous media

The ultrasonic properties (reflection/transmission and bulk attenuation/speed) of porous and permeable media saturated with a Newtonian fluid, namely water, are considered. The frequency dependence of the transmission amplitudes of pulses is measured through a slab of thickness d1, repeated for another slab of thickness d2 for a given material. With these two measurements on two different thicknesses, it is possible in principle to separate bulk losses from reflection/transmission losses for compressional waves in these materials. The bulk properties are calculated from the Biot theory for which all of the input parameters have been measured separately; the attenuations are particularly sensitive to the values of Λ, determined from second‐sound attenuation measurements reported in the companion article. There is excellent quantitative agreement between the theoretical and experimental values in the cases considered; there are no adjustable parameters involved. The reflection and transmission coefficients ...

Damping of first sound and viscosity of liquid 3He-B near the superfluid transition

Abstract The viscosity of normal fluid component of liquid 3He-B in the vicinity of the superfluid transition temperature has been measured by using the damping of first sound in a cylindrical resonator. The measured reduced viscosity η(T)/η(T)c) is in qualitative agreement with recent theoretical studies and with the results of torsion oscillator experiments.

The metal-insulator transition in La1.85Sr0.15CuO4 with various substitutions for Cu

The authors have determined the concentrations x of Zn, Ga, Fe, and Ni impurities which lead to a metal-insulator transition when substituted for Cu in La1.85Sr0.15CuO4. In each case these values of x are larger than those which are associated with the disappearance of superconductivity.

Effects of 4He coating on the size effects in superfluid 3He as measured by fourth sound

Abstract We studied the effects of 4He coating of the surfaces of alumina powder grains on the superfluid fraction of 3He. The superfluid fraction was measured by fourth sound techniques. The observed size effects in pure 3He were similar to our previous results. When 1 layer of 4He coverage was adsorbed onto the powder, no change in the superfluid fraction was observed. The attenuation of fourth sound was not affected either. When 3 layers of 4He coverage was adsorbed, a substantial increase in the superfluid fraction was observed. An increase in the attenuation of fourth sound was also seen.

The Stress Driven Rearrangement Instabilities in Electronic Materials and in Helium Crystals

At present, there is a consensus that various Stress Driven Rearrangement Instabilities (SDRI) are the implications of the mathematically rigorous theoretical Gibbs thermodynamics. Many applied researchers and practitioners believe that SDRI are also universal physical phenomena occurring over a large range of length scales and applied topics. There is a multitude of publications claiming experimental observation of the SDRI based phenomena. This opinion is challenged by other highly respected scholars claiming theoretical inconsistencies and multiple experimental counterexamples. Such an uncertainty is too costly for further progress on the SDRI topic. The ultimate goal of our project is to resolve this controversy. The project includes experimental, theoretical, and numerical studies. Among various plausible manifestations of SDRI, the authors focused only on two most promising for which the validity of the SDRI has already been claimed by other researchers: a) stress driven corrugations of the solid-melt phase interface in macroscopic quantum 4 He and b) the dislocation-free Stranski-Krastanov pattern of growth of semiconductor quantum dots. We devised a program and experimental set-ups for testing applicability of the SDRI mechanisms using the same physical systems as before but using implications of the SDRI theory for 2D patterning which have never been tested in the past.

Interdigital capacitor as solid 4He height detector

Abstract A compact interdigital capacitor is developed as a level detector of solid 4 He . The capacitor consists of 38 interlaced 50 μm wide and 3.8 mm long gold films separated by 50 μm and deposited onto a flat 5×5 mm 2 sapphire substrate. The background capacitance is 6.5 pF . The capacitance change is 1.3×10 −3 pF/mm of solid 4 He height change. A height change of 10 μm is detectable. Observations at 1.2 K in a 6.7×8.7×9.1 mm 3 cell show over pressures (measured with a low temperature strain gauge) up to 25 mbar prior to nucleation of solid. The solid height may be controlled by varying heat applied to a pressure bomb cooled to 77 K .

Superconductivity and metal-insulator transition in zinc-substituted La1.85Sr0.15CuO4

When Zn is substituted for Cu in La 1.85 Sr 0.15 CuO 4 , T c goes to zero near 3 at %. The metal-insulator transition occurs at a higher concentration, so that there is a non-superconducting but metallic range of impurity concentration.

Unresolved issues in spin-polarized superfluid flow dynamics

Abstract Spin (accompanied by mass) flows can be induced magnetically in its “ferromagnetic” spin-polarized superfluid 3 He A 1 phase. Observations have been made by applying magnetic field gradients across a spin filter and measuring the induced pressure gradient. There are issues which remain unresolved. An ideal static magnetic fountain effect has not been seen. The observed effect decays with a characteristic relaxation time. The relaxation time jumps by a factor of 2 near the middle of the A1 phase. The presence of A1/A2 phase interface has been speculated to cause the jump. The measured relaxation time increases with the applied field up to 1.4 T . The origins of the temperature and field dependence are not understood. Planned experiments to probe these issues are described.

Effects of4He coverage for the boundary condition of superfluid3He.

Abstract We present measurements of4He coating effects on the boundary wall of superfluid3He in confined geometries for four different pore sizes. Using the fourth sound technique, the systematic study has been carried out for both the superfluid density and the Q-value of the resonance signal.