Hikota Akimoto

MAGNETIC PHASE DIAGRAM OF BCC 3HE

Abstract A complete magnetic phase diagram of bcc solid 3He at a molar volume of 22.7 cm3/mol has been determined by pressure measurements at temperatures down to 250 μK in magnetic fields up to 7 T. The transition temperature between the paramagnetic phase and the high-field antiferromagnetic ordered phase increases with field and reaches a maximum (=0.96 mK) at 3 T. Above this field, it turns to decrease and seems to be zero at 5.4 T, i.e., the upper critical field Bc2(0). The transition is not the first-order one from 1 T to Bc2(0). The observed reentrant phase diagram strongly supports the multiple spin exchange model, but reveals a severe quantitative inaccuracy of the mean field approximation even at high fields near Bc2(0).

Dielectric properties of glasses at ultra low temperatures

A temperature dependence in the dielectric constant of vitreous silica has been measured down to a few hundred microkelvin at frequencies between 110Hz and 10kHz. Homosil glass shows a logarithmic increase below 10 mK down to the lowest temperature of 0.61 mK. On the other hand, Suprasil glass exibits a saturation behavior following a logarithmic increase above 3 mK. These phenomena are discussed on the basis of the two level system.

Hydrodynamic properties on superfluid3He in high magnetic fields up to 12 tesla

Using a torsional oscillator, superfluid fraction and viscosity of superfluid3He were measured in the A1 and the a2 phase up to 12 Tesla below 1 mK in the pressure of 29 bar. This is the first measurement in the high magnetic field region in a well defined geometry and in a controlled texture. The a1 and the a2 transition were seen clearly in the torsional oscillator frequency and amplitude. Two GL parameters, β245 and β24, are independent of field up to 12 Tesla. β245 is 1.18 from the A2 phase measurements, which agree with those from heat capacity measurement. While β24 is 3.08 from the a1 phase, which is smaller than that from recent spin entropy wave measurement.

Torsional oscillator for high magnetic field experiments

A new type of torsional oscillator for experiments in the high magnetic field of 12 Tesla is reported. A beryllium copper alloy (BeCu25) was chosen as a torsion rod for its reliable mechanical properties. Two non-metallic materials were tested for a torsion head except a small amount of silver paste for the electrode. For a quartz glass head, liquid3He was successfully cooled down below 0.5 mK at 12 Tesla. The quality factor was 2 × 104 even at the highest field. On the other hand, a Stycast 1266 head in a field of 12 Tesla caused a large temperature difference between the liquid in the head and in the open space, in spite of a comparable quality factor with the quartz head.

On superconductivity of α-(BEDT-TTF)2KHg(SCN)4 under uniaxial stress below 1K

Resistance measurement of α-(BEDT-TTF)2KHg(SCN)4 in the conducting plane has been made under uniaxial stress perpendicular to the BEDT-TTF molecular layers at temperatures down to 34 mK. The in-plane resistance at about 1 kbar starts to decrease at around 1 K and shows zero resistance below 0.1 K. The I-V characteristics exibits nonohmic behavior V ∝ Iα in the same temperature region. The value of α increases from 1 at 1 K to 2 at 34 mK, suggesting the existence of vortex motion.

3He-3He quasiparticle interaction in3He-4He mixtures under pressure

Based on our recent phase separation curve of3He-4He solution at elevated pressures, we propose new3He-3He quasiparticle interaction potentials, which reproduce the existing experimental results pretty well except for3He effective mass under pressure.

Positive Ion Mobility in Normal and Superfluid 3He at High Magnetic Field

We have measured the mobility of positive ions (“snowballs”) in liquid3He at magnetic fields H = 0 − 12 T and temperatures T = 0.3 − 10 mK. In normal3He, with increasing magnetic field the mobility first increases and then decreases nearly quadratically. The initial rise of the mobility is most strongly seen at the lowest temperatures (T = 2.5−3 mK) and high pressures (p = 29 bar) where at H = 6 T the mobility reaches a maximum enhancement of about 6 %. We attribute this effect to the suppression of the exchange scattering with magnetic field and thus infer the value of the exchange scattering cross-section. In the superfluid A1and A2phases, the relative increase of the ion mobility, as the sample is cooled below the transition temperature, is about twice as small as in the A or B phases.

Andreev reflection at free surface of superfluid3He

Quasiparticle reflection at the free surface of superfluid3He-B has been investigated in the ballistic temperature region by use of a blackbody radiator with an orifice of 0.2 mm in diameter and 1 mm long. In order to cut off the normal reflection and to increase Andreev reflection rate, a quasiparticle beam is produced to hit the surface at small angle of 20 degree. The observed Andreev reflection seems to be smaller than that expected from the theory.

Pressure dependence of the superfluid fraction in3He-A

The superfluid fraction of the A phase was measured with a torsional oscillator at 0.6 Tesla in the wide pressure range of 0–29 bar and in the whole temperature region. After a Fermi liquid correction, a strong coupling gap enhancement factor was derived in the GL region. The value of 1.02 at 0 bar is consistent with a weak coupling limit, and increases with pressure up to the value of 1.30 at 29 bar. These results are consistent with those from the specific heat measurements at pressures above a policritical point(PCP)(22 bar) and those from the field dependence of the A-B transition temperature below the PCP.

Nuclear cooling of hcp solid3He

Nuclear adiabatic demagnetization has successfully been done for hcp solid3He with exchange interaction of the order of 10 μK. Isochoric pressure change during the demagnetization is compared with that for bcc sample with a similar exchange interaction. The bcc sample showed a clear pressure change during the demagnetization at the upper and the lower critical fields, while the hcp one had no pressure change down to zero magnetic field. These results give a strong evidence that the ground state in hcp3He is ferromagnetic.