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Spin pump for boosting spin polarization of superfluid 3He A1 phase

yuki aoki, 智 村川, S. Murakawa

Published Aug 1, 2009• 0 citations
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Abstract

Mechanical pumping and filtering of spin-polarized condensate were realized in the superfluid 3He A1 phase by the pneumatic pumping action of an electrostatically actuated diaphragm. Spin pumping increased the net spin polarization by 20 – 50 % as measured by the induced pressure change during spin pumping. The observed spin relaxation time was consistent with the increased spin polarization. These observations demonstrate the feasibility of using spin pumping to substantially increase the effective magnetic field to which the A1 phase is exposed.

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Journal of Physics Conference Series•Feb 1, 2009•1 citation
Mechanical pumping and filtering of spin-polarized condensate were realized in the superfluid 3He A1 phase by the pneumatic pumping action of an electrostatically actuated diaphragm. Spin pumping increased the net spin polarization by 20 – 50 % as measured by the induced pressure change during spin pumping. The observed spin relaxation time was consistent with the increased spin polarization. These observations demonstrate the feasibility of using spin pumping to substantially increase the effective magnetic field to which the A1 phase is exposed.
Mechanical pumping and filtering of spin-polarized condensate were realized in the superfluid 3He^{3}\text{He}3He A1{\text{A}}_{1}A1​ phase by the pneumatic pumping action of an electrostatically actuated diaphragm. Spin pumping increased the net spin polarization by 20−−50 20--50\text{ }%20−−50  as measured by the induced pressure change during spin pumping. The observed spin relaxation time was consistent with the increased spin polarization. These observations demonstrate the feasibility of using spin pumping to substantially increase the effective magnetic field to which the A1{\text{A}}_{1}A1​ phase is exposed.
The superfluid 3He A1 phase contains a spin-polarized condensate. This property allows novel superfluid spin current experiments. In the mechano-spin effect of the A1 phase a mechanically applied pressure gradient and a superleak-spin filter enable to directly boost spin polarization of 3He in a small chamber. Using a flexible membrane as an electrostatically actuated pump, we carried out such experiments and observed 50% enhancement of spin density. Here we report on a new 3He-hydraulic actuator for achieving greater enhancement of spin density. The actuator consists of two liquid 3He chambers located at a 4.2 K plate and in the interior of the cell. The pressure in the 4.2 K chamber is heater-controlled and it transmits a force onto a membrane in the cell. The motion of the membrane induces spin-polarized current into an accumulation chamber.
Under applied magnetic field, the originally single superfluid 3 He transition near 3 mK in zero field splits into two transitions between which a new A 1 phase emerges. The two second order transitions are marked by abrupt changes in viscosity, zero sound attenuation and nuclear magnetic resonance. To date, the maximum magnetic field for producing A 1 phase is 15 T. The A 1 phase has been identified with a spin-polarized (ferromagnetic) superfluid system which breaks the relative symmetry between spin, orbit and gauge spaces. A superfluid mass current in A 1 is simultaneously a spin current resulting in the propagation of spin-entropy wave. Experiments with spin-entropy wave provide measurements of anisotropic superfluid density and strong coupling parameters, spin diffusion coefficient and texture transformations. Owing to the spin-polarized nature, superflows may be generated by applied magnetic field gradients and measured from the induced magnetic fountain pressure. The mechanical spin density detector is developed to measure the spin relaxation in A 1 phase. The observed unexpected temperature dependence of the spin relaxation time gives evidence that the A 1 phase contains a small amount of the predicted minority spin condensate from dipolar interaction energy.
Measurements on spin relaxation time in superfluid 3A1^{3}\mathit{A}_{1}3A1​ were carried out at a pressure of 22.4 bar and a static field of 7.5 kG. The influence of surface coverage by 4He^{4}\mathrm{He}4He was systematically explored. The 4He^{4}\mathrm{He}4He coverage is used to eliminate the spin relaxation at the boundary walls and to extract the spin relaxation time in the warmer half of bulk A1{\mathit{A}}_{1}A1​ phase. The bulk spin relaxation time is much shorter than expected from Leggett-Takagi theory applied to the A1{\mathit{A}}_{1}A1​ phase. In the colder half of the phase, 4He^{4}\mathrm{He}4He coverage anomalously has no effect on the spin relaxation time.
We carried out experiments on a spin polarized 3He- 4He mixture with 3He concentration x 3 = 6.26 × 10−4, and on pure 3He liquid. Spin polarization affects the transport properties of these Fermi systems. The effect on momentum transport was studied by using a vibrating-wire viscometer to measure viscosity of the 3He-4He mixture over the temperature range 6.09 mK–100 mK in 7.96 T and 1.00 T magnetic fields. A large viscosity increase was observed upon application of the 7.96 T magnetic field for temperature T < TF(TF = 19.5 mK is the Fermi temperature). The observed viscosity is in very good agreement with theoretical calculations for a dilute Fermi gas by Jeon and Mullin [1988, 1989] and Mullin and Jeon [1992]. The polarization effect on spin transport was investigated by measuring the transverse diffusion coefficient D ⊥ in pure 3He liquid at saturated vapor pressure and at 15.85 bar over the temperature range 4.5 mK–159 mK in a 7.96 T magnetic field. We used a pulsed NMR spin echo technique in a field gradient of 16.0 G/cm to do the measurements and fits to the Leggett equations [1970] to obtain D⊥. For T < 20 mK, we found D⊥ is less than measured in earlier experiments at lower magnetic fields. D⊥ does not increase with decreasing temperature as 1/T2, but appears to approach a constant as T → 0 while it is expected that the longitudinal spin diffusion coefficient D∥ ∝ 1/ T2. This is called spin diffusion anisotropy and it was observed for the first time in our 3He liquid experiments. The anisotropy temperature we determined for 3He liquid was Ta = 16.4 ± 2.2 mK at saturated vapor pressure and in a 7.96 T magnetic field. The transverse spin diffusion in 3 He results agree qualitatively with theories proposed by Meyerovich and Musaeflan [1992, 1994]. They also agree qualitatively with theories proposed by Golosov and Ruckenstein [1995, 1998] by extrapolation of the dilute theory to a strongly interacting system.
The magnetic fountain effect occurring in superfluid 3 He A 1 phase is a unique phenomenon in which the pressure and magnetic field gradients in the chemical potential are balanced. The effect has been applied extensively to investigate the intrinsic spin relaxation. We constructed a new improved sample cell. The new cell includes an inner detector and an outer reservoir chamber made of "Macor" which was helpful to reduce the heat release possibly arising from proton nuclei under high magnetic fields. The measured temperature difference between the two chambers was cut to less than 5 μK. The measured relaxation time τ of the fountain pressure decreases monotonically and smoothly as the temperature is decreased from T c1 (normal- A 1 boundary) towards T c2 (A 1 - A 2 boundary). As the temperature approaches T c2 , τ tends to vanish smoothly.