- Activity Report - Research Highlights 2010
A. Yamaguchi, Y. Aoki and M. Kubota
Spin Current Manipulation in Superfluid 3He A1 Phase
Fig.1. MSE response in A2 and A1 phases (green and red curves, respectively). Temperatures are indicated by normalized reduced temperature r=(T-TC1)/(TC1-TC2). Induced spin-pressure differences (δP) (left ordinate) are plotted. δP for each r is displaced by 4000 dyne/cm2 for clarity. The right ordinates show the computed fractional increase in spin-density difference 12 δS /Sn. The liquid pressure is 28 bar and the applied static magnetic field is 5 T. Inset: Phase diagram of superfluid 3He at a constant liquid pressure. The MSE process corresponds to the red arrow.
The superfluid phases of liquid 3He appear below a pressure-dependent transition temperature Tc. In zero applied magnetic field two superfluid phases known as A and B phases appear. In applied magnetic fields, a new A1 phase emerges between the two transition temperatures TC1 and TC2 where TC2< TC< TC1 at all pressures. The A1 phase has been regarded as a “ferromagnetic” superfluid phase whose condensate involves spin polarized pairs with the energy gap Δ↑↑>0 but Δ↓↓≡0. Such a unique phase like A1 phase has not been observed in any other condensed-matter system yet. Therefore, it is quite interesting to investigate this magnetically ordered superfluid A1 phase from the aspect of fundametal physics. The unique hydrodynamics of the A1 phase originating in the broken relative spin-orbit-gauge symmetry allows spin superflow to be created either by magnetic field or pressure gradient; In the magnetic fountain effect (MFE), an applied magnetic field gradient across a superleak is accompanied by a pressure gradient, creating spin-flow through the superleak; In the mechano-spin effect (MSE), on the other hand, the mechanically applied pressure gradient and the superleak serving as spin filter enable us to directly create spin supercurrent and to boost spin polarization of 3He in a small chamber. Recently, we have extensively studied the spin fluid dynamics in the A1 phase using both the MFE and MSE techniques [1-5]. One very important issue in the spin fluid dynamics is the origin of the unexpected spin relaxation that we observed in MFE experiments . Understanding this spin relaxation would yield important clues in designing a spin-pumping device for boosting the spin polarization to much greater level than feasible by available static magnetic fields.So far, the MSE experiments have been carried out to observe the accumulated spin density in the small chamber by using a glass capillary array (to increase the spin/mass flux) as the superleak and a flexible membrane as an electrostatically actuated pneumatic pump . The change in spin density was deduced from the measured differential pressure. Measurements in an applied static magnetic field of 5 T indicate that the superfluid polarization increases by 20 ~ 50% from that produced by the static field (Figure 1). That is, the effective field increased to 6 ~ 7.5 T. Remarkably, even a tiny electrical voltage of several hundreds volts applied to the actuator, exerts a tesla-scale effecitive field on the system. The increase in polarization is corroborated by the observed increase in the spin relaxation time. By making improvements in (1)superleak, (2)decreasing the chamber volume and (3)improving the pneumatic pumping mechanism, even greater increase in polarization should be possible.
We are currently developing a new 3He -hydraulic actuator for achieving greater enhancement of spin density. The actuator consists of two small 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 flexible membrane in the cell. The motion of the membrane induces spin-polarized current through an array of superleak capillary tubes into an accumulation chamber. At the same time, a high field-3He-NMR detector is tested to detect the polarization increase directly.
The superfluidity of liquid 3He in the higher magnetic fields is one of the long-standing unexplored topics in the ultra-low temperature physics. It might open up a new technique for manipulating the spin super current and for studying A1 phase in much greater effective field than the maximum 15 T static field so far achieved.