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Minority spin condensate in the spin-polarized superfluid 3He A1 phase


The magnetic properties of 3He in its various phases originate from the interactions among the nuclear spins1. The spin-polarized ‘ferromagnetic’ superfluid 3He A1 phase2 (which forms below 3 mK between two transition temperatures, Tc1 and Tc2, in an external magnetic field) serves as a material in which theories of fundamental magnetic processes and macroscopic quantum spin phenomena may be tested. Conventionally, the superfluid component of the A1 phase is understood3,4,5,6 to contain only the majority spin condensate, having energetically favoured paired spins directed along the external field and no minority spin condensate having paired spins in the opposite direction. Because of difficulties in satisfying both the ultralow temperature and high magnetic field required to produce a substantial phase space, there exist few studies of spin dynamics phenomena that could be used to test the conventional view of the A1 phase. Here we develop a mechanical spin density detector that operates in the required regime, enabling us to perform measurements of spin relaxation in the A1 phase as a function of temperature, pressure and magnetic field. Our mechanical spin detector is based in principle on the magnetic fountain effect7; spin-polarized superfluid motion can be induced both magnetically and mechanically, and we demonstrate the feasibility of increasing spin polarization by a mechanical spin filtering process. In the high temperature range of the A1 phase near Tc1, the measured spin relaxation time is long, as expected2,8,9. Unexpectedly, the spin relaxation rate increases rapidly as the temperature is decreased towards Tc2. Our measurements, together with Leggett–Takagi theory5, demonstrate that a minute presence of minority spin pairs is responsible for this unexpected spin relaxation behaviour. Thus, the long-held conventional view2 that the A1 phase contains only the majority spin condensate is inadequate.

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Figure 1: Schematic cross-sectional view of our spin density detector.
Figure 2: Time response of the detector membrane displacement Z to an applied step in field gradient at time t  = 0.
Figure 3: Measured relaxation time versus normalized reduced temperature at 21 bar.
Figure 4: Dependence of longitudinal relaxation rate on reduced temperature relative to Tc2.


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We thank R. Masutomi and K. Kimura for their contributions in the early stages of the experiments and A. Leggett for correspondence. This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Areas of MEXT Japan and JPSJ, and by the Condensed Matter Physics and East Asia and Pacific Programs of the US National Science Foundation.

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Correspondence to H. Kojima.

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Yamaguchi, A., Kobayashi, S., Ishimoto, H. et al. Minority spin condensate in the spin-polarized superfluid 3He A1 phase. Nature 444, 909–912 (2006).

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