Minority Spin Condensate in Spin-Polarized Superfluid ${}^3$He-A${}_1$ Phase

Abstract

The spin-polarized superfluid ${}^3$He A${}_1$ phase, which forms below 3 mK in external magnetic field, serves as a material in which theoretical notions of fundamental magnetic processes and macroscopic quantum spin phenomena may be tested. Conventionally, the superfluid component of the A${}_1$ phase is understood 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. We have developed a novel mechanical spin density detector to measure the spin relaxation in the A${}_1$ phase as functions of temperature, pressure and magnetic field. The mechanical spin detector is based in principle on the unique magnetic fountain effect occurring only in the A${}_1$ phase (occurring between two transition temperatures, T${}_{c1}$ and T${}_{c2}$). Unexpectedly, the spin relaxation rate increases rapidly as the temperature is decreased towards T${}_{c2}$. Our measurements, together with Leggett-Takagi and Monien-Tewordt theories, demonstrate that a minute presence of minority spin pairs is responsible for the unexpected spin relaxation phenomena in A${}_1$ phase.

13 pages, 4 figures. Accepted for publication in Nature