Spin-Polarized Quantum Fluids and Solids
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ations, Castaing and Nozieres4 proposed a technique to rapidly melt highly polarized solid 3He. The idea was to melt the solid faster than the spin relaxation time, Tj, thereby conserving the magnetization after melting. This is made possible in liquid helium since the bulk Tl is governed by the weak (spin nonconserving) dipolar interactions. Moreover, surface relaxation can be suppressed at low T by coating the walls of the cell containing the 3He with superfluid 4He. The polarizations produced by this method were originally close to 20%5 and have recently risen to as high as 65%.6 Unfortunately, the resulting spinpolarized liquid phase was at a temperature of 200 mK or more. To study the Fermi liquid behavior and the superfluidity as a function of polarization, much lower temperatures are needed. To get around this heating problem, a method that is a compromise between high polarization and low temperature was developed by Frossati's group in Leiden.7 This method starts with a mixture of highly polarized solid and weakly polarized liquid 3He. After melting, the only other source of heating would be due to the heat of mixing between the highly polarized liquid and the weakly polarized liquid. If the spin diffusion is more rapid than the melting, this will produce no additional heating.' In fact, cooling could occur in the solid-toliquid conversion if the solid could be spin polarized close to 100%. In this case, the solid entropy would drop below that of the liquid, and cooling would occur since the entropy would be conserved in the melting. To date, this technique has produced polarizations of approximately 25% (this is equivalent to B = 60 T) and temperatures less than 40 mK.7 With the low temperatures and high polarizations, we can begin to explore some theoretical predictions for the magneticequation of state (MEOS) for spin-polarized liquid 3He. There are three viable candidates for the MEOS for liquid 3He: the "nearly metamagnetic" (NM),8 the "nearly localized" (NL),9 and the "nearly ferromagnetic"1" (NF) models. The models are characterized by how close the liquid is to a given type of instability. The most complete picture of the MEOS for liquid 3He is the NM model of Bedell and SanchezCastro.8 We can characterize the MEOS using the polarization (A) dependence of the spin susceptibility, #(A). In the NM model, the ^-(A) initially increases, reaching a maximum of around A — 0.35, then falls to zero for A = 1.0. That the magnetic tendencies are enhanced with increasing A in the NM model suggest 3He is close to a field-induced metamagnetic phase transition. With the increase in ^(A), there is a
MRS BULLETIN/AUGUST 1993
Spin-Polarized Quantum Fluids and Solids
further enhancement of the ferromagnetic correlations, which in turn enhances the triplet pairing enhancement of the ferromagnetic correlations, which in turn enhances the triplet pairing tendencies.8 This is one of the most striking predictions of the NM model, predicting a Tc as high as 14 mK at A « 0.35 for a pressure of 25 bar.7 The NL and NF model
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