EXPERIMENTAL CONSIDERATIONS ON PRODUCING HIGHLY POLARIZED LIQUID 3He IN A MATRIX OF SOLID 4He

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EXPERIMENTAL CONSIDERATIONS ON

PRODUCING HIGHLY POLARIZED LIQUID 3He IN A MATRIX OF SOLID 4He

A. Greenberg, B. Hébral, M. Papoular, M. Béal-Monod

To cite this version:

A. Greenberg, B. Hébral, M. Papoular, M. Béal-Monod. EXPERIMENTAL CONSIDERATIONS ON PRODUCING HIGHLY POLARIZED LIQUID 3He IN A MATRIX OF SOLID 4He. Journal de Physique Colloques, 1980, 41 (C7), pp.C7-79-C7-82. �10.1051/jphyscol:1980713�. �jpa-00220150�

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EXPERIMENTAL CONSIDERATIONS ON PRODUCING HIGHLY POLARIZED LIQUID He IN A M A T R I X OF SOLID 4 ~ e

A . S. ~reenber~+, B. ~~bral', M. Papoulart , and M.T. B6al-Monod

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+

t CRTBT, CAPS, B.P. 166 X, 38042 GrenobZe Cedes, France.

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Rdsum6.- On rappelle brisvement les rgsultats obtenus dans deux expdriences oC des gouttes de 3 ~ e dtaient formdes dans 4He solide. Ces experiences montrent que de telles conditions sont favorables pour la production de 3 ~ e liquide fortement polarisd quasi-stable. Une solution solide de 3lie dilud dans 4 ~ e est proposde comme devant ttre un systSme prometteur pour l'obtention expdrimentale de 3He liquide hautement polaris6 en utilisant une ddcompression analogue B celle proposde par Castaing et NoziSres.

Abstract.- Two experiments are briefly reviewed in which droplets of 3 ~ e were formed in solid 4 ~ e . These experiments indicate such conditions are favorable for the production of quasi-stable highly polarized liquid 3He. A solid solution of dilute 3 ~ e in 4 ~ e is proposed as a promising system to produce experimentally realizable highly polarized liquid 3 ~ e using the Castaing-Nozisres decom- pression.

One of the major problems in producing quasi-- stable highly polarized spin systems is the effi- ciency of wall or interface relaxation mechanisms.

Perhaps a promising system in which to produce highly polarized liquid 3 ~ e is the phase separated dilute or concentrated liquid 3 ~ e phase contained

in solid He. In this paper we briefly review both 4 an early and a recent experiment which are perhaps relevant to the problem of producing, by a method similar to one first suggested by Castaing and NoziPres, experimentally realizable polarized liquid 3 ~ e .

While liquid mixtures of 3 ~ e and He have 4 been well studied, especially near the critical point, less well investigated are the solid solu- tions of 3 ~ e and He for which a phase separation 4 was first discovered in 1962 by Edwards,

Mc Williams and

aunt.^

1. Cornell experiment.- In an experiment performed 3 at Cornell University in 1970, bulk samples,dia- meter 0 % 6 mm, containing between 1 and 2 % 3 ~ e

were observed to undergo an abrupt change in nnr relaxation times at a temperature of about 180 mK.

In one sample,at a pressure of 27.3 bar and con- taining 2 % 3 ~ e , the spin lattice relaxation time, T,decreased from 220 s in the diluted solid phase,

1

to 40 s in the separated phase, while T2, the spin- spin relaxation time, went from 4 ms to 1.2 s. A strain gauge in the cell simultaneously measured a pressure inctteane of 0.91 bar. Since the final pressure of the sample was lower than the pressure of the pure 3 ~ e melting curve, the He had to be 3 in a liquid phase in equilibrium with solid 413e.

The large value of T I was an early indication of how efficient a 4 ~ e "container" is in preserving the He polarization. 3

The liquid phase at that time was assumed to be nearly pure, the equilibrium being between nearly pure hcp 4 ~ e and liquid 3 ~ e with inpurity

concentrations given by the coexistence curve of ref. 2. Furthermore the liquid phase was believed to be dispersed throughout the solid in the form of small droplets because of the long diffusion times inhibited gravitational stratification :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980713

JOURNAL DE PHYSIQUE

*34 the

=-% ² a2 10-]~.,2where a = 3.27x10-* cm is

T T

34 - ^{1 5}

lattice spacing, and ( ~ T T ~ ~ ) = 10 Hz is the measured3 3-4 hopping frequency. This gives rise to a droplet size d = x1'3/~2D34; 5 5 p supposing that the droplets were formed during the experi- mentally observed equilibrium time, t % ^{500 s.}

If the droplet was not rich in 3 ~ e , but dilute as we discuss below, the above estimate of the droplet size would have to be increased.

2. Grenoble experiment.- Another experiment that was recently performed in Grenoble was the measure- ment of the specific heat of the droplets confined in the He matrix at a pressure of 4 27 bar.4 However the Grenoble experiment differed from the previous experiment in that a confined geometry was used with average open pore size of several microns, and

the initial concentration of the 3 ~ e was smaller, only 1000 ppm. The result was a smaller sized dro- plet, of typical diameter lo3 ;,assuming only one droplet present in each pore.

A linear specific heat was found for tempera- tures below the Solid Phase Separation (SPS), TSpS

2

and for pressures between 25 bar and 34 bar.

However arguments4s5 similar ro the BBP argu- ments for liquid 3 ~ e - 4 He solutions indicate that it is possible to have liquid 4 ~ e diluted with He up 3 to pressures slightly greater than 25 bar. In fact

I . - P k a e diagkam i n (T , x) pLane

%-

n owug expeded pke~nwre dependence

04

chitical p4anwre p h a e .

3 ~ e in 4 ~ e at T = 0. This dilute phase would per- sist only for pressures less than a critical pressure, p

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between the pure hcp 4 ~ e and essentially pure 3 ~ e liquid droplets. As shown in fig. I the characte- ristic,concentrations x and x o join at T = 0 for P = p

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with increasing p. ~vidence' that such a dilute phase was in fact observed in the Cornell experi- ment is the large value of the pressure shift Ap = 0.9 bar. Experimental measurements are clearly necessary to map out the phase diagram at low temperatures.

3. Dilution cooling.- We would like to also point out that there is the possibility of refrigeration in passing from just above

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1. This is so if the initial 3 ~ e concentration x is tion. It is seen that in fact starting from initial less than a critical concentration xc(p) that must temperature T in the concentrated liquid ( 3 He, or be less than the maximum solubility x,(p) of liquid 3 ~ e +), the temperature is reduced to Tb as the

pressure is lowed to below p

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This is the process of dilution cooling.

An alternative interpretation of the specific heat, is to suppose the droplets to be pure 3 ~ e . The Grenoble group then found an effective mass mR/m = 10.2 ; the Wheatley group found m*/m

-

at p = 34 bar, which is 7 bar more than this expe- riment. This could be explained by cpin fluctua- tions, that is paramagnons in a very nearly magne- tic Fermi liquid which in turn are enhanced by surface effects. If this is the case, then such a large effective mass would also mean an enhancement of the susceptibility by nearly two orders of magnitude above the Pauli value. Thus, even for

fields available in the laboratory it may be ~ossible to have an equilibrium polarization of something like 10%.

4. Experimental possibilities.- By combining the two effects discussed above it may be possible to produce a nearly stable highly polarized liquid.

The starting point would be the phase separated solids, that is initial pressure slightly greater

probably in small (depending on geometry) pure clusters and polarized to about 72 ",early inde- pendent of the temperature below 10 u K . ~ A rather gentle decompression of less than I bar (the amount of the decompression will depend on the polarized melting curve which has to be determined ) will

lead to highly polarized liquid droplets in the 4 ~ e matrix. This is similar to the pure solid decom- pression of Castaing and NoziSres.' Because the 4 ~ e

3 wall does not efficiently relax the 3 ~ e spins and because the finallevel of polarization will be higher, this should give the optimum experimental conditions for polarized 3 ~ e . It should be interes- ting for example to measure the difference in molar volume between the polarized and non-polarized phase estimated to be 0.1 cm3/mole near melting pressures.' In a I cm cell containing only 3 I % 3 ~ e

at constant volume this would be a change in pressure of % 5x10-~, relatively easy to detect ,with modern strain gauges. Other experiments such

as specific heat and nmr should be possible since the solid 4 ~ e acts essentially as a vacuum at low T.

The dieutc? liquid phase inside the solid 4 ~ e matrix may also lead to a conveniently polarized Fermi-liquid system. As pointed out by Nozisres in this Conference, the relaxation time T I increases

1 1

as -(-) for the degenerate (classical) dilute x 2

solution of coficentration x. A slight decompression

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from p.>p to pf<p , starting from the prepolarized pure liquid 3 ~ e phase, would then lead to a highly stable polarized dilute Fermi liquid of concentra- tion x = x(p ) lying between 0 and x o , depending

f

on the final pressure pf. This dilute liquid would have still higher probability than pure 3 ~ e of being "quasi-solid" rather than "quasi-

JOURNAL DE PHYSIQUE

f erromagneticl' (see ref. 1). 6. 1 I . T . Bdal-?lonod and A. Theumann, Comm. at the So far we have discussed producing polarized

Znt. Conf. "Ordering in 2-dimensions" Lake concentrated or dilute 3 ~ e liquid by decompressions

Geneva, Wisconsin, May 1980, to be published.

However the phase diagram offers another means for J.P. Eluscat, M.T. Bdal-Monod, D.M. Newns, producing the polarized state within the liquid and D. Spanjaard, Phys. Rev.

11,

relaxation time T1. The process proposed here is to

7. H.,Godfrin, G. Frossati, A.S. Greenberg, start with a solid solution of 3 ~ e diluted in 4 ~ e .

B. HBbral, 3. Thoulouze (to be published).

If the initial pressure is below the pure 3 ~ e melting curve then the 3 ~ e will phase separate into

liquid droplets as we discussed in the Cornell and Grenoble experiments. However if this is done in a strong magnetic field then the 'He spins will be initially polarized at low temperatures. Thus the resulting liquid 3 ~ e droplets should be in a highly polarized state. For practical reasons it would be necessary to start with very small concentrations of 3 ~ e , x

6

2

We appreciate useful comments from M. Bernier, A. Landesman, G. Frossati, H. Godfrin, and

D. Thoulouze.

References

1 . B. Castaing and P. Noziires, J. Physique 60, ²⁵⁷

(1979).

2. D.O. Edwards, A.S. Mc Williams, and J.G. Daunt, Phys. Rev. Lett.

9,

3. A.S. Greenberg, W.C. Thomlinson, and

R.C. Richardson, J. Low Temp. Phys. 8, ^{3 (1972).}

4. B. Hdbral, A.S. Greenberg, M.T. Bdal-Monod, M. Papoular, G. Frossati, H. Godfrin, and D. Thoulouze, (to be published).

5. 1:. Papoular, A.S. Greenberg, B. Hdbral (paper submitted to Surface Chenistry Conference, San Srancisco, August 1980).