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Submitted on 1 Jan 1978
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SPIN RELAXATION IN 3He-B FAR BELOW Tc
O. Avenel, P. Berglund, Daniel Bloyet, E. Varoquaux, C. Vibet
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-26
SPIN RELAXATION IN
3He-B FAR BELOW T
c0. Avenel
0, P . Berglund
0, D. Bloyet*, E. Varoquaux and C. Vibet .
° Ovme des Merisiers, CEN-Saclay, 91190 Gif-Sur-Yvette _
XK34
Xnstitut d'Eleotronique Fondamentale et 'Laboratoire de Physique dee Solides ,
VniversitS Paris Sud, 91405 Orsay, Fvccnoe
Résumé.- Nous avons étudié l'amortissement de la résonance nucléaire longitudinale de l*3He-B à température basse devant T .
c
Abstract.- We have studied the damping of the magnetization ringing in 3He-B at temperatures small with respect to T .
The longitudinal ringing of magnetization in superfluid He is, close to the transition tempera-ture T , a rather heavily damped phenomenon /l/, apart from the occurence of a "wall-pinned mode" in certain geometries in 3He-B. /2/ The damping is
caused by the presence of normal quasi-particles and disappears as the temperature is decreased. As has been shown by R. Combescot and H. Ebisawa /3/, and A. Leggett and S. Takagi / 4 / , the ringing damping time T is related to an intrinsic quasi-particle lifetime T by
ce J
1 _ Y + 2 f Tc E "
T„fi " 2(2+Y) |l'+Fa (2+Y) I 1+T"Si" U '
R „
_0_ cK
||
3
At low temperature ( T « T ) , the Yosida function Y behaves asymptotically as (2TT A/T) 2
exp (-A/T) and the Combescot-Ebisawa function f as (2 7r T / A )1'2 exp (-A/T). The first factor of the right hand side in eq. (1) contains the fact that •the portion of the magnetization which causes the relaxation diminishes rapidly as the temperature is lowered. The disappearance of the normal fluid - or quasi-particle-component is also reflected on the low T behaviour of T, evaluated by R. Combescot lb I
and by others /(>/, which can be shown on general grounds to go as :
T = A ( A / T )l / 2 exp (A/T) (2)
where A is a constant considered below. Thus, the collision time becomes long at low temperature. The
Present address: Low Temperature Laboratory, Helsinki University of Technology, Espoo 15 SF 02150 - Finland
Laboratoires associes au CNRS
lorentzian function of eq. (I) also acts, in the collision-less limit, in such a way as to increase T . So, it is expected that the magnetization
rin-K
ging becomes a long lived phenomenon at sufficien-tly low temperature.
We have performed direct observations, briefly reported elsewhere (7), of the longitudinal NMR signals in 3He-B cooled to below 0.6 mK by a nuclear
demagnetization apparatus at various pressures. The n-vector is aligned by a stack of mylar plates. 0.4 mm apart, 9 mm in diameter. The temperature is measured by NMR in Pt in a field of 270 Oe. This field can be reduced to zero or increased to 400 Oe with only minor thermal effects. The ringing is excited by rf pulses of one full-cycle with ampli-tudes up to 5 Oe. Before reaching the intrinsic relaxation, we have run across several other eff ecJEs (1) In an applied H such that yH » Q|(, the
obser-ved ringing frequency is fl = S2 |cos8|, 9 being the angle between n and H Even a small fan-out in the directions of n across the sample causes an appre-ciable loss of coherence of the ringing signal. This effect disappears in zero field.
(2) When the excitation field Hj is increased, we observe a reduction of T^ and a progressive distor-tion of the exponential behaviour. At high exita-tion levels (TjyU1/n„ ^ 0.1), the signal decay is fast and approximately linear in time. No exponen-tial tail with the small signal behaviour is obser-ved. This effect is partially due to the fact that Hj is inhomogeneous and varies by ^ 20 % over the
3He sample. Thus, after the excitation, different initial conditions prevail at various points of the sample, and a dephasing occurs, due to the non-linear character of the Leggett equations. A
rical evaluation of this effect shows that it may
in wihich
V(o)is the density of states at thepermi
be held responsible for only part of the anomalous
surface for both spin directions and n is the num-
decay at high excitation levels.
ber density of 3 ~ e
atom.
(3) The other part comes from a memory effect which
represents an internal behaviour of the superfluid.
This effect is probed as follows. A large excita-
tion pulse is sent at t=O, followed by a short, non
exponential free-ringing decay. Then, at time
8o7
asmall pulse is shown on the perturbed sample. If
gois small the ringing observed after the small pulse
is shorter than the small-excitation free ringing
decay. It recovers in a characteristic time T
M
'
which measures the memory of the system. T
,
is of
.A
the order of 2 ms at 29.3 bar in 270 Oe and shows
(1
+
2
)versus 1/T, T in
mK, T
little temperature dependence between 0.6 and 1.lmK
1
R
in us. T
.
-
=2.45 mK at 20.0 bar.
This effect is linked to the existence of magneti
-
zation gradients in the sample and is being investi-
We find for the quantity expressed by (3) thevalue
gated at present.
4.5
+
0.3 at 20 bar and 6.8
+
0.3 at 29.3 bar.
The experimental observed values of the
These values are tobe compared with the
intrinsic rin ing time TR are shown in fig.
(1)and
%
theoretical estimate which is 10.
(2) where Ln
pR
(I
+ 2:)I
is plotted versus I/T.
According to the low-temperature limit of eq. (I),
Further experiments are in progress at 10
the slope of the straight lines obtained in the
and
1bar.
figure yields 2A.
P. 29 bar
