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Submitted on 1 Jan 1978
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NUCLEAR SPIN-LATTICE RELAXATION AND
SUPERCONDUCTIVITY IN V(1-x)Gax
COMPOUNDS
A. Gevers, N.J. Poulis, H. Khan, Ch. Raub
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-404
NUCLEAR SPIN-LATTICE RELAXATION AND SUPERCONDUCTIVITY IN V/, <>Ga COMPOUNDS v l - x / x
A. Gevers , N . J . Poulis , H.R. Khan and Ch.J. Raub
+ ++
Kammerlingh Onnes Laborator-Cum dev Rijkswiivers-Lt&t Leiden, Leiden, The Netherlands. Forschungs-•institut fiir Edelmetalle imd Metdllehemie, 7070 Schw'abisoh Gmiind, West-Germany.
Résumé.- La relaxation non exponentielle, observée dans les alliages V. _ .Ga peut-être décrite avec deux temps de relaxation, l'un correspondant à la phase A-15et l'astre à une deuxième phase présente dans les échantillons. La valeur de (TjT)- 1, correspondant à la structure A-15, est forte-ment dépendante de la température dans l'état normal. La variation des (TjT)-1 de V et de Ga, en
fonction de la concentration x, est en accord avec la variation de T avec x. c
Abstract.- The observed non-exponential relaxation behaviour in V, _ ..Ga compounds can be described with two relaxation times, one corresponding to the A 15 phase, tne§the? one to an additional phase present in the samples. The value of ( T ^ )- 1 corresponding to the A-15 structure shows a strong
tem-perature dependence in the normal state. The composition dependence of (TjT)~ for both V and Ga in the A-15 structure is in agreement with the composition dependence of T .
INTRODUCTION.- The NMR study (Knight shift and rela-xation rate) on the system V. .Ga is attractive
(i-x) x for two reasons :
1° Both the V and Ga nuclei can be observed with NMR
2° The A-15 phase has a broad homogeneity range (ex-tending from 20 to 33 at % Ga /1,2/.
Knight shift measurements of both V and Ga show a clear minimum at x = 25 at % /3/. Superconducting transition temperature T in this series has a ma-ximum value for the stoichiometric composition (as can be seen in figure 2 ) . These combined results can be explained by a maximum value in the N,(Ef) at the Fermi level of the V3d electrons. N,(E,) should also be reflected in the relaxation parameter R = (T T) 1. (T is the nuclear spin-lattice relaxation
time).
RELAXATION MEASUREMENTS.- From the relaxation mea-surements made by the spin-echo technique it ap-pears that the recovery of the nuclear magnetization in all our V, _ .Ga samples is non exponential. Therefore accurate values of T for these samples
l
are difficult to obtain. To check the origin of this non-exponential behaviour, we further investigated an as cast sample in which the A-15 structure was confirmed by X-ray diffraction and the composition was analysed being V Ga . T of the sample was
6 73 27 C
found to be 12.75 K.
From a careful analysis of the recoveries of 5lV in
the normal state (from T up to 300 K) it turns out that these curves can be described - within
experi-mental accuracy - with two relaxation times : M^ - Mz(t) mx = A exp (-t/T ) + B exp (-t/T !)(1) in which M z( t ) is the magnetization along the
exter-nal magnetic field and M^ is the equilibrium magne-tization. It appears that for the V Ga sample A
77 23
= 0.81, B = 0.19 and T x obeys the Korringa relation
(TJT) = constant = 1.61 sK. Temperature dependence of (T T) might indicate that the sample contains
l
(at least) two phases : a main phase (A = 0.81) with a large and temperature dependent R = (T T ) ~ and a second phase with a small and temperature indepen-dent R1 = (T ' T )- 1 (figure 1). Both relatively large
values of R and the anomalous temperature dependence in the normal state is characteristic for the A-15 structure. Considering that A-15 becomes supercon-ducting and the second phase is not a superconductor then T corresponding to the A-15 phase should con-siderably increase below T because of BCS electron pairing, whereas T 1 for the second phase should continue to follow the Korringa relation. Consequen-tly, below T both relaxation times must become equal to each other. A nearly exponential recovery of the magnetization at a temperature around 4 K. Figure 1 confirms this. Below this temperature the relaxation time corresponding to the A-15 structure must be derived from the tail of the recovery curve. The temperature dependence fo this T (figure I) was found to be consistent with an energy gap 2A = 3.4 k_T which is in reasonable agreement with the BCS prediction 2A = 3.5 k T . An analysis of the re-covery of the Ga signal like 5 V one, is not useful
because of poor signal to noise ratio. Actually the menon is being further investigated on 8-15 structu- Ga recoveries appear to differ less from exponential re compounds.
behaviour that the V ones.
Fig. 1 Temperature dependence of the relaxation ra- te of
'V
for the A-15 structure and the second phase in V Ga.
7 3 2 7
THE SECOND PHASE.- The presence of
a
second phase (V-Ga solid solution) in V Ga was confirmed by :7 3 27
l o X-ray diffraction 2' the NMR spectrum of 'Iv, consisting of a component without quadrupole inter- action, indicating the presence of a structure with V in cubic symmetry 1 3 1 .
Relaxation in the V Ga system
-
R-measurements(1-1 x
in the series V Gax(as cast) are analysed in the (1-x)
same way as before. In figure 2 , R is plotted as a function of x for T = 300 K, 77 K and 20 K. R =
R(x) for 'l~a at T = 20 K is also shown. In all si- tuations R exhibits a maximum for the stoichiometric composition. This follows the composition dependence of Knight shift and T
.
Composition dependence of Knight shift and R of Ga are similiar to that 0f.V. Apparently the V3d electrons are strongly admixed in the Ga and are also resonsible for the Knight shift and the relaxation of Ga. A similiar conclusim was drawn from our R-measurements on V(l-xlPtx sys-tem.
Temperature dependence of R in the normal state
-
A remarkable result is the strong temperature depen- dence of R in the normal state (figures I andZ),
which cannot be described with the lowest order T~ corrections. This indicates that the Fermi level lies near a very sharp peak in the N(Ef). The peak should have a width of the % kgT in order to account for
the observed effects / 3 , 4 / . This interesting pheno-
Fig. 2 : Composition dependence of R for 5 1and ~ 7 1 ~ a at different temperatures in V Ga conk-
pounds. Composition dependence of ~1'~~)alfo shown.
References
/ I / Van Vucht, J . H . N . , Brunning, H.A.C.M., Donker- sloot, H.C. and Gomes de Mesquita, A.H., Philips Res. Rep. (1 964) 407
/ 2 / Flckiger, R. and Standenmann, J.L., J. Less- Common Metals
50
(1976) 273/ 3 / Wulffers, L.A.G.M., Thesis, Leiden (1977)
