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Submitted on 1 Jan 1978
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NMR- AND SUSCEPTIBILITY INVESTIGATIONS
ON B.C.C. SUPERCONDUCTING SOLID
SOLUTIONS OF Mo(1-x)Vx
H. Khan, U. Köbler, K. Lüders, Ch. Raub, Z. Szücs
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-465
NMR- AND SUSCEPTIBILITY INVESTIGATIONS ON B . C . C . SUPERCONDUCTING SOLID SOLUTIONS OF M0( i -x) V x
+ ++ .. +++ + .. +++
H.R. Khan , U. Kobler , K. Luders , C h . J . Raub and Z. Szucs
+ Foreohungsinst. fiii> Edelmetalle und Metallohemie, SahuSbisoh Grmind,FRG
+* Institut fur FesthSvpevfovsahung dev Kernfovsehungsanlage Jiilioh3FRG +++Faohbereioh Physik, Freie Univevsifab Bevlin,FRG
Résumé.- On a mesuré le Knight-shift, les taux de relaxation et la susceptibilité d'une série de so-lutions solides de Mo(i-x)Vx. Pour les comparer aux Tc correspondantes, on a séparé des contributions
s-, d- et orbitales et on a calculé les densités d'état Ns(Ef) et Nd(Ef). On peut conclure que Tc
est relié à N,(E,.) . d £
Abstract.- Knight-Shift-, relaxation rate, and susceptibility measurements were performed at a series of b.c.c. solid solution of Mo(j_x)Vx. For comparison with the corresponding Tc-values the s-, d-,
and orbital contributions were separated and the densities of states Ns(Ef) and N<j(Ef) calculated. It can be concluded, that T is related to N (E ) .
c a t
For A15-structure compounds a maximum of the superconducting transition temperature Tc is obser-ved at the stoichiometric composition A3B. This ma-ximum is related to the mama-ximum in the d-electron density of states at the Fermi surface, Nd( Ef) , as
shown by Knight-shift and spin-lattice relaxation time measurements on systems like Pt.. .V /1,2/ and Ga,, .V /3/. In this paper similar
investiga-(1 x) x
tions onb.c.c. superconducting solid solutions of Mo, NV are reported. This system forms a
conti-(]-x) x
nuous series of b.c.c. solid solutions and T shows a minimum. Measurements of the Knight-shift of 9 SMo
and 5 1V and the relaxation rate of 5 1V along with
magnetic susceptibility measurements as a function of composition were performed in order to find con-nections of these quantities with T , especially the influence of N (E ) on T£ in b.c.c. alloys of transition metals.
The Mo,. -V solid solutions were prepared (1-x) x
by arc melting in argon atmosphere and were homoge-nized by heat treatment at 1300°C for a period of 6 hours. The alloys were found to be homogeneous as checked by optical microscopy. Lattice parameters were determined from X-ray diffraction patterns using
the CuK radiation. The lattice parameter increases linearly from a = 3.024 A to a = 3.150 A with in-creasing Mo concentration. The Knight-shifts of
5 1V and 9 5Mo as a function of composition were mea-sured by cw spectroscopy, and for measuring the spin-lattice relaxation time of S 1V a pulsed NMR
spectro-meter was used. The susceptibility measurements were performed with a Faraday magnetometer I hi.
The superconducting transition temperature T as a function of the composition is plotted in
c
figure 1 /5/. From the value of pure V T decrea-ses with increasing concentration of Mo and after going through a minimum again increases to the va-lue of pure Mo.
1 0 i i 1 — - • 1 x 0.8- \ w \ fo.7- 51 ^ ^ " ~ * ^ \ \ 0.6 k ^ - ^ m A
^
-5 t _ _D - J <-111 CL h £\ ^ - '
V 80 60 40 20 Mo V concentration , at % F i g . 1 : V a r i a t i o n of Tc and K n i g h t - s h i f t s of 5 1V and 95Mo a s a f u n c t i o n of V c o n c e n t r a t i o n i n M o . , . V . xIn the same figure the Knight-shifts of 95Mo and 5 1V are also plotted as a function of composition. The Knight-shift of 9 5Mo shows a maximum at a
composi-tion of Mo^pVgQ whereas the maximum of the
shift of 5 1 ~ lies at the composition M O ~ ~ V ~ ~ . These curves suggest already a correlation of the Knight-
shift maximum to the minimum of Tc, especially in
the case of
"v.
Furthermore, with the usual assum-ption, that the contribution of the d-electrons to the Knight-shift is negative, this is a first indi- cation for the dominant role of the d-electrons for superconductivity in these alloys.
The relaxation rate R = (T~T)-' as a function
of the Mo concentration is plotted in figure 2.
Fig. 2 : Variation of relaxation rate of 5 1and ~
the magnetic susceptibility as a function of V con-
centration in Mo V
(I-x) x '
TI is the spin-lattice relaxation time of 5 1 except ~
for pure Mo, where relaxation time of 9 5 ~ 0 is plot-
ted. R decreases, linearly with increasing Mo con- centration. Contrary to K and X, which vary linear- ly with the density of states at the Fermi surface, R is proportional to the square of the density of
states, N~(E ) . In the same figure the magnetic
f
susceptibility, X , as a function of composition is
also plotted. Like R,
x
also decreases continuauslywith increasing Mo concentration, but not linearly. On the Mo side the slope of the curve decreases.
R as well as
x
do not show any direct correlationwith the observed minimum in Tc.
With the aim of calculating the density of
states at the Fermi surface, Ns (Ef) and Nd(Ef), a
separation of the s-, d-, and orbital contributions
to the measured quantities K, R and
x
,
is perfor-using their connections via the hyperfine fields Ylf/6,7/. The resulting system of nine equations with fifteen variables can be used for the separa-
tion, if at least six of the fifteen quantities are known. Three are the measured quantities K, R, and
x
as shown in the figures. Further three quantities,the hyperf ine fields Hhf (s)
,
Ylf
(d), and Hhf (orb), areknown for the pure metals V /8/ and Yo 171. For the
solid solutions it is ~lausible to assume a monotonous
variation with concentration. We used a modified li- near dependence with the influence of the pure me- tal hyperfine fields predominating in the low con- centration regions. This assumption agrees fairly well with specific heat measurements.
Using the susceptibility contributions
x
and
xd,
the corresponding densities of states canbe calculated. The results are given in table I.
Table I
V 80 60 40 20 Mo
N~ (E~)/D= erg atom 7.26 5.47 4.63 2.13 1.20 1.52
states
Ns (Ef)/DU erg atom 0.74 0.68 0.44 0.32 0.21 0.21
Ns(Ef) is small compared with Nd(Ef) and decreases linearly with increasing Mo concentration. Contraty
to this, N (E ) shows a minimum at 80 at-% Mo. This
d f
agrees qualitatively with the T -behaviour. Due to the lack of experimental values, the exact position of the T -minimum, which can be somewhere in the re-
gion between 50 and 90 at-% (figure I ) , is not known.
Nevertheless, from our results it can be concluded, that T is related to the d-electron density of sta-
tes N (E ) for the investigated system.
d f
References
/ I / Wulffers,L.A.G.M., Poulis, N.J., Khan,H.R.and
Raub, Ch.J., ((1975) 85.
/2/ Wulffers, L.A.G.M., Thesis Leiden (1977)
/3/ Gevers,A., Poulis, N.J., Kahn, H.R.and Raub,Ch.J.
Submitted to the conference in Grenoble.
/4/ KEbler,U., Deloie,F., Rep. Jul-1305, KFA Julich (1976).
/ S f Andres,K., Sucher,E., Maita,J.P., Sherwood,R.C.,
Phys. Rev.
178
(1969) 702./6/ Karcher,R., ~Gders,~., Roth,G., Sziics,Z. Phps.
Stat. Sol. (b)
-
79 (1977) 519./7/ Karcher,R., K;bler,U., ~;ders,K., Sziics,Z.,
Phys. Stat. Sol. (b)
86
(1977) 189./ 8 / Hechtfischer,D., Z. Physik
