SUPERCONDUCTIVITY OF V3Si UNDER HYDROSTATIC PRESSURES TO 45 kBAR

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SUPERCONDUCTIVITY OF V3Si UNDER

HYDROSTATIC PRESSURES TO 45 kBAR

G. Fasol, J. Schilling, B. Seeber

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-41C

SUPERCONDUCTIVITY OF V

3S i UNDER HYDROSTATIC PRESSURES TO ^5 kBAR

G. Fasol, J . S c h i l l i n g and B. Seeber .

Inst, fiir Experimentalphysik IV, Ruhr-Universitdt Boahvm, Germany,

Dept.de Phys.de la Mat.Condensee, Univ.de GenSve, Suisse.

Résumé.- La température de transition supraconductrice de cristaux de V3Si qui ne montrent pas la transformation Batterman-Barrett a été mesurée sous pressions hydrostatiques jusqu'à 45 kbar dans un appareil récemment construit. Un changement important de la pente de T (P) à 30 kbar est attibué à la transformation prédite par Larsen et Ruoff. c

Abstract.- The superconducting transition temperature of two V3Si single crystals which do not show a Batterman-Barett transformation at zero pressure was measured resistively with a newly deve-loped hydrostatic pressure cell to 45 kbar. A clear slope change in T (P) at 30 kbar is attributed to the pressure-induced phase transformation predicted by Larsen and fiuoff.

Most known superconductors with high super-conducting transition temperatures T crystallize in the cubic A15 structure and possess anomalous elec-tronic and lattice properties, structural instabili-ties, and phase transitions IM. To study the appa-tent interdependence of these anomalous properties and high temperature superconductivity, V Si proves especially interesting because a cubic-to-tetragonal phase transition /2/ and superconductivity occur in the same temperature range (17 - 22 K ) . Pressure measurements are useful to check predictions of theo-retical models /3/ since pressure allows the varia-tion of parameters on a single sample without the difficulties occuring in alloy series. Contrary to the behaviour of normal metals, in V Si the shear modulus c = (c - c )/2 decreases drastically

to-s 11 12

wards lower temperatures, leading to the phase tran-sition in some crystals at a temperature TT ^ 2 1 K

121, as shown in the lower curve in figure 1. The upper curve applies to a crystal which is slightly stiffer due to defects and internal stresses so that

no transformation occurs before the crystal becomes superconducting. Superconductivity stops further softening and stabilizes the crystal preventing a lattice transformation for T < T . As the arrows in

c

figure 1 indicate, a transforming crystal is found to stiffen at low temperatures under hydrostatic pressure /4/.A crystal which does not show a Batterman-Barrett transformation at zero pressure behaves differently. Larsen and Ruoff /5/ find that

the shear modulus c of such a crystal softens un-der hydrostatic pressure at low temperatures. They predict that a pressure of 28 kbar would cause c

s to vanish above T so that a new phase

transforma-t i o n should occur.

I ' ' 1 — ' ' ~~" ' 97—' 0 6 - / / 05

" v

3

si / J

**°*~**

A /

2£ / / o nontransforming y / 0*03- / U transforming

/ \

A L 1 1 1 1 I 1 U • ' u2 i 6 8 10 T ( K | 20 40 60 SO WO

Fig. 1 : Relative decrease of shear modulus c for both transforming and nontransforming V Si single crystals /I/. Arrows indicate the change of c un-der hydrostatique pressure /4/, /5/.

To verify this hypothesis it was necessary to de-velop a new pressure cell with which it is possible

to carry out four-terminal resistivity measurements under hydrostatic pressures well above 30 kbar on V Si single crystal and a lead sample as a manome-ter. As in the diamond anvil cell /6/ we use two opposed anvils which press into a metal gasket con-taining the pressure transmitting fluid (1:4 metha-nol :ethametha-nol) and the samples. Details of this hy-drostatic cell will be given elswhere.

We measured the hydrostatic pressure dependence of T up to 45 kbar of two V.Si single crystals which, as shown by X-ray studies 111 do not show the Bat-terman-Barrett transformation at zero pressure. Figure 2 shows some very narrow (10 mK) transition

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c u r v e s a n d f i g u r e 3 shows t h e p r e s s u r e dependence of Tc, which r i s e s e x a c t l y l i n e a r l y w i t h p r e s s u r e up t o a b o u t 30 k b a r f o r both samples.

F t g . 2 : Superconducting t r a n s i t i o n c u r v e s of V S i s l n g l e c r y s t a l . The anomalous shape of t h e p = 10.9 k b a r c u r v e i s caused by an e r r o r i n t h e c o o l i n g pro- c e d u r e . The s t r u c t u r e a t t h e t o p of t h e t r a n s i t i o n s i s t h o u g h t t o be caused by l e a d conttact e f f e c t s .

Above 30 k b a r Tc of sample N r . 1 (measurements 1 t o 8 i n f i g u r e 3) c o n t i n u e s t o r i s e w i t h a much s m a l l e r s l o p e , whereas f o r sample N r . 2 (measurements 9. t o

19') a c l e a r b r e a k

i n

t h e Tc(P) c u r v e seems t o o c c u r . The r e d u c t i o n i n dTc/dP above 30 k b a r i s s t r o n g e v i - dence f o r t h e o c c u r r e n t e £0 t h e l a t t i c e transforma- t i o n p r e d i c t e d by L a r s e n and Ruoff / 5 / f o r s u f f i c i e n - t l y h i g h p r e s s u r e s which s h o u l d l e a d t o a r e d u c t i o n of Tc a s e x p e c t e d by t h e e l e c t r o n i c model of B i l b r o and McMillan / 8 / . As i t is accompanied by t h e s o f - t e n i n g of t h e same e l a s t i c c o n s t a n t s , t h i s p r e s s u r e induced t r a n s f o r m a t i o n should have t h e same charac- t e r a s t h e Batterman-Barrett t r a n s f o r m a t i o n , i. e

.

c u b i c - t o - t e t r a g o n a l .

Referendes

/ 1 / T e s t a r d i , L.R., Rev. Mod. Phys.

67

(1975) 637. / 2 / Batterman, B.W., B a r r e t t , C.S., Phys. Rev.

3

(1966) 296.

/ 3 / Weger, M., Goldberg, I.B., S o l i d S t a t e P h y s i c s , ed. by E h r e n t e i c h H . , S e i t z , F., T u r n b u l l , D.

(Acad. P r e s s , New York)

8

(1973) I .

/ 4 / C a r c i a , P.F., Barsch, G.R., Phys. S t a t u s . S o l i d i

(b)

2

( 1973) 595.

/ 5 / L a r s e n , R.E., Ruoff, A.L., J. Appl. Phys.

44

(1973) 1'21.

/ 6 / P i e r m a r i n i , G.J., Block, S., Rev. S c i . I n s t r u m . 46 (1975) 973.

-

/7/ D r . HESS, B a y r i s c h e Akademie d e r Wissenschaften, Garching, Munich, p r i v a t e communication

/ 8 / B i l b r o , G., McMillan, W.L., Phys. Rev. B

14

(1976) 1887.

F i g . 3 : s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e s Tc d e f i n e d by R(T,) = R(18 K)/2, v e r s u s h y d r o s t a t i c p r e s s u r e and r e l a t i v e volume / 5 / . Two c r y s t a l s were measured (numbers primed and unprimed), numbers g i - v i n g o r d e r s of measurements. The b a r s i n d i c a t e 10 %