SPECIFIC HEAT OF SUPERCONDUCTING 2H-NbSe2 DILUTELY DOPED WITH Fe

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SPECIFIC HEAT OF SUPERCONDUCTING 2H-NbSe2 DILUTELY DOPED WITH Fe

M. Ikebe, N. Kobayashi, S. Morohashi, H. Nakatsuji, Y. Muto

To cite this version:

M. Ikebe, N. Kobayashi, S. Morohashi, H. Nakatsuji, Y. Muto. SPECIFIC HEAT OF SUPERCON-

DUCTING 2H-NbSe2 DILUTELY DOPED WITH Fe. Journal de Physique Colloques, 1978, 39 (C6),

pp.C6-1641-C6-1642. �10.1051/jphyscol:19786612�. �jpa-00218108�

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JOURNAL DE PHYSIQUE

Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-1641 INSTEAD OF the a r t i c l e printed in the Volume I I page C6-878 ; READ the new p r i n t i n g of the same a r t i c l e .

SPECIFIC HEAT OF SUPERCONDUCTING 2H"NbSe2 DILUTELY DOPED WITH Fe

M. Ikebe, N. Kobayashi, S. Morohashi, H. Nakatsuji and Y. Muto

The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai, 980 Japan

Résumé.- L'action des impuretés de Fe sur les propriétés supraconductrices de NbSe^ a été étudiée par des mesures de la chaleur spécifique. Les résultats sont compatibles avec la théorie d'Abriko- sov-Gor'kov et indiquent que le fort champ cristallin d'anisotropie est responsable de la diminu- tion de l'anomalie de Kondo dans ce supraconducteur à structure en couches.

Abstract.- The effect of Fe impurities on superconducting properties of NbSe2 is examined by specific heat measurements. The results are rather consistent with the Abrikosov-Gor'kov theory and the strong crystalline anisotropy field is indicated to be responsible for the apparent quenching of the Kondo anomaly in this layered superconductor.

A good deal of interest and investigations has been accumulated for a typical layered superconductor NbSe2(T =7.3 K) which shows various peculiar properties such as the strong anisotropy in both normal and superconducting states and CDW transi- tions, etc. On the other hand, several 3-d transition metals (Fe, Ni, Co) /l/ are believed to bear localized magnetic moments in NbSe2. Especially, Fe- doped NbSe2 shows the most drastic depression of the superconducting transition temperature T as well as the resistance minimum /2/ between 8-20 K indicating the existence of the Kondo anomaly. In this note we present thermodynamic studies to clarify the effect of Fe spins on the superconductivity by measuring the specific heat of Fe-doped NbSe2 single crystals.

Single crystals of 2H-Fe NbSe2 were prepared by an iodine vapor transport method. The specific heat measurements were performed on a single crystal

CVJO mg wt) by use of a thermal relaxation method /3/

between 1.6 K and 9 K. In the measurements of the normal state, magnetic fields 40-45 kOe were' applied in a direction perpendicular to the layers. Fe con- centrations of the samples studied are, nominally, 0.05, 0.24, 0.34 and 0.44 mole %.

As usually done, the specific heat in the normal and superconducting states (C and C ) were analysed by the following formulae, C = yl +.0T³ and Cg = C + gT3, where y and 6 are coefficients of the electron and lattice contributions, respecti- vely, C be£ng tK'e superconducting electronic contributions. Several fundamental parameters determined from the specific heat are summarized in table I, together with those of pure NbSe2 /3/. For the normal state specific heat of Fe-doped NbSe2, upward

deviations were observed below 4 K. This additional specific heat is probably due to localized moments of Fe ions but detailed analysis are to be made.

Table 1

concentratlon(moleJS) 0 0.05 0.2lt# 1 0 . 2 l /2 0.3k O.UU TC(K) 7.25 6.70 5.06 5.Oil U.3>* h.39 ACbnJ/mole K) 252 2 l 6 1 2 3 110 96 9 7 Y(mJ/mole K2) 18.0 19.7 20.5 20.3 20.0 19.8 B(mJ/mole 0.506 0*675 0.U16 0.1(19 0.603 O.U88 40/YTC 1.93 1.61* 1.19 1.08 1.11 1.12 2 A ( 0 ) / Y TO 3.03 2.59 2.51 2.57 1.88 1.8U

In figure ! the ratio of the specific heat jump, AC/AC , is shown as a function of the ratio of transition temperature T /T , where AC and T represent those of pure NbSe2. For comparison, theoretical curves of Shiba /4/ based on the Ander- son model are also illustrated for Ys=l ai»d Ys-3- Here y is the parameter defined by Shiba, Y^{s =}' corresponding to weak pair breaking limit (=A-G results) and y =0» to strong limit. Our results seem to be explicable within the Shiba's theory which does not take into account the Kondo effect, while the estimation of the Kondo temperature T, from d(AC/ACQ)/d(T /T C O) | TC« TC O following the MHZ theory /5/ gives T =0.015 or 1300 K for S = -| and T, = 0.004 K or 4500 K for S=2 /6/. These values of Tfe

are contradictory with T. = 0.5-4 K from transport /3/ and magnetic measurements 76/ in the normal state and seem to he inacceptable.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786612

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JOURNAL DE PHYSIQUE

Fig. 1 : The normalized s p e c i f i c h e a t jump AC/ACo v e r s u s Tc/Tco. The s o l i d curves a r e t h e t h e o r e t i c a l r e s u l t s c a l c u l a t e d by Shiba.

I n f i g u r e 2 i s p l o t t e d t h e c r i t i c a l tempera- t u r e v e r s u s t h e Fe c o n c e n t r a t i o n x determined from t h e s p e c i f i c h e a t . Although some a m b i g u i t i e s remain due t o t h e p o s s i b l e inaccuracy i n

x,

t h e d i s c r e p a n c y i s c l e a r between t h e experiment and t h e MHz t h e o r y w i t h Tk < Tco. The p r s e s e n t r e s u l t s a r e r a t h e r con- s i s t e n t w i t h t h e A-G t h e o r y w i t h some d i s c r e p a n c i e s which m y b e noted a t h i g h e r c o n c e n t r a t i o n s .

Fig. 2 : T r a n s i t i o n temperature Tc a s a f u n c t i o n of i m p u r i t y c o n c e n t r a t i o n x. The dashed curve and t h e c h a i n

(---I

curve r e p r e s e n t t h e r e s u l t s of c a l c u l a - t i o n s based on t h e MHZ theory f o r S = w i t h T = 0.015 K and Tk = 4 K, r e s p e c t i v e l y . The A-G pre!hc- t i o n is given by t h e s o l i d curve.

Thus, t h e e f f e c t of t h e Kondo anomaly due t o Fe l o c a l i z e d moments i s a p p a r e n t l y quenched o u t i n t h e l a y e r e d superconductor NbSez. The r e s u l t s of HC2 /7/ of Fe-doped NbSee a r e e x p l a i n e d i n t h e f r a - mework of t h e Fulde-Maki t h e o r y /8/ and a l s o s u p p o r t t h e quenching o u t of t h e Kondo anomaly. The r e s u l t s o f t h e magnetic s u s c e p t i b i l i t y / 6 / of Fe-doped NbSen and TaSe2 i n d i c a t e t h a t Fe i m p u r i t i e s do n o t e x i s t

i n S - s t a t e i o n s (3d5 e l e c t r o n i c c o n f i g u r a t i o n ) b u t i n P - s t a t e (3d6) w i t h s t r o n g a n i s o t r o p i c c r y s t a l l i - ne f i e l d s . I n f a c t i t i s v e r y p l a u s i b l e t h a t ' a n i s o - t r o p y f i e l d s from s e v e r a l cm-' t o about 10 cm-' should a c t on t h e non-S-state Fe s p i n s i n t h e i n t e r - l a y e r o c t a h e d r a l h o l e . Accordingly, one should take account of t h e r e d u c t i o n of t h e s p i n - f l i p s c a t t e r - i n g a s a consequence of t h e a n i s o t r o p i c c r y s t a l l i n e f i e l d r a t h e r than t h e e f f e c t of i n t e r a c t i o n of t h e l o c a l i z e d s p i n s , because t h e l a t t e r h a s n o t been confirmed i n t h e very d i l u t e d Fe c o n c e n t r a t i o n re- g i o n s s t u d i e d i n t h e p r e s e n t work. More q u a n t i t a t i v e a n a l y s i s a r e under way.

References

/ I / Hauser, J.J., Robbins, M., and DiSalvo, F.J., Phys. Rev. B z (1973) 1038.

/2/ Whitney, D.A., Fleming, R.M., and Coleman, R.V., Phys. Rev. B E (1977) 3405.

/3/ Kobayashi, N . , Noto, K., and Muto, Y., J. Low Temp. Phys.

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(1977) 217.

/ 4 / S h i b a , H., Prog. Theor. Phys.

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(1973) 50.

/5/ See Muller-Hartman, E . , i n Magnetism, Suhl H., ed. (Academic P r e s s ) , v o l . 5 (1973), p. 353.

/6/ Coleman, R.V., Fleming, R.M., Domb, E.R., and Sellmyer, D . J . , AIP Conf. Proc.

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(1976) 400.

/7/ Muto, Y., Noto, K., N a k a t s u j i , H., and Toyota, N., Nuovo Cimento, E B , N.2 (1977) 503.

/8/ Fulde, P., and Maki, K., Phys. Rev.

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(1966) 275.