HAL Id: jpa-00218840
https://hal.archives-ouvertes.fr/jpa-00218840
Submitted on 1 Jan 1979
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Superconducting critical field and low temperature heat capacity of americium
J. Smith, G. Stewart, C. Huang, R. Haire
To cite this version:
J. Smith, G. Stewart, C. Huang, R. Haire. Superconducting critical field and low temperature heat capacity of americium. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-138-C4-139.
�10.1051/jphyscol:1979444�. �jpa-00218840�
JOURNAL DE PHYSIQUE Colloque C4, supplément au n° 4, Tome 40, avril 1979, page C4-138
Superconducting critical field and low temperature heat capacity of ameri- cium (*)
J. L . S m i t h , G. R. S t e w a r t , C . Y . H u a n g a n d R. G. H a i r e (*)
Los Alamos Scientific Laboratory of the University of California, Los Alamos, N.tyl. 87545, U.S.A.
(f) Oak Ridge National Laboratory, Oak Ridge, TN 37830, U.S.A.
Résumé. — Des mesures à basse température sur le supraconducteur américium donnent un coefficient de chaleur spécifique électronique y = 2 ± 2 mj/mole • K et un champ critique HQ(0i) ~ 500 Oe. Le premier est plutôt petit tandis que le champ critique élevé est inattendu.
Abstract. — Low temperature measurements on the superconductor americium yield an electronic heat capacity coefficient of y = 2 ±2 mJ/mole • K2 and a critical field of ffo(0) ~ 500 Oe. The former is rather small while the critical field is unexpectedly large.
1. Introduction. — T h e e l e m e n t a m e r i c i u m h a s r e c e n t l y b e e n s h o w n t o b e a s u p e r c o n d u c t o r [1].
Since general t h e r m o d y n a m i c p r o p e r t i e s of super- c o n d u c t i n g e l e m e n t s a r e a n aid t o u n d e r s t a n d i n g s u p e r c o n d u c t i v i t y , w e d e c i d e d t o m e a s u r e t h e criti- cal field ( H J a n d electronic h e a t capacity coefficient (•y) of a m e r i c i u m . T h e s e t w o m e a s u r e m e n t s are t h e r m o d y n a m i c a l l y related a n d w e h o p e d t h a t t h e y w o u l d c o n s t i t u t e a c h e c k of t h e e x p e r i m e n t a l self-consistency. T h e only p r e v i o u s m e a s u r e m e n t of this n a t u r e o n A m w a s t h e value of y = 3 ± 3 m J / m o l e . K2 from t h e r e c e n t resistivity a n d h e a t capacity w o r k on a m e r i c i u m b y Miiller et al. [2].
O u r m e a s u r e m e n t s p r o v e d m o r e difficult t h a n w e h a d anticipated d u e t o t h e heating (6.8 m W / g ) and r a d i a t i o n d a m a g e of t h e M 3Am m e t a l . F u r t h e r m o r e , t h e r e s u l t s w e r e m o r e interesting than e x p e c t e d a n d t h u s , w o r k is still in p r o g r e s s .
2 . Experimental results and discussion. — T h e s a m p l e s w e r e a n e w r e d u c t i o n of a m e r i c i u m using t h e t e c h n i q u e s of r e f e r e n c e [1]. T h e h e a t capacity m e a s u r e m e n t s w e r e p e r f o r m e d with a 5 mg b a r e s a m p l e g r e a s e d t o a sapphire platform that w a s ther- mally weakly-tied t o a 1.38 K liquid helium b a t h . T h e h e a t capacity w a s d e t e r m i n e d b y t h e i n c r e a s e in t h e r m a l relaxation t i m e t h a t t h e s a m p l e a d d e d t o t h e platform w h e n it w a s h e a t e d with p u l s e s a b o v e its equilibrium t e m p e r a t u r e [3]. T h e m e a s u r e m e n t w a s t h u s i n d e p e n d e n t of t h e s a m p l e heating r a t e w h i c h w e d e t e r m i n e d t o b e —6.8 m W / g . H o w e v e r , t h e self-heating limited t h e l o w e s t t e m p e r a t u r e attained t o ~ 7 K . O u r r e s u l t s from 7 t o 19 K a r e s h o w n in figure 1. T h e e l e c t r o n i c h e a t capacity coefficient
Fig. 1. — Heat capacity of M!Am at low temperatures.
w a s d e t e r m i n e d from t h e linear e x t r a p o l a t i o n of t h e C/T versus T2 plot to b e 2 ± 2 = m J / m o l e . K2. T h e large u n c e r t a i n t y is d u e t o t h e fact t h a t e v e n at 7 K t h e electronic contribution is still only 4 % of t h e total specific h e a t . T h e d e t e r m i n a t i o n of y t h e n allows t h e D e b y e t e m p e r a t u r e s (0D) t o b e calculated a n d t h e y a r e s h o w n in t h e figure as a function of T.
T h e s e values of flD s h o w a d r a m a t i c d r o p f r o m higher t e m p e r a t u r e values [2]. This softening could reflect a highly elastically a n i s o t r o p i c s t r u c t u r e or m a y b e d u e t o a m a r t e n s i t i c t r a n s f o r m a t i o n t h a t could b e a s s o c i a t e d w i t h t h e 69 K h e a t capacity a n o m a l y [2].
T h e critical field d e t e r m i n a t i o n s w e r e m a d e in t h e dilution refrigerator d e s c r i b e d in r e f e r e n c e [1] with t h e addition of a s u p e r c o n d u c t i n g solenoid c e n t r e d on t h e s a m p l e s . M o s t of t h e m e a s u r e m e n t s w e r e m a d e o n a s a m p l e 0.3 m m thick, 0.76 m m w i d e , a n d
1.02 m m long t h a t w a s cut from t h e h e a t capacity s a m p l e . T h e magnetic field w a s applied p e r p e n d i c u - lar t o t h e large faces of t h e s a m p l e . A d e m a g n e t i z - ing coefficient of 0.56 w a s a s s u m e d yielding an (*) Work performed under the auspices of the Department of
Energy.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979444
SUPERCONDUCTING CRITICAL FIELD AND LOW TEMPERATURE HEAT CAPACITY C4-139
Hi,,,d
-
2.3 Happlied. (One attempt to run a 2.3 mg sphere was limited in temperature to 0.38 K by the poor geometry for cooling.)Some typical traces of ac susceptibility versus applied field at constant temperature are shown in the inset of figure 2. These are very unusual. A non-radioactive, pure, strain-free superconductor should yield a trace resembling a step function with the step either occurring at the critical field (H,) or ending at the upper critical field (H,,). The observed curves for Am are a result of two competing mecha- nisms, both of which are extremely complicated to deal with in detail for a particular geometry. First of all the heat generated within the sample must main- tain the centre at a relatively high temperature, probably it is in the normal state [I]. Secondly, the application of a magnetic field at first drives part of the surface normal (raising its thermal conductivity non-linearly), and in this case (since no sign of mag- netic hysteresis is seen) finally breaks the sample into regions of normal and superconducting metal before finally driving it completely normal at H,,.
Now a determination of the normal and supercon- ducting regions of the sample taking into account the thermal and magnetic restrictions for a practical geometry is almost impossible.
We believe that something useful can be extract- ed. It must be recalled that superconducting regions
Hwp~xea (00) 0 250 M O 750
I I I " '
Fig. 2. - Critical field points for Am (likely H,,) as a function of temperature. The inset shows recorder traces. The curve is
where HJO) = 530 Oe and T, = 0.625 K. Also
can shield normal regions making the ac susceptibili- ty, possibly, unrealistically sensitive to subtle chan- ges in the superconducting state. For this reason we disregard the ends of the traces. (We also could not extract any sensible information from the ends.) All of the traces exhibit a change of slope in the central portion (marked by the arrows in the figure) which we believe is the onset of the mixed state ( H , , ) caused by the applied field.
Taking this viewpoint results in the critical field curve shown in figure 2 which has a very believable shape. For purposes of further discussion in this limited space we shall call this Hc(T) rather than Hc,(T) since it represents a lower limit of H,(T) which as we shall see is rather large. Finally our values of Hc increase by 2-4 %/day presumably due to stored radiation damage. In support of reference [2], annealing at 75 K restores most of the changes.
To barely touch on theoretical considerations we first go to McMillan [4]. Starting with
y = 2.0 rnJ/mole
-
K2, T, = 0.625 K (from Fig. 2),and assuming p
*
= 0.13 (probably high) we arrive at a density of states (N(0)) of 0.57 stateslev-atom which is very small for an elemental superconductor and we obtain an electron-phonon interaction pairing potential (V) of 0.32 eV-atom which is rather large.As the most unusual result, consider the expression
y = (1/2 7 ~ ) (Hf ( T = 0)/ T 3
which is a result of the simple two-fluid mo- del [5]. Using Hc(T = 0) = 530 Oe, we derive y = 200 mJ/mole KZ. Clearly americium is some type of a high critical field material. Calculations of the coherence length, penetration depth, and the mean free path are rather dependent on the choice of starting parameters and are no more satisfying than the two-fluid model result.
Although there is a great deal of uncertainty in our determinations of y and H , they are not large enough to make the superconducting properties of americium straightforward. As mentioned in reference [I] the superconductivity of americium is
surprising.
We thank B . T. Matthias for helpful conversa- tions.
References
[I] SMITH, J. L. and HAIRE, R. G., Science 200 (1978) 535. 133 STEWART, G. R., Cryogenics 18 (1978) 120 ; STEWART, G. R.
[Z] MULLER, W., SCHENKEL, R., SCHMIDT, H. E., SPIRLET, J. C., and GIORGI, A. L., Phys. Rev. B 17 (1978) 3534.
MCELROY, D. L., HALL, R. 0. A. and MORTIMER, M. J., [4] MCMILLAN, W. L., Phys. Rev. 167 (1968) 331.
J. Low Temp. Phys. 30 (1978) 561 ; [5] LYNTON, E. A., Superconductivity 3rd edition (Chapman and HALL, R. 0 . A., MORTIMER, M. J . , MCELROY, D. L . , Hall, London) 1969, p. 22.
MULLER, W. and SPIRLET, J . C., Transplutonium Ele- ments, eds. Miiller, W . and Lindner, R. (North Holland, Amsterdam) 1976, p. 139.
