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Submitted on 1 Jan 1979
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Transport properties, susceptibility and specific heat of UAl2
H. Armbrüster, W. Franz, W. Schlabitz, F. Steglich
To cite this version:
H. Armbrüster, W. Franz, W. Schlabitz, F. Steglich. Transport properties, susceptibility and specific heat of UAl2. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-150-C4-151.
�10.1051/jphyscol:1979449�. �jpa-00218845�
JOURNAL DE PHYSIQUE Colloque C4, supplément au n° 4, Tome 40, avril 1979, ppge C4-150
Transport properties, susceptibility and specific heat of UA1
2(*)
H . A r m b r i i s t e r , W . F r a n z , W . Schlabitz a n d F . Steglich
II. Physikalisches Institut der Universitat zu Koln, Zulpicher Str. 77, D-5000 Koln 41, F.R.G.
Résumé. — Nous discutons les résultats de mesures de susceptibilité, de chaleur spécifique et des propriétés de transport (conductivité électrique et thermique et pouvoir thermoélectrique) pour UA12 polycristàllin.
Malgré le désordre très important des échantillons, nous avons observé des anomalies dans les grandeurs mesurées qui sont intrinsèques à UA12. La plus grande partie de ces anomalies peut être expliquée par l'existence d'une bande 5f étroite au niveau de Fermi et par la prise en compte d'excitations de paramagnons dans cette bande 5f.
Abstract. — We discuss measurements of the susceptibility, specific heat, and transport properties (electrical and thermal resistivity, thermoelectric power) performed on polycrystalline UA12 samples. Despite the high disorder of the samples, we observed anomalies in the various quantities, which are believed to be intrinsic to UA12. Most of these anomalies can be interpreted with the existence of a narrow 5f band at the Fermi level and of paramagnon excitations within this 5f band.
T h e intermetallic c o m p o u n d UA12 is t h e only k n o w n (atomically o r d e r e d ) m e t a l for w h i c h t h e e x i s t e n c e of f e r r o m a g n e t i c spin fluctuations (with Tst — 25 K ) w a s inferred f r o m s u c h different quanti- ties as specific h e a t , c, susceptibility, x> a n d resisti- v i t y , p , w h i c h w e r e m e a s u r e d a b o v e = 1 K [1].
I n this article, w e p r e s e n t n e w results c o n c e r n i n g t r a n s p o r t p r o p e r t i e s (thermal resistivity, T R , a n d t h e r m o e l e c t r i c p o w e r , T E P ) t o g e t h e r with r e s u l t s of t h e quantities m e n t i o n e d a b o v e ( c , x, p)-
W e c o m m e n t on t h e \ a n d c m e a s u r e m e n t s first.
I n b o t h c a s e s , t h e t e m p e r a t u r e regime w a s e x t e n d e d t o well b e l o w 1 K . Details of t h e s e e x p e r i m e n t s will b e published e l s e w h e r e . H e r e , w e only w a n t t o state t h a t t h e analysis of x a n d c ( b e t w e e n 0.3 a n d 2.2 K ) confirms t h e e x i s t e n c e of p a r a m a g n o n s ; h o w e v e r Tst being 4-7 K , i.e. considerably smaller t h a n
Fig. 1. — Electrical (left) and thermal (right) resistivities of UAU vs. temperature.
Ts ( = 25 K published b y B r o d s k y [1]. A smaller v a l u e (— 10 K ) w a s r e c e n t l y also f o u n d b y F o u r n i e r [2].
B e s i d e s t h e d i s a g r e e m e n t in t h e Ts£ values w e h a v e also f o u n d d i s a g r e e m e n t in t h e y coefficients of t h e low t e m p e r a t u r e specific h e a t , o u r y values being 10-15 % smaller t h a n B r o d s k y ' s [1]. F u t u r e e x p e r i m e n t s m u s t s h o w t o w h a t e x t e n t t h e metallur- gical quality of t h e s a m p l e s is r e s p o n s i b l e for this d i s c r e p a n c y .
I n figures 1 a n d 2 w e p r e s e n t t h e r e s u l t s of t h e t r a n s p o r t m e a s u r e m e n t s . A m o r e detailed discussion will b e published e l s e w h e r e . T h e salient f e a t u r e s of t h e figures a r e :
1) A giant i n c r e a s e of p f r o m its (large) residual v a l u e pn = 26 ixfl.cm t o 190 (xfi.cm at 300 K , w h i c h c a n n o t b e e x p l a i n e d b y e l e c t r o n - p h o n o n scattering [3]. This h a s b e e n a t t r i b u t e d t o t h e u n u s u a l scatter- ing p r o c e s s of t h e c o n d u c t i o n e l e c t r o n s from t h e p a r a m a g n o n excitations of t h e 5f b a n d [4].
2) V e r y large values of t h e L o r e n t z ratio L = pkT-1 ,
Fig. 2. — Thermoelectric power of UA12 vs. temperature.
(*) Work performed within the research program of the Sonderforschungsbereich 125 Aachen/Julich/Koln.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979449
TRANSPORT PROPERTIES, SUSCEPTIBILITY AND SPECIFIC HEAT OF UA12 C4-151
namely L = 2.7 Lo at 300 K and 4.1 Lo at 10 K where Lo = 2.45 x lo-* WO.Kd2 is the Sommerfeld constant. This is an obvious result of the low metal- lurgical quality of the sample, resulting in a large phonon thermal conductivity, k,, in relation to the electronic one, k, = k - k,.
3) The measured TR, k-I, is proportional to T-' below 15 K (resulting from the Wiedemann Franz part of k, and from k,) and also above 200 K. Below 50 K , k-' shows a distinct plateau. A tentative substraction of k, from the measured k(T) data leads to an electronic TR, k,', which is much larger than the raw data in figure 2. In addition, the plateau is changed into a flat maximum, whereas the T-' dependence at room temperature seems to be pre- served. The latter features are in qualitative agree- ment with the expectations from paramagnon theory [5].
4) The TEP assumes large values (40 FV/K) at room temperature. This could also be explained by the above mentioned theory including paramagnon Umklapp processes and non-sphericity of the Fermi
surface [6]. Below 100 K, the TEP falls off rapidly, changes sign at 30 K , reaching a negative peak of - 12.5 pV/K at 16 K and finally vanishes proportio- nally to T with S / T = - 1.13 pV.K-2. At present, there is no theoretical explanation for these low temperature anomalies of the TEP. One could think of the negative peak as being due to a paramagnon drag [7] which would also imply the correct T- dependence for T -+ O. However, if we identify this linear S ( T ) law with a diffusion TEP, we arrive at a degeneracy temperature T* = 250 K. This is of the same order as the Curie-Weiss temperature from susceptibility and the residual ( T + 0) half-width of the quasi-elastic neutron line in UAl, [8].
To conclude, most of the results presented here support the idea of paramagnons within a narrow Sf band in UAl,. However further investigations, espe- cially on the role of sample quality on the properties of UA1, are desirable. Also, the low temperature TEP must be better understood.
We are grateful to Dr. R. F. Hoyt for a critical reading of this manuscript.
References
[I] BRODSKY, M. B . and TRAINOR, R. J., Physica 91B (1977) 271 [6] IGLESIAS-SICARDI, J. R . , JULLIEN, R. and COQBLIN, B., Phys.
and further references cited therein. Rev. B 17 (1978) 2366.
121 FOURNIER, J. M., to be published. [7] KAISER, A. B., AIP Conf. Proc. 29 (1976) 364.
[3] FRANZ, W., Diplorn thesis, Cologne 1975 (unpublished). 181 LOEWENHAUPT, M., HORN, S., STEGLICH, F., HOLLAND- 141 JULLIEN, R., BEAL-MONOD, M. T. and COQBLIN, B., Phys. MORITZ, E. and LANDER, G. H., J. Physique Colloq. 40
Rev. Lett. 30 (1973) 1057. (1979) C4-142.
[S] JULLIEN, R. and COQBLIN, B., J. LOW Temp. Phys. 19 (1975) 59.
