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Electronic specific heat of Nd2−xCexCuO4−δ
Physlea C 235-240 (1994) 1747-1748
PHYSICA
North-Holland
Electronic specific heat of Nd2-xCexCuO4-8 C. M a r c e n a t , J.Y. Henry a n d R. Calemczuk C E A / D ~ p a r t e m e n t de Recherche Fondamentale sur la Matibre Condensde /SPSMS/LCP 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
We p r e p a r e d a batch of ceramics of N d 2 - x C e x C u O 4 - b by the citrate route and the concentration in Ce ranges between 0.13 < xCe < 0.18. The specific heat of these s a m p l e s was m e a s u r e d by adiabatic, high resolution calorimetry between 15 and 200K. We observe a specific heat a n o m a l y at the s u p e r c o n d u c t i n g transition temperature for the samples with xCe = 0.146, 0.153, a n d 0.157. The a m p l i t u d e of the best defined j u m p i* ~C/'[c-= 6 mJ.K-2mole -1. The metallic samples, s u p e r c o n d u c t i n g or n o t , have an extra contribution to the specific heat, m comparison to the insulating samples. This contribuhon is observed between 15 and 70K and is not of electronic origin ( not linear in T ).
Smce the discovery of s u p e r c o n d u c t o r s with electron carriers, Ln2-xCexCuO4 (Ln = Pr, Nd, Sm a n d Eu) 1l], intensive studies have been d e v o t e d to understand their properhes. These c o m p o u n d s have the K2NIFu structure where each Cu atom is s u r r o u n d e d only by a s q u a r e planar array of oxygen atoms, which makes these c o m p o u n d s very attractive to s t u d y CuO2-1ayered superconductors.
200K. We put a lot of effort in achlevmg a very good reproauctbd,ty (~10 -3) m order to be able to m e a s u r e a c c u r a t e l y the small differences m C of the different samples above 100K.
0.50
N d 2 _ x C e \ C u O 4 _ ~ (NCCO) c e r a m i c s were p r e p a r e d by the citraie route. One needs a very precise and h o m o g e n e o u s control of the two parameters x and ~ smce bulk s u p e r c o n d u c t i v i t y occurs for a very. narrow range, 0.14<\<0.18 and small changes in the o x y g e n stoich|ometry have dramatic effects. The quality and h o m o g e m ~ of our samples is p r o v e d by the observation of a specific h e a t jump at Tc for the superconductmg samples 121. In this cont"ibutmn, we s t u d y the evolution of the specific heat, e s p e c i a l l y that of t h e e l e c t r o m c part, accross the m e t a l - i n s u l a t o r border hne when changing the Ce doping We p r e p a r e d a batch of c e r a m i c s ~ l t h Ce concentrahon m changing from \=0 129 to 0.182 in ste W, of ~- 0.006-0.007. All the samples were reduced in the same way, close to the chemical stabihty, 6--- 0.03 The specific heat of these NCCO ceramics was m e a s u r e d by adlabahc, high resolution calorimetry between 16 and
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Figure 1 Vanatuon of the specific heat of Nd2-x Cex Cu04-6 upon Ce doping in a CFF3 versus T plot We represented the data in figure 1 in a C / T 3 versus T plot for the temperature range 16K
0921-4534/94/S07 00 © 1994 - F.lsevter Science B V All rights reserved SSDI 0921-4534(94)014-10-X
C. Mmcenat et al /Physaca C 235-240 (1994) 1747-1748
1748
to the non s u p e r c o n d u c t i n g u n d e r d o p e d c o m p o u n d s , x=0.129 and x=0.136. The two higher curves o r r e s p o n d to metallic n o n superconducting overdoped compounds, x--0.175 and \=0.182. The rest of the curves in the middle correspond to superconducmg samples, x=0.146, 0.153, 0.157 and 0.160. The 3 former samples present a superconducting transition within the measured range, and we observe anomalies at Tc-25K, the same temperature for all the 3 concentrations. The best defined jump, ACp/Tc~6 m J / m o l e . K 2, is obtained for x-0.153. See ref. [2] for more details about the b e h a v i o r m magnetic fields and for the different t h e r m o d y n a m i c estimates that we can d e r i v e from this v a l u e . T h e striking feature is obviously the discontinuity of the specific heat C (even above Tc) as a funchon of x at the msulator-superconduchng transihon and at the superconducting -non superconducting metal trangltion. 40
30
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above 701(, A C p / T c is constant and could be interpretated as an electronic contribution. We obtained ¥-~6+_2 m J / K 2 tool for t h e superconducting samples and 14+2 mJ / K2mol for the metallic ones. This is significantly smaller than the value of 53 m J / m o i K2 published in ref. [31. In this reference, to obtain their eshmate, they subtracted the data for N d 2C u O 4 from the data for N d 1 . 8 5 C e 0 . 1 5 C u O 4 - i ) which was fitted between 23K and 30K. From our work, it is clear that this difference is not of electronic origin (v',t constant in temperature) and that an extra contribuhon is present. This extracontribution, present in metallic compounds, x~0.146, has a maximum centered at roughfly 30K, independent of x within the experimental uncertainty. The observation of this d i s c o n h n u o u s change of the specific heat u p o n Ce doping must be related to the abrupt onset of the superconductivity around Xc=0.141 [4] and would be the signature of a subtle magnetic or structural transition induced by cerium doping. A very interesting speculation is that, at a critical concentration, the local distorslons m the CuO 2 planes around the Ce atoms percolate, originating new phonon frequencies that give rise to superconductivity through electron-phonon couphng. To clarify the s~tuahon, more measurements are underway to see if this extra contribution we found in metallic compounds is sensitive to magnetic field and to oxygen content. REFERENCES
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40
60 80100
T(K)
Figure 2 Thermal variation of t h e difference AC/T =C/T(x)-C/T(x=0.129) for the d~fferent concentrations x m Ce More insight can be obtained from fig. 2, where we display, as a funchon of T, the difference aC/T =C/T(x)-C/T(x=0.129). Fwo d~shnct regions are apparent. At temperatures
11] Y. Tokura, H. Takagl and S. Uchlda, Nature 331 (1989), 345. 12] C Marcenat, R Calemczuk, A.F. Khoder, E Bonlour, C. Marm and I.Y. H e n r y Ph7 slca C 216 (1993), 443. 131 s. Ghamaty, B.W Lee, ]."I. M'~rakert, E.A. Early, 1-. Blovnholm, C.I_. S~aman and M.B Maple, Physlca C (1989), 217. [41 H. Takagl, S Uchlda and Y. Tokura Phys. Rex'. Lett. 62 (1989), 1197.
PHYSICA
North-Holland
Electronic specific heat of Nd2-xCexCuO4-8 C. M a r c e n a t , J.Y. Henry a n d R. Calemczuk C E A / D ~ p a r t e m e n t de Recherche Fondamentale sur la Matibre Condensde /SPSMS/LCP 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
We p r e p a r e d a batch of ceramics of N d 2 - x C e x C u O 4 - b by the citrate route and the concentration in Ce ranges between 0.13 < xCe < 0.18. The specific heat of these s a m p l e s was m e a s u r e d by adiabatic, high resolution calorimetry between 15 and 200K. We observe a specific heat a n o m a l y at the s u p e r c o n d u c t i n g transition temperature for the samples with xCe = 0.146, 0.153, a n d 0.157. The a m p l i t u d e of the best defined j u m p i* ~C/'[c-= 6 mJ.K-2mole -1. The metallic samples, s u p e r c o n d u c t i n g or n o t , have an extra contribution to the specific heat, m comparison to the insulating samples. This contribuhon is observed between 15 and 70K and is not of electronic origin ( not linear in T ).
Smce the discovery of s u p e r c o n d u c t o r s with electron carriers, Ln2-xCexCuO4 (Ln = Pr, Nd, Sm a n d Eu) 1l], intensive studies have been d e v o t e d to understand their properhes. These c o m p o u n d s have the K2NIFu structure where each Cu atom is s u r r o u n d e d only by a s q u a r e planar array of oxygen atoms, which makes these c o m p o u n d s very attractive to s t u d y CuO2-1ayered superconductors.
200K. We put a lot of effort in achlevmg a very good reproauctbd,ty (~10 -3) m order to be able to m e a s u r e a c c u r a t e l y the small differences m C of the different samples above 100K.
0.50
N d 2 _ x C e \ C u O 4 _ ~ (NCCO) c e r a m i c s were p r e p a r e d by the citraie route. One needs a very precise and h o m o g e n e o u s control of the two parameters x and ~ smce bulk s u p e r c o n d u c t i v i t y occurs for a very. narrow range, 0.14<\<0.18 and small changes in the o x y g e n stoich|ometry have dramatic effects. The quality and h o m o g e m ~ of our samples is p r o v e d by the observation of a specific h e a t jump at Tc for the superconductmg samples 121. In this cont"ibutmn, we s t u d y the evolution of the specific heat, e s p e c i a l l y that of t h e e l e c t r o m c part, accross the m e t a l - i n s u l a t o r border hne when changing the Ce doping We p r e p a r e d a batch of c e r a m i c s ~ l t h Ce concentrahon m changing from \=0 129 to 0.182 in ste W, of ~- 0.006-0.007. All the samples were reduced in the same way, close to the chemical stabihty, 6--- 0.03 The specific heat of these NCCO ceramics was m e a s u r e d by adlabahc, high resolution calorimetry between 16 and
B
i
0.48
,I |
,11|
" : •
@
I
.:.
° °
.~:. -: -~
-•.
E 0.46
•
"
i DI
E
,\\
,~ 0.44
':.
%',,:..
".
o.,
"~
k~ 0 . 4 2 0.40
I
,.,,., Lu
I 25
I ~U "~"
,"3r
DO
T(K)
Figure 1 Vanatuon of the specific heat of Nd2-x Cex Cu04-6 upon Ce doping in a CFF3 versus T plot We represented the data in figure 1 in a C / T 3 versus T plot for the temperature range 16K
0921-4534/94/S07 00 © 1994 - F.lsevter Science B V All rights reserved SSDI 0921-4534(94)014-10-X
C. Mmcenat et al /Physaca C 235-240 (1994) 1747-1748
1748
to the non s u p e r c o n d u c t i n g u n d e r d o p e d c o m p o u n d s , x=0.129 and x=0.136. The two higher curves o r r e s p o n d to metallic n o n superconducting overdoped compounds, x--0.175 and \=0.182. The rest of the curves in the middle correspond to superconducmg samples, x=0.146, 0.153, 0.157 and 0.160. The 3 former samples present a superconducting transition within the measured range, and we observe anomalies at Tc-25K, the same temperature for all the 3 concentrations. The best defined jump, ACp/Tc~6 m J / m o l e . K 2, is obtained for x-0.153. See ref. [2] for more details about the b e h a v i o r m magnetic fields and for the different t h e r m o d y n a m i c estimates that we can d e r i v e from this v a l u e . T h e striking feature is obviously the discontinuity of the specific heat C (even above Tc) as a funchon of x at the msulator-superconduchng transihon and at the superconducting -non superconducting metal trangltion. 40
30
~zo E ~o
above 701(, A C p / T c is constant and could be interpretated as an electronic contribution. We obtained ¥-~6+_2 m J / K 2 tool for t h e superconducting samples and 14+2 mJ / K2mol for the metallic ones. This is significantly smaller than the value of 53 m J / m o i K2 published in ref. [31. In this reference, to obtain their eshmate, they subtracted the data for N d 2C u O 4 from the data for N d 1 . 8 5 C e 0 . 1 5 C u O 4 - i ) which was fitted between 23K and 30K. From our work, it is clear that this difference is not of electronic origin (v',t constant in temperature) and that an extra contribuhon is present. This extracontribution, present in metallic compounds, x~0.146, has a maximum centered at roughfly 30K, independent of x within the experimental uncertainty. The observation of this d i s c o n h n u o u s change of the specific heat u p o n Ce doping must be related to the abrupt onset of the superconductivity around Xc=0.141 [4] and would be the signature of a subtle magnetic or structural transition induced by cerium doping. A very interesting speculation is that, at a critical concentration, the local distorslons m the CuO 2 planes around the Ce atoms percolate, originating new phonon frequencies that give rise to superconductivity through electron-phonon couphng. To clarify the s~tuahon, more measurements are underway to see if this extra contribution we found in metallic compounds is sensitive to magnetic field and to oxygen content. REFERENCES
ZO
40
60 80100
T(K)
Figure 2 Thermal variation of t h e difference AC/T =C/T(x)-C/T(x=0.129) for the d~fferent concentrations x m Ce More insight can be obtained from fig. 2, where we display, as a funchon of T, the difference aC/T =C/T(x)-C/T(x=0.129). Fwo d~shnct regions are apparent. At temperatures
11] Y. Tokura, H. Takagl and S. Uchlda, Nature 331 (1989), 345. 12] C Marcenat, R Calemczuk, A.F. Khoder, E Bonlour, C. Marm and I.Y. H e n r y Ph7 slca C 216 (1993), 443. 131 s. Ghamaty, B.W Lee, ]."I. M'~rakert, E.A. Early, 1-. Blovnholm, C.I_. S~aman and M.B Maple, Physlca C (1989), 217. [41 H. Takagl, S Uchlda and Y. Tokura Phys. Rex'. Lett. 62 (1989), 1197.