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Specific heat, susceptibility and resistivity measurements of high-Tc superconductors YBa2Cu3O7−x
Physica 148B (1987) 305-308 North-Holland, Amsterdam
SPECIFIC HEAT, SUSCEPTIBILITY AND RESISTIVITY MEASUREMENTS OF HIGH-To SUPERCONDUCTORS YBa2CuaO7_ x C. AYACHE, B. BARBARA*, E. BONJOUR, P. BURLET, R. CALEMCZUK, M. COUACH, M.J.G.M. JURGENS*, J.Y. HENRY and J. ROSSAT-MIGNOD D~partement de Recherche Fondamentale, C.E.N. Grenoble, 85 X - 38041 Grenoble Cedex, France *Laboratoire Louis-N&l, C.N.R.S., 166 X - 38042 Grenoble Cedex, France
Received 31 August 1987
Different single phase polycrystalline samples of high-T c superconducting compounds YBa2CU3OT_ x have been prepared and characterized. They have been investigated by ac and dc susceptibility, resistivity and specific heat experiments to study the influence of the oxygen content. The y-value and the T-independent susceptibility are found to be not sensitive to the oxygen content.
I. Introduction Since the discovery of high temperature superconductors in ceramic compounds La2(Ba , Sr) CuO 4 [1] with Tc = 4 0 K and more recently in BaaYCu307 x [2] with Tc = 95 K a large amount of experimental works have been devoted to these compounds. A systematic investigation of the structural, magnetic and superconducting properties of these ceramic compounds have been undertaken at the Departement de Recherche Fondamentale of the Centre d'Etudes Nucl6aires de Grenoble. Preliminary results on the specific heat anomaly and critical fields of superconducting compounds [3] have already been published, together with their structural study [4, 5]. In this paper we report more extensive studies on the influence of the oxygen content on the magnetic and superconducting properties of YBa2Cu307_ x compounds.
2. Sample preparation and crystal structure analysis The samples we have investigated have been * On leave from Kamerlingh Onnes Laboratorium Leiden, The Netherlands. 0378-4363/87/$03.50 (~) Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation
prepared according to the following procedures. For samples with the largest oxygen content, YBaaCu306.92, a mixture of BaCO3, CuO and Y203 is heated in air at 950°C during 12h, quenched, annealed at 480°C under 1.2 bar of oxygen and then slowly cooled to room temperature in 12h. Samples with the formula YBa2fu30 6 are obtained by heating up the O7_x-compound under vacuum to get a sample resistivity larger than 1 MII cm at 25°C. Samples with intermediate oxygen concentration (from x = 0 to 1) have been prepared by heating under an argon atmosphere the appropriate mixture of 0 6 and O6.92-samples. All samples have been characterized by X-ray powder diffraction and parasitic phases were not detected within an accuracy of a few per cent. The chemically determined stoichiometry has been controlled by the refinement of the crystal structure from powder neutron diffraction experiments. We have shown [4] that the reduced compound has a tetragonal structure (space group P4/mmm) with a = 3.857 ,~ and c = 11.83 A. The well defined stoichiometry YBa2Cu306. 0 was confirmed and the oxygen site 0(4) along the Cu-chains (Cu(1)) was found to be completely empty yielding two-fold coordinated Cu ÷ ions, Cu in the CuO 2 planes remaining with a Cu 2+ valency. From 06. 0 to O6.92-compounds the main ef-
306
C. Ayache et al. / Thermodynamical properties o f Y B a 2 C u 3 0 v ~ compounds
fects of the oxygen intercalation is to induce an orthorombic distorsion above x = 0.5 and a decrease of the c - p a r a m e t e r (from 11.83A to 11.67 A). A careful analysis of these structures indicates that the main change concerns the Ba 2+ positions. They m o v e towards the O(1) plane as the oxygen content increases because of the increase of the electrostatic interaction between Ba 2+ and additional 02 in the linear chain 0 ( 4 ) site. The positions of the other ions remain almost unchanged because of strong covalent bondings between copper and oxygen. This analysis of the different structures of YBa2Cu307_ x c o m p o u n d s is in agreement with those recently determined by different groups [6, 7].
0 -0.2
Y Ba 2 Cu 3 06.92
mO.4 01
-0.6 005
-0.8
0
-1
.< t,,,q v"
-
-1.2 .
.
0
_ _
y-
-02
~X
-O.t,
0A
-0.6
~-
0.05
-08
/ -1
Hac =
3. Results
70e
0
~/
-12
:
Many samples with various oxygen content have been investigated by ac and dc susceptibility, resistivity and specific heat experiments. The sharpest superconducting transition at T c = 91 K was obtained for the c o m p o u n d containing the largest (x = 0.08) oxygen content. Typical ac susceptibility results are given in fig. 1 which reports both the real and imaginary part of the susceptibility for excitation fields of 0.01 and 7 0 e . For Hac = 0.01 Oe the transition is very narrow (about 1 K), X' and X" show a sharp diamagnetic signal and a single peak, respectively. M o r e o v e r X" exhibits an unusual t e m p e r a t u r e independent contribution in the superconducting state. At higher ac fields a new feature appears below Tc consisting in a shoulder for X' and a correlated broad peak for X"- This b r o a d peak can easily be shifted by applying a static field, in contrast with the narrow p e a k which remains unchanged. These observations lead us to associate the narrow peak of X" to the superconducting phase transition within "grains", and the broad p e a k of X" to a distribution of superconducting junctions linking the "grains". The excess of dissipation (X") and the relative decrease of the diamagnetism signal ( X ' ) appearing around 82 K therefore attributed to the transition of the superconducting junctions to a resistive state.
', 20
z
;
t,O
;
~ ,
t
60
80
I
I00
Temperature ( K ) Fig. 1. Real and imaginary parts of the ac susceptibility as a function of temperature at different excitation fields of the superconducting compound YBa2Cu30~ 92. The zero field resistivity m e a s u r e m e n t on the oxygen rich sample (x = 0.08) gives the same transition t e m p e r a t u r e with the same width than by ac susceptibility experiments. The c o m p o u n d with x = 0.34 is found to be superconducting at a lower t e m p e r a t u r e T c = 50 K. YBazCu306 is an insulating c o m p o u n d which exhibits a semiconducting behaviour with an activation energy of 0.19 eV (see fig. 2). We have also p e r f o r m e d dc susceptibility measurements in the t e m p e r a t u r e range 6 0 - 4 0 0 K using a S Q U I D magnetometer. The values of X T as a function of t e m p e r a t u r e are reported in fig. 3 for various oxygen stoichiometries (0.08 < x < 1). A correction for the diamagnetism of the ions has been performed. At low t e m p e r a t u r e ( T < 100 K), X T is constant indicating that the susceptibility is dominated by a Curie-like contribution C / T with C = 0 . 0 6 e m u / m o l which could correspond to about 5% of isolated Cu 2+ ions. It must be emphasized that this contribution is very much sample dependent and a value down to C - = - 0 . 0 2 e m u / m o l has been obtained. At high
307
C. Ayache et al. / Thermodynamical properties of YBa2Cu307_ ~ compounds
Y Ba
Y Ba 2 Cu 3 07_ x
IE
60-
¢w
£] tY .S
V
o
ai -6 E
Eo = 2200K = 0.19eV
o
o
~7
t,0-
0
~7
0~7 0~7 v D
E I t,
0 3
I 5
I
c~vo~
Y Ba 2 Cu 3 OT-x
¢ o 0 o
0.12
A
E
Oo o °
0,06
B @
o1@~
oooDqb o °O ~' AZ~ ~0 ~ 0 o
0
x=0.3
o
x:0.5
". x : O . 7 m x=l
0.04 H : 5WOe 0.02
0 0
I 100
I 200
x=l
I
I
I
25
50
75
o
D
o o o
x : O,3Z,
Fig. 4. CIT as a function of T 2 for different oxygen contents in the Y B a 2 C u 3 0 ` compounds.
o
0
8
o 13
T z ( K2 1
o
8 0.10
~7 x : 0,08
20
0 o
0.14
0.08
OO~7
i.J
Fig. 2, Resistivity of YBa2Cu306 as a function of 1/T.
o E
00 00~'
I--
6
( K-1)
IO00/T
~
D
I 300
~00
Temperature ( K )
Fig. 3. X T as a function of temperature. temperature a more or less linear behaviour is observed characteristic of a T-independent susceptibility X0 = 2.5 10 -4 e m u / m o l . Actually a slight increase of X0 is observed in increasing temperature especially for Y B a 2 f u 3 0 6. L o w temperature specific heat results are shown in fig. 4. The behaviour of C / T as a
function of T 2 is very surprising because whether the samples are superconducting or not they give almost the same linear contribution to the specific heat ( 7 - 10 mJ K -2 mo1-1) within about 10%. These observations allow us to conclude that the linear contribution to the specific heat is not due to a parasitic phase, nor to a residual normal phase. H o w e v e r below T = 2 K a systematic Schottky-like contribution has been observed the amplitude of which is sample dependent and may have the same origin as the Curie constant. M o r e o v e r the /3 T3-contribution (lattice) is also very similar for the different samples, only a slight decrease is observed when the oxygen content increases. This result implies a stiffening of the p h o n o n spectrum probably correlated with the lattice contraction, observed in structural studies. The average/3-value for the three investigated samples is about 0 . 5 4 m J m o 1 - 1 K - 4 yielding a mean sound velocity 6 - - 2850 m / s . Assuming that all vibrational m o d e s are included in a D e b y e distribution, a D e b y e temperature
308
C. Ayache et al. / Thermodynamical properties o f YBa2Cu30 v x compounds
OD=360K is d e t e r m i n e d . A well d e f i n e d specific heat jump with = 70 -+ 15 m J / m o l K 2 has b e e n o b s e r v e d at T c = 93 K in t h e b e s t s a m p l e YBa2Cu306.92.
References
ACp/T~
4. Conclusion In c o n c l u s i o n w e w a n t to u n d e r l i n e t h e following results: i) T h e i m a g i n a r y p a r t of t h e ac s u s c e p t i b i l i t y is a g o o d w a y for i n v e s t i g a t i n g t h e g r a n u l a r t y p e b e h a v i o u r of t h e s e s u p e r c o n d u c t i n g c e r a m i c s . ii) T h e l i n e a r c o n t r i b u t i o n to t h e specific h e a t ( y ) is f o u n d to b e i n d e p e n d e n t o f t h e o x y g e n content. iii) Y B a 2 C u 3 0 6 is f o u n d to b e an i n s u l a t o r with a g a p o f a b o u t 0 . 2 e V b u t s u r p r i s i n g l y it e x h i b i t s a small v a l u e o f t h e s u s c e p t i b i l i t y a n d a l i n e a r c o n t r i b u t i o n to t h e specific h e a t . This unusual b e h a v i o u r c o u l d s u p p o r t t h e i d e a o f a singlet liquid s t a t e ~ la A n d e r s o n .
[l] Y.G. Bednorz and K.A. Miiller, Z. Phys. B 64 (1986) 189. [2] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58 (1987) 908. [3] C. Ayache, B. Barbara, E. Bonjour, R. Calemczuk, M. Couach, J.H. Henry and J. Rossat-Mignod, Solid State Commun. in press. [4] J.Y. Henry, P. Burlet, A. Bourret, G. Roult, P. Bacher, M.G.B.M. Jurgens and J. Rossat-Mignod, Solid State Commun. in press. [5] C. Ayache, B. Barbara, P. Burlet, E. Bonjour, A. Bourret, R. Calemczuk, M. Couach, J.H. Henry, M.G.B.M. Jurgens, J. Rossat-Mignod and G. Roult, European Workshop on High-Tc superconductors Genova 1-2-3 July 1987. [6] J.J. Capponi, C. Chaillout, A.W. Hewat, P. Lejay, M. Marezio, H. Nguyen, B. Raveau, J.L. Soubeyroux, J.L. Tholence and R. Tournier, Europhys. Lett. 3 (1987) 1301-8. [7] M.A. Beno, L. Soderholm, D.W. Capone lI, D.G. Hinks, J.D. Jorgensen, I.K. Schuller, C.U. Segre, K. Zhang and J.D. Grace, Appl. Phys. Lett., in press (1987).
SPECIFIC HEAT, SUSCEPTIBILITY AND RESISTIVITY MEASUREMENTS OF HIGH-To SUPERCONDUCTORS YBa2CuaO7_ x C. AYACHE, B. BARBARA*, E. BONJOUR, P. BURLET, R. CALEMCZUK, M. COUACH, M.J.G.M. JURGENS*, J.Y. HENRY and J. ROSSAT-MIGNOD D~partement de Recherche Fondamentale, C.E.N. Grenoble, 85 X - 38041 Grenoble Cedex, France *Laboratoire Louis-N&l, C.N.R.S., 166 X - 38042 Grenoble Cedex, France
Received 31 August 1987
Different single phase polycrystalline samples of high-T c superconducting compounds YBa2CU3OT_ x have been prepared and characterized. They have been investigated by ac and dc susceptibility, resistivity and specific heat experiments to study the influence of the oxygen content. The y-value and the T-independent susceptibility are found to be not sensitive to the oxygen content.
I. Introduction Since the discovery of high temperature superconductors in ceramic compounds La2(Ba , Sr) CuO 4 [1] with Tc = 4 0 K and more recently in BaaYCu307 x [2] with Tc = 95 K a large amount of experimental works have been devoted to these compounds. A systematic investigation of the structural, magnetic and superconducting properties of these ceramic compounds have been undertaken at the Departement de Recherche Fondamentale of the Centre d'Etudes Nucl6aires de Grenoble. Preliminary results on the specific heat anomaly and critical fields of superconducting compounds [3] have already been published, together with their structural study [4, 5]. In this paper we report more extensive studies on the influence of the oxygen content on the magnetic and superconducting properties of YBa2Cu307_ x compounds.
2. Sample preparation and crystal structure analysis The samples we have investigated have been * On leave from Kamerlingh Onnes Laboratorium Leiden, The Netherlands. 0378-4363/87/$03.50 (~) Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation
prepared according to the following procedures. For samples with the largest oxygen content, YBaaCu306.92, a mixture of BaCO3, CuO and Y203 is heated in air at 950°C during 12h, quenched, annealed at 480°C under 1.2 bar of oxygen and then slowly cooled to room temperature in 12h. Samples with the formula YBa2fu30 6 are obtained by heating up the O7_x-compound under vacuum to get a sample resistivity larger than 1 MII cm at 25°C. Samples with intermediate oxygen concentration (from x = 0 to 1) have been prepared by heating under an argon atmosphere the appropriate mixture of 0 6 and O6.92-samples. All samples have been characterized by X-ray powder diffraction and parasitic phases were not detected within an accuracy of a few per cent. The chemically determined stoichiometry has been controlled by the refinement of the crystal structure from powder neutron diffraction experiments. We have shown [4] that the reduced compound has a tetragonal structure (space group P4/mmm) with a = 3.857 ,~ and c = 11.83 A. The well defined stoichiometry YBa2Cu306. 0 was confirmed and the oxygen site 0(4) along the Cu-chains (Cu(1)) was found to be completely empty yielding two-fold coordinated Cu ÷ ions, Cu in the CuO 2 planes remaining with a Cu 2+ valency. From 06. 0 to O6.92-compounds the main ef-
306
C. Ayache et al. / Thermodynamical properties o f Y B a 2 C u 3 0 v ~ compounds
fects of the oxygen intercalation is to induce an orthorombic distorsion above x = 0.5 and a decrease of the c - p a r a m e t e r (from 11.83A to 11.67 A). A careful analysis of these structures indicates that the main change concerns the Ba 2+ positions. They m o v e towards the O(1) plane as the oxygen content increases because of the increase of the electrostatic interaction between Ba 2+ and additional 02 in the linear chain 0 ( 4 ) site. The positions of the other ions remain almost unchanged because of strong covalent bondings between copper and oxygen. This analysis of the different structures of YBa2Cu307_ x c o m p o u n d s is in agreement with those recently determined by different groups [6, 7].
0 -0.2
Y Ba 2 Cu 3 06.92
mO.4 01
-0.6 005
-0.8
0
-1
.< t,,,q v"
-
-1.2 .
.
0
_ _
y-
-02
~X
-O.t,
0A
-0.6
~-
0.05
-08
/ -1
Hac =
3. Results
70e
0
~/
-12
:
Many samples with various oxygen content have been investigated by ac and dc susceptibility, resistivity and specific heat experiments. The sharpest superconducting transition at T c = 91 K was obtained for the c o m p o u n d containing the largest (x = 0.08) oxygen content. Typical ac susceptibility results are given in fig. 1 which reports both the real and imaginary part of the susceptibility for excitation fields of 0.01 and 7 0 e . For Hac = 0.01 Oe the transition is very narrow (about 1 K), X' and X" show a sharp diamagnetic signal and a single peak, respectively. M o r e o v e r X" exhibits an unusual t e m p e r a t u r e independent contribution in the superconducting state. At higher ac fields a new feature appears below Tc consisting in a shoulder for X' and a correlated broad peak for X"- This b r o a d peak can easily be shifted by applying a static field, in contrast with the narrow p e a k which remains unchanged. These observations lead us to associate the narrow peak of X" to the superconducting phase transition within "grains", and the broad p e a k of X" to a distribution of superconducting junctions linking the "grains". The excess of dissipation (X") and the relative decrease of the diamagnetism signal ( X ' ) appearing around 82 K therefore attributed to the transition of the superconducting junctions to a resistive state.
', 20
z
;
t,O
;
~ ,
t
60
80
I
I00
Temperature ( K ) Fig. 1. Real and imaginary parts of the ac susceptibility as a function of temperature at different excitation fields of the superconducting compound YBa2Cu30~ 92. The zero field resistivity m e a s u r e m e n t on the oxygen rich sample (x = 0.08) gives the same transition t e m p e r a t u r e with the same width than by ac susceptibility experiments. The c o m p o u n d with x = 0.34 is found to be superconducting at a lower t e m p e r a t u r e T c = 50 K. YBazCu306 is an insulating c o m p o u n d which exhibits a semiconducting behaviour with an activation energy of 0.19 eV (see fig. 2). We have also p e r f o r m e d dc susceptibility measurements in the t e m p e r a t u r e range 6 0 - 4 0 0 K using a S Q U I D magnetometer. The values of X T as a function of t e m p e r a t u r e are reported in fig. 3 for various oxygen stoichiometries (0.08 < x < 1). A correction for the diamagnetism of the ions has been performed. At low t e m p e r a t u r e ( T < 100 K), X T is constant indicating that the susceptibility is dominated by a Curie-like contribution C / T with C = 0 . 0 6 e m u / m o l which could correspond to about 5% of isolated Cu 2+ ions. It must be emphasized that this contribution is very much sample dependent and a value down to C - = - 0 . 0 2 e m u / m o l has been obtained. At high
307
C. Ayache et al. / Thermodynamical properties of YBa2Cu307_ ~ compounds
Y Ba
Y Ba 2 Cu 3 07_ x
IE
60-
¢w
£] tY .S
V
o
ai -6 E
Eo = 2200K = 0.19eV
o
o
~7
t,0-
0
~7
0~7 0~7 v D
E I t,
0 3
I 5
I
c~vo~
Y Ba 2 Cu 3 OT-x
¢ o 0 o
0.12
A
E
Oo o °
0,06
B @
o1@~
oooDqb o °O ~' AZ~ ~0 ~ 0 o
0
x=0.3
o
x:0.5
". x : O . 7 m x=l
0.04 H : 5WOe 0.02
0 0
I 100
I 200
x=l
I
I
I
25
50
75
o
D
o o o
x : O,3Z,
Fig. 4. CIT as a function of T 2 for different oxygen contents in the Y B a 2 C u 3 0 ` compounds.
o
0
8
o 13
T z ( K2 1
o
8 0.10
~7 x : 0,08
20
0 o
0.14
0.08
OO~7
i.J
Fig. 2, Resistivity of YBa2Cu306 as a function of 1/T.
o E
00 00~'
I--
6
( K-1)
IO00/T
~
D
I 300
~00
Temperature ( K )
Fig. 3. X T as a function of temperature. temperature a more or less linear behaviour is observed characteristic of a T-independent susceptibility X0 = 2.5 10 -4 e m u / m o l . Actually a slight increase of X0 is observed in increasing temperature especially for Y B a 2 f u 3 0 6. L o w temperature specific heat results are shown in fig. 4. The behaviour of C / T as a
function of T 2 is very surprising because whether the samples are superconducting or not they give almost the same linear contribution to the specific heat ( 7 - 10 mJ K -2 mo1-1) within about 10%. These observations allow us to conclude that the linear contribution to the specific heat is not due to a parasitic phase, nor to a residual normal phase. H o w e v e r below T = 2 K a systematic Schottky-like contribution has been observed the amplitude of which is sample dependent and may have the same origin as the Curie constant. M o r e o v e r the /3 T3-contribution (lattice) is also very similar for the different samples, only a slight decrease is observed when the oxygen content increases. This result implies a stiffening of the p h o n o n spectrum probably correlated with the lattice contraction, observed in structural studies. The average/3-value for the three investigated samples is about 0 . 5 4 m J m o 1 - 1 K - 4 yielding a mean sound velocity 6 - - 2850 m / s . Assuming that all vibrational m o d e s are included in a D e b y e distribution, a D e b y e temperature
308
C. Ayache et al. / Thermodynamical properties o f YBa2Cu30 v x compounds
OD=360K is d e t e r m i n e d . A well d e f i n e d specific heat jump with = 70 -+ 15 m J / m o l K 2 has b e e n o b s e r v e d at T c = 93 K in t h e b e s t s a m p l e YBa2Cu306.92.
References
ACp/T~
4. Conclusion In c o n c l u s i o n w e w a n t to u n d e r l i n e t h e following results: i) T h e i m a g i n a r y p a r t of t h e ac s u s c e p t i b i l i t y is a g o o d w a y for i n v e s t i g a t i n g t h e g r a n u l a r t y p e b e h a v i o u r of t h e s e s u p e r c o n d u c t i n g c e r a m i c s . ii) T h e l i n e a r c o n t r i b u t i o n to t h e specific h e a t ( y ) is f o u n d to b e i n d e p e n d e n t o f t h e o x y g e n content. iii) Y B a 2 C u 3 0 6 is f o u n d to b e an i n s u l a t o r with a g a p o f a b o u t 0 . 2 e V b u t s u r p r i s i n g l y it e x h i b i t s a small v a l u e o f t h e s u s c e p t i b i l i t y a n d a l i n e a r c o n t r i b u t i o n to t h e specific h e a t . This unusual b e h a v i o u r c o u l d s u p p o r t t h e i d e a o f a singlet liquid s t a t e ~ la A n d e r s o n .
[l] Y.G. Bednorz and K.A. Miiller, Z. Phys. B 64 (1986) 189. [2] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58 (1987) 908. [3] C. Ayache, B. Barbara, E. Bonjour, R. Calemczuk, M. Couach, J.H. Henry and J. Rossat-Mignod, Solid State Commun. in press. [4] J.Y. Henry, P. Burlet, A. Bourret, G. Roult, P. Bacher, M.G.B.M. Jurgens and J. Rossat-Mignod, Solid State Commun. in press. [5] C. Ayache, B. Barbara, P. Burlet, E. Bonjour, A. Bourret, R. Calemczuk, M. Couach, J.H. Henry, M.G.B.M. Jurgens, J. Rossat-Mignod and G. Roult, European Workshop on High-Tc superconductors Genova 1-2-3 July 1987. [6] J.J. Capponi, C. Chaillout, A.W. Hewat, P. Lejay, M. Marezio, H. Nguyen, B. Raveau, J.L. Soubeyroux, J.L. Tholence and R. Tournier, Europhys. Lett. 3 (1987) 1301-8. [7] M.A. Beno, L. Soderholm, D.W. Capone lI, D.G. Hinks, J.D. Jorgensen, I.K. Schuller, C.U. Segre, K. Zhang and J.D. Grace, Appl. Phys. Lett., in press (1987).