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Influence of oxygen deficiency on the superconducting properties of YBa2Cu3O7−δ
IIII
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Physica C 185- 189 (1991) 755-756 North-Holland
INFLUENCE OF OXYGEN DEFICIENCYON rilE SUPERCONDUCTINGPROPERTIESOF YBa2Cu3074i H, W0hl, R. Benischke*, M. Braun*, B. Frank*, O. Kraut*, R. Ahrens, G. Briuchle* H. Clausa, A. Erb*, W.H. Fietz, C. Meingast, G. M011er-Vogt*, and T. Wolf Kernforschungszentrum Karlsruhe, Institut for Technische Physik and * UniversitSt Karlsruhe, Fakult~t for Physik, W-7500 Karlsruhe, FRG Specific heat, thermal expansion, and pressure dependence of Tc have been investigated on polycrystals and single crystals (twinned and untvyinned) of YBa2Cu30~-~with different oxygen content. For ceramic samples in the concentration range ot the 90 K plateau, a strong decrease of the specific heat jump &CDat the superconducting transition is observed with decreasing oxygen concentration, whereas ~he expansivity jump &o clearly increases. This behavior is thermodynamically consistent with the ex~erimental!y determined pressure dependence of Tc. Measurements on single crystals reveal that the Ao s for the three crystallographic axes are different in magnitude and sign, and strongly dependent on oxygen content. Thus, the hydrostatic pressure dependence of Tc is the result of a delicate balance of the uniaxial pressure dependences. Alloying is a classical method for the investigation of electron-density-of-states effects on super* conductivity. An example is the removal of oxygen from Cu-O chains in YBa2Cu3OT-8and much effort has been put forward into understandincj ~he phase diagram of oxygen deficient YBazCu307~. In this paper we report first results of a systematic study of the specific heat jump, ACp (To),the thermal expansivity jump, Aa (Tc), and the pressure dependence of Tc of oxygen deficient YBa2Cu307-6 single crystalsas well as some ceramic samples. The ceramicswere prepared by standard procedures, then annealed in air at temperatures Ta between 300 and 700 °C and subsequently quenched in liquid nitrogen. The oxygen content was obtained by the weight change starting from fully oxygenated (x = 6.9) samples with sharp, well developed ACp jumps. Our Ta - x dependence agrees very well with the phase diagram of Ref. 1. The single crystals were grown in Y203 stabilized ZrO2 crucibles and subsequently fully oxygenated as described elsewhere.2 The oxygen concentration was reduced by annealing the crystals at 520 °C in various static oxygen pressures ranging from below 1 Torr to 1 atm, followed by a quench to room temperature. The oxygen concentration was estimated from known Tc vs. ~idependences for single crystals.3 Some of the crystals were successfully detwinned
1
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60 ;20
FIGURE 1 Specific heat vs. temperature for two polycrystalline samples with 8 = 6.9 and 6.5 (inset). The normai state specific heat, Cpn,as determined from non superconducting Zn doped YBa2Cu307.6is subtracted
by applying uniaxial pressure in the a-b plane during the 520 °C anneals under reduced oxygen pressures. The specific heat was measured with a continuous-heating adiabatic calorimeter, the exp~nsivity with a high-resolution capacitance dilatometer 4 and the pressu,e dependence of Tc inductively in a hydrostatic He-gas high-pressure cell. In Fig. 1 typical specific heat anomalies associated with the superconducting transitions are shown
a permanent address: Department of Physics, University of Illinois at Chicago 0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V..MI rights .-,-dcrved.
H. W~hl et a L / $uperconducting properties of YBa2Cu~07.~
756
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8O 6O
|
40 %4
.,= A¢Xa= -1.3.10 "s K"1'
•
7-,
4O
2O
o
2
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X
~
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:
80
A~Xb= + 1.4,10 "6 K"1
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90
100 T [K]
FIGURE 2 FIGURE 3
Specific heat jump, &Cp/Tc, and transition temperature, To, vs. oxygen concentration for t w o poiycryst~lline samples with oxygen concentrations of x = 6.9 and 6.5. The Tc = 92 K transition is characterizer °.by fluctuations. A mean field transition istaken to be ACp/T c = 40 mJ/mol K2 (see Fig. 1). For x = 6.5 the transition is considerably reduced but is still observable with a &Cp/Tc = 11 mJ/mol K2 (see inset in Fig. !) The concentration dependence of ACp and Tc are plotted in Fig. 2. The transition temperature follows the well-known two-plateau behavior. ACp, however, drops sharply in the narrow concentration range where T¢ remains nearly constant at 90 K in agreement with earlier results.S When these results are combined with the thermal expansivityjump Ao atTc with a = (1leo) d£/dT and the hydrostatic pressure dependence of To, measured on the same ceramic samples, the Ehrenfest relation dTc/dP = 3V Aa/(ACfrc) is satisfactorily obeyed. Thus, we exclude inhomogeneities from causing the strong decrease in ACp, in contrast to earlier interpretations, s The investigation of fully oxygenized, detwinned single crystals reveals a very strong anisotropy i n Aa along the a and baxis (Fig. 3). Aaa and Aab are unexpectedly large and opposite in sign. In comparisonAac is negligible.4 Via the Ehrenfest relation uniaxial pressure dependences dTc/dpa = -3.4 K/GPa and dTc/dPb = 3.7 K/GPa are to be excpected which nearly compensate under hydrostatic pressure, yielding the experimentally observed dTc/dp = 0.3 K/GPa. Preliminary results indicate that for dn oxygen deficient detwinned single crystal with Tc = 57 K (x = 6.55) the a-b anisotropy in Aa still persists, al-
Thermal expansivity in the two crystalographic basal-plane directions vs. temperature, for a detwinned single crystal with x = 6.9
though the magnitudes of the jumps are strongly reduced. At the same time a significant increase in &ac is seen ( = 2 x 10-7 K-l). As an interesting consequence the vo;ume thermal expansion jump is now dominated by Aac, in contrast to x = 6.9 where Aa¢ made no contribution. Again, for x = 6.55 there is good agreement with the experimentally observed hydrostatic dTddp of about 3 K/GPa. In conclusion, the strong reduction of the specific heat jump atTc in O-deficient YBa2Cu307-8 in a concentration range where Tc is nearly constant at 90 K implies that at least parts of an anisotropic energy gap are drastically reduced, possibly leading to a gapless state. The highly anisotropic expansivity jumps point to complex mechanism(s) responsible for T¢ changes under pressure.
1. T. Graf, G. Triscone, and J. Muller J. Less-Common Met 159 (1990) 349. . A. Zepold, M. D(Jrrler, H. P. Geserich, A. Erb, and G. MSller-Vogt, Physica C 177 (1990) 151. .
B. W. Veat, A. P. Paulikas, Hoydoo You, Hao Shi, Y. Fang, J. W. Downey, Phys. Rev. B42 (1990) 6305.
. C. Meingast, B. Blank, H. BSrkle, B. Obst, T. Wolf, and H. W~ihl, Phys. Rev. B41 (1990) 11299. . A. Junod, D. Eckert, T. Graf, G. Triscone, and J. Muller, Physica C 162-164 (1989) 482.
roll
fllltilU
Physica C 185- 189 (1991) 755-756 North-Holland
INFLUENCE OF OXYGEN DEFICIENCYON rilE SUPERCONDUCTINGPROPERTIESOF YBa2Cu3074i H, W0hl, R. Benischke*, M. Braun*, B. Frank*, O. Kraut*, R. Ahrens, G. Briuchle* H. Clausa, A. Erb*, W.H. Fietz, C. Meingast, G. M011er-Vogt*, and T. Wolf Kernforschungszentrum Karlsruhe, Institut for Technische Physik and * UniversitSt Karlsruhe, Fakult~t for Physik, W-7500 Karlsruhe, FRG Specific heat, thermal expansion, and pressure dependence of Tc have been investigated on polycrystals and single crystals (twinned and untvyinned) of YBa2Cu30~-~with different oxygen content. For ceramic samples in the concentration range ot the 90 K plateau, a strong decrease of the specific heat jump &CDat the superconducting transition is observed with decreasing oxygen concentration, whereas ~he expansivity jump &o clearly increases. This behavior is thermodynamically consistent with the ex~erimental!y determined pressure dependence of Tc. Measurements on single crystals reveal that the Ao s for the three crystallographic axes are different in magnitude and sign, and strongly dependent on oxygen content. Thus, the hydrostatic pressure dependence of Tc is the result of a delicate balance of the uniaxial pressure dependences. Alloying is a classical method for the investigation of electron-density-of-states effects on super* conductivity. An example is the removal of oxygen from Cu-O chains in YBa2Cu3OT-8and much effort has been put forward into understandincj ~he phase diagram of oxygen deficient YBazCu307~. In this paper we report first results of a systematic study of the specific heat jump, ACp (To),the thermal expansivity jump, Aa (Tc), and the pressure dependence of Tc of oxygen deficient YBa2Cu307-6 single crystalsas well as some ceramic samples. The ceramicswere prepared by standard procedures, then annealed in air at temperatures Ta between 300 and 700 °C and subsequently quenched in liquid nitrogen. The oxygen content was obtained by the weight change starting from fully oxygenated (x = 6.9) samples with sharp, well developed ACp jumps. Our Ta - x dependence agrees very well with the phase diagram of Ref. 1. The single crystals were grown in Y203 stabilized ZrO2 crucibles and subsequently fully oxygenated as described elsewhere.2 The oxygen concentration was reduced by annealing the crystals at 520 °C in various static oxygen pressures ranging from below 1 Torr to 1 atm, followed by a quench to room temperature. The oxygen concentration was estimated from known Tc vs. ~idependences for single crystals.3 Some of the crystals were successfully detwinned
1
0,6 6: 0.4 0.2
.,.:" ~,.:..~;,,,:. .'(
0.9 Ij
i
: %:~,
£0
40 10D
v [K.]
60 ;20
FIGURE 1 Specific heat vs. temperature for two polycrystalline samples with 8 = 6.9 and 6.5 (inset). The normai state specific heat, Cpn,as determined from non superconducting Zn doped YBa2Cu307.6is subtracted
by applying uniaxial pressure in the a-b plane during the 520 °C anneals under reduced oxygen pressures. The specific heat was measured with a continuous-heating adiabatic calorimeter, the exp~nsivity with a high-resolution capacitance dilatometer 4 and the pressu,e dependence of Tc inductively in a hydrostatic He-gas high-pressure cell. In Fig. 1 typical specific heat anomalies associated with the superconducting transitions are shown
a permanent address: Department of Physics, University of Illinois at Chicago 0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V..MI rights .-,-dcrved.
H. W~hl et a L / $uperconducting properties of YBa2Cu~07.~
756
i
i
i
i
L
- -
Q~e~--'-/P
"'/ //
8O 6O
|
40 %4
.,= A¢Xa= -1.3.10 "s K"1'
•
7-,
4O
2O
o
2
0
X
~
'
~
:
80
A~Xb= + 1.4,10 "6 K"1
|
--
"'
:
"
90
100 T [K]
FIGURE 2 FIGURE 3
Specific heat jump, &Cp/Tc, and transition temperature, To, vs. oxygen concentration for t w o poiycryst~lline samples with oxygen concentrations of x = 6.9 and 6.5. The Tc = 92 K transition is characterizer °.by fluctuations. A mean field transition istaken to be ACp/T c = 40 mJ/mol K2 (see Fig. 1). For x = 6.5 the transition is considerably reduced but is still observable with a &Cp/Tc = 11 mJ/mol K2 (see inset in Fig. !) The concentration dependence of ACp and Tc are plotted in Fig. 2. The transition temperature follows the well-known two-plateau behavior. ACp, however, drops sharply in the narrow concentration range where T¢ remains nearly constant at 90 K in agreement with earlier results.S When these results are combined with the thermal expansivityjump Ao atTc with a = (1leo) d£/dT and the hydrostatic pressure dependence of To, measured on the same ceramic samples, the Ehrenfest relation dTc/dP = 3V Aa/(ACfrc) is satisfactorily obeyed. Thus, we exclude inhomogeneities from causing the strong decrease in ACp, in contrast to earlier interpretations, s The investigation of fully oxygenized, detwinned single crystals reveals a very strong anisotropy i n Aa along the a and baxis (Fig. 3). Aaa and Aab are unexpectedly large and opposite in sign. In comparisonAac is negligible.4 Via the Ehrenfest relation uniaxial pressure dependences dTc/dpa = -3.4 K/GPa and dTc/dPb = 3.7 K/GPa are to be excpected which nearly compensate under hydrostatic pressure, yielding the experimentally observed dTc/dp = 0.3 K/GPa. Preliminary results indicate that for dn oxygen deficient detwinned single crystal with Tc = 57 K (x = 6.55) the a-b anisotropy in Aa still persists, al-
Thermal expansivity in the two crystalographic basal-plane directions vs. temperature, for a detwinned single crystal with x = 6.9
though the magnitudes of the jumps are strongly reduced. At the same time a significant increase in &ac is seen ( = 2 x 10-7 K-l). As an interesting consequence the vo;ume thermal expansion jump is now dominated by Aac, in contrast to x = 6.9 where Aa¢ made no contribution. Again, for x = 6.55 there is good agreement with the experimentally observed hydrostatic dTddp of about 3 K/GPa. In conclusion, the strong reduction of the specific heat jump atTc in O-deficient YBa2Cu307-8 in a concentration range where Tc is nearly constant at 90 K implies that at least parts of an anisotropic energy gap are drastically reduced, possibly leading to a gapless state. The highly anisotropic expansivity jumps point to complex mechanism(s) responsible for T¢ changes under pressure.
1. T. Graf, G. Triscone, and J. Muller J. Less-Common Met 159 (1990) 349. . A. Zepold, M. D(Jrrler, H. P. Geserich, A. Erb, and G. MSller-Vogt, Physica C 177 (1990) 151. .
B. W. Veat, A. P. Paulikas, Hoydoo You, Hao Shi, Y. Fang, J. W. Downey, Phys. Rev. B42 (1990) 6305.
. C. Meingast, B. Blank, H. BSrkle, B. Obst, T. Wolf, and H. W~ihl, Phys. Rev. B41 (1990) 11299. . A. Junod, D. Eckert, T. Graf, G. Triscone, and J. Muller, Physica C 162-164 (1989) 482.