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Anomalous peaks in the thermopower of single crystal YBa2Cu3O1−X
Physica B 165&166 (1990) 1369-1370 North-Holland
ANOMALOUS PEAKS IN THE THERMOPOWER OF SINGLE CRYSTAL YBa 2 Cu 3 0,-x A J Lowe, S E Regan and M A Howson Department of Physics, University of Leeds, Leeds LS2 9JT, UK We present results for the thermopower of two 'single' crystals of YBa 2 Cu 3 0,-x from the transition temperature to 110K. The crystals were of variable 'quality' with Tc's ranging from 73K to 92K. One of the crystals had three distinct domains with Tc's of 73K, 81K and 90K as observed using ac susceptibility measurements. Although the thermopowers were qualitatively similar, they showed anomalous peaks in the vicinity of Tc ' We have argued that the peaks are due to fluctuation effects, but the signature of the peaks, characteristic of a particular crystal, is the result of inhomogeneity, and suggests that the crystals are made up of a small number of domains with distinct Tc's. 1
INTRODUCTION In previous papers (1,2) we have presented results for the thermopower of YBCO crystals which have shown the presence of sharp peaks in the Vicinity of Tc . We have argued that this peak is due to superconducting fluctuation effects. In this paper we present further measurements on two crystals with different Tc's; sample A had a Te of 92.6K, while sample B had three distinct transitions of 90K, 81K and 73K, as seen from ac susceptibility measurements. 2
EXPERIMENTAL DETAILS The crystals are grown using a self flux method and were typically lmm x lmm x SOpm in size. The resistivity of sample A was measured using a 4 probe ac method. The thermopower was measured relative to a Pb reference using a similar ac technique as outlined in Howson et al (2). The method was different only in that the chopped light heating was provided by a laser diode, the light from which was passed into the cryostat and directed onto the crystal through a 200pm silica glass fibre. This modification allows us to be able to vary the frequency of the ac heating. The wavelength of light used was 780nm. Since we only use very small temperature gradients the technique allows us to measure the thermopower to a precision of 0.3% and a temperature resolution of less than 100mK. 3
RESULTS AND DISCUSSION The main feature in the thermopower is the series of sharp peaks around Tc (Figures 1 and 2). In Howson et al. (2) we argued that the peaks were due to a fluctuation enhancement of the thermal diffusion current. The fact that each crystal has a number of peaks suggests that the crystals are in fact made up of an equal number of domains with different transition temperatures. This is
0921-4526/90/$03.50
© 1990 -
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90 95 100 Temperature (K) FIGURE 1 The thermopower (~VjK) of sample A VS. temperature
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70 80 90 100 110 120 Temperature (K)
FIGURE 2 The thermopower (pVjK) of sample B temperature
Elsevier Science Publishers B.V. (North-Holland)
VS.
1370
A.J. Lowe, S.E. Regan, M.A. Howson
seen very clearly with sample B. Here the ac susceptibility (Figure 3) clearly shows three
Magnetic Susceptibility
o
70
60
80
ge(
100
110
Temperature (K)
thermopower peaks as might be expected if the cluster were due to domains. It is also interesting to note that while the resistivity appears to go to zero at around 9lK the thermopower is still appreciable and shows structure down to below 90K. While we have argued that each peak is the result of superconducting fluctuation effects, the fact that each crystal has a number of peaks is evidence that the samples are far from being true 'single' crystals but are made up of a small number of domains with distinctly different Tc's. The thermopower turns out to be a very sensitive probe of the presence of such domains. For example, the thermopower in Figure 2 was measured following oxygen annealing over 5 days at 400 0 C, while the measurements in Figure 5 show the thermopower on the same specimen prior to that treatment. However, it is only the fact that we are using a sensitive ac technique that has allowed us to detect such 'narrow' peaks.
FIGURE 3 The ac susceptibility of Sample B well separated transitions, while the thermopower shows three clusters of peaks in the vicinity of each transition. Sample A has one cluster of peaks with a total width of about 2.5K. The resistivity was also measured for this crystal and in Figure 4 we show the
... ."
vV~~4
10
",
5
o 0.03
...
70
..
0.02
0.00 92.5
94.0
Temperature (K) FIGURE 4 The derivatives of the resistance temperatures for Sample A
90
100
110
FIGURE 5 The thermopower (~V/K) of Sample B temperature before annealing in O2
0.01
91.5
80
Temperature (K)
VS.
derivative dR/dT. The peaks in dR/dT have a total width of about 2 to 3K with a cluster of peaks which seem to correlate with the
VS.
REFERENCES (1) M A Howson, M B Salamon, T A Friedmann, S E Inderhees, J PRice, D M Ginsberg and K M Ghiron, J.Phys. ,Condens.Matter 1, (1989) 465-71. (2) M A Howson, M B Salaman, T A Friedmann and D Ginsberg, Phys.Rev.B., 41, 1 (1990) 300-306
ANOMALOUS PEAKS IN THE THERMOPOWER OF SINGLE CRYSTAL YBa 2 Cu 3 0,-x A J Lowe, S E Regan and M A Howson Department of Physics, University of Leeds, Leeds LS2 9JT, UK We present results for the thermopower of two 'single' crystals of YBa 2 Cu 3 0,-x from the transition temperature to 110K. The crystals were of variable 'quality' with Tc's ranging from 73K to 92K. One of the crystals had three distinct domains with Tc's of 73K, 81K and 90K as observed using ac susceptibility measurements. Although the thermopowers were qualitatively similar, they showed anomalous peaks in the vicinity of Tc ' We have argued that the peaks are due to fluctuation effects, but the signature of the peaks, characteristic of a particular crystal, is the result of inhomogeneity, and suggests that the crystals are made up of a small number of domains with distinct Tc's. 1
INTRODUCTION In previous papers (1,2) we have presented results for the thermopower of YBCO crystals which have shown the presence of sharp peaks in the Vicinity of Tc . We have argued that this peak is due to superconducting fluctuation effects. In this paper we present further measurements on two crystals with different Tc's; sample A had a Te of 92.6K, while sample B had three distinct transitions of 90K, 81K and 73K, as seen from ac susceptibility measurements. 2
EXPERIMENTAL DETAILS The crystals are grown using a self flux method and were typically lmm x lmm x SOpm in size. The resistivity of sample A was measured using a 4 probe ac method. The thermopower was measured relative to a Pb reference using a similar ac technique as outlined in Howson et al (2). The method was different only in that the chopped light heating was provided by a laser diode, the light from which was passed into the cryostat and directed onto the crystal through a 200pm silica glass fibre. This modification allows us to be able to vary the frequency of the ac heating. The wavelength of light used was 780nm. Since we only use very small temperature gradients the technique allows us to measure the thermopower to a precision of 0.3% and a temperature resolution of less than 100mK. 3
RESULTS AND DISCUSSION The main feature in the thermopower is the series of sharp peaks around Tc (Figures 1 and 2). In Howson et al. (2) we argued that the peaks were due to a fluctuation enhancement of the thermal diffusion current. The fact that each crystal has a number of peaks suggests that the crystals are in fact made up of an equal number of domains with different transition temperatures. This is
0921-4526/90/$03.50
© 1990 -
!
l\
-6
:.:: ...... >;:l.
jL
~
-4
000000000 000
~
~
U,l
-2
~ 8
0 85
90 95 100 Temperature (K) FIGURE 1 The thermopower (~VjK) of sample A VS. temperature
~--~-!
12
I
10
:~
I
8 6 4 2
o 60
70 80 90 100 110 120 Temperature (K)
FIGURE 2 The thermopower (pVjK) of sample B temperature
Elsevier Science Publishers B.V. (North-Holland)
VS.
1370
A.J. Lowe, S.E. Regan, M.A. Howson
seen very clearly with sample B. Here the ac susceptibility (Figure 3) clearly shows three
Magnetic Susceptibility
o
70
60
80
ge(
100
110
Temperature (K)
thermopower peaks as might be expected if the cluster were due to domains. It is also interesting to note that while the resistivity appears to go to zero at around 9lK the thermopower is still appreciable and shows structure down to below 90K. While we have argued that each peak is the result of superconducting fluctuation effects, the fact that each crystal has a number of peaks is evidence that the samples are far from being true 'single' crystals but are made up of a small number of domains with distinctly different Tc's. The thermopower turns out to be a very sensitive probe of the presence of such domains. For example, the thermopower in Figure 2 was measured following oxygen annealing over 5 days at 400 0 C, while the measurements in Figure 5 show the thermopower on the same specimen prior to that treatment. However, it is only the fact that we are using a sensitive ac technique that has allowed us to detect such 'narrow' peaks.
FIGURE 3 The ac susceptibility of Sample B well separated transitions, while the thermopower shows three clusters of peaks in the vicinity of each transition. Sample A has one cluster of peaks with a total width of about 2.5K. The resistivity was also measured for this crystal and in Figure 4 we show the
... ."
vV~~4
10
",
5
o 0.03
...
70
..
0.02
0.00 92.5
94.0
Temperature (K) FIGURE 4 The derivatives of the resistance temperatures for Sample A
90
100
110
FIGURE 5 The thermopower (~V/K) of Sample B temperature before annealing in O2
0.01
91.5
80
Temperature (K)
VS.
derivative dR/dT. The peaks in dR/dT have a total width of about 2 to 3K with a cluster of peaks which seem to correlate with the
VS.
REFERENCES (1) M A Howson, M B Salamon, T A Friedmann, S E Inderhees, J PRice, D M Ginsberg and K M Ghiron, J.Phys. ,Condens.Matter 1, (1989) 465-71. (2) M A Howson, M B Salaman, T A Friedmann and D Ginsberg, Phys.Rev.B., 41, 1 (1990) 300-306