The absolute thermopower of single phase YBa2Cu3O7 and Y0.8Er0.2Ba2Cu3O7 has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show a large enhancement of thermopower in the range 150 K down to Tc. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms. © 1987 Indian Academy of Sciences.

Sankaranarayanan V

Department Of Physics

Upadhyayula V Varadaraju

Department Of Chemistry

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IIT Madras

Enhancement of thermopower in the highT c superconductor YBa2Cu3O7 and related compounds

Pramana

The title compounds have been synthesized and studied with a view to stabilize the 123 structure and achieve single phase formation. Single phase and high TC are obtained with Sr content as low as x=0.1 in the case of Yb. The single phase region extends only up to x=0.5, in contrast to the Sr substituted Y-123 system wherein the single phase region has been established up to x=1.2. The TC, zero decreases systematically with increase in x. Preliminary results of thermopower on two of the single phase compounds (x=0.1 and 0.2) show a negative thermopower at all temperatures above TC with temperature dependence similar to that of Y-123. In the case of Lu, Sr substitution does not completely stabilise the 123 phase for any x. However, the quantity of the 123 phase is increased significantly for x=0.4 -0.5 when compared to x=0.0. Structural, electrical and magnetic data are discussed. © 1992.

Materials Research Bulletin

Structure and superconductivity studies on LnBa2-xSrxCu3O7 (Ln=Yb and Lu; 0.0≤x≤0.5)

Single-phase 2122 samples of thallium and bismuth superconductors were made by the precursor matrix method. The thermopower of these samples was measured in the temperature range 250 K-Tc . The thermopower was positive and decreased linearly with increasing temperature above Tc (onset). The exponential enhancement of thermopower seen in the undoped and doped YBCO was not observed in these samples. The linear variation of thermopower can be explained on the basis of either a two-band model or a narrow band model. © 1991 Indian Academy of Sciences.

Fulltext

Bulletin of Materials Science

Thermoelectric power of single-phase samples of Tl2CaBa2Cu2Oy and Ba2CaSr2Cu2Oy

Absolute thermopower measurements on well oxygen annealed YBa2Cu3-xZnxO7-y (x=0.025, 0.05, 0.075, 0.1, 0.15, 0.2, 0.3, and 0.4) were made in the temperature range 250 K to the superconducting transition temperature, Tc (zero). X-ray analysis showed the specimens to be single phase. The Tc decreased drastically with increasing zinc concentration. The thermopower was found to be positive and an enhancement relative to the linear electron-diffusion contribution was observed over a range of temperatures from 125 K down to the onset of the superconducting transition temperature, Tc (onset). The enhancement showed a steep exponential drop as the temperature increases from Tc (onset). There exists unambiguous evidence that phonon drag is not responsible for the peak. The enhancement of thermopower in all these specimens varies more steeply with temperature than what one would expect from electron-phonon interaction. Hence, the cause of the enhancement must be other than the electron-phonon interaction. © 1989 The American Physical Society.

Physical Review B

Thermopower of zinc-doped Y-Ba-Cu-O

Resistance and thermopower measurements have been made on a series of compounds, YBa2Cu3-xZnx07_Yl, with x - 0.025, 0.05, 0.1, 0.15 and 0.2. The superconducting transition temperature decreases as the zinc concentration increases. In a range of temperatures below TM, the mid point of the transition, the resistance shows an exponential temperature dependence fitting the phenomenological formula proposed by Ausloos et ai From the plot of logarithm of resistivity vs. (TM- T)1/2/T, one deduces a value of the average dimension of the Josephson junction to be a few tens of A, suggesting the microtwin boundaries to be the location of the junctions. The thermopower shows a peak always just above Tc. This conclusively shows that phonon drag is not the cause of the peak. The temperature dependence of the thermopower appears to resemble closely the earlier observations of Srinivasan et al. on yttrium barium copper oxide. Single-particle tunneling measurements carried out for two concentrations, x = 0 and 0.05, appear to indicate that the energy gap parameter scales with Tc, and 2Δ/kTchas an approximate value of 5.5. © 1989, Taylor &amp; Francis Group, LLC. All rights reserved.

Phase Transitions

Resistivity, Thermopower And Single-Particle Tunneling Studies On Some Zinc-Doped Yttrium Barium Copper Oxide Superconductors

The specific heat of superconducting oxide compound, YBa2Cu3O7 -x, is studied using a quasi-adiabatic calorimeter from 4.2 to 60 K. The analysis of the specific heat data below 15 K gives a value of 17 mJ/mole K2 for the electronic heat capacity coefficient. The value of θD(0) is determined to be 397±8 K. The variation of θD with temperature was calculated in the temperature range 4.2 to 60 K. © 1988 Indian Academy of Sciences.

Journal	Data powered by TypesetPramana
Publisher	Data powered by TypesetSpringer India
ISSN	03044289
Open Access	No