IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

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IIT Madras

The nonequilibrium behaviour of the intermediate state of non-ellipsoidal specimens of type-I superconductors is well documented. We present the identification of a signal due to an octupole moment of this intermediate state in the magnetization measurements on a disc shaped Pb specimen. We can also identify a quasi-irreversibility temperature, like the HTSC materials, in the magnetization data on a Pb disc. However, its precise value at a given H depends on the orientation of the plane of the disc with respect to the applied field and it cannot be treated as a material characteristics of Pb. © 1990.

Department Of Physics

Physica C: Superconductivity and its applications

A reappraisal of irreversible behaviour in the intermediate state of a type-I superconductor

Single phase compounds of Nd2-xCexCuO4-y(0.14≤x≤0.18) were made by the standard solid state reaction. The as-prepared nonsuperconducting samples were further annealed in various reducing atmospheres to induce superconductivity. All of the samples, except that with a concentration x=0.18, exhibited superconductivity after an annealing treatment at 850°C for 6 h in a vacuum of 10-5 Torr. Magnetisation measurments confirmed the Meissner fraction to be around 20% for these compounds. Resistivity measurements were performed on samples which were annealed under different conditions. The resistivity showed a ln T dependence, which was prominent in those samples in which superconductivity was not observed. In the samples showing zero resistivity, the ln T term is absent or present with a much smaller coefficient than in the nonsuperconducting samples. © 1990.

Logarithmic temperature dependence of the resistivity of Nd2-xCexCuO4-y

In ceramic superconductors, where the intergrain coupling is weak, the phase transition at Tco corresponds to the temperature at which phase coherence occurs in all the grains. The region between Tm, the mid point of the superconducting transition, and Tco, the temperature at which the resistivity goes to zero, is called the paracofioerence region. The resistivity behaviour of single phase Tl2CaBa2Cu2O8, CaLaBaCu3O7−y and CaSmBaCu3O7−y has been studied in the above region and the results indicate that the excess conductivity varies as (T − Tco)−γ. © 1990, Elsevier Science Publishers B.V. (North-Holland). All rights reserved.

GENERAL

Physica B: Condensed Matter

Paracoherence studies in Tl2CaBa2Cu2O8,CaLaBaCu3O7−y and CaSmBaCu3O7−y

We report the first measurement of contributions from multipole moments higher than the dipole of a non-uniformly magnetized intermediate state of a type I superconductor. In the specimen shape studied, these contributions are small compared to that from the dipole and they do not affect in a significant way the magnetization curve obtained using a commercial SQUID magnetometer. © 1990.

Solid State Communications

Measurement of higher multipole moments of an inhomogeneous state of a superconductor

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

The electrical resistivity of the granular superconductors, YBa2Cu3-xZnxO7-y with x=0.05, 0.075, 0.1, 0.15, and 0.2, has been measured precisely below the midpoint of the superconducting transition. The expression for electrical resistivity due to Ausloos fits the data over a small temperature region. Besides, the value estimated for the average volume of the intergranular junction on the basis of the above fit is about a few thousands of A3 which is physically not reasonable. On the other hand, the results obtained could be explained better on the basis of a model that assumes that the fraction of the grain boundaries not becoming superconducting at a temperature T follows a Gaussian distribution. © 1989 The American Physical Society.

Grain-boundary effects on the resistivity 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.

Department Of Chemistry

Phase Transitions

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

Two samples of single phase thallium-calcium-barium-copper-oxide were prepared using a precursor matrix technique. The resistivity of these samples was determined from 300 K to the temperature of zero resistance, Tzero, using the Montgomery technique. The room temperature resistivity of sample 2 was approximately seven times the room temperature resistivity of sample 1. From these measurements it was found that the temperature dependence of the fluctuation-induced excess conductivity in both the samples was in agreement with the Aslamazov-Larkin expression for three dimensional fluctuations. © 1989.

Fluctuation induced excess conductivity in Tl2CaBa2Cu2O8

The specific heat of samples of Y1-xPrxBa2Cu3O7-y for x = 0.1 to 0.5 has been measured from 4.5 to 65 K using a quasi-adiabatic calorimeter. A plot of C/T vs T2 shows a linear behavior below 20 K. The excess specific heat due to praseodymium in all the compounds is obtained by subtracting the specific heat of YBa2Cu3O7-x from the that of doped compound. The excess specific heat shows a peak near 19 K and its height decreases with increase in x. The excess specific heat could be interpreted on the basis of Schottky anomalies due to Pr3+ and Pr4+ ions, the peak at 19 K arising from the former. The fraction of Pr3+ ion obtained from a fit to the specific heat curves decreases from 0.30 for x = 0.1 to 0.05 for x = 0.5. © 1988.

Specific heat of praseodymium doped yttrium barium copper oxide

Studies on the title compounds show that while La and Pr form complete solid solutions, Tb does not substitute for Y. Superconducting transition temperature (Tc) decreased with increasing x from 91K but pronounced effect is noted for Ln=Pr. PrBa2Cu3O7 exhibits a semiconductor behavior. © 1988 Elsevier Science Publishers B.V.

Superconductivity studies on (Y1-xLnx)Ba2Cu3O7, Ln=La,Pr,Tb

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.

Pramana

Specific heat of YBa2Cu3O7-x from 4.2 to 60 K

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.

Enhancement of thermopower in the high Tc superconductor YBa2Cu3O7 and related compounds

The absolute thermoelectric power measurements on AgMo6S8, Mo6S8, pbMo6S8, SnMo6S8, SnMo6S8 were made from 4.2 to 300 k. All the compounds have positive thermoelectric power near room temperature. Mo6S8 and the compounds containing small metal ion such as Cu and Ag show a negative peak and the compounds containing large metal ion such as Pb, Sn show a positive peak in the thermopower vs. temperature graph. There seems to be a correlation between low temperature peak in the thermopower and delocalisation of the metal ion in the compound. Two mechanisms may account for the peaks in the thermopower viz. phonon drag effect and the enhancement of thermopower caused by the renormalisation of energy, velocity and relaxation time due to electron-phonon interaction. In the present case the mechanism mentioned later aoes not seem to provide an explanation for the negative peaks in the thermopower. © 1987 The Japan Society of Applied Physics.

Fulltext

Japanese Journal of Applied Physics

Thermoelectric Power Measurements on Some Chevrel Phase Superconductors

The normal state electrical resistivity of the A-15 superconductor, Ti//3Sb, as a function of temperature from 6 to 300 K is reported. The results are analysed on the basis of the generalised diffraction model which incorporates the Pippard-Ziman condition namely that the phonons with wavelength exceeding the mean free path of the electron are ineffective in scattering of electrons.

World University root node

NORMAL STATE ELECTRICAL RESISTIVITY OF THE A-15 SUPERCONDUCTOR, Ti//3Sb.

Normal-state electrical resistivity data from 12-300K on Chevrel-phase superconductors of the type Cu1.8Mo6S8-ySe y, 0&lt;or=y&lt;or=8, are reported. The temperature dependence of the normal-state resistivity has been analysed on the basis of models proposed for A15 superconductors. It is found that the 'parallel resistor' model based on s-d scattering accounts for the observed variation satisfactorily in most of the compounds, provided the lower limit of the transport theory integral is taken to have a non-zero value obtained by a fit to the experimental data. Physically, this limit corresponds to the minimum energy required for a phonon to scatter an electron from the s band into the d band. The electrical resistivity below 40K could also be interpreted on the basis of interband electron-electron scattering.

Journal of Physics F: Metal Physics

Normal-state electrical resistivity of Chevrel-phase superconductors of the type Cu1.8Mo6S8-ySey, 0≤y≤8

The thermoelectric power, S, of the Chevrel phase compounds PbMo//6S//8 and EuMo//6S//8 were measured on sintered pellets from liquid helium temperature to room temperature. In PbMo//6S//8 S is positive and is a sum of electron diffusion (S//e) and phonon drag (S//p//h) parts. From S//e, using the rigid band model of Mott, the slope of the density of states curve is calculated. The result is compared with existing band structure calculation. In EuMo//6S//8 the thermoelectric power shows a sharp rise at around 100K which is the temperature at which a structural transformation takes place. The thermoelectric power reverses sign at 92K and 33K.

THERMOELECTRIC POWER OF PbMo//6S//8 AND EuMo//6S//8.

Normal state electrical resistivity of the Chevrel phase compounds of the type Cu1.8Mo6S8-y Sey, 0≤y≤8 and Cu1.8Mo6S8-y Tey, 0≤y≤4 is analysed on the basis of the generalized diffraction model which incorporates a postulate on electron-phonon interaction, viz phonons with wavelength exceeding the electron mean-free path are ineffective electron scatterers. Fit obtained by this model was found to be superior to other models based on the interband s-d scattering of electrons. © 1984 Indian Academy of Sciences.

Electron transport in Chevrel phase superconductors, Cu1.8Mo6S8-y Sey, 0≤y≤8 and Cu1.8Mo6S8-y Tey, 0≤y≤4-Fit to Cote-Meisel theory

The superconducting transition temperatures and normal state residual electrical resistivities of the ternary Chevrel phase compounds Cu1.8Mo6S8-YSey (0 ≤y ≤ 8) and Cu1.8Mo6S8-y Tey (0 ≤ y ≤4) are reported. The data are examined for possible correlations with structural parameters of the rhombohedral unit call and the residual resistivity values. It is found that Tc decreases as the c/a ratio increases and as the rhombohedral angle approaches 90°. There is also an inverse correlation between Tc and the normal residual resistivity of the samples. © 1982.