K. M. Sivakumar

Effect of Mn doping on the specific heat of the high TC superconductor REBa2Cu3Oy?(RE = Y,Gd)

We present measurements of specific heat in Mn-doped compounds REBa2(Cu1−xMnx)3Oy (RE = Y and Gd) with 0≤x≤2%. It is found that the transition temperature of Mn-doped YBa2(Cu1−xMnx)3Oy (YBaCuMnO) compounds does not change appreciably. On the other hand, in the case of GdBa2(Cu1−xMnx)3Oy (GdBaCuMnO) samples, the transition temperature decreases noticeably with the increase in Mn concentration. In REBaCuMnO, a jump in specific heat at the superconducting transition was observed for a low concentration of Mn; however, only a small change in slope was noticed for Mn concentrations above 1%. Replacing 0.5% Cu by Mn has only a marginal effect on the transition temperature, but a three-fold suppression of the specific heat jump is observed with such a substitution. This finding clearly demonstrates that these constituents are being incorporated into the superconductors as a whole and not in the form of a local cluster.

Thermal transport in (Y,Gd)Ba2(Cu1−xMnx)3O7−δ for x≤0.02

Thermal conductivity ?(T) and thermoelectric power S(T) studies on (Y,Gd)Ba2(Cu1?xMnx)3O7?? (x?0.02) superconductors are presented here. Thermal conductivity for all the samples exhibits a hump below the superconducting transition temperature Tc. The peak height of the hump decreases with the Mn content in both the Y-?and Gd-based systems, barring GdBa2(Cu0.99Mn0.01)3O7??. The peak height reduction in the Gd-based cuprates is much faster (~one?fourth) compared to the Y-based samples. The thermoelectric power (TEP) of the Y-based samples for x?0.0075 is electron-like (up to ~140?K) whereas it turns to hole-like even at x = 0.005 for the Gd-based system. On the basis of the structure of the thermal conductivity hump, and of the electron-?or hole-like nature of the thermopower, it has been argued that, in the Y-based system up to x = 0.0075, Mn produces qualitatively the same effect as Gd in the Gd-based system. An analysis of the thermal conductivity data in terms of lattice theory, and the TEP data in terms of a narrow-band picture, has been made to invoke the role of Mn in these systems. Boundary scattering, point defects and sheet-like faults (from ?(T) data analysis) and chemical potential (from S(T) data analysis) support different roles of Mn for x?0.0075 and x>0.0075.

Transport properties of Ti-Zr-Ni quasicrystalline and glassy alloys

We report on measurements of the temperature dependence of the electrical resistivity (ρ), thermopower (S), and thermal conductivity (κ) of Ti-Zr-Ni alloys between 10 and 300 K. A series of Ti-Zr-Ni quasicrystals (QCs) Ti40Zr40Ni20, Ti45Zr35Ni20, and Ti50Zr30Ni20 and metallic glasses Ti35Zr45Ni20 and Ti40Zr40Ni20 was prepared to systematically study the compositional and structural dependences of their transport properties. The resistivity of all these alloys was found to be very weakly temperature dependent with a negative temperature coefficient of resistance. Further, the observed increase in electrical resistivity with increasing Ti/Zr ratio is most likely due to the increase in disorder. The S/T against temperature curves exhibited a maximum between 20 and 50 K and a noticeable deviation from the expected linear behavior in S(T) at higher temperatures. Such observations in the thermopower of QCs have been attributed to the electron-phonon enhancement and phonon drag effect. The measured thermal condu...

Effects of Ge substitution on the thermoelectric properties and pseudogap characteristics of Fe2VGa

We report the effects of partial substitution of Ge onto the Ga sites of Fe 2 VGa by measuring electrical resistivity, Seebeck coefficient and thermal conductivity as a function of temperature. It is found that Ge substitution effectively dopes electrons to the system and thus causes a dramatic decrease in the electrical resistivity. The Seebeck coefficient changes sign from positive to negative upon replacing Ga by Ge, which is in good agreement with a simple band filling picture. In addition, the magnitude of the Seebeck coefficient gradually increases and attains a maximum value of 85 μV K -1 at around 120 K for Fe 2 VGa 0.9 Ge 0.1 . Such a variation of Seebeck coefficient can be understood by means of rigid band-like shifting of the Fermi level across the pseudogap. The thermal conductivity is also reduced and a detailed analysis based on the Debye approximation indicates that the extrinsic disorder introduced by Ge substitution in Fe 2 VGa has a minor contribution to the point defect scattering. Other lattice imperfections, such as antisite disorder, may be the main source for the point defect scattering which shows no systematic variation with Ge concentration. While the thermoelectric performance improves with the partial substitution of Ge, the largest figure-of-merit (ZT) value among these presently investigated alloys is still an order of magnitude lower than the conventional thermoelectric materials.

First-order phase transition in the ternary sulfide NiV2S4

We report our investigations of the electrical resistivity, Seebeck coefficient, thermal conductivity, and heat capacity on the ternary sulfide NiV2S4 as a function of temperature. All measured physical quantities exhibit pronounced anomalies at around 165 K, with a significant hysteresis of about 10 K. The results of electrical transport measurements show conventional metallic-like characteristics below and above the transition, indicative of the first-order metal–metal transition in NiV2S4. The anomalous features of NiV2S4 are compared to those of other thiospinels, and possible origins responsible for the present transition are provided.

The influence of Mn doping on the thermal expansion of the high TC superconductor YBa2(Cu1−xMnx)3Oy

Measurement of the linear thermal expansion α has been carried out for Mn substituted YBa2(Cu1−xMnx )3Oy (0 � x � 2%) using a high resolution dilatometer over the temperature range 10–300 K. Across the superconducting transition, the jump in the coefficient of linear thermal expansion �α was found to decrease with increasing Mn content. For the pure sample, we observed a negative jump. While a threefold decrease in �α with x = 0.5% was observed, only slight changes in the oxygen content and transition temperature have been noted, as a function of the Mn concentration. The above observations clearly suggest that the substituent Mn is being incorporated into the superconductors as a whole and not in the form of a local cluster. Further, from the Ehrenfest relations, the pressure dependence of TC (dTC/dP) and the discontinuity in the compressibility, �K, are expected to decrease with the Mn concentration