Biennial Report 2005-2007 - Saha Institute of Nuclear Physics

112 Biennial Report 2005-07being developed for the data collection by the computer. The work is in progress.S Das, A Pal, A Chakrabarty, BG GhoshECMP4.1.1.18 Anomalous transport properties of Co-site impurity doped Na x CoO 2The effect of substitution of Cu, Zn, Ga, Mn and Ni for Co on resistivity (ρ), magnetization (M)and thermopower (S) in layered Na x CoO 2 has been investigated. At low temperature, ρ remainsmetallic (dρ/dT >0) for Cu, Zn and Ga impurities over the entire range of doping while for Mnand Ni a metal-insulator transition occurs at or above 3 percent doping. For Cu-doped samples,ρ(T) shows strong anomaly and hysteresis at around 280 K where a first-order structural phasetransition occurs due to the long-range ordering of Na-vacancy clusters. The magnetic susceptibilityexhibits Curie-Weiss law. The Neel temperature is insensitive to impurity while Curie constant andWeiss constant (θ) depend both on the nature and amount of impurity. A small enhancement ofmagnetization above 280 K and the increase of θ with Cu doping are the signatures of Na-vacancyordering. In contrast to ρ and M, S changes systematically with doping and temperature, anddoes not show anomalous behavior around 280 K.Prabhat MandalECMP4.1.1.19 Transport properties of doped RuSr 2 GdCu 2 O 8 superconductorThe transport properties of Ru 1−x M x Sr 2 GdCu 2 O 8 (Ru-1212), with M = Ti, Rh and Sn for 0 ≤x ≤ 0.2, have been studied. Both thermoelectric power (S) and resistivity (ρ) measurementsare discussed and compared with those of Ru(Sr 1−x L x )GdCu 2 O 8 , where L is La and Na, with0 ≤ x ≤ 0.10. The superconducting and normal state properties of Ru-1212 depend strongly on thetype of dopant and the substitutional site. It is noticed that T c is reduced for all doped compoundseven for Ti, Rh and Na doping where a small decrease in S is observed. The behavior of both S(T)and ρ(T) indicates that Ru-1212 superconductor is similar to other underdoped cuprate systems.A Hassen, Prabhat MandalECMP4.1.1.20 Volume contraction and resistivity drop at the Jahn-Teller transition inLa 1−x Ca x MnO 3We have investigated the Jahn-Teller transition accompanied by orbital order-disorder transitionin La 1−x Ca x MnO 3 by high temperature X-ray powder diffraction with synchrotron radiation andresistivity measurements. The unit cell volume of LaMnO 3 decreases with increasing temperaturein a narrow temperature range below T JT =750 K and then undergoes a volume collapse at T JT .The transition is first order. Similar behavior is also obtained in Ca-doped La 1−x Ca x MnO 3 forx=0.025 and 0.075. The amount of volume collapse, however, decreases with the doping and alsothe first order discontinuous transition crosses over to a quasi-continuous transition with doping.We interpret the volume contraction at the transition is due to a more efficient packing of theMnO 6 octahedra in the orbitally liquid state and the crossover from the discontinuous to the