A. Poddar

The influence of hydrostatic pressure on the temperature dependence of the resistivity in ND23Sr13MnO3−δ

Abstract The temperature dependence of the resistivity of Nd 2 3 Sr 1 3 MnO 3−δ was studied under hydrostatic pressure up to 15 kbar. Two magnetic phase transition temperatures, T1 and T2 are identified, they both increase under pressure with nearly the same rate d T 1, 2 d P = 2.4 K/kbar . It is conccluded that T1 increases under pressure because the electron overlap integral increases and with it the double-exchange transfer integra, while a substitution with large RE elements (“negative chemical pressure”) should decrease the transfer but could still increase T1 by increasing the Zener carrier density.

Order parameter of the metal to insulator phase transition in the thio spinel CuIr2S4

The temperature dependences of the resistivity, the magnetoresistance and the Hall effect of the Mott compound CuIr2S4−y have been measured through the metal insulator transition temperature Tmi, in the temperature range 4.2 K<T<300 K and in magnetic fields up to 5 Tesla. We observed carrier concentrations of 1020cm−3 in the insulating and of 1022 cm−3 in the metallic state. Anomalies in the resistivity curve ρ(T), an anomalous shift of Tmi in magnetic fields, dTmi/dH=+0.3 K/T, and a structure of the magnetoresistance close to Tmi were observed and interpreted in terms of quasi-particle formation.

Anomalies of Hall effect and magnetoresistance in oxygen deficient La2/3A1/3MnO3-δ epitaxial layers

The effect of oxygen disorder on the magnetic moment, resistivity, Hall effect and magnetoresistance of oxygen deficient La 2/3 Al 1/3 InO 3-δ epitaxial thin films has been studied in magnetic fields up to 20 T and temperatures down to 4.2 K. Our results show, that?(T, H) of these thin films depends on δ but differs in detail from that of equivalent nonstoichiometric bulk samples. In particular, changes in sign and magnitude of the magnetoresistance ΔQ/Q occur which suggest positive and negative shifts of a magnetic order-oder transition temperature in a magnetic field, dT 1 /dH, depending on δ. The Hall measurements give a hole carrier concentration of about 3 x 10 21 cm -3 for near stoichiometric compounds.

Magnetic moment, thermal and electrical transport in the inverse Mott systems BaCo0.9Ni0.1S2-y and CuIr2S4

The antiferromagnetic BaCo0.9Ni0.1S2−y and the paramagnetic CuIr2S4 Mott systems show an inverse conductivity behavior, i.e., the metallic phase occurs at low or high temperatures, respectively, which also suggests inverse behavior in other transport properties. In this contribution we present magnetization data, conductivity, thermal conductivity, and Seebeck effect in the temperature range 4.2 K<T<350 K and in particular through the metal-insulator transition at TMI. We recover the anomalies in χ(T),σ(T), which are discussed using Jahn-Teller and anti-Jahn-Teller models; also, we compare the anomalies in σ(T), S(T), and of the heat conductivity κ(T) at TMI. In particular, for BaCo0.9Ni0.1S1.95, we find a step in κ of almost one order of magnitude (times 6) and a large pressure shift, ⩽−24 K/kbar, which makes this material a potential pressure activated heat switch, while CuIr2S4 shows a small step of κ (times 1.2) and a pressure shift of +2.8 K/kbar.

Heat Diffusivity of the Mott Systems BaCo0.9Ni0.1S2—y and CuIr2S4

The thermal diffusivity D(T), the Seebeck-coefficient S(T) and specific heat capacity c p (T) of BaCo 0.9 Ni 0.1 S 2-y and of CuIr 2 S 4 were investigated in the temperature range 100 K < T < 350 K and in particular through their metal-insulator transition at T MI . We find steps in S(T); D( T) and in the heat conductivity K( T) at T MO = 230 and 228 K, respectively. In particular, for BaCo 0.9 Ni 0.1 S 1.95 , we find a step in K of almost one order of magnitude. Since a large pressure shift had been discovered earlier for BaCo 0.9 Ni 0.1 S 1.9 , these crystals are potential pressure activated heat switches.

The effect of annealing conditions on the physical properties of Nd0.67Sr0.33MnO3-δ

A study of the influence of annealing in atmospheres with defined oxygen partial pressures (1.5×10-6 Pa ≤ PO2 ≤ 10+5 Pa) on the electrical resistivity ρ(T), magnetic susceptibility χ(T), and heat capacity cp(T) of ceramic samples of Nd0.67Sr0.33MnO3-δ is presented. ρ(T) at 1000 °C shows no appreciable change for 10+2 Pa ≤ PO2 ≤ 10+5 Pa. For samples annealed in air a sharp peak is found in cp(T) and in ρ(T) at the ferromagnetic transition (TC≈238 K). The insulator-metal transition is observed below TC as a broad peak in ρ(T). Annealing at lower PO2 leads to an increase of ρ whereas TC decreases only by ≈2 K. Annealing at PO2 ≤ 6.3×10-2 Pa suppresses TC and results in spin canting and a vanishing of the peak in cp(T). The specific heat (2-300 K) for samples annealed in air and in Ar/5% H2 is determined and thermodynamic standard values are calculated. The problem of the separation of cp(T) into lattice, electronic, and Mn and Nd magnetic contributions is discussed. cp(T < 20 K) is dominated by a Schottky-like anomaly from the Zeeman-split ground-state doublet of Nd3+. A large quasi-linear term is mimicked by a broadening of the Schottky-like anomaly and by magnetic contributions from Mn in oxygen-deficient samples. The results are discussed in the context of oxygen deficiency δ, carrier concentration, defect chemistry, and disorder.

Excess conductivity analysis of the Bi2Sr2Ca1-xYxCu2O8+y system: an estimation of interlayer coupling strength

The excess conductivity of the Bi2Sr2CaCu2O8+y system is measured as a function of carrier concentration. The analysis shows that the system is highly anisotropic and the interlayer coupling strength decreases very rapidly with the decrease of holes. In the low-hole-concentration region Tc depends strongly on the interlayer coupling as compared to that in the optimally and heavily-doped regions. From the value of the interlayer coupling, the out-of-plane coherence length and dimensional crossover temperature are estimated.

Thermodynamic fluctuation analysis in Tl2Ba2Ca1-xYxCu2O8+y (x=0-0.25) system

We report systematic conductivity fluctuation measurements above the mean-field transition temperature Tc for Tl2Ba2Ca1-xYxCu2O8+y (x=0-0.25). The excess conductivity has been analysed in terms of the Aslamazov-Larkin, Lawrence-Doniach (LD) and Maki-Thompson (MT) models. The analysis of the LD fit shows that the system is highly anisotropic and the interlayer coupling J decreases with decrease in the holes in the system. In the under-doped region the dependence of Tc on J seems to be more comparable with that in the optimally doped region. Incorporating the MT term into the LD model fits our data over quite a high-temperature range. We have also observed that the MT contribution increases with decrease in the interlayer coupling strength.

Heat diffusivity of Nd1−xSrxMnO3−δ and La1−xCaxMnO3−δ compounds

The thermal diffusivity of bulk and epitaxial layer samples of the giant magnetoresistance compounds Nd0.67Sr0.33MnO3−δ and La0.67Ca0.33MnO3−δ is investigated in a comparative study for temperatures between 35 and 350 K in order to detect preparational differences and to correlate the thermal transport with magnetization M and resistivity ρ. We find for Nd–Sr an anomaly of the heat diffusivity D near 145 K (80 K La–Ca) which we attribute to a temperature-induced canted metal to AFM insulator transition and a second anomaly in D(T) near 250 K (300 K) which we assign to the transition to paramagnetism. The low-temperature anomaly can be a step of up to five times in the heat conductivity, which makes these compounds potential magnetic field activated heat switches, as this transition can also be induced by a magnetic field.

Anomalous Metal–Insulator Transition in BaCo1—xNixS2—y as a Triggered Phase Transition

The anomalous metal-insulator transition in BaCo 1-x Ni x S 2-y is identified as a triggered phase transition based on strong coupling between a magnetic and a structural order parameter. In particular, we observe a second-order paramagnetic-antiferromagnetic transition at T N followed by a first-order metal-insulator transition at T MI . The field-induced shifts of the sequential transition temperatures are determined for y = 0.05: dT N /dB = (-0.8 ± 0.2) K/T, and (-0.1 ± 0.1) K/T ≤ dT MI /dB ≤ (-0.5 ± 0.1) K/T.