Sunil Panwar

A variational theory of zero field electrical resistivity of colossal magnetoresistive manganites (Re1-xAxMnO3)

Using a simple variational method, we have studied the zero field electrical resistivity ρ(T) of rare earth manganites doped with alkaline earths namely Re1-xAxMnO3 which exhibit colossal magnetoresistance (CMR), metal–insulator transition and many other poorly understood phenomena. We take the two band model Hamiltonian for manganites in the strong electron-lattice Jahn–Teller (JT) coupling regime. This model is constructed for the doped manganites which exhibit colossal magnetoresistance (CMR) involving a broad spin-majority (eg-spins) conduction band (b-band) as well as nearly localized spin-minority (t2g-spins) electron states (l-band). Two band models involving itinerant and localized states were also suggested earlier by both experimentalists and theorists. We have also studied the temperature dependence of electrical resistivity ρ(T) at H = 0 of these materials and observed the role of the model parameters e.g. local Coulomb repulsion U, strong ferromagnetic Hunds rule coupling JH between eg and t2g spins and hybridization V between l-polarons and b-electrons of the same spins on ρ(T). We find from the resistivity results that as the temperature is lowered below a critical temperature Tc (~ 200 K), there is a sudden drop in electrical resistivity ρ(T) at H = 0 resembling with the key feature of many CMR compounds like La2/3 (Pb, Ca)1/3MnO3 and (Sm1-yGdy)0.55Sr0.45MnO3 at y = 0.5. This anomaly in ρ(T) arises from the onset of magnetic ordering at 200 K and vanishes on increasing V or JH value. T-dependence of ρ(T) is metallic-like below Tc (~ 200 K), above which it shows insulator/semiconducting-like behavior.

Hall effect in heavy fermion systems

Abstract We study Hall effect in heavy fermion systems using periodic Anderson model. A variational method has been used to calculate Hall constant R H arising from skew scattering in cerium compounds. Since the scattering phase shift is strongly magnetic field dependent (the constant of proportionality is the magnetic susceptibility), the Hall constant approximately follows the magnetic susceptibility. We obtain good agreement with the experimental results except at low temperatures.

Transport properties of heavy fermions and mixed-valence systems

Abstract Representing the heavy fermion systems by the periodic Anderson Model (PAM), we have used a variational method to study the temperature dependence of electronic transport properties of these systems. The electrical resistivity ϱ ( T ) and thermoelectric power Q ( T ) calculated show the features experimentally observed in these materials. In the low temperature region ϱ ( T ) and Q ( T ) increase rapidly. Towards higher temperature Q ( T ) changes sign.

Electrical resistivity and thermoelectric power of heavy fermions and mixed‐valence systems

Representing the heavy fermions and mixed‐valence systems by the periodic Anderson model, we have used the variational method to study the temperature dependence of electronic transport properties of these systems. The electrical resistivity ρ(T) and thermoelectric power Q(T) calculated show the features experimentally observed in these materials. In the low‐temperature region ρ(T) and Q(T) increase rapidly. Toward high‐temperature region, Q(T) changes sign.

Magnetic susceptibility and electronic specific heat of Anderson lattice with finite f‐band width

We study an extension of the periodic Anderson model by considering finite f‐band width. A variational method recently developed, has been used to study the temperature dependence of the average valence of magnetic susceptibility χs and electronic specific heat Cv for different values of the f‐band width. As f‐band width increases, the low‐temperature peak in χs and Cv becomes more broad and shifts towards the high‐temperature region.