A variational theory of zero field electronic specific heat of Anderson lattice model: An application to colossal magnetoresistive manganites (Re1-xAxMnO3)

Abstract

Using a simple variational method, we have studied the zero field electronic specific heat (Cv) as a function of temperature of Anderson lattice model used for rare earth manganites doped with alkaline earths namely 𝑹𝒆𝟏-𝒙𝑨𝒙𝑴𝒏𝑶𝟑( where Re=La, Pr, Nd etc., and A= Ca, Sr, Ba etc.) which exhibit colossal magnetoresistance (CMR), metal- insulator transition & many other poorly understood phenomena . We have already used this variational method to study the zero field electronic & magnetic properties like electrical resistivity & magnetic susceptibility of doped CMR manganites. We have taken two band (l-b) Anderson lattice model Hamiltonian for manganites in the strong electron- lattice Jahn- Teller (JT) coupling regime to study the temperature dependence of C vin these compounds. We have also observed the role of the model parameters e.g. local Coulomb repulsion U, strong ferromagnetic Hund's Rule coupling JH between eg & t2g spins & hybridization Vk between l– polarons & d - electrons of the same spins on Cv & linear coefficient Cv/ T. We find from our results that as the temperature is lowered below a critical temperature Tc (-100 K), there is an anomaly (sharp peak) in both Cv(T) &Cv/T beyond which it falls off for particular values of JH, V & doping values x resembling with the key feature of many CMR compounds La0.815 Sr0.185 MnO3 & Pr0.6-xBixSr0.4MnO3(x = 0.0- 0.15). The observed low temperature peak in Cv(T) becomes progressively broader & shifts to higher temperature region on increasing Vk or JH or doping x value. The specific heat anomaly near Tc ∼ 100 K is related to magnetic ordering due to the paramagnetic- ferromagnetic transitions and magnetic inhomogeneity in these manganites.