STUDY OF STRONGLY INTERACTING ELECTRON SYSTEMS : HEAVY FERMIONS

Abstract

The turn of the last two decades has seen a continued growth in the literature on studies related to a new class of metals with strongly correlated electrons, called Heavy Fermions. These contain Ce, Yb, U or Np as one of their constituents, i.e., there are always 4f or 5f electrons involved. Characteristic examples are CeA13, CeCu2Si2, CeRu2Si2, CeCu6, CeB6, YbA13, YbCu2Si2, UBei3, UPt3, UCdi 1, U2Zni7 and NpBe13. Below a characteristic temperature, T*, the heavy fermions show Fermi-liquid behavior with huge effective masses, m* ( > 100 me ) of the quasiparticles. Consequently, the low-temperature specific heat C ( = y T) shows enormously large y coefficients. ln the same temperature range Pauli paramagnetic susceptibility is greatly enhanced. As the temperature goes above T*, the specific heat levels off, susceptibility changes from Pauli to a Curie-like behavior and the quasiparticles lose their heavy mass. The Ce and U ions now behave more and more like ions with well localized f-electrons. Based on their low temperature behavior, these materials may be broadly classified into following categories : (i) Materials like CeCu6, CeAl3, UA12, do not order down to the lowest temperature and show Fermi liquid behavior at low temperature. (ii) Materials like CeB6, CeAl2, UCcIll, NpSn3 order anti-ferromagnetically at low temperature with TN = a few Kelvin. (iii) The materials like UPt3, UBe13, CeCu2Si2 have been found to show superconductivity at low temperature with Tc — 1 K and above this T, show antiferromagnetic ordering. The aim of the present thesis is to understand some of the anomalous electronic and magnetic properties of the Heavy Fermion systems, viz., electronic specific heat, (i) magnetic susceptibility and electrical resistivity. Now during last one and a half decades these Heavy Fermion systems, which have strongly interacting f-electrons and wide band conduction electrons, have been very successfully described by the Periodic Anderson Model (PAM) or the Anderson Lattice. Therefore in this work we have used PAM. The first part of the thesis deals with the study of above mentioned properties using PAM and employing the variational technique. The second part deals with Quantum Monte Carlo Simulation study of finite Anderson Lattice and Hubbard Lattice.