Paramagnetic susceptibility of highly conducting polyaniline: Disordered metal with weak electron-electron interactions (Fermi glass)

Electron-spin-resonance studies of films of the conducting form of polyaniline (PANI), doped with camphor-sulfonic acid (CSA) and cast from metacresol solutions, show a temperature-independent Pauli susceptibility between 300 and 50 K; in PANI-CSA, there is no Curie contribution to the paramagnetic susceptibility at high temperatures. Below 50 K, a Curie-like contribution to the electronic susceptibility ({χ}{∝}1/T) is observed, indicative of singly occupied states at the Fermi energy. Thus, at approximately 50 K, the electron-electron interactions are comparable to the thermal energy in this disordered metal near the boundary of the metal-insulator transition. We conclude from this crossover that the intrasite electron-electron interaction parameter U is {~}50--60 K ({~}4--5 meV). In many samples, evidence of a spin-glass transition is observed around 10 K due to spin-spin coupling between the singly occupied states. The average spin-spin coupling (random in sign) is thereby estimated to be 0.5--1 meV. The indirect exchange interaction between localized moments via {π}-electrons is proposed to dominate at these low temperatures.