Anisotropies in the optical ac and dc conductivities in lightly dopedLa2 − xSrxCuO4: the role of deep and shallow acceptor states

Abstract

We investigate the origin of the optical ac and dc conductivity anisotropiesobserved in the low temperature orthorhombic phase of lightly doped, untwinnedLa2 − xSrxNiO4 single crystals. We show that these anisotropies can be naturally ascribed to the emergenceof two odd parity, rotational-symmetry-broken, localized impurity acceptor states, onedeeper and one shallower, resulting from the trapping of doped holes by the Coulombpotential provided by the Sr ions. These two lowest-energy, p-wave-like states are split byorthorhombicity and are partially filled with holes. This leaves a unique imprint in theoptical ac conductivity, which shows two distinct far-infrared continuum absorptionenergies corresponding to the photoionization of the deep and shallow acceptor states.Furthermore, we argue that the existence of two independent and orthogonal channels forhopping conductivity, directly associated with the two orthorhombic directions, alsoquantitatively explains the observed low temperature anisotropies in the dc conductivity.