Electronic transport in low dimensional LaAlO3 SrTiO3 heterointerface

Abstract

The large variety of phenomena exhibited by the interface between two band-insulators LaAlO3 and SrTiO3 has attracted intense research activity in recent years.The interplay of different symmetries, viz. point group symmetry, charge inversion symmetry, U(1) gauge symmetry and spin rotation symmetry in heterostructures of these complex perovskite oxides [1] lead to the co-existence of a host of intriguing properties - ferroelasticity, ferroelectricity, superconductivity and ferromagnetism [2, 3]. In this thesis, we attempt to address the physics of some of these these phases by probing the resistance and resistance fluctuations in LaAlO3/SrTiO3 heterostructures. In a two dimensional (2D) superconductor, the vortices induced by thermal fluctuations appear as a bound pairs below a characteristic temperature: the Berezinskii-Kosterlitz-Thouless transition temperature (TBKT ). Conventional techniques that probe for signatures of BKT transition such as the measurement of discontinuity in the superfluid density near the transition [4, 5] cannot be applied in the case of heterostructures where the charge carriers are buried at an interface. In this thesis, we use a fluctuation based kinetic probe namely electrical resistance fluctuation to study the BKT transition in 2D superconductors. LaAlO3/SrTiO3 heterostructure exhibits superconductivity at low temperatures with a charge carrier density dependent TBKT [6, 7]. We have experimentally studied low frequency resistance fluctuations (noise) and its higher order statistics near the superconducting transition region by varying the temperature, gate voltage and magnetic field. From the analysis of the higher order statistics of resistance fluctuations, we find large non-Gaussian components (NGC) in resistance fluctuations near TBKT , which signifies strong correlations among interacting vortices in the system. The NGC are found to be completely absent above mean field temperature TC. Theoretical simulations indicate that the large non-Gaussian resistance fluctuations are ...