| dc.description.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 the
manifestation of a percolative transition of a Josephson-coupled superconducting network.
LaAlO3/SrTiO3 interface exhibits coexistence of superconductivity and ferromagnetism. In general, superconductivity and
ferromagnetism are antagonistic to each other. So, the appearance of two co-existing phase at the interface has opened up a
new direction of research in condensed matter physics. In this thesis, we address this physics by analyzing the results of the
measurements of perpendicular field magnetotransport above superconducting transition temperature (T=TBKT 2 ). From
our experiments, we identify a gate voltage tunable Lifshitz transition between single band to multiband transport. We find
that ferromagnetism is strongest below Lifshitz transition. We observed a novel transient superconducting state (TSS) in the
presence of a magnetic field applied perpendicular to the interface. We show that the TSS is a consequence of the interplay
between ferromagnetism, superconductivity and the finite relaxation time of the in-plane magnetization in this system.
We also studied the electrical noise above TBKT to look for possible signatures of effect of multiband transport. In this
thesis, we present results of the measurements of noise at T 2TBKT as the chemical potential is tuned across the Lifshitz
transition. We demonstrate that the noise have strikingly different features on either side of the Lifshitz transition. Below the
Lifshitz transition, noise is dominated by carrier density fluctuations arising from trapping-detrapping of charge carriers from
defects in the underlying SrTiO3 substrate. Above the Lifshitz transition, we propose that the noise presumably originate from
the scattering of carriers from different available conduction channels. | en_US |
