Superconductivity in Oxide heterostructure
by Prof. Aveek Bid (Department of Physics, Indian Institute of Science, Bangalore)
Monday, January 15, 2018 from to (Asia/Kolkata)
The mutual interplay of point group symmetry, charge inversion symmetry, U(1) gauge symmetry and spin rotation symmetry in heterostructure of complex perovskite oxides lead to the co-existence of a host of intriguing properties - ferroelasticity, ferroelectricity, super-conductivity and ferromagnetism . Superconductivity and magnetism are generally considered to be incompatible with each other and hence the observation of the co-existence of these two phases in the conducting electronic layer formed at the interface of two insulating oxides LaAlO3 (LAO) and SrTiO3 (STO) has opened up a new direction of research in condensed matter physics. Despite intensive research over the last decade, there is no clear understanding of theorigin of superconductivity and ferromagnetism in this system.There is now overwhelming evidence that superconductivityis mediated by phonons and is conventional BCS-like. Scanning superconductingquantum interference device (SQUID) measurements have revealed that the superconductivity in these systems is probably spatially inhomogeneous although more recent experiments may suggest otherwise. While torque measurements show a large in-field magnetization, scanning SQUID experiments show that there are only spatially inhomogeneous patches of localmoments with no net magnetization. Although various scenarios have been invoked to reconcile theseapparently contradictory experimental observations, a clear picture of the magnetization and superconducting behaviour ofthis system is yet to emerge. We have observed a novel magnetic field assisted apparent transient superconducting state in the two-dimensional electron gas existing at the interface of LaAlO3/SrTiO3 heterostructure. This transient state appears when a relaxing normal magnetic field reduces the magnetization of the system to a value such that electron pairing becomes energetically favorable. The metastable state thus reached depends critically on the doping density in the parent compound. We find that at ultra-low temperatures and at moderate doping levels, superconductivity is a hidden order and is masked by the in-plane magnetization - appearing only whenthe net magnetization is sufficiently low.Our results clearly demonstrate the inherently metastable nature of the superconducting state competing with a magnetic order in these systems. --- Figure 1: (a) Magnetoresistance of the device at different gate voltages ranging from -200 V till 170 V measured at 245 mK. (b) Magnetoresistance at gate voltage Vg = -200 V showing hysteresis at low magnetic fields. The arrows denote the direction of magnetic field sweep.(c) Hysteresis in magnetoresistance as a function of gate voltage and magnetic field at 245 mK. Note that the hysteresis gradually disappears with increasing Vg.