Potential Fluctuations at Low Temperatures in Mesoscopic-Scale SmTiO₃/SrTiO₃/SmTiO₃ Quantum Well Structures

Abstract

Heterointerfaces of SrTiO 3 with other transition metal oxides make up an intriguing family of systems with a bounty of coexisting and competing physical orders. Some examples, such as LaAlO 3 /SrTiO 3 , support a high carrier density electron gas at the interface whose electronic properties are determined by a combination of lattice distortions, spin-orbit coupling, defects, and various regimes of magnetic and charge ordering. Here, we study electronic transport in mesoscale devices made with heterostructures of SrTiO 3 sandwiched between layers of SmTiO 3 , in which the transport properties can be tuned from a regime of Fermi-liquid like resistivity (ρ ∝ T 2 ) to a non-Fermi liquid (ρ ∝ T 5/3 ) by controlling the SrTiO 3 thickness. In mesoscale devices at low temperatures, we find unexpected voltage fluctuations that grow in magnitude as T is decreased below 20 K, are suppressed with increasing contact electrode size, and are independent of the drive current and contact spacing distance. Magnetoresistance fluctuations are also observed, which are reminiscent of universal conductance fluctuations but not entirely consistent with their conventional properties. Candidate explanations are considered, and a mechanism is suggested based on mesoscopic temporal fluctuations of the Seebeck coefficient.An improved understanding of charge transport in these model systems, especially their quantum coherent properties, may lead to insights into the nature of transport in strongly correlated materials that deviate from Fermi liquid theory. Interest in oxide-based heterostructures has intensified in recent years, in large part due to advances in epitaxial film growth and the discovery of a two-dimensional conductive interface between the insulating perovskite materials LaAlO 3 (LAO) and SrTiO 3 (STO). 1,2 The electron gas at this interface exhibits strong electronic correlations, resulting in competing orders that can be tuned by various external parameters (e.g., temperature, magnetic field, pressure, chemical doping, and electrostatic gating). [3][4][5][6][7][8] Techniques for growing high-quality films and atomically sharp interfaces of these transition metal oxides (TMOs) and their characterization are central to many ongoing research efforts. 9 The possibility for rich interplay between charge, spin, and orbital ordering makes oxide heterostructure systems excellent tools in the study of correlation phenomena with wide-ranging implications, from understanding high-T c superconductivity 10 to engineering materials with desirable functional properties. [11][12][13][14] Many aspects of these and related oxide heterostructures remain, however, only partially understood, including the nature of quantum coherence in transport, and relationship of coherence with other effects such as spinorbit coupling, localization (both weak and strong), and charge and magnetic ordering. KeywordsMotivating the present study is a body of prior work focused on quantum wells (QWs) in epitaxial STO layers sandwiched between layers of eit...