Coexistence of magnetic order and two-dimensional superconductivity at LaAlO3/SrTiO3 interfaces

Abstract

A two-dimensional electronic system forms at the interface between the band insulators 1,2 LaAlO 3 and SrTiO 3 . Samples fabricated until now have been found to be either magnetic or superconducting, depending on growth conditions 3,4 . Combining high-resolution magnetic torque magnetometry and transport measurements, we report here magnetization measurements providing direct evidence of magnetic ordering of the two-dimensional electron liquid at the interface. The magnetic ordering exists from well below the superconducting transition to up to 200 K, and is characterized by an in-plane magnetic moment. Surprisingly, despite the presence of this magnetic ordering, the interface superconducts below 120 mK. This is unusual because conventional superconductivity rarely exists in magnetically ordered metals 5,6 . Our results suggest that there is either phase separation or coexistence between magnetic and superconducting states. The coexistence scenario would point to an unconventional superconducting phase as the ground state.Superconductivity and magnetic order are in general mutually exclusive phenomena. Nonetheless, the coexistence of magnetism and superconductivity has been suggested for finite-momentum pairing states 5,6 . Coexistence of magnetism and superconductivity has been reported in a few three-dimensional superconducting systems 7-9 , such as RuSr 2 GdCu 2 O 8 and UGe 2 . The question remains if such coexistence can occur in a two-dimensional electronic system. An intriguing candidate is the interface between the two band insulators LaAlO 3 (LAO) and SrTiO 3 (STO). At their ntype interface a conducting two-dimensional electron liquid is generated. Moreover, the LAO/STO interface was also reported to have a two-dimensional superconducting ground state 3 . For this system, magnetic ordering was suggested in ref. 4, the authors of which deduced the presence of magnetic scattering centres from the temperature dependence of the interface resistance R and a hysteresis of R during the sweep of magnetic field H . Different magnetotransport studies indicate an antiferromagnetic order 10 or a non-uniform field-induced magnetization and strong magnetic anisotropy 11 . Recently, it was found that, at both chemically treated STO bulk and LAO/STO interfaces, charges are electronically phase separated into regions containing a quasi-two-dimensional electron-gas phase, a ferromagnetic phase persisting above room temperature or a diamagnetic/paramagnetic phase 12 below 60 K. On the theoretical side, electronic-structure calculations yield complicated pictures for the magnetism at the interface layers [13][14][15][16] . Specifically, the calculations do not support magnetically ordered moments at the interface of an LAO/STO bilayer covered by Coexistence of magnetism and superconductivity has not been reported at the LAO/STO interfaces. The ground state was found to be controlled by growth conditions, carrier concentration 18 and external magnetic field 19 . These experimental observations based on transport properties ...

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Two-dimensional electron gas at the LaAlO3/SrTiO3 inteface with a potential barrier

“…[1][2][3] It has been shown, that even though the constituent oxides are ordinary band insulators with well-known electronic properties, 4 their interfaces can exhibit a variety of phenomena -from two-dimensional (2D) metallic conductivity, [5][6][7][8][9] and superconductivity, 10 to ferromagnetism 11,12 and coexistence of magnetic order and superconductivity. 1,13,14 Beginning with the paper by Ohtomo and Hwang, 5 the physical properties of the LaAlO 3 /SrTiO 3 interface have been intensively studied in recent years, both experimentally and theoretically. The main peculiarity of such an interface is its perfectly metallic conductivity, with a relatively high electronic mobility, that can be described by the model of 2D electron gas.…”

Theory of Multi-Orbital Superconductivity in SrTiO3/LaAlO3Interface

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