Magnetic and superconducting ordering in one-dimensional nanostructures at the LaAlO3/SrTiO3interface

Abstract

We formulate a model for magnetic and superconducting ordering in one-dimensional nanostructures at LaAlO3/SrTiO3 interfaces containing both localized magnetic moments and itinerant electrons. Though these both originate in Ti 3d orbitals, the former may be due to electrons more tightly-bound to the interface while the latter are extended over several layers. Only the latter contribute significantly to metallic conduction and superconductivity. In our model, the interplay between the two types of electrons, which is argued to be ferromagnetic, combined with strong spin-orbit coupling of the itinerant electrons, leads to magnetic ordering. Furthermore, we propose a model for interfacial superconductivity, consisting of random superconducting grains in the bulk STO driven, via coupling to the interface conduction band, towards long-ranged or quasi-long-ranged order. Most interestingly, the magnetic order and strong spin orbit coupling can lead in this manner to unconventional interfacial superconductivity, yielding a possible realization of Majorana physics.

Gate-tunable superconducting weak link and quantum point contact spectroscopy on a strontium titanate surface

“…Our structure consists of two superconducting STO banks flanking a nanoscale STO weak link, which is tunable at low temperatures from insulating to superconducting behaviour by a local metallic gate. At low gate voltages, our device behaves as a quantum point contact that exhibits a minimum conductance plateau of e 2 /h in zero applied magnetic field, half the expected value for spin-degenerate electrons, but consistent with predictions [8][9][10][11][12] and experimental signatures [13][14][15][16][17][18] of a magnetically ordered ground state. The quantum point contact mediates tunnelling between normal and superconducting regions, enabling lateral tunnelling spectroscopy of the local superconducting state.…”

Room-temperature electronically-controlled ferromagnetism at the LaAlO3/SrTiO3 interface

“…At higher carrier densities (and lower temperatures), a distinct class of magnetic behaviour is observed, with signatures such as anomalous Hall effect, anisotropic magnetoresistance and coexistence of superconductivity. Above the Lifshitz transition, local moments are predicted to align ferromagnetically with the d xz /d yz electrons, from Hund's rule coupling 20,32,33 . Such properties are distinct from the room-temperature ferromagnetism reported here.…”

“…The mechanism depends on the relative density of localized versus itinerant electrons, as well as their orbital character 20,31,32 . Fidkowski et al 32 describe a ferromagnetic Kondo model in which local d xy moments couple ferromagnetically to delocalized d xz /d yz carriers; models with ferromagnetic exchange are also described by Joshua et al 20 and Bannerjee et al 33 Michaeli et al 34 describe a model based on Zener (antiferromagnetic) exchange between localized and delocalized d xy carriers 35 .…”

“…The local moments themselves are postulated to arise either from electronic correlations 36 or interfacial disorders 32,34 , or are related to oxygen vacancies 31,37 . Extrinsic sources of magnetic impurities have been ruled out experimentally 11,12,15 .…”

A polarity-induced defect mechanism for conductivity and magnetism at polar–nonpolar oxide interfaces

“…However, it is yet unclear whether such Ti 3 þ ions reside in the interface within SrTiO 3 side, or LaAlO 3 side, or both sides. Theoretically, it has been argued that the Ti 3 þ ions arise in SrTiO 3 side, owing to the occupation of the low-energy Ti-d xylike sub-bands caused by the interfacial splitting of orbital degeneracy 31 , or interfacial disorder 32,33 , or interfacial oxygen vacancies 34 . However, these scenarios are difficult to explain the fact that magnetism occurs at p-type interfaces and n-type interfaces with a critical thickness (L c ) similar to that for 2DEG.…”