Electrolyte Gate-Controlled Kondo Effect inSrTiO3

Abstract

We report low-temperature, high-field magnetotransport measurements of SrTiO 3 gated by an ionic gel electrolyte. A saturating resistance upturn and negative magnetoresistance that signal the emergence of the Kondo effect appear for higher applied gate voltages. This observation, enabled by the wide tunability of the ionic gel-applied electric field, promotes the interpretation of the electric field-effect-induced 2D electron system in SrTiO 3 as an admixture of magnetic Ti 3þ ions, i.e., localized and unpaired electrons, and delocalized electrons that partially fill the Ti 3d conduction band.The Coulomb interaction amongst electrons and ions in a solid can spontaneously generate internal magnetic fields and effective magnetic interactions. Unexpected magnetic phenomena may emerge whenever we consider a new system where interactions are important. In recent years, predictions for and observations of magnetism originating in the two-dimensional (2D) system of electrons at the interface between SrTiO 3 (STO) and LaAlO 3 (LAO) have attracted much attention [1][2][3][4][5][6][7], particularly the prediction of charge disproportionation and the emergence of þ3-valent Ti sites with unpaired spin [1] and direct measurements of in-plane magnetization [6,7]. The conducting electrons at the LAO=STO interface are believed to be induced by polar LAO's strong internal electric fields, and to reside on the Ti sites on the STO side of the interface, partially filling the lowest-lying Ti 3d bands [8-10]. Questions remain, however, over the role of the growth process, in particular, whether oxygen vacancy formation or cation intermixing are in fact responsible for the observed n-type conduction [11][12][13][14].Other than growing a polar overlayer, a 2D system of electrons in STO can be made by chemical doping with Nb, La, or oxygen vacancies [15][16][17][18], or purely electrostatic charging in an electric double layer transistor (EDLT) [19,20]. If electronic reconstruction in response to overlayer polarity is an accurate description for LAO=STO, then that system can be closely modeled by field-effectinduced electrons in undoped STO, where confounding questions over growth conditions do not arise, and the applied electric field can be widely tuned.In this Letter, we expand on the body of evidence for Ti 3þ magnetism in STO that conducts in two dimensions. We demonstrate a gate-controlled Kondo effect in the 2D electron system in undoped STO formed beneath the bare surface by the electric field from an ionic gel electrolyte, and interpret this system as an admixture of magnetic Ti 3þ ions (unpaired and localized electrons) and delocalized electrons partially filling the Ti 3d conduction band, as predicted theoretically [2,21]. The Kondo effect is an archetype for the emergent magnetic interactions amongst localized and delocalized electrons in conducting alloys [22,23], and the ability to produce and tune the effect by purely electrostatic means in any conducting system is of interest in its own right [24,25]. The observed appear...

Spin injection and detection in lanthanum- and niobium-doped SrTiO3 using the Hanle technique

“…19). Indeed, evidence for such scattering is found from the Kondo effect observed in 2DEGs formed at the surface of STO single crystals when electrolyte gated 20,21 and the observation of magnetism at the LAO/STO interface [22][23][24] , which have both been attributed to the formation of Ti 3 þ magnetic centres 20,21,25,26 .…”

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

“…Although the experiment of Ref. 44 suggests an antiferromagnetic coupling between conduction and impurity spins, we believe that it is more natural, according to Hund's rule, take the interaction between d xy electrons and d xz , d yz electrons to be ferromagnetic. In either case, the Hamiltonian takes the form…”

“…Although we believe that it is more natural, according to Hund's rule, to have a ferromagnetic coupling, J < 0, we will also, for completeness, consider the case of antiferromagnetic coupling, J > 0, with a magnitude of roughly one third of the bandwidth, as obtained in Ref. 44.…”

“…We thus model these electrons as localized spins. Further indirect evidence for a picture involving localized spins in LAO/STO comes from transport experiments on a related system consisting of pure STO, with the doping effect of the LAO simulated by an polarized gel overlayer 44 . The longitudonal resistance in this system exhibits a Kondo minimum as a function of temperature, indicative of the presence of localized impurity spins.…”

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

“…Inducing a two-dimensional (2D) electron density near 10 13 cm −2 creates a metal, and several times that density creates an optimally doped superconducting state 4 . To modulate the density on this scale, we use the electric double-layer transistor technique [4][5][6][7] . A schematic of our sample is shown in Fig.…”