title: Electron pairing without superconductivity.
abstract: Strontium titanate (SrTiO3) is the first and best known superconducting semiconductor. It exhibits an extremely low carrier density threshold for superconductivity, and possesses a phase diagram similar to that of high-temperature superconductors--two factors that suggest an unconventional pairing mechanism. Despite sustained interest for 50 years, direct experimental insight into the nature of electron pairing in SrTiO3 has remained elusive. Here we perform transport experiments with nanowire-based single-electron transistors at the interface between SrTiO3 and a thin layer of lanthanum aluminate, LaAlO3. Electrostatic gating reveals a series of two-electron conductance resonances-paired electron states--that bifurcate above a critical pairing field Bp of about 1-4 tesla, an order of magnitude larger than the superconducting critical magnetic field. For magnetic fields below Bp, these resonances are insensitive to the applied magnetic field; for fields in excess of Bp, the resonances exhibit a linear Zeeman-like energy splitting. Electron pairing is stable at temperatures as high as 900 millikelvin, well above the superconducting transition temperature (about 300 millikelvin). These experiments demonstrate the existence of a robust electronic phase in which electrons pair without forming a superconducting state. Key experimental signatures are captured by a model involving an attractive Hubbard interaction that describes real-space electron pairing as a precursor to superconductivity.
authors.affiliation: 1] Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA [2] Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, USA.
authors.forename: Guanglei
authors.initials: G
authors.lastname: Cheng
authors.affiliation: 1] Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA [2] Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, USA.
authors.forename: Michelle
authors.initials: M
authors.lastname: Tomczyk
authors.affiliation: 1] Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA [2] Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, USA.
authors.forename: Shicheng
authors.initials: S
authors.lastname: Lu
authors.affiliation: Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA.
authors.forename: Joshua P
authors.initials: JP
authors.lastname: Veazey
authors.affiliation: 1] Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA [2] Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, USA.
authors.forename: Mengchen
authors.initials: M
authors.lastname: Huang
authors.affiliation: 1] Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA [2] Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, USA.
authors.forename: Patrick
authors.initials: P
authors.lastname: Irvin
authors.affiliation: Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Sangwoo
authors.initials: S
authors.lastname: Ryu
authors.affiliation: Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Hyungwoo
authors.initials: H
authors.lastname: Lee
authors.affiliation: Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Chang-Beom
authors.initials: CB
authors.lastname: Eom
authors.affiliation: Center for Computational Materials Science, Naval Research Laboratory, Washington DC 20375, USA.
authors.forename: C Stephen
authors.initials: CS
authors.lastname: Hellberg
authors.affiliation: 1] Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA [2] Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania 15260, USA.
authors.forename: Jeremy
authors.initials: J
authors.lastname: Levy