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.

Tunable Electron-Electron Interactions in LaAlO3/SrTiO3 Nanostructures

“…This setup is geometrically similar to the one reported in Ref. [27], but here we investigate higher electron densities on the QD and different gap structures in the leads. We observe a dramatic change in the transport properties as we tune the electron density on the QD using electrostatic gating (by a sketched side gate).…”

“…The general phenomenology of transitioning from strong to weak pairing interactions, known as the "BEC-BCS crossover," has been thoroughly investigated both theoretically and experimentally in ultracold atoms [23][24][25][26]. Recently, the hallmark of BEC-regime physicselectron pairing without superconductivity-was observed at the LAO=STO interface [27]. Specifically, it was found that electron pairs persist up to pairing temperatures of T p ∼ 1-10 K and magnetic fields of B p ∼ 1-10 T, far higher than the superconducting critical temperature T c ∼ 0.3 K and upper critical magnetic field μ 0 H c2 ∼ 0.3 T. The ratio of pairing temperature to Fermi temperature T p =T F ∼ 0.1-0.8 is much larger than that of conventional BCS superconductors, indicating that the pairing interactions in low-density STO are indeed quite strong and attractive, and hence are in the BEC-BCS crossover.…”

Research Update: Conductivity and beyond at the LaAlO3/SrTiO3 interface

“…Let us mention that recent measurements on quantum dots suggest pairing without phase coherence at temperature as high as 900 mK. 59 Remarkably the pseudogap line appears to merge with the superconducting boundary beyond the top of the dome, suggestive of a more three-dimensional type of transition in this region of doping. In order to test this proposal one can overlay the T c curves of LAO/STO interfaces and of bulk, oxygen reduced, STO crystals by plotting T c /T max c versus σ/σ(T max c ).…”

Large nanoscale electronic conductivity in complex oxide heterostructures with ultra high electron density

“…Moreover, these heterostructures exhibit intriguing physical phenomena and hold potential for novel electronic devices with promising properties including high breakdown voltages and extremely large electron sheet densities. 7,8 Other interesting properties such as ferroelectricity, 9-11 superconductivity, [12][13][14][15] and negative magnetoresistance 16,17 have also been reported in such systems. In this context, complex oxide 2DEGs have been shown to support electron densities that are approximately two orders of magnitude larger than those in traditional semiconductor heterostructures.…”