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

Abstract

Two-dimensional electron systems in gallium arsenide and graphene have enabled ground-breaking discoveries in condensed-matter physics, in part because they are easily modulated by voltages on nanopatterned gate electrodes. Electron systems at oxide interfaces hold a similarly large potential for fundamental studies of correlated electrons and novel device technologies1,2,3, but typically have carrier densities too large to control by conventional gating techniques. Here we present a quantum transport study of a superconducting strontium titanate (STO) interface, enabled by a combination of electrolyte4,5,6,7 and metal-oxide gating. 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 e2/h in zero applied magnetic field, half the expected value for spin-degenerate electrons, but consistent with predictions8,9,10,11,12 and experimental signatures13,14,15,16,17,18 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. Our work provides a generic scheme for quantum transport studies of STO and other surface electron liquids.

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Figure 1: Device properties.
Figure 2: Tunable superconducting weak link.
Figure 3: Quantum point contact.
Figure 4: Quantum point contact spectroscopy of the superconductor.

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Acknowledgements

We thank J-M. Triscone and J. Mannhart for helpful discussions concerning this work, and S. Ilani for broader discussions concerning the properties of STO. Sample fabrication was supported by the Air Force Office of Science Research, Award No. FA9550-12-1-02520. Sample measurement was supported by the MURI Program of the Army Research Office, Grant No. W911-NF-09-1-0398. Development of the ionic liquid gating technique was supported by the Center on Nanostructuring for Efficient Energy Conversion (CNEEC) at Stanford University, an Energy Frontier Research Center funded by the US Department of Energy, Office of Basic Energy Sciences under Award No. DE-SC0001060. P.G. acknowledges support from the DOE Office of Science Graduate Fellowship Program. M.L. acknowledges support from Samsung and Stanford University. J.R.W. and D.G-G. acknowledge support from the W. M. Keck Foundation.

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P.G. designed the experiment, fabricated the sample, and performed the measurements. All authors contributed to data analysis. P.G. prepared the manuscript with input from all authors.

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Correspondence to David Goldhaber-Gordon.

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The authors declare no competing financial interests.

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Gallagher, P., Lee, M., Williams, J. et al. Gate-tunable superconducting weak link and quantum point contact spectroscopy on a strontium titanate surface. Nature Phys 10, 748–752 (2014). https://doi.org/10.1038/nphys3049

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