Interface superconductor with gap behaviour like a high-temperature superconductor

Abstract

The physics of the superconducting state in two-dimensional (2D) electron systems is relevant to understanding the high-transition-temperature copper oxide superconductors and for the development of future superconductors based on interface electron systems1. But it is not yet understood how fundamental superconducting parameters, such as the spectral density of states, change when these superconducting electron systems are depleted of charge carriers. Here we use tunnel spectroscopy with planar junctions to measure the behaviour of the electronic spectral density of states as a function of carrier density, clarifying this issue experimentally. We chose the conducting LaAlO3–SrTiO3 interface2 as the 2D superconductor, because this electron system can be tuned continuously with an electric gate field3. We observed an energy gap of the order of 40 microelectronvolts in the density of states, whose shape is well described by the Bardeen–Cooper–Schrieffer superconducting gap function. In contrast to the dome-shaped dependence of the critical temperature, the gap increases with charge carrier depletion in both the underdoped region and the overdoped region. These results are analogous to the pseudogap behaviour of the high-transition-temperature copper oxide superconductors and imply that the smooth continuation of the superconducting gap into pseudogap-like behaviour could be a general property of 2D superconductivity.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Figure 1: Device layout.
Figure 2: Large-range tunnel spectra and the superconducting gap.
Figure 3: Dependence of the tunnel spectra on gate voltage.
Figure 4: Dependences of Tc and Tgap on gate voltage.
Figure 5: Comparison between phase diagrams for LaAlO3–SrTiO3 and copper oxide superconductors.

References

  1. 1

    Pereiro, J., Petrovic, A., Panagopoulos, C. & Božović, I. Interface superconductivity: history, development and prospects. Phys. Express 1, 208–241 (2011)

    CAS  Google Scholar 

  2. 2

    Ohtomo, A. & Hwang, H. Y. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. Nature 427, 423–426 (2004)

    ADS  CAS  Article  Google Scholar 

  3. 3

    Caviglia, A. D. et al. Electric field control of the LaAlO3/SrTiO3 interface ground state. Nature 456, 624–627 (2008)

    ADS  CAS  Article  Google Scholar 

  4. 4

    Warren, W. W. et al. Cu spin dynamics and superconducting precursor effects in planes above Tc in YBa2Cu3O6. 7 . Phys. Rev. Lett. 62, 1193–1196 (1989)

    ADS  CAS  Article  Google Scholar 

  5. 5

    Ding, H. et al. Spectroscopic evidence for a pseudogap in the normal state of underdoped high-Tc superconductors. Nature 382, 51–54 (1996)

    ADS  CAS  Article  Google Scholar 

  6. 6

    Damascelli, A., Hussain, Z. & Shen, Z.-X. Angle-resolved photoemission studies of the cuprate superconductors. Rev. Mod. Phys. 75, 473–541 (2003)

    ADS  CAS  Article  Google Scholar 

  7. 7

    Lee, P. A., Nagaosa, N. & Wen, X.-G. Doping a Mott insulator: physics of high-temperature superconductivity. Rev. Mod. Phys. 78, 17–85 (2006)

    ADS  CAS  Article  Google Scholar 

  8. 8

    Tranquada, J. M., Sternlieb, B. J., Axe, J. D., Nakamura, Y. & Uchida, S. Evidence for stripe correlations of spins and holes in copper oxide superconductors. Nature 375, 561–563 (1995)

    ADS  Article  Google Scholar 

  9. 9

    Kivelson, S. A., Fradkin, E. & Emery, V. J. Electronic liquid-crystal phases of a doped Mott insulator. Nature 393, 550–553 (1998)

    ADS  CAS  Article  Google Scholar 

  10. 10

    Hinkov, V. et al. Electronic liquid crystal state in the high-temperature superconductor YBa2Cu3O6. 45 . Science 319, 597–600 (2008)

    CAS  Article  Google Scholar 

  11. 11

    Fauqué, B. et al. Magnetic order in the pseudogap phase of high-Tc superconductors. Phys. Rev. Lett. 96, 197001 (2006)

    ADS  Article  Google Scholar 

  12. 12

    Ghiringhelli, G. et al. Long-range incommensurate charge fluctuations in (Y,Nd)Ba2Cu3O6+x . Science 337, 821–825 (2012)

    ADS  CAS  Article  Google Scholar 

  13. 13

    Norman, M. R., Pines, D. & Kallin, C. The pseudogap: friend or foe of high-Tc? Adv. Phys. 54, 715–733 (2005)

    ADS  CAS  Article  Google Scholar 

  14. 14

    Orenstein, J. & Millis, A. J. Advances in the physics of high temperature superconductors. Science 288, 468–474 (2000)

    ADS  CAS  Article  Google Scholar 

  15. 15

    Renner, Ch., Revaz, B., Genoud, J.-Y., Kadowaki, K. & Fisher, Ø. Pseudogap precursor of the superconducting gap in under- and overdoped Bi2Sr2CaCu2O8+δ . Phys. Rev. Lett. 80, 149–152 (1998)

    ADS  CAS  Article  Google Scholar 

  16. 16

    Miyakawa, N. et al. Predominantly superconducting origin of large energy gaps in underdoped Bi2Sr2CaCu2O8+δ from tunneling spectroscopy. Phys. Rev. Lett. 83, 1018–1021 (1999)

    ADS  CAS  Article  Google Scholar 

  17. 17

    Emery, V. J. & Kivelson, S. A. Importance of phase fluctuations in superconductors with small superfluid density. Nature 374, 434–437 (1995)

    ADS  CAS  Article  Google Scholar 

  18. 18

    Giaever, I. Energy gap in superconductors measured by electron tunneling. Phys. Rev. Lett. 5, 147–148 (1960)

    ADS  Article  Google Scholar 

  19. 19

    Servoin, J. L., Luspin, Y. & Gervais, F. Infrared dispersion in SrTiO3 at high temperature. Phys. Rev. B 22, 5501–5506 (1980)

    ADS  CAS  Article  Google Scholar 

  20. 20

    Dynes, R. C., Narayanamurti, V. & Garno, J. P. Direct measurement of quasiparticle-lifetime broadening in a strong-coupled superconductor. Phys. Rev. Lett. 41, 1509–1512 (1978)

    ADS  CAS  Article  Google Scholar 

  21. 21

    Binnig, G., Baratoff, A., Hoenig, H. E. & Bednorz, J. G. Two-band superconductivity in Nb-doped SrTiO3 . Phys. Rev. Lett. 45, 1352–1355 (1980)

    ADS  CAS  Article  Google Scholar 

  22. 22

    Kalisky, B. et al. Locally enhanced conductivity due to tetragonal domain structure in LaAlO3/SrTiO3 heterointerfaces. Nature Mater (in the press)

  23. 23

    Uemura, Y. J. et al. Universal correlations between Tc and n s/m* (carrier density over effective mass) in high-Tc cuprate superconductors. Phys. Rev. Lett. 62, 2317–2320 (1989)

    ADS  CAS  Article  Google Scholar 

  24. 24

    Bert, J. A., et al. Gate-tuned superfluid density at the superconducting LaAlO3/SrTiO3 interface. Phys. Rev. B 86, 060503(R) (2012)

    ADS  Article  Google Scholar 

  25. 25

    Fischer, Ø., Kugler, M., Maggio-Aprile, I., Berthod, C. & Renner, Ch. Scanning tunneling spectroscopy of high-temperature superconductors. Rev. Mod. Phys. 79, 353–419 (2007)

    ADS  CAS  Article  Google Scholar 

  26. 26

    Joshua, A., Pecker, S., Ruhman, J., Altman, E. & Ilani, S. A universal critical density underlying the physics of electrons at the LaAlO3/SrTiO3 interface. Nature Commun. 3, 1129 (2012)

    ADS  Article  Google Scholar 

  27. 27

    Berner, G. et al. Direct k-space mapping of the electronic structure in an oxide-oxide interface. Phys. Rev. Lett. 110, 247601–247605 (2013)

    ADS  CAS  Article  Google Scholar 

  28. 28

    Sacépé, B. et al. Pseudogap in a thin film of a conventional superconductor. Nature Commun. 1, 140 (2010)

    Article  Google Scholar 

  29. 29

    Feld, M., Fröhlich, B., Vogt, E., Koschorreck, M. & Köhl, M. Observation of a pairing pseudogap in a two-dimensional Fermi gas. Nature 480, 75–78 (2011)

    ADS  CAS  Article  Google Scholar 

  30. 30

    Schneider, C. W. et al. The origin of oxygen in oxide thin films: role of the substrate. Appl. Phys. Lett. 97, 192107 (2010)

    ADS  Article  Google Scholar 

Download references

Acknowledgements

We acknowledge discussions with M. Beasley, A. P. Kampf and T. Kopp, technical support from M. Hagel, I. Hagel, M. Schmid and D. Zhang, and financial support from the German Science Foundation (TRR 80). Electron microscopy and spectroscopy was performed at the Cornell Center for Materials Research (CCMR), which is an NSF MRSEC supported by NSF grant DMR-1120296.

Author information

Affiliations

Authors

Contributions

C.R., R.J. and C.W.S. prepared the samples. C.R., H.B., W.D. and E.F.-S. performed the transport measurements. L.F.K. and D.A.M. performed the electron microscopy. F.L. and J.R.K. provided theoretical input for the analysis and the modelling of the tunnel characteristics. J.M. supervised the research. C.R., H.B. and J.M. wrote the manuscript. All authors contributed to the discussion and provided feedback on the manuscript.

Corresponding author

Correspondence to J. Mannhart.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Scanning transmission electron microscopy.

a, b, High-angle annular dark-field STEM image (a) and simultaneously recorded electron energy-loss spectroscopic map (b) of a Au–LaAlO3–SrTiO3 tunnel junction. The spectroscopic image maps the concentration of La in green, that of Ti in blue and that of O in red. c, Ti, O and La concentration profiles were computed by averaging the elemental map from b parallel to the interface. The LaAlO3–SrTiO3 interface shows a small amount of cation interdiffusion. No variations of the oxygen concentration across the interface are observed.

Extended Data Figure 2 Temperature dependence of the gap.

Best fits (solid lines) used to extract the temperature dependence of the superconducting gap, Δ, from the measured tunnel spectra (dots). The model used is based on a lifetime-broadened s-wave quasiparticle density of states (equation (1)) with no additional conductance contributions (G0 = 0 in equation (2)). The inset shows the obtained fit results for Δ and the quasiparticle decay-rate parameter, Γ. The solid line is the prediction of the BCS model for Tc = 0.28 K and 2Δ/kBTc = 3.3.

Extended Data Figure 3 Dependence of the tunnelling spectra on gate voltage.

ac, Temperature dependence of the gap at VG = −200 V (a), 0 V (b) and 200 V (c). Above Tgap, the spectra are independent of temperature, but a depression of the DOS at EF is still present. d, The T = 0.07 K data together with the best fits (solid lines). The spectra above Tgap were used as a background in the fitting routine.

Extended Data Figure 4 Temperature dependence of the 2DEL sheet resistance for different gate voltages.

The inset shows a sketch of the device layout.

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Richter, C., Boschker, H., Dietsche, W. et al. Interface superconductor with gap behaviour like a high-temperature superconductor. Nature 502, 528–531 (2013). https://doi.org/10.1038/nature12494

Download citation

Further reading

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing