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.
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Pereiro, J., Petrovic, A., Panagopoulos, C. & Božović, I. Interface superconductivity: history, development and prospects. Phys. Express 1, 208–241 (2011)
Ohtomo, A. & Hwang, H. Y. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. Nature 427, 423–426 (2004)
Caviglia, A. D. et al. Electric field control of the LaAlO3/SrTiO3 interface ground state. Nature 456, 624–627 (2008)
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)
Ding, H. et al. Spectroscopic evidence for a pseudogap in the normal state of underdoped high-Tc superconductors. Nature 382, 51–54 (1996)
Damascelli, A., Hussain, Z. & Shen, Z.-X. Angle-resolved photoemission studies of the cuprate superconductors. Rev. Mod. Phys. 75, 473–541 (2003)
Lee, P. A., Nagaosa, N. & Wen, X.-G. Doping a Mott insulator: physics of high-temperature superconductivity. Rev. Mod. Phys. 78, 17–85 (2006)
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)
Kivelson, S. A., Fradkin, E. & Emery, V. J. Electronic liquid-crystal phases of a doped Mott insulator. Nature 393, 550–553 (1998)
Hinkov, V. et al. Electronic liquid crystal state in the high-temperature superconductor YBa2Cu3O6. 45 . Science 319, 597–600 (2008)
Fauqué, B. et al. Magnetic order in the pseudogap phase of high-Tc superconductors. Phys. Rev. Lett. 96, 197001 (2006)
Ghiringhelli, G. et al. Long-range incommensurate charge fluctuations in (Y,Nd)Ba2Cu3O6+x . Science 337, 821–825 (2012)
Norman, M. R., Pines, D. & Kallin, C. The pseudogap: friend or foe of high-Tc? Adv. Phys. 54, 715–733 (2005)
Orenstein, J. & Millis, A. J. Advances in the physics of high temperature superconductors. Science 288, 468–474 (2000)
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)
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)
Emery, V. J. & Kivelson, S. A. Importance of phase fluctuations in superconductors with small superfluid density. Nature 374, 434–437 (1995)
Giaever, I. Energy gap in superconductors measured by electron tunneling. Phys. Rev. Lett. 5, 147–148 (1960)
Servoin, J. L., Luspin, Y. & Gervais, F. Infrared dispersion in SrTiO3 at high temperature. Phys. Rev. B 22, 5501–5506 (1980)
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)
Binnig, G., Baratoff, A., Hoenig, H. E. & Bednorz, J. G. Two-band superconductivity in Nb-doped SrTiO3 . Phys. Rev. Lett. 45, 1352–1355 (1980)
Kalisky, B. et al. Locally enhanced conductivity due to tetragonal domain structure in LaAlO3/SrTiO3 heterointerfaces. Nature Mater (in the press)
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)
Bert, J. A., et al. Gate-tuned superfluid density at the superconducting LaAlO3/SrTiO3 interface. Phys. Rev. B 86, 060503(R) (2012)
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)
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)
Berner, G. et al. Direct k-space mapping of the electronic structure in an oxide-oxide interface. Phys. Rev. Lett. 110, 247601–247605 (2013)
Sacépé, B. et al. Pseudogap in a thin film of a conventional superconductor. Nature Commun. 1, 140 (2010)
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)
Schneider, C. W. et al. The origin of oxygen in oxide thin films: role of the substrate. Appl. Phys. Lett. 97, 192107 (2010)
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.
The authors declare no competing financial interests.
Extended data figures and tables
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.
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.
a–c, 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.
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
Nature Physics (2020)
A Local Maximum in the Superconducting Transition Temperature of Nb-Doped Strontium Titanate Under Uniaxial Compressive Stress
Journal of Superconductivity and Novel Magnetism (2020)
Precision Measurements of the AC Field Dependence of the Superconducting Transition in Strontium Titanate
Journal of Superconductivity and Novel Magnetism (2020)
Controlling a Van Hove singularity and Fermi surface topology at a complex oxide heterostructure interface
Nature Communications (2019)
Nature Materials (2019)