Discovery of superconductivity in KTaO3 by electrostatic carrier doping

Journal name:
Nature Nanotechnology
Volume:
6,
Pages:
408–412
Year published:
DOI:
doi:10.1038/nnano.2011.78
Received
Accepted
Published online

Abstract

Superconductivity at interfaces has been investigated since the first demonstration of electric-field-tunable superconductivity in ultrathin films in 19601. So far, research on interface superconductivity has focused on materials that are known to be superconductors in bulk1, 2, 3, 4, 5, 6, 7, 8, 9. Here, we show that electrostatic carrier doping can induce superconductivity in KTaO3, a material in which superconductivity has not been observed before10, 11. Taking advantage of the large capacitance of the self-organized electric double layer that forms at the interface between an ionic liquid and KTaO3 (ref. 12), we achieve a charge carrier density that is an order of magnitude larger than the density that can be achieved with conventional chemical doping. Superconductivity emerges in KTaO3 at 50 mK for two-dimensional carrier densities in the range 2.3 × 1014 to 3.7 × 1014 cm−2. The present result clearly shows that electrostatic carrier doping can lead to new states of matter at nanoscale interfaces.

At a glance

Figures

  1. Electric double-layer (EDL) transistor.
    Figure 1: Electric double-layer (EDL) transistor.

    a,b, Schematic diagrams (a) and photograph (b) of the EDL transistor with an ionic liquid electrolyte, DEME-BF4. DEME+ ions comprise the cations and BF4 ions ore the anions. The device was fabricated on a KTaO3 single crystal. Source, drain and gate electrodes were fabricated on the crystal (black area in the photograph), and the entire surface of the crystal, except for the channel area and electrodes, was covered by separator layer (yellow area in the photograph). A small amount of the ionic liquid was dropped on the crystal so that it covered the channel region (KTaO3 surface) and the gate electrode. c, Molecular and crystal structures for the anion, cation and KTaO3.

  2. Characterization of EDL transistors.
    Figure 2: Characterization of EDL transistors.

    a, Superconducting critical temperature Tc as a function of three-dimensional charge carrier density for chemically doped superconductivity in 11 different material systems (filled symbols), and electrostatically induced superconductivity in two of these (open symbols). The lower panel shows the electronic phases appearing in KTaO3 as a function of carrier density up to the maximum density that can be achieved with chemical doping: much higher densities are possible with EDL transistors (dashed red vertical line). b, Sheet resistance RS (on a logarithmic scale) versus temperature T at six different gate voltages VG for an EDL transistor in which the channel is a single crystal of KTaO3. The channel shows metallic conduction for values of VG higher than a threshold of 2.75 V. c, Two-dimensional charge carrier density n2D (top) and carrier mobility (bottom) versus T for five values of VG: both n2D and mobility were evaluated by Hall measurements.

  3. Transport properties.
    Figure 3: Transport properties.

    a, Two-dimensional charge carrier density n2D, deduced from the Hall coefficient at 100 K, versus gate voltage VG for four different EDL transistors in which the channel is a layer of KTaO3. b, Mobility at 2 K versus n2D for the same four samples. c, Mobility versus three-dimensional carrier density n3D, deduced from the estimated depth distribution of carriers (see Supplementary Information) for the same four samples: note that both axes are logarithmic. Solid and open symbols correspond to the data deduced from the three-dimensional carrier density n3D determined by the Hall coefficient measured at 100 K and 2 K, respectively. Data for chemically doped bulk KTaO3 crystals from ref. 10 are also shown. Chemical doping in KTaO3 cannot access values of n3D in the shaded area.

  4. Superconducting properties.
    Figure 4: Superconducting properties.

    a, Sheet resistance RS versus temperature T at gate voltage VG = 5 V in an EDL transistor in which the channel is a layer of KTaO3. The solid line denotes the mid-point of the superconducting transition. b, RS versus magnetic field μ0H at 20 mK. c, Current I versus differential voltage V at 20 mK, measured in a four-terminal geometry.

  5. Transport properties and critical parameters of superconductivity.
    Figure 5: Transport properties and critical parameters of superconductivity.

    a, Sheet resistance RS versus temperature T at five values of the gate voltage VG. b, Two-dimensional carrier density n2D (top panel), mid-point critical temperature Tcmid (middle panel), critical magnetic field μ0Hc (bottom panel, blue; left axis) and critical current density Jc (bottom panel, green; right axis) as a function of gate voltage VG. Green and grey points in the top panel correspond to data deduced from the Hall coefficient RH at 100 K and 2 K, respectively. The KTaO3 channel remained insulating at VG = 2.5 V, and n2D was close to zero because of the low temperature. The bar for VG = 4 V in the middle panel indicates uncertainty owing to the minimum accessible temperature of 20 mK. Each critical parameter was deduced from the mid-point of the transition.

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Author information

Affiliations

  1. WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

    • K. Ueno &
    • M. Kawasaki
  2. PRESTO, Japan Science and Technology Agency, Tokyo 102-0075, Japan

    • K. Ueno
  3. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

    • S. Nakamura &
    • T. Nojima
  4. Center for Low Temperature Science, Tohoku University, Sendai 980-8577, Japan

    • S. Nakamura,
    • N. Kimura,
    • T. Nojima &
    • H. Aoki
  5. Quantum-Phase Electronics Center and Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan

    • H. Shimotani,
    • H. T. Yuan,
    • Y. Iwasa &
    • M. Kawasaki
  6. Department of Physics, Tohoku University, Sendai 980-8578, Japan

    • N. Kimura &
    • H. Aoki
  7. CREST, Japan Science and Technology Agency, Tokyo 102-0075, Japan

    • Y. Iwasa &
    • M. Kawasaki

Contributions

K.U. performed planning, sample fabrication, measurements and analysis. S.N., N.K., T.N. and H.A. assisted with cryogenic transport measurements. H.S. and H.T.Y. assisted with planning. Y.I. and M.K. performed planning and analysis.

Competing financial interests

The authors declare no competing financial interests.

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