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Evidence for Eight-Node Mixed-Symmetry Superconductivity in a Correlated Organic Metal

Daniel Guterding, Sandra Diehl, Michaela Altmeyer, Torsten Methfessel, Ulrich Tutsch, Harald Schubert, Michael Lang, Jens Müller, Michael Huth, Harald O. Jeschke, Roser Valentí, Martin Jourdan, and Hans-Joachim Elmers
Phys. Rev. Lett. 116, 237001 – Published 7 June 2016
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  • Introduction.—
  • Theory.—
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    Abstract

    We report on a combined theoretical and experimental investigation of the superconducting state in the quasi-two-dimensional organic superconductor κ−(ET)2Cu[N(CN)2]Br. Applying spin-fluctuation theory to a low-energy, material-specific Hamiltonian derived from ab initio density functional theory we calculate the quasiparticle density of states in the superconducting state. We find a distinct three-peak structure that results from a strongly anisotropic mixed-symmetry superconducting gap with eight nodes and twofold rotational symmetry. This theoretical prediction is supported by low-temperature scanning tunneling spectroscopy on in situ cleaved single crystals of κ−(ET)2Cu[N(CN)2]Br with the tunneling direction parallel to the layered structure.

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    • Received 30 October 2015

    DOI:https://doi.org/10.1103/PhysRevLett.116.237001

    © 2016 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Electronic structurePairing mechanisms
    1. Physical Systems
    Organic superconductorsTunnel junctions
    1. Techniques
    Scanning tunneling spectroscopy
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Daniel Guterding1, Sandra Diehl2,3, Michaela Altmeyer1, Torsten Methfessel3, Ulrich Tutsch4, Harald Schubert4, Michael Lang4, Jens Müller4, Michael Huth4, Harald O. Jeschke1, Roser Valentí1, Martin Jourdan3, and Hans-Joachim Elmers3,*

    • 1Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Strasse 1, 60438 Frankfurt am Main, Germany
    • 2Graduate School Materials Science in Mainz, Staudingerweg 9, 55128 Mainz, Germany
    • 3Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudingerweg 7, 55128 Mainz, Germany
    • 4Physikalisches Institut, Goethe-Universität Frankfurt, Max-von-Laue-Strasse 1, 60438 Frankfurt am Main, Germany

    • *elmers@uni-mainz.de

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    Issue

    Vol. 116, Iss. 23 — 10 June 2016

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    Images

    • Figure 1
      Figure 1

      Ab initio calculated results for (a) superconducting gap Δ on the Fermi surface, (b) absolute value |Δ| of the superconducting gap as a function of the angle φ in the kxky plane measured with respect to the kx direction, and (c) quasiparticle DOS ρqp in the superconducting state of κ−Br. The maxima of the superconducting gap magnitude are indicated by arrows A, B, and C, while the positions of nodes are indicated by arrows α and β. The energy scale is set to Δ0=10  meV. A small broadening of Γ=0.07  meV is included here.

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    • Figure 2
      Figure 2

      (a) (30×30)  nm2 STM image of the crystal surface with a tunneling direction parallel to the layered structure of the κ−Br crystal, revealing a stripe pattern (T=5  K, I=60  pA, Utip=30  mV). (b) The height profile along the marked area in (a) reveals an average stripe width of 29.9 Å. The red stripes indicate the position of the insulating layers. (c) Conductance spectra dI/dV/T(V) at T=5  K (the black dots) measured at the ET layer and (d) the Cu[N(CN)2]Br layer with −eUtip=eV=E−EF. The function B(V) is also plotted in (c) (see the text for a discussion).

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    • Figure 3
      Figure 3

      (a) Second derivative of the conductance spectrum dI/dV/T(V) at T=5  K shown in Fig. 2. Arrows labeled by A, B, and C indicate three pairs of minima which are symmetric to the origin, corresponding to the coherence peaks partially seen in (b). (b) Conductance spectra S(V)=1/[B(V)T(V)]dI/dV of the superconducting state as a function of eV=E−EF at different temperatures measured parallel to the layered crystal structure. The red lines show mappings of Eq. (2) to the measured data.

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