Location via proxy:   [ UP ]  
[Report a bug]   [Manage cookies]                

Specific heat of Ca0.32Na0.68Fe2As2 single crystals: Unconventional s± multiband superconductivity with intermediate repulsive interband coupling and sizable attractive intraband couplings

S. Johnston, M. Abdel-Hafiez, L. Harnagea, V. Grinenko, D. Bombor, Y. Krupskaya, C. Hess, S. Wurmehl, A. U. B. Wolter, B. Büchner, H. Rosner, and S.-L. Drechsler
Phys. Rev. B 89, 134507 – Published 14 April 2014
PDFHTMLExport Citation
Abstract
Authors
Article Text
  • INTRODUCTION
  • METHODS
  • RESULTS
  • DISCUSSION
  • SUMMARY AND CONCLUSIONS
  • ACKNOWLEDGEMENTS
  • APPENDICES
    • References

    Abstract

    We report a low-temperature specific heat study of high-quality single crystals of the heavily hole-doped superconductor Ca0.32Na0.68Fe2As2. This compound exhibits bulk superconductivity with a transition temperature Tc≈34 K, which is evident from the magnetization, transport, and specific heat measurements. The zero-field data manifest a significant electronic specific heat in the normal state with a Sommerfeld coefficient γ≈53 mJ/mol K2. Using a multiband Eliashberg analysis, we demonstrate that the dependence of the zero-field specific heat in the superconducting state is well described by a three-band model with an unconventional s± pairing symmetry and gap magnitudes Δi of approximately 2.35, 7.48, and −7.50 meV. Our analysis indicates a non-negligible attractive intraband coupling, which contributes significantly to the relatively high value of Tc. The Fermi surface averaged repulsive and attractive coupling strengths are of comparable size and outside the strong coupling limit frequently adopted for describing high-Tc iron pnictide superconductors. We further infer a total mass renormalization of the order of five, including the effects of correlations and electron-boson interactions.

    • Figure
    • Figure
    • Figure
    • Figure
    • Figure
    • Figure
    • Figure
    1 More
    • Received 13 November 2013
    • Revised 14 February 2014

    DOI:https://doi.org/10.1103/PhysRevB.89.134507

    ©2014 American Physical Society

    Authors & Affiliations

    S. Johnston1,2,3, M. Abdel-Hafiez1, L. Harnagea1, V. Grinenko1, D. Bombor1, Y. Krupskaya1, C. Hess1, S. Wurmehl1, A. U. B. Wolter1, B. Büchner1,4, H. Rosner5,1, and S.-L. Drechsler1

    • 1Leibniz-Institute for Solid State and Materials Research, (IFW)-Dresden, D-01171 Dresden, Germany
    • 2Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
    • 3Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4
    • 4Physical Department, University of Technology Dresden, Germany
    • 5Max-Planck Institute for Chemical Physics of Solids (MPI-CPfS), Dresden, Germany

    Article Text (Subscription Required)

    Click to Expand

    References (Subscription Required)

    Click to Expand
    Issue

    Vol. 89, Iss. 13 — 1 April 2014

    Reuse & Permissions
    Access Options
    Author publication services for translation and copyediting assistance advertisement

    Authorization Required

    Log In
    Other Options
    ×

    Images

    • Figure 1
      Figure 1

      (a) The T dependence of the magnetization of CaFe2As2 and Ca0.32Na0.68Fe2As2 single crystals, measured under an applied magnetic field of 1 T parallel to the crystallographic basal plane in zero-field cooled conditions. (b) T dependence of the in-plane electrical resistivity in zero field up to 300 K. The inset presents a zoom of the resistivity data around Tc for the Ca0.32Na0.68Fe2As2 sample.

      Reuse & Permissions
    • Figure 2
      Figure 2

      The temperature dependence of the specific heat Cp measured in zero-field conditions for CaFe2As2 and Ca0.32Na0.68Fe2As2. The inset shows the low-temperature behavior of Cp/T as a function of T2. The straight lines are the linear fits to Cp/T=γ+βT2 (see text).

      Reuse & Permissions
    • Figure 3
      Figure 3

      The T dependence of the specific heat Cp/T of Ca0.32Na0.68Fe2As2 single crystal measured in magnetic fields applied along the crystallographic c axis. For reference, the specific heat of CaFe2As2 measured in zero-field conditions is also shown. The inset shows Cp/T of Ca0.32Na0.68Fe2As2 near Tc.

      Reuse & Permissions
    • Figure 4
      Figure 4

      The electronic specific heat of Ca0.32Na0.68Fe2As2 after subtracting the phonon contribution as a function of reduced temperature t=T/Tc. In the inset, the normal and superconducting state entropies are shown.

      Reuse & Permissions
    • Figure 5
      Figure 5

      (a) The assumed spectrum of phonons (black/dark) and spin fluctuations (red/light). (b) The temperature dependence of the gap functions Δi(ωn=π/β). The dashed red line in (b) is a rescaled version of the solid red line, to show the non-BCS-like temperature dependence of the superconducting gap for this band (see main text). (c) A comparison between the calculated (thick black line) and measured (open ◯) change in electronic specific heat ΔCel(T) as a function of the reduced temperature T/Tc. The individual band contributions are also shown for T<Tc, following the color scheme of (b).

      Reuse & Permissions
    • Figure 6
      Figure 6

      (a) The electronic density of state (DOS) from LDA-FPLO calculation for Ca0.32Na0.68Fe2As2 with the various elemental contributions from Fe 3d, As 4p, and Ca 4s states. (b) The Fermi surface sheets (FSSs) band-resolved contributions. Bands h1−3 include the three hole-type FSSs centered around the Γ point (see Fig. 7) and e1,2 are the two electron-type FSSs (see Fig. 8), respectively. (c) Orbital resolved partial DOS for the outer hole FSSs h3 (see Fig. 7), lower panel). The 3dxz and 3dyz are degenerate in the present tetragonal symmetry and have the same weight. The notation of orbitals is the same as that used in the ARPES and dHvA literature for Fe pnictides, i.e., there is a 45∘ rotation of the x and y axes with respect of the original tetragonal axis of the the Bravais cell.

      Reuse & Permissions
    • Figure 7
      Figure 7

      The three hole Fermi surface sheets (FSSs) around the Γ point formed by the bands h1 (upper), h2 (middle), and h3 (bottom) according to our LDA calculation. The color denotes the magnitude of the Fermi velocities. Within scenario I, we tentatively assign the upper (h1) and the middle (h2) FSSs to band 1 in our three-band Eliashberg analysis (see subsection D) and the lower FSS (h3) to band 2. Within scenario II, the band h2, only, forms band 1, whereas the two remaining hole bands h1 and h3 form the effective band 2. The color scale denotes the magnitude of the calculated Fermi velocities.

      Reuse & Permissions
    • Figure 8
      Figure 8

      The two electron FSSs resulting from bands e1 and e2 according to both scenarios I and II are tentatively assigned to to the effective band 3 in our Eliashberg analysis of the specific heat (see Sec. 3 D).

      Reuse & Permissions
    ×

    Sign up to receive regular email alerts from Physical Review B

    Log In

    Cancel
    ×

    Search

    Article Lookup

    Paste a citation or DOI
    Enter a citation
    ×