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Disorder-sensitive nodelike small gap in FeSe

Yue Sun, Shunichiro Kittaka, Shota Nakamura, Toshiro Sakakibara, Peng Zhang, Shik Shin, Koki Irie, Takuya Nomoto, Kazushige Machida, Jingting Chen, and Tsuyoshi Tamegai
Phys. Rev. B 98, 064505 – Published 13 August 2018
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  • INTRODUCTION
  • EXPERIMENT
  • RESULTS AND DISCUSSION
  • CONCLUSIONS
  • ACKNOWLEDGMENTS
    • Supplemental Material
    References

    Abstract

    We investigate the band structure, nematic state, and superconducting gap structure of two selected FeSe single crystals containing different amounts of disorder. Transport and angle-resolved photoemission spectroscopy measurements show that the small amount of disorder has little effect on the band structure and the nematic state of FeSe. However, temperature and magnetic field dependencies of specific heat for the two samples are quite different. Wave-vector-dependent gap structure is obtained from the three-dimensional field-angle-resolved specific heat measurements. A small gap with two vertical-line nodes or gap minima along the kz direction is found only in the sample with higher quality. Such symmetry-unprotected nodes or gap minima are found to be smeared out by a small amount of disorder, and the gap becomes isotropic in the sample of lower quality. Our study reveals that the reported controversy on the gap structure of FeSe is due to the disorder-sensitive nodelike small gap.

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    • Received 3 March 2018
    • Revised 7 July 2018

    DOI:https://doi.org/10.1103/PhysRevB.98.064505

    ©2018 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Band gapHall effectSpecific heatSuperconducting gapSuperconducting order parameterSuperconductivity
    1. Physical Systems
    High-temperature superconductorsUnconventional superconductors
    1. Techniques
    Angle-resolved photoemission spectroscopySpecific heat measurements
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Yue Sun1,*, Shunichiro Kittaka1, Shota Nakamura1, Toshiro Sakakibara1, Peng Zhang1, Shik Shin1, Koki Irie2, Takuya Nomoto3, Kazushige Machida2, Jingting Chen4, and Tsuyoshi Tamegai4

    • 1Institute for Solid State Physics (ISSP), The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
    • 2Department of Physics, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
    • 3RIKEN Center for Emergent Matter Science (CEMS), Hirosawa, Wako, Saitama 351-0198, Japan
    • 4Department of Applied Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan

    • *sunyue@issp.u-tokyo.ac.jp

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    Issue

    Vol. 98, Iss. 6 — 1 August 2018

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    Images

    • Figure 1
      Figure 1

      Temperature dependences of the (a) normalized magnetization at 5 Oe, (b) resistivity at zero field below 20 K, (c) specific heat at zero field, (d) resistivity in the temperature range of 0–300 K, (e) first derivative of ρ−T for the two samples A (red) and B (blue). (f) XRD patterns for sample A and B. The inset is the enlarged part of the (004) peaks.

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    • Figure 2
      Figure 2

      ARPES intensity plot of (a) sample A and (b) sample B at 10 K measured with p-polarized photons. The bands with dyz orbital character are revealed because of the matrix element effect [32]. (c) Extracted band structures from (a) and (b). The band structures are extracted by fitting the momentum distribution curves (MDCs) with Lorentzian peaks. Fitting functions with four Lorentzian peaks are used for the MDCs in the range of −18 meV ∼ 0 meV, while fitting functions with two Lorentzian peaks are used for the MDCs in the range of −25 meV ∼ −10 meV. The inset shows the details of the fitting on the MDCs indicated by the blue solid lines in (a) and (b). (d) Temperature dependence of Hall coefficients for samples A and B.

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    • Figure 3
      Figure 3

      (a) Normalized zero-field electronic-specific heat, Ce/γnT vs T, for sample A (red) and B (blue). Inset is the enlarged low temperature part. Magnetic-field-induced changes in the specific heat divided by temperature ΔC/T (ΔC=C(H)−C(0 T)) for H∥c and H∥ab at 0.6 K for sample (b) A and (c) B.

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    • Figure 4
      Figure 4

      (a) Azimuthal angle dependence of the specific heat ΔC(ϕ)/T measured under various fields at 0.33 K for samples A (left panel) and B (right panel). ΔC(ϕ)/T is defined as C(ϕ)/T−C(−45∘)/T, and each subsequent curve is shifted vertically by 0.3mJ/molK2. (b) Polar angle dependence of the specific heat ΔC(θ)/T measured under various fields at 0.33 K for samples A (left panel) and B (right panel). ΔC(θ)/T is defined as C(θ)/T−C(−90∘)/T, and each subsequent curve is shifted vertically by 2mJ/molK2. Black-outlined symbols are the measured data; the others are mirrored points to show the symmetry more clearly. The in-plane (azimuthal) angle ϕ is defined as the angle away from [100] direction, while the out-of-plane (polar) angle θ is defined as the angle away from the [001] direction as shown in the insets of (a) and (b), respectively.

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