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Evidence of Andreev bound states as a hallmark of the FFLO phase in κ-(BEDT-TTF)2Cu(NCS)2

Nature Physics volume 10, pages 928–932 (2014)Cite this article

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Abstract

Superconductivity is a quantum phenomenon arising, in its simplest form, from the pairing of fermions with opposite spin into a state with zero net momentum. Whether superconductivity can occur in fermionic systems with an unequal number of two species distinguished by spin or flavour presents an important open question in condensed-matter physics or quantum chromodynamics1. In condensed matter the imbalance between spin-up and spin-down electrons that form the Cooper pairs is induced by the magnetic field. Such an imbalanced system can lead to exotic superconductivity in which pairs acquire finite momentum2,3. This momentum leads to a spatially inhomogeneous state consisting of periodically alternating ‘normal’ and ‘superconducting’ regions. Here, we establish that the hallmark of this state is the appearance of spatially localized and spin-polarized quasiparticles forming the so-called Andreev bound states (ABS). These are detected through our nuclear magnetic resonance (NMR) measurements.

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Figure 1: (H,T) phase diagram of κ-(BEDT-TTF)2Cu(NCS)2.
Figure 2: NMR relaxation rate in the normal and superconducting states.
Figure 3: Enhancement of the NMR relaxation rate in the FFLO state.
Figure 4: Field dependence of the electronic spin polarization and NMR relaxation rate at low temperatures.
Figure 5: Schematic of the properties of the modulated superconductivity.

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Acknowledgements

We would like to thank A. Vorontsov, Y. Yanase and M. Sigrist for illuminating discussions, and I. Sheikin for providing raw data for the phase diagram. This research is supported by funds from the French ANR grant 06-BLAN-0111, the EuroMagNET II network under EU Contract No. 228043, the visiting faculty program of Université Joseph Fourier (V.F.M.), and ADVANCE HRD-0548311 (V.F.M.).

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Authors and Affiliations

  1. Laboratoire National des Champs Magnétiques Intenses, LNCMI - CNRS (UPR 3228), UJF, UPS and INSA, BP 166, 38042 Grenoble Cedex 9, France

    H. Mayaffre, S. Krämer, M. Horvatić & C. Berthier

  2. Department of Applied Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan

    K. Miyagawa & K. Kanoda

  3. Department of Physics, Brown University, Providence, Rhode Island 02912, USA

    V. F. Mitrović

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  1. H. Mayaffre
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  2. S. Krämer
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  3. M. Horvatić
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  4. C. Berthier
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  5. K. Miyagawa
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  7. V. F. Mitrović
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Contributions

K.M. and K.K. prepared the samples. H.M., S.K., K.M. and V.F.M. performed the experiments. S.K. and M.H. developed and operated the high-field NMR facility. H.M. created software for the spectrometers. H.M. and V.F.M. analysed the data. C.B. provided conceptual advice and contributed to the planning of the project. H.M., C.B., M.H. and V.F.M. developed the data interpretation. V.F.M. wrote the paper and supervised the project. All authors discussed the results and commented on and edited the manuscript.

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Correspondence to V. F. Mitrović.

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Mayaffre, H., Krämer, S., Horvatić, M. et al. Evidence of Andreev bound states as a hallmark of the FFLO phase in κ-(BEDT-TTF)2Cu(NCS)2. Nature Phys 10, 928–932 (2014). https://doi.org/10.1038/nphys3121

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