• Open Access

Balancing Act: Evidence for a Strong Subdominant d-Wave Pairing Channel in Ba0.6K0.4Fe2As2

T. Böhm, A. F. Kemper, B. Moritz, F. Kretzschmar, B. Muschler, H.-M. Eiter, R. Hackl, T. P. Devereaux, D. J. Scalapino, and Hai-Hu Wen
Phys. Rev. X 4, 041046 – Published 18 December 2014

Abstract

We present detailed measurements of the temperature-dependent Raman spectra of optimally doped Ba0.6K0.4Fe2As2 and analyze the low-temperature spectra based on local-density-approximation band-structure calculations and the subsequent estimation of effective Raman vertices. Experimentally, a narrow, emergent mode appears in the B1g (dx2y2) Raman spectra only below Tc, well into the superconducting state and at an energy below twice the energy gap on the electron Fermi-surface sheets. The Raman spectra can be reproduced quantitatively with estimates for the magnitude and momentum-space structure of an A1g (s-wave) pairing gap on different Fermi-surface sheets, as well as the identification of the emergent sharp feature as a Bardasis-Schrieffer exciton. Formed as a Cooper-pair bound state in a subdominant dx2y2 channel, the binding energy of the exciton relative to the gap edge shows that the coupling strength in the subdominant channel is as strong as 60% of that in the dominant s-wave channel. This result suggests that dx2y2 may be the dominant pairing symmetry in Fe-based superconductors that lack central hole bands.

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  • Received 18 August 2014

DOI:https://doi.org/10.1103/PhysRevX.4.041046

This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Authors & Affiliations

T. Böhm1,2,3, A. F. Kemper4, B. Moritz3, F. Kretzschmar1,2, B. Muschler1,2,†, H.-M. Eiter1,2, R. Hackl1,*, T. P. Devereaux3,5, D. J. Scalapino6, and Hai-Hu Wen7

  • 1Walther Meissner Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany
  • 2Physik-Department E23, Technische Universität München, 85748 Garching, Germany
  • 3Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
  • 4Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA
  • 5Geballe Laboratory for Advanced Materials and Department of Applied Physics, Stanford University, California 94305, USA
  • 6Physics Department, University of California, Santa Barbara, California 93106-9530, USA
  • 7National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China

  • *Corresponding author. Hackl@wmi.badw.de
  • Present address: Zoller & Fröhlich GmbH, Simoniusstrasse 22, 88239 Wangen im Allgäu, Germany.

Popular Summary

Superconductivity arises from a mutual attraction, or pairing, between electrons. In conventional superconductors such as mercury and lead, pairing is mediated by lattice vibrations; however, in unconventional, high-temperature superconductors like those based on copper oxides, iron arsenides, or chalcogenides, the attraction is believed to result from magnetic or electronic interactions, or perhaps even the cooperation of several mechanisms. These intertwined mechanisms in unconventional materials lead to rich and varied phase diagrams with superconductivity accompanied by magnetism, charge order, and nematic or stripe phases that can be tuned via the application of fields or pressure, changing the crystal structure or number of electrons, which may even tip the balance between dominant and subdominant pairing channels with different symmetry. We demonstrate how Raman light scattering can provide an effective tool for identifying and quantifying competing pairing channels through the observation and analysis of various modes.

We obtain temperature-dependent Raman spectra of a single crystal of Ba0.6K0.4Fe2As2 over 8–46 K and find a sharp, nearly resolution limited mode at 140cm1 in the midinfrared regime, which disappears upon approaching the superconducting transition at 38.5 K. The mode is located at an energy less than twice the largest energy gap of the Fermi-surface sheets. Based on a band structure derived from density functional theory, we calculate the Raman spectra of the sample and conclude that the mode arises from an excitonlike state, similar to those found in semiconductors. Here, the state originates from broken superconducting pairs that reform in a subdominant channel. Our theoretical treatment provides a quantitative estimate for the relative strength between the two channels.

We expect that our results will pave the way for new studies of materials engineering in unconventional superconductors.

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Vol. 4, Iss. 4 — October - December 2014

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It is not necessary to obtain permission to reuse this article or its components as it is available under the terms of the Creative Commons Attribution 3.0 License. This license permits unrestricted use, distribution, and reproduction in any medium, provided attribution to the author(s) and the published article's title, journal citation, and DOI are maintained. Please note that some figures may have been included with permission from other third parties. It is your responsibility to obtain the proper permission from the rights holder directly for these figures.

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