Probing the Unconventional Superconducting State of LiFeAs by Quasiparticle Interference

Torben Hänke, Steffen Sykora, Ronny Schlegel, Danny Baumann, Luminita Harnagea, Sabine Wurmehl, Maria Daghofer, Bernd Büchner, Jeroen van den Brink, and Christian Hess
Phys. Rev. Lett. 108, 127001 – Published 19 March 2012
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Abstract

A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well-suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high Tc18K. Our data reveal that in LiFeAs the quasiparticle scattering is governed by a van Hove singularity at the center of the Brillouin zone which is in stark contrast to other pnictide superconductors where nesting is crucial for both scattering and s± superconductivity. Indeed, within a minimal model and using the most elementary order parameters, calculations of the QPI suggest a dominating role of the holelike bands for the quasiparticle scattering. Our theoretical findings do not support the elementary singlet pairing symmetries s++, s±, and d wave. This brings to mind that the superconducting pairing mechanism in LiFeAs is based on an unusual pairing symmetry such as an elementary p wave (which provides optimal agreement between the experimental data and QPI simulations) or a more complex order parameter (e.g., s+id wave symmetry).

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  • Received 13 August 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.127001

© 2012 American Physical Society

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Vol. 108, Iss. 12 — 23 March 2012

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