title: Probing the unconventional superconducting state of LiFeAs by quasiparticle interference.
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 T(c)∼18  K. 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).
authors.affiliation: IFW Dresden, 01171 Dresden, Germany.
authors.forename: Torben
authors.initials: T
authors.lastname: Hänke
authors.forename: Steffen
authors.initials: S
authors.lastname: Sykora
authors.forename: Ronny
authors.initials: R
authors.lastname: Schlegel
authors.forename: Danny
authors.initials: D
authors.lastname: Baumann
authors.forename: Luminita
authors.initials: L
authors.lastname: Harnagea
authors.forename: Sabine
authors.initials: S
authors.lastname: Wurmehl
authors.forename: Maria
authors.initials: M
authors.lastname: Daghofer
authors.forename: Bernd
authors.initials: B
authors.lastname: Büchner
authors.forename: Jeroen
authors.initials: J
authors.lastname: van den Brink
authors.forename: Christian
authors.initials: C
authors.lastname: Hess