title: Dirac surface states and nature of superconductivity in Noncentrosymmetric BiPd.
abstract: In non-magnetic bulk materials, inversion symmetry protects the spin degeneracy. If the bulk crystal structure lacks a centre of inversion, however, spin-orbit interactions lift the spin degeneracy, leading to a Rashba metal whose Fermi surfaces exhibit an intricate spin texture. In superconducting Rashba metals a pairing wavefunction constructed from these complex spin structures will generally contain both singlet and triplet character. Here we examine the possible triplet components of the order parameter in noncentrosymmetric BiPd, combining for the first time in a noncentrosymmetric superconductor macroscopic characterization, atomic-scale ultra-low-temperature scanning tunnelling spectroscopy, and relativistic first-principles calculations. While the superconducting state of BiPd appears topologically trivial, consistent with Bardeen-Cooper-Schrieffer theory with an order parameter governed by a single isotropic s-wave gap, we show that the material exhibits Dirac-cone surface states with a helical spin polarization.
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany.
authors.forename: Zhixiang
authors.initials: Z
authors.lastname: Sun
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany.
authors.forename: Mostafa
authors.initials: M
authors.lastname: Enayat
authors.affiliation: 1] Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany [2] School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Ana
authors.initials: A
authors.lastname: Maldonado
authors.affiliation: 1] School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK [2] School of Physics and Astronomy, University of Edinburgh, Mayfield Road, Edinburgh, EH9 3JZ UK.
authors.forename: Calum
authors.initials: C
authors.lastname: Lithgow
authors.affiliation: School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Ed
authors.initials: E
authors.lastname: Yelland
authors.affiliation: 1] Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany [2] Center for Correlated Electron Systems, Institute for Basic Science, Seoul National University, Seoul 151-747 Korea.
authors.forename: Darren C
authors.initials: DC
authors.lastname: Peets
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Yaresko
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany.
authors.forename: Andreas P
authors.initials: AP
authors.lastname: Schnyder
authors.affiliation: 1] Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany [2] SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Peter
authors.initials: P
authors.lastname: Wahl
references.citation: Rev Sci Instrum. 2013 Jan;84(1):013708
references.pmid: 23387660
references.citation: Phys Rev Lett. 2011 Aug 26;107(9):097001
references.pmid: 21929261
references.citation: Phys Rev Lett. 1990 May 28;64(22):2711-2714
references.pmid: 10041790
references.citation: Phys Rev Lett. 2004 Jan 16;92(2):027003
references.pmid: 14753961
references.citation: Phys Rev Lett. 2008 Sep 19;101(12):127003
references.pmid: 18851405
references.citation: Phys Rev Lett. 2010 Nov 19;105(21):217001
references.pmid: 21231342
references.citation: Phys Rev Lett. 2010 Aug 27;105(9):097002
references.pmid: 20868185
references.citation: Phys Rev Lett. 1989 Jan 9;62(2):214-216
references.pmid: 10039952
references.citation: Science. 2009 Feb 13;323(5916):923-6
references.pmid: 19164709