title: Anomalous critical fields in quantum critical superconductors.
abstract: Fluctuations around an antiferromagnetic quantum critical point (QCP) are believed to lead to unconventional superconductivity and in some cases to high-temperature superconductivity. However, the exact mechanism by which this occurs remains poorly understood. The iron-pnictide superconductor BaFe2(As(1-x)P(x))2 is perhaps the clearest example to date of a high-temperature quantum critical superconductor, and so it is a particularly suitable system to study how the quantum critical fluctuations affect the superconducting state. Here we show that the proximity of the QCP yields unexpected anomalies in the superconducting critical fields. We find that both the lower and upper critical fields do not follow the behaviour, predicted by conventional theory, resulting from the observed mass enhancement near the QCP. Our results imply that the energy of superconducting vortices is enhanced, possibly due to a microscopic mixing of antiferromagnetism and superconductivity, suggesting that a highly unusual vortex state is realized in quantum critical superconductors.
authors.affiliation: H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK.
authors.forename: C
authors.initials: C
authors.lastname: Putzke
authors.affiliation: H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK.
authors.forename: P
authors.initials: P
authors.lastname: Walmsley
authors.affiliation: National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK.
authors.forename: J D
authors.initials: JD
authors.lastname: Fletcher
authors.affiliation: H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK.
authors.forename: L
authors.initials: L
authors.lastname: Malone
authors.affiliation: Laboratoire National des Champs Magnétiques Intenses (CNRS-INSA-UJF-UPS), 31400 Toulouse, France.
authors.forename: D
authors.initials: D
authors.lastname: Vignolles
authors.affiliation: Laboratoire National des Champs Magnétiques Intenses (CNRS-INSA-UJF-UPS), 31400 Toulouse, France.
authors.forename: C
authors.initials: C
authors.lastname: Proust
authors.affiliation: Laboratoire National des Champs Magnétiques Intenses (CNRS-INSA-UJF-UPS), 31400 Toulouse, France.
authors.forename: S
authors.initials: S
authors.lastname: Badoux
authors.affiliation: National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK.
authors.forename: P
authors.initials: P
authors.lastname: See
authors.affiliation: Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: H E
authors.initials: HE
authors.lastname: Beere
authors.affiliation: Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: D A
authors.initials: DA
authors.lastname: Ritchie
authors.affiliation: Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Kasahara
authors.affiliation: 1] Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan [2] Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8561, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Mizukami
authors.affiliation: 1] Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan [2] Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8561, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Shibauchi
authors.affiliation: Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Matsuda
authors.affiliation: H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK.
authors.forename: A
authors.initials: A
authors.lastname: Carrington