Events in Physics
CMP Seminar - Tony Carrington (Bristol)
Quantum criticality and gap structure in iron-pnictide superconductors
Antony Carrington
HH Wills Physics laboratory, University of Bristol
Iron pnictide/chalcogenide superconductors present a so-far unique new perspective on the field of unconventional superconductivity. The materials have a relatively high superconducting transition temperature and in contrast to the cuprates, multi-band, mult-orbital physics plays a key role but Mott physics less so. One unique property is that the structure of the superconducting gap function has an unprecedented variation between the different compounds. In some cases, there is a clearly identifiable quantum critical point in the temperature – doping phase diagram and this presents an excellent opportunity to study how quantum critical fluctuations affect (or perhaps cause) the superconducting state.
In this talk I will review experiments identifying the superconducting gap structure and quantum critical point (QCP) in the iron-pnictide superconductor series BaFe2(As1-xPx)2. Specific heat and magnetic penetration depth measurements are used to identify the structure of the superconducting energy gap which has been suggested to be a key test of whether the pairing is due to magnetic or orbital order fluctuations. de Haas-van Alphen, magnetic penetration depth and heat capacity results all show clear signatures of a diverging mass at its QCP. The proximity of the QCP yields unexpected anomalies in the superconducting critical fields. We find that both the lower and upper critical fields strongly violate the expectations from conventional theory taking into account the observed mass enhancement near the QCP. This implies that the energy of superconducting vortices is enhanced suggesting that a highly unusual vortex state is realised in quantum critical superconductors.