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(As1−xPx)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.
Highlights
Fluctuations around an antiferromagnetic quantum critical point (QCP) are believed to lead to unconventional superconductivity and in some cases to high-temperature superconductivity
The normal state of these materials has been widely studied and close to their QCPs non-Fermi liquid behaviour of transport and thermodynamic properties are often found, comparatively little is known about how the quantum critical fluctuations affect the superconducting state[4]
A strong peak in m* at the QCP should result in a corresponding increase in Hc2 as well as the slope of Hc2 at Tc h0 1⁄4 ðdHc2=dTÞTc
Summary
Fluctuations around an antiferromagnetic quantum critical point (QCP) are believed to lead to unconventional superconductivity and in some cases to high-temperature superconductivity. The normal state of these materials has been widely studied and close to their QCPs non-Fermi liquid behaviour of transport and thermodynamic properties are often found, comparatively little is known about how the quantum critical fluctuations affect the superconducting state[4] This is important as it is the difference in energy between the normal and superconducting state that determines the critical temperature Tc. Among the various iron-pnictide superconductors, BaFe2(As1 À xPx)[2] has proved to be the most suitable family for studying the influence of quantum criticality on the superconducting state. Measurements of superconducting state properties that show signatures of quantum critical effects include the magnetic penetration depth l and the heat capacity jump at Tc, DC9,10 Both of these quantities show a strong increase as x tends to 0.30, and it is shown that this could be explained by an underlying approximately sixfold increase in the quasiparticle effective mass m* at the QCP10. This latter quantity is often more accessible experimentally because of the very high Hc2 values in compounds such as iron pnictides for TooTc and because the values of Hc2 close to Tc are not reduced by the effect of the magnetic field on the electron spin (Pauli limiting effects)
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