## Abstract

In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λ_{L}. In two-dimensional systems, this ratio n/m* (∼1/λ_{L} ^{2}) determines the effective Fermi temperature, T_{F}. We report a sharp peak in the x-dependence of λ_{L} at zero temperature in clean samples of BaFe_{2}(As_{1-x}P _{x})_{2} at the optimum composition x = 0.30, where the superconducting transition temperature T_{c} reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of T_{c}/T_{F} at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.

Original language | English |
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Pages (from-to) | 1554-1557 |

Number of pages | 4 |

Journal | Science |

Volume | 336 |

Issue number | 6088 |

DOIs | |

Publication status | Published - 2012 Jun 22 |

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