SPEAKER: Dr. Fahad Mahmood, Johns Hopkins University
TITLE: “Locating the missing superconducting electrons in overdoped cuprates”
ABSTRACT: High-Tc superconductivity in overdoped cuprates has been widely believed to be within the standard Bardeen-Cooper-Schrieffer (BCS) paradigm describing conventional superconductors. This view was strongly challenged by recent measurements of the superfluid density which found that even as the carrier concentration increases on the overdoped side, the superfluid density decreases – Cooper pairs seem to be ‘missing’ from the condensate. Finding these ‘missing’ carriers and understanding why they fail to condense are crucial questions in explaining the superconducting transition in high-Tc cuprates. In this talk I will discuss our recent measurements of the superfluid stiffness in MBE grown films of the overdoped cuprate La2-xSrxCuO4 over a broad frequency range (kHz to THz). By combining kHz range mutual inductance measurements and time-domain THz spectroscopy on the same films, we track and quantify the distribution of carriers. In this way we directly locate the “missing” carriers and discover that they display an exceedingly large metal-like response even deep into the superconducting state. An analysis of this distribution in terms of a quantum Debye-Waller factor demonstrates the prominent role of quantum superconducting phase fluctuations as the critical doping is approached.
Host: Rick Greene