G. Grissonnanche, O. Cyr-Choiniere, F. Laliberte, S. Rene de Cotret, A. Juneau-Fecteau, S. Dufour-Beausejour, M. -E. Delage, D. LeBoeuf, J. Chang, B. J. Ramshaw, D. A. Bonn, W. N. Hardy, R. Liang, S. Adachi, N. E. Hussey, B. Vignolle, C. Proust, M. Sutherland, S. Kramer, J. -H. Park, D. Graf, N. Doiron-Leyraud, Louis Taillefer
The factors that control the exceptional strength of superconductivity in cuprates are not understood. The upper critical field Hc2 is a fundamental measure of the pairing strength, yet there is no agreement on its magnitude, and only indirect, model-dependent estimates are available. In underdoped YBa2Cu3Oy, a remarkably low critical field Hvs is sufficient to suppress the state of zero resistance, but specific heat data have been interpreted to imply that Hc2 is much greater than Hvs. A wide separation of Hc2 and Hvs is often regarded as a defining property of underdoped cuprates. Here we report the direct determination of Hc2 in ultraclean crystals of YBa2Cu3Oy and YBa2Cu4O8 from high-field thermal conductivity measurements. These reveal that there is no vortex liquid phase at T -> 0, where Hc2 = Hvs. Given this equality, we use high-field resistivity measurements to map out Hc2 in YBa2Cu3Oy as a function of doping p. We find that Hc2(p) consists of two domes, peaked at p1 ~ 0.08 and p2 ~ 0.18, each straddling an underlying critical point at which the Fermi surface of YBa2Cu3Oy undergoes a transformation. These coincide with the onset of spin order and charge order, respectively. Our findings highlight the separate roles of spin and charge orders in the rise and fall of superconductivity in cuprates.
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http://arxiv.org/abs/1303.3856
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