Thermal conductivity as a direct probe of the upper critical field Hc2 in cuprate superconductors

The value of the upper critical field $H_{\mathrm{c2}}$ in cuprate superconductors is an open question, subject to much debate [1]. Owing to its sensitivity to vortex scattering, the thermal conductivity is a powerful technique to directly measure the upper critical field $H_{\mathrm{c2}}$ in a clean type-II superconductor [2]. Here we report measurements of the thermal conductivity in the underdoped cuprate superconductor YBCO in magnetic fields up to 45 T, from which we can directly extract $H_{\mathrm{c2}}$. We find that $H_{\mathrm{c2}}$ is remarkably low at a doping $p \quad =$ 0.11, showing that quantum oscillations [3, 4] are observed above $H_{\mathrm{c2}}$, in a normal state without vortices.\\[4pt] [1] J. Chang \textit{et al}., Nat. Phys. \textbf{8}, 751 (2012).\\[0pt] [2] A. B. Vorontsov and I. Vekhter, Phys. Rev. B \textbf{75}, 224502 (2007).\\[0pt] [3] N. Doiron-Leyraud \textit{et al}., Nature \textbf{447}, 565 (2007).\\[0pt] [4] S. C. Riggs \textit{et al}., Nat. Phys. \textbf{7}, 332 (2011).