Transport of charge and entropy in the normal state of an optimally doped cuprate at T=0 limit

The Wiedemann-Franz (WF) law strictly relates the conduction of heat and charge in a Fermi liquid at T=0 limit. Its validity or violation in the normal state of high-Tc cuprates has become a subject of intense investigation. However, until present, the normal state of hole-doped cuprates at optimal doping level has not been explored due to the difficulties of measuring sub-Kelvin thermal conductivity at very high magnetic fields. We present the first verification of the WF law in an optimally doped cuprate. By measuring the low-temperature thermal and electric conductivities of Bi2Sr2CuO6+δ at 25 T, we found that within experimental resolution, the ground state of high-Tc cuprates at optimal doping level can not be distinguished from a Fermi liquid