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Thermal transport as a probe of low-energy excitations in the high-Tc cuprates

Low temperature thermal transport measurements have proved to be particularly useful for probing the superconducting state of the high Tc's. We present thermal conductivity data across the phase diagram of the cuprates, focusing particularly on the extreme underdoped regime where superconductivity gives way to long-range magnetic order. We find that in ultra-clean samples of YBCO, a finite residual conductivity exists in non-superconducting samples, suggesting the presence of a nodal excitation spectrum without superconducting order. This is accompanied by a Bosonic mode associated with an ordered phase which appears to onset at a doping level equal to the threshold value for the onset of superconductivity. We compare these results with similar measurements on other cuprates and discuss their implication for our understanding of the cuprate phase diagram.