In-plane Microwave Conductivity of the Single-Layer Cuprate Tl₂Ba₂CuO_6+d

We have measured the in-plane microwave conductivity of nearly optimally: doped single crystals (T-c = 78 K) of Tl2Ba2CuO6+delta (Tl-2201) at 14.4, 24.8, and 35.9 GHz using cavity perturbation methods. At low temperatures, the in-plane penetration depth has a strong, linear temperature dependence, indicative of an unconventional pairing state with line nodes. The real part of the conductivity shows a broad, frequency-dependent peak near 30 K, similar to that seen in YBa2Cu3O7-delta and Bi2Sr2CaCu2O8 crystals. With tetragonal crystal symmetry and a single CuO2 plane per unit cell, Tl-2201 is the simplest structure so far to display these features. [S0163-1829(97)50246-3].