Evidence of $d_{x^2-y^2}$ symmetry in the tunneling conductance density of states of ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CuO}}_6$

Tunnel junctions have been made on single crystals of ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CuO}}_6$ with a $T_c$∼91 K using a mechanical point-contact method. Both superconductor–insulator–normal-metal (SIN) and superconductor-insulator-superconductor (SIS) junctions were obtained using Au and Nb tips, respectively. The tunneling conductances obtained are reproducible, with sharp, well-defined energy gap structures and are characterized by a cusplike feature at zero bias. These conductances can be fit to a simple, momentum-averaged density of states (DOS) obtained from a superconducting order parameter with $d_{x^2-y^2}$ symmetry. The SIS data display a weak Nb gap feature which is consistent with the subgap DOS observed in the SIN data.