TUNNELING SPECTROSCOPY OF TRILAYER HIGH-TC CUPRATE, <font>TlBa</font>2<font>Ca</font>2<font>Cu</font>2<font>O</font>10-δ

We present point contact tunneling spectroscopy measured on TlBa 2 Ca 2 Cu 3 O 10-δ ( Tl 1223) with three CuO 2 planes in a unit cell. Our samples of Tl 1223 with Tc~91 K are heavily overdoped at the outer CuO 2 planes (OP), but slightly underdoped at the inner CuO 2 planes (IP). The tunneling conductances on Tl 1223 exhibit two kinds of gaps that originate from crystallographically inequivalent IP and OP. The overall spectral shape of tunneling conductance for Tl 1223 is consistent with that observed on a bilayer Bi 2 Sr 2 CaCu 2 O 8+δ, that is, unusual peak-dip-hump (PDH) structures are observed. The origin of PDH structures is controversial, but these results, especially, the observation of PDH for trilayer Tl 1223 in which interlayer effects should be quite different from that in bilayer, put severe constraints on any theoretical model for the origin of these structures.