Unexpected 18-Fold Overlapped Feathery Fermi Pockets in Typical Thermoelectric Bi0.5Sb1.5Te3

Bi_{0.5}Sb_{1.5}Te_{3} is the most widely used p-type commercial thermoelectric materials over six decades, yet its complex electronic structure remains uncertain especially in band degeneracy and k_{z} dispersions. Here we show an unexpected band structure of 18-fold overlapped feathery Fermi pockets through substantial angle-resolved photoelectron spectroscopy data. Complemented with transport tests, we suggest that the high performance originates in the cooperation of four electronic features-momentum overlap of Fermi pockets, 18-fold band degeneracy, ultrasharp k_{y} dispersions, and heavy k_{z} bands. This cooperation of band features proposes a new paradigm for promising thermoelectrics.