Materials Science, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA *e-mail: jsnyder@caltech.edu Fundamental to the eld of thermoelectric materials is the need to optimize a variety of conicting properties. To maximize the e eective mass of the charge carrier provides another conict as large eective masses produce high thermopower but low electrical conductivity. e m* in equation (1) refers to the density-of-states (1) 22 105 EFFECTIVE MASS 2