Enhanced Thermopower in Hole-doped Pyrochlore Iridates: Correlated Metal with Quadratic Band Touching

Quadratic band touching (QBT) is a band structure in which the parabolic valence and conduction band touch at a point, protected by spatial and time inversion symmetry. The possibility that QBT becomes the source of exotic electronic phases such as Weyl semimetal has been discussed. The existence of QBT has been confirmed in Pr₂Ir₂O₇ (PIO), which is a member of pyrochlore-type iridate R₂Ir₂O₇ (RIO, R=Rare earth). Since QBT in PIO is stabilized by the crystal symmetry, we can consider the possibility of the universal existence of QBT in the paramagnetic metallic phase of RIO. Therefore, we made hole-doped samples of insulator Eu₂Ir₂O₇ to compare its electronic properties with PIO.
Using the charge transport measurement and ab-initio calculation, we have explored the thermopower and electronic structure of correlated metal with QBT in hole-doped pyrochlore iridates. We found that the hole-doped QBT yields a thermopower peak, which can be enhanced exceeding 40 V/K below 50 K by tuning the band-filling and lattice distortion. The thermopower peak is observed even in the doping induced metallic phase created from Mott insulator, indicating that the QBT is robust against the strong electron correlation as well as the lattice distortion while keeping the cubic symmetry.