Existence of electron and hole pockets and partial gap opening in the correlated semimetal Ca 3 Ru 2 O 7

The electronic band structure of correlated Ca3Ru2O7 featuring an antiferromagnetic (AFM) as well as a structural transition has been determined theoretically at high temperatures, which has led to the understanding of the remarkable properties of Ca3Ru2O7, such as the bulk spin-valve effects. However, its band structure and Fermi surface (FS) below the structural transition have not been resolved, even though a FS consisting of electron pockets was found experimentally. Here we report magnetoelectrical transport and thermoelectric measurements with the electric current and temperature gradient directed along the a and b axes, respectively, of an untwined single crystal of Ca3Ru2O7. The thermopowers obtained along the two crystal axes were found to show opposite signs at low temperatures, demonstrating the presence of both electron and hole pockets on the FS. In addition, how the FS evolves across T∗=30K at which a distinct transition from coherent to incoherent behavior occurs was also inferred: the Hall and Nernst coefficient results suggest a temperature- and momentum-dependent partial gap opening in Ca3Ru2O7 below the structural transition with a possible Lifshitz transition occurring at T∗. The experimental demonstration of a correlated semimetal ground state in Ca3Ru2O7 calls for further theoretical studies of this remarkable material.