Electronic structure of the quadrupolar ordered heavy-fermion compound YbRu2Ge2 measured by angle-resolved photoemission

We studied the electronic structure of the heavy-fermion compound Yb(Ru1-xRhx)2Ge2 with x=0 and nominally x=0.125 using ARPES and LDA calculations. We find a valence band structure of Yb corresponding to a noninteger valence close to 3+. The three observed crystal electric field levels with a splitting of 32 and 75 meV confirm the suggested configuration with a quasiquartet ground state. The experimentally determined band structure of the conduction electrons with predominantly Ru 4d character is well reproduced by our calculations. YbRu2Ge2 undergoes a nonmagnetic phase transition into a ferroquadrupolar ordered state below 10.2 K and then to an antiferromagnetically ordered state below 6.5 K. A small hole Fermi surface shows nesting features in our calculated band structure and its size determined by ARPES is close to the magnetic ordering wave vector found in neutron scattering. The transitions are suppressed when YbRu2Ge2 is doped with 12.5% Rh. The electron doping leads to a shift of the band structure and successive Lifshitz transitions.