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

## TL;DR

This study investigates the electronic structure of the heavy fermion compound YbRu2Ge2 using ARPES and theoretical calculations, revealing details about its valence state, crystal electric field levels, and Fermi surface features related to its phase transitions.

## Contribution

The paper combines ARPES measurements with LDA calculations to elucidate the electronic structure and phase transition mechanisms in YbRu2Ge2, including effects of Rh doping.

## Key findings

- Yb has a valence close to 3+ with a quasi-quartet ground state.
- The conduction band structure matches theoretical predictions.
- Doping with Rh suppresses phase transitions and causes Lifshitz transitions.

## Abstract

We studied the electronic structure of the heavy fermion compound Yb(Ru$_{1-x}$Rh$_{x}$)$_2$Ge$_2$ with $x=0$ and nominally $x=0.125$ using ARPES and LDA calculations. We find a valence band structure of Yb corresponding to a non-integer 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 quasi-quartet ground state. The experimentally determined band structure of the conduction electrons with predominantly Ru $4d$ character is well reproduced by our calculations. YbRu$_2$Ge$_2$ undergoes a non-magnetic 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 YbRu$_2$Ge$_2$ is doped with 12.5% Rh. The electron doping leads to a shift of the band structure and successive Lifshitz transitions.