Microscopic description of triaxiality in Ru isotopes with covariant energy density functional theory

The triaxiality in nuclear low-lying states has attracted great interests for many years. Recently, the reduced transition probabilities for levels near the ground state in $^{110}$Ru have been measured and provided strong evidences for a triaxial shape of this nucleus. The aim of this work is to provide a microscopic study of low-lying states for the Ru isotopes with $A\sim100$ and to examine in detail the role of triaxiality, and the evolution of quadrupole shapes with the isospin and spin degrees of freedom. The low-lying excitation spectra and transition probabilities of even-even Ru isotopes are described at the beyond mean-field level by solving a five-dimensional collective Hamiltonian with parameters determined by constrained self-consistent mean-field calculations based on the relativistic energy density functional PC-PK1. The calculated energy surfaces, low-energy spectra, intraband and interband transition rates, as well as some characteristic collective observables, such as $E(4_{\rm g.s.}^+)/E(2^+_{\rm g.s.})$, $E(2^+_\gamma)/E(4^+_{\rm g.s.})$, $B(E2; 2^+_{\rm g.s.}\to 0^+_{\rm g.s.})$, and $\gamma$ band staggerings are in a good agreement with the available experimental data. The main features of the experimental low-lying excitation spectra and electric transition rates are well reproduced, and thus strongly support the onset of triaxiality in the low-lying excited states of the Ru isotopes around $^{110}$Ru.