  Microscopic signatures of nuclear ground-state shape phase transitions in Nd
isotopes are studied using excitation spectra and collective wave functions
obtained by diagonalization of a five-dimensional Hamiltonian for quadrupole
vibrational and rotational degrees of freedom, with parameters determined by
constrained self-consistent relativistic mean-field calculations for triaxial
shapes. As a function of the physical control parameter -- the number of
nucleons, energy gaps between the ground state and the excited vibrational
states with zero angular momentum, isomer shifts, and monopole transition
strengths, exhibit sharp discontinuities at neutron number N=90, characteristic
of a first-order quantum phase transition.
