This work is concerned with the excited state quantum phase transitions
(ESQPTs) defined in Ann.Phys. 323, 1106 (2008). In many-body models that
exhibit such transitions, the ground state quantum phase transition (QPT)
occurs in parallel with a singularity in the energy spectrum that propagates to
higher energies as the control parameter increases beyond the QPT critical
point. The analysis of the spectrum has been a main tool for the detection of
these ESQPTs. Studies of the effects of this transition on the system dynamics
are more limited. Here, we extend our previous works and show that the
evolution of an initial state with energy close to the ESQPT critical point may
be extremely slow. This result is surprising, because it may take place in
systems with long-range interactions, where the dynamics is usually expected to
be very fast. A timely example is the one-dimensional spin-1/2 model with
infinite-range Ising interaction studied in experiments with ion traps. Its
Hamiltonian has a U(2) algebraic structure. More generally, the slow dynamics
described here occurs in two-level bosonic or fermionic models with pairing
interactions and a U(v+1) Hamiltonian exhibiting a QPT between its limiting
U(v) and SO(v+1) dynamical symmetries. In this work, we compare the results for
v=1, 2, and 3.Comment: 6 pages, 4 figures, accepted at the FQMT15 special volume of the
Fortschritte der Physi