  We have developed a novel ab initio Gamow in-medium similarity
renormalization group (Gamow IMSRG) in the complex-energy Berggren framework.
The advanced Gamow IMSRG is capable of describing the resonance and nonresonant
continuum properties of weakly bound and unbound nuclear many-body systems. As
test grounds, carbon and oxygen isotopes have been calculated with chiral two-
and three-nucleon forces from the effective field theory. Resonant states
observed in the neutron-dripline 24O are well reproduced. The halo structure of
the known heaviest Borromean nucleus 22C is clearly seen by calculating the
density distribution in which the continuum s channel plays a crucial role.
Furthermore, we predict low-lying resonant excited states in 22C. The Gamow
IMSRG provides tractable ab initio calculations of weakly bound and unbound
open quantum systems.
