  Orthogonal metal is a new quantum metallic state that conducts electricity
but acquires no Fermi surface (FS) or quasiparticles, and hence orthogonal to
the established paradigm of Landau's Fermi-liquid (FL). Such a state may hold
the key of understanding the perplexing experimental observations of quantum
metals that are beyond FL, i.e., dubbed non-Fermi-liquid (nFL), ranging from
the Cu- and Fe-based oxides, heavy fermion compounds to the recently discovered
twisted graphene heterostructures. However, to fully understand such an exotic
state of matter, at least theoretically, one would like to construct a lattice
model and to solve it with unbiased quantum many-body machinery. Here we
achieve this goal by designing a 2D lattice model comprised of fermionic and
bosonic matter fields coupled with dynamic $\mathbb Z_2$ gauge fields, and
obtain its exact properties with sign-free quantum Monte Carlo simulations. We
find that as the bosonic matter fields become disordered, with the help of
deconfinement of the $\mathbb Z_2$ gauge fields, the system reacts with
changing its nature from the conventional normal metal with an FS to an
orthogonal metal of nFL without FS and quasiparticles and yet still responds to
magnetic probe like an FL. Such a quantum phase transition from a normal metal
to an orthogonal metal, with its electronic and magnetic spectral properties
revealed, is calling for the establishment of new paradigm of quantum metals
and their transition with conventional ones.
