  Metal-to-insulator transitions (MITs) are a dramatic manifestation of strong
electron correlations in solids1. The insulating phase can often be suppressed
by quantum tuning, i.e. varying a nonthermal parameter such as chemical
composi- tion or pressure, resulting in a zero-temperature quantum phase
transition (QPT) to a metallic state driven by quantum fluctuations, in
contrast to conventional phase transitions driven by thermal fluctuations.
Theories of exotic phenomena known to occur near the Mott QPT such as quantum
criticality and high-temperature superconductivity often assume a second-order
QPT, but direct experimental evidence for either first- or second-order
behavior at the magnetic QPT associated with the Mott transition has been
scarce and further masked by the superconducting phase in unconventional
superconductors. Most measurements of QPTs have been performed by
volume-integrated probes, such as neutron scattering, magnetization, and
transport, in which discontinuous behavior, phase separation, and spatially
inhomogeneous responses are averaged and smeared out, leading at times to
misidentification as continuous second-order transitions. Here, we demonstrate
through muon spin relaxation/rotation (MuSR) experiments on two archetypal Mott
insulating systems, composition-tuned RENiO3 (RE=rare earth element) and
pressured-tuned V2O3, that the QPT from antiferromagnetic insulator to
paramagnetic metal is first-order: the magnetically ordered volume fraction
decreases to zero at the QPT, resulting in a broad region of intrinsic phase
separation, while the ordered magnetic moment retains its full value across the
phase diagram until it is suddenly destroyed at the QPT. These findings call
for further investigation into the role of inelastic soft modes and the nature
of dynamic spin and charge fluctuations underlying the transition.
