  Metal-insulator (MI) transitions in correlated electron systems have long
been a central and controversial issue in material science. Vanadium dioxide
(VO2) exhibits a first-order MI transition at 340 K. For more than half a
century, it has been debated whether electronic correlation or the structural
instability due to dimerised V ions is the more essential driving force behind
this MI transition. Here, we show that an ultrahigh magnetic field of 500 T
renders the insulator phase of tungsten (W)-doped VO2 metallic. The spin Zeeman
effect on the d electrons of the V ions dissociates the dimers in the
insulating phase, resulting in the delocalisation of electrons. Because the
Mott-Hubbard gap essentially does not depend on the spin degree of freedom, the
structural instability is likely to be the more essential driving force behind
the MI transition.
