  We report a de Haas-van Alphen (dHvA) oscillation study on IrTe2 single
crystals showing complex dimer formations. By comparing the angle dependence of
dHvA oscillations with band structure calculations, we show distinct Fermi
surface reconstruction induced by a 1/5-type and a 1/8-type dimerizations. This
verifies that an intriguing quasi-two-dimensional conducting plane across the
layers is induced by dimerization in both cases. A phase transition to the 1/8
phase with higher dimer density reveals that local instabilities associated
with intra- and interdimer couplings are the main driving force for complex
dimer formations in IrTe2.
