Effects of 4He substitution on low temperature heat capacities of 2D 3He adsorbed on graphite

We measured heat capacities of the second layer 3He admixed with a small amount of 4He (= 0.9 nm−2) on graphite preplated with a monolayer 4He. The aim of this study is to elucidate possible 4He substitution effects on a recently proposed new quantum phase existing at densities just below that (ρ4/7) for the 4/7 registered phase. This phase is supposed to be the hole (zero-point vacancy) doped Mott localized phase. The substitution gives rise to an excess heat capacity (Cex) with unusual temperature and density dependencies which is most pronounced at a slightly lower density than ρ4/7. The Cex stays relatively large even at ρ4/7, and suddenly disappears at a slightly higher density. One possible scenario, which is consistent with the present data, is that the Cex comes from the isotopic mixing effect existing in a narrow density range around ρ4/7 and that there exists an excess particle phase just above ρ4/7 in addition to the hole doped phase below. The result may suggest finite fluidity or unexpectedly large band widths for the isotopic impurities in the 4/7 phase.