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 (ρ_4/7) 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 (C_ex) with unusual temperature and density dependencies which is most pronounced at a slightly lower density than ρ_4/7. The C_ex stays relatively large even at ρ_4/7, and suddenly disappears at a slightly higher density. One possible scenario, which is consistent with the present data, is that the C_ex comes from the isotopic mixing effect existing in a narrow density range around ρ_4/7 and that there exists an excess particle phase just above ρ_4/7 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.