Davide Galli - Università di Milano #
<sup>3</sup>He and <sup>4</sup>He on graphene-fluoride and graphane: 
prediction of novel fluid, superfluid and supersolid phases #
We present a study of submonolayer He adsorbed on two derivatives of
graphene: graphene-fluoride (GF) and graphane (GH). A semiempirical 
interaction with the
substrate is used in state of the art quantum simulations. We predict that 
both isotopes 
<sup>3</sup>He and <sup>4</sup>He 
form anisotropic fluid states at low 
coverage. The commensurate 
state analogous to
the standard &radic; &times; &radic; R30 &deg;
phase that preempts fluid states on graphite 
turns out to be
unstable relative to a fluid state. The commensurate insulating ground 
state on GF and GH is
disfavored by the much smaller inter-site distance (below 1.5 Å) compared 
to graphite (2.46
Å), implying a large energy penalty for localizing He atoms. The 4He 
ground state on both
substrates is a self-bound anisotropic superfluid with anisotropic roton 
excitations and with a
superfluid density ρs reduced from 100% due to the corrugation of the 
adsorption potential. In
the case of GF such corrugation is so large that ρs=57% at T=0K and the 
superfluid is
essentially restricted to move in a multiconnected space, along the bonds 
of a honeycomb
lattice. We predict a superfluid transition temperature T≈0.25(1.1)K for 
4He on GF (GH). At
higher coverages we find two kinds of solids, an incommensurate triangular 
one as well as a
novel commensurate state at filling factor 2/7 with 4 atoms in the unit 
cell. We have evidence
that this 2/7 state is supersolid. We conclude that these new platforms 
for adsorption studies
offer the possibility of studying novel phases of quantum condensed matter 
like an anisotropic
Fermi fluid, possibly superfluid, an anisotropic Bose superfluid and a 
commensurate
supersolid.