Davide Galli - Università di Milano #
3He and 4He 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
3He and 4He
form anisotropic fluid states at low
coverage. The commensurate
state analogous to
the standard √×√ R30°
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.