Condensed Matter Physics Seminar
Note Special Time:
10:30 a.m., Tuesday, August 9, 2005
Room 1201, Physics Building
Quantum Phase Transitions in Two Dimensional 3He
John Saunders
(University of London)
Abstract: Helium films on graphite are atomically layered. This
allows a wide variety of studies of strong correlations in two dimensions with
density as a continuously tunable parameter. Studies of a monolayer of 3He
adsorbed on graphite plated by a bi-layer of HD find a divergence of effective
mass with increasing density, corresponding to a Mott-Hubbard transition between
a 2D Fermi liquid and a quantum spin liquid phase. While the Fermi liquid
survives in 2D, non-Fermi liquid features remain; we show that experimentally
the subleading corrections to FL behaviour of heat capacity scale as T2,
in agreement with recent theories, which find that this correction arises from
the spin component of the backscattering amplitude. Elsewhere it has been argued
that the correlations drive the formation of a “fermion condensate”,
corresponding to a flattening of the single-particle dispersion relation in the
vicinity of the Fermi surface; we discuss experimental evidence for this.
In another experiment a 3He film is grown on graphite plated by a
bi-layer of 4He. The first 3He layer only solidifies in
the presence of an overlayer. However in the regime in which the system
comprises a 3He fluid bilayer, we observe a striking maximum in the
temperature dependence of both heat capacity and magnetisation. This feature is
driven towards T = 0 with increasing film coverage, suggestive of a magnetic
instability with a quantum critical point at 9.2 nm-2. Below the heat
capacity maximum the temperature dependence of the heat capacity is consistent
with a partially gapped Fermi surface. Well below the maximum a linear
temperature dependence of the heat capacity is recovered; the coverage
dependence of the effective mass identifies a (bandwidth driven) Mott-Hubbard
transition at 9.8 nm-2.
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Host: Yakovenko
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