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.

Host:  Yakovenko
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