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Quantum phase transitions and quantum criticality in two-dimensional helium

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Two dimensional helium films provide simple model systems for the investigation of quantum phase transitions in two dimensions. Monolayer 3He absorbed on graphite, with various pre-platings, behaves as a two dimensional Mott-Hubbard system, complete with a density driven “metal-insulator” transition [1, 2]. In two dimensions the corrections to the temperature dependence of the heat capacity, beyond the term linear in T, are anomalous and attributed to quasi-1D scattering [3]. Bi-layer 3He films adsorbed on the surface of graphite show evidence of heavy fermion behaviour and quantum criticality [4, 5]. Here the metal insulator transition appears to arise from tuning both the bandwidth of the lower layer and the Kondo coupling between layers. A theoretical model in terms of a “Kondo breakdown selective Mott transition” has recently been suggested [6]. On the other hand 4He films adsorbed on graphite allow the study of the superfluid-insulator transition [7], related to the superfluid-Mott insulator transition observed in cold atoms [8]. In the present case the transition is between a Kosterlitz-Thouless 2D superfluid and a (possibly hole-doped) Mott insulator. Both “density driven” and “bandwidth driven” transitions are experimentally accessible. Here the superfluid density is being studied using torsional oscillator techniques; the possibility of a 2D supersolid phase will be discussed.

  • In collaboration with: A Casey, M Neumann, J Nyeki, J Parpia, B Cowan.

[1] Evidence for a Mott-Hubbard Transition in a Two-Dimensional 3He Fluid Monolayer, A. Casey, H. Patel, J. Nyéki, B. P. Cowan, and J. Saunders Phys. Rev. Lett. 90, 115301 (2003) [2] D Tsuji et al. J. Low Temp. Phys. 134, 31 (2004) [3] A V Chubukov et al. Phys. Rev. B71 , 205112 (2005) [4] Bilayer 3He; a simple two dimensional heavy fermion system with quantum criticality, Michael Neumann, Jan Nyeki, Brian Cowan, John Saunders. Science 317, 1356 (2007) [5] Heavy fermions in the original Fermi liquid. Christopher A Hooley and Andrew P Mackenzie. Science 317, 1332 (2007) [6] C Pepin, Phys. Rev. Lett. 98, 206401 (2007) and A Benlagra and C Pepin, arXiv: 0709.0354 [7] M P A Fisher et al. Phys. Rev. B40 , 546 (1989) [8] M Greiner et al. Nature 415, 39 (2002)

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