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METALS, SUPERCONDUCTORS... 2009Metal-non-metal transition in the charge transfer salt(BEDT-TTF) 8 [Hg 4 Br 12 (C 6 H 5 Br) 2 ]At room temperature and ambient pressure, all the membersof the family of charge transfer salts (BEDT-TTF) 8 [Hg 4 X 12 (C 6 H 5 Y) 2 ] (where X, Y = Cl, Br and BEDT-TTF stands for bis-ethylenedithio-tetrathiafulvalene) areisostructural. According to band structure calculations,their Fermi surface is composed of one electron and onehole compensated orbit, coupled by magnetic breakdown(MB). However, even though a metallic ground-state is observedfor X = Cl, a metal-non metal transition occurs as thetemperature is lowered for X = Br, as displayed in figure 96.(2003), Audouard et al. Euro. Phys. Lett. 71 783 (2005)]),only few MB orbits are observed for X = Br. This is due toa larger MB gap between electron and hole orbits in the lattercase, in line with band structure calculations. The mostsalient feature is the sizeable decrease of the effective masslinked to the compensated orbits observed as the appliedpressure increases (roughly a factor of two in the exploredrange). In addition, the effective mass scales with the coefficient(A) of the T 2 law. Such a behaviour is reminiscent ofa Brinkman-Rice scenario which predicts the divergence ofthe effective mass as approaching a Mott transition. In thatrespect, no structural phase transition can be inferred fromX-ray data down to 100 K at ambient pressure, for X = Br.Figure 96: Temperature dependence of the interlayer resistivityfor (a) X = Cl and (b) X = Br. Even though the two compounds areisostructural at room temperature, a metal-non metal transition isobserved for X = Br at low pressure. T 2 dependence of the interlayerresistivity at low temperature for (c) the metallic compoundwith X = Cl and (d) the pressure-induced metallic state of X =Br. The Fermi surface for X = Br is displayed in the inset of (b).Electron (red) and hole (blue) orbits are compensated.We have studied the interlayer magnetoresistance of the twocompounds with Y = Br under applied pressure of up to 1.1GPa. For X = Cl, a metallic ground-state is observed in allthe pressure range explored while for X = Br, a pressureinducedmetallic state is observed at a few tenth of GPa.For both compounds, a T 2 variation of the zero-field resistance(ρ = ρ 0 + A×T 2 ), typical of correlated Fermi liquids,is observed in the metallic state. Shubnikov-de Haas (SdH)oscillations are observed for both X = Cl (see figure 97)and X = Br. While many frequency combinations, typicalof coupled orbits networks are observed in the former case(for more details see [Vignolles et al. Eur. Phys. J. B 31 53Figure 97: (a) Field-dependent resistance for X = Cl at 1.1 GPaand (b) corresponding Fourier spectra. Label a stands for the compensatedorbits (see figure 96), the other labels corresponds to eitherfrequency combinations or Fermi surface pieces located in--between the orbits. (c) The coefficient (A) of the T 2 law of thezero-field resistivity scales with the square of the effective mass(m ∗ ) deduced from SdH oscillations.D. Vignolles, A. Audouard, F. Duc, M. NardoneR.B. Lyubovskii, R.N. Lyubovskaya (IPCP, Chernogolovka, Russian Federation), E. Canadell (ICMAB, Barcelona,Spain)70