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**Presentation**

Due to their rather simple Fermi surface, organic metals provide a powerful playground for the investigation of quantum oscillation physics. Indeed, in most cases, their FS can be regarded as a network of orbits coupled by magnetic breakdown giving rise to quantum oscillations spectra with numerous frequency combinations that cannot be accounted for by the semi-classical model of Falicov and Stachowiak. This phenomenon which is attributed to either the formation of Landau bands or (and) the oscillation of the chemical potential in magnetic field needs to be better understood.

**Members of the laboratory implied in this activity**:

A. Audouard, M. Nardone, D. Vignolles

**Collaborations:**

V.N Laukhin, E. Canadell (ICMAB Barcelona)

R.B. Lyubovskii, R.N. Lyubovskaya, E.B. Yagubskii (IPCP, Chernogolovka)

J.-Y. Fortin (Institut Jean Lamour, Nancy)

**Frequency combinations in linear chains of orbits with high scattering rate**

As displayed on the opposite figure, the Fermi surface of (BEDO)Ni(CN) 3CH(OH) corresponds to a linear chain of quasi-two-dimensional orbits () coupled to quasi-1D sheets by magnetic breakdown. Remarkably, the scattering rate is consistently deduced from the Shubnikov-de Haas oscillations data relevant to both the basic orbits, the second harmonic 2 and the magnetic breakdown-induced orbits. Its large value points to a significant reduction of the chemical potential oscillations. Despite of this feature, the oscillations spectrum exhibits many frequency combinations. Their temperature dependence (i.e. the effective mass) and (or) field dependence (i.e. the scattering rate) are not in agreement with either the predictions of the quantum interference model (β-α, β-2α and 2(β-α) orbits) or the semi-classical model (β+α obits).

**Frequency combinations in networks of compensated orbits**

We have evidenced that, chemical potential oscillations are strongly reduced for 2D metals with compensated orbits. According to band structure calculations, such networks are achieved in the family (ET)[HgX(CHY)] where (X, Y) = (Cl, Br). As displayed in the opposite figure, the two orbits labelled a, with different shapes, are compensated while the and pieces are forbidden orbits. However, even though the compounds with X = Cl have a metallic ground state, a metal-non metal transition is observed as the temperature decreases for X = Br. For X = Cl and Y = Br (metallic state), all the frequency combinations observed in early de Haas-van Alphen (dHvA) oscillations spectra up to 28 T data are consistent with the semi-classical picture. Oppositely, recent dHvA spectra obtained from magnetic torque measurements up to 55 T exhibit many frequency combinations, most of them being forbidden within the semi-classical framework (see the opposite figure). Otherwise, the effective mass linked to the basic orbits (a) strongly decrease under applied pressure. The obtained results suggest for this family a Brinkman-Rice type scenario which predicts a divergence of the effective mass as approaching a Mott transition.

**Selected publications**

D. Vignolles, A. Audouard, V.N. Laukhin, J. Béard, E. Canadell, N.G. Spitsina and E.B. Yagubskii, Eur. Phys. J. B. **55** 383 (2007).

J.-Y. Fortin and A. Audouard, Phys. Rev. B **77** 134440 (2008).

D. Vignolles, A. Audouard, R.B. Lyubovskii, M. Nardone1, E. Canadell, E.I. Zhilyaeva and R.N. Lyubovskaya ,Eur. Phys. J. B. **66** 489 (2008).

D. Vignolles, A. Audouard, V.N. Laukhin, E. Canadell, T.G. Prokhorova and E.B. Yagubskii, Eur. Phys. J. B. (2009).