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2009 METALS, SUPERCONDUCTORS...Coexistence of closed orbit and quantum interferometer with the same crosssection in the organic metal β”-(BEDT-TTF) 4 (H 3 O)[Fe(C 2 O 4 ) 3 ]·C 6 H 4 Cl 2The family of quasi-two-dimensional charge transfer saltsβ”-(BEDT-TTF) 4 (A)[M(C 2 O 4 ) 3 ]Solv (where BEDT-TTFstands for bis-ethylenedithio-tetrathiafulvalene, A is amonovalent cation, M is a trivalent cation and Solv is asolvent) have raised great interest in particular because ityielded, more than ten years ago, the first organic superconductorat ambient pressure with magnetic ions.due to the relatively narrow field range in which these latteroscillations can be observed (B > 20 T).The Lifshitzs-Kosevich formalism accounts for the fieldand temperature dependence of both the SdH and dHvAdata over all the explored range. However, a very weak thermaldamping of the Fourier component F b , with the highestamplitude, is evidenced for SdH spectra above about 6 K(see figure 95). As a result, magnetoresistance oscillationsare observed at temperatures higher than 30 K. Taking intoaccount the temperature dependence of the scattering rate,this feature, which is not observed for dHvA oscillations(recorded up to 15 K), is in line with the coexistence, at leastin the temperature range around 6 K, of a closed orbit b anda symmetric (i.e. with a zero effective mass) quantum interferencepath with the same area (keeping in mind that dHvAoscillations are only sensitive to the density of states). Thisresult, which cannot be interpreted in the framework of theFermi surface displayed in figure 94(d), points to a Fermisurface reconstruction in this compound. For details, see[Vignolles et al. Eur. Phys. J. B 71 203 (2009)].Figure 94: (a) Field-dependent interlayer resistance ofβ”-(BEDT-TTF) 4 (H 3 O)[Fe(C 2 O 4 ) 3 ]·C 6 H 4 Cl 2 for θ = 0 ◦ (θ is theangle between the field direction and the normal to the conductingplane). (b) Fourier analysis deduced from the oscillatory part ofthe magnetoresistance displayed in the inset. The field range is18-54 T. Marks are calculated with F a = 74 T and F b = 348 T. (c)Magnetic torque at θ = 29 ◦ . Corresponding Fourier analysis aredisplayed in the inset. The field range is 30-53.5 T and 38-53.5 Tbelow and above 9 K, respectively. (d) Textbook case of Fermisurface accounting for the frequencies a, b and b-a.Magnetoresistance and magnetic torque of the salt with A= H 3 O + , M = Fe 3+ and Solv = C 6 H 4 Cl 2 have been investigatedin pulsed magnetic fields of up to 54 T. Shubnikov-deHaas (SdH) oscillations reveal three basic frequencies F a ,F b and F b−a , which, in line with band structure calculations,can be interpreted on the basis of three compensated closedorbits originating from a hole orbit with an area equal to thatof the first Brillouin zone (see figure 94). Only F a and F bare observed in de Haas-van Alphen (dHvA) spectra, likelyFigure 95: Temperature dependence of the amplitude of the boscillations for dHvA and SdH data. Empty and solid symbolscorrespond to a mean field value of 44.6 T and 30 T/cos(θ), respectively(θ is the angle between the field direction and the normal tothe conducting plane). Solid lines are best fits of the Lifshitzs-Kosevichformula. A zero-effective mass and a temperature-dependentscattering rate are considered for the SdH data in the hightemperature range.D. Vignolles, A. AudouardV.N. Laukhin, E. Canadell (ICMAB, Barcelona, Spain), E.B. Yagubskii (IPCP, Chernogolovka, Russian Federation)69