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2009 CARBON ALLOTROPESHow perfect can graphene be?We have identified the cyclotron resonance (CR) responseof purest graphene ever investigated, which can be found innature on the surface of bulk graphite, in form of decoupledlayers from the substrate material, and which have been recentlydiscovered in scanning tunnelling experiments [G.Li et al., Phys. Rev. Lett. 103, 176804 (2009)]. Probingsuch flakes in the THz range at very low magnetic fields, wedemonstrate a superior electronic quality of these ultra lowdensity layers (n 0 ≈ 3×10 9 cm −2 ) expressed by the carriermobility in excess of 10 7 cm 2 /(V.s) or scattering time ofτ ≈ 20 ps. These values significantly exceed those reportedin any kind of manmade graphene samples.The cyclotron resonance of electrons in these grapheneflakes has been measured in a high-frequency electron paramagneticresonance setup in double-pass transmission configuration,using the magnetic-field-modulation technique.A flake of natural graphite was placed in the variable temperatureinsert of the superconducting solenoid and viaquasi-optics exposed to the linearly polarized microwaveradiation emitted by a Gun diode.A typical example of our experimental finding is illustratedin figure 19a, where the derivative of the magnetoabsorptionspectrum of decoupled graphene on the surfaceof a natural graphite specimen at T = 25 K, measured asa function of the magnetic field at a fixed microwave frequency.The interpretation is schematically illustrated inpart (b). The observed spectral lines are assigned to cyclotronresonance transitions between adjacent Landau levelswith energies: E n = sign(n)˜c √ 2eB|n|, characteristicof massless Dirac fermions in graphene sheets with an effectiveFermi velocity ˜c. This velocity is the only adjustableparameter required to match the energies of theobserved and calculated CR transitions and reaches ˜c =(1.00 ± 0.02) × 10 6 m.s −1 .Since the well-defined Landau level quantization in ourgraphene flakes is spotted down to |B| = 1 mT we obtainvia the semi-classical quantization condition µB > 1 thecarrier mobility µ > 10 7 cm 2 /(V.s), almost two orders ofmagnitude higher in comparison to any other reported values.The scattering time τ ≈ 20 ps derived from the typicalCR width in our spectra also significantly exceeds valuesreported in any kind of graphene samples. This scatteringtime gives an independent estimation for the mobilityµ = eτ ˜c 2 /E F ≈ 3 × 10 7 cm 2 /(V.s) in good agreementwith the estimate above. Interestingly, bearing in mindthis exceptional quality, one may conclude that Landaulevel quantization should survive in studied graphene layersdown to the field of B = (/(E 1 · τ)) 2 ≈ 1 µT. Hence,the magnetic field of the Earth of ∼ 50 µT is no longer negligiblysmall. Instead, the energy gap up to ∆ ≈ 0.3 meVshould appear at the Dirac point, depending on the sampleorientation.Figure 19: (Derivative of) a typical magneto-absorptionspectrum measured at T = 25 K and microwave frequencyω = 1.171 meV (a) in comparison with the Landau level fanchart (b), where the observed CR transitions are shown by arrows.The occupation of individual levels is given by the Fermi-Diracdistribution plotted in the part (c). For simplicity, we consideredonly n-type doping with E F = 6.5 meV. The dashed lines showpositions of resonances assuming ˜c = 1.00 × 10 6 m.s −1 .Moreover, the estimated mobility should not decrease withtemperature, as no broadening of CRs is observed up toT = 50 K, when CR intensities become comparable withthe noise. This extremely high value of mobility combinestwo effects: the long scattering time τ and a very small effectivemass m = E F / ˜c 2 ≈ 2 × 10 −4 m 0 E F [meV]. Remarkably,the same scattering time in a moderate density sample(n 0 = 10 11 cm −2 ), would imply the mobility still remaininghigh, around µ ≈ 5×10 6 cm 2 /(V.s), and comparable to bestmobilities of two-dimensional electron gas in GaAs structuresat these densities.To conclude, our measurement significantly shifts the currentlimits of intrinsic mobility in graphene and poses aquest for further development in the technology of its fabrication.For details see, P. Neugebauer et al., Phys. Rev. Lett. 103,136403 (2009).P. Neugebauer, M. Orlita, C. Faugeras, A.-L. Barra, M. Potemski15