Approaching the Dirac Point in High-Mobility Multilayer Epitaxial Graphene

Multilayer epitaxial graphene is investigated using far infrared transmission experiments in the different limits of low magnetic fields and high temperatures. The cyclotron-resonance-like absorption is observed at low temperature in magnetic fields below 50 mT, probing the nearest vicinity of the Dirac point. The carrier mobility is found to exceed $250000{\mathrm{cm}}^2/(\mathrm{V}·\mathrm{s})$. In the limit of high temperatures, the well-defined Landau level quantization is observed up to room temperature at magnetic fields below 1 T, a phenomenon unusual in solid state systems. A negligible increase in the width of the cyclotron resonance lines with increasing temperature indicates that no important scattering mechanism is thermally activated.

Figure 1

Part (a) The FIR transmission $\mathcal{T}$ plotted as $-\ln \mathcal{T}$ as a function of the magnetic field at $T=2.0\mathrm{K}$. The dashed lines denote the expected transitions for $\tilde{c}=1.02\times {10}^6\mathrm{m}·{\mathrm{s}}^{-1}$. The inset shows the transmission spectrum at $B=0.3\mathrm{T}$. Part (b): FIR transmission taken at $T=2\mathrm{K}$ in low magnetic fields. Successive spectra are shifted vertically by 0.05. The part (c) shows the peak position, width and area for the $L_{-1(0)}\to L_{0(1)}$ transition. The dashed line in part (c) is a least squares fit to the peak positions.

Figure 2

Temperature dependence of the transmission spectra taken at $B=0.8$ and 1.0 T in parts (a) and (b), respectively, showing the $L_{-1(0)}\to L_{0(1)}$ transition up to room temperature. The blue curves represent the Lorentzian fits whose parameters are plotted in Fig. 3. Successive spectra in both parts are shifted vertically by 0.15.

Figure 3

Parameters of the Lorentzian fits to the transmission spectra in Fig. 2. Red and blue color represent data for $B=0.8$ and 1.0 T, respectively. Whereas position and width of absorption lines in parts (a) and (b) show none or a very weak temperature dependence, the peak area in the part (c) decreases with $T$, nearly following the theoretical expectation $1–2f_1$ shown by solid lines. The inset schematically shows the LL occupation and dipole-allowed transitions between adjacent LLs at finite temperature in undoped graphene.