Quasiparticle Transformation during a Metal-Insulator Transition in Graphene

Here we show, with simultaneous transport and photoemission measurements, that the graphene-terminated SiC(0001) surface undergoes a metal-insulator transition upon dosing with small amounts of atomic hydrogen. We find the room temperature resistance increases by about 4 orders of magnitude, a transition accompanied by anomalies in the momentum-resolved spectral function including a non-Fermi-liquid behavior and a breakdown of the quasiparticle picture. These effects are discussed in terms of a possible transition to a strongly (Anderson) localized ground state.

Figure 1

Fermi surfaces (upper) and associated bandstructure cuts (lower) through the graphene K point for (a) clean, and (b)–(d) as a function of $n_{\mathrm{H}}$ indicated in H atoms per ${\mathrm{cm}}^2$. The dashed lines schematically show the Fermi surface contours. The solid lines in the lower row are the peak position determined by fits to the momentum distribution curves. The data were acquired with a slow flux of H atoms to compensate for photodesorption of H atoms.

Figure 2

(a)–(d) Variation of (a) DOS, fitted to lines, (b) momentum linewidth, (c) Energy distribution curves (EDCs) at $k_F$ and (d) EDCs at the K symmetry point. The series were acquired over 5 minutes under constant dosing conditions for the hydrogen coverages ($\times {10}^{12}{\mathrm{cm}}^{-2}$) indicated in (d) with each trace averaged over 4 spectra at slightly different coverages. The horizontal lines indicate the baselines for each trace which are shifted for clarity. (e) Angle-integrated spectra for clean and dosed graphene, together with the difference spectrum, acquired $\sim 0.3{\AA }^{-1}$ away from the K point.

Figure 3

(a) The momentum distribution at $E_F$ as a function of coverage $n_{\mathrm{H}}$. (b) The momentum width $\Delta k$ as a function coverage $n_{\mathrm{H}}$ and $n_{\mathrm{K}}$ for atomic H or K, respectively, compared to the resistance $R$ determined by photocurrent-induced voltage change as a function of $n_{\mathrm{H}}$. The data were taken simultaneously with the data in Fig. 2. The error bars for $R$ are the same or smaller than the symbols. (c) The relevant length scales as a function of atomic H coverage $n_{\mathrm{H}}$: the defect length scale $L_{\mathrm{H}}$, the inverse momentum width $1/\Delta k$, and the inverse Fermi wave vector $1/k_F$. The crossing of the latter two curves defines the Ioffe, Regel, and Mott (IR&M) criterion for the localization transition; the observed MIT occurs at the coverage indicated by the gold line. The red and blue lines are guides to the eye.

Figure 4

(a) The $I\mathrm{\text{−}}V$ characteristics at a single spot on the sample. The slope of the $I\mathrm{\text{−}}V$ curve defines the sample resistance $R$. (b) The resistance plotted logarithmically against $(1/T{)}^{1/3}$, which should be a straight line in the VRH model.