The advent of Dirac materials has made it possible to realize two-dimensional gases of relativistic fermions with unprecedented transport properties in condensed matter. Their photoconductive control with ultrafast light pulses is opening new perspectives for the transmission of current and information. Here we show that the interplay of surface and bulk transient carrier dynamics in a photoexcited topological insulator can control an essential parameter for photoconductivity-the balance between excess electrons and holes in the Dirac cone. This can result in a strongly out of equilibrium gas of hot relativistic fermions, characterized by a surprisingly long lifetime of more than 50 ps, and a simultaneous transient shift of chemical potential by as much as 100 meV. The unique properties of this transient Dirac cone make it possible to tune with ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that is impossible with conventional optoelectronic materials.

Both transport τtr and elastic τe scattering times are experimentally determined from the carrier density dependence of the magnetoconductance of monolayer and bilayer graphene. Both times and their dependences on carrier density are found to be very different in the monolayer and the bilayer. However, their ratio τtr/τe is found to be close to 1.8 in both systems and nearly independent of the carrier density. These measurements give insight on the nature (neutral or charged) and range of the scatterers. Comparison with theoretical predictions suggests that the main scattering mechanism in our samples is due to strong (resonant) scatterers of a range shorter than the Fermi wavelength; likely candidates being vacancies, voids, ad-atoms or short-range ripples.PACS numbers: 63.22. Np, 73.23.Hk, 73.21.Ac Since the discovery of the fascinating electronic properties of graphene [1] due to its electronic spectrum with linear dispersion and a perfect electron-hole symmetry at the Fermi level [2], the nature of defects has been shown to play an essential role in determining the carrier density (n c ) dependence of the conductance. The wavevector and energy dependences of the impurity potential are known to determine the characteristic scattering times of the carriers. It is important to distinguish the transport time τ tr , which governs the current relaxation and enters the Drude conductivity (σ), from the elastic scattering time τ e , which is the lifetime of a plane wave state [3]. Since τ tr and τ e involve different angular integrals of the differential cross section, they differ as soon as the Fourier components of the potential depend on the wavevector q. A large ratio τ tr /τ e indicates that scattering is predominantly in the forward direction, so that transport is not affected much by this type of scattering. This is the case in 2D electron gases (2DEG) confined to GaAs/GaAlAs heterojunctions with the scattering potential produced by remote charged Si donors [4], where τ tr /τ e is found to be larger than 10.The nature of the main scattering mechanism limiting the carrier mobility in graphene is still subject to controversy. It has indeed been shown [5][6][7] that "white noise" (q independent) scattering leads to a weak (logarithmic) dependence of σ(n c ), in contradiction with experiments which typically find a linear increase. In contrast, scattering on charged impurities originates from a q dependent screened Coulomb potential described in the Thomas Fermi approximation [8][9][10]. This leads to a linear σ(n c ) both for a monolayer (ML) and a moderately doped bilayer (BL). Recent experiments performed to probe this question measured the change in σ upon immersion of graphene samples in high-K dielectric media. Their conclusions differ [11]. Alternate explanations involve resonant scattering centers with a large energy mismatch with the Fermi energy of carriers [7,12].In order to gain insight into the scattering mechanism in graphene, we have extracted τ e and τ tr from magnetotransport in monolayer and b...