Intraband carrier dynamics in Landau-quantized multilayer epitaxial graphene

Abstract

We investigate the low-energy carrier dynamics in Landau quantized multilayer epitaxial graphene on (0001) SiC, using 14 meV photons. The THz absorption is dominated by Landau-level transitions within the conduction bands of several graphene layers with different doping. Varying the magnetic field allows us to tune the THz-induced response from induced transmission around B = 0 to induced absorption at intermediate fields (1.5 T-3.3 T) and back to induced transmission at higher fields (3.3 T-7 T). The main features of this complex response are explained by a strong dependence of the absorption on the electron temperature. Furthermore a prolonged relaxation at high fields, which is attributed to reduced scattering via optical phonons, is observed.

Doping-dependent intraband carrier dynamics in Landau-quantized graphene

“…and the spin s = ±1/2. To minimize the effect of an initial thermal excitation of the system, a low temperature of T = 10 K is assumed, corresponding to the value in the previous experimental investigation of the intraband carrier dynamics in Landau-quantized graphene [14]. The appearing Rabi frequency if contains the optical excitation field as well as the optical matrix element, which determines the optical selection rules.…”

“…Since the optical phonon energies are much larger than the low-energy photons [15], the phononinduced relaxation should be also negligible [6]. However, to account for the rather fast experimentally observed decay of differential transmission [13,14], carrier-phonon scattering is included on a phenomenological level.…”

“…In good approximation, it results in a Landau-level broadening of γ = v F e 0 B/A imp that depends on the Fermi velocity, the free-electron charge e 0 , the magnetic field B, and the parameter A imp determining the carrier-impurity interaction strength [22]. We set A imp = 165 to obtain a broadening of 2 meV at B = 1 T, corresponding to the values in a recent pump-probe experiment [14]. According to the Fermi-Dirac distribution, the initial values of the occupations depend on the Fermi energy E F .…”

“…The nonequidistant Landau-level spectrum and the optical selection rules of Landau-quantized graphene allow the excitation of specific inter-LL transitions using circularly polarized light. As a result, one can track the path of excited electrons towards a thermalized Fermi distribution [5][6][7][8][9][10][11][12][13][14][15][16].…”

“…The applied microscopic approach is based on the density matrix formalism, a wellestablished method for modeling the many-particle-induced carrier dynamics in graphene with and without the influence of a magnetic field [9,15,[17][18][19]. In accordance with a recent pump-probe experiment [14], we consider terahertz photons with an energy of 14 meV which is well below the cyclotron energy ω c 36 meV √ B at magnetic field strengths of B = 1 T. In conventional semiconductors with an equidistant Landau quantization, the absorption of low-energy terahertz photons is generally strongly suppressed, unless the cyclotron energy coincides with the photon energy. In contrast, one can always find an approximately resonant transition LL n → LL n+1 matching the photon energy in graphene due to the nonequidistant Landau-level spacing (see Fig.…”