Thermodynamic picture of ultrafast charge transport in graphene

Abstract

The outstanding charge transport properties of graphene enable numerous electronic applications of this remarkable material, many of which are expected to operate at ultrahigh speeds. In the regime of ultrafast, sub-picosecond electric fields, however, the very high conduction properties of graphene are not necessarily preserved, with the physical picture explaining this behaviour remaining unclear. Here we show that in graphene, the charge transport on an ultrafast timescale is determined by a simple thermodynamic balance maintained within the graphene electronic system acting as a thermalized electron gas. The energy of ultrafast electric fields applied to graphene is converted into the thermal energy of its entire charge carrier population, near-instantaneously raising the electronic temperature. The dynamic interplay between heating and cooling of the electron gas ultimately defines the ultrafast conductivity of graphene, which in a highly nonlinear manner depends on the dynamics and the strength of the applied electric fields.

Terahertz Spectroscopy of Nanomaterials: a Close Look at Charge‐Carrier Transport

“…[167] The carrier-carrier scattering in graphene is very efficient due to the linear momentum-energy dispersion such that a thermalization of a nonthermal distribution of the electronic subsystem occurs on sub-100 fs timescale; [169,172] it means, that, from the point of view of the THz probing, the thermal distribution of electrons sets quasi-instantaneously after an ultrafast photoexcitation. The quantum origin of (23) is also illustrated by the explicit presence of the Planck constant.…”

“…In the case of photoexcited semiconductors, the appropriate Drude amplitude must take into account the concentrations of photoexcited electrons and holes given by the Fermi-Dirac statistics with appropriate quasi-Fermi levels. [172,174] The interband recombination of carriers occurs on timescales varying from units to hundreds of ps [167,[175][176][177][178] depending on temperature and on electron and hole concentrations; the principal recombination channels involve intra-and intervalley scattering of carriers on optical phonons. The energy of optical phonon branch is very high in graphene, ≈200 meV at Γ-point and ≈163 meV at K and K′.…”

“…Mics et al [172] used a weakly doped CVD graphene for studying its ultrafast conductivity properties in a nonlinear THz regime. The strong THz electric field excites carriers in graphene and, due to a very efficient carrier-carrier scattering, the electronic gas quasi-instantaneously enters into a thermal regime.…”

Evidence of Fermi level pinning at the Dirac point in epitaxial multilayer graphene

“…where | | is the energy-dependent density of states in graphene, is the electron momentum scattering time, v F is the Fermi velocity and f FD the Fermi Dirac distribution [ 20 ]. µ(T) is the chemical potential that is a decreasing function of the temperature calculated considering the conservation of the total particle number in the system.…”

Strong field transient manipulation of electronic states and bands

“…In recent works, the nonlinear optical properties of graphene have been modeled using a phenomenological thermodynamic model for a hot electron gas 104,105 . In particular, in Ref.…”