Published: Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

By time- and angle-resolved photoemission spectroscopy we determined the evolution of the out-of-equilibrium electronic structure of the topological insulator Bi₂Se₃.
We found that the energy dependence of the nonequilibrium charge population is solely determined by the analytical form of the effective Fermi-Dirac distribution.

A. Crepaldi et al., Physical Review B 86, 205133 (2012).

Figure Caption: ARPES band dispersion of Bi2Se3 acquired with the 4th harmonic of our laser system, at 6.3 eV. In the figure the topological surface state (SS) and conduction band (CB) are clearly visible. The chemical potential energy is marked with μ and a green line. The graph shows the snapshot for one particular delay time of the the pump-probe tr-ARPES. The signal is obtained as the difference between the ARPES image at +600 fs and an ARPES image at a negative delay. Red (blue) represents an increase (decrease) of the spectral weight.

The response of the Fermi-Dirac distribution to ultrashort IR laser pulses has been studied by modeling the dynamics of hot electrons after optical excitation. We disentangled a large increase in the effective temperature (T*) from a shift of the chemical potential (μ*), which is consequence of the ultrafast photodoping of the conduction band. We demonstrated that the relaxation dynamics of T* and μ* are k independent and these two quantities uniquely define the evolution of the excited charge population.