Time-resolved magneto-Raman study of carrier dynamics in low Landau levels of graphene

We study the relaxation dynamics of the electron system in graphene flakes under a Landau quantization regime using an approach of time-resolved Raman scattering. The nonresonant character of the experiment allows us to analyze the field dependence of the relaxation rate. Our results clearly evidence a sharp increase in the relaxation rate upon the resonance between the energy of the Landau transition and the G band and shed light on the relaxation mechanism of the Landau-quantized electrons in graphene beyond the previously studied Auger scattering.

Figure 1

Scheme presenting the spectrum of Landau levels in graphene placed in an external magnetic field. Color saturation denotes the relative population of LLs (a) directly after the pump pulse tuned to a transition between certain low-lying hole and electron LLs, and (b) after a few-hundred fs following a relatively high-energy (near-infrared) laser pulse, during which the system reaches its quasiequilibrium state due to fast Auger scattering.

Figure 2

Magnetic field dependence of the Raman scattering spectrum measured at $T=200$ K under excitation with a cw Ti:sapphire laser at $\lambda =781$ nm. Each spectrum was corrected by subtracting 80% of the zero-field spectrum in order to spotlight the field-induced changes. The white dashed lines mark two example Landau-level transitions originating from the graphene domain. The signal below 400 ${\mathrm{cm}}^{-1}$ is suppressed due to the long-pass filter placed in the detection path.

Figure 3

A series of Raman scattering spectra measured using a pulsed laser of different intensity at $B=10$ T. Each spectrum was normalized using intensity of the G-band peak as the reference.

Figure 4

(a) Spectrum of the changes induced by the pump beam in the Raman spectrum at $T=200$ K and $B=10$ T. The color reflects a difference between the measured Raman signal and the base signal at the same energy determined for the negative delay. (b) The reference Raman spectrum on top of the map marks the position of various peaks in the spectrum. (c) Transient of the integrated intensity of the $L_{-1,1}$ peak.

Figure 5

(a) Electron relaxation times as a function of the magnetic field. A set of square-root functions are marked by dashed lines as a guide to the eye. Two arrows indicate the resonant fields discussed in the text. (b) Pump-induced change in the signal for two magnetic fields demonstrating the difference in relaxation time. The straight lines represent the exponential-decay profiles fitted to the experimental data.