Skyrmion Effect on the Relaxation of Spin Waves in a Quantum Hall Ferromagnet

Spin relaxation of two-dimensional electrons in a GaAs/AlGaAs quantum well was studied by time-resolved Kerr rotation measurements using a two-color pump and probe technique. In quantum Hall ferromagnets, the spin-wave relaxation is strongly influenced by the photogenerated Skyrmion and anti-Skyrmion pairs. By tuning the pump and probe lights to the lowest optical transition, an intrinsic filling factor dependence of spin relaxation is obtained without photogeneration of Skyrmions. The relaxation time of the spin wave presents a sharp peak at odd filling factors, accompanied by dips on both sides of it. The peculiar filling factor dependence of the spin-wave relaxation around quantum Hall ferromagnets can be explained by the interaction between the spin wave and Skyrmion. Observation of a similar feature around $\nu =1$, 3, and 5 may suggest the existence of Skyrmions around higher odd filling factors.

Figure 1

(a) Filtered and unfiltered spectrum of a Ti:sapphire laser used for the selective excitation. (b),(c) PLE spectra obtained in the Faraday geometry at 6.8 T ($\nu =0.97$) for ${\sigma }^{+}$ and ${\sigma }^{-}$ polarized light, respectively. (d),(e) 2D contour plots of photoinduced Kerr signals at $\nu =1$ (6.6 T) as a function of time delay and photon energy of the pump beam for ${\sigma }^{+}$ and ${\sigma }^{-}$ polarization, respectively. (f) A cartoon of the possible recombination processes of the photoexcited electron-hole pair.

Figure 2

(a) TRKR signals for $\nu =1$ in the 60° tilted-field geometry under the selective excitations of the optical transitions $\mathrm{HH}\Rightarrow {\mathrm{LL}}_0$ and $\mathrm{HH}\Rightarrow {\mathrm{LL}}_1$. (b) Logarithmic plots of the oscillation amplitude of the TRKR signals. Solid lines are exponential fittings for the tail parts.

Figure 3

Filling factor dependence of $T_2^{*}$ around $\nu =1$ in the 60° tilted-field geometry. Open and filled circles show the spin relaxation time obtained with the pumping of the lowest absorption line $\mathrm{HH}\Rightarrow {\mathrm{LL}}_0$ and the higher optical absorption line $\mathrm{HH}\Rightarrow {\mathrm{LL}}_1$, respectively.

Figure 4

Filling factor dependence of $T_2^{*}$ around $\nu =3$ and 5 in the 45° tilted-field geometry. Open circles show the spin relaxation time obtained under the pumping of the lowest absorption lines. Closed circles show the data obtained under the pumping of higher optical absorption line around $\nu =3$ only.