Spin Texture and Magnetoroton Excitations at $\nu =1/3$

Neutral spin texture (ST) excitations at $\nu =1/3$ are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin flips. At low magnetic fields, the ST energy is below that of the magnetoroton minimum. With increasing in-plane magnetic field these mode energies cross at a critical ratio of the Zeeman and Coulomb energies of ${\eta }_c=0.020\pm 0.001$. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magnetoroton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at $\nu =1/3$.

Figure 1

(a) Dispersion of the lowest magnetoroton branch for excitation in the charge degree of freedom at $\nu =1/3$. The ST is lower in energy than the MR for $\eta <{\eta }_c$ and larger at $\eta >{\eta }_c$. (b) Experimental setup. (c) Energies of the ST and MR as the total field $B$ increases at constant $B_{\perp }$. Circles (squares) denote peaks resonant with $S_1$ ($T_B$). The diamond is the activated transport gap for $B=B_{\perp }=7.8\mathrm{T}$ at $\nu =1/3$. Dashed lines are fits.

Figure 2

(a),(b) Excitation spectra for different laser energies ${\omega }_L$ at 70 mK, $\nu =1/3$, and total field $B=8\mathrm{T}$. The peak corresponding to ${\Delta }_0$ is observed at $\omega =0.93\mathrm{meV}$, outside the scope of the plot (see a recent paper on the same sample [6]). (c) Spectra such as those in (a),(b) as a function of $\omega$ and ${\omega }_S$. Dark red means high intensity. (d) Resonant enhancement profiles of the ST and MR modes (dots) superimposed to the photoluminescence spectrum (black line). Dashed lines are guides to the eye.

Figure 3

Spectra at 40 mK and $\nu =1/3$ as in Fig. 2c, but at the higher total field of $B=10.1\mathrm{T}$ ($B_{\perp }$ is unchanged). The white line is the photoluminescence spectrum (PL).

Figure 4

Temperature dependence of the intensity of the MR, ${\Delta }_0$, and ST peaks at $\nu =1/3$ at different values of $B$. Each mode was normalized independently for clarity. The dashed lines are fits with $T_c=0.85\mathrm{K}$, $\alpha =2.84$, $n_0(0)=0.78$. (Insets) Spectra at two temperatures.