Missing conductivity peak in a surface acoustic wave measurement at $\nu =\frac{2}{3}$

Surface acoustic waves (SAWs) were used to study the spin transition of the fractional quantum Hall effect at a filling factor of $2∕3$. Although very distinct and hysteretic anomalies were observed at the transition in the transport measurements, no indication was visible in the SAW data. This holds true not only for the small current regime but also for the large current regime where dynamic nuclear-spin polarization enhances the effect. We suggest that this result can be accounted for by highly conducting domain walls with thicknesses of at most 1% of the typical domain size.

Figure 1

(Color online) (a) Longitudinal resistance measured using standard quasi-dc technique as a function of the magnetic field for both sweep directions. (b) Shift in the velocity of the SAW detected during the same magnetic field sweeps of (a). The vertical dotted line indicates the magnetic field corresponding to a filling factor of $2∕3$. The sample is of type A.

Figure 2

(Color online) Conductivity in the magnetic-field range around a filling factor of $2∕3$ for both sweep directions. (a) shows the conductivity calculated from the resistivities measured with a quasi-dc technique. (b) shows the conductivity determined from the velocity shift of surface acoustic waves of different wavelengths. The curves are labeled with the wavelength of the respective acoustic waves and are shifted vertically by $2.5{\sigma }_m$ for clarity. The vertical dashed line indicates the position of $\nu =2∕3$, which is extrapolated from the higher filling factors.

Figure 3

(Color online) Conductivity measured with a $200\mathrm{nA}$ currrent in a magnetic-field range around a filling factor of $2∕3$ for both sweep directions. The black (red/gray) curves are measured for an upward (downward) field sweep. (a) shows the conductivity calculated from the quasi-dc resistivity. (b) shows the conductivity determined from the velocity shift of a surface acoustic wave with a wavelength of $12.6\mu \mathrm{m}$. The vertical dashed line marks the position of $\nu =2∕3$. The measurements of (a) and (b) were done simultaneously.

Figure 4

(Color online) (a) Longitudinal resistance of a sample of type B measured using quasi-dc technique as a function of the magnetic field in the HLR regime using two different sweep speeds. (b) Shift in the SAW velocity during the field sweeps of (a).