What's Happening at Half-Filling in the FQHE?

The recent discovery of the first even-denominator quantum number at v=5/2 in the fractional quantum Hall effect (FQHE) has created considerable uncertainty as to the consequences of electron-electron correlation at half-filling. With the v=5/2 state being a v=1/2 state in the first excited Landau level, it is of importance to examine the ground Landau level around v=1/2 and v=3/2. We performed low-temperature magneto-transport experiments on extremely high-mobility 2D electron systems.