Ultra-Low Temperature Behaviour of the upsilon =5/2 Fractional Quantum Hall Effect.

The newly-discovered even-denominator fractional quantum Hall effect (FQHE) at filling factor nu =5/2 has been studied at ultra-low temperatures. While rho sub xx is not found to vanish in the temperature range studied, the minimum in rho sub xx is seen to drop at the lowest temperatures. While this drop alone is insufficient to determine the energy gap, it may be combined with the temperature dependence of the background resistivity to give a value of DELTA=26mK. Due to the high electron-phonon relaxation rate, tau sub epsilon sup (-1)=2. 91x10 sup 9T sup 3sec sup (-1)K sup(-3), a minimum electron temperature of 9mK could be obtained with a residual heat leak of 8x10 sup (-14) W. It appears likely that rho sub xx should approach zero at T=0 in this state as in the ordinary FQHE.