Observation of the fractional quantum Hall effect under hydrostatic pressure

We report on the first observation of the fractional quantum Hall effect (FQHE) under hydrostatic pressure. The FQHE has been observed at a range of two-dimensional electron-gas densities, ${\mathit{n}}_{\mathit{s}}$, in a high-quality ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs heterostructure at temperatures between 0.28 and 1.2 K and at hydrostatic pressures of 1 bar, 1.4 kbar, 4.5 kbar, and 6.2 kbar. We observe an increase of the activation energies for the 5/3 and 7/5 fractional states with increasing pressure and a very strong enhancement of the 4/3 state both in ${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$ and in the gradient of the Hall resistance d${\mathrm{\rho }}_{\mathrm{xy}}$/dB.