Evidence of low-lying gapped excitations in the $\nu =$5/2 quantum fluid

The competition between quantum phases that dictate the physics in the second Landau level (SLL) results in striking phenomena. A highly fascinating state is the even denominator fractional quantum Hall (FQHE) state at filling $\nu =$5/2 that is widely believed to support non-Abelian quasi-particle excitations. Our work explores the low-lying neutral excitation modes in the SLL by resonant inelastic light scattering measurements. At 5/2 the spectra revealed a band of gapped modes with peak intensity at energy of 0.07meV. These modes are interpreted as a roton minimum in the wave vector dispersion of spin-conserving excitations. The intensity of the roton band significantly diminishes by increasing the temperature to 250mK and it fully collapses for T\textgreater\ 250mK. This temperature dependence is consistent with activated magneto-transport of the incompressible quantum fluid at 5/2. A long wavelength spin wave mode (SW) is seen at the bare Zeeman energy, indicating that there is non-zero spin-polarization. Both, roton and SW modes appear only in a very narrow filling factor range of less than $\nu $\textless\ 5/2 $\pm$ 0.01. A gapless continuum of low-lying excitations emerges at filling factors slightly away from 5/2. This demonstrates a transition from an incompressible quantum Hall fluid at exactly $\nu =$5/2 to compressible states at very close filling factors. This work is in collaboration with A. Pinczuk, A. Levy, K. West, L. Pfeiffer, S. Mondal, J. Watson and M. Manfra.