Low-Energy Excitations in the Second LL: Fundamental Insights from Inelastic Light Scattering*

The competition between quantum phases that dictates the physics in the second Landau level (SLL) results in striking phenomena. Our work explores this fascinating interaction physics by measurements of low-lying neutral excitation modes in the SLL from resonant inelastic light scattering experiments. We focus here on the marked differences of the low-lying collective excitation spectra of the even-denominator state at $\nu$=5/2 with those in the range 5/2>$\nu$>2. Filling factor 5/2 is characterized by the presence of gapped modes, a spin mode exactly at $E_{Z}$ and the absence of a continuum of low-lying excitations. In contrast, a continuum of low-lying excitations and gapped modes are coexistent at $\nu$=2+1/3, 2+3/8 and 2+2/5 and the spin-modes appear significantly below $E_{Z}$. All observed modes weakens with smallest variations in filling factor substantiating the transition from an incompressible quantum Hall fluid to compressible states.