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.