Microwave spectroscopic observation of distinct electron solid phases in wide quantum wells

In high magnetic fields ($B$), two dimensional electron systems (2DESs) can form a number of phases in which interelectron repulsion plays the central role, since the kinetic energy is frozen out by Landau quantization. These phases include the well-known liquids of the fractional quantum Hall effect (FQHE), as well as solid phases with broken spatial symmetry and crystalline order. Solids can occur at the low Landau filling ($\nu$) termination of the FQHE series, but also within integer quantum Hall effects (IQHEs). Here, we present microwave spectroscopy studies of wide quantum wells (WQWs). The spectra clearly reveal two distinct solid phases, hidden within what in dc transport would be the zero diagonal conductivity of an integer quantum Hall effect state. Explanation of these solids is not possible with the simple picture of a Wigner solid (WS) of ordinary (quasi) electrons or holes.