This talk will cover many-body liquids and solids occurring in two-dimensional electron systems  (2DESs)  confined  to  GaAs  and  AlAs 

quantum  wells  subjected  to  a  large perpendicular magnetic field and cooled to very low temperatures. In AlAs 2DESs,  we investigated the Wigner solid and fractional quantum Hall liquid competition.

Also, the very low-disorder GaAs 2DESs allow us to probe the thermal melting of bubble phases, a type of Wigner solid.

In the first part of my talk,  I will present the exploration of the  fate  of  the  ground  state  of  a  2DES  at  very  low  Landau  level  filling factors where interaction reigns supreme.  We report experimental data for a new 2DES where the electrons are confined to an AlAs quantum well.  The exceptionally high quality of the samples and the large electron effective mass allow us to determine the liquid-solid phase diagram for the 2D electrons in a large range of  filling  factors  near 1/3  and 1/5. In the second part of my talk, I will present  the  screening  properties  of  bubble phases, probed via a simple capacitance technique where the 2DES is placed between a  top  and  a  bottom  gate  and  the  electric  field  penetrating  through  the  2DES  is measured.   The  bubbles  formed  at  very  low  temperatures  screen  the  electric  field poorly as they are pinned by the residual disorder potential, allowing a large electric field to reach the top gate.