Metal-insulator transition in two-dimensional electron systems

4 problems

• Does the effective mass diverge at the 2D metal-insulator transition?

  Contested

  Near the transition, electrons in silicon devices behave as if they become many times heavier. Whether this effective mass (the mass that sets how electrons respond to forces) truly diverges at a critical density, and whether that density coincides with the transition, is unsettled.

• What causes the metallic resistance drop in dilute 2D electron systems?

  Contested

  In clean silicon devices with few electrons, resistance falls by up to about ten times on cooling below a few kelvin, the opposite of what weak-disorder theory predicts. Which physical process causes this drop is disputed.

• Why does an in-plane field suppress the 2D metallic state?

  Open

  A magnetic field along the sheet, which aligns electron spins but does not bend their paths, raises the resistance many times and in silicon transistors removes the metal-like behavior. Why spin alignment has such a large effect, and why some ultra-clean samples stay metallic even with all spins aligned, is not understood.

• Does a two-dimensional metal survive to zero temperature?

  Contested

  Scaling theory says a thin disordered sheet of non-interacting electrons is always an insulator at absolute zero. The question is whether strong repulsion allows a true metal, or whether the observed metal turns insulating at temperatures not yet reached.