Subhasish Chakraborty

High-scan-range cryogenic scanning probe microscope

We have designed and constructed a scanning probe microscope operable at temperatures down to 260 mK within a top-loading helium-3 cryostat. It achieves a large scan range with the sample situated near the bottom of the scanning head—maximizing the cooling efficiency of the liquid helium. The scan head is completely thermally compensated, thus eliminating thermal expansion and contraction on cooling and warm-up, as well as thermal drift during operation. We demonstrate the performance using two distinct scanning probe methods: scanning tunneling microscopy and charge accumulation imaging.

Direct observation of micron-scale ordered structure in a two-dimensional electron system

We have extended the subsurface charge accumulation imaging method to directly resolve the interior structure of a GaAs/AlGaAs two-dimensional electron system in a tunneling geometry. We find that the application of a perpendicular magnetic field can induce surprising density modulations that are not static as a function of the field. Near six and four filled Landau levels, stripelike structures emerge with a characteristic length scale ∼2 μm. Present theories do not account for ordered density modulations on this scale.

Tunneling images of a 2D electron system in a quantizing magnetic field

Abstract We have applied a scanning probe method, subsurface charge accumulation imaging, to resolve the local structure of the interior of a semiconductor two-dimensional electron system (2DES) in a tunneling geometry. Near magnetic fields corresponding to integer Landau level filling, submicron scale spatial structure in the out-of-phase component of the tunneling signal becomes visible. In the images presented here, the structure repeats itself when the filling factor is changed from ν =6 to 7. Therefore, we believe the images reflect small modulations in the 2DES density caused by the disorder in the sample.