Strongly Interacting Two-dimensional Hole Systems

2D metal-insulator transition

In two-dimensional charge carrier systems, it is well known  that any amount of disorder in the absence of interactions between the carriers will localize the carriers, leading to an insulator with zero conductivity as the temperature T is decreased to zero. Recent experiments on high mobility dilute 2D systems, on the other hand, have shown a ‘‘metallic’’ behavior at low T, characterized by an increasing conductivity with decreasing T, and an apparent metal-insulator transition (MIT) as the carrier density is lowered. The ratio of the interaction energy to the Fermi energy in these systems, which is rs , is around 10 or higher, implying that an interaction must be playing a role.

We have fabricated a novel  device that hosts a ultra high mobility 2D hole system. The mobility at a hole density of 3x1010 cm-2 is around 1.8x106 cm2/Vs. This high mobility allows us to lower the density to a very dilute regime where the interaction effects become the highest. In this system, we have observed the MIT at a critical density of pc=3x109 cm-2, lowest ever observed.

Magnetic field induced insulating phases

Exploring a backgated low density two-dimensional hole sample in the large rs regime we found a surprisingly rich phase diagram. At the highest densities, beside the n = 1/3, 2/3, and 2/5 fractional quantum Hall states, we observe both of the previously reported high field insulating and the reentrant insulating phases. As the density is lowered, the reentrant insulating phase initially strengthens, then it unexpectedly starts weakening until it completely dissapears. At even lower densities another insulating phase appears that is different from the reetrant insulator. Finally, at the lowest densities the terminal quantum Hall state moves from n = 1/3 to n = 1. The intricate behavior of the insulating phases can be explained by a non-monotonic melting line in the n - rs phase space.

Coulomb Drag Experiment

Coulomb drag between dilute 2D hole layers

Coulomb drag experiment is a novel transport measurement technique, which directly probes the electron-electron scattering rate between two independent electron systems. Many of the Coulomb drag experiments have been performed with double layer two-dimensional electron or hole systems. Carrier densities in earlier drag experiments were high enough that the correlation effects could be ignored.  Recently, we have performed drag measurements on dilute double layer two-dimensional hole systems, where the correlation effects are important with rs value ranging from 19 to 39. We observed a strong enhancement of the drag over the simple Boltzmann calculations and deviations from the T2 dependence which cannot be explained by phonon mediated, plasmon-enhanced, or disorder-related processes. We suggest that this deviation results from correlation effects in the dilute regime.

Coulomb drag between 1D electron systems

With the technical development in the fabrication of nanowires with various materials, it is possible to realize the Coulomb drag experiment in one dimension. In one-dimensional electron systems, electron-electron interactions make the system described by the Luttinger liquid theory. This will lead to unique behaviors in the T-dependence of drag at low temperatures. There have been a lot of theoretical studies and predictions on the one-dimensional drag, while a successful experiment has yet to be performed. Very few attempts reported so far had technical problems such as inter-wire tunneling effects. In our laboratory, we employ the state-of-the-art nano fabrication technologies to make devices that consist of parallel nanowires separated by a few tens of nanometers, and use them in Coulomb drag experiment. Nanowires used in this study include carbon nanotubes and various semiconductor nanowires.