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 3x10 ^{10}
cm^{-2} is around 1.8x10^{6} cm^{2}/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 p_{c}=3x10^{9}
cm^{-2}, lowest ever observed. |

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 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 T^{2}
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. |

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. |