We report experiment and theory on an ambipolar gate-controlled Si-vacuum field effect transistor (FET) where we study electron and hole (low-temperature 2D) transport in the same device simply by changing the external gate voltage to tune the system
from being a 2D electron system at positive gate voltage to a 2D hole system at negative gate voltage. The electron (hole) conductivity manifests strong (moderate) metallic temperature dependence with the conductivity decreasing by a factor of 8 (2) between 0.3 K and 4.2 K with the peak electron mobility ($sim 18$ m$^2$/Vs) being roughly 20 times larger than the peak hole mobility (in the same sample). Our theory explains the data well using RPA screening of background Coulomb disorder, establishing that the observed metallicity is a direct consequence of the strong temperature dependence of the effective screened disorder.
We develop a theory for graphene magnetotransport in the presence of carrier spin polarization as induced, for example, by the application of an in-plane magnetic field ($B$) parallel to the 2D graphene layer. We predict a negative magnetoresistance
$sigma propto B^2$ for intrinsic graphene, but for extrinsic graphene we find a non-monotonic magnetoresistance which is positive at lower magnetic fields (below the full spin-polarization) and negative at very high fields (above the full spin-polarization). The conductivity of the minority spin band $(-)$ electrons does not vanish as the minority carrier density ($n_-$) goes to zero. The residual conductivity of $(-)$ electrons at $n_- = 0$ is unique to graphene. We discuss experimental implications of our theory.
We calculate the temperature dependent conductivity of graphene in the presence of randomly distributed Coulomb impurity charges arising from the temperature dependent screening of the Coulomb disorder without any phonons. The purely electronic tempe
rature dependence of our theory arises from two independent mechanisms: the explicit temperature dependence of the finite temperature dielectric function $epsilon(q,T)$ and the finite temperature energy averaging of the transport scattering time. We find that the calculated temperature dependent conductivity is non-monotonic, decreasing with temperature at low temperatures, and increasing at high temperatures. We provide a critical comparison with the corresponding physics in semiconductor-based parabolic band 2D electron gas systems.
We investigate transport and Coulomb drag properties of semiconductor-based electron-hole bilayer systems. Our calculations are motivated by recent experiments in undoped electron-hole bilayer structures based on GaAs-AlGaAs gated double quantum well
systems. Our results indicate that the background charged impurity scattering is the most dominant resistive scattering mechanism in the high-mobility bilyers. We also find that the drag transresistivity is significantly enhanced when the electron-hole layer separation is small due to the exchange induced renormalization of the single layer compressibility.
We calculate the screening function in bilayer graphene (BLG) both in the intrinsic (undoped) and the extrinsic (doped) regime within random phase approximation, comparing our results with the corresponding single layer graphene (SLG) and the regular
two dimensional electron gas (2DEG). We find that the Kohn anomaly is strongly enhanced in BLG. We also discuss the Friedel oscillation and the RKKY interaction, which are associated with the non-analytic behavior of the screening function at $q=2k_F$. We find that the Kohn anomaly, the Friedel oscillation, and the RKKY interaction are all qualitatively different in the BLG compared with the SLG and the 2DEG.
We theoretically calculate the phonon scattering limited electron mobility in extrinsic (i.e. gated or doped with a tunable and finite carrier density) 2D graphene layers as a function of temperature $(T)$ and carrier density $(n)$. We find a tempera
ture dependent phonon-limited resistivity $rho_{ph}(T)$ to be linear in temperature for $Tagt 50 K$ with the room temperature intrinsic mobility reaching values above $10^5$ cm$^2/Vs$. We comment on the low-temperature Bloch-Gr{u}neisen behavior where $rho_{ph}(T) sim T^4$ for unscreened electron-phonon coupling.
