30 research outputs found
Quantum computing based on space states without charge transfer
An implementation of a quantum computer based on space states in double
quantum dots is discussed. There is no charge transfer in qubits during
calculation, therefore, uncontrollable entan-glement between them due to
long-range Coulomb interaction is suppressed. Other plausible sources of
decoherence caused by interaction with phonons and gates could be substantially
suppressed in the structure too. We also demonstrate how all necessary quantum
logic operations, initialization, writing, and read-out could be carried out in
the computer.Comment: 7 pages, 4 figures, RevTeX forma
Hydrodynamic model for electron-hole plasma in graphene
We propose a hydrodynamic model describing steady-state and dynamic electron
and hole transport properties of graphene structures which accounts for the
features of the electron and hole spectra. It is intended for electron-hole
plasma in graphene characterized by high rate of intercarrier scattering
compared to external scattering (on phonons and impurities), i.e., for
intrinsic or optically pumped (bipolar plasma), and gated graphene (virtually
monopolar plasma). We demonstrate that the effect of strong interaction of
electrons and holes on their transport can be treated as a viscous friction
between the electron and hole components. We apply the developed model for the
calculations of the graphene dc conductivity, in particular, the effect of
mutual drag of electrons and holes is described. The spectra and damping of
collective excitations in graphene in the bipolar and monopolar limits are
found. It is shown that at high gate voltages and, hence, at high electron and
low hole densities (or vice-versa), the excitations are associated with the
self-consistent electric field and the hydrodynamic pressure (plasma waves). In
intrinsic and optically pumped graphene, the waves constitute quasineutral
perturbations of the electron and hole densities (electron-hole sound waves)
with the velocity being dependent only on the fundamental graphene constants.Comment: 11 pages, 6 figure
Effect of Coulomb scattering on graphene conductivity
The effect of Coulomb scattering on graphene conductivity in field effect
transistor structures is discussed. Inter-particle scattering
(electron-electron, hole-hole, and electron-hole) and scattering on charged
defects are taken into account in a wide range of gate voltages. It is shown
that an intrinsic conductivity of graphene (purely ambipolar system where both
electron and hole densities exactly coincide) is defined by strong
electron-hole scattering. It has a universal value independent of temperature.
We give an explicit derivation based on scaling theory. When there is even a
small discrepancy in electron and hole densities caused by applied gate voltage
the conductivity is determined by both strong electron-hole scattering and weak
external scattering: on defects or phonons. We suggest that a density of
charged defects (occupancy of defects) depends on Fermi energy to explain a
sub-linear dependence of conductivity on a fairly high gate voltage observed in
experiments. We also eliminate contradictions between experimental data
obtained in deposited and suspended graphene structures regarding graphene
conductivity.Comment: 4 pages, 3 figures, to be published in JETP Let