227 research outputs found
Wigner crystallization in quantum electron bilayers
The phase diagram of quantum electron bilayers in zero magnetic field is
obtained using density functional theory. For large electron densities the
system is in the liquid phase, while for smaller densities the liquid may
freeze (Wigner crystallization) into four different crystalline phases; the
lattice symmetry and the critical density depend on the the inter-layer
distance. The phase boundaries between different Wigner crystals consist of
both first and second order transitions, depending on the phases involved, and
join the freezing curve at three different triple points.Comment: To appear in Europhys. Lett. (11 pages in REVTEX + 2 figures in
postscript
Statics and dynamics of phase segregation in multicomponent fermion gas
We investigate the statics and dynamics of spatial phase segregation process
of a mixture of fermion atoms in a harmonic trap using the density functional
theory. The kinetic energy of the fermion gas is written in terms of the
density and its gradients. Several cases have been studied by neglecting the
gradient terms (the Thomas-Fermi limit) which are then compared with the
Monte-Carlo results using the full gradient corrected kinetic energy. A linear
instability analysis has been performed using the random-phase approximation.
Near the onset of instability, the fastest unstable mode for spinodal
decomposition is found to occur at . However, in the strong coupling
limit, many more modes with decay with comparable time scales.Comment: 14 figure
A Gate-Induced Switch in Zigzag Graphene Naoribbons and Charging Effects
Using non-equilibrium Green's function formalism, we investigate nonlinear
transport and charging effects of gated graphene nanoribbons (GNRs) with even
number of zigzag chains. We find a negative differential resistance (NDR) over
a wide range of gate voltages with on/off ratio for narrow enough
ribbons. This NDR originates from the parity selection rule and also
prohibition of transport between discontinues energy bands. Since the external
field is well screened close to the contacts, the NDR is robust against the
electrostatic potential. However, for voltages higher than the NDR threshold,
due to charge transfer through the edges of ZGNR, screening is reduced such
that the external potential can penetrate inside the ribbon giving rise to
smaller values of off current. Furthermore, on/off ratio of the current depends
on the aspect ratio of the length/width and also edge impurity. Moreover,
on/off ratio displays a power law behavior as a function of ribbon length.Comment: 8 pages, 9 figure
Wigner Crystallization of a two dimensional electron gas in a magnetic field: single electrons versus electron pairs at the lattice sites
The ground state energy and the lowest excitations of a two dimensional
Wigner crystal in a perpendicular magnetic field with one and two electrons per
cell is investigated. In case of two electrons per lattice site, the
interaction of the electrons {\em within} each cell is taken into account
exactly (including exchange and correlation effects), and the interaction {\em
between} the cells is in second order (dipole) van der Waals approximation. No
further approximations are made, in particular Landau level mixing and {\em
in}complete spin polarization are accounted for. Therefore, our calculation
comprises a, roughly speaking, complementary description of the bubble phase
(in the special case of one and two electrons per bubble), which was proposed
by Koulakov, Fogler and Shklovskii on the basis of a Hartree Fock calculation.
The phase diagram shows that in GaAs the paired phase is energetically more
favorable than the single electron phase for, roughly speaking, filling factor
larger than 0.3 and density parameter smaller than 19 effective Bohr
radii (for a more precise statement see Fig.s 4 and 5). If we start within the
paired phase and increase magnetic field or decrease density, the pairs first
undergo some singlet- triplet transitions before they break.Comment: 11 pages, 7 figure
Ground-state energy and compressibility of a disordered two-dimensional electron gas
Two-dimensional (2D) electron systems in the presence of disorder are of interest in connection with the observed metal-insulator transition in such systems. We use density functional theory in its local-spin density approximation (LSDA) to calculate the ground-state energy of a 2D electron system in the presence of remote charged impurities which up on averaging provides disorder. The inverse compressibility calculated from the ground-state energy exhibits a minimum at a critical density controlled by the disorder strength. Our findings are in agreement with experimental results. © World Scientific Publishing Company
Cold-Adapted Signal Proteins: NMR Structures of Pheromones from the Antarctic Ciliate Euplotes nobilii
Cell type-specific signal proteins, known as pheromones, are
synthesized by ciliated protozoa in association with their self/nonself mating-type systems, and are utilized to control the vegetative growth and mating stages of their life cycle. In species of the most ubiquitous ciliate, Euplotes, these pheromones form families of structurally homologous molecules, which are constitutively secreted into the
extracellular environment, from where they can be isolated in
sufficient amounts for chemical characterization. This paper
describes the NMR structures of En-1 and En-2, which are
members of the cold-adapted pheromone family produced by
Euplotes nobilii, a species inhabiting the freezing coastal waters of Antarctica. The structures were determined with the proteins from the natural source, using homonuclear 1H NMR techniques in combination with automated NOESY peak picking and NOE assignment. En-1 and En-2 have highly homologous global folds, which consist of a central three-a-helix bundle with an up-down-up topology and a 310-helical turn near the N-terminus. This fold is stabilized by four disulfide bonds and the helices are connected by
bulging loops. Apparent structural specificity resides in the variable C-terminal regions of the pheromones.TheNMRstructures ofEn-1 and
En-2 provide novel insights into the cold-adaptive modifications that distinguish the E. nobilii pheromone family from the closely related E. raikovi pheromone family isolated from temperate waters
Negative differential resistance in molecular junctions: application to graphene ribbon junctions
Using self-consistent calculations based on Non-Equilibrium Green's Function
(NEGF) formalism, the origin of negative differential resistance (NDR) in
molecular junctions and quantum wires is investigated. Coupling of the molecule
to electrodes becomes asymmetric at high bias due to asymmetry between its
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) levels. This causes appearance of an asymmetric potential
profile due to a depletion of charge and reduction of screening near the source
electrode. With increasing bias, this sharp potential drop leads to an enhanced
localization of the HOMO and LUMO states in different parts of the system. The
reduction in overlap, caused by localization, results in a significant
reduction in the transmission coefficient and current with increasing bias. An
atomic chain connected to two Graphene ribbons was investigated to illustrate
these effects. For a chain substituting a molecule, an even-odd effect is also
observed in the NDR characteristics.Comment: 8 pages, 8 figure
Multiple Functionality in Nanotube Transistors
Calculations of quantum transport in a carbon nanotube transistor show that
such a device offers unique functionality. It can operate as a ballistic
field-effect transistor, with excellent characteristics even when scaled to 10
nm dimensions. At larger gate voltages, channel inversion leads to resonant
tunneling through an electrostatically defined nanoscale quantum dot. Thus the
transistor becomes a gated resonant tunelling device, with negative
differential resistance at a tunable threshold. For the dimensions considered
here, the device operates in the Coulomb blockade regime, even at room
temperature.Comment: To appear in Phys. Rev. Let
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