24,774 research outputs found
First-principles Analysis of Photo-current in Graphene PN Junctions
We report a first principles investigation of photocurrent generation by
graphene PN junctions. The junctions are formed by either chemically doping
with nitrogen and boron atoms, or by controlling gate voltages. Non-equilibrium
Green's function (NEGF) formalism combined with density functional theory (DFT)
is applied to calculate the photo-response function. The graphene PN junctions
show a broad band photo-response including the terahertz range. The dependence
of the response on the angle between the light polarization vector and the PN
interface is determined. Its variation against photon energy is
calculated in the visible range. The essential properties of chemically doped
and gate-controlled PN junctions are similar, but the former shows fingerprints
of dopant distribution.Comment: 7 pages, 6 figure
Nature of the Quantum Phase Transition in Quantum Compass Model
In this work, we show that the quantum compass model on an square lattice can
be mapped to a fermionic model with local density interaction. We introduce a
mean-field approximation where the most important fluctuations, those
perpendicular to the ordering direction, are taken into account exactly. It is
found that the quantum phase transition point at marks a first order
phase transition. We also show that the mean field result is robust against the
remaining fluctuation corrections up to the second order.Comment: 7 pages, 10 fig
Optical Monitoring of the Seyfert Galaxy NGC 4151 and Possible Periodicities in the Historical Light Curve
We report B, V, and R band CCD photometry of the Seyfert galaxy NGC 4151
obtained with the 1.0-m telescope at Weihai Observatory of Shandong University
and the 1.56-m telescope at Shanghai Astronomical Observatory from 2005
December to 2013 February. Combining all available data from literature, we
have constructed a historical light curve from 1910 to 2013 to study the
periodicity of the source using three different methods (the Jurkevich method,
the Lomb-Scargle periodogram method and the Discrete Correlation Function
method). We find possible periods of P_1=4\pm0.1, P_2=7.5\pm0.3 and
P_3=15.9\pm0.3 yr.Comment: 8 pages, 5 figures, Accepted by Research in Astronomy and
Astrophysic
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