2,531 research outputs found
Mode mixing induced by disorder in graphene PNP junction in a magnetic field
We study the electron transport through the graphene PNP junction under a
magnetic field and show that modes mixing plays an essential role. By using the
non-equilibrium Green's function method, the space distribution of the
scattering state for a specific incident modes as well the elements of the
transmission and reflection coefficient matrixes are investigated. All elements
of the transmission (reflection) coefficient matrixes are very different for a
perfect PNP junction, but they are same at a disordered junction due to the
mode mixing. The space distribution of the scattering state for the different
incident modes also exhibit the similar behaviors, that they distinctly differ
from each other in the perfect junction but are almost same in the disordered
junction. For a unipolar junction, when the mode number in the center region is
less than that in the left and right regions, the fluctuations of the total
transmission and reflection coefficients are zero, although each element has a
large fluctuation. These results clearly indicate the occurrence of perfect
mode mixing and it plays an essential role in a graphene PNP junction
transport
Charge ordering and phase separation in the infinite dimensional extended Hubbard model
We study the extended Hubbard model with both on-site (U) and nearest
neighbor (V) Coulomb repulsion using the exact diagonalization method within
the dynamical mean field theory. For a fixed U (U=2.0), the T-n phase-diagrams
are obtained for V=1.4 and V=1.2, at which the ground states of n=1/2 system is
charge-ordered and charge-disordered, respectively. In both cases, robust
charge order is found at finite temperature and in an extended filling regime
around n=1/2. The order parameter changes non-monotonously with temperature.
For V=1.4, phase separation between charge-ordered and charge-disordered phases
is observed in the low temperature and n < 0.5 regime. It is described by an
"S"-shaped structure of the n-/mu curve. For V=1.2, the ground state is
charge-disordered, and a reentrant charge-ordering transition is observed for
0.42 < n < 0.68. Relevance of our results to experiments for doped manganites
is discussed.Comment: 9 pages, 7 figures, submitted to Phys. Rev.
Mott-Hubbard transition in infinite dimensions
We analyze the unanalytical structure of metal-insulator transition (MIT) in
infinite dimensions. By introducing a simple transformation into the dynamical
mean-field equation of Hubbard model, a multiple-valued structure in Green's
function and other thermodynamical quantities with respect to the interaction
strength are found at low temperatures. A unified description of stable,
metastable and unstable phases is obtained in the regime
, and the Maxwell construction is performed to evaluate
the MIT line . We show how the first-order MIT at
for evolves into second-order one at for . The phase
diagram near MIT is presented.Comment: 5 pages with 3 figures, text and figures revise
Statistical analysis for a penalized EM algorithm in high-dimensional mixture linear regression model
The expectation-maximization (EM) algorithm and its variants are widely used
in statistics. In high-dimensional mixture linear regression, the model is
assumed to be a finite mixture of linear regression and the number of
predictors is much larger than the sample size. The standard EM algorithm,
which attempts to find the maximum likelihood estimator, becomes infeasible for
such model. We devise a group lasso penalized EM algorithm and study its
statistical properties. Existing theoretical results of regularized EM
algorithms often rely on dividing the sample into many independent batches and
employing a fresh batch of sample in each iteration of the algorithm. Our
algorithm and theoretical analysis do not require sample-splitting, and can be
extended to multivariate response cases. The proposed methods also have
encouraging performances in numerical studies
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