28,944 research outputs found
Broken-Symmetry States of Dirac Fermions in Graphene with A Partially Filled High Landau Level
We report on numerical study of the Dirac fermions in partially filled N=3
Landau level (LL) in graphene. At half-filling, the equal-time density-density
correlation function displays sharp peaks at nonzero wavevectors . Finite-size scaling shows that the peak value grows with electron
number and diverges in the thermodynamic limit, which suggests an instability
toward a charge density wave. A symmetry broken stripe phase is formed at large
system size limit, which is robust against purturbation from disorder
scattering. Such a quantum phase is experimentally observable through transport
measurements. Associated with the special wavefunctions of the Dirac LL, both
stripe and bubble phases become possible candidates for the ground state of the
Dirac fermions in graphene with lower filling factors in the N=3 LL.Comment: Contains are slightly changed. Journal reference and DOI are adde
Odd-Integer Quantum Hall Effect in Graphene: Interaction and Disorder Effects
We study the competition between the long-range Coulomb interaction, disorder
scattering, and lattice effects in the integer quantum Hall effect (IQHE) in
graphene. By direct transport calculations, both and IQHE
states are revealed in the lowest two Dirac Landau levels. However, the
critical disorder strength above which the IQHE is destroyed is much
smaller than that for the IQHE, which may explain the absence of a
plateau in recent experiments. While the excitation spectrum in the
IQHE phase is gapless within numerical finite-size analysis, we do find and
determine a mobility gap, which characterizes the energy scale of the stability
of the IQHE. Furthermore, we demonstrate that the IQHE state is a Dirac
valley and sublattice polarized Ising pseudospin ferromagnet, while the
state is an plane polarized pseudospin ferromagnet.Comment: 5 pages, 5 figure
Classical integrability of the O(N) nonlinear Sigma model on a half-line
The classical integrability the O(N) nonlinear sigma model on a half-line is
examined, and the existence of an infinity of conserved charges in involution
is established for the free boundary condition. For the case N=3 other possible
boundary conditions are considered briefly.Comment: 12 Pages. Latex file (process twice
Quantum Spin Hall Effect and Topologically Invariant Chern Numbers
We present a topological description of quantum spin Hall effect (QSHE) in a
two-dimensional electron system on honeycomb lattice with both intrinsic and
Rashba spin-orbit couplings. We show that the topology of the band insulator
can be characterized by a traceless matrix of first Chern integers.
The nontrivial QSHE phase is identified by the nonzero diagonal matrix elements
of the Chern number matrix (CNM). A spin Chern number is derived from the CNM,
which is conserved in the presence of finite disorder scattering and spin
nonconserving Rashba coupling. By using the Laughlin's gedanken experiment, we
numerically calculate the spin polarization and spin transfer rate of the
conducting edge states, and determine a phase diagram for the QSHE.Comment: 4 pages and 4 figure
Spatially Resolved Nonlinearity Measurements of YBaCuO Bi-crystal Grain Boundaries
We have developed a near-field microwave microscope to locally excite a
superconducting film and measure second and third harmonic responses at
microwave frequencies. We study the local nonlinear response of a
YBaCuO thin film grown on a bi-crystal SrTiO substrate. The
location of the bi-crystal grain boundary is clearly identified by the
microscope through higher harmonic response, and the spatial resolution is on
the order of the magnetic loop probe size, about 500. The harmonic power
and spatial resolution are successfully modeled with a one-dimensional extended
Josephson junction simulation. From the model, the 2nd order harmonic response
is dominated by Josephson vortex generation and flow. A geometry-free nonlinear
scaling current density is also exstracted
from the data, indicating that the grain boundary weak link is the dominant
nonlinear source in this case.Comment: 4pages, 4figure
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A niching memetic algorithm for simultaneous clustering and feature selection
Clustering is inherently a difficult task, and is made even more difficult when the selection of relevant features is also an issue. In this paper we propose an approach for simultaneous clustering and feature selection using a niching memetic algorithm. Our approach (which we call NMA_CFS) makes feature selection an integral part of the global clustering search procedure and attempts to overcome the problem of identifying less promising locally optimal solutions in both clustering and feature selection, without making any a priori assumption about the number of clusters. Within the NMA_CFS procedure, a variable composite representation is devised to encode both feature selection and cluster centers with different numbers of clusters. Further, local search operations are introduced to refine feature selection and cluster centers encoded in the chromosomes. Finally, a niching method is integrated to preserve the population diversity and prevent premature convergence. In an experimental evaluation we demonstrate the effectiveness of the proposed approach and compare it with other related approaches, using both synthetic and real data
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