1,018 research outputs found
Opacity of graphene independent of light frequency and polarization due to the topological charge of the Dirac points
The opacity of graphene is known to be approximately given by the
fine-structure constant times . We point out the fact that the
opacity is roughly independent of the frequency and polarization of the light
can be attributed to the topological charge of the Dirac points. As a result,
one can literally see the topological charge by naked eyes from the opacity of
graphene, and moreover it implies that the fine-structure constant is
topologically protected. A similar analysis suggests that 3D topological
insulator thin films of any thickness also have opacity in the
infrared region owing to the topological surface states, indicating that one
can see the surface states by naked eyes through an infrared lens. For 3D Dirac
or Weyl semimetals, the optical absorption power is linear to the frequency in
the infrared region, with a linearity given by the fine-structure constant and
the topological charge of Weyl points.Comment: 9 pages, 3 figure
Fractional Supersymmetry and Infinite Dimensional Lie Algebras
In an earlier work extensions of supersymmetry and super Lie algebras were
constructed consistently starting from any representation \D of any Lie
algebra \g. Here it is shown how infinite dimensional Lie algebras appear
naturally within the framework of fractional supersymmetry. Using a
differential realization of \g this infinite dimensional Lie algebra,
containing the Lie algebra \g as a sub-algebra, is explicitly constructed.Comment: 8 pages, D.V.Volkov Memorial Conference ``Supersymmetry and Quantum
Field Theory'', Kharkov, July 25-29, 2000), two figure
J/psi D*D* vertex from QCD sum rules
We calculated the strong form factor and coupling constant for the vertex in a QCD sum rule calculation. We performed a double Borel sum
rule for the three point correlation function of vertex considering both
and mesons off--shell. The form factors obtained are very
different, but they give the same coupling constant.Comment: 7 pages and 4 figures, replaced version accepted for publication in
Phys. Lett.
Pentaquark Decay in QCD Sum Rules
In a diquark-diquark-antiquark picture of the pentaquark we study the decay
within the framework of QCD sum rules. After evaluation of
the relevant three-point function, we extract the coupling
which is directly related to the pentaquark width. Restricting the decay
diagrams to those with color exchange between the meson-like and baryon-like
clusters reduces the coupling constant by a factor of four. Whereas a small
decay width might be possible for a positive parity pentaquark, it seems
difficult to explain the measured width for a pentaquark with negative parity.Comment: 14pages, 5 eps figures. Contribution to the proceedings of LC200
Magnetoelectric torque and edge currents in spin-orbit coupled graphene nanoribbons
For graphene nanoribbons with Rashba spin-orbit coupling, the peculiar
magnetic response due to the presence of a magnetization and geometric
confinement are analyzed within a tight-binding model. We observe a sizable
transverse susceptibility that can be considered as a gate voltage-induced
magnetoelectric torque without the need of a bias voltage, with different
directions for zigzag and armchair ribbons. The local torque generates
non-collinear spin polarization between the two edges and/or along the ribbon,
and the net torque averages to zero if the magnetization is homogeneous.
Nevertheless, a nonzero net torque can appear in partially magnetized
nanoribbons or in nanoflakes of irregular shapes. The equilibrium spin current
produced by the spin-orbit coupling also appears in nanoribbons, but the
component flowing in the direction of confinement is strongly suppressed. Even
without the magnetization, an out-of-plane polarized chiral edge spin current
is produced, resembling that in the quantum spin Hall effect. Moreover, a
magnetization pointing perpendicular to the edge produces a laminar flow of
edge charge currents, whose flow direction is symmetric (non chiral) or
antisymmetric (chiral) between the two edges depends on whether the
magnetization points in-plane or out-of-plane.Comment: 10 pages, 5 figure
Measure, dimension, and complexity of escape in open Hamiltonian systems
In this work, we introduce the escape measure, a finite-time version of the
natural measure, to investigate the transient dynamics of escape orbits in open
Hamiltonian systems. In order to numerically calculate the escape measure, we
cover a region of interest of the phase space with a grid and we compute the
visitation frequency of a given orbit on each box of the grid before the orbit
escapes. Since open systems are not topologically transitive, we also define
the mean escape measure, an average of the escape measure on an ensemble of
initial conditions. We apply these concepts to study two physical systems: the
single-null divertor tokamak, described by a two-dimensional map; and the
Earth-Moon system, as modeled by the planar circular restricted three-body
problem. First, by calculating the mean escape measure profile, we visually
illustrate the paths taken by the escape orbits within the system. We observe
that the choice of the ensemble of initial conditions may lead to distinct
dynamical scenarios in both systems. Particularly, different orbits may
experience different stickiness effects. After that, we analyze the mean escape
measure distribution and we find that these vary greatly between the cases,
highlighting the differences between our systems as well. Lastly, we define two
parameters: the escape correlation dimension, that is independent of the grid
resolution, and the escape complexity coefficient, which takes into account
additional dynamical aspects, such as the orbit's escape time. We show that
both of these parameters can quantify and distinguish between the diverse
transient scenarios that arise.Comment: 12 pages, 12 figures, 2 table
Mapping quantum geometry and quantum phase transitions to real space by a fidelity marker
The quantum geometry in the momentum space of semiconductors and insulators,
described by the quantum metric of the valence band Bloch state, has been an
intriguing issue owing to its connection to various material properties.
Because the Brillouin zone is periodic, the integration of quantum metric over
momentum space represents an average distance between neighboring Bloch states,
of which we call the fidelity number. We show that this number can further be
expressed in real space as a fidelity marker, which is a local quantity that
can be calculated directly from diagonalizing the lattice Hamiltonian. A linear
response theory is further introduced to generalize the fidelity number and
marker to finite temperature, and moreover demonstrates that they can be
measured from the global and local optical absorption power against linearly
polarized light. In particular, the fidelity number spectral function in 2D
systems can be easily measured from the opacity of the material. Based on the
divergence of quantum metric, a nonlocal fidelity marker is further introduced
and postulated as a universal indicator of any quantum phase transitions
provided the crystalline momentum remains a good quantum number, and it may be
interpreted as a Wannier state correlation function. The ubiquity of these
concepts is demonstrated for a variety of topological insulators and
topological phase transitions in different dimensions.Comment: 11 pages, 5 figure
Functional analysis of cancer-associated EGFR mutants using a cellular assay with YFP-tagged EGFR intracellular domain
<p>Abstract</p> <p>Background</p> <p>The presence of EGFR kinase domain mutations in a subset of NSCLC patients correlates with the response to treatment with the EGFR tyrosine kinase inhibitors gefitinib and erlotinib. Although most EGFR mutations detected are short deletions in exon 19 or the L858R point mutation in exon 21, more than 75 different EGFR kinase domain residues have been reported to be altered in NSCLC patients. The phenotypical consequences of different EGFR mutations may vary dramatically, but the majority of uncommon EGFR mutations have never been functionally evaluated.</p> <p>Results</p> <p>We demonstrate that the relative kinase activity and erlotinib sensitivity of different EGFR mutants can be readily evaluated using transfection of an YFP-tagged fragment of the EGFR intracellular domain (YFP-EGFR-ICD), followed by immunofluorescence microscopy analysis. Using this assay, we show that the exon 20 insertions Ins770SVD and Ins774HV confer increased kinase activity, but no erlotinib sensitivity. We also show that, in contrast to the common L858R mutation, the uncommon exon 21 point mutations P848L and A859T appear to behave like functionally silent polymorphisms.</p> <p>Conclusion</p> <p>The ability to rapidly obtain functional information on EGFR variants of unknown relevance using the YFP-EGFR-ICD assay might prove important in the future for the management of NSCLC patients bearing uncommon EGFR mutations. In addition, our assay may be used to determine the response of resistant EGFR mutants to novel second-generation TKIs.</p
Cracking KD-Tree: The first multidimensional adaptive indexing
Workload-aware physical data access structures are crucial to achieve short response time with (exploratory) data analysis tasks as commonly required for Big Data and Data Science applications. Recently proposed techniques such as automatic index advisers (for a priori known static workloads) and query-driven adaptive incremental indexing (for a priori unknown dynamic workloads) form the state-of-the-art to build single-dimensional indexes for single-attribute query predicates. However, similar techniques for more demanding multi-attribute query predicates, which are vital for any data analysis task, have not been proposed, yet. In this paper, we present our on-going work on a new set of workload-adaptive indexing techniques that focus on creating multidimensional indexes. We present our proof-of-concept, the Cracking KD-Tree, an adaptive indexing approach that generates a KD-Tree based on multidimensional range query predicates. It works by incrementally creating partial multidimensional indexes as a by-product of query processing. The indexes are produced only on those parts of the data that are accessed, and their creation cost is effectively distributed across a stream of queries. Experimental results show that the Cracking KD-Tree is three times faster than creating a full KD-Tree, one order of magnitude faster than executing full scans and two orders of magnitude faster than using uni-dimensional full or adaptive indexes on multiple columns
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