1,564 research outputs found
Hidden Symmetries of Higher Dimensional Black Holes and Uniqueness of the Kerr-NUT-(A)dS spacetime
We prove that the most general solution of the Einstein equations with the
cosmological constant which admits a principal conformal Killing-Yano tensor is
the Kerr-NUT-(A)dS metric. Even when the Einstein equations are not imposed,
any spacetime admitting such hidden symmetry can be written in a canonical form
which guarantees the following properties: it is of the Petrov type D, it
allows the separation of variables for the Hamilton-Jacobi, Klein-Gordon, and
Dirac equations, the geodesic motion in such a spacetime is completely
integrable. These results naturally generalize the results obtained earlier in
four dimensions.Comment: 5 pages, no figure
AC Hopping Magnetotransport Across the Spin Flop Transition in Lightly Doped La_2CuO_4
The weak ferromagnetism present in insulating La_{2}CuO_4 at low doping leads
to a spin flop transition, and to transverse (interplane) hopping of holes in a
strong external magnetic field. This results in a dimensional crossover 2D
3D for the in-plane transport, which in turn leads to an increase of the
hole's localization length and increased conduction. We demonstrate
theoretically that as a consequence of this mechanism, a frequency-dependent
jump of the in-plane ac hopping conductivity occurs at the spin flop
transition. We predict the value and the frequency dependence of the jump.
Experimental studies of this effect would provide important confirmation of the
emerging understanding of lightly doped insulating La_{2-x}Sr_xCuO_4.Comment: 4 pages, 1 figur
Negative Hopping Magnetoresistance and Dimensional Crossover in Lightly Doped Cuprate Superconductors
We show that, due to the weak ferromagnetism of LaSrCuO, an
external magnetic field leads to a dimensional crossover 2D 3D for the
in-plane transport. The crossover results in an increase of the hole's
localization length and hence in a dramatic negative magnetoresistance in the
variable range hopping regime. This mechanism quantitatively explains puzzling
experimental data on the negative magnetoresistance in the N\'eel phase of
LaSrCuO.Comment: 6 pages, 3 figures; published versio
c-Src drives intestinal regeneration and transformation
The non‐receptor tyrosine kinase c‐Src, hereafter referred to as Src, is overexpressed or activated in multiple human malignancies. There has been much speculation about the functional role of Src in colorectal cancer (CRC), with Src amplification and potential activating mutations in up to 20% of the human tumours, although this has never been addressed due to multiple redundant family members. Here, we have used the adult <i>Drosophila</i> and mouse intestinal epithelium as paradigms to define a role for Src during tissue homeostasis, damage‐induced regeneration and hyperplasia. Through genetic gain and loss of function experiments, we demonstrate that Src is necessary and sufficient to drive intestinal stem cell (ISC) proliferation during tissue self‐renewal, regeneration and tumourigenesis. Surprisingly, Src plays a non‐redundant role in the mouse intestine, which cannot be substituted by the other family kinases Fyn and Yes. Mechanistically, we show that Src drives ISC proliferation through upregulation of EGFR and activation of Ras/MAPK and Stat3 signalling. Therefore, we demonstrate a novel essential role for Src in intestinal stem/progenitor cell proliferation and tumourigenesis initiation <i>in vivo.</i>
Screening of Coulomb Impurities in Graphene
We calculate exactly the vacuum polarization charge density in the field of a
subcritical Coulomb impurity, , in graphene. Our analysis is based on
the exact electron Green's function, obtained by using the operator method, and
leads to results that are exact in the parameter , where is
the "fine structure constant" of graphene. Taking into account also
electron-electron interactions in the Hartree approximation, we solve the
problem self-consistently in the subcritical regime, where the impurity has an
effective charge , determined by the localized induced charge. We find
that an impurity with bare charge Z=1 remains subcritical, , for any , while impurities with and higher can become
supercritical at certain values of .Comment: 4 pages, 2 figure
1/N Expansion in Correlated Graphene
We examine the 1/N expansion, where N is the number of two-component Dirac
fermions, for Coulomb interactions in graphene with a gap of magnitude . We find that for , where is graphene's "fine
structure constant", there is a crossover as a function of distance from
the usual 3D Coulomb law, , to a 2D Coulomb interaction, , for . This effect
reflects the weak "confinement" of the electric field in the graphene plane.
The crossover also leads to unusual renormalization of the quasiparticle
velocity and gap at low momenta. We also discuss the differences between the
interaction potential in gapped graphene and usual QED for different coupling
regimes.Comment: 7 pages, 2 figures; expanded presentation, references adde
Parallel-propagated frame along null geodesics in higher-dimensional black hole spacetimes
In [arXiv:0803.3259] the equations describing the parallel transport of
orthonormal frames along timelike (spacelike) geodesics in a spacetime
admitting a non-degenerate principal conformal Killing-Yano 2-form h were
solved. The construction employed is based on studying the Darboux subspaces of
the 2-form F obtained as a projection of h along the geodesic trajectory. In
this paper we demonstrate that, although slightly modified, a similar
construction is possible also in the case of null geodesics. In particular, we
explicitly construct the parallel-transported frames along null geodesics in
D=4,5,6 Kerr-NUT-(A)dS spacetimes. We further discuss the parallel transport
along principal null directions in these spacetimes. Such directions coincide
with the eigenvectors of the principal conformal Killing-Yano tensor. Finally,
we show how to obtain a parallel-transported frame along null geodesics in the
background of the 4D Plebanski-Demianski metric which admits only a conformal
generalization of the Killing-Yano tensor.Comment: 17 pages, no figure
Stationary strings near a higher-dimensional rotating black hole
We study stationary string configurations in a space-time of a
higher-dimensional rotating black hole. We demonstrate that the Nambu-Goto
equations for a stationary string in the 5D Myers-Perry metric allow a
separation of variables. We present these equations in the first-order form and
study their properties. We prove that the only stationary string configuration
which crosses the infinite red-shift surface and remains regular there is a
principal Killing string. A worldsheet of such a string is generated by a
principal null geodesic and a timelike at infinity Killing vector field. We
obtain principal Killing string solutions in the Myers-Perry metrics with an
arbitrary number of dimensions. It is shown that due to the interaction of a
string with a rotating black hole there is an angular momentum transfer from
the black hole to the string. We calculate the rate of this transfer in a
spacetime with an arbitrary number of dimensions. This effect slows down the
rotation of the black hole. We discuss possible final stationary configurations
of a rotating black hole interacting with a string.Comment: 13 pages, contains additianal material at the end of Section 8, also
small misprints are correcte
Analysing Molecular Mechanism Related to Therapy- Resistance in In-vitro Models of Ovarian Cancer
Ovarian cancer is among the most common cause of cancer death and ranks first in the number of deaths each year in the field of gynaecological malignancies. This is due to its late diagnosis and the development of chemoresistance. Platinum derivates, including cisplatinum and carboplatin in combination with paclitaxel, are the first-line chemotherapeutic agents. Platinum derivates irreversibly intercalates into the DNA and creates inter- and intra-strand DNA cross-links. During cell division, platinum-DNA-adducts block the replication machinery, inducing DNA damage and apoptosis. Nearly all patients respond to first-line chemotherapy before it comes later to recurrence of the disease. At time of recurrence, tumours are usually more aggressive, form metastasis in secondary tissues and acquire resistance to conventional chemotherapeutics. Drug resistance is a common problem in tumour therapy not only restricted to ovarian cancer. It is characterized by gene mutations, increased DNA repair, reduced drug efficacy and enhanced drug clearance and detoxification. Up to now the complex molecular mechanism of chemoresistance is not well understood. Increasing evidence points towards AKT over-expression and alteration of the PI3K/AKT/mTOR cascade as a central mechanistic reason for this resistance
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