86 research outputs found
Caustics due to a negative refractive index in circular graphene p-n junctions
We show that the wave functions form caustics in circular graphene
p-n
junctions which in the framework of geometrical optics can be
interpreted with a negative refractive index
Electron Flow in Circular n-p Junctions of Bilayer Graphene
We present a theoretical study of electron wave functions in ballistic
circular n-p junctions of bilayer graphene. Similarly to the case of a circular
n-p junction of monolayer graphene, we find that (i) the wave functions form
caustics inside the circular region, and (ii) the shape of these caustics are
well described by a geometrical optics model using the concept of a negative
refractive index. In contrast to the monolayer case, we show that the strong
focusing effect is absent in the bilayer. We explain these findings in terms of
the angular dependence of Klein tunneling at a planar n-p junction.Comment: Published version 6 pages, 5 figure
A valley-spin qubit in a carbon nanotube
Although electron spins in III-V semiconductor quantum dots have shown great
promise as qubits, a major challenge is the unavoidable hyperfine decoherence
in these materials. In group IV semiconductors, the dominant nuclear species
are spinless, allowing for qubit coherence times that have been extended up to
seconds in diamond and silicon. Carbon nanotubes are a particularly attractive
host material, because the spin-orbit interaction with the valley degree of
freedom allows for electrical manipulation of the qubit. In this work, we
realise such a qubit in a nanotube double quantum dot. The qubit is encoded in
two valley-spin states, with coherent manipulation via electrically driven spin
resonance (EDSR) mediated by a bend in the nanotube. Readout is performed by
measuring the current in Pauli blockade. Arbitrary qubit rotations are
demonstrated, and the coherence time is measured via Hahn echo. Although the
measured decoherence time is only 65 ns in our current device, this work offers
the possibility of creating a qubit for which hyperfine interaction can be
virtually eliminated
Enhanced NMR relaxation of Tomonaga-Luttinger liquids and the magnitude of the carbon hyperfine coupling in single-wall carbon nanotubes
Recent transport measurements [Churchill \textit{et al.} Nat. Phys.
\textbf{5}, 321 (2009)] found a surprisingly large, 2-3 orders of magnitude
larger than usual C hyperfine coupling (HFC) in C enriched
single-wall carbon nanotubes (SWCNTs). We formulate the theory of the nuclear
relaxation time in the framework of the Tomonaga-Luttinger liquid theory to
enable the determination of the HFC from recent data by Ihara \textit{et al.}
[Ihara \textit{et al.} EPL \textbf{90}, 17004 (2010)]. Though we find that
is orders of magnitude enhanced with respect to a Fermi-liquid
behavior, the HFC has its usual, small value. Then, we reexamine the
theoretical description used to extract the HFC from transport experiments and
show that similar features could be obtained with HFC-independent system
parameters.Comment: 5 pages plus 2 supplementary material
Valley-spin blockade and spin resonance in carbon nanotubes
Manipulation and readout of spin qubits in quantum dots made in III-V
materials successfully rely on Pauli blockade that forbids transitions between
spin-triplet and spin-singlet states. Quantum dots in group IV materials have
the advantage of avoiding decoherence from the hyperfine interaction by
purifying them with only zero-spin nuclei. Complications of group IV materials
arise from the valley degeneracies in the electronic bandstructure. These lead
to complicated multiplet states even for two-electron quantum dots thereby
significantly weakening the selection rules for Pauli blockade. Only recently
have spin qubits been realized in silicon devices where the valley degeneracy
is lifted by strain and spatial confinement. In carbon nanotubes Pauli blockade
can be observed by lifting valley degeneracy through disorder. In clean
nanotubes, quantum dots have to be made ultra-small to obtain a large energy
difference between the relevant multiplet states. Here we report on
low-disorder nanotubes and demonstrate Pauli blockade based on both valley and
spin selection rules. We exploit the bandgap of the nanotube to obtain a large
level spacing and thereby a robust blockade. Single-electron spin resonance is
detected using the blockade.Comment: 31 pages including supplementary informatio
Overexpression of CD44 accompanies acquired tamoxifen resistance in MCF7 cells and augments their sensitivity to the stromal factors, heregulin and hyaluronan
Background: Acquired resistance to endocrine therapy in breast cancer is a significant problem with relapse being associated with local and/or regional recurrence and frequent distant metastases. Breast cancer cell models reveal that endocrine resistance is accompanied by a gain in aggressive behaviour driven in part through altered growth factor receptor signalling, particularly involving erbB family receptors. Recently we identified that CD44, a transmembrane cell adhesion receptor known to interact with growth factor receptors, is upregulated in tamoxifen-resistant (TamR) MCF7 breast cancer cells. The purpose of this study was to explore the consequences of CD44 upregulation in an MCF7 cell model of acquired tamoxifen resistance, specifically with respect to the hypothesis that CD44 may influence erbB activity to promote an adverse phenotype. Methods: CD44 expression in MCF7 and TamR cells was assessed by RT-PCR, Western blotting and immunocytochemistry. Immunofluorescence and immunoprecipitation studies revealed CD44-erbB associations. TamR cells (± siRNA-mediated CD44 suppression) or MCF7 cells (± transfection with the CD44 gene) were treated with the CD44 ligand, hyaluronon (HA), or heregulin and their in vitro growth (MTT), migration (Boyden chamber and wound healing) and invasion (Matrigel transwell migration) determined. erbB signalling was assessed using Western blotting. The effect of HA on erbB family dimerisation in TamR cells was determined by immunoprecipitation in the presence or absence of CD44 siRNA. Results: TamR cells overexpressed CD44 where it was seen to associate with erbB2 at the cell surface. siRNA-mediated suppression of CD44 in TamR cells significantly attenuated their response to heregulin, inhibiting heregulin-induced cell migration and invasion. Furthermore, TamR cells exhibited enhanced sensitivity to HA, with HA treatment resulting in modulation of erbB dimerisation, ligand-independent activation of erbB2 and EGFR and induction of cell migration. Overexpression of CD44 in MCF7 cells, which lack endogenous CD44, generated an HA-sensitive phenotype, with HA-stimulation promoting erbB/EGFR activation and migration. Conclusions: These data suggest an important role for CD44 in the context of tamoxifen-resistance where it may augment cellular response to erbB ligands and HA, factors that are reported to be present within the tumour microenvironment in vivo. Thus CD44 may present an important determinant of breast cancer progression in the setting of endocrine resistance
Trastuzumab emtansine: mechanisms of action and drug resistance
Trastuzumab emtansine (T-DM1) is an antibody-drug conjugate that is effective and generally well tolerated when administered as a single agent to treat advanced breast cancer. Efficacy has now been demonstrated in randomized trials as first line, second line, and later than the second line treatment of advanced breast cancer. T-DM1 is currently being evaluated as adjuvant treatment for early breast cancer. It has several mechanisms of action consisting of the anti-tumor effects of trastuzumab and those of DM1, a cytotoxic anti-microtubule agent released within the target cells upon degradation of the human epidermal growth factor receptor-2 (HER2)-T-DM1 complex in lysosomes. The cytotoxic effect of T-DM1 likely varies depending on the intracellular concentration of DM1 accumulated in cancer cells, high intracellular levels resulting in rapid apoptosis, somewhat lower levels in impaired cellular trafficking and mitotic catastrophe, while the lowest levels lead to poor response to T-DM1. Primary resistance of HER2-positive metastatic breast cancer to T-DM1 appears to be relatively infrequent, but most patients treated with T-DM1 develop acquired drug resistance. The mechanisms of resistance are incompletely understood, but mechanisms limiting the binding of trastuzumab to cancer cells may be involved. The cytotoxic effect of T-DM1 may be impaired by inefficient internalization or enhanced recycling of the HER2-T-DM1 complex in cancer cells, or impaired lysosomal degradation of trastuzumab or intracellular trafficking of HER2. The effect of T-DM1 may also be compromised by multidrug resistance proteins that pump DM1 out of cancer cells. In this review we discuss the mechanism of action of T-DM1 and the key clinical results obtained with it, the combinations of T-DM1 with other cytotoxic agents and anti-HER drugs, and the potential resistance mechanisms and the strategies to overcome resistance to T-DM1.BioMed Central open acces
MONONUCLEAR OR POLYNUCLEAR SURFACE SPECIES - MODELS AND TENDENCIES WITH COBALT AND RHODIUM
Models of surface reactions of cobalt and rhodium carbonyls with oxide or thiolate type supports were prepared and characterized. For Co carbonyls it was found that simple noncarbonyl ligands that can be formed in reductive CO reactions may promote clusterification under conditions where the support alone causes declusterification. Model complexes of the interaction of rhodium with silica and aromatic thiolate supports were structurally characterized by X-ray diffraction. Model compounds indicate that organic thiolate supports promote clusterification with cobalt and declusterification with rhodium
NICKEL-COMPLEXES WITH SULFUR-CONTAINING LIGANDS AS INITIATORS OF THE POLYMERIZATION OF PHENYLACETYLENE
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