2,874 research outputs found
Activated O2 dissociation and formation of oxide islands on the Be(0001) surface: Another atomistic model for metal oxidation
By simulating the dissociation of O2 molecules on the Be(0001) surface using
the first-principles molecular dynamics approach, we propose a new atomistic
model for the surface oxidation of sp metals. In our model, only the
dissociation of the first oxygen molecule needs to overcome an energy barrier,
while the subsequent oxygen molecules dissociate barrierlessly around the
adsorption area. Consequently, oxide islands form on the metal surface, and
grow up in a lateral way. We also discover that the firstly dissociated oxygen
atoms are not so mobile on the Be(0001) surface, as on the Al(111) surface. Our
atomistic model enlarges the knowledge on metal surface oxidations by perfectly
explaining the initial stage during the surface oxidation of Be, and might be
applicable to some other sp metal surfaces.Comment: 5 pages, 4 figure
Probing the inter-layer exciton physics in a MoS/MoSe/MoS van der Waals heterostructure
Stacking atomic monolayers of semiconducting transition metal dichalcogenides
(TMDs) has emerged as an effective way to engineer their properties. In
principle, the staggered band alignment of TMD heterostructures should result
in the formation of inter-layer excitons with long lifetimes and robust valley
polarization. However, these features have been observed simultaneously only in
MoSe/WSe heterostructures. Here we report on the observation of long
lived inter-layer exciton emission in a MoS/MoSe/MoS trilayer van
der Waals heterostructure. The inter-layer nature of the observed transition is
confirmed by photoluminescence spectroscopy, as well as by analyzing the
temporal, excitation power and temperature dependence of the inter-layer
emission peak. The observed complex photoluminescence dynamics suggests the
presence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. We
show that circularly polarized optical pumping results in long lived valley
polarization of inter-layer exciton. Intriguingly, the inter-layer exciton
photoluminescence has helicity opposite to the excitation. Our results show
that through a careful choice of the TMDs forming the van der Waals
heterostructure it is possible to control the circular polarization of the
inter-layer exciton emission.Comment: 19 pages, 3 figures. Just accepted for publication in Nano Letters
(http://pubs.acs.org/doi/10.1021/acs.nanolett.7b03184
Thermodynamics and magnetic field profiles in low-kappa type-II superconductors
Two-dimensional low-kappa type-II superconductors are studied numerically
within the Eilenberger equations of superconductivity. Depending on the
Ginzburg-Landau parameter \kappa=\lambda/\xi vortex-vortex interaction can be
attractive or purely repulsive. The sign of interaction is manifested as a
first (second) order phase transition from Meissner to the mixed state.
Temperature and field dependence of the magnetic field distribution in
low-kappa type-II superconductors with attractive intervortex interaction is
calculated. Theoretical results are compared to the experiment.Comment: 4 pages, 3 figure
Dependence of the flux creep activation energy on current density and magnetic field for MgB2 superconductor
Systematic ac susceptibility measurements have been performed on a MgB
bulk sample. We demonstrate that the flux creep activation energy is a
nonlinear function of the current density , indicating a
nonlogarithmic relaxation of the current density in this material. The
dependence of the activation energy on the magnetic field is determined to be a
power law , showing a steep decline in the activation
energy with the magnetic field, which accounts for the steep drop in the
critical current density with magnetic field that is observed in MgB. The
irreversibility field is also found to be rather low, therefore, the pinning
properties of this new material will need to be enhanced for practical
applications.Comment: 11 pages, 6 figures, Revtex forma
Gravity as Backreaction
Quadratic theory of gravity is a complicated constraint system. We
investigate some consequences of treating quadratic terms perturbatively
(higher derivative version of backreaction effects). This approach is shown to
overcome some well known problems associated with higher derivative theories,
i.e., the physical gravitational degree of freedom remains unchanged from those
of Einstein gravity.
Using such an interpretation of gravity, we investigate a
classical and Wheeler DeWitt evolution of gravity for a
particular sign of , corresponding to non- tachyon case. Matter is
described by a phenomenological . It is concluded that
both the Friedmann potential () and the
Wheeler DeWitt potential () develop repulsive barriers near for
(i.e., ). The interpretations is clear. Repulsive
barrier in implies that a contracting FRW universe ()
will bounce to an expansion phase without a total gravitational collapse.
Repulsive barrier in means that is a classically forbidden
region. Therefore, probability of finding a universe with the big bang
singularity ( ) is exponentially suppressed.Comment: Accepted for publication in Phy. Rev. D.,18 pages, 6 figures, Latex
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Targeting Btk/Etk of prostate cancer cells by a novel dual inhibitor.
Btk and Etk/BMX are Tec-family non-receptor tyrosine kinases. Btk has previously been reported to be expressed primarily in B cells and has an important role in immune responses and B-cell malignancies. Etk has been shown previously to provide a strong survival and metastasis signal in human prostate cancer cells, and to confer androgen independence and drug resistance. While the role of Etk in prostate carcinogenesis is well established, the functions of Btk in prostate cancer have never been investigated, likely due to the perception that Btk is a hematopoietic, but not epithelial, kinase. Herein, we found that Btk is overexpressed in prostate cancer tissues and prostate cancer cells. The level of Btk in prostate cancer tissues correlates with cancer grades. Knockdown of Btk expression selectively inhibits the growth of prostate cancer cells, but not that of the normal prostate epithelial cells, which express very little Btk. Dual inhibition of Btk and Etk has an additive inhibitory effect on prostate cancer cell growth. To explore Btk and Etk as targets for prostate cancer, we developed a small molecule dual inhibitor of Btk and Etk, CTN06. Treatment of PC3 and other prostate cancer cells, but not immortalized prostate epithelial cells with CTN06 resulted in effective cell killing, accompanied by the attenuation of Btk/Etk signals. The killing effect of CTN06 is more potent than that of commonly used inhibitors against Src, Raf/VEGFR and EGFR. CTN06 induces apoptosis as well as autophagy in human prostate cancer cells, and is a chemo-sensitizer for docetaxel (DTX), a standard of care for metastatic prostate cancer patients. CTN06 also impeded the migration of human prostate cancer cells based on a 'wound healing' assay. The anti-cancer effect of CTN06 was further validated in vivo in a PC3 xenograft mouse model
Adsorption and dissociation of O at Be(0001): First-principles prediction of an energy barrier on the adiabatic potential energy surface
The adsorption and dissociation of O molecules at the Be(0001) surface
is studied by using density-functional theory within the generalized gradient
approximation and a supercell approach. The physi- and chemisorbed molecular
precursor states are identified to be along the parallel and vertical channels,
respectively. It is shown that the HH-Z (see the text for definition) channel
is the most stable channel for the molecular chemisorption of O. The
electronic and magnetic properties of this most stable chemisorbed molecular
state are studied, which shows that the electrons transfer forth and back
between the spin-resolved antibonding molecular orbitals and the
surface Be states. A distinct covalent weight in the molecule-metal bond
is also shown. The dissociation of O is determined by calculating the
adiabatic potential energy surfaces, wherein the T-Y channel is found to be the
most stable and favorable for the dissociative adsorption of O.
Remarkably, we predict that unlike the other simple metal surfaces such as
Al(111) and Mg(0001), the \textit{adiabatic} dissociation process of O at
Be(0001) is an activated type with a sizeable energy barrier.Comment: 21 pages, 10 figure
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