5,114 research outputs found
Magnetic circular dichroism from the impurity band in III-V diluted magnetic semiconductors
The magnetic circular dichroism of III-V diluted magnetic semiconductors,
calculated within a theoretical framework suitable for highly disordered
materials, is shown to be dominated by optical transitions between the bulk
bands and an impurity band formed from magnetic dopant states. The theoretical
framework incorporates real-space Green's functions to properly incorporate
spatial correlations in the disordered conduction band and valence band
electronic structure, and includes extended and localized electronic states on
an equal basis. Our findings reconcile unusual trends in the experimental
magnetic circular dichroism in III-V DMSs with the antiferromagnetic p-d
exchange interaction between a magnetic dopant spin and its host.Comment: 5 pages, 4 figure
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Gas-Phase Complexes of Americium and Lanthanides with a Bis-triazinyl Pyridine: Reactivity and Bonding of Archetypes for F-Element Separations.
Bis-triazinyl pyridines (BTPs) exhibit solution selectivity for trivalent americium over lanthanides (Ln), the origins of which remain uncertain. Here, electrospray ionization was used to generate gas-phase complexes [ML3]3+, where M = La, Lu, or Am and L is EtBTP 2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)-pyridine. Collision-induced dissociation (CID) of [ML3]3+ in the presence of H2O yielded a protonated ligand [L(H)]+ and hydroxide [ML2(OH)]2+ or hydrate [ML(L-H)(H2O)]2+, where (L-H)- is a deprotonated ligand. Although solution affinities indicate stronger binding of BTPs toward Am3+ versus Ln3+, the observed CID process is contrastingly more facile for M = Am versus Ln. To understand the disparity, density functional theory was employed to compute potential energy surfaces for two possible CID processes, for M = La and Am. In accordance with the CID results, both the rate determining transition state barrier and the net energy are lower for [AmL3]3+ versus [LaL3]3+ and for both product isomers, [ML2(OH)]2+ and [ML(L-H)(H2O)]2+. More facile removal of a ligand from [AmL3]3+ by CID does not necessarily contradict stronger Am3+-L binding, as inferred from solution behavior. In particular, the formation of new bonds in the products can distort kinetics and thermodynamics expected for simple bond cleavage reactions. In addition to correctly predicting the seemingly anomalous CID behavior, the computational results indicate greater participation of Am 5f versus La 4f orbitals in metal-ligand bonding
Adiabatic and non-adiabatic perturbations for loop quantum cosmology
We generalize the perturbations theory of loop quantum cosmology to a
hydrodynamical form and define an effective curvature perturbation on an
uniform density hypersurfaces . As in the classical cosmology,
should be gauge-invariant and conservation on the large scales. The
evolutions of both the adiabatic and the non-adiabatic perturbations for a
multi-fluids model are investigated in the framework of the effective
hydrodynamical theory of loop quantum cosmology with the inverse triad
correction. We find that, different from the classical cosmology, the evolution
of the large-scales non-adiabatic entropy perturbation can be driven by an
adiabatic curvature perturbation and this adiabatic source for the
non-adiabatic perturbation is a quantum effect. As an application of the
related formalism, we study a decay model and give out the numerical results.Comment: 10 pages, 3 figure
Imaginary polarization as a way to surmount the sign problem in ab initio calculations of spin-imbalanced Fermi gases
From ultracold atoms to quantum chromodynamics, reliable ab initio studies of
strongly interacting fermions require numerical methods, typically in some form
of quantum Monte Carlo calculation. Unfortunately, (non)relativistic systems at
finite density (spin polarization) generally have a sign problem, such that
those ab initio calculations are impractical. It is well-known, however, that
in the relativistic case imaginary chemical potentials solve this problem,
assuming the data can be analytically continued to the real axis. Is this
feasible for nonrelativistic systems? Are the interesting features of the phase
diagram accessible in this manner? By introducing complex chemical potentials,
for real total particle number and imaginary polarization, the sign problem is
avoided in the nonrelativistic case. To give a first answer to the above
questions, we perform a mean-field study of the finite-temperature phase
diagram of spin-1/2 fermions with imaginary polarization.Comment: 5 pages, 2 figures; published versio
Quantum interference induced by multiple Landau-Zener transitions in a strongly driven rf-SQUID qubit
We irradiated an rf-SQUID qubit with large-amplitude and high frequency
electromagnetic field. Population transitions between macroscopic distinctive
quantum states due to Landau-Zener transitions at energy-level avoided
crossings were observed. The qubit population on the excited states as a
function of flux detuning and microwave power exhibits interference patterns.
Some novel features are found in the interference and a model based on rate
equations can well address the features.Comment: 6 pages, 3 figures, comments are welcom
Band Narrowing and Mott Localization in Iron Oxychalcogenides La2O2Fe2O(Se,S)2
Bad metal properties have motivated a description of the parent iron
pnictides as correlated metals on the verge of Mott localization. What has been
unclear is whether interactions can push these and related compounds to the
Mott insulating side of the phase diagram. Here we consider the iron
oxychalcogenides La2O2Fe2O(Se,S)2, which contain an Fe square lattice with an
expanded unit cell. We show theoretically that they contain enhanced
correlation effects through band narrowing compared to LaOFeAs, and we provide
experimental evidence that they are Mott insulators with moderate charge gaps.
We also discuss the magnetic properties in terms of a Heisenberg model with
frustrating J1-J2-J2' exchange interactions on a "doubled" checkerboard
lattice.Comment: 4 pages, 5 eps figures. Version to appear in Phys. Rev. Let
Differential and Joint Effects of Metformin and Statins on Overall Survival of Elderly Patients with Pancreatic Adenocarcinoma: A Large Population-Based Study.
Background: Published evidence indicates that individual use of metformin and statin is associated with reduced cancer mortality. However, their differential and joint effects on pancreatic cancer survival are inconclusive.Methods: We identified a large population-based cohort of 12,572 patients ages 65 years or older with primary pancreatic ductal adenocarcinoma (PDAC) diagnosed between 2008 and 2011 from the Surveillance, Epidemiology, and End Results (SEER)-Medicare-linked database. Exposure to metformin and statins was ascertained from Medicare Prescription Drug Event files. Cox proportional hazards models with time-varying covariates adjusted for propensity scores were used to assess the association while controlling for potential confounders.Results: Of 12,572 PDAC patients, 950 (7.56%) had used metformin alone, 4,506 (35.84%) had used statin alone, and 2,445 (19.45%) were dual users. Statin use was significantly associated with improved overall survival [HR, 0.94; 95% confidence interval (CI), 0.90-0.98], and survival was more pronounced in postdiagnosis statin users (HR, 0.69; 95% CI, 0.56-0.86). Metformin use was not significantly associated with overall survival (HR, 1.01; 95% CI, 0.94-1.09). No beneficial effect was observed for dual users (HR, 1.00; 95% CI, 0.95-1.05).Conclusions: Our findings suggest potential benefits of statins on improving survival among elderly PDAC patients; further prospective studies are warranted to corroborate the putative benefit of statin therapy in pancreatic cancer.Impact: Although more studies are needed to confirm our findings, our data add to the body of evidence on potential anticancer effects of statins. Cancer Epidemiol Biomarkers Prev; 26(8); 1225-32. ©2017 AACR
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