4,132 research outputs found
Effect of Samarium doping on the nucleation of fcc-Aluminum in undercooled liquids
The effect of Sm doping on the fcc-Al nucleation was investigated in Al-Sm
liquids with low Sm concentrations (xSm) with molecular dynamics simulations.
The nucleation in the moderately undercooled liquid is achieved by the recently
developed persistent-embryo method. Systematically computing the nucleation
rate with different xSm (xSm=0%, 1%, 2%, 3%, 5%) at 700 K, we found Sm dopant
reduces the nucleation rate by up to 25 orders of magnitudes with only 5%
doping concentration. This effect is mostly associated with the increase in the
free energy barrier with a minor contribution from suppression of the
attachment to the nucleus caused by Sm doping.Comment: 4 figure
Overcoming the Time Limitation in Molecular Dynamics Simulation of Crystal Nucleation: A Persistent-Embryo Approach
The crystal nucleation from liquid in most cases is too rare to be accessed within the limited time scales of the conventional molecular dynamics (MD) simulation. Here, we developed a âpersistent embryoâ method to facilitate crystal nucleation in MD simulations by preventing small crystal embryos from melting using external spring forces. We applied this method to the pure Ni case for a moderate undercooling where no nucleation can be observed in the conventional MD simulation, and obtained nucleation rate in good agreement with the experimental data. Moreover, the method is applied to simulate an even more sluggish event: the nucleation of the B2 phase in a strong glass-forming Cu-Zr alloy. The nucleation rate was found to be 8 orders of magnitude smaller than Ni at the same undercooling, which well explains the good glass formability of the alloy. Thus, our work opens a new avenue to study solidification under realistic experimental conditions via atomistic computer simulation
Phase diagram and neutron spin resonance of superconducting NaFe1âxCuxAs
We use transport and neutron scattering to study the electronic phase diagram and spin excitations of NaFe1âxCuxAs single crystals. Similar to Co- and Ni-doped NaFeAs, a bulk superconducting phase appears near xâ2% with the suppression of stripe-type magnetic order in NaFeAs. Upon further increasing Cu concentration the system becomes insulating, culminating in an antiferromagnetically ordered insulating phase near xâ50%. Using transport measurements, we demonstrate that the resistivity in NaFe1âxCuxAs exhibits non-Fermi-liquid behavior near xâ1.8%. Our inelastic neutron scattering experiments reveal a single neutron spin resonance mode exhibiting weak dispersion along c axis in NaFe0.98Cu0.02As. The resonance is high in energy relative to the superconducting transition temperature Tc but weak in intensity, likely resulting from impurity effects. These results are similar to other iron pnictides superconductors despite that the superconducting phase in NaFe1âxCuxAs is continuously connected to an antiferromagnetically ordered insulating phase near xâ50% with significant electronic correlations. Therefore, electron correlations is an important ingredient of superconductivity in NaFe1âxCuxAs and other iron pnictides
Trends and challenges of multi-drug resistance in childhood tuberculosis
Drug-resistant tuberculosis (DR-TB) in children is a growing global health concern, This review provides an overview of the current epidemiology of childhood TB and DR-TB, including prevalence, incidence, and mortality. We discuss the challenges in diagnosing TB and DR-TB in children and the limitations of current diagnostic tools. We summarize the challenges associated with treating multi-drug resistance TB in childhood, including limitations of current treatment options, drug adverse effects, prolonged regimens, and managing and monitoring during treatment. We highlight the urgent need for improved diagnosis and treatment of DR-TB in children. The treatment of children with multidrug-resistant tuberculosis will be expanded to include the evaluation of new drugs or new combinations of drugs. Basic research is needed to support the technological development of biomarkers to assess the phase of therapy, as well as the urgent need for improved diagnostic and treatment options
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