152 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
Tailorable polymer waveguides for miniaturized bio-photonic devices via two-polymer microtransfer molding
Traditional optical fibers have been developed to achieve novel characteristics for both macro- and micro-applications. Inorganic optical waveguides using two-dimensional photonic crystals and silicon-on-insulator technology are examples of recent trends for macro- and micro-scale optical applications, respectively. As bio-photonics devices operate mostly with visible light, visible-transparent materials such as metal oxides and polymers are preferred as the guiding medium. Although polymers have tremendous potential because of their enormous variation in optical, chemical and mechanical properties, their application for optical waveguides is limited by conventional lithography. We present a non-optical lithographic technique, called two-polymer microtransfer molding, to fabricate polymer nano-waveguides, on-chip light sources and couplers. Micro-sources using quantum dots emitting red light (625nm) are successfully embedded in a waveguides array as the on-chip light sources. Fabrication of a grating coupler is also attempted for various external light sources including lasers and white light. We have quantified propagation losses of the waveguides using CCD photometry. The guiding loss is approximately 1.7dB/mm. We also demonstrated that the surface roughness of the fabricated waveguides can be reduced by chemical etching. We demonstrate that low cost, high yield, high fidelity, and tailorable fabrication of bio-photonic devices are achievable by the combination of the presented techniques
Teleportation for atomic entangled state by entanglement swapping with separate measurements in cavity QED
Experimentally feasible scheme for teleportation of atomic entangled state
via entanglement swapping is proposed in cavity quantum electrodynamics (QED)
without joint Bell-state measurement (BSM). In the teleportation processes the
interaction between atoms and a single-mode nonresonant cavity with the
assistance of a strong classical driving field substitute the joint
measurements. The discussion of the scheme indicates that it can be realized by
current technologies.Comment: 5 pages, no figur
Photonic crystal: energy-related applications
We review recent work on photonic-crystal fabrication using soft-lithography techniques. We consider applications of the resulting structures in energy-related areas such as lighting and solar-energy harvesting. In general, our aim is to introduce the reader to the concepts of photonic crystals, describe their history, development, and fabrication techniques and discuss a selection of energy-related applications
A self-contained algorithm for determination of solid-liquid equilibria in an alloy system
We describe a self-contained procedure to evaluate the free energy of liquid
and solid phases of an alloy system. The free energy of a single-element solid
phase is calculated with thermodynamic integration using the Einstein crystal
as the reference system. Then, free energy difference between the solid and
liquid phases is calculated by Gibbs-Duhem integration. The central part of our
method is the construction of a reversible alchemical path connecting a pure
liquid and a liquid alloy to calculate the mixing enthalpy and entropy. We have
applied the method to calculate the free energy of solid and liquid phases in
the Al-Sm system. The driving force for fcc-Al nucleation in Al-Sm liquid and
the melting curve for fcc-Al and Al3Sm are also calculated.Comment: 15 pages, 7 figures, submitting to a journa
A comparative study of Sm networks in Al-10 at.%Sm glass and associated crystalline phases
The Al–Sm system is selected as a model system to study the transition process from liquid and amorphous to crystalline states. In recent work, we have shown that, in addition to long-range translational periodicity, crystal structures display well-defined short-range local atomic packing motifs that transcends liquid, amorphous and crystalline states. In this paper, we investigate the longer range spatial packing of these short-range motifs by studying the interconnections of Sm–Sm networks in different amorphous and crystalline samples obtained from molecular dynamics simulations. In our analysis, we concentrate on Sm–Sm distances in the range ~5.0–7.2 Å, corresponding to Sm atoms in the second and third shells of Sm-centred clusters. We discover a number of empirical rules characterising the evolution of Sm networks from the liquid and amorphous states to associated metastable crystalline phases experimentally observed in the initial stages of devitrification of different amorphous samples. As direct simulation of glass formation is difficult because of the vast difference between experimental quench rates and what is achievable on the computer, we hope these rules will be helpful in building a better picture of structural evolution during glass formation as well as a more detailed description of phase selection and growth during devitrification
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 “persistent embryo” 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
First-principles calculation of correlated electron materials based on Gutzwiller wave function beyond Gutzwiller approximation
We propose an approach that is under the framework of Gutzwiller wave function but goes beyond the commonly adopted Gutzwiller approximation to improve the accuracy and flexibility in treating the correlation effects. Detailed formalism is described for a dimer which is straightforwardly generalized later to more complicated periodic bulk systems. The accuracy of the approach is demonstrated by evaluating the potential energy curves of spin-singlet N2 dimer, spin-triplet O2 dimer, and 1D hydrogen chain. The computational workload of the approach can be easily handled by efficient parallel computing
- …