Biomedical subjects

M Asta

Publications and source records attributed to M Asta.

At least 19 recordsLinked to original sources

PubMed2006

Equilibrium adsorption at crystal-melt interfaces in Lennard-Jones alloys.

Although the properties of crystal-melt interfaces have been extensively studied in pure materials, effects of alloying on the interfacial free energy remain relatively poorly understood. In this work we make use of Monte Carlo computer simulations for model binary Lennard-Jones alloys to explore the effects which variations in atomic-size mismatch and the chemical contributions to mixing energies have upon density and composition profiles, as well as the resulting magnitudes of equilibrium adsorption coefficients in concentrated alloys. We study four different model systems covering a range of chemical and size mismatch, finding relatively small adsorption values which are nevertheless statistically different from zero.

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PubMed2006

Orientation dependence of strained-Ge surface energies near (001): role of dimer-vacancy lines and their interactions with steps.

Recent experiments and calculations have highlighted the important role of surface-energy (gamma) anisotropy in governing island formation in the Ge/Si(001) system. To further elucidate the factors determining this anisotropy, we perform atomistic and continuum calculations of the orientation dependence of gamma for strained-Ge surfaces near (001), accounting for the presence of dimer-vacancy lines (DVLs). The net effect of DVLs is found to be a substantial reduction in the magnitude of the slope of gamma vs orientation angle, relative to the highly negative value derived for non-DVL, dimer-reconstructed, strained-Ge(001) surfaces. The present results thus point to an important role of DVLs in stabilizing the (001) surface orientation of a strained-Ge wetting layer.

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PubMed2005

Competition between order and phase separation in Au-Ni.

We have measured and theoretically analyzed the diffuse scattering in the binary alloy system Au-Ni, which has been proposed as a testing ground for theories of alloy phase stability. We found strong evidence that in the alloys Au3Ni and Au3Ni2, fluctuations of both ordering- and clustering-type are competing with each other. Our results resolve a long-standing controversy on the balance of relaxation and mixing energies in this alloy system and explain recent findings of ordering in thin Au-Ni films.

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PubMed2005

Role of strain-dependent surface energies in Ge/Si(100) island formation.

Formation energies for Ge/Si(100) pyramidal islands are computed combining continuum calculations of strain energy with first-principles-computed strain-dependent surface energies. The strain dependence of surface energy is critically impacted by the presence of strain-induced changes in the Ge {100} surface reconstruction. The appreciable strain dependencies of rebonded-step {105} and dimer-vacancy-line-reconstructed {100} surface energies are estimated to give rise to a significant reduction in the surface contribution to island formation energies.

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PubMed2003

Mg segregation at Al/Al3Sc heterophase interfaces on an atomic scale: experiments and computations.

Microscopic factors governing solute partitioning in ternary two-phase Al-Sc-Mg alloys are investigated combining three-dimensional-atom-probe (3DAP) microscopy measurements with first-principles computations. 3DAP is employed to measure composition profiles with subnanometer-scale resolution, leading to the identification of a large enhancement of Mg solute at the coherent alpha-Al/Al(3)Sc (fcc/L1(2)) heterophase interface. First-principles calculations establish an equilibrium driving force for this interfacial segregation reflecting the nature of the interatomic interactions.

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PubMed2002

"Devil's staircases" in bulk-immiscible ultrathin alloy films.

Ground-state phase diagrams of ultrathin epitaxial alloy films are studied within the framework of a discrete lattice-model Hamiltonian incorporating competing elastic and chemical interactions. For bulk-immiscible alloy systems an infinite number of commensurate, long-period stripe-superstructure ground states are obtained as a function of chemical potential. The average periodicity of these stripe superstructures is found to be a nonmonotonic function of alloy composition, in contrast to the predictions of continuum theories for two-dimensional systems with competing interactions.

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PubMed2002

Elastic relaxations in ultrathin epitaxial alloy films.

Elastic interactions responsible for the stability of nanometer-scale patterns in ultrathin, bulk-immiscible-alloy films are analyzed within the context of a hybrid atomistic-continuum model. Two apparently different descriptions of alloy film behavior, a continuum elasticity theory describing a deformable substrate and a rigid substrate atomistic scheme, emerge naturally as limiting cases on long and short length scales, respectively. Quantitative first-principles calculations explain the origin of recently observed nanoscale patterns in Co-Ag/Ru(0001), and reveal a surprising failure of the continuum model.

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PubMed2001

Method for computing the anisotropy of the solid-liquid interfacial free energy.

We present a method to compute accurately the weak anisotropy of the solid-liquid interfacial free energy, a parameter which influences dendritic evolution in materials with atomically rough interfaces. The method is based on monitoring interfacial fluctuations during molecular dynamics simulation and extracting the interfacial stiffness which is an order of magnitude more anisotropic than the interfacial free energy. We present results for pure Ni with interatomic potentials derived from the embedded atom method.

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PubMed2001

Role of stress in thin film alloy thermodynamics: competition between alloying and dislocation formation.

Using scanning tunneling microscopy (STM) and first-principles local-spin-density-approximation calculations to study submonolayer films of Co (1-c)Ag (c)/Ru(0001) alloys, we have discovered a novel phase-separation mechanism. When the Ag concentration c exceeds 0.4, the surface phase separates between a dislocated, pure Ag phase and a pseudomorphically strained Co(0.6)Ag (0.4) surface alloy. We attribute the phase separation to the competition between two stress relief mechanisms: surface alloying and dislocation formation. The agreement between STM measurements and our calculated phase diagram supports this interpretation.

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PubMed2001

Large vibrational effects upon calculated phase boundaries in Al-Sc.

The fcc portion of the Al-Sc phase diagram is calculated from first principles including contributions to alloy free energies associated with ionic vibrations. It is found that vibrational entropy accounts for a 27-fold increase in the calculated solubility limits for Sc in fcc Al at high temperatures, bringing calculated and measured values into very good agreement. The present work gives a clear example demonstrating a large effect of vibrational entropy upon calculated phase boundaries in substitutional alloys.

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PubMed2000

Test of the universal scaling law for the diffusion coefficient in liquid metals.

The recently proposed scaling law relating the diffusion coefficient and the excess entropy of a liquid [M. Dzugutov, Nature (London) 381, 137 (1996)] is tested for several metals using molecular dynamics simulations. Interatomic potentials derived from the embedded atom method are used to study Ag, Au, Cu, Ni, Pd, Pt, Ni(3)Al, and AuPt and the angular dependent Stillinger-Weber form is used to investigate Si.

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PubMed1996