Biomedical subjects

S I Simak

Publications and source records attributed to S I Simak.

7 recordsLinked to original sources

PubMed2004

High-pressure melting of molybdenum.

The melting curve of the body-centered cubic (bcc) phase of Mo has been determined for a wide pressure range using both direct ab initio molecular dynamics simulations of melting as well as a phenomenological theory of melting. These two methods show very good agreement. The simulations are based on density functional theory within the generalized gradient approximation. Our calculated equation of state of bcc Mo is in excellent agreement with experimental data. However, our melting curve is substantially higher than the one determined in diamond anvil cell experiments up to a pressure of 100 GPa. An explanation is suggested for this discrepancy.

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PubMed2004

Fluctuating lattice constants of indium under high pressure.

Recent high-pressure investigations of elemental In have yielded controversial results. We show that the observed high-pressure face-centered orthorhombic (fco) structure can be explained as an intermediate state between two body-centered tetragonal (bct) structures with different c/a ratios (c/a < square root [2] and c/a > square root [2], respectively). In a pressure range from about 50 to 200 GPa these two bct structures correspond to local minima of the total energy with respect to orthorhombic distortion of the ground-state bct In structure. The fco saddle point represents a tiny barrier and even at low temperatures rapid structural fluctuations should occur. Such a situation has not been identified in any other elemental metal.

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PubMed2003

Origin of magnetic anisotropy of Gd metal.

Using first-principles theory, we have calculated the energy of Gd as a function of spin direction, theta, between the c and a axes and found good agreement with experiment for both the total magnetic anisotropy energy and its angular dependence. The calculated low temperature direction of the magnetic moment lies at an angle of 20 degrees to the c axis. The calculated magnetic anisotropy energy of Gd metal is due to a unique mechanism involving a contribution of 7.5 microeV from the classical dipole-dipole interaction between spins plus a contribution of 16 microeV due to the spin-orbit interaction of the conduction electrons. The 4f spin polarizes the conduction electrons via exchange interaction, which transfers the magnetic anisotropy of the conduction electrons to the 4f spin.

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PubMed2003

Van der Waals density functional for layered structures.

To understand sparse systems, we must account for both strong local atom bonds and weak nonlocal van der Waals forces between atoms separated by empty space. A fully nonlocal functional form [Phys. Rev. B 62, 6997 (2000)]] of density-functional theory (DFT) is applied here to the layered systems graphite, boron nitride, and molybdenum sulfide to compute bond lengths, binding energies, and compressibilities. These key examples show that the DFT with the generalized-gradient approximation does not apply for calculating properties of sparse matter, while use of the fully nonlocal version appears to be one way to proceed.

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PubMed2003

Metal-nonmetal transition in the boron group elements.

Structural competition in boron group elements has been studied by means of ab initio calculations. For boron we predict a structural change alpha-B-->alpha-Ga accompanied by a nonmetal-metal transition at a pressure of about 74 GPa. For Al and Ga we find an icosahedron based elemental modification (alpha-B) 0.22 and 0.05 eV/atom, respectively, higher in energy than the corresponding metallic ground state structures. In particular, the low energy difference for Ga raises expectations into the experimental feasibility of new modifications for these elements, especially in nanosized systems.

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PubMed2002

Quantum origin of the oxygen storage capability of ceria.

The microscopic mechanism behind the extraordinary ability of ceria to store, release, and transport oxygen is explained on the basis of first-principles quantum mechanical simulations. The oxygen-vacancy formation energy in ceria is calculated for different local environments. The reversible CeO2-Ce2O3 reduction transition associated with oxygen-vacancy formation and migration is shown to be directly coupled with the quantum process of electron localization.

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PubMed2001

From V8Ga36.9Zn4.1 and Cr8Ga29.8Zn11.2 to Mn8Ga27.4Zn13.6: a remarkable onset of Zn-cluster formation in an intermetallic framework.

The series of isotypic compounds V8Ga41 --> V8Ga36.9Zn4.1 --> Cr8Ga29.5Zn11.2 --> Mn8Ga27.4Zn13.6 with the V8Ga41 structure type (space group R3, Z = 3) was prepared and structurally characterised by X-ray diffraction experiments (V8Ga41: a 13.9351(5), 14.8828(12); V8Ga36.9Zn4.1: a = 13.9244(7), c = 14.8660(9): Cr8Ga29.8Zn11.2: 13.7153(5), c = 14.6872(9); Mn8Ga27.4Zn13.6: a = 13.6033(6), c = 14.6058(16)). The site occupancies of the ternary compounds were refined from neutron powder-diffraction data and exposed a startling segregation of Zn and Ga, which finally resulted in the formation of separated Zn13 cluster entities-corresponding to almost ideal centred cuboctahedra or small pieces of fcc metal-in the Mn compound, which has the highest Zn content in the series. The homogeneity ranges of the underlying phases T8Ga41 xZnx were determined to be 0 < x < 4.1(3), 8.7(3) < x < 11.2(3) and 13.6(4) < x < 16.5(3) for T = V, Cr and Mn, respectively. The different ranges of composition of the phases reflect the requirement of an optimum electron concentration for a stable V8Ga41-type structure, which is in the narrow range between 159 and 165 electrons per formula unit. First-principles electronic-structure calculations could explain this fact by the occurrence of a pseudo gap in the density of states at which the Fermi level is put for this particular electron concentration. Furthermore the nature of the Zn/Ga segregation was revealed: T-Zn interactions were found to be considerably weaker than those for T-Ga. This places the Zn atoms as far as possible from the T atoms, thus leading to the formation of cuboctahedral Zn13 entities.

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