arXiv subjects

Michael J. Mehl

Publications and source records attributed to Michael J. Mehl.

26 records · Page 2Linked to original sources

arXiv2010

Density Functional Study of the L1_0 - alpha-IrV Transition in IrV and RhV

Both IrV and RhV crystallize in the alpha-IrV structure, with a transition to the higher symmetry L1_0 structure at high temperature, or with the addition of excess Ir or Rh. Here we present evidence that this transition is driven by the lowering of the electronic density of states at the Fermi level of the alpha-IrV structure. The transition has long been thought to be second order, with a simple doubling of the L1_0 unit cell due to an unstable phonon at the R point (0 1/2 1/2). We use first-principles calculations to show that all phonons at the R point are, in fact, stable, but do find a region of reciprocal space where the L1_0 structure has unstable (imaginary frequency) phonons. We use the frozen phonon method to examine two of these modes, relaxing the structures associated with the unstable phonon modes to obtain new structures which are lower in energy than L1_0 but still above alpha-IrV. We examine the phonon spectra of these structures as well, looking for instabilities, and find further instabilities, and more relaxed structures, all of which have energies above the alpha-IrV phase. In addition, we find that all of the relaxed structures, stable and unstable, have a density comparable to the L1_0 phase (and less than the alpha-IrV phase), so that any transition from one of these structures to the ground state will have a volume change as well as an energy discontinuity. We conclude that the transition from L1_0 to alpha-IrV is probably weakly first order.

cond-mat.mtrl-sci↗

arXiv2004

Absence of Metastable States in Strained Monatomic Cubic Crystals

A tetragonal (Bain path) distortion of a metal with an fcc (bcc) ground state will initially cause an increase in energy, but at some point along the Bain path the energy will again decrease until a local minimum is reached. Using a combination of parametrized tight-binding and first-principles LAPW calculations we show that this local minimum is unstable with respect to an elastic distortion, except in the rare case that the minimum is at the bcc (fcc) point on the Bain path. This shows that body-centered tetragonal phases of these materials, which have been seen in epitaxially grown thin films, must be stabilized by the substrate and cannot be free-standing films.

cond-mat.mtrl-sci↗

arXiv2003

Superconductivity in Compressed Potassium and Rubidium

Calculations of the electron-phonon interaction in the alkali metals, Potassium and Rubidium, using the results of band theory and BCS theory-based techniques suggest that at high pressures K and Rb would be superconductors with transition temperatures approaching 10 K.

cond-mat.supr-con↗

arXiv1999

Ab Initio Calculation of Spin Gap Behavior in CaV4O9

Second neighbor dominated exchange coupling in CaV4O9 has been obtained from ab initio density functional (DF) calculations. A DF-based self-consistent atomic deformation model reveals that the nearest neighbor coupling is small due to strong cancellation among the various superexchange processes. Exact diagonalization of the predicted Heisenberg model yields spin-gap behavior in good agreement with experiment. The model is refined by fitting to the experimental susceptibility. The resulting model agrees very well with the experimental susceptibility and triplet dispersion.

cond-mat.mtrl-sci↗

arXiv1996

Applications of a tight-binding total energy method for transition and noble metals: Elastic Constants, Vacancies, and Surfaces of Monatomic Metals

A recent tight-binding scheme provides a method for extending the results of first principles calculations to regimes involving $10^2 - 10^3$ atoms in a unit cell. The method uses an analytic set of two-center, non-orthogonal tight-binding parameters, on-site terms which change with the local environment, and no pair potential. The free parameters in this method are chosen to simultaneously fit band structures and total energies from a set of first-principles calculations for monatomic fcc and bcc crystals. To check the accuracy of this method we evaluate structural energy differences, elastic constants, vacancy formation energies, and surface energies, comparing to first-principles calculations and experiment. In most cases there is good agreement between this theory and experiment. We present a detailed account of the method, a complete set of tight-binding parameters, and results for twenty-nine of the alkaline earth, transition and noble metals.

mtrl-th↗

arXiv1995

First-Principles Calculation of the Structure of Mercury

Mercury has perhaps the strangest behavior of any of the metals. Although the other metals in column IIB have an $hcp$ ground state, mercury's ground state is the body centered tetragonal $β$Hg phase. The most common phase of mercury is the rhombohedral $α$Hg phase, which is stable from 79K to the melting point and meta-stable below 79K. Another rhombohedral phase, $γ$Hg, is believed to exist at low temperatures. First-principles calculations are used to study the energetics of the various phases of mercury. Even when partial spin-orbit effects are included, the calculations indicate that the hexagonal close packed structure is the ground state. It is suggested that a better treatment of the spin-orbit interaction might alter this result.

mtrl-th↗

arXiv1995

Application of a new Tight-Binding method for transition metals: Manganese

A new tight-binding total energy method, which has been shown to accurately predict ground state properties of transition and noble metals, is applied to Manganese, the element with the most complex ground state structure among the $d$ metals. We show that the tight-binding method correctly predicts the ground state structure of Mn, and offers some insight into the magnetic properties of this state.

mtrl-th↗

arXiv1992

Pressure Dependence of the Elastic Moduli in Aluminum Rich Al-Li Compounds

I have carried out numerical first principles calculations of the pressure dependence of the elastic moduli for several ordered structures in the Aluminum-Lithium system, specifically FCC Al, FCC and BCC Li, L1_2 Al_3Li, and an ordered FCC Al_7Li supercell. The calculations were performed using the full potential linear augmented plane wave method (LAPW) to calculate the total energy as a function of strain, after which the data was fit to a polynomial function of the strain to determine the modulus. A procedure for estimating the errors in this process is also given. The predicted equilibrium lattice parameters are slightly smaller than found experimentally, consistent with other LDA calculations. The computed elastic moduli are within approximately 10% of the experimentally measured moduli, provided the calculations are carried out at the experimental lattice constant. The LDA equilibrium shear modulus C11-C12 increases from 59.3 GPa in Al, to 76.0 GPa in Al_7Li, to 106.2 GPa in Al_3Li. The modulus C_44 increases from 38.4 GPa in Al to 46.1 GPa in Al_7Li, then falls to 40.7 GPa in Al_3Li. All of the calculated elastic moduli increase with pressure with the exception of BCC Li, which becomes elastically unstable at about 2 GPa, where C_11-C_12 vanishes.

cond-mat↗