John E. Klepeis

First-principles elastic constants and electronic structure of α-Pt2Si and PtSi

We have carried out a first-principles study of the elastic properties and electronic structure for two roomtemperature stable Pt silicide phases, tetragonal a-Pt2Si, and orthorhombic PtSi. We have calculated all of the equilibrium structural parameters for both phases: the a and c lattice constants for a-Pt2Si and the a, b, and c lattice constants and four internal structural parameters for PtSi. These results agree closely with experimental data. We have also calculated the zero-pressure elastic constants, confirming prior results for pure Pt and Si and predicting values for the six ~nine! independent, nonzero elastic constants of a-Pt2Si ~PtSi!. These calculations include a full treatment of all relevant internal displacements induced by the elastic strains, including an explicit determination of the dimensionless internal displacement parameters for the three strains in a-Pt2Si for which they are nonzero. We have analyzed the trends in the calculated elastic constants, both within each material as well as among the two silicides and the pure Pt and Si phases. The calculated electronic structure confirms that the two silicides are poor metals with a low density of states at the Fermi level, and consequently we expect that the Drude component of the optical absorption will be much smaller than in good metals such as pure Pt. This observation, combined with the topology found in the first-principles spin-orbit split band structure, suggests that it may be important to include the interband contribution to the optical absorption, even in the infrared region.

Structure and stability of germanium nanoparticles

In order to tailor the properties of nanodots, it is essential to separate the effects of quantum confinement from those due to the surface, and to determine the mechanisms by which preparation conditions can influence the properties of the dot. We address these issues for the case of small Ge clusters (1--2.5 nm), using a combination of empirical and first-principles molecular-dynamics techniques. Our results show that over a wide temperature range, the diamond structure is more stable than tetragonal-like structures for clusters containing more than 40 atoms; however, the magnitude of the energy difference is strongly dependent on the structure and termination of the surface. On the basis of our calculations, we propose a possible mechanism for the formation of metastable tetragonal clusters observed in vapor deposition experiments, by cold quenching of amorphous nanoparticles exhibiting unsaturated, reconstructed surfaces.

Shear-induced anisotropic plastic flow from body-centred-cubic tantalum before melting

There are many structural and optical similarities between a liquid and a plastic flow. Thus, it is non-trivial to distinguish between them at high pressures and temperatures, and a detailed description of the transformation between these phenomena is crucial to our understanding of the melting of metals at high pressures. Here we report a shear-induced, partially disordered viscous plastic flow from body-centred-cubic tantalum under heating before it melts into a liquid. This thermally activated structural transformation produces a unique, one-dimensional structure analogous to a liquid crystal with the rheological characteristics of Bingham plastics. This mechanism is not specific to Ta and is expected to hold more generally for other metals. Remarkably, this transition is fully consistent with the previously reported anomalously low-temperature melting curve and thus offers a plausible resolution to a long-standing controversy about melting of metals under high pressures.

Fermi surface nesting and pre-martensitic softening in V and Nb at high pressures

First-principles total-energy calculations were performed for the trigonal shear elastic constant (C44) of body-centred cubic (bcc) V and Nb. A mechanical instability in C44 is found for V at pressures of ~2 Mbar which also shows a softening in Nb at pressures of ~0.5 Mbar. We argue that the pressure-induced shear instability (softening) of V (Nb) is due to the intra-band nesting of the Fermi surface.

Coverage dependence of Schottky barrier formation

When the metal coverage at the surface of a semiconductor is sufficiently small so that there are isolated metal adatoms and therefore localized states, there is a distinction between the energy (0,+) to remove an electron from a neutral adatom and the energy (−,0) to add an electron to a neutral adatom. The acceptor level (−,0) is higher in energy than the donor level (0,+) by an energy U*, just as in the free atom, but here U* is modified by the presence of the surface. For these isolated metal adatoms, the donor level (0,+) frequently lies in the semiconductor gap with the acceptor level (−,0) coming in the conduction band. This circumstance leads to an asymmetry between the low‐coverage band bending for n‐type semiconductors versus that for p‐type. As the number of adatoms is increased, the (0,+) and (−,0) levels shift relative to the semiconductor bands at the surface. These shifts can be understood in terms of the polar bonds between the metal adatoms and the semiconductor surface atoms. At sufficie...

Elastic constants and volume changes associated with two high-pressure rhombohedral phase transformations in vanadium

We present results from ab-initio electronic-structure calculations of mechanical properties of the rhombohedral phase of vanadium reported in recent experiments (R Ia), and other predicted high-pressure phases (R Ib and bcc), focusing on properties relevant to dynamic experiments. We find that of the three transitions the largest volume collapse (1.3%) is for the R Ia to R Ib transition. Calculations of the single crystal and polycrystal elastic constants reveal a remarkably small discontinuity across the phase transitions even at zero temperature where the transitions are first order.

Phonon and magnetic structure in δ-plutonium from density-functional theory

We present phonon properties of plutonium metal obtained from a combination of density-functional-theory (DFT) electronic structure and the recently developed compressive sensing lattice dynamics (CSLD). The CSLD model is here trained on DFT total energies of several hundreds of quasi-random atomic configurations for best possible accuracy of the phonon properties. The calculated phonon dispersions compare better with experiment than earlier results obtained from dynamical mean-field theory. The density-functional model of the electronic structure consists of disordered magnetic moments with all relativistic effects and explicit orbital-orbital correlations. The magnetic disorder is approximated in two ways: (i) a special quasi-random structure and (ii) the disordered-local-moment method within the coherent potential approximation. Magnetism in plutonium has been debated intensely, but the present magnetic approach for plutonium is validated by the close agreement between the predicted magnetic form factor and that of recent neutron-scattering experiments.

Multiphase improved Steinberg–Guinan model for vanadium

Vanadium has been observed recently to transform from the body-centered cubic (bcc) crystal structure to a rhombohedral structure at high pressure (∼0.69 Mbar) [Y. Ding et al., Phys. Rev. Lett. 98, 085502 (2007)]. Recent theoretical work predicts a transformation to a second rhombohedral phase at 1.2 Mbar before transforming back to the bcc structure at 2.8 Mbar at absolute zero temperature [B. Lee et al., Phys. Rev. B 75, 180101(R) (2007)]. Here we develop an analytic model for the shear modulus in these phases based on ab initio calculations of the single-crystal elastic moduli and a finite element based homogenization technique. The form of the shear modulus is suited to application in strength models such as in the Steinberg–Guinan form and other analogous continuum-level models.

Valence band study of the PtSi by synchrotron radiation photoelectron spectroscopy

Abstract The electronic structure of PtSi was measured using synchrotron radiation-based valence band (VB) photoelectron spectroscopy (PES). We have compared our experimental results to local density approximation (LDA) calculations and have found that, contrary to previously suggested electronic structure models of localized Pt d-states amidst Si s and p electrons, the electronic structure of the silicide is more complex with evidence of two- and three-center covalent bonds between Pt and Si. The agreement between theory and experiment is discussed within the context of bond order hybridization between platinum and silicon.

Electronic structure of small coverages of column III metals on silicon [100]

We calculate the electronic structure of geometrically ideal interfaces between column III metals and silicon. For a single neutral adatom the associated metallic Fermi energy must lie between the energy (0,+), for the removal of an electron, and the energy (0,−), for adding an electron, but is otherwise unrestricted. This circumstance also holds for many uncoupled adatoms and will produce pinning if the (0,+) and (0,−) levels are in the gap and the semiconductor Fermi energy is outside this range. At still larger coverages the occupied (0,+) levels and the (0,−) levels shift and broaden in energy until the gap between them disappears, pinning n‐type and p‐type semiconductors at the same energy. We find that the (0,+) level for a single aluminum adatom is in the silicon gap and that the (0,−) level is in the conduction band. In addition, the (0,+) level moves towards the valence band maximum as the metal coverage is increased.