J. W. Davenport

Electron coherent and incoherent pairing instabilities in inhomogeneous bipartite and nonbipartite nanoclusters

Abstract Exact calculations of collective excitations and charge/spin (pseudo) gaps in an ensemble of bipartite and nonbipartite clusters yield level crossing degeneracies, spin-charge separation, condensation and recombination of electron charge and spin driven by interaction strength, inter-site couplings and temperature. Near crossing degeneracies, the electron configurations of the lowest energies control the physics of electronic pairing, phase separation and magnetic transitions. Rigorous conditions are found for the smooth and dramatic phase transitions with competing stable and unstable inhomogeneities. Condensation of electron charge and spin degrees at various temperatures offers a new mechanism of pairing and a possible route to superconductivity in inhomogeneous systems, different from the BCS scenario. Small bipartite and frustrated clusters exhibit charge and spin inhomogeneities in many respects typical for nano and heterostructured materials. The calculated phase diagrams in various geometries may be linked to atomic scale experiments in high T c cuprates, manganites and other concentrated transition metal oxides.

Alloy heats of formation: theory versus experiment

Comparison is made among calculated heats of formation of ordered transition metal compounds, experimental heats obtained by Kleppa and coworkers, and parameters appropriate to CALPHAD constructs. The calculated heats are precise representatives of band theory granted the class of local density potential which was employed; comparison with experiment suggests that they are less accurate than is desired. The issue of the stability of excited phases against local distortion is explored indicating that some systems,which are normally presumed to be metastable, are not. This has consequences for CALPHAD phase diagram constructs and suggests that entropy is an important contribution to the local stability of some high- temperature phases.

First principles calculations: The elemental transition metals and their compounds

If done with sufficient care, present day a priori theory yields calculated enthalpies of formation whose agreement with experiment (when such data is available) is of the order of the experimental scatter. Comparisons will be made for the Pt-Ti system for which such data exist and for which one crystal structure involves atomic sites of low symmetry. Two other cases will be considered for which there is no direct experimental heats data. The first of these will be the structural stabilities of the 4d elemental metals. Such structural stabilities have been an issue of contention between electronic structure theorists and those who construct phase diagrams for some 25 years. The second involves the energetics of forming metal adlayers and artificial multilayers. The distortion energies associated with the requirement that adlayers (or multilayers) conform to some given substrate are often the controlling factors in the fabrication of multilayer materials. This contribution is best understood by invoking a combination of elemental structural promotion energies plus elastic distortions from these structures. As will be seen, the fabrication of multilayers also involves a term not normally encountered in bulk phase diagram considerations, namely the difference in surface energies of the two multilayer constituents.

Ionic charge reduction at the surfaces of crystals

Abstract Because the atomic constituents of a compound have lower coordination in the surface of a solid than in the interior one expects that compounds are less ionic at crystal surfaces than in the bulk, e.g. that cations at surfaces are in lower states of oxidation. A general expression is obtained here for a bound on such charge reduction at nonpolar surfaces. This shows that surface atoms, as a rule, have their ionic charge reduced relative to the value in the bulk by at most a factor of two.

Spin-charge separation and electron pairing instabilities in Hubbard nanoclusters

Electron charge and spin pairing instabilities in various cluster geometries for attractive and repulsive electrons are studied exactly under variation of interaction strength, electron doping and temperature. The exact diagonalization, level crossing degeneracies, spin-charge separation and separate condensation of paired electron charge and opposite spins yield intriguing insights into the origin of magnetism, ferroelectricity and superconductivity seen in inhomogeneous bulk nanomaterials and various phenomena in cold fermionic atoms in optical lattices. Phase diagrams resemble a number of inhomogeneous, coherent and incoherent nanoscale phases found recently in high-T(c) cuprates, manganites and multiferroic nanomaterials probed by scanning tunneling microscopy. Separate condensation of electron charge and spin degrees at various crossover temperatures offers a new route for superconductivity, different from the BCS scenario. The calculated phase diagrams resemble a number of inhomogeneous paired phases, superconductivity, ferromagnetism and ferroelectricity found in Nb and Co nanoparticles. The phase separation and electron pairing, monitored by electron doping and magnetic field surprisingly resemble incoherent electron pairing in the family of doped high-T(c) cuprates, ruthenocuprates, iron pnictides and spontaneous ferroelectricity in multiferroic materials.

Cohesive and Structural Energies for the 5d Elements, Lu through Au

We have calculated the cohesive energy and the energy difference between the fcc and bcc phases of the 5d elements Lu-Au using the recently developed Linear Augmented Slater Type Orbital method. We find the crystal structure to be correctly predicted and the structural energy differences to be significantly larger than the most commonly quoted values. We also find that the cohesive energies of these elements can be understood within the local spin density functional theory provided the experimental promotion energy from the local density dns configuration to the preferred dn-1s2 configuration is included.

Theoretical determination of surface magnetism (invited)

The theoretical determination of the magnetic structure of surfaces within the (local) spin‐density formalism is briefly described. The feasibility of using such methods for determining delicate magnetic quantities is illustrated by calculation of (1) the Knight shift of the paramagnetic Pt(001) surface, (2) the magnetization of the clean and Ag‐covered Fe(001) surface, and (3) the effect of a p(1×1) H overlayer on the magnetization of a Ni(001) surface. These results demonstrate that it is possible not only to make quantitative predictions for real systems, but more importantly, to gain insight into the underlying physics at surfaces.

Charge and spin pairing instabilities in Hubbard nanoclusters

Electron coherent and incoherent pairings and other types of correlations are studied exactly for the ensemble of small clusters with different geometries under variation of interaction strength, electron doping and temperature. Charge and spin collective excitations yield intriguing insights into level crossing degeneracies, phase separation and condensation. Criteria for spin-charge separation and recombination driven by interaction strength and temperature are found. Phase diagrams resemble a number of inhomogeneous, nanoscale phases and pseudogap regions seen recently in high TC cuprates, manganites, multiferroics and CMR nanomaterials.

First Principles Molecular Dynamics Studies of Liquid and Solid Sodium

One of the outstanding problems in materials science is the lack of a fundamental theory of melting and freezing. Such a theory would be extremely useful in the search for new materials. For example, the high-temperature properties of alloys almost always limit the efficiency that can be achieved in an engine or power plant. Also the mechanical strength of materials is often determined by the distribution of impurities, which in turn is a result of the cooling (or annealing) that occurs during pro cessing. To address these problems we have used the technique of molecular dynamics simulation in which 50 to 100 atoms are followed on the computer as they move. From these simulations it is possible to calculate the important thermal properties such as the rate of dif fusion, the internal energy, the free energy, and the volume of the material above and below the melting temperature. These calculations make use of a new theory that combines molecular dynamics simulation with calculations of the electron states and therefore re quires no adjustable parameters as inputs.

The metallic bond for monolayer transition metal layers on transition metal surfaces

This present work discusses changes in the electronic and chemical properties of some transition metal layers on transition metal substrates, and presents a simple framework for understanding the results.