Herbert B. Shore

Calculation of ground state surface properties of simple metals including non-local effects of the exchange and correlation energy

Abstract The energy density functional formalism is used to calculate the ground state surface energies and work functions for simple metals. Non-local corrections to the exchange and correlation energies were included by means of a gradient expansion in the density. Good agreement is obtained with experiment.

THE ACADEMY UNDER SIEGE: Informing the Public about the Merits of Academic Tenure

It is argued that the academy has been reluctant to discuss and defend the institution of academic tenure with the general public and that if the institution is to be saved, this reticence to talk about tenure must give way to a reasoned discussion of the merits of tenure. To this end, the authors offer a definition of what tenure is and a discussion on how it is achieved. They also discuss what tenure gives a faculty member, why it is in the best interests of society for university professors to be protected by tenure, and why it is in the best interest of American universities to retain the tenure sytem. The authors conclude with a discussion of when budget exigency should override tenure and when it should not.

Valence of the elemental transition metals

We propose that the recently introduced bonding valence is the appropriate valence for describing the gross energetics of the elemental transition metals. In support of this proposal, we show that the trends in the experimental cohesive energies, surface energies and melting points of the noble and transition metals are simple linear functions of the bonding valence. The trends in the cohesive energies of the elemental transition metals are given by 2¢ 45ZBi3¢ 47 eV, where ZB is the bonding valence.

Role of oxygen vacancies in anodic TiO2 thin films

Abstract Defects play an important role in the electronic and optical properties of amorphous solids in general. Here we present both experimental and theoretical investigations on the nature and origin of defect states in anodic rutile TiO 2 thin films (thickness 5–20 nm). There is experimental evidence that the observed gap state at 0.7 eV below the edge of the conduction band is due to an oxygen vacancy. For this reason oxygen vacancies are used in our model. A comparison of the calculated bulk-photoconductivity to photospectroscopy experiment reveals that the films have bulk-like transport properties. On the other hand, a fit of the surface density of states to the scanning tunneling microscopy (STM) on the (001) surfaces has suggested a surface defect density of 5% of oxygen vacancies. To resolve this discrepancy, we calculated the DC-conductivity where localization effects are included. Our results show an impurity band formation at about p c =9% of oxygen vacancies. We concluded that the gap states seen in STM are localized and the oxygen vacancies are playing the role of trapping centers (deep levels) in the studied films.

Ground state energy of small electron-hole drops

Abstract The ground state energy of small electron-hole drops is calculated for droplets ranging in size from 10 to 10,000 pairs. A new value for the bending energy of 1.1×10 −10 erg cm is derived. We also give a simple highly accurate formula for the total energy per pair. The surface energy is extracted from the total energy and found to agree well with a previous self-consistent calculation. The density at the center of the drop remains essentially constant over the entire range of N, indicating that the drop is not dramatically compressed by the surface tension.

Equation of motion method for the electronic structure of disordered transition metal oxides

Abstract The equation of motion method is very well suited for studying the electronic density of states of disordered systems, especially those described by a tight binding Hamiltonian. The Hamiltonian problem is solved in direct space, hence the method can be applied to the systems with high substitutional disorder (oxygen vacancies, dopants), surfaces and interfaces and to study the local electronic environment in the presence of disorder. The presented version of the program was used to obtain the local, total and surface electronic density of states of rutile TiO 2- x with up to x = 0.2 oxygen vacancies concentration.

Size effects in small electron-hole drops

Abstract The self-consistent formulation of density functional theory is used to calculate the single-particle properties of electron-hole droplets in Ge with between 10 and 120 pairs. Results are presented for the recombination luminescence lineshape, the electron and hole density profiles and the work function. As in the physics of the nucleus there are strong effects associated with the filling of the various electron and hole shells. In particular, the luminescence line narrows and oscillates as N decreases for a drop with less than 120 pairs. If N = 70 the width is 70% of the bulk value; while for N = 50 it is 80%. The density profiles and work functions for different N are rather varied and show very strong shell effects.

Attachment of an electron-hole droplet to a donor

Abstract The attachment of an electron-hole droplet to a donor in Ge is considered using the density-functional method. We find the heat of solution of a donor in a droplet to be about 7 meV, and the electron density at the donor to be 32 times the carrier density in the droplet far from the donor. Effects on droplet pinning and recombination are discussed.

INVARIANCE OF THE MOBILITY EDGE IN ANODIC TITANIUM OXIDES

We present a theoretical investigation to explain the electronic and optical properties of anodic rutile TiO2 thin films of different thicknesses (ranging from 5nm to 20nm). There is experimental evidence that the observed gap state at 0.7eV below the edge of conduction-band is due to an oxygen vacancy. For this reason, oxygen vacancies are used as defects in our model. A comparison of the calculated bulk-photoconductivity to photospectroscopy experiment reveals that the films have bulk-like transport properties with a bandgap Eg = 3.0eV. On the other hand, a fit of the surface density of states to the scanning tunneling microscopy (STM) experiment on the (001) surfaces has suggested a surface defect density of 5% of oxygen vacancies. To resolve this discrepancy, we calculated the dc-conductivity where localization effects are included. Our results show an impurity band formation at about pc = 9% of oxygen vacancies. We concluded that the studied films have defect densities below the threshold of impurity...