Benjamin Welber

Mechanism of the ferroelastic transition of BiVO4

Abstract BiVO4 has a pure ferroelastic transition at Tc = 528°K and atmospheric pressure. We elucidated the mechanism of this transition by studying the q≈0 soft optical phonon, with the symmetry of the ferroelastic strain, under large hydrostatic pressures at room temperature. A free-energy analysis, including the optical-acoustical phonon couplings, shows that the transition is driven by the q≈0 soft optical phonon.

High pressure photoluminescence and resonant Raman study of GaAs

Abstract Photoluminescence and resonant Raman scattering have been studied in GaAs under hydrostatic stress up to 72 kbar. From the pressure dependence of the photoluminescence intensity the pressure coefficient of the X 6 conduction band minimum in GaAs has been determined. The pressure dependence of zone edge phonon energies in GaAs is also reported.

Dependence of the indirect energy gap of silicon on hydrostatic pressure

A diamond anvil optical cell is employed to measure the pressure dependence of the fundamental indirect Γ-X transition of crystalline Si. The result is E (eV) = (1.110 ± 0.002) − (1.41 ± 0.06) × 10−3P where the pressure P is in kbar. Between 115 and 126 a transformation takes place to a phase opaque to electromagnetic radiation of wavelength between 0.3 and 2.0 μm. We believe this is the same phase transition reported by Minomura and Drickamer. A pseudopotential calculation assuming a relatively soft core is carried out and its results are in rather good agreement with experiment.

Optical microspectroscopic system for use with a diamond anvil high pressure cell to 200 kilobar

We describe an optical system which has been used in conjunction with a high pressure diamond anvil cell to obtain absorption, reflectance, and photoluminescence spectra. The system is shown to yield useful results to pressures at least as high as 200 kilobar.

Electron-electron interactions in the fulvalene family of organic metals

Abstract Magnetic susceptibility as well as optical reflectivity data in the metallic regime of the conductivity of HMTTF-TCNQ are presented. It is shown that: 1. (1) The spin susceptibility of the donor stack is temperature independent in the temperature range of the metallic conductivity 50 ⩽T⩽300 K. 2. (2) By comparing the magnitude of this susceptibility with the value estimated from plasma frequency measurements based on the model of non-interacting electrons, we conclude that the donor stack susceptibility is not enhanced. 3. (3) A comparison of data on the three compounds (HMTTF, TTF, TSeF)-TCNQ leads to the conclusion that the donor stack susceptibility is enhanced only in TTF-TCNQ.

Reversible Phototransfer of Electrons between Rare‐Earth Ions in CaF2

This paper describes the observation of a charge‐transfer phenomenon occurring between divalent and trivalent ions incorporated into the CaF2 lattice. It is shown that, in one of the two cases considered, the phenomenon is reversible at 4°K.

Lattice vibrational study of the phase transition in the plastic crystal adamantane (C10H16)

Abstract Using the Raman technique we have studied the lattice vibrations of the prototype plastic crystal adamantane (C 10 H 16 ). The measurements, as a function of temperature through the phase transition (208K), show splittings in certain lines. At room temperature using hydrostatic pressure, we also induced a phase transition. The transition is detected by Raman measurements, and we observe splittings of the very same vibrational lines. This indicates that the pressure induced phase transition yields the same crystal structure as the low temperature structure. Some analysis of the pressure and temperature derivatives of the vibrational modes is presented.

Micro‐optic system for reflectance measurements at pressures to 70 kilobar

An optical system which makes it possible to obtain reflectance spectra of solids in the range of hydrostatic pressures 0–70 kilobar and higher is described. The system is used in conjunction with a diamond anvil pressure cell, and has a spectral range of 0.6–4.5 eV. An example is furnished illustrating its use in an investigation of the shift with pressure of the plasma reflectance edge of TTF‐TCNQ.

Direct Charge Transfer by X Irradiation in the System CaF2:Eu, Tm

Starting with CaF2 containing Eu2+ and Tm3+ ions, we have employed optical techniques to observe the decrease in valence of the Tm ion under x irradiation. We find that the number of Tm2+ ions produced corresponds closely to the number of Eu2+ ions which disappear, and we suggest that the underlying mechanism is a direct transfer of an electron between the two types of ions.

Pressure‐induced changes in the reflectivity of SmSe and some comments on Nd‐substituted SmSe

Pressure induced changes in the reflectivity of SmSe have been investigated up to 70 kbar hydrostatic pressure, using the gasketted diamond anvil high‐pressure x‐ray apparatus in conjunction with an optical microspectroscopic system. The effect of Nd substitution on the lattice parameter, resistivity, and color of SmSe has also been studied. Reflectivity data for GdSe at ambient pressure have been obtained for comparison with the metallic phase of SmSe. With increasing pressure the color of the sample (as observed in reflected light) goes through a sequence of changes, from black, steel‐blue, deep purple, reddish purple, copperlike metallic color to bronze yellow at the highest pressure. From the reflectivity data, the static conductivity has been evaluated and is compared with the dc conductivity, previously measured on SmSe single crystals under hydrostatic pressure. Deviations from Drude theory indicate that the interband contribution to the dielectric constant plays an important role in determining the sequence of metallic colors observed in metallic rare‐earth monochalcogenides.