J. S. Helman

Surface spin-exchange-scattering of polarized electrons emitted from magnetic materials

Abstract It is known that a paramagnetic surface sheet persists far below the Curie temperature in the ferromagnetic semiconductors EuO and EuS. We show that the observed spin-polarization of the photo- and field-emitted electrons is largely determined by spin-exchange collisions in the surface sheet. A single paramagnetic atomic layer may have a depolarizing effect of up to 50 per cent. Other circumstances in which the proposed model might apply are suggested.

Polycrystalline zinc selenide films deposited by a low‐temperature close‐spaced‐vapor‐transport method

Polycrystalline films of ZnSe, pure and doped with Cu and Ga, have been deposited by a modified close‐spaced‐vapor‐transport method which requires low temperatures (500 °C) and no subsequent annealing. The resulting grains are of the order of 1 μm. The films were characterized by their photoluminescence emission in the temperature range 10–290 K. The undoped and Cu‐doped films display the usual Cu‐green and Cu‐red and near edge emission bands at low temperatures. In the Ga‐doped films only a mildly temperature dependent band centered at ∼620 nm appears at all temperatures. The films have very similar characteristics to those prepared by more sophisticated techniques.

Resonant tunneling through Schottky barriers

The model of the Schottky barrier is based on the presence of ionized donors within the barrier. The empty energy levels corresponding to these donors lie in the barrier. Thus, there can be resonant tunneling through these empty energy levels. This is an inherent effect associated with Schottky barriers. We show how resonant tunneling depends on the ionization energy of the donors and the conditions under which resonant tunneling prevails over common tunneling.

Optical and conductivity measurements on MoO3:H

Abstract Results of optical absorption and conductivity measurements on hydrogen doped MoO 3 are reported. A simple band model based on ligand-field theory is used to assign the optical transitions and explain the origin of the activation energy obtained from the conductivity data.

Magnetic field effects on the fluorescence of Cr3+ in GdAlO3

Abstract The fluorescence spectrum of Cr3+ in GdAlO3 has been examined at 4.2 K as a function of magnetic field up to 60 kG. The resulting splitting of the 2E → 4A2 emission lines are explained in terms of a modified molecular field approximation, which incorporates the effect of the spin fluctuations. The exchange constant in the relaxed excited state is found to be 1.2 cm-1, which differs from that reported from absorption data. It is suggested that the difference may be related to the Frank-Condon effect.

Evidence for the magnetic Franck-Condon effect in a system of an interstitial magnetic impurity in an antiferromagnetic matrix : Cr(3+) in GdAlO3

Absorption and emission measurements of the 4A2-2E transitions of Cr3+ in GdAlO3 at T iU4.2 K, in magnetic fields up to 6 T parallel to the easy axis, are consistent with a magnetic Franck-Condon principle for which the energy difference between the absorption and emission lines arises mainly from the spin relaxation, rather than the neighboring ion space coordinates around the impurity. The Cr-Gd exchange-coupling const. changes with the applied magnetic field from J = 2.1 cm-1 at H = 0 to Jsat = 1.46 cm-1 at high field.

Asymmetry of the splitting of the fluorescence spectrum of Cr3+ in GdAlO3

Abstract The fluorescence spectrum of Cr 3+ in the antiferromagnet GdAlO 3 consists of four broad bands whose peak energies are not perfectly equidistant. A theory recently proposed for the splittung of the fluorescence of a magnetic impurity in a magnetic matrix accounts for the small asymmetry observed.

Critical resistivity of antiferromagnets below Tc

Abstract It is shown that the critical resistivity of antiferromagnets has a magnetic energy-like behavior, both above and below the critical temperature, Tc.

Multiphonon contribution to the structure factor of metals

Abstract The structure factor of Na is calculated including two-phonon terms and the Debye-Waller factor. The result is compared with the one-phonon approximation usually employed to evaluate the electronic transport coefficients. This multiphonon contribution can amount to 13 per cent at the melting point and 10 per cent at room temperature in the transport sensitive region of wave vector 1.5 k F q k F , where k F is the Fermi wave vector. We conclude that calculations of electronic transport coefficients of metals intended to attain a precision better than 10 per cent above the Debye temperature must take into account the contributions of the Debye-Waller factor and the two phonon terms.

The dielectric constant of liquid noble metals and their alloys

Abstract The nearly-free-electron part of the dielectric constant of liquid Cu, Ag and Au are calculated and the results are compared with available experimental data. The formulae are generalized for the case of liquid alloys.