Electrical conductivity,absorption and luminescence measurements in cubic boron nitride

Electrical and optical measurements were carried out on tiny crystals of cubic boron nitride. The dark current i_D was found to change exponentially with T, with activation energies in the range 0·2–0·4 eV. A red electroluminescence, of intensity i_EL. proportional to i_D was observed. Upon illumination at low temperatures a photocurrent i_p proportional to the square root of the excitation intensity appeared. It varied exponentially with T, with an activation energy of 0.05 ± 0.01 eV. The crystals exhibited a red thermoluminescence with several unresolved peaks covering the temperature range 100–400K, and having activation energies in the range 0·15–0·40 eV.     

NMR study of pure and doped β-LiAl

The NMR spin-lattice relaxation time, T_I, has been measured as a function of temperature for both ⁷Li and ²⁷Al in pure and doped β-LiAl alloys. Compositions with ⁷Li concentration in the range 48.3–54.5% and doping in the form Li₅₀Al_50?xM_x, where M = Ag or In, were studied. The relaxation rates T₁^?1 for the ²⁷Li and the ²⁷Al resonances were found to be peaked functions of temperature with the maxima for ⁷Li appearing at composition dependent temperatures. The ²⁷Al maxima always appeared at a lower temperature, independent of composition, and the ²⁷Al maximum relaxation rate was a strong function of composition in contrast with ⁷Li where the maximum rate was only weakly dependent on composition. The principle relaxation mechanisms are identified as dipole-dipole coupling in the ⁷Li and coupling of the ²⁷Al quadrupole moment to electric field gradients. The temperature dependence of these rates is attributed to the thermally activated diffusion of vacancies of a non-thermal origin in the Li sub-lattice. These vacancies are also responsible for the fluctuating electric field gradients. The results have been analyzed to give the Li diffusion coefficients with associated activation energies and estimates of the vacancy concentration as functions of alloy composition.     

Comments on D.C. conductivity in amorphous semiconductors

A recent paper in this journal argued that the electrical conductivity arising from thermal hopping between localized states obeys the law in $\text{ln}\text{σ∝ — (T}{}_{\mathrm{<mtext>0</mtext>}}\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$, at low temperatures. That result is found to be based on several unjustified assumptions, however, and the correct temperature dependence is $\text{ln}\text{σ∝ — (T}{}_{\mathrm{<mtext>0</mtext>}}\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$, as originally proposed by Mott.     

The A-emission of KBr : Tl

The line shapes and relative efficiencies of A_T and A_X emission bands of KBr : Tl have been studied over a wide temperature range (10–500 K). The behavior of A_T and A_X emissions has been described phenomenologically in terms of the coexistence of two different kinds of minima on the adiabatic potential energy surface of the ³T_1u relaxed excited state. The line shape parameters and the A-emission temperature dependence show that T and X minima belong to distortions of different symmetry. A model considering the emission processes from T and X minima separately, in spaces of different distortions, is shown to account well for the experimental results. The emission at very low temperatures (below 50 K) was found to be sensitive to trace impurities.     

On the temperature dependence of the visible emission in thallium doped KCl and KBr crystals

The visible emission of KBr : Tl and KCl : Tl crystals was measured in the temperature range 10–300 K, on excitation in the low-energy tails of the corresponding A-bands. The peculiar temperature dependence of the visible emission is explained in terms of absorption changes due to thermal broadening of the A₁-side band itself and due to the growing overlap of A₁-band with the A-band on warming. A quantitative analysis is given which enables the computation of some parameters of the A₁-side band.     

A perturbation scheme for spherical arrangements with application to molecular modeling

We describe a software package for computing and manipulating the subdivision of a sphere by a collection of (not necessarily great) circles and for computing the boundary surface of the union of spheres. We present problems that arise in the implementation of the software and the solutions that we have found for them. At the core of the paper is a novel perturbation scheme to overcome degeneracies and precision problems in computing spherical arrangements while using floating point arithmetic. The scheme is relatively simple, it balances between the efficiency of computation and the magnitude of the perturbation, and it performs well in practice. In one O(n) time pass through the data, it perturbs the inputs necessary to insure no potential degeneracies and then passes the perturbed inputs on to the geometric algorithm. We report and discuss experimental results. Our package is a major component in a larger package aimed to support geometric queries on molecular models; it is currently employed by chemists working in “rational drug design”. The spherical subdivisions are used to construct a geometric model of a molecule where each sphere represents an atom.     

Superfluidity and dimerization in a multilayered system of fermionic polar molecules

We consider a layered system of fermionic molecules with permanent dipole moments aligned perpendicular to the layers by an external field. The dipole interactions between fermions in adjacent layers are attractive and induce interlayer pairing. Because of the competition for pairing among adjacent layers, the mean-field ground state of the layered system is a dimerized superfluid, with pairing only between every other layer. We construct an effective Ising-XY lattice model that describes the interplay between dimerization and superfluid phase fluctuations. In addition to the dimerized superfluid ground state, and high-temperature normal state, at intermediate temperature, we find an unusual dimerized "pseudogap" state with only short-range phase coherence. We propose light-scattering experiments to detect dimerization.     

Stokes-Einstein relation in supercooled aqueous solutions of glycerol

The diffusion of glycerol molecules decreases with decreasing temperature as its viscosity increases in a manner simply described by the Stokes-Einstein relation. Approaching the glass transition, this relation breaks down as it does with a number of other pure liquid glass formers. We have measured the diffusion coefficient for binary mixtures of glycerol and water and find that the Stokes-Einstein relation is restored with increasing water concentration. Our comparison with theory suggests that adding water postpones the formation of frustration domains.