Persistent tunneling currents and magnetic-field effects in glasses

Recent experiments on insulating glasses showed an unexpected variation of the dielectric constant with a static magnetic field and, in particular, oscillations with an apparent flux quantum of about 10(-5)h/e. We propose an explanation for these phenomena in terms of a model based on pairs of two-level systems. A weak interaction is shown to give rise to quantum states with a net circular current and a linear field dependence of the energy levels. Under experimental conditions, the dynamic susceptibility shows oscillatory behavior, with an effective flux quantum of the order of 10(-5)h/e.     

Mode-coupling theory for relaxation of coupled two-level systems

A Mori approach to the dynamics of coupled tunnelling units in defect crystals is presented. Transverse and longitudinal correlation functions of a two-level system are given as continued fractions; the memory kernels are evaluated in a usual decoupling approximation. Due to the random configuration of the defects on the host lattice both two-level splitting and relaxation rates show a broad distribution; the corresponding distribution function is derived for the case of a dipolar interaction of the defects. The theory covers both weak and strong coupling; the average interaction energy turns out to be the essential parameter. Dependence on frequency, temperature and concentration of the dynamical susceptibility is discussed. When passing from weak to strong coupling, the zero-temperature susceptibility shows a crossover from a constant value to a decrease with the third power of inverse defect concentration; there is quite a strong relaxational peak in the susceptibility. The theory accounts for several features observed in a recent low-frequency experiment on KCl:Li.     

Systematically weak temperature dependence of electric field gradients in distorted cubic metals

The temperature dependence of the electric field gradient for radiation-damaged copper and for the dilute alloy Zn$\text{Ag}$ has been measured using the perturbed angular correlation technique. A systematically weak temperature dependence is found for distorted cubic metals compared to non-cubic metals. The strong temperature dependence for the Zn$\text{Ag}$, In$\text{Ag}$ and Sn$\text{Ag}$ alloys as reported in literature has not been confirmed in the present experiment.     

Electric-field induced capillary interaction of charged particles at a polar interface

We study the electric-field induced capillary interaction of charged particles at a polar interface. The algebraic tail of the electrostatic pressure of each charge results in a deformation of the interface u approximately r(-4), where r is the lateral distance. The capillary interaction of nearby particles is repulsive and varies as rho(-6) with their distance rho. As a consequence, electric-field induced capillary forces cannot be at the origin of the secondary minimum observed recently for charged poly(methyl methacrylate) particles at an oil-water interface.     

Photon antibunching and collective effects in the fluorescence of single bichromophoric molecules

The fluorescence of individual pairs of perylenemonoimide chromophores coupled via a short rigid linker is investigated. Photon antibunching is reported, indicating collective effects in the fluorescence, which are further substantiated by the observation of collective triplet off times and triplet lifetime shortening. The experimental findings are analyzed in terms of singlet-singlet and singlet-triplet annihilation based on F?rster type energy transfer. The results reported here demonstrate that the statistical properties of the emission light of isolated single quantum systems can serve as a hallmark of intermolecular interactions.     

Synthesis of Carbohydrate Derived α-Methylene-γ-lactones by Diasterecseldctive,Low Temperature Reformatsky-Type Reactions

Hexopyranoside- and hexofuranose uloses with either ethyl 2-(branamethyl)acrylate in the presence of laminar Zn/Ag-graphite or ethyl 2-(trimethylsilylinethyl)acrylate/tetra-n-butylanrnonium fluoride undergo stereoselective branching, mainly with formation of spiro α-methylene-γ-lactones.