The intensity of triplet-exciton annihilation luminescence in anthracene crystals at room temperature increases in weak magnetic fields up to a maximum increase of 5 % at 350 Oe. For stronger fields the intensity decreases, finally leveling off at 80 % of the zero-field value for H k, 5 kOe. Sharp dips are found in the high-field intensity for field orientations at which a particular kind of level crossing occurs for pairs of triplet excitons. These effects result solely from field-induced changes in the rate constant for triplet-triplet annihilation.We wish to report the discovery of some remarkable effects of magnetic fields on the intensity of fluorescence resulting from mutual annihilation of triplet excitons 1 (delayed fluorescence) in anthracene crystals at room temperature.Experiments to measure the dependence of the intensity of delayed fluorescence on the magnitude and direction of a steady magnetic field were carried out on an anthracene crystal situated in the uniform field of an electromagnet. The crystal was irradiated with unpolarized red light with wavelengths longer than 5900 A obtained by passing the steady focused light from a 150-W xenon lamp through two Corning C.S. 2-62 filters and heat-absorbing glass. Blue delayed fluorescence (A«4400 A) from the crystal was led through a fiber-optics light guide and red-absorbing, blue transmitting filters (C.S. 5-56, 5-57, and 4-72) to a photomultiplier tube, the output of which was amplified and recorded by conventional means. In all cases, it was verified that the delayed fluorescence intensity was proportional to the square of the intensity of the incident light; this insured that the triplet-exciton population was controlled by monomolecular decay processes rather than by the bimolecular annihilation process. x ' 2 The crystals were melt-grown 3 from highly purified natural anthracene. The effects of stray magnetic field on the xenonlamp output and photomultiplier output were minimized by locating these devices more than 1 m away from the sample and by using appropriate magnetic shielding. Transmission of red light (with crystal) and blue light (without crystal) through the apparatus was established to be independent of magnetic field strength.A typical result of a measurement at room temperature (T«295°K) is shown in Fig. 1.In this case, an anthracene crystal (roughly 7x9x14 mm 3 in size) which had been oriented with x rays was placed so that the magnetic field lay in the ac plane and the red light was incident normal to the ac plane. The triplet-exciton lifetime in this crystal was 15 msec. We distinguish two regions of the magnetic field effect. The low-field effect is an increase in delayed fluorescence which occurs in anthracene for field strengths between 0 and about 350 Oe. The high-field effect is a decrease in delayed fluorescence which occurs in anthracene for field strengths greater than this value. Qualitatively similar behavior was observed in UJ 1.0 -0.8h LU O I 0.6h ) UJ cr o Z) 0.4h S 0.2 < UJ Q 0 1 .,,,..,,, . . 1 . . . . 1 . -5.0 1...
A new superconductor that displays onset behavior near 120 K has been identified as Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y), with x ranging from about 0.4 to 0.9. Single crystal x-ray diffraction data were used to determine a pseudo-tetragonal structure based on an A-centered orthorhombic subcell with a = 5.399 A, b= 5.414A, and c = 30.904 A. The structure contains copper-oxygen sheets as in La(2)CuO(4) and YBa(2)Cu(3)O(7), but the copper-oxygen chains present in YBa(2)Cu(3)O(7) do not occur in Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y). The structure is made up of alternating double copper-oxygen sheets and double bismuth-oxygen sheets. There are Ca(2+) and Sr(2+) cations between the adjacent Cu-O sheets; Sr(2+) cations are also found between the Cu-O and Bi-O sheets. Electron microscopy studies show an incommensurate superstructure along the a axis that can be approximated by an increase of a factor of 5 over the subcell dimension. This superstructure is also observed by x-ray diffraction on single crystals, but twinning can make it appear that the superstructure is along both a and b axes. Flux exclusion begins in our samples at about 116 K and is very strong by 95 K. Electrical measurements on a single crystal of Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y) show a resistivity drop at about 116 K and apparent zero resistivity at 91 K.
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