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An overview of the recent development of the superconducting FeSe1x and related compounds is presented. Methods to synthesize high purity polycrystalline samples, single crystals and thin films with preferred orientation are described. In addition to the synthesis of FeSe and FeSeTe, the effects of various partial chemical substitutions on Fe and Se/Te site are described. It was found that the effects of chemical doping to the Se-site or Fe-site are rather different. Ionic size of the dopant is found to play a critical role on the occurrence of superconductivity. We also review the physical properties, including transport, magnetic, and thermal properties. There exist interesting transport anomalies in the resistivity at low temperature, and it was found that a structural distortion at low temperature is critical to the occurrence of superconductivity in these materials. However, the exact origin of these observed anomalies is not clear, and the exact pairing symmetry in FeSe-based superconductors is also still in question. 2009 Published by Elsevier B.V.

The triboluminescence (TL) intensity of unirradiated Ba, Sr, Ca and Pb doped NaCl and LiF single crystals decreases with temperature and disappears completely at 105+or-5 and 180+or-10 degrees C respectively. The temperature Tc at which TL disappears changes slightly with increasing concentration of the dopants. The temperature dependence of total TL intensity follows the relation IT=IT0(1-T/Tc)n, where IT0 is a constant and n lies between 0.5 and 0.6 for the unirradiated divalent impurity doped NaCl and LiF crystals. It is concluded that the decrease in the TL intensity with temperature may be primarily due to the decrease in charge density of the newly created surfaces. The increase in TL intensity with increasing dopant concentration is attributed to the increasing hardness of the crystals.

A new peak at about 650°C has been observed in the thermoluminescence emission of gamma irradiated natural fluorites containing significant quantities of lanthanide rare-earth (RE) elements as impurities. Thermal activation energy of the corresponding trap has been evaluated to be 2·99 eV with a frequency factor of 1016 sec−1. The optical activation energy, as deduced from photo-transfer-induced-TL from this trap, is 5·36 eV. Many types of natural fluorites as well as synthetically grown CaF2 crystals doped with single and pairs of rare earth elements have been studied and the results indicate that the high temperature peak is associated with Sm impurity coexisting with Y, La or Ce in CaF2. There are indications that this newly observed TL peak can be gainfully employed in ultraviolet dosimetry and geological dating of fluorite deposits.

Dc conductivity and stimulated thermo-current (STC) studies on VO2+ doped lithium hydrazinium sulphate Li(N2H5)SO4 are reported. The activation energies obtained from conductivity studies in extrinsic and intrinsic regions are found to be 0.65 and 0.84 eV. A pair of STC peaks occurring both at low and high temperature regions with activation energies of 0.40 and 0.88 eV, respectively, are recorded. The variation in these energies with temperature is explained on the basis of a “one-dimensional conductor” model of this crystal, and the interchannel jumps of proton vacancies formed due to the divalent impurity. Large values of l/τ0 may be due to the dynamics of hydrogen groups in this system. Gleichstromleitfahigkeitsmessungen und Untersuchungen des stimulierten Thermostroms (STC) an VO2+-dotierten Lithiumhydrazinsulfat Li(N2H5)SO4 werden durchgefuhrt. Es wird gefunden, das die aus der Leitfahigkeit in extrinaischen und intrinsischen Bereichen erhaltenen Aktivierungsenergien 0,65 und 0,84 eV betragen. Es wird ein STC-Maximumpaar gefunden, das sowohl in Nieder- als auch in Hochtemperaturbereichen auftritt mit Aktivierungsenergien von 0,40 bzw. 0,88 eV. Die Anderung dieser Energien mit der Temperatur wird auf der Grundlage eines „eindimensionalen Leiter”-Modells dieses Kristalls und Interkanalsprungen von Protonenleerstellen, die infolge der zweiwertigen Storstelle gebildet werden, erkart. Grose Werte von 1/τ0 konnen durch die Dynamik von Wasserstoffgruppen in diesem System hervorgerufen werden.

The undoped samples exhibit five glow peaks at 60, 105, 145, 175 and 285 degrees C. The 60 degrees C peak is tentatively attributed to F' centres, the 105, 175 and 285 degrees C peaks to F centres and the 145 degrees C peak to hole centres. Doped samples exhibit six glow peaks at 85, 106, 145, 177, 230 and 320 degrees C. The 106, 145 and 177 degrees C peaks of doped samples are respectively the same as the 105, 145 and 175 degrees C peaks of the undoped samples, and are explained in a similar fashion. The 85 and 230 degrees C peaks are attributed to F centres and the 320 degrees C peak to hole centres. The TL emission spectra of the doped samples consist of two bands at 410-420 and 570 nm at all temperatures, the 410-420 nm emission being due to the intrinsic luminescence perturbed or allowed by the impurities, and the 570 nm emission being due, at least in the case of the first five peaks, to the host-sensitised secondary luminescence with Mn acting as activator. The emission spectrum observed when irradiated doped samples are excited with 342 nm (F band) light also contains bands at 420 and 570 nm.