Self-trapped electrons and holes in PbBr2_2 crystals

We have directly observed self-trapped electrons and holes in PbBr$_{2}$ crystals with electron-spin-resonance (ESR) technique. The self-trapped states are induced below 8 K by two-photon interband excitation with pulsed 120-fs-width laser light at 3.10 eV. Spin-Hamiltonian analyses of the ESR signals have revealed that the self-trapping electron centers are the dimer molecules of Pb$_2$$^{3+}$ along the crystallographic a axis and the self-trapping hole centers are those of Br$_2$$^-$ with two possible configurations in the unit cell of the crystal. Thermal stability of the self-trapped electrons and holes suggests that both of them are related to the blue-green luminescence band at 2.55 eV coming from recombination of spatially separated electron-hole pairs.Comment: 8 pages (7 figures, 2 tables), ReVTEX; revised the text and figures 1, 4, and

Self-trapped states and the related luminescence in PbCl2_2 crystals

We have comprehensively investigated localized states of photoinduced electron-hole pairs with electron-spin-resonance technique and photoluminescence (PL) in a wide temperature range of 5-200 K. At low temperatures below 70 K, holes localize on Pb$^{2+}$ ions and form self-trapping hole centers of Pb$^{3+}$. The holes transfer to other trapping centers above 70 K. On the other hand, electrons localize on two Pb$^{2+}$ ions at higher than 50 K and form self-trapping electron centers of Pb$_2$$^{3+}$. From the thermal stability of the localized states and PL, we clarify that blue-green PL band at 2.50 eV is closely related to the self-trapped holes.Comment: 8 pages (10 figures), ReVTEX; removal of one figure, Fig. 3 in the version

Advances in ab-initio theory of Multiferroics. Materials and mechanisms: modelling and understanding

Within the broad class of multiferroics (compounds showing a coexistence of magnetism and ferroelectricity), we focus on the subclass of "improper electronic ferroelectrics", i.e. correlated materials where electronic degrees of freedom (such as spin, charge or orbital) drive ferroelectricity. In particular, in spin-induced ferroelectrics, there is not only a {\em coexistence} of the two intriguing magnetic and dipolar orders; rather, there is such an intimate link that one drives the other, suggesting a giant magnetoelectric coupling. Via first-principles approaches based on density functional theory, we review the microscopic mechanisms at the basis of multiferroicity in several compounds, ranging from transition metal oxides to organic multiferroics (MFs) to organic-inorganic hybrids (i.e. metal-organic frameworks, MOFs)Comment: 22 pages, 9 figure

Crystal growth, ionic conductivity, and photolysis of pure and impurity-doped lead bromide single crystals

In this paper an account is given of the preparation of lead bromide single crystals undoped and doped with monovalent and trivalent cations and with oxygen. The conductivity of these crystals has been measured in the temperature region 50–300°C. The thermal disorder appeared to be of the same Schottky type as in lead chloride. From the slopes of the conductivity curves the activation heat for the anion vacancy migration has been calculated to be 0.29 ± 0.04 eV and the heat required for the formation of one set of vacancies 1.4 ± 0.1 eV. Irradiation of pure lead bromide with ultraviolet light gives a characteristic damaging as is made visible by electron and optical microscopy. The increase of the optical density varies as the square root of the irradiation time

Photoconductivity in lead chloride and lead bromide

Photoconductivity measurements on PbCl2 and PbBr2 at liquid nitrogen temperature are described. It was found that the holes are the dominant mobile charge carriers in these crystals. The range per unit field was estimated to be 1.2 × 10-4 cm2/V for PbBr2. For PbCl2 the ranges are 4 × 10-4 cm2/V and 0.3 × 10-4 cm2/V for surface and bulk conductivity, respectively. The wavelength dependence indicated that the first maximum at the long wavelength side of the fundamental absorption of PbBr2 and PbCl2 is an exciton peak

Charge transport in oxygen-doped polysilicon layers on Si

It is shown that layers of polysilicon doped with oxygen (polydox) can be used for the passivation of underlying p-n junctions. The conduction mechanism was derived from measurements in layers directly deposited on to silicon crystals. At room temperature we found Poole-Frenkel conduction changing at higher temperatures, presumably, to hopping in localized states

Density of oxidation-induced stacking faults in damaged silicon

A model for the relation between density and length of oxidation-induced stacking faults on damaged silicon surfaces is proposed, based on interactions of stacking faults with dislocations and neighboring stacking faults. The model agrees with experiments.