We report a comprehensive study of ultrafast carrier dynamics in single crystals of multiferroic BiFeO3. Using femtosecond optical pump-probe spectroscopy, we find that the photoexcited electrons relax to the conduction band minimum through electron-phonon coupling with a ∼1 picosecond time constant that does not significantly change across the antiferromagnetic transition. Photoexcited electrons subsequently leave the conduction band and primarily decay via radiative recombination, which is supported by photoluminescence measurements. We find that despite the coexisting ferroelectric and antiferromagnetic orders in BiFeO3, the intrinsic nature of this chargetransfer insulator results in carrier relaxation similar to that observed in bulk semiconductors. Bismuth ferrite (BFO) is one of the most actively studied multiferroic materials due to its room temperature coexistence of ferroelectric (FE) (T c ∼1100 K) and antiferromagnetic (AFM) (T N ∼640 K) orders. Much research has focused on enhancing their weak mutual coupling, particularly by using growth techniques that vary the structure or strain in BFO films. 1-6 This could allow both control of magnetism with electric fields and control of electric polarization with magnetic fields, which would lead to a variety of potential applications in optoelectronics, spintronics, and data storage.Despite the intense research on this material, relatively few studies of its optical properties have been done to date. However, these studies have uncovered several unique phenomena that are linked to FE order in BFO. For example, calculated and measured absorption spectra reveal the optical band gap to be ∼2.6-2.8 eV at 300 K, 7-9 arising from the dipole-allowed O 2p to Fe 3d charge transfer (CT) transition. These measurements have also revealed strong absorption edge "smearing" 7,8,10 which was attributed to low-lying electronic 8,11 or defect states, 11,12 both of which can strongly impact the ferroelectric response. Terahertz (THz) emission spectroscopy indicates that ultrafast depolarization of the FE order causes the observed emission, 13,14 although the detailed mechanism is not entirely clear. A substantial zero-bias photovoltaic effect has also been observed in BFO with near/above band gap illumination, and the photocurrent preferentially moves along the direction of FE polarization. 15 Deeper insight into these and other phenomena, as well as their potential for applications (e.g., in determining switching speeds in BFO-based devices), can be gained by tracking carrier dynamics in BFO on an ultrafast timescale, 16 particularly after excitation of the p-d CT transitions that dominate the near band gap optical response and have been linked to FE properties. 8 This can be done using ultrafast optical spectroscopy (UOS), a technique that is capable of tracking the interplay between carrier, spin and lattice degrees of freedom by using femtosecond optical pulses to photoexcite materials and probing the response in the time domain. [17][18][19] In this letter, we use UOS, in...

This research aimed at developing comprehensive assessments of physicochemical quality of groundwater for drinking and irrigation purposes at Dalcheon in Ulsan City, Korea. The mean concentration of major ions represented as follows: Ca (94.3 mg/L) > Mg (41.7 mg/L) > Na (19.2 mg/L) > K (3.2 mg/L) for cations and SO4 (351 mg/L) > HCO3 (169 mg/L) > Cl (19 mg/L) for anions. Thematic maps for physicochemical parameters of groundwater were prepared, classified, weighted, and integrated in GIS method with fuzzy logic. The maps exhibited that suitable zone of drinking and irrigation purpose occupied in SE, NE, and NW sectors. The undesirable zone of drinking purpose was observed in SW and central parts and that of irrigation was in the western part of the study area. This was influenced by improperly treated effluents from an abandoned iron ore mine, irrigation, and domestic fields. By grouping analysis, groundwater types were classified into Ca(HCO3)2, (Ca,Mg)Cl2, and CaCl2, and CaHCO3 was the most predominant type. Grouping analysis also showed three types of irrigation water such as C1S1, C1S2, and C1S3. C1S3 type of high salinity to low sodium hazard was the most dominant in the study area. Equilibrium processes elucidated the groundwater samples were in the saturated to undersaturated condition with respect to aragonite, calcite, dolomite, and gypsum due to precipitation and deposition processes. Cluster analysis suggested that high contents of SO4 and HCO3 with low Cl was related with water-rock interactions and along with mining impact. This study showed that the effluents discharged from mining waste was the main sources of groundwater quality deterioration.

No abstract

No abstract

a b s t r a c tMagnetic susceptibility, magnetization and neutron diffraction measurements have been performed to study structure and magnetic states of crystalline and amorphous Nd 2 Fe 14 B and Er 2 Fe 14 B alloys. In the crystalline state there exists a large (about 20%) anisotropy of Er-sublattice magnetization. Values of the magnetic anisotropy constant of Er ions and the Er-Fe exchange-coupling parameter were estimated using temperature dependence of the Er-and Fe-sublattice magnetizations. Amorphous state of the samples was obtained by irradiation of fast (E eff P 1 MeV) neutrons with a fluence up to the 1.2 Â 10 20 n/cm À2 at 340 K. It is shown that antiferromagnetic coupling between the rare-earth and iron spins is kept in the amorphous state. Amorphization of the samples is found to result in reduction of the Curie temperature (T C ) by about 200 K and almost total absence of coercivity. We suggest that the strong decrease of T C is a consequence of enhancement of negative Fe-Fe interactions as a result of dispersion of interatomic distances, which is a characteristic feature of the amorphous state.