SHG microscopic observations of polar state in Li-doped KTaO3 under electric field

Incipient ferroelectric KTaO3 with off-center Li impurity of the critical concentration of 2.8 mol% was investigated in order to clarify the dipole glass state under electric field. Using optical second-harmonic generation (SHG) microscope, we observed a marked history dependence of SHG intensity through zero-field cooling (ZFC), zero-field heating (ZFH), field heating after ZFC (FH/ZFC) and FH after field cooling (FH/FC). These show different paths with respect to temperature: In the ZFC/ZFH process, weak SHG was observed at low temperature, while in the FH/ZFC process, relatively high SHG appears in a limited temperature range below TF depending on the field strength, and in the FC and FH/FC processes, the SHG exhibits ferroelectric-like temperature dependence: it appears at the freezing temperature of 50K, increases with decreasing temperature and has a tendency of saturation. These experimental results strongly suggest that dipole glass state or polar nano-clusters which gradually freezes with decreasing temperature is transformed into semi-macroscopic polar state under the electric field. However at sufficiently low temperature, the freezing is so strong that the electric field cannot enlarge the polar clusters. These experimental results show that the polar nano-cluster model similar to relaxors would be more relevant in KTaO3 doped with the critical concentration of Li. Further experiments on the anisotropy of SHG determine that the average symmetry of the field-induced polar phase is tetragonal 4mm or 4, which is also confirmed by the X-ray diffraction measurement.Comment: 26 pages, 8 figures, 1 tabl

The Energetics of Li Off-Centering in K1−x_{1-x}Lix_xTaO3_3; First Principles Calculations

K$_{1-x}$Li$_{x}$TaO$_3$ (KLT) solid solutions exhibit a variety of interesting physical phenomena related to large displacements of Li-ions from ideal perovskite A-site positions. First-principles calculations for KLT supercells were used to investigate these phenomena. Lattice dynamics calculations for KLT exhibit a Li off-centering instability. The energetics of Li-displacements for isolated Li-ions and for Li-Li pairs up to 4th neighbors were calculated. Interactions between nearest neighbor Li-ions, in a Li-Li pair, strongly favor ferroelectric alignment along the pair axis. Such Li-Li pairs can be considered "seeds" for polar nanoclusters in KLT. Electrostriction, local oxygen relaxation, coupling to the KT soft-mode, and interactions with neighboring Li-ions all enhance the polarization from Li off-centering. Calculated hopping barriers for isolated Li-ions and for nearest neighbor Li-Li pairs are in good agreement with Arrhenius fits to experimental dielectric data.Comment: 14 pages including 10 figures. To Physical Review B. Replaced after corrections due to referees' remark

Nuove metodologie per le Scienze Forensi: la spettroscopia Raman nella rilevazione di tracce ematiche

Le attività nell’ambito delle Scienze forensi dell’Università di Pavia sono state oggetto di un Workshop internazionale dal titolo “Improving the death scene investigation: advanced multidisciplinary approaches and their use in court” con la partecipazione del Direttore del Servizio di Polizia Scientifica e dei Direttori dei Reparti Speciali della Polizia di Stato e del Comandante del Racis che è stata espressione dell’importanza dell’approccio interdisciplinare alle tematiche forensi e della consolidata collaborazione tra i differenti attori. In tal senso va sottolineata la nascita di una piattaforma universitaria interdipartimentale che si occupa di Scienze Forensi (FIND – Forensic Interdisciplinary Networking Domain) aperta al contributo di molti attori esterni. Alla base di questo progetto vi è la condivisa convinzione che l’unico modo di rendere le investigazioni scientifiche davvero probanti ai fini giudiziali sia la continua collaborazione tra enti differenti e tra professionalità differenti, ciascuna al servizio di un fine comune, non raggiungibile se perseguito individualmente come peraltro evidenziato a proposito della spettroscopia micro-Raman. In aggiunta ai risultati mostrati nell’articolo si riporta nel seguito un interessante risultato che indica come la tecnica possa diventare valido strumento complementare alla classica prova del Luminol. L’analisi micro-Raman è stata condotta su di un’ascia arrugginita che presentava alcune macchie apparentemente attribuibili a materiale ematico. In tal caso l’analisi al Luminol porterebbe a risultati non affidabili poiché notoriamente la presenza di ruggine, ossido di Ferro α-Fe2O3, produce un cosidetto falso positivo al Luminol, cioè la reazione luminescente viene attivata non dalla presenza di materiale ematico ma dalla ruggine stessa. In questo la spettroscopia micro-Raman consente di discriminare molto rapidamente la differente natura di tracce a composizione differente come è facile notare dagli spettri riportati nella figura sottostante e prevelati da due zone differenti come mostrato dalle frecc

Optical Properties of Cubic Stabilized Zirconia

Optical absorption across the fundamental edge and the associated photoluminescence, have been studied in single crystals of yttrium stabilized zirconia (YSZ) of varying yttrium concentration. The results allow discrimination of a region of intrinsic transitions (above 5.2 eV) from a low energy region, where disorder-induced transitions are dominating. The size of extra absorption in this range shows an inverse correlation to the stoichiometric ratio, to be interpreted in terms of progressive ordering of the vacancy system

Microstructural modification of LiNbO3 crystals induced by femtosecond laser irradiation

Microstructural modifications were induced by femtosecond laser irradiation on lithium mobate (LN) crystals and investigated by means of optical microscopy and micro-Raman spectroscopy. Commercial z-cut congruent LN substrates were irradiated with focused ultra-short laser pulses at 810 nm, generated by a Ti:Sapphire oscillator or an amplified Ti:Sapphire laser system. At the focus region, either refractive index changes or material removal were observed by varying the irradiation conditions. The Raman spectrum shows in the illuminated regions the insurgence of Raman modes, which are forbidden in the used-scattering geometry. This phenomenon increases gradually by approaching the ablation region, where nobium oxides and Li-deficient surfaces are formed for the low energy and high repetition rate pulses from the Ti:Sapphire oscillator. Amorphous surfaces are obtained for high energy and low repetition rate pulses from the amplified Ti:Sapphire laser system. (c) 2005 Elsevier B.V. All rights reserved

Structural investigation of high-Tc superconductor EuBa2Cu3Oy by microRaman spectroscopy

Polycrystalline EuBa2Cu3Oy (EuBCO) samples with various oxygen contents (y 6.25, 6.38 and 6.8) have been analysed by means of microRaman spectroscopy. The usual Raman features of REBa2Cu3Oy high temperature superconductors (RE Y, Eu, Sm, Er¼) have been con®rmed also for EuBCO. A correlation between the crystal structure and the conductivity properties as a function of the oxygen content has been established by studying in detail the features of the different Raman spectra. In particular, the analysis of the peak pro®les of lattice vibrations involving oxygen atoms provides important information to understand the anomalous superconducting properties found in the sample with y 6.38. In addition, the appearance in the superconducting samples of a new mode centred at 223 cm21 may be correlated to ordering phenomena which have been called to justify the conductivity results

New occurrence of fire opal from Bemia, Madagascar

Twenty-two gem opals from a new volcanic deposit located near Bemia, in southeastern Madagascar, were investigated by classical gemological methods, SEM-EDS, powder X-ray diffraction analysis, LA-ICP-MS, and Raman and IR spectroscopy. Although none of the opals show play-of-color, they exhibit a wide variety of hues-- including those typical of fire opal--that are related to iron content, mainly from Fe-rich inclusions. Consistent with their volcanic origin, these samples are microcrystalline and composed of opal-CT or opal-C. Among the inclusions were ilmenite needles, clay minerals, and iron oxides and hydroxides. The RI and, in particular, SG values were higher than those typical of natural fire opal (e.g., from Mexico) and some synthetic fire opal, allowing for a rapid separation