Infrared and THz studies of polar phonons and improper magnetodielectric effect in multiferroic BFO3 ceramics

BFO3 ceramics were investigated by means of infrared reflectivity and time domain THz transmission spectroscopy at temperatures 20 - 950 K, and the magnetodielectric effect was studied at 10 - 300 K, with the magnetic field up to 9 T. Below 175 K, the sum of polar phonon contributions into the permittivity corresponds to the value of measured permittivity below 1 MHz. At higher temperatures, a giant low-frequency permittivity was observed, obviously due to the enhanced conductivity and possible Maxwell-Wagner contribution. Above 200 K the observed magnetodielectric effect is caused essentially through the combination of magnetoresistance and the Maxwell-Wagner effect, as recently predicted by Catalan (Appl. Phys. Lett. 88, 102902 (2006)). Since the magnetodielectric effect does not occur due to a coupling of polarization and magnetization as expected in magnetoferroelectrics, we call it improper magnetodielectric effect. Below 175 K the magnetodielectric effect is by several orders of magnitude lower due to the decreased conductivity. Several phonons exhibit gradual softening with increasing temperature, which explains the previously observed high-frequency permittivity increase on heating. The observed non-complete phonon softening seems to be the consequence of the first-order nature of the ferroelectric transition.Comment: subm. to PRB. revised version according to referees' report

Origin of magnetoelectric behavior in BiFeO3_3

The magnetoelectric behavior of BiFeO$_3$ has been explored on the basis of accurate density functional calculations. The structural, electronic, magnetic, and ferroelectric properties of BiFeO$_3$ are predicted correctly without including strong correlation effect in the calculation. Moreover, the experimentally-observed elongation of cubic perovskite-like lattice along the [111] direction is correctly reproduced. At high pressure we predicted a pressure-induced structural transition and the total energy calculations at expanded lattice show two lower energy ferroelectric phases, closer in energy to the ground state phase. Band-structure calculations show that BiFeO$_3$ will be an insulator in A- and G-type antiferromagnetic phases and a metal in other magnetic configurations. Chemical bonding in BiFeO$_3$ has been analyzed using various tools and electron localization function analysis shows that stereochemically active lone-pair electrons at the Bi sites are responsible for displacements of the Bi atoms from the centro-symmetric to the noncentrosymmetric structure and hence the ferroelectricity. A large ferroelectric polarization (88.7 $\mu$C/cm$^{2}$) is predicted in accordance with recent experimental findings. The net polarization is found to mainly ($>$ 98%) originate from Bi atoms. Moreover the large scatter in experimentally reported polarization values is due to the large anisotropy in the spontaneous polarization.Comment: 19 pages, 12 figures, 4 table

Magnetoelectric ordering of BiFeO3 from the perspective of crystal chemistry

In this paper we examine the role of crystal chemistry factors in creating conditions for formation of magnetoelectric ordering in BiFeO3. It is generally accepted that the main reason of the ferroelectric distortion in BiFeO3 is concerned with a stereochemical activity of the Bi lone pair. However, the lone pair is stereochemically active in the paraelectric orthorhombic beta-phase as well. We demonstrate that a crucial role in emerging of phase transitions of the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order types belongs to the change of the degree of the lone pair stereochemical activity - its consecutive increase with the temperature decrease. Using the structural data, we calculated the sign and strength of magnetic couplings in BiFeO3 in the range from 945 C down to 25 C and found the couplings, which undergo the antiferromagnetic-ferromagnetic transition with the temperature decrease and give rise to the antiferromagnetic ordering and its delay in regard to temperature, as compared to the ferroelectric ordering. We discuss the reasons of emerging of the spatially modulated spin structure and its suppression by doping with La3+.Comment: 18 pages, 5 figures, 3 table

Polar and antipolar polymorphs of metastable perovskite BiFe0.5Sc0.5O3

A metastable perovskite BiFe0.5Sc0.5O3 synthesized under high-pressure (6 GPa) and high-temperature (1500 K) conditions was obtained in two different polymorphs, antipolar Pnma and polar Ima2, through an irreversible behavior under a heating/cooling thermal cycling. The Ima2 phase represents an original type of a canted ferroelectric structure where Bi3+ cations exhibit both polar and antipolar displacements along the orthogonal [110](p) and [1 (1) over bar0](p) pseudocubic directions, respectively, and are combined with antiphase octahedral tilting about the polar axis. Both the Pnma and the Ima2 structural modifications exhibit a long-range antiferromagnetic ordering with a weak ferromagnetic component below T-N similar to 220 K. Analysis of the coupling between the dipole, magnetic, and elastic order parameters based on a general phenomenological approach revealed that the weak ferromagnetism in both phases is mainly caused by the presence of the antiphase octahedral tilting whose axial nature directly represents the relevant part of Dzyaloshinskii vector. The magnetoelectric contribution to the spontaneous magnetization allowed in the polar Ima2 phase is described by a fifth-degree free-energy invariant and is expected to be small

Quantification of the 2-Deoxyribonolactone and Nucleoside 5 '-Aldehyde Products of 2-Deoxyribose Oxidation in DNA and Cells by Isotope-Dilution Gas Chromatography Mass Spectrometry: Differential Effects of gamma-Radiation and Fe2+-EDTA

The oxidation of 2-deoxyribose in DNA has emerged as a critical determinant of the cellular toxicity of oxidative damage to DNA, with oxidation of each carbon producing a unique spectrum of electrophilic products. We have developed and validated an isotope-dilution gas chromatography-coupled mass spectrometry (GC−MS) method for the rigorous quantification of two major 2-deoxyribose oxidation products: the 2-deoxyribonolactone abasic site of 1′-oxidation and the nucleoside 5′-aldehyde of 5′-oxidation chemistry. The method entails elimination of these products as 5-methylene-2(5H)-furanone (5MF) and furfural, respectively, followed by derivatization with pentafluorophenylhydrazine (PFPH), addition of isotopically labeled PFPH derivatives as internal standards, extraction of the derivatives, and quantification by GC−MS analysis. The precision and accuracy of the method were validated with oligodeoxynucleotides containing the 2-deoxyribonolactone and nucleoside 5′-aldehyde lesions. Further, the well-defined 2-deoxyribose oxidation chemistry of the enediyne antibiotics, neocarzinostatin and calicheamicin γ1I, was exploited in control studies, with neocarzinostatin producing 10 2-deoxyribonolactone and 300 nucleoside 5′-aldehyde per 106 nt per μM in accord with its established minor 1′- and major 5′-oxidation chemistry. Calicheamicin unexpectedly caused 1′-oxidation at a low level of 10 2-deoxyribonolactone per 106 nt per μM in addition to the expected predominance of 5′-oxidation at 560 nucleoside 5′-aldehyde per 106 nt per μM. The two hydroxyl radical-mediated DNA oxidants, γ-radiation and Fe2+−EDTA, produced nucleoside 5′-aldehyde at a frequency of 57 per 106 nt per Gy (G-value 74 nmol/J) and 3.5 per 106 nt per μM, respectively, which amounted to 40% and 35%, respectively, of total 2-deoxyribose oxidation as measured by a plasmid nicking assay. However, γ-radiation and Fe2+−EDTA produced different proportions of 2-deoxyribonolactone at 7% and 24% of total 2-deoxyribose oxidation, respectively, with frequencies of 10 lesions per 106 nt per Gy (G-value, 13 nmol/J) and 2.4 lesions per 106 nt per μM. Studies in TK6 human lymphoblastoid cells, in which the analytical data were corrected for losses sustained during DNA isolation, revealed background levels of 2-deoxyribonolactone and nucleoside 5′-aldehyde of 9.7 and 73 lesions per 106 nt, respectively. γ-Irradiation of the cells caused increases of 0.045 and 0.22 lesions per 106 nt per Gy, respectively, which represents a 250-fold quenching effect of the cellular environment similar to that observed in previous studies. The proportions of the various 2-deoxyribose oxidation products generated by γ-radiation are similar for purified DNA and cells. These results are consistent with solvent exposure as a major determinant of hydroxyl radical reactivity with 2-deoxyribose in DNA, but the large differences between γ-radiation and Fe2+−EDTA suggest that factors other than hydroxyl radical reactivity govern DNA oxidation chemistry.National Institute of Environmental Health Sciences (ES002109)National Center for Research Resources (U.S.) (RR023783-01)National Center for Research Resources (U.S.) (RR017905-01)National Cancer Institute (U.S.) (CA103146

Breast cancer patients' clinical outcome measures are associated with Src kinase family member expression

<p>BACKGROUND: This study determined mRNA expression levels for Src kinase family (SFK) members in breast tissue specimens and assessed protein expression levels of prominent SFK members in invasive breast cancer to establish associations with clinical outcome. Ki67 was investigated to determine association between SFK members and proliferation.</p> <p>METHODS: The mRNA expression levels were assessed for eight SFK members by quantitative real-time PCR. Immunohistochemistry was performed for c-Src, Lyn, Lck and Ki67.</p> <p>RESULTS: mRNA expression was quantified in all tissue samples. SRC and LYN were the most highly expressed in malignant tissue. LCK was more highly expressed in oestrogen receptor (ER)-negative, compared with ER-positive tumours. High cytoplasmic Src kinase protein expression was significantly associated with decreased disease-specific survival. Lyn was not associated with survival at any cellular location. High membrane Lck expression was significantly associated with improved survival. Ki67 expression correlated with tumour grade and nuclear c-Src, but was not associated with survival.</p> <p>CONCLUSIONS: All eight SFK members were expressed in different breast tissues. Src kinase was highest expressed in breast cancer and had a negative impact on disease-specific survival. Membrane expression of Lck was associated with improved clinical outcome. High expression of Src kinase correlated with high proliferation.</p&gt