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

Magnetodielectric effect and optic soft mode behaviour in quantum paraelectric EuTiO3 ceramics

Infrared reflectivity and time-domain terahertz transmission spectra of EuTiO3 ceramics revealed a polar optic phonon at 6 - 300K, whose softening is fully responsible for the recently observed quantum paraelectric behaviour. Even if our EuTiO3 ceramics show lower permittivity than the single crystal due to a reduced density and/or small amount of secondary pyrochlore Eu2Ti2O7 phase, we confirmed the magnetic field dependence of the permittivity, also slightly smaller than in single crystal. Attempt to reveal the soft phonon dependence at 1.8K on the magnetic field up to 13T remained below the accuracy of our infrared reflectivity experiment

Polar phonons in some compressively stressed epitaxial and polycrystalline SrTiO3 thin films

Several SrTiO3 (STO) thin films without electrodes processed by pulsed laser deposition, of thicknesses down to 40 nm, were studied using infrared transmission and reflection spectroscopy. The complex dielectric responses of polar phonon modes, particularly ferroelectric soft mode, in the films were determined quantitatively. The compressed epitaxial STO films on (100) La0.18Sr0.82Al0.59-Ta0.41O3 substrates (strain 0.9%) show strongly stiffened phonon responses, whereas the soft mode in polycrystalline film on (0001) sapphire substrate shows a strong broadening due to grain boundaries and/or other inhomogeneities and defects. The stiffened soft mode is responsible for a much lower static permittivity in the plane of the compressed film than in the bulk samples.Comment: 11 page

Fast polarization mechanisms in the uniaxial tungsten-bronze relaxor strontium barium niobate SBN-81

The high-frequency dielectric response of the uniaxial strontium barium niobate crystals with 81% of Sr has been studied from 1 kHz to 30 THz along the polar c axis by means of several techniques (far infrared, time domain terahertz, high-frequency and low-frequency dielectric spectroscopies) in a wide temperature interval 20–600 K. Relaxor properties were observed in the complex dielectric response and four main excitations were ascertained below the phonon frequencies. These fast polarization mechanisms take place at THz, GHz and MHz ranges and show different temperature evolution. The central mode excitation in the THz range, related to anharmonic dynamics of cations, slightly softens from high temperatures and then hardens below T ~ 400 K. Below the phase transition (at T ~ 330 K) an additional microwave excitation appears near 10 GHz related to micro domain wall oscillations. The strongest relaxation appears in the GHz range and slows down on cooling according to the Arrhenius law. Finally, another relaxation, present in the MHz range at high temperatures, also slows down on cooling at least to the kHz range. These two relaxations are due to polar fluctuations and nanodomains dynamics. Altogether, the four excitations explain the dielectric permittivity maximum in the kHz range

Первое описание семейного случая спиноцеребеллярной атаксии 14-го типа в России

Spinocerebellar ataxia type 14 (SCA14) is a rare neurodegenerative disease with a predominant cerebellar affection and autosomal dominant inheritance. A characteristic clinical presentation is slowly progressive cerebellar ataxia, hyperreflexia, cognitive impairment and movement disorders (dystonia and myoclonus). Clinical and genetic characteristics of the first familial case of SCA14 in Russia (a 77‑year‑old female patient) caused by heterozygous pathogenic mutation c.155G>C (p.Cys52Ser) in exon 1 in PRKCG gene (NM_002739.1) are presented. The total duration of the disease was 47 years, and the follow‑up period was 32 years. The disease phenotype corresponded to isolated ataxia with a slow rate of progression; brain MRI revealed atrophy of the cerebellar vermis and hemispheres, symmetrical hyperintensity of the dentate nucleus on T2‑weighted images. The features of the SCA14 clinical presentation and the effect of mutations in the regulatory and kinase domains of protein kinase C gamma on the formation of pure and complex phenotypes are discussed.Спиноцеребеллярная атаксия 14‑го типа (СЦА14) – редкое нейродегенеративное заболевание с преимущественным поражением мозжечка и аутосомно‑доминантным типом наследования. Характерная клиническая картина включает медленно прогрессирующую мозжечковую атаксию, гиперрефлексию, когнитивные и двигательные нарушения (дистония, миоклонус). Представлены клинико‑генетические характеристики первого семейного случая СЦА14 в России (пробанд – пациентка 77 лет), обусловленного патогенной гетерозиготной мутацией c.155G>C (p.Cys52Ser) в экзоне 1 гена PRKCG (NM_002739.1). Общая продолжительность заболевания составила 47 лет, катамнез наблюдения – 32 года. Фенотип заболевания соответствовал изолированной атаксии с медленным темпом прогрессирования, при проведении магнитно‑резонансной томографии головного мозга выявлены признаки атрофии червя и полушарий мозжечка, симметричный гиперинтенсивный сигнал от зубчатых ядер в режиме Т2. Обсуждаются особенности клинической картины СЦА14 и влияние мутаций в регуляторном и киназном доменах протеинкиназы С‑гамма на формирование изолированного и комплексного фенотипов

Lattice dynamics and dielectric response of undoped, soft and hard PbZr0.42Ti0.58O3

In this study, ceramic samples of lead zirconate titanate Pb(Zr0.42Ti0.58)O3 (PZT 42/58) doped with Fe (hard) and Nb (soft) were studied by Raman, infrared (IR) and THz spectroscopy in the temperature range from 900 to 20 K. From the evaluation of the IR and Raman phonons in undoped ceramics, we conclude that tetragonal PZT ceramics undergo a low-temperature phase transition to a tilted phase, revealed by anomalies in phonon parameters and the appearance of new modes. Doped samples also presented similar behaviour. The main differences between undoped and doped ceramics lie in the dielectric behaviour below phonon frequency range, where several mechanisms appear: a soft central mode (CM) located near 1 THz and two relaxations in the GHz range. Both types of doping raise the permittivity values below the phonon frequencies, not only increasing the dielectric contribution of the CM, but also modifying the dielectric response near the GHz range. Soft ceramics show higher permittivity with logarithmic increase at low frequencies, corresponding to an almost frequency-independent value of the dielectric losses below 0.1 GHz

Data publication: Unusual dynamics of the ferroelectric phase transition in K1−xLixTaO3 crystals

SHG temperature dependence and dielectric spectroscopy data on KLT measured by A. Pashki