Experimental evidence for an intermediate phase in the multiferroic YMnO3

We have studied YMnO$_{3}$ by high-temperature synchrotron X-ray powder diffraction, and have carried out differential thermal analysis and dilatometry on a single crystal sample. These experiments show two phase transitions at about 1100K and 1350K, respectively. This demonstrates the existence of an intermediate phase between the room temperature ferroelectric and the high temperature centrosymmetric phase. This study identifies for the first time the different high-temperature phase transitions in YMnO$_{3}$.Comment: 10 pages 5 figures. New version, Additional data, Journal of Physics: Condensed Matter, in Pres

Frequency dependent polarisation switching in h-ErMnO3_3

We report an electric-field poling study of the geometric-driven improper ferroelectric h-ErMnO$_3$. From a detailed dielectric analysis we deduce the temperature and frequency dependent range for which single-crystalline h-ErMnO$_3$ exhibits purely intrinsic dielectric behaviour, i.e., free from extrinsic so-called Maxwell-Wagner polarisations that arise, for example, from surface barrier layers. In this regime ferroelectric hysteresis loops as function of frequency, temperature and applied electric fields are measured revealing the theoretically predicted saturation polarisation in the order of 5 - 6 $\mu$C/cm$^2$. Special emphasis is put on frequency-dependent polarisation switching, which is explained in terms of domain-wall movement similar to proper ferroelectrics. Controlling the domain walls via electric fields brings us an important step closer to their utilization in domain-wall-based electronics.Comment: 5 pages, 3 figure

Conflict, cooperation or competition in the Caspian Sea region:A critical review of the New Great Game paradigm

This article critically reviews the New Great Game image of the Caspian Sea region and the assumptions, concepts, and mechanisms (revolving around actors, aims, and motivations) this image is based on. More specifically, this review essay answers the following questions: How does the academic literature interpret the impact of competition between great powers on social, political and economic developments in the Caspian Sea region? Which actors are presented as the dominant players? The essay also introduces the existing criticism of the New Great Game concept and alternatives to it that have already been put forward. By identifying the gaps and limits of existing scholarship, this article offers new avenues for alternative theoretical and empirical interpretations. More specifically, this article argues that the New Great Game literature promotes unsystematic and shallow discussion as it ignores and misunderstands historical, material, political, economic, and normative differences in the Caspian Sea region. Within this discussion, actors, interests, identities, social contexts, and principles are taken to be fixed, i.e. not prone to change or to any sort of adjustmen

High-temperature phase transitions of hexagonal YMnO3

We report a detailed high-resolution powder neutron diffraction investigation of the structural behaviour of the multiferroic hexagonal polymorph of YMnO3 between room temperature and 1403 K. The study was aimed at resolving previous uncertainties regarding the nature of the paraelectric- ferroelectric transition and the possibilities of any secondary structural transitions. We observe a clear transition at 1258 +/- 14 K corresponding to a unit cell tripling and a change in space group from centrosymmetric P6_3/mmc to polar P6_3cm. Despite the fact that this symmetry permits ferroelectricity, our experimental data for this transition analysed in terms of symmetry-adapted displacement modes clearly supports previous theoretical analysis that the transition is driven primarily by the antiferrodistortive K3 mode. We therefore verify previous suggestions that YMnO3 is an improper ferrielectric. Furthermore, our data confirm that the previously suggested intermediate phase with space group P6_3/mcm does not occur. However, we do find evidence for an isosymmetric phase transition (i.e. P6_3cm to P6_3cm) at ~920 K which involves a sharp decrease in polarization. This secondary transition correlates well with several previous reports of anomalies in physical properties in this temperature region and may be related to Y-O hybridization.Comment: Submitted to PR

Hidden order in hexagonal RMnO3 multiferroics

Hexagonal RMnO3 manganites are improper ferroelectrics in which the electric polarization is a by-product of the tripling of the unit cell. In YMnO3, there is a second transition at ~ 920K whose nature remains unexplained. We argue that this transition can be seen as a sort of hidden order in which a residual symmetry displayed by the trimerization order parameter is spontaneously broken. This additional order gives rise to twelve structural domains instead of six, and structural domain boundaries that can be either ferroelectric or non-ferroelectric domain walls