Landau Theory of Domain Wall Magnetoelectricity

We calculate the exact analytical solution to the domain wall properties in a multiferroic system with two order parameters that are coupled bi-quadratically. This is then adapted to the case of a magnetoelectric multiferroic material such as BiFeO3, with a view to examine critically whether the domain walls can account for the enhancement of magnetization reported for thin films fo this material, in view of the correlation between increasing magnetization and increasing volume fraction of domain walls as films become thinner. The present analysis can be generalized to describe a class of magnetoelectric devices based upon domain walls rather than bulk properties.Comment: 9 pages, 4 figure

Independent ferroelectric contributions and rare-earth-induced polarization reversal in multiferroic TbMn2O5

Three independent contributions to the magnetically induced spontaneous polarization of multiferroic TbMn2O5 are uniquely separated by optical second harmonic generation and an analysis in terms of Landau theory. Two of them are related to the magnetic Mn3+/4+ order and are independent of applied fields of up to 7 T. The third contribution is related to the long-range antiferromagnetic Tb3+ order. It shows a drastic decrease upon the application of a magnetic field and mediates the change of sign of the spontaneous electric polarization in TbMn2O5. The close relationship between the rare-earth long-range order and the non-linear optical properties points to isotropic Tb-Tb exchange and oxygen spin polarization as mechanism for this rare-earth induced ferroelectricity.Comment: 8 pages, 5 figure

Pretransitional phenomena in dilute crystals with first-order phase transition

Pretransitional phenomena at first-order phase transition in crystals diluted by 'neutral' impurities (analogue of nonmagnetic atoms in dilute magnets) are considered. It is shown that field dependence of order parameter becomes nonanalytical in the stability region of the ordered phase, while smeared jumps of thermodynamic parameters and anomalous (non-exponential) relaxation appear near transition temperature of pure crystal.Comment: 4 page

Finite strain Landau theory of high pressure phase transformations

The properties of materials near structural phase transitions are often successfully described in the framework of Landau theory. While the focus is usually on phase transitions, which are induced by temperature changes approaching a critical temperature T-c, here we will discuss structural phase transformations driven by high hydrostatic pressure, as they are of major importance for understanding processes in the interior of the earth. Since at very high pressures the deformations of a material are generally very large, one needs to apply a fully nonlinear description taking physical as well as geometrical nonlinearities (finite strains) into account. In particular it is necessary to retune conventional Landau theory to describe such phase transitions. In Troster et al (2002 Phys. Rev. Lett. 88 55503) we constructed a Landau-type free energy based on an order parameter part, an order parameter-(finite) strain coupling and a nonlinear elastic term. This model provides an excellent and efficient framework for the systematic study of phase transformations for a wide range of materials up to ultrahigh pressures

Rehabilitación de las instalaciones eléctricas en los edificios destinados principalmente a viviendas

The objective of the present work is to report about the complexity of the renovation of the electric installation of a building, in the frame of a entire building renovation. We will also discuss the regulations that must be taken into account in the project and that the electrician will have to follow. During the design of the renovation of the electric installation of a building, it is necessary to consider what we want to renovate and which use will be given to the building afterwards. Further, the work must be intended for, at least, 30 to 50 years of utilization. We have also made a review of all the components of the connection installation in a building, applying regulations and way of implementation. About the interior electric equipment of housings, it is described the circuits that must be implemented in any case according to the low voltage regulation from 2002, presently in effect. These circuits fit out the buildings for the electrification currently needed in housings. The renovation works in electric installations require good management of the present regulations, both national and regional, and wide experience on how to apply them in each case. Note that the characteristics of the buildings must be preserved and the renovation work must not cause the building as a whole or its more significant parts to deteriorate.Este trabajo tiene como objetivo difundir la complejidad que conlleva la rehabilitación de las instalaciones eléctricas de un edificio dentro de la rehabilitación integral de los mismos, así como la normativa que la regula y que el proyectista, y posteriormente el instalador electricista que realiza la obra, deben de cumplir. A la hora de pensar en la rehabilitación de la instalación eléctrica de un edificio debemos plantearnos que es lo que se va a rehabilitar y cual va a ser el uso de esas instalaciones después de la rehabilitación, teniendo presente que el trabajo que se realice tiene que ser de utilidad al menos para 30 – 50 años. Se hace un repaso de todas las partes que componen las instalaciones eléctricas de enlace de un edificio, normativa a aplicar y forma de ejecución. En las instalaciones interiores de las viviendas se describen los circuitos que como mínimo se deben ejecutar de acuerdo con el reglamento electrotécnico para baja tensión de 2002, actualmente en vigor, y que habilitan a esas viviendas para el nivel de electrificación que requieren actualmente las viviendas. Los trabajos de rehabilitación de instalaciones eléctricas requieren un manejo importante de la legislación actual, tanto a nivel nacional como autonómica, y una gran experiencia en cómo aplicar esta norma en los edificios, por cuyas características debemos conservar, haciendo unas instalaciones que no deterioren el edificio en su conjunto ni en sus partes más significativas

Two and Three Dimensional Incommensurate Modulation in Optimally-Doped Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta}

X-ray scattering measurements on optimally-doped single crystal samples of the high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ reveal the presence of three distinct incommensurate charge modulations, each involving a roughly fivefold increase in the unit cell dimension along the {\bf b}-direction. The strongest scattering comes from the well known (H, K$\pm$ 0.21, L) modulation and its harmonics. However, we also observe broad diffraction which peak up at the L values complementary to those which characterize the known modulated structure. These diffraction features correspond to correlation lengths of roughly a unit cell dimension, $\xi_c$$\sim$20 $\AA$ in the {\bf c} direction, and of $\xi_b$$\sim$ 185 $\AA$ parallel to the incommensurate wavevector. We interpret these features as arising from three dimensional incommensurate domains and the interfaces between them, respectively. In addition we investigate the recently discovered incommensuate modulations which peak up at (1/2, K$\pm$ 0.21, L) and related wavevectors. Here we explicitly study the L-dependence of this scattering and see that these charge modulations are two dimensional in nature with weak correlations on the scale of a bilayer thickness, and that they correspond to short range, isotropic correlation lengths within the basal plane. We relate these new incommensurate modulations to the electronic nanostructure observed in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ using STM topography.Comment: 8 pages, 8 figure

Salivary androgens in adolescence and their value as a marker of puberty: results from the SCAMP cohort

Context: Salivary androgens represent non-invasive biomarkers of puberty that may have utility in clinical and population studies. Objective: To understand normal age-related variation in salivary sex steroids and demonstrate their correlation to pubertal development in young adolescents. Design, Setting, and participants: School-based cohort study of 1,495 adolescents at two time points for collecting saliva samples approximately two years apart. Outcome measures: The saliva samples were analyzed for five androgens (testosterone, androstenedione (A4), 17-hydroxyprogesterone (17-OHP), 11-ketotestosterone (11-KT) and 11β-hydroxyandrostenedione (11-OHA4)) using LC-MS/MS; in addition, salivary dehydroepiandrosterone (DHEA) and oestradiol (OE2) were analyzed by ELISA. Pubertal staging was self-reported using the pubertal development scale (PDS). Results: In 1,236 saliva samples from 903 boys aged between 11-16 years, salivary androgens except DHEA exhibited an increasing trend with an advancing age (ANOVA, p<0.001), with salivary testosterone and A4 concentration showing the strongest correlation (r=0.55, p<0.001 and r=0.48, p<0.001, respectively). In a subgroup analysis of 155 and 63 saliva samples in boys and girls, respectively morning salivary testosterone concentrations showed the highest correlation with composite PDS scores and voice-breaking category from PDS self-report in boys (r=0.75, r=0.67, respectively). In girls, salivary DHEA and OE2 had negligible correlations with age or composite PDS scores. Conclusion: In boys aged 11-16 years, increase in salivary testosterone and A4 is associated with self-reported pubertal progress and represent valid non-invasive biomarkers of puberty in boys

Observation of a continuous phase transition in a shape-memory alloy

Elastic neutron-scattering, inelastic x-ray scattering, specific-heat, and pressure-dependent electrical transport measurements have been made on single crystals of AuZn and Au_{0.52}Zn_{0.48} above and below their martensitic transition temperatures (T_M=64K and 45K, respectively). In each composition, elastic neutron scattering detects new commensurate Bragg peaks (modulation) appearing at Q = (1.33,0.67,0) at temperatures corresponding to each sample's T_M. Although the new Bragg peaks appear in a discontinuous manner in the Au_{0.52}Zn_{0.48} sample, they appear in a continuous manner in AuZn. Surprising us, the temperature dependence of the AuZn Bragg peak intensity and the specific-heat jump near the transition temperature are in favorable accord with a mean-field approximation. A Landau-theory-based fit to the pressure dependence of the transition temperature suggests the presence of a critical endpoint in the AuZn phase diagram located at T_M*=2.7K and p*=3.1GPa, with a quantum saturation temperature \theta_s=48.3 +/- 3.7K.Comment: 6 figure