Goldstone-Mode Phonon Dynamics in the Pyrochlore Cd2Re2O7

We have measured the polarized Raman scattering spectra of Cd2Re2O7, the first superconducting pyrochlore, as a function of temperature. For temperatures below the cubic-to-tetragonal structural phase transition (SPT) at 200K, a peak with B1 symmetry develops at zero frequency with divergent intensity. We identify this peak as the first observation of the Goldstone phonon in a crystalline solid. The Goldstone phonon is a collective excitation that exists due to the breaking of the continuous symmetry with the SPT. Its emergence coincides with that of a Raman-active soft mode. The order parameter for both features derives from an unstable doubly-degenerate vibration (with Eu symmetry) of the O1 atoms which drives the SPT.Comment: 4+ pages, 4 figures. Updated figures and text. Accepted to PR

Excitons in coupled InAs/InP self-assembled quantum wires

Optical transitions in coupled InAs/InP self-assembled quantum wires are studied within the single-band effective mass approximation including effects due to strain. Both vertically and horizontally coupled quantum wires are investigated and the ground state, excited states and the photoluminescence peak energies are calculated. Where possible we compare with available photo-luminescence data from which it was possible to determine the height of the quantum wires. An anti-crossing of the energy of excited states is found for vertically coupled wires signaling a change of symmetry of the exciton wavefunction. This crossing is the signature of two different coupling regimes.Comment: 8 pages, 8 figures. To appear in Physical Review

Effect of elicitors on holm oak somatic embryo development and efficacy inducing tolerance to Phytophthora cinnamomi

Holm oak trees (Quercus ilex L.) mortality is increasing worryingly in the Mediterranean area in the last years. To a large degree this mortality is caused by the oomycete Phytophthora spp., which is responsible for forest decline and dieback in evergreen oak forest areas of the southwestern Iberian Peninsula. This study is based on the possibility of applying chemical elicitors or filtered oomycete extracts to holm oak somatic embryos (SE) in order to induce epigenetic memory, priming, that may increase tolerance to the pathogen in future infections. To this end, we first examined the effect of priming treatments on SE development and its oxidative stress state, to avoid elicitors that may cause damage to embryogenic tissues. Both, the sterile oomycete extracts and the chemical elicitor methyl jasmonate (MeJA) did not produce any detrimental effect on SE growth and development, unlike the elicitors benzothiadiazole (BTH) and p-aminobenzoic acid (PABA) that reduced the relative weight gain and resulted in necrotic and deformed SE when were applied at high concentrations (25 µM BTH or 50 µM PABA) in accordance with their high malondialdehyde content. No significant differences among elicitation treatments were found in dual culture bioassays,  although those SEs elicited with 50 µM MeJA increased H2O2 production after challenged against active oomycete indicating the activation of stress response. Since this elicitation treatment did not produce any adverse effect in the embryogenic process we suggest that could be used in further priming experiments to produce holm oak plants adapted to biotic stress

Pressure effects on the transport coefficients of Ba(Fe1-xCox)2As2

We report the temperature dependence of the resistivity and thermoelectric power under hydrostatic pressure of the itinerant antiferromagnet BaFe2As2 and the electron-doped superconductor Ba(Fe0.9Co0.1)2As2. We observe a hole-like contribution to the thermopower below the structural-magnetic transition in the parent compound that is suppressed in magnitude and temperature with pressure. Pressure increases the contribution of electrons to transport in both the doped and undoped compound. In the 10% Co-doped sample, we used a two-band model for thermopower to estimate the carrier concentrations and determine the effect of pressure on the band structure

Critical Strain Region Evaluation of Self-Assembled Semiconductor Quantum Dots

A novel peak finding method to map the strain from high resolution transmission electron micrographs, known as the Peak Pairs method, has been applied to In(Ga) As/AlGaAs quantum dot (QD) samples, which present stacking faults emerging from the QD edges. Moreover, strain distribution has been simulated by the finite element method applying the elastic theory on a 3D QD model. The agreement existing between determined and simulated strain values reveals that these techniques are consistent enough to qualitatively characterize the strain distribution of nanostructured materials. The correct application of both methods allows the localization of critical strain zones in semiconductor QDs, predicting the nucleation of defects, and being a very useful tool for the design of semiconductor device

Synthesis, biological evaluation, and structure-activity relationships of potent noncovalent and nonpeptidic cruzain Inhibitors as anti-trypanosoma cruzi agents

The development of cruzain inhibitors has been driven by the urgent need to develop novel and more effective drugs for the treatment of Chagas' disease. Herein, we report the lead optimization of a class of noncovalent cruzain inhibitors, starting from an inhibitor previously cocrystallized with the enzyme (K-i = 0.8 mu M). With the goal of achieving a better understanding of the structure-activity relationships, we have synthesized and evaluated a series of over 40 analogues, leading to the development of a very promising competitive inhibitor (8r, IC50 = 200 nM, K-i = 82 nM). Investigation of the in vitro trypanocidal activity and preliminary cytotoxicity revealed the potential of the most potent cruzain inhibitors in guiding further medicinal chemistry efforts to develop drug candidates for Chagas' disease57623802392CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPsem informação2013/07600-3; 2012/02230-0; 2010/16778-

Low thermal conductivity of the layered oxide (Na,Ca)Co_2O_4: Another example of a phonon glass and an electron crystal

The thermal conductivity of polycrystalline samples of (Na,Ca)Co_2O_4 is found to be unusually low, 20 mW/cmK at 280 K. On the assumption of the Wiedemann-Franz law, the lattice thermal conductivity is estimated to be 18 mW/cmK at 280 K, and it does not change appreciably with the substitution of Ca for Na. A quantitative analysis has revealed that the phonon mean free path is comparable with the lattice parameters, where the point-defect scattering plays an important role. Electronically the same samples show a metallic conduction down to 4.2 K, which strongly suggests that NaCo_2O_4 exhibits a glass-like poor thermal conduction together with a metal-like good electrical conduction. The present study further suggests that a strongly correlated system with layered structure can act as a material of a phonon glass and an electron crystal.Comment: 5 pages 3 figures, to be published in Phys. Rev.

Weak ferromagnetism and field-induced spin reorientation in K2V3O8

Magnetization and neutron diffraction measurements indicate long-range antiferromagnetic ordering below TN=4 K in the 2D, S=1/2 Heisenberg antiferromagnet K2V3O8. The ordered state exhibits ``weak ferromagnetism'' and novel, field-induced spin reorientations. These experimental observations are well described by a classical, two-spin Heisenberg model incorporating Dzyaloshinskii-Moriya interactions and an additional c-axis anisotropy. This additional anisotropy can be accounted for by inclusion of the symmetric anisotropy term recently described by Kaplan, Shekhtman, Entin-Wohlman, and Aharony. This suggests that K2V3O8 may be a very unique system where the qualitative behavior relies on the presence of this symmetric anisotropy.Comment: 5 pages, 4 ps figures, REVTEX, submitted to PR

First Principles Study of Zn-Sb Thermoelectrics

We report first principles LDA calculations of the electronic structure and thermoelectric properties of $\beta$-Zn$_{4}$Sb$_{3}$. The material is found to be a low carrier density metal with a complex Fermi surface topology and non-trivial dependence of Hall concentration on band filling. The band structure is rather covalent, consistent with experimental observations of good carrier mobility. Calculations of the variation with band filling are used to extract the doping level (band filling) from the experimental Hall number. At this band filling, which actually corresponds to 0.1 electrons per 22 atom unit cell, the calculated thermopower and its temperature dependence are in good agreement with experiment. The high Seebeck coefficient in a metallic material is remarkable, and arises in part from the strong energy dependence of the Fermiology near the experimental band filling. Improved thermoelectric performance is predicted for lower doping levels which corresponds to higher Zn concentrations.Comment: 5 pages, 6 figure