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We report the temperature dependent THz to mid-infrared response of hexagonal-ErMnO3 using absorption, reflectivity, and emissivity techniques from 2 K to 1700 K. At low temperatures, lowest frequency vibrational modes coexist with paramagnon excitations associated with well-defined crystal field Rare Earth pure magnetic replicas in an intriguing phonon-magnetic convergence. Increasing the temperature, a number of vibrational bands close to the space group predicted undergo profile broadening and softening. In particular, a distinctive set of bands in the 288-329 cm-1 (300 K) range has a component whose profile is carrier screened becoming nearly fully blurred in the intermediate phase between ~830 K and ~1500 K. Below TC ~830 K this asymmetric band further splits as spin phonon interaction and the tripling of the unit cell takes place revealing a delicate balance of long- and short-range interactions. Ambient Raman scattering brings up evidence of a Fr��hlich resonance due to Coulomb interactions between carriers and the macroscopic field of the longitudinal optical phonon mode. We found it is dynamically correlated to the hexagonal c-axis negative thermal expansion. Mid-infrared (MIR) optical conductivity show that also plays a role in small polarons and mediates in high temperature bipolarones.At higher temperatures a low frequency Drude contribution is triggered by electron hopping signaling an insulator-metal phase transition at ~1600 K while the MIR response suggests coexistence between single small polarons and bipolarons. We draw a parallel with improper ferroelectrics sustaining a lattice incommensurate intermediate phase and unit cell tripling. We argue that in the h-RMnO3 (R=Rare Earth, Y) family of compounds the intermediate phase be considered incommensurate with onset at TINC ~1500 K and ferroelectric lock-in at TC ~830 K delimiting this regime in h-ErMnO3., Manuscript and Supplemental Material (66 pages)

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Multitrophic interactions are powerful forces shaping the structure of living communities. Plants encounter a great diversity of organisms in their environment: some of these interactions are beneficial (e.g. symbiotic fungi and insect pollinators) while some are detrimental (e.g. herbivorous insects and pathogenic micro-organisms). Multitrophic interactions between below-ground and above-ground organisms are receiving increasing attention because they may influence plant defences against biotic and abiotic stresses. In this study we show that an arbuscular mycorrhizal symbiosis makes tomato plants significantly more resistant towards aphids, by enhancing both direct defences, both attractivity towards aphid parasitoids.

The aim of this work was to assess the effects of a combined inoculum of a rhizobacterium and an arbuscular mycorrhizal (AM) fungus on plant responses to phytoplasma infection, and on phytoplasma multiplication and viability in Chrysanthemum carinatum plants infected by chrysanthemum yellows phytoplasma (CY). Combined inoculation with Glomus mosseae BEG12 and Pseudomonas putida S1Pf1Rif resulted in some resistance to phytoplasma infection (about 30%), delayed symptom expression in nonresistant plants, improved growth of the aerial part of the infected plants (+68Æ1%), and altered root morphology (root tip number: +49Æ9%; branching degree: +82Æ8%). Combined inoculation with the two beneficial microorganisms did not alter CY multiplication and viability. In inoculated and infected plants, phytoplasma morphology was typical of senescent cells. A more active and efficient root system in double-inoculated plants probably mediated the effects of the two rhizospheric microorganisms in the infected plants. The practical application of rhizospheric microorgan- isms for mitigating phytoplasma damage, following evaluation under field conditions, represents an additional tool for the integrated management of phytoplasmosis

Se analizan las propiedades ópticas de nanocristales de ZrO2 puros de tamaño de 10 nm, puros y dopados con 2, 4 y 6% mol de itria, mediante la técnica FT-IR en el rango de frecuencias 400-1000 cm-1. El espectro muestra un pico marcado en frecuencias cercanas a los 460 cm-1, que es asociado a nanocristales de estructura tetragonal. Se realiza también una discusión de las propiedades de simetría de los modos activos IR y Raman Fil: Casali, Ricardo Antonio. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura (UNNE-FaCENA). Corrientes. Argentina Fil: Massa, N. E.. Universidad Nacional de La Plata - CONICET. Centro de Química Inorgánica "Dr. Pedro J. Aymonino" (CEQUINOR). Buenos Aires. Argentina Fil: Ponce, C. A.. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura (UNNE-FaCENA). Corrientes. Argentina

6 pages, 7 figures.-- Communication presented at the 6th World Multiconference on Systemics, Cybernetics and Informatics and 8th International Conference on Information System Analysis and Synthesis (SCI/ISAS 2002, Orlando, Florida, Jul 14-18, 2002)., We report on electron-phonon interactions as a main common property in simple oxides deduced from infrared measurements, and search, using X-ray absorption techniques, a local structure counterpart associated to it. We found that temperature dependent electron localization is determinant in explaining the conductivity in RNiO3 (R= rare earth) and in La(0.67)Ca(0.33)MnO3 and their metal-insulator phase transitions. This is also associated to a lower symmetry lattice distortion in the insulating phase. The mechanism established from comparison of the experimental conductivity obtained from reflectivity spectra with that from Reik’s theory [1] suggests small polaron hopping as the principle for electron displacement in an oxygen high polarizable lattice. We also comment our preliminary results on the temperature dependent consequence of Jahn- Teller and GdFeO3 lattice distortions in the insulator YVO3.

We report on the preparation dependence of the La0.67C0.33MnO3 insulator-metal phase transition. We have found that while resistivity measurements show that sol-gel techniques yield samples with a sharp transition with the lowest Tc at ≈ 180 K in a highly resistive medium, those prepared under 20 kbar and 1000 °C have an increment of about 80 degrees in their insulator-metal transition at ≈254 K. Intermediate to these results we found that the 1300 °C temperature cycling of sol-gel samples results in two phase compounds with their resistivities peaking at ≈180 K and ≈254 K. Although this also suggests that in these last ones there is a miscibility gap of two distinct ferromagnetic phases, susceptibility measurements only show ferromagnetic saturation at lower temperatures. The 77 K far infrared reflectivity of these compounds is typical of conducting oxides, their Drude component scaling with the overall resistivities.

We report temperature dependent infrared and Raman spectra of pyrochlores Tl2-xBixMn2O7 (x = 0.00, 0.10, 0.50). When Tl is substituted by Bi the magnetoresistance increases dramatically. This shows up in our reflectivity spectra for x = 0.10 as a weaker Drude contribution implying a downscaling of the number of active carriers at the insulator-metal phase transition (TIM) and lower temperatures. A system magnetically frustrated with no defined Tc is found for nominal x = 0.50. While in this last case the overall infrared features are maintained there is a weakening and broadening in the bands centered at ≈250 cm-1. A relative increase in intensity of the band at ≈900 cm-1, assigned to lattice polarons, is understood as indicative of stronger localization. Raman spectra are characterized by phonon bands sensitive to the bismuth content and for a band at ≈816 cm-1 that weakens when approaching TIM. We tentatively associate its origin to a bound state due to a polaron related octahedral distortion.

En esta comunicación presentamos resultados preliminares del espectro Raman de Rb₂SeO₄. Este compuesto, que pertenece al grupo espacial Pnam es isomorfo a temperatura ambiente con K₂SeO₁. Este último es prototipo de la conocida familia de compuestos que tiene por característica las transiciones de fase en secuencia paraeléctrica - inconmensurada - ferroeléctrica. El espectro Raman de los modos internos de Rb₂SeIO₄, tiene, como era de esperar, características similares a las de K₂SO₄ observándose un corrimiento hacia frecuencias más bajas del orden de 5 cmˉ¹. Los modos de la red muestran que este compuesto también puede considerarse pseudohexagonal ya que la cantidad de modos Big es menor que la calculada por análisis de grupo factor. Más importante, en estas medidas que ahora están siendo llevadas a cabo, no se observa hasta 98 K ningún modo blando que pueda ser reminiscente al comportamiento del modo amplitud característico de la fase inconmensurada en el compuesto prototípico. Nuestros resultados son coincidentes con cálculos teóricos recientes que identifican el incremento del radio catiónico (de K a Rb) como un factor inhibidor de la secuencia de transiciones de fase arriba mencionada Fil: Massa, N. E.. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física y Química (UNLP-FCE). Buenos Aires. Argentina Fil: Igartua Aldamiz, J. M.. Universidad del País Vasco. Departamento de Física de la Materia Condensada (UPV-EHU). Bilbao. España Fil: Echarri, A. L.. Universidad del País Vasco. Departamento de Física de la Materia Condensada (UPV-EHU). Bilbao. España Fil: Breczewski, T.. Universidad del País Vasco. Departamento de Física de la Materia Condensada (UPV-EHU). Bilbao. España

Si bien con espectroscopía infrarroja y/o Raman no es posible determinar unívocamente si una transición sólido- solido conduce a una fase inconmensurada, estas técnicas son de extrema importancia para su caracterización. En esta comunicación mostramos que modos solamente activos debajo de T ing en un sistema modulado sinusoidalmente en una dimensión, tal como en K₂SeO₁, crecen en intensidad con dependencia en temperatura cercana al exponente crítico 2ẞ. Además mostraremos que, por el contrario, en aquellos cuasiunidimensionales, como X₀.₃MoO₃,(X=K, Rb), a temperaturas debajo de la transición de Peierls se forman "clusters" de material conmensurado que permite delinear con difusión Raman densidades de estados fonónicas en una dimensión, en los cuales las singularidades de Van Hove son los nuevos modos activos de la fase incomensurada Fil: Massa , N. E.. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física y Química (UNLP-FCE). Buenos Aires. Argentina

Es conocido que los óxidos superconductores de alta temperatura tienen como característica principal el orden antiferromagnético de los planos de CuO₂. Esto permite detectar, con técnicas comunes a espectroscopía Raman, scattering inelástico de pares de magnones como una banda ancha centrada alrededor de 2500 cmˉ¹. En esta comunicación presentamos medidas preliminares de dichas excitaciones en pastillas superconductoras de Bi-2212 nominalmente puras y semiconductoras dopada con Fe bajo una corriente DC a temperaturas inferiores a Tₑ. Encontramos que bajo estas condiciones aumenta el ancho medio a media altura del perfil aproximadamente simétrico de la banda de dos magnones de la pastilla pura y que del lado de más altas frecuencias hay un significativo incremento en su intensidad. Estas medidas sugieren una explicación según la cual los portadores involucrados en la corriente continua afectarían la interacción de intercambio a través de una interacción electrón magnón Fil: Massa, N. E.. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física y química (UNLP-FCE). Buenos Aires. Argentina Fil: Etchegoin,P. G.. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche. Instituto Balseiro (CNEA-CAB). Río Negro. Argentina Fil: Fainstein, C.. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche. Instituto Balseiro (CNEA-CAB). Río Negro. Argentina