On-site magnetization in open antiferromagnetic chains: a classical analysis versus NMR experiments in a spin-1 compound

The response of an open spin chain with isotropic antiferromagnetic interactions to a uniform magnetic field is studied by classical Monte Carlo simulations. It is observed how the induced on-site magnetization is non uniform, due to the occurrence of edge staggered terms which decay exponentially over a distance equal to the zero field correlation length of the infinite chain. The total magnetic moment associated to each staggered term is found to be about half of the original single-spin magnitude and to decrease as the inverse of temperature (i.e. to behave as a Curie-like moment). The numerical results are compared to recent NMR findings in spinless-doped Y(2)BaNiO(5); the remarkable agreement found shows that, for temperatures above the Haldane gap, the classical approach gives a correct picture of the boundary effects observed in the Heisenberg S=1 chain.Comment: 4 pages, 4 eps figures; minor changes in the text; added reference

Comparison of S=0 and S=1/2 Impurities in Haldane Chain Compound, Y2BaNiO5Y_{2}BaNiO_{5}

We present the effect of Zn (S=0) and Cu (S=1/2) substitution at the Ni site of S=1 Haldane chain compound $Y_{2}BaNiO_{5}$. $^{89}$Y NMR allows us to measure the local magnetic susceptibility at different distances from the defects. The $^{89}$Y NMR spectrum consists of one central peak and several less intense satellite peaks. The shift of the central peak measures the uniform susceptibility, which displays a Haldane gap $Delta$$equiv$100 K and it corresponds to an AF coupling J$equiv$260 K between the near-neighbor Ni spins. Zn or Cu substitution does not affect the Haldane gap. The satellites, which are evenly distributed on the two sides of the central peak, probe the antiferromagnetic staggered magnetization near the substituted site, which decays exponentially. Its extension is found identical for both impurities and corresponds accurately to the correlation length $xi$(T) determined by Monte Carlo (QMC) simulations for the pure compound. In the case of non-magnetic Zn, the temperature dependence of the induced magnetization is consistent with a Curie law with an "effective" spin S=0.4 on each side of Zn, which is well accounted by Quantum Monte Carlo computations of the spinless-defect-induced magnetism. In the case of magnetic Cu, the similarity of the induced magnetism to the Zn case implies a weak coupling of the Cu spin to the nearest- neighbor Ni spins. The slight reductionin the induced polarization with respect to Zn is reproduced by QMC computations by considering an antiferromagnetic coupling of strength J'=0.1-0.2 J between the S=1/2 Cu-spin and nearest-neighbor Ni-spin.Comment: 15 pages, 18 figures, submitted to Physical Review

Possible Localized Modes in the Uniform Quantum Heisenberg Chains of Sr2CuO3

A model of mobile-bond defects is tentatively proposed to analyze the "anomalies" observed on the NMR spectrum of the quantum Heisenberg chains of Sr2CuO3. A bond-defect is a local change in the exchange coupling. It results in a local alternating magnetization (LAM), which when the defect moves, creates a flipping process of the local field seen by each nuclear spin. At low temperature, when the overlap of the LAM becomes large, the defects form a periodic structure, which extends over almost all the chains. In that regime, the density of bond-defects decreases linearly with T.Comment: 4 pages + 3 figures. To appear in Physical Review

Magnetization profiles and NMR spectra of doped Haldane chains at finite temperatures

Open segments of S=1 antiferromagnetic spin chains are studied at finite temperatures and fields using continuous time Quantum Monte Carlo techniques. By calculating the resulting magnetization profiles for a large range of chain lengths with fixed field and temperature we reconstruct the experimentally measured NMR spectrum of impurity doped Y$_2$BaNi$_{1-x}$Mg$_x$O$_5$. For temperatures above the gap the calculated NMR spectra are in excellent agreement with the experimental results, confirming the existence of $S=1/2$ excitations at the end of open S=1 chain segments. At temperatures below the gap, neglecting inter chain couplings, we still find well defined peaks in the calculated NMR spectra corresponding to the $S=1/2$ chain end excitations. At low temperatures, inter chain couplings could be important, resulting in a more complicated phase.Comment: 7 pages, 5 figures, minor correction

Puesta en marcha y estudio de la operación en campo de microinversores de potencia para aplicaciones fotovoltaicas

En este trabajo se estudia el sistema asociado a la operación y monitoreo de dos micro-inversores de potencia, desarrollados por el Laboratorio de Instrumentación y Control de la Universidad Nacional de Mar del Plata y Lytron SA. Cada microinversor inyecta a la red la energía generada por un panel fotovoltaico de 290 Wp. Ambos paneles, cada uno con su respectivo micro-inversor, se encuentran emplazados en la azotea de la Facultad. Además, se realiza una comparativa del desempeño de los micro-inversores individuales frente al de inversores de mayor potencia que inyectan actualmente la energía generada por arreglos de 6 paneles fotovoltaicos de las mismas características. Los mismos forman parte de la planta piloto de la Facultad, que integra el proyecto IRESUD. Se describe el tratamiento de los datos de irradiancia a fin de estimar el recurso solar en el plano de los paneles fotovoltaicos. Se presentan los datos de recurso y producción obtenidos durante tres semanas en el mes de abril. Luego, se toman como objeto de comparación dos días típicos, uno nublado y el otro soleado, relevando curvas de recurso y producción de los sistemas solares bajo estudio. Finalmente, se concluye que este tipo de inversores tiene un rendimiento aproximado al de los inversores convencionales, dejando en evidencia que el precio por unidad de potencia sería el factor determinante al momento de la elección del inversor que adoptará un sistema fotovoltaico.This paper studies the system involved in the operation and monitoring of two micro-inverters, developed by the Laboratorio de Instrumentación y Control of the Facultad de Ingeniería of the Universidad Nacional de Mar del Plata and Lytron SA. Each micro-inverter delivers to the grid the power generated by single photovoltaic panels with 290 Wp. Both, panels and micro-inverters, are settled in the roof of the building of the engineering school. Also, a comparison is made of the performance between individual inverters with others of higher power, of which the latter are injecting power from arrays of six identical photovoltaic panels. They are part of the photovoltaic solar plant of the engineering school, integrating the IRESUD project. Is described the data treatment of solar irradiance which aim to estimate the solar resource in the tilted plane of the panels. Resource and production data from three weeks of April are shown. Then, two typical days, one cloudy and one sunny, are taken as object of comparison displaying trends of resource and production from the solar systems under study. Finally, it is concluded that this type of inverters has an approximate performance to conventional inverters, manifesting that the price per watt peak unit would be the main factor for choosing the inverter which makes up a photovoltaic system.Tema 4: Energía solar, conversión fotovoltaica.Facultad de Arquitectura y Urbanism

Puesta en marcha y estudio de la operación en campo de microinversores de potencia para aplicaciones fotovoltaicas

En este trabajo se estudia el sistema asociado a la operación y monitoreo de dos micro-inversores de potencia, desarrollados por el Laboratorio de Instrumentación y Control de la Universidad Nacional de Mar del Plata y Lytron SA. Cada microinversor inyecta a la red la energía generada por un panel fotovoltaico de 290 Wp. Ambos paneles, cada uno con su respectivo micro-inversor, se encuentran emplazados en la azotea de la Facultad. Además, se realiza una comparativa del desempeño de los micro-inversores individuales frente al de inversores de mayor potencia que inyectan actualmente la energía generada por arreglos de 6 paneles fotovoltaicos de las mismas características. Los mismos forman parte de la planta piloto de la Facultad, que integra el proyecto IRESUD. Se describe el tratamiento de los datos de irradiancia a fin de estimar el recurso solar en el plano de los paneles fotovoltaicos. Se presentan los datos de recurso y producción obtenidos durante tres semanas en el mes de abril. Luego, se toman como objeto de comparación dos días típicos, uno nublado y el otro soleado, relevando curvas de recurso y producción de los sistemas solares bajo estudio. Finalmente, se concluye que este tipo de inversores tiene un rendimiento aproximado al de los inversores convencionales, dejando en evidencia que el precio por unidad de potencia sería el factor determinante al momento de la elección del inversor que adoptará un sistema fotovoltaico.This paper studies the system involved in the operation and monitoring of two micro-inverters, developed by the Laboratorio de Instrumentación y Control of the Facultad de Ingeniería of the Universidad Nacional de Mar del Plata and Lytron SA. Each micro-inverter delivers to the grid the power generated by single photovoltaic panels with 290 Wp. Both, panels and micro-inverters, are settled in the roof of the building of the engineering school. Also, a comparison is made of the performance between individual inverters with others of higher power, of which the latter are injecting power from arrays of six identical photovoltaic panels. They are part of the photovoltaic solar plant of the engineering school, integrating the IRESUD project. Is described the data treatment of solar irradiance which aim to estimate the solar resource in the tilted plane of the panels. Resource and production data from three weeks of April are shown. Then, two typical days, one cloudy and one sunny, are taken as object of comparison displaying trends of resource and production from the solar systems under study. Finally, it is concluded that this type of inverters has an approximate performance to conventional inverters, manifesting that the price per watt peak unit would be the main factor for choosing the inverter which makes up a photovoltaic system.Tema 4: Energía solar, conversión fotovoltaica.Facultad de Arquitectura y Urbanism

Puesta en marcha y estudio de la operación en campo de microinversores de potencia para aplicaciones fotovoltaicas

En este trabajo se estudia el sistema asociado a la operación y monitoreo de dos micro-inversores de potencia, desarrollados por el Laboratorio de Instrumentación y Control de la Universidad Nacional de Mar del Plata y Lytron SA. Cada microinversor inyecta a la red la energía generada por un panel fotovoltaico de 290 Wp. Ambos paneles, cada uno con su respectivo micro-inversor, se encuentran emplazados en la azotea de la Facultad. Además, se realiza una comparativa del desempeño de los micro-inversores individuales frente al de inversores de mayor potencia que inyectan actualmente la energía generada por arreglos de 6 paneles fotovoltaicos de las mismas características. Los mismos forman parte de la planta piloto de la Facultad, que integra el proyecto IRESUD. Se describe el tratamiento de los datos de irradiancia a fin de estimar el recurso solar en el plano de los paneles fotovoltaicos. Se presentan los datos de recurso y producción obtenidos durante tres semanas en el mes de abril. Luego, se toman como objeto de comparación dos días típicos, uno nublado y el otro soleado, relevando curvas de recurso y producción de los sistemas solares bajo estudio. Finalmente, se concluye que este tipo de inversores tiene un rendimiento aproximado al de los inversores convencionales, dejando en evidencia que el precio por unidad de potencia sería el factor determinante al momento de la elección del inversor que adoptará un sistema fotovoltaico.This paper studies the system involved in the operation and monitoring of two micro-inverters, developed by the Laboratorio de Instrumentación y Control of the Facultad de Ingeniería of the Universidad Nacional de Mar del Plata and Lytron SA. Each micro-inverter delivers to the grid the power generated by single photovoltaic panels with 290 Wp. Both, panels and micro-inverters, are settled in the roof of the building of the engineering school. Also, a comparison is made of the performance between individual inverters with others of higher power, of which the latter are injecting power from arrays of six identical photovoltaic panels. They are part of the photovoltaic solar plant of the engineering school, integrating the IRESUD project. Is described the data treatment of solar irradiance which aim to estimate the solar resource in the tilted plane of the panels. Resource and production data from three weeks of April are shown. Then, two typical days, one cloudy and one sunny, are taken as object of comparison displaying trends of resource and production from the solar systems under study. Finally, it is concluded that this type of inverters has an approximate performance to conventional inverters, manifesting that the price per watt peak unit would be the main factor for choosing the inverter which makes up a photovoltaic system.Tema 4: Energía solar, conversión fotovoltaica.Facultad de Arquitectura y Urbanism

Puesta en marcha y estudio de la operación en campo de micro-inversores de potencia para aplicaciones fotovoltaicas

En este trabajo se estudia el sistema asociado a la operación y monitoreo de dos micro-inversores de potencia, desarrollados por el Laboratorio de Instrumentación y Control de la Universidad Nacional de Mar del Plata y Lytron SA. Cada micro- inversor inyecta a la red la energía generada por un panel fotovoltaico de 290 Wp. Ambos paneles, cada uno con su respectivo micro-inversor, se encuentran emplazados en la azotea de la Facultad. Además, se realiza una comparativa del desempeño de los micro-inversores individuales frente al de inversores de mayor potencia que inyectan actualmente la energía generada por arreglos de 6 paneles fotovoltaicos de las mismas características. Los mismos forman parte de la planta piloto de la Facultad, que integra el proyecto IRESUD. Se describe el tratamiento de los datos de irradiancia a fin de estimar el recurso solar en el plano de los paneles fotovoltaicos. Se presentan los datos de recurso y producción obtenidos durante tres semanas en el mes de abril. Luego, se toman como objeto de comparación dos días típicos, uno nublado y el otro soleado, relevando curvas de recurso y producción de los sistemas solares bajo estudio. Finalmente, se concluye que este tipo de inversores tiene un rendimiento aproximado al de los inversores convencionales, dejando en evidencia que el precio por unidad de potencia sería el factor determinante al momento de la elección del inversor que adoptará un sistema fotovoltaico.This paper studies the system involved in the operation and monitoring of two micro-inverters, developed by the Laboratorio de Instrumentación y Control of the Facultad de Ingeniería of the Universidad Nacional de Mar del Plata and Lytron SA. Each micro-inverter delivers to the grid the power generated by single photovoltaic panels with 290 Wp. Both, panels and micro-inverters, are settled in the roof of the building of the engineering school. Also, a comparison is made of the performance between individual inverters with others of higher power, of which the latter are injecting power from arrays of six identical photovoltaic panels. They are part of the photovoltaic solar plant of the engineering school, integrating the IRESUD project. Is described the data treatment of solar irradiance which aim to estimate the solar resource in the tilted plane of the panels. Resource and production data from three weeks of April are shown. Then, two typical days, one cloudy and one sunny, are taken as object of comparison displaying trends of resource and production from the solar systems under study. Finally, it is concluded that this type of inverters has an approximate performance to conventional inverters, manifesting that the price per watt peak unit would be the main factor for choosing the inverter which makes up a photovoltaic system.Asociación Argentina de Energías Renovables y Medio Ambiente (ASADES

Susceptibility and dilution effects of the kagome bi-layer geometrically frustrated network. A Ga-NMR study of SrCr_(9p)Ga_(12-9p)O_(19)

We present an extensive gallium NMR study of the geometrically frustrated kagome bi-layer compound SrCr_(9p)Ga_(12-9p)O_(19) (Cr^3+, S=3/2) over a broad Cr-concentration range (.72<p<.95). This allows us to probe locally the kagome bi-layer susceptibility and separate the intrinsic properties due to the geometric frustration from those related to the site dilution. Our major findings are: 1) The intrinsic kagome bi-layer susceptibility exhibits a maximum in temperature at 40-50 K and is robust to a dilution as high as ~20%. The maximum reveals the development of short range antiferromagnetic correlations; 2) At low-T, a highly dynamical state induces a strong wipe-out of the NMR intensity, regardless of dilution; 3) The low-T upturn observed in the macroscopic susceptibility is associated to paramagnetic defects which stem from the dilution of the kagome bi-layer. The low-T analysis of the NMR lineshape suggests that the defect can be associated with a staggered spin-response to the vacancies on the kagome bi-layer. This, altogether with the maximum in the kagome bi-layer susceptibility, is very similar to what is observed in most low-dimensional antiferromagnetic correlated systems; 4) The spin glass-like freezing observed at T_g=2-4 K is not driven by the dilution-induced defects.Comment: 19 pages, 19 figures, revised version resubmitted to PRB Minor modifications: Fig.11 and discussion in Sec.V on the NMR shif