110 research outputs found
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,
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 NMR allows us to
measure the local magnetic susceptibility at different distances from the
defects. The 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 100 K and it
corresponds to an AF coupling J260 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 (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 YBaNiMgO. For
temperatures above the gap the calculated NMR spectra are in excellent
agreement with the experimental results, confirming the existence of
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 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 energÃa 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 energÃa generada por arreglos de 6 paneles fotovoltaicos de las mismas caracterÃsticas. 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 dÃas tÃpicos, 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 serÃa 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 IngenierÃa 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: EnergÃa 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 energÃa 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 energÃa generada por arreglos de 6 paneles fotovoltaicos de las mismas caracterÃsticas. 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 dÃas tÃpicos, 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 serÃa 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 IngenierÃa 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: EnergÃa 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 energÃa 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 energÃa generada por arreglos de 6 paneles fotovoltaicos de las mismas caracterÃsticas. 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 dÃas tÃpicos, 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 serÃa 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 IngenierÃa 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: EnergÃa 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 energÃa 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 energÃa generada por arreglos de 6 paneles fotovoltaicos de las mismas caracterÃsticas. 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 dÃas tÃpicos, 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 serÃa 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 IngenierÃa 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 EnergÃas 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
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