Fluctuaciones de valencia entre dos configuraciones magnéticas
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1982
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Universidad Nacional de Cuyo (Argentina). Instituto de Física "Dr. J. A. Balseiro
Comisión Nacional de Energía Atómica (Argentina). Centro Atómico Bariloche.
Comisión Nacional de Energía Atómica (Argentina). Centro Atómico Bariloche.
Resumen
La mayoría de los modelos microscópicos desarrollados para compuestos de tierra rara con valencia intermedia (V.I), describen fluctuaciones de valencia de los iones (cationes) entre una configuración magnética (J#0) y una nomagnética (J=0). Estos modelos, basados en niveles resonantes, no pueden explicar las propiedades físicas de los compuestos de Tulio(Tm) que sean de V.I. La peculiaridad del Tm con V.I. dentro de la familia de tierras-raras con V.I (SmS, YbCu2Si2, &-Ce) está en evidencia en distintas propiedades del TmSe. Por ejemplo: susceptibilidad magnética, el espectro de dispersión de neutrones, calor específico. Asimismo también ocurren cambios drásticos e varias propiedades bajo la acción de un campo magnético moderado. Esto no tiene lugar en otros compuestos de V.I. La gran sensibilidad a campos magnéticos de TmSe aparece en la resistividad, el calor específico, el parámetro de red, expansión térmica, etc. Es importante destacar el gran cambio que ocurre en TmSe al aplicar un campo magnético de 10kOe. La correspondiente energía de Zeeman es del orden de 0.001 eV, mucho menor que cualquier parámetro de la valencia intermedia. El propósito de esta tesis es el estudio de un modelo microscópico que describe al TmSe a través de su característica más importante: las fluctuaciones de valencia tiene lugar entre dos configuraciones magnéticas (Tm2+: 4f12, 2f772;Tm3+:4f13,3H6). El capítulo I es una revisión general de las propiedades físicas más importantes de los sistemas de tierra experimentales y modelos teóricos de los compuestos de Tm. En el capítulo I se present y discute el Hamiltoniano modelo. Asimísmo se analiza la pérdida de la invarancia rotaciones. Además en este capítulo se estudia "el limiteatómico" del Hamiltoniano: Los espector de fluctuaciones de spind y carga son los resultados más sorprendentes y permiten explicar conceptualmente el espectro de dispersión magnético de neutrones y tal vez el espectro Raman. El capítulo III esta dedicado al estudio de los compuestos no- estequiméticos TmxSe. Se muestra que ser considerado como una mezcla de TmSe(Sistema de V.I) y Tm3+0.87Se. La susceptibilidad magnética, el calor específico y la ocupación 4f son calculadas a segundo orden en la hibirdización usando la teoría de perturbaciones. En el capítulo IV se calcula las susceptibilidades de spin y carga. Los resultados obtenidos permiten exlicar las caracteristicas esenciales del espectro de dispersión magnética de neutrones en TmSe. Se muestra que la energía del pico inelástico hallado en dicho espectro está relacionada con la energía de fluctuaciones de carga. En el capítulo V se presenta un Hamiltoniano periódico que es resoluble exactamente. Este modelo predice: i) El sistema se ordena magnéticamente a T=0. ii) Lasm uestras estequiométricas se ordenan antiferromagnéticamente. iii) Las muestras no-estequiométricas pueden ordenarse ferromagnéticamente. iv) El estado ferromagnético es metálico. v) Las muestras estequiométricas son aisladores (con orden A.F.) y tienen una transición metal-aislador a T=Tn ó H=Hc. Estas conclusiones son muy importantes para comprender las peculiares propiedades de los compuestos de Tm. Finalmente el alcance del modelo propuesto para describir los compuestos de Tm se discute en el capítulo VI. Al final de los capítulos II,III, IV y V hay un resumen de los conceptos desarrollados y resultados obtenidos en cada uno de ellos.
Most microscopic models developted for intermediate valence of rare earth compounds are devided to describe valence fluctuations between one magnetic (J # 0) and one non-magnetic (J = 0) configurations of the ions. Models based on resonant levels, can not explain the physical properties of intermediate valence (I.V) Thulium (Tm) compounds. The peculiarity of I.V. Tm within the family of I.V. rare earths (SmS, YbCu2Si2, &-Ce) is evidenced in several properties of TmSe. For example: magnetic susceptibility neutron scattering spectrum and specific heat. Also, radical changes occur under moderate magnetic fields in several of the properties of TmSe that do not take place in other I.V. compounds.This sensitivity to magnetic fields appears in the resistivity, specific heat, thermal expansion,latice parameter, etc. It is important to notice that large change occur in TmSe when a magnetic field of 10kOe is applied. The corresponding Zeeman energy is of the order of 0.001 eV, much smaller than any parameter characteristic of intermediate valence. The subjecte of this work is the study of a microscopy model which incorporates the most important feature TmSe: valence fluctuations between two magnetic configurations (Tm2: 4f12, 2f772; Tm3: 4f13,3H6). Chapter I is a general review of the most important physical properties of rare-earth I.V. systems, and a general description of the experimental results and theoretical models of Tm compounds. In Chapter II the model Hamiltonian is presented and discussed. The consequences of the lack rotational invariance are pointed out. Besides in this chapter the "atomic limit" of the Hamiltonian is studied. The moststriking results are the spin and charge power spectra which can conceptually explain the neutron scattering spectrum and eventually the Raman spectrum. Chapter III is dedicated to the study of non-stoichiometric TmxSe. It is shown that can be considered as a mixture of TmSe (IV System) and Tm3 0.87Se. Magnetic susceptibility, specific heat and 4f occupation of stoichimetric TmSe are calculated in second order perturbation theory on the hybridization using . In chapter IV the spin and load susceptibilities are calculated. The results obtained allow us to explain the essential characteristics of the neutron magnetic scattering spectrum in TmSe. It is shown that the energy of the inelastic peak found in said spectrum is related to the energy of charge fluctuations. In Chapter V a periodic Hamiltonian is presented that is solvable exactly. This model predicts: i) The system is magnetically ordered at T = 0. ii) Stoichiometric samples are arranged antiferromagnetically. iii) Non-stoichiometric samples can be ordered ferromagnetically. iv) The ferromagnetic state is metallic. v) Stoichiometric samples are insulators (with order A.F.) and have a metal-insulator transition at T = Tn or H = Hc. These conclusions are very important to understand the peculiar properties of Tm compounds. Finally, the scope of the proposed model to describe Tm compounds is discussed in chapter VI. At the end of chapters II, III, IV and V there is summary of the results and the concepts developed in each of them.
Most microscopic models developted for intermediate valence of rare earth compounds are devided to describe valence fluctuations between one magnetic (J # 0) and one non-magnetic (J = 0) configurations of the ions. Models based on resonant levels, can not explain the physical properties of intermediate valence (I.V) Thulium (Tm) compounds. The peculiarity of I.V. Tm within the family of I.V. rare earths (SmS, YbCu2Si2, &-Ce) is evidenced in several properties of TmSe. For example: magnetic susceptibility neutron scattering spectrum and specific heat. Also, radical changes occur under moderate magnetic fields in several of the properties of TmSe that do not take place in other I.V. compounds.This sensitivity to magnetic fields appears in the resistivity, specific heat, thermal expansion,latice parameter, etc. It is important to notice that large change occur in TmSe when a magnetic field of 10kOe is applied. The corresponding Zeeman energy is of the order of 0.001 eV, much smaller than any parameter characteristic of intermediate valence. The subjecte of this work is the study of a microscopy model which incorporates the most important feature TmSe: valence fluctuations between two magnetic configurations (Tm2: 4f12, 2f772; Tm3: 4f13,3H6). Chapter I is a general review of the most important physical properties of rare-earth I.V. systems, and a general description of the experimental results and theoretical models of Tm compounds. In Chapter II the model Hamiltonian is presented and discussed. The consequences of the lack rotational invariance are pointed out. Besides in this chapter the "atomic limit" of the Hamiltonian is studied. The moststriking results are the spin and charge power spectra which can conceptually explain the neutron scattering spectrum and eventually the Raman spectrum. Chapter III is dedicated to the study of non-stoichiometric TmxSe. It is shown that can be considered as a mixture of TmSe (IV System) and Tm3 0.87Se. Magnetic susceptibility, specific heat and 4f occupation of stoichimetric TmSe are calculated in second order perturbation theory on the hybridization using . In chapter IV the spin and load susceptibilities are calculated. The results obtained allow us to explain the essential characteristics of the neutron magnetic scattering spectrum in TmSe. It is shown that the energy of the inelastic peak found in said spectrum is related to the energy of charge fluctuations. In Chapter V a periodic Hamiltonian is presented that is solvable exactly. This model predicts: i) The system is magnetically ordered at T = 0. ii) Stoichiometric samples are arranged antiferromagnetically. iii) Non-stoichiometric samples can be ordered ferromagnetically. iv) The ferromagnetic state is metallic. v) Stoichiometric samples are insulators (with order A.F.) and have a metal-insulator transition at T = Tn or H = Hc. These conclusions are very important to understand the peculiar properties of Tm compounds. Finally, the scope of the proposed model to describe Tm compounds is discussed in chapter VI. At the end of chapters II, III, IV and V there is summary of the results and the concepts developed in each of them.