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ESR results for DEM(TCNQ)₂ and for MEM(TCNQ)₂ are presented as examples of TCNQ salts with and without level crossing effects. In the case of DEM(TCNQ)₂ we are performing ESR on a system with two types of spins. The latter are due to the two kinds of stacks of the acceptor TCNQ molecules in DEM(TCNQ)₂. The level crossing occurs when the g value of these two kinds of electrons are the same. This is compared to the case of MEM(TCNQ)₂ where there is only one kind of TCNQ stack and consequently no level crossing effect. It is found that g values and susceptibilities of the ESR spectra for DEM(TCNQ)₂ can be fitted very well to an interaction between the stacks of the form J ∑i,j,Si,Sj; with J > 0. This gives an antiferromagnetic coupling between every spin on one type of stack with every spin on the other type of stack. This type of interaction is also in qualitative agreement with the published results for HMM(TCNQ)₂. The experimental results in DEM(TCNQ)₂ and MEM(TCNQ)₂ also show that there are small but significant differences in the g tensors when the temperature or the crystalline environment are changed, and consequently the g tensor in these compounds does not depend only on the orientation of TCNQ molecule in the magnetic field as previously assumed. These differences in g are typically less than 2 x 10⁻⁴. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Orientador: Carlos Rettori / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-04T01:45:05Z (GMT). No. of bitstreams: 1 Urbano_RicardoRodrigues_D.pdf: 7003500 bytes, checksum: 302763535eafe1664ae875c23c64eaec (MD5) Previous issue date: 2004 / Resumo: Esta tese é o resultado da investigação de dois temas principais: o composto supercondutor MgB2 e o sistema semicondutor Ca1-xRxB6 dopado com terras raras R. O composto MgB2 é conhecido desde 1950, porém, só recentemente foi descoberto como sendo um supercondutor (tipo II) com uma temperatura crítica Tc @ 40 K. Esta descoberta realimentou o interesse nos materiais supercondutores não-óxidos, iniciando assim a procura por supercondutividade em vários outros compostos como, por exemplo, MgCNi3 e recentemente, diamante. A motivação do ponto de vista de Ressonância Paramagnética Eletrônica (RPE) se dá pela possibilidade de estudar o estado supercondutor misto numa ampla faixa de temperatura, via a ressonância dos elétrons de condução (CESR). Utilizando as freqüências de microondas de 4.1 GHz (banda S) e 9.5 GHz (banda X), o campo de ressonância (H r para g ~ 2:00) assume valores entre Hc1< Hr « Hirr £ Hc2 e, conseqüentemente, os elétrons de condução certamente estarão sondando a Distribuição de Campo Magnético Interno (DCMI) gerada pela rede de vórtices de MgB2. Com a atenção voltada às propriedades do estado misto deste supercondutor, apresentamos nesse trabalho a primeira, direta e não ambígua observação da DCMI, juntamente com o efeito de desancoramento da rede de linhas de fluxo. Determinamos a temperatura de desancoramento Tp (v), a qual separa o regime de movimento viscoso do regime de aprisionamento de vórtices. Também propomos um diagrama de fases supercondutor para a região de baixos campos magnéticos para nossa amostra estequiométrica. Comparativamente ao estudo do composto estequiométrico, estudamos amostras de Mg1-xB2(0 £ x £ 0.15). Para x ~ 0.15, encontramos a presença de duas linhas de ressonância, sugerindo uma possível segregação de fases. Além disso, a maior Tp encontrada, e o mais rápido alargamento e distorção da largura de linha do CESR no estado misto, comparados à amostra estequiométrica, são ambos consistentes com uma maior concentração de defeitos. Os hexaboretos RB6 (R = terra rara ou metais alcalinos terrosos) pertencem a uma classe de materiais que está sendo extensivamente estudada atualmente. Eles cristalizam numa rede cúbica simples tipo CsCl, e exibem um vasto universo de fascinantes propriedades físicas. Metais de bandas simples (LaB6), magnetos de momento local convencional (GdB6), sistemas de denso comportamento Kondo (CeB6) ou ainda ferromagnetismo (FM) de baixa densidade de portadores em sistemas com momentos localizados como EuB6, fazem parte deste grupo. Esta riqueza de fenômenos, combinada à simplicidade cristalográfica, torna os hexaboretos um sistema ideal para se estudar as propriedades magnéticas e eletrônicas destes compostos. Recentemente, foi encontrado que CaB6 dopado com impurezas de La 3+ (0.5%) exibe um FM (0.07 m B/La) em alta temperatura (Tc ~ 600 - 900 K), sem que os elementos constituintes tenham orbitais d ou f parcialmente preenchidos. Enorme esforço tanto teórico quanto experimental foi devotado para esclarecer esta intrigante propriedade, na tentativa de estabelecer a origem deste WF e seu relacionamento com a natureza condutora real de CaB6 dopado com R. Particularmente, encontramos em nosso estudo através da técnica de RPE em monocristais de CaB6 dopados com Gd 3+, que o processo de dopagem é não-homogêneo, apresentando diferentes regimes de concentração, inclusive coexistência de fases isolante e metálica. Este resultado sugere que uma abrupta mudança no ambiente local do Gd 3+ , associada à transição metal-isolante, pode estar ocorrendo. A configuração eletrônica de Eu 2+ é idêntica a do Gd 3+ (4f 7 , 8 S 7/2). No entanto, o efeito da dopagem de Gd 3+ e Eu 2+ em CaB6 é diferente: Eu 2+ tem a mesma valência de Ca 2+ enquanto Gd 3+ doa um elétron extra ao sistema, criando um estado doador tipo-hidrogênio. Acreditamos que a transição isolante/metal revelada pela forma de linha de RPE possa ser alcançada quando estes estados ligados de Gd doadores se sobrepõem e começam a formar uma rede percolativa. Desde que nem todos os sítios de Gd participam desta rede, uma coexistência de regiões metálicas e isolantes não é uma observação improvável, como observado em nossos resultados. De um outro lado, a substituição de Ca2+ por impurezas de Eu 2+ não produz um estado ligado doador, mas a invariância translacional quebrada da rede deve introduzir um estado localizado separado da banda de valência/condução. Supõe-se então que a dopagem de Eu 2+ forma uma banda de impurezas. No entanto, somente em um regime de muito alta concentração, estas impurezas formariam o estado percolativo, o que também encontra suporte em nossos resultados experimentais. Em contraste ao caso de CaB6, o composto EuB6 é um material semi-metálico bem estabelecido que apresenta um ordenamento FM em Tc » 15 K. Investigações das propriedades físicas de EuB6 foram realizadas essencialmente através de experimentos de RPE e de transporte. Nosso estudo mostrou que os comportamentos observados na largura de linha DH dos espectros de RPE, e na anisotropia da magneto-resistência, estão associados à presença de pólarons magnéticos e efeitos de superfície de Fermi, uma vez que esta última é de fato anisotrópica (elipsóides de revolução nos pontos X da Zona de Brillouin). Concluímos do estudo em Ca1-xRxB6 (R = Gd 3+ e Eu 2+ ) que, uma vez alcançado o estado percolativo (estado metálico), efeitos da anisotrópica superfície de Fermi podem ser observados através de DH, uma vez assumido que o processo de relaxação dominante é o mecanismo de espalhamento de spin eletrônico (spin-flip scattering). Para o caso particular de Eu 2+ em CaB6, efeitos polarônicos, a exemplo de EuB6, são também observados em DH / Abstract: This thesis results from the scientific investigation of two different systems: the MgB2 superconductor (SC) compound and the rare earth (R) doped Ca1-xRxB6 semiconductor material. The MgB2 compound is known since 1950, however, just recently discovered as a conventional (type II) SC with a critical temperature Tc @ 40 K. This discovery has revived the interest in non-oxides SC and initiated a search for superconductivity in related materials as, for exemple, MgCNi3. The motivation in the ESR point of view was the imprecedent possibility of studying the SC mixed-state in a wide temperature range by conduction electron spin resonance (CESR). Taking the 4.1 GHz (S band) and 9.5 GHz (X band) microwave frequencies, the resonance field (H r for g ~ 2:00) ranges between Hc1< Hr « Hirr £ Hc2 and, consequently, the conduction electrons will certainly probe the Internal Magnetic Field Distribution (IMFD) in the vortex lattice of MgB2. Focusing our attention on the properties of the mixed-state of this superconductor, we present in this work the first, direct and unambiguously observation of the IMFD together the flux line lattice depinning effects in MgB2. We have determined the depinning temperature Tp (v) which separates the viscous vortex motion regime to the pinning regime. Also, we have proposed a superconductor phase diagram for the low field region for our stoichiometric MgB2 sample. Comparatively to the stoichiometric sample study, we have studied Mg1-xB2 (0 £ x £ 0.15) samples. For x ~ 0.15, we found the presence of two resonance lines indicating phase segregation. Furthermore, the Tp found is higher than the obtained for the stoichiometric sample and, the CESR line width in the mixed-state presented a faster broadening when decreasing temperature, consistent with the larger defects concentration. Currently, the hexaboride compounds RB6 (R = earth rare or earth alcalines) belong to a material class which has been extensively studied. They crystallizes in a simple cubic lattice as CsCl, and show a wide variety of fascinating physical properties. Simple band metals (LaB6), conventional local moments magnets (GdB6), system with dense Kondo behavior (CeB6) or even low carrier density FM systems with local moments as EuB6, make part of this group. This richness of interesting physical properties, together with their simple crystallographic structure, make the hexaborides an ideal system to study. Recently, it was found that the La 3+ (0.5%) impurity doped CaB6 shows a WF (0.07 m B/La) in high temperature (Tc ~ 600 - 900 K) without the elements having half-filled d or f orbitals, which is usually required for FM. Enormous theoretical and experimental efforts have been devoted to clarify this intriguing property, in the sense to establish the origin of this WF and its relationship with the real conductive nature of the R doped CaB6. Particularly, we have found in our ESR study in single crystals of Gd 3+ doped CaB6 that the doping process is inhomogeneous, presenting different concentration regimes, including insulating and metallic phase coexistence. This result suggests that an abrupt change in the Gd 3+ local environment associated with the metal-insulator transition can be happening. The electronic configuration of Eu 2+ ions ( 8S7/2) is identical to that of the Gd 3+ ions. However, the effect of Gd 3+ and Eu 2+ doping in CaB6 is different: Eu 2+ has the same valence as Ca 2+ while Gd 3+ delivers an extra electron to the system, creating a hydrogen-like donor state. We believe that the insulator-metal transition revealed by the ESR line shape can be reached when these Gd donor bound states overlap and start to form a percolative network. Since not all Gd-sites participate in this network, a coexistence of metallic and insulating local environments are not impossible to be observed, as shown in our experimental results. In the other side, the substitution of Ca 2+ by Eu 2+ impurities does not yield a donor bound state, but the broken translational invariance of the lattice introduces a localized split-off state from the valence/conduction band. Then, it is supposed that an impurity band for Eu 2+ hen only forms at much higher concentrations as for Gd 3+ , as it is indeed observed experimentally. In contrast to the CaB6, the EuB6 compound is a well established semi-metallic material which presents a FM ordering at Tc » 15 K. Investigations of the physical properties of EuB6 were performed by ESR and transport experiments. Our study showed that the observed anisotropy and field-dependence of the ESR line width D H and of the magneto-resistence are associated with magnetic polarons and Fermi surface effects, since the latter is really anisotropic. We conclude in this study that, once reached the percolative state (metallic state), the anisotropic Fermi surface effects can be observed by the D H since the dominating relaxation process is the spin-flip scattering mechanism. For the particular case of Eu 2+ in CaB6, polaronic effects can be also observed in the D H, such as in EuB6 / Doutorado / Física / Doutor em Ciências

In the present work three different proteins were investigated, namely IGPS, the SRII/HtrII complex and ChR2. EPR spectroscopy combined with SDSL were applied to investigate the relation of the respective protein dynamics and their functions. In case of IGPS spin label side chain dynamics were recorded and allow a classification of loop dynamics during catalysis. For SRII/HtrII an equilibrium of spin label side chain dynamics was found to reflect the equilibrium of the activated and deactivated states, which are responsible for signal transfer through the complex. Time-resolved analyses of side chain dynamics and trapping of intermediates of the light-triggered channel ChR2 identified conformational changes to occur during channel function. Thereby, the EPR spectroscopy proves to be a powerful tool to investigate proteins in context of structural changes during their protein function.

Polymer-derived Ceramics (PDCs) represent a unique class of high-temperature stable materials synthesized directly by the thermal decomposition of polymers. This research first focuses on the fabrication of high temperature stable siliconcarbonitride (SiCN) fibers by electrospinning for ceramic matrix composite (CMC) applications. Ceraset™ VL20, a commercially available liquid cyclosilazane, was functionalized with aluminum sec-butoxide in order to be electrospinnable. The surface morphology of the electrospun fibers was investigated using the fibers produced from solvents. The electrospun fibers produced from the chloroform/N,N-dimethylformamide solutions had hierarchical structures that led to superhydrophobic surfaces. A “dry skin” model was proposed to explain the formation of micro/- and nanostructures. The second objective of the research is to align the multiwalled carbon nanotubes (MWCNTs) in PDC fibers. For this purpose, a non-invasive approach to disperse carbon nanotubes in polyaluminasilazane chloroform solutions was developed using a conjugated block copolymer synthesized by ATRP. The effect of the polymer and CNT concentration on the fiber structure and morphology was also examined. Detailed characterization using SEM and TEM was performed to demonstrate the orientation of CNTs inside the ceramic fibers. Additionally, the electrical properties of the ceramic fibers were investigated. Finally, the structural evolution of polymer-derived amorphous siliconborocarbonitride (SiBCN) ceramics with pyrolysis temperatures was studied by solid-state NMR, Raman and EPR spectroscopy. Results suggested the presence of three major components: (i) hexagonal boron nitride (h-BN), (ii) turbostratic boron nitride (t-BN), and (iii) BN2C groups in the final ceramic. iv The pyrolysis at higher temperature generated boron nitride (BN3) with a simultaneous decomposition of BN2C groups. A thermodynamic model was proposed to quantitatively explain the conversion of BN2C groups into BN3 and “free” carbon. Such structure evolution is believed to be the reason that the crystallization of Si4.0B1.0 ceramics starts at 1500 ° C, whereas Si2.0B1.0 ceramics is stable upto 1600 ° C.

A long standing problem in catalysis is the identification and characterization of the active sites, i.e. an atom or an ensemble of atoms spouse on the surface of a catalyst.[Taylor1925] One relevant case, that is treated in this thesis, is constituted by the Phillips catalyst.[Hogan1958] For several reasons, even though this catalyst has be applied at industrial scale for decades and is accounted for a majority of the high density polyethylene (HDPE) production, the identification and the mechanism of the active sites are still under debate. This work was initiated in the framework of the PARACAT project, which is dedicated to study the paramagnetic species in catalysis, and focuses on 'The role of Cr paramagnetic states in olefin polymerization over Phillips catalyst.' In the course of the study, I brought this research to a larger scale which included but was not limited to the Phillips catalyst itself. Considering the relevance of the interaction between transition metal ions (TMI) and the support to the catalytic activity, I worked on systems that cover a number of oxide-supported TMIs by means of electron paramagnetic resonance (EPR) spectroscopy. In this thesis I investigated the paramagnetic Cr(V) and Cr(III) species in the Phillips catalyst. The Cr(III) species were suggested with possible relevance to the catalytic reaction while Cr(V) species were suggested as just the spectators in the reaction.[McDaniel2010, Groppo2018] Nevertheless, Cr(V) species were used in this work as spin probes to provide more information on the overall system. Field-sweep methods including continuous wave (CW) EPR and echo detected field sweep (EDFS) showed that the there are two Cr(V) species with different local geometries. Quantitative analysis of the CW EPR showed that these two Cr(V) species have different reactivity with ethylene. The instantaneous diffusion analysis were performed on the Cr(V) species to provide information on the dispersion of the Cr on the silica surface and the results suggested clusters were formed locally. Besides studying the Phillips catalyst itself, I studied also the silica supported organometallic-Cr catalyst, Cr[CH(SiMe3)2]3/SiO2, which served as a model system to investigate the catalytic active Cr species with well-defined oxidation states and geometry. Two categories of the Cr(III) species were assigned to the active sites for olefin oligomerization and polymerization. The assignment were done by comparing their distortion of the local geometries with that of the different precursors. On the other hand, microporous materials including zeolites and zeotype materials such as aluminophosphate (AlPO) can be engineered with different physical and chemical properties in terms of chemical composition and provide a relevant example of structure sensitivity of a heterogeneous catalyst.[Hartmann2002, Hartmann1999] Such structure sensitivity is highly relevant in catalysis and can be very well studies with EPR spectroscopy. In this regard, I investigated a series of SAPO-5 materials doped with different TMI. In the first place, the incorporation of Cr in SAPO-5 was studied focuses on the discrimination of isomorphous substitution at framework sites and extra-framework sites. In the hyperfine sublevel correlation (HYSCORE) spectrum, large hyperfine interaction (hfi) of 27Al with the matrix 31P signal provide solid evidence for the isomorphous substitution of Cr(V) at framework sites. In addition to the Cr-incorporated SAPO-5, a method to prepare a bi-metallic Mo/V-SAPO-5 system was developed and the metal-metal synergy was validated with a single electron transfer reaction and the short range hyperfine interaction. HYSOCRE spectra showed large \textit{hfi} of both 27Al and 31P and suggested the V species grafted at extra-framework sites. Moreover, the HYSCORE spectrum showed signals at low frequency region which were attributed to the 95,97Mo species with large hfi, confirming the short range interaction. Finally, the surface properties of SAPO-5 were studied by adsorbing NO radicals in the pores and investigating their interaction with the surface. Different adsorption sites of NO molecules according to different activation conditions were first discriminated by the g-factors obtained from the CW EPR. From the 27Al HYSCORE spectra, it is observed that when the activation temperature is higher the NO molecules are situated in vicinity of some defect Al sites. However, the dominant Al species were observed either in samples activated at lower temperature or by increasing the NO dosage. This is postulated as that the defect sites were blocked by residual water molecules or saturated by excessive NO molecules. The presence of water molecules were validated by 1H HYSCORE experiments and the coordination of NO-water was estimated from the hfi structure. [Taylor1925] Taylor, H. S. Proc. R. Soc. London. Ser. A, Contain. Pap. a Math. Phys. Character 1925, 108, 105-111 [Hogan1958] Hogan, J. P.; Banks, R. L. Polymers and production thereof US Patent 2,825,721, 1958. [McDaniel2010] McDaniel M. P. Advances in Catalysis, 1st Ed. 2010; Vol. 52, pp 123-606 [Groppo2018] Groppo, E.; Martino, G. A.; Piovano, A.; Barzan, C. ACS Catal. 2018, 8, 10846-10863 [Hartmann2002] Hartmann, M.; Kevan, L. Res. Chem. Intermed. 2002, 28, 625-695 [Hartmann1999] Hartmann, M.; Kevan, L. Chem. Rev. 1999, 99, 635-663

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