Relevance of electron spin dissipative processes to dynamic nuclear polarization via thermal mixing

The available theoretical approaches aiming at describing Dynamic Nuclear spin Polarization (DNP) in solutions containing molecules of biomedical interest and paramagnetic centers are not able to model the behaviour observed upon varying the concentration of trityl radicals or the polarization enhancement caused by moderate addition of gadolinium complexes. In this manuscript, we first show experimentally that the nuclear steady state polarization reached in solutions of pyruvic acid with 15 mM trityl radicals is substantially independent from the average internuclear distance. This evidences a leading role of electron (over nuclear) spin relaxation processes in determining the ultimate performances of DNP. Accordingly, we have devised a variant of the Thermal Mixing model for inhomogenously broadened electron resonance lines which includes a relaxation term describing the exchange of magnetic anisotropy energy of the electron spin system with the lattice. Thanks to this additional term, the dependence of the nuclear polarization on the electron concentration can be properly accounted for. Moreover, the model predicts a strong increase of the final polarization on shortening the electron spin-lattice relaxation time, providing a possible explanation for the effect of gadolinium doping.Comment: 13 pages, 12 figure

Role of the glassy dynamics and thermal mixing in the dynamic nuclear polarization and relaxation mechanisms of pyruvic acid

The temperature dependence of $^1$H and $^{13}$C nuclear spin-lattice relaxation rate $1/T_1$ has been studied in the 1.6 K - 4.2 K temperature range in pure pyruvic acid and in pyruvic acid containing trityl radicals at a concentration of 15 mM. The temperature dependence of $1/T_1$ is found to follow a quadratic power law for both nuclei in the two samples. Remarkably the same temperature dependence is displayed also by the electron spin-lattice relaxation rate $1/T_{1e}$ in the sample containing radicals. These results are explained by considering the effect of the structural dynamics on the relaxation rates in pyruvic acid. Dynamic nuclear polarization experiments show that below 4 K the $^{13}$C build up rate scales with $1/T_{\text{1e}}$, in analogy to $^{13}$C $1/T_1$ and consistently with a thermal mixing scenario where all the electrons are collectively involved in the dynamic nuclear polarization process and the nuclear spin reservoir is in good thermal contact with the electron spin system.Comment: 14 pages, 13 figure

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

Spin dynamics in hole-doped two-dimensional S=1/2 Heisenberg antiferromagnets: ^{63}Cu NQR relaxation in La_{2-x}Sr_xCuO_4 for x≤0.04x\leq 0.04

The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the paramagnetic phase of La_{2-x}Sr_xCuO_4 (for $x \lesssim 0.04$) due to the injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation time T_1 measurements. The results are discussed in the framework of the connection between T_1 and the in-plane magnetic correlation length $\xi_{2D}(x,T)$. It is found that at high temperatures the system remains in the renormalized classical regime, with a spin stiffness constant $\rho_s(x)$ reduced by small doping to an extent larger than the one due to Zn doping. For $x\gtrsim 0.02$ the effect of doping on $\rho_s(x)$ appears to level off. The values for $\rho_s(x)$ derived from T_1 for $T\gtrsim 500$ K are much larger than the ones estimated from the temperature behavior of sublattice magnetization in the ordered phase ($T\leq T_N$). It is argued that these features are consistent with the hypothesis of formation of stripes of microsegregated holes.Comment: 10 pages, 3 figure

La crisis catalana y el Tribunal Constitucional. Anatomía politológica de la sentencia 31/2010

Desde la aprobación del nuevo Estatut de Catalunya (2006) hasta la sentencia del Tribunal Constitucional (2010), que lo rechazó parcialmente, pasaron cuatro años. Este rechazo, en las partes más políticas como, y sobre todo, la del reconocimiento de Cataluña como nación, determinó una volcánica, que a día de hoy aún no ha terminado, y que llevó a la fractura definitiva con Madrid y a la elección de la vía independentista (el procés). No obstante, precisamente la sentencia de rechazo del TC nº 31 de 2010, debido en parte a las dinámicas políticas que nacieron en el seno de la Sala que conoció del asunto durante la redacción del pronunciamiento, constituye un documento de investigación, cuyo análisis puede convertirse en una pieza útil para la reconstrucción histórica de las dramáticas decisiones políticas que se sucedieron.Desde la aprobación del nuevo Estatut de Catalunya (2006) hasta la sentencia del Tribunal Constitucional (2010), que lo rechazó parcialmente, pasaron cuatro años. Este rechazo, en las partes más políticas como, y sobre todo, la del reconocimiento de Cataluña como nación, determinó una volcánica, que a día de hoy aún no ha terminado, y que llevó a la fractura definitiva con Madrid y a la elección de la vía independentista (el procés). No obstante, precisamente la sentencia de rechazo del TC nº 31 de 2010, debido en parte a las dinámicas políticas que nacieron en el seno de la Sala que conoció del asunto durante la redacción del pronunciamiento, constituye un documento de investigación, cuyo análisis puede convertirse en una pieza útil para la reconstrucción histórica de las dramáticas decisiones políticas que se sucedieron

Stato, governi e crescita del debito pubblico in Italia negli anni ottanta. Un approccio storico-istituzionale

Analisi dei rapporti politico-istituzionali nello stato e nel governo italiani di fronte alla crescita rapida del debito pubblico alla met\ue0 degli anni ottanta del Novecento

Maria Santos-Sainz, Albert Camus, journaliste - Reporter à Alger, éditorialiste à Paris

Il volume si propone come un manuale per giornalisti e futuri giornalisti. L’autrice Maria Santos-Sainz si avvale della figura di Albert Camus, al fine di presentarla come modello del mestiere. A tale scopo, viene raccontato il percorso biografico del giornalista franco-algerino in un insieme di sette capitoli, i quali coincidono con la ricostruzione puntuale della sua carriera di reporter presso diversi quotidiani dell’epoca. Ma non solo. Più si avanza nella lettura del testo più il profilo ..

Trajetórias Familiares e Práticas de Gestão: Estudo de Caso da Família Gotardo em Guarapari.

Inserido em uma perspectiva subjetiva de pesquisa sobre empresas familiares, este trabalho, busca somar-se ao o esforço para interpretação de dinâmicas sócio-culturais, contrapondo-se aos que tratam apenas assuntos referentes à sucessão. A pesquisa tem caráter qualitativo e investiga como as práticas sócio-culturais de gestão da família Gotardo configuraram as suas trajetórias empresariais. Para tal, utiliza como procedimento metodológico à análise da história oral desenvolvida sobre a transcrição de entrevistas com os gestores de empresas pertencentes à referida família, em Guarapari-ES. O referencial teórico é desenvolvido em temáticas essenciais para direcionar a pesquisa: gestão como prática social; estudos sobre empresas familiares e trajetórias empresariais e; estudos sobre famílias.Inserted in a subjective perspective of research on family companies, this work search to add to the effort for interpretation of partner-cultural dynamics, opposing to the that just treat referring subjects about the succession. The research has qualitative character and investigates as the social practices of the Gotardo family set its businesses trajectory. For such, it uses as methodological procedure the analysis of the oral history developed about the transcription of interviews with the managers of the companies that belong the referred family in Guarapari-ES. The theoretical thought is developed in essential thematic to set the research: management as social practice; studies on family companies; managerial paths and studies on families