Électron célibataire

Résumé

vignette| redresse=2| Tableau périodique indiquant le nombre des électrons célibataires des différents éléments chimiques (neutres). Un électron célibataire (ou non apparié) est un électron qui est seul à occuper une orbitale atomique ou moléculaire, sans former une paire de Lewis. Chaque orbitale atomique d'un atome (spécifiée par les trois nombres quantiques n, l et m) peut comporter deux électrons ayant des spins opposés (un doublet électronique, ou paire de Lewis). La formation d'une paire (isolée autour d'un atome ou assurant une liaison covalente entre deux atomes) étant souvent favorable énergétiquement, les électrons célibataires sont relativement rares : une entité qui en porte un est généralement assez réactive, et réagit en devenant un anion ou en formant une liaison covalente (ce qui apparie l'électron), ou bien en formant un cation (ce qui fait disparaître l'électron célibataire du cortège électronique). En chimie organique, les éle

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https://fr.wikipedia.org/wiki/%C3%89lectron_c%C3%A9libataire

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Low-temperature spin dynamics of a valence bond glass in Ba2YMoO6

Julian Piatek, Henrik Moodysson Rønnow

We carried out ac magnetic susceptibility measurements and muon spin relaxation spectroscopy on the cubic double perovskite Ba2YMoO6, down to 50 mK. Below similar to 1 K the muon relaxation is typical of a magnetic insulator with a spin-liquid type ground state, i.e. without broken symmetries or frozen moments. However, the ac susceptibility revealed a dilute-spin-glass-like transition below similar to 1 K. Antiferromagnetically coupled Mo5+ 4d(1) electrons in triply degenerate t(2g) orbitals are in this material arranged in a geometrically frustrated fcc lattice. Bulk magnetic susceptibility data has previously been interpreted in terms of a freezing to a heterogeneous state with non-magnetic sites where 4d(1) electrons have paired in spin-singlets dimers, and residual unpaired Mo5+ 4d(1) electron spins. Based on the magnetic heat capacity data it has been suggested that this heterogeneity is the result of kinetic constraints intrinsic to the physics of the pure system (possibly due to topological overprotection) leading to a self-induced glass of valence bonds between neighbouring 4d(1) electrons. The muon spin relaxation (mu SR) unambiguously points to a heterogeneous state with a static arrangement of unpaired electrons in a background of (valence bond) dimers between the majority of Mo5+ 4d electrons. The ac susceptibility data indicate that the residual magnetic moments freeze into a dilute-spin-glass-like state. This is in apparent contradiction with the muon-spin decoupling at 50 mK in fields up to 200 mT, which indicates that, remarkably, the time scale of the field fluctuations from the residual moments is similar to 5 ns. Comparable behaviour has been observed in other geometrically frustrated magnets with spin-liquid-like behaviour and the implications of our observations on Ba2YMoO6 are discussed in this context.

Institute of Physics (IoP) and Deutsche Physikalische Gesellschaft2013

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We carried out ac magnetic susceptibility measurements and muon spin relaxation spectroscopy on the cubic double perovskite Ba2YMoO 6, down to 50 mK. Below ∼1 K the muon relaxation is typical of a magnetic insulator with a spin-liquid type ground state, i.e. without broken symmetries or frozen moments. However, the ac susceptibility revealed a dilute-spin-glass-like transition below ∼1 K. Antiferromagnetically coupled Mo5+ 4d1 electrons in triply degenerate t2g orbitals are in this material arranged in a geometrically frustrated fcc lattice. Bulk magnetic susceptibility data has previously been interpreted in terms of a freezing to a heterogeneous state with non-magnetic sites where 4d1 electrons have paired in spin-singlets dimers, and residual unpaired Mo5+ 4d1 electron spins. Based on the magnetic heat capacity data it has been suggested that this heterogeneity is the result of kinetic constraints intrinsic to the physics of the pure system (possibly due to topological overprotection) leading to a self-induced glass of valence bonds between neighbouring 4d1 electrons. The muon spin relaxation (μSR) unambiguously points to a heterogeneous state with a static arrangement of unpaired electrons in a background of (valence bond) dimers between the majority of Mo5+ 4d electrons. The ac susceptibility data indicate that the residual magnetic moments freeze into a dilute-spin-glass-like state. This is in apparent contradiction with the muon-spin decoupling at 50 mK in fields up to 200 mT, which indicates that, remarkably, the time scale of the field fluctuations from the residual moments is ∼5 ns. Comparable behaviour has been observed in other geometrically frustrated magnets with spin-liquid-like behaviour and the implications of our observations on Ba 2YMoO6 are discussed in this context. © IOP Publishing and Deutsche Physikalische Gesellschaft.

Institute of Physics (IoP) and Deutsche Physikalische Gesellschaft2013

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Dynamic nuclear polarisation via the integrated solid effect II: experiments on naphthalene-h(8) doped with pentacene-d(14)

Tim Rolf Eichhorn

In dynamic nuclear polarisation (DNP), also called hyperpolarisation, a small amount of unpaired electron spins is added to the sample containing the nuclear spins, and the polarisation of these unpaired electron spins is transferred to the nuclear spins by means of a microwave field. Traditional DNP polarises the electron spin of stable paramagnetic centres by cooling down to low temperature and applying a strong magnetic field. Then weak continuous wave microwave fields are used to induce the polarisation transfer. Complicated cryogenic equipment and strong magnets can be avoided using short-lived photo-excited triplet states that are strongly aligned in the optical excitation process. However, a much faster transfer of the electron spin polarisation is needed and pulsed DNP methods like nuclear orientation via electron spin locking (NOVEL) and the integrated solid effect (ISE) are used. To describe the polarisation transfer with the strong microwave fields in NOVEL and ISE, the usual perturbation methods cannot be used anymore. In the previous paper, we presented a theoretical approach to calculate the polarisation transfer in ISE. In the present paper, the theory is applied to the system naphthalene-h(8) doped with pentacene-d(14) yielding the photo-excited triplet states and compared with experimental results.

Taylor & Francis2014

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Low-temperature spin dynamics of a valence bond glass in Ba2YMoO6

Julian Piatek, Henrik Moodysson Rønnow

Institute of Physics (IoP) and Deutsche Physikalische Gesellschaft2013