J. Nasser

Monte Carlo simulations of spin-crossover transitions using the two-level model. I: Mononuclear single sublattice case

We study by Monte-Carlo technique the Hamiltonian associated with the phenomenological two level Ising-like model proposed to explain the thermal behaviour of the spin-crossover compounds. We propose an adaptation of the Metropolis algorithm to take into account the different degeneracies of the two levels.

Size and pressure effects in the atom-phonon coupling model for spin crossover compounds

The smallest size of a spin crossover particle which still exhibits hysteric behavior is an open question. It is clear that spin crossover is a cooperative phenomenon, which implies the dependence of the high spin fraction variation on the number of molecules within the system. We use the “atom-phonon coupling” model to explain the different results that have been recently published with regard to the critical dimension of a spin crossover system. In the second part of the paper, the applicability of the model to the pressure induced transition has been developed, and the results are in good agreement with what has been observed experimentally.

Monte Carlo simulations of spin-crossover transitions using the two level model. II: Binuclear case

Abstract We show preliminary results of the Monte Carlo simulations on spin-crossover binuclear systems. The two-level Ising-like model was used with two interaction parameters: intermolecular (‘ferro’) and intramolecular (‘antiferro’). The obtained two step transitions are analysed in term of LS-LS, LS-HS HS-1HS populations.

Intramolecular aspects of the electron transfer in the biferrocenium mixed-valence cation, using PKS theory

Abstract The electron transfer properties of the mixed-valence biferrocenium cation are investigated using the Piepho-Krausz-Schatz (PKS) vibronic theory. Parameters of the model are deduced from both vibrational spectroscopy (stiffness constant) and MO calculations (transfer integral and vibronic coupling parameter); the computed asymmetric distortion agrees with structural data; a specific heat Schottky anomaly, recently reported, is also explained in the frame of that model. The low-temperature transfer rate is also computed. it ranges from 1.5 × 10 10 to 8.7 x 10 12 s −1 , according to the nature of the HOMO, and definitely lies higher than the Mossbauer timescale (≈ 10 8 s −1 ).

Excited metastables electronic spin states in spin crossover compounds studies by atom-phonon coupling model: Gradual and two-step transition cases

This contribution reports the study of metastable spin states in spin-crossover materials in the framework of the atom-phonon coupling model. Using this model for a linear chain we show theoretically for the first time that, even if the cooperativity is not strong enough to obtain hysteresis at high temperatures, metastable states exist at low temperatures for a high spin (HS) fraction of nHS=1. This gradual thermal transition featuring metastable states at low temperature has been observed experimentally by [Letard et al., J. Phys.: Conf. Ser. 21, 23 (2005)]. Moreover, for compounds showing a two-step thermal transition, we show that metastable states, corresponding to a HS fraction nHS=0.5, are present. This metastable states appear up to 50 K, fact that was observed by [Moussa et al., Phys. Rev. Lett. 94, 107205 (2005)] and [Matsumoto et al., J. Phys.: Conf. Ser. 148, 012029 (2009)].

Re-entrance phase and excited metastable electronic spin states in one-dimensional spin crossover compounds explained by atom-phonon coupling model

An unusual behavior of the disappearance and the reappearance of the hysteresis loop of the spin crossover compounds under external pressure was observed experimentally by Garcia et al. [Hyperfine Interact. 139/140, 543 (2002)] and modeled within the framework of the “atom-phonon coupling” model. In this contribution we explain the evolution of the hysteresis width versus the applied pressure. Using the same model, we also show the existence of stable as well as metastable and unstable states at very low temperatures. These excited metastable electronic spin states at low temperature can be associated to the “out of equilibrium” states in the light-induced electronic spin state trapping.An unusual behavior of the disappearance and the reappearance of the hysteresis loop of the spin crossover compounds under external pressure was observed experimentally by Garcia et al. [Hyperfine Interact. 139/140, 543 (2002)] and modeled within the framework of the “atom-phonon coupling” model. In this contribution we explain the evolution of the hysteresis width versus the applied pressure. Using the same model, we also show the existence of stable as well as metastable and unstable states at very low temperatures. These excited metastable electronic spin states at low temperature can be associated to the “out of equilibrium” states in the light-induced electronic spin state trapping.

Specific heat of two-step spin conversions

Abstract The specific heat anomaly associated with two-step spin conversions has been computed in the frame of a mean-field Ising-like model, implicitly accounting for intramolecular vibrations through an ‘electro-vibrational’ degeneracy ratio.

Application of A Spin-1 Model to Describe Localized Delocalized Transitions in Mixed Valence Molecular Solids

Abstract A S=1 Ising interacting system described by a Blume-Emery-Griffith Hamiltonian is proposed to reproduce the different temperatures-dependencies of mixed valence biferrocenium derivatives.

Absolute method for an optical measurement of the earth gravitational axis: application to Watt balance

We show how a Fabry Perot interferometer could be used to determine the gravitational axis of the earth g with an high sensitivity. Our method lie g axis to an optical beat frequency value between two lasers. The principle is described and a budget error is presented. The sensitivity of the sensor is estimated to be below of 85 nrad/MHz. This method could be used as a high sensitivity and absolute tilt sensor or even as a seismometer. This sensor is dedicated to the french watt balance project as described in this paper.

Improved miniature wavelength meter based on polarisation state measurements

We propose an alternative design to the wavemeters based on scanning Michelson interferometers. Using realistic experimental parameters, we show that a relative accuracy of one part in 6x10-8 can be reached with a displacement of the target mirror of only 360 &mgr;m. This improvment becomes possible thanks to the significant advances in polarimetry that permit measurement of the ellipsometric parameters &psgr; and &Dgr; with an accuracy of 0.07° with readily-available commercial equipment. This leads to an interpolation rate of &lgr;/10000. The proposed method has been setup and the chief parameters limiting its accuracy are determined.