J.P. Ader

The effect of a magnetic field on the inversion temperature of a spin crossover compound revisited

The compound MnIII[(pyrol)3tren] where (pyrol)3tren is the trianionic Schiff base resulting from the condensation of pyrolle-2-carboxaldehyde with 2,2′,2′′-tris(ethylamino)amine exhibits an abrupt LS (S=1) ↔ HS (S=2) spin conversion with an inversion temperature in low field of T1/2 = 43.7 K. The temperature dependence of the heat capacity has been measured. The enthalpy and entropy variations associated with the spin conversion have been found as ΔH=0.60 kJ mol−1 and ΔS=13.8 J K−1 mol−1. The temperature dependence of the magnetization has been measured under various magnetic fields up to 23 T. T1/2 has been found to decrease as the field increases, first in a smooth fashion, then more rapidly. Under 23 T, T1/2 is equal to 42.1 K. A theoretical model based on a mean field approximation of the spin crossover phenomenon has been compared with the experimental findings.

A Regge pole model fit of pion and K meson photoproduction

Abstract Differential cross sections in the forward direction for π + , π o and K + photoproduction are evaluated with Regge trajectories exchange in the t -channel. Kinematical singularities, constraints among helicity amplitudes, conspiracy and evasion, have been studied very carefully. A very good fit is obtained for γ p → π o p. assuming ω and B exchange and with only three parameters, for 2 ⩽ Eγ ⩽ 5 GeV and transfer momentum t up to −3 (GeV/ c ) 2 . Good agreement is also observed for γ p → π + n with π and B exchange, and for γ p → Λ o K + and γ p → Σ o K + with K and K ∗ exchange. Present experimental data seem to exhibit violations of the SU 3 symmetry.

Does cooperativity influence the lifetime of the photo-induced HS state ?

We have first recalled the T(LIESST) procedure which consists to determine the temperature above which the photo-magnetic effect is erased. In addition we have selected to series of iron(II) spin crossover complexes, the [Fe(PM-L)2(NCS)2] and [Fe(bpp)2]X2·nH2O families, to analyse the influence of the cooperativity on the stability of the photo-induced HS state. Some of these complexes exhibit gradual thermal spin crossover behaviours while some others undergo an abrupt thermal transition, with and without hysteresis. Interestingly, whatever the cooperativity effect on the thermal spin crossover transition, the lifetime of the metastable state of all these derivates remains governed by the T(LIESST) = T0 - 0.31 T1/2 relation. Finally, we have investigated the magnetic and the photomagnetic properties of a [Fe(bpp)2]-Nafion film. Once more the role of the cooperativity on the stability of the photoinduced HS state appears minor. Conversely, the influence of the nature and the geometry of the inner coordination sphere appears preponderant.

Gauged BRST symmetry for the free bosonic string

Abstract A gauged version of the BRST symmetry for the free bosonic string is constructed by an iterated Noether procedure. The gauge-fixed action invariant under the resulting local BRST symmetry is derived.

Gauge invariance presciptions for photoproduction amplitudes

Abstract We show that the gauge condition for a massless particle should be considered as an usual kinematical constraint, in fact a low-energy theorem, which has no dynamical implications for photoproduction reactions, and in particular that it gives no prescription on the nature of dynamical singularities for the scattering amplitude (fixed poles, moving poles, conspiring poles, cuts, Kronecker-delta singularities, etc...) which are all allowed. We also prove that no peculiar philosophy with regard to duality is imposed by gauge invariance. A special attention is devoted to π-exchange and we show that π−pϱtr0n s have structure in the forward direction, expected in very particular case where ϱ0-polarization is measured in the Jackson frame. We then deduce some implications for vector dominance, and more generally we give a very simple prescription to compute scattering amplitudes describing vector-meson production (or electroproduction) which goes smoothly to the photoproduction amplitude when the vector-meson mass goes to zero.

Orthodoxvs. unorthodoxB\bar B widths of diquoniumswidths of diquoniums

SummaryThe quantitative status of the

Do we need “mock” diquoniums to explain narrow widths into BB channels?

B\bar B

A BRST lagrangian quantization of reducible gauge theories: Non-abelian p -forms and string field theories

widths of true diquoniums as predicted by various models is critically analysed. It is shown that—contrary to a recent claim—unorthodox values of ≃10 MeV for the