Magnetic and resonant X-ray scattering investigations of strongly correlated electron systems

Resonant X-ray scattering is a method which combines high- resolution X-ray elastic diffraction and atomic core-hole spectroscopy for investigating electronic and magnetic long-range ordered structures in condensed matter. During recent years the development of theoretical models to describe resonant X-ray scattering amplitudes and the evolution of experimental techniques, which include the control and analysis of linear photon polarization and the introduction of extreme environment conditions such as low temperatures, high magnetic field and high pressures, have opened a new field of investigation in the domain of strongly correlated electron systems. To cite this article: L. Paolasini, F. de Bergevin, C. R. Physique 9 (2008).     

X radiation sources based on accelerators

Light sources based on accelerators aim at producing very high brilliance coherent radiation, tuneable from the infrared to X-ray range, with picosecond or femtosecond light pulses.The first synchrotron light sources were built around storage rings in which a large number of relativistic electrons produce “synchrotron radiation” when their trajectory is subjected to a magnetic field, either in bending magnets or in specific insertion devices (undulators), made of an alternating series of magnets, allowing the number of curvatures to be increased and the radiation to be reinforced.These “synchrotron radiation” storage rings are now used worldwide (there are more than thirty), and they simultaneously distribute their radiation to several tens of users around the storage ring.The most effective installations in term of brilliance are the so-called 3rd generation synchrotron radiation light sources. The radiation produced presents pulse durations of the order of a few tens of ps, at a high rate (of the order of MHz); it is tuneable over a large range, depending on the magnetic field and the electron beam energy and its polarisation is adjustable (in the VUV-soft-X range). Generally, a very precise spectral selection is made by the users with a monochromator.The single pass linear accelerators can produce very short electron bunches (). The beam of very high electronic density is sent into successive undulator modules, reinforcing the radiation's longitudinal coherence, produced according to a Free Electron Laser (FEL) scheme by the interaction between the electron bunch and a light wave. The very high peak brilliance justifies their designation as 4th generation sources. The number of users is smaller because an electron pulse produces a radiation burst towards only one beamline. Energy Recovery Linacs (ERL) let the beam pass several times in the accelerator structures either to recover the energy or to accelerate the electrons during several turns, and thus provide subpicosecond beams for a greater number of users.A state-of-the-art of X sources using conventional (and not laser plasma based) accelerators is given here, underlying the performance already reached or forecast and the essential challenges. To cite this article: M.-E. Couprie, J.-M. Filhol, C. R. Physique 9 (2008).     

Tree-level contributions to the rare decays , , and in the Standard Model

The tree-level contributions to the rare decays , , and are analyzed and compared to those occurring in , , and . It is shown that these purely long-distance contributions, arising from the exchange of a charged lepton, can be significant in B⁺ decays for an intermediate τ, potentially blurring the distinction between the modes used to extract B⁺→τ⁺ν_τ and those used to probe the genuine short-distance and FCNC transitions. Numerically, the tree-level contributions are found to account for 98%, 12% and 14% of the total , , and rates, respectively.     

Phase transitions in the quantum Hall liquid in a quantum wire

Two scenarios for the collapse of the ν=1 quantum Hall liquid (QHL) state, with the effective quantum wire (QW) width defined by the Fermi vector k_F, are studied. Here, ν for the QW is defined as the filling factor of Landau levels (LL) at the center of the QW. In the first one there is no electron redistribution at critical magnetic field , where the Fermi energy, E_F, coincides with the bottom of the empty upper spin-split LL. For the ν=1 state is unstable due to exchange-correlation effects and lateral confinement. In the second scenario, a transition to the ν=2 state occurs, with much smaller width, at . The latter scenario is analyzed in the Hartree–Fock approximation (HFA). Here the Hartree contribution to the total energy affects drastically due to strong electron redistribution in the QW. In both scenarios, the exchange-enhanced g-factor is suppressed at B_cr. The critical fields, activation energy, and optical g-factor obtained in the first scenario are very close to the measured ones.     

Intensity transformation of vector Bessel beams using a multilayer system

We theoretically investigate the generation of vector Bessel beams of the order m using a phase shifted superposition of TE and TM electromagnetic Bessel beams. Such Bessel beams are characterized by the intensity profile described by the superposition of squared Bessel functions of the orders (m-1) and (m+1). We derive the conditions for creating the special distributions of the intensity, which are determined by only one Bessel function, or . We offer the approach of intensity transformation based on the Bessel beam transmission through a multilayer system. Finally, we reveal the perfect intensity transformation transferring the whole energy from - to -profile of intensity distribution.     

Three equations of state for solids considering thermal effect

 The Einstein model to consider thermal effect in universal equations of state (UEOS) is modified. It is proposed that the zero-point vibration term should be deleted in a thermal UEOS, and the parameters cannot be directly taken as experimental data at a reference temperature, V_R, B_R, and , but their values at absolute zero temperature, V₀, B₀, and . An approach is proposed to solve V₀, B₀, and from V_R, B_R, and . The approaches are applied to three typical universal EOSs, including the Baonza, mGLJ and Morse EOSs. The numerical results show that the solved values of parameters are almost identical for different EOSs. And the thermo-physical properties predicted through different EOSs are almost identical at zero- and low-pressure conditions, once the same approach and input experimental data are used to solve the parameters. It is concluded that the prediction of thermo-physical properties at zero- and low-pressure conditions cannot be taken as the criteria to judge the applicability of a universal EOS.     

Study on improving output capability of high-power lasers providing short pulses by using different laser glasses together in the amplifier chain

We have discussed the optical and laser properties of different laser glass types. According to the properties of the gain and the B integral in the high-power laser amplifier chain, an optimal configuration is pointed out to improve the output capability at short pulses based on reducing nonlinear refractive index n₂. In this way, high gain glasses and low n₂ glasses are used in different amplifier stages simultaneously. The simulated results show that on the condition that the maximal output energies of both the configurations are same at 3 and 5 ns, the maximal output capability growth ratio of 30.48% () and 42.24% () for the optimal configuration can be obtained at 1 ns, respectively.     

Polarized neutron scattering studies of the kagomé lattice antiferromagnet

We report polarized neutron scattering studies of spin-wave excitations and spin fluctuations in the lattice antiferromagnet KFe₃(OH)₆(SO₄)₂ (jarosite). Inelastic polarized neutron scattering measurements at 10 K on a single crystal sample reveal two spin gaps, associated with in-plane and out-of-plane excitations. The polarization analysis of quasi-elastic scattering at 67 K shows in-plane spin fluctuations with XY symmetry, consistent with the disappearance of the in-plane gap above the Néel temperature . Our results suggest that jarosite is a promising candidate for studying the 2D XY universality class in magnetic systems.     

Magnetostructural phase transitions of DyFe4Ge2: Part II Neutron diffraction

We present the temperature magnetic phase diagram of the compound DyFe₄Ge₂ determined from neutron diffraction data for the entire magnetically ordered regime. DyFe₄Ge₂ undergoes at a simultaneous structural and magnetic transition of second order (or weakly first order) followed by two subsequent isostructural first-order magnetic transitions at and T_ic1=28K:

The re-entrant lock-in magnetic phase is stable in the high-temperature range T_ic2–T_N and in the low-temperature range 1.5 K–T_ic1 while the incommensurately modulated magnetic phase is sandwiched in the intermediate range T_ic1–T_ic2 between the two commensurate phases. The wave vector q₂ has a temperature-dependent length with a minimum in the middle of the incommensurate range and corresponds to a multiaxial amplitude modulated phase. Symmetry analysis leads for both propagation vectors in Cmmm to a twofold and fourfold splitting of the tetragonal Dy 2b site and the Fe 8i sites, respectively. The low temperature and the phases correspond to 3D canted magnetic structures described by the irreducible representations (Irreps) Γ₂+Γ₃ while the high-temperature q₁ phase to 2D canted magnetic structures described by a single Irrep Γ₂. The T_ic2 transition is connected with reorientations of both Fe and Dy moments.     

Search for the rare decays , , and

Rare decay modes , J/ψ→D⁻π⁺+c.c., and are searched for using events collected with the BESII detector at the BEPC. No signal above background is observed. We present upper limits on the branching fractions of , B(J/ψ→D⁻π⁺)<7.5×10⁻⁵, and at the 90% confidence level.