P. Garoche

Modulation of the beam intensity for high-dynamic-range low energy electron diffraction

We describe a low-energy electron diffraction process with a periodical modulation of the electron beam intensity. The conditions to get a coherent modulated beam are investigated, first, by simulating the electron trajectories and second, by analyzing the diffraction pattern oscillation. The large increase in the dynamic range is exemplified by diffraction of the electrons by mica surface nanostructures.

Specific heat measurements on single-phased AlCuFe quasicrystals

Abstract We have measured the low-temperature specific heat of samples of quasicrystalline AlCuFe alloys using an ac method. We observed weak electronic contributions in all the samples and excess vibrational contributions in phason-strained ones. The electronic contribution can account for the anomalously high resistivity using a simple pseudogap model with a realistic electronic mean free path value. The evolution of the vibrational contribution suggests that phason strains induce a softening of low-energy transverse phonon modes by decreasing the elastic shear modulus. Some implications are discussed.

Electronic density of states, stability, and coherency effects between resonant states in quasicrystalline Al73.2Mn21Si5.8

In quasicrystalline AlMnSi alloys, the spin‐glass contribution makes difficult the correct evaluation of the electronic density of states (DOS) at the Fermi level. The present investigation shows that the low temperature specific heat contribution varies as the inverse of the square of the magnetic field, and so enables us to separate the spin‐glass contribution from the electronic one. The d DOS at the Fermi level is reduced to about 70% of the value observed in the AlMn solid solution. This reduction can be understood as resulting from a sp‐d resonant effect which opens a pseudogap at the center of the virtual bound state. This pseudogap, along with the icosahedral symmetry, can explain the origin of the stability of this phase.

Fractional quantized nesting for the spin‐density waves induced by a magnetic field

Recent experiments on magnetic field‐induced spin‐density waves (SDW) in the highly anisotropic organic conductor (TMTSF)2ClO4 have led us to build from thermodynamic data a phase diagram with a series of magnetic subphases. This series of transitions results from the jump of the nesting vector of the SDW. In this paper we relate our investigations of the phase diagram by magnetocaloric experiments. The measurement of the entropy variations were performed at fixed temperature by sweeping the magnetic field on an oriented single crystal. A vanishing latent heat is observed at the transition between different subphases, but this experiment reveals that each subphase is split by several phase transitions. This new complexity explains the discrepancy between phase diagrams obtained from various techniques. It is proposed that a third characteristic length, the lattice periodicity, is responsible for this complexity, thus implying a fractional quantization of the SDW wave vector.

Improving the coherence of a low-energy electron beam by modulation

In a standard low-energy electron generator the beam is formed by particles traveling close to the axis of symmetry. However some electron trajectories are unstable and strongly dependent on the initial conditions. Numerical ray tracing shows that ultimate beam coherence is limited by these trajectories that pass far from the symmetry axis. This contribution can be partly eliminated by modulating the initial conditions and selecting the modulated response. This is exemplified with a low-energy electron beam used for electron diffraction, where the beam current modulation produces a modulated diffraction pattern that displays noteworthy improvement (sevenfold) in wave vector resolution.