K. H. Bennemann

Theory for the Ultrafast Ablation of Graphite Films

The physical mechanisms for damage formation in graphite films induced by femtosecond laser pulses are analyzed using a microscopic electronic theory. We describe the nonequilibrium dynamics of electrons and lattice by performing molecular dynamics simulations on time-dependent potential energy surfaces. We show that graphite has the unique property of exhibiting two distinct laser-induced structural instabilities. For high absorbed energies ( >3.3 eV/atom) we find nonequilibrium melting followed by fast evaporation. For low intensities above the damage threshold ( >2.0 eV/atom) ablation occurs via removal of intact graphite sheets.

Dynamics of excited electrons in copper and ferromagnetic transition metals: Theory and experiment

Both theoretical and experimental results for the dynamics of photoexcited electrons at surfaces of Cu and the ferromagnetic transition metals Fe, Co, and Ni are presented. A model for the dynamics of excited electrons is developed, which is based on the Boltzmann equation and includes effects of photoexcitation, electron-electron scattering, secondary electrons (cascade and Auger electrons), and transport of excited carriers out of the detection region. From this we determine the time-resolved two-photon photoemission (TR-2PPE). Thus a direct comparison of calculated relaxation times with experimental results by means of TR-2PPE becomes possible. The comparison indicates that the magnitudes of the spin-averaged relaxation time

THEORY FOR THE EXCITATION SPECTRUM OF HIGH-TC SUPERCONDUCTORS : QUASIPARTICLE DISPERSION AND SHADOWS OF THE FERMI SURFACE

\ensuremath{\tau}

A theory for the size and structural dependence of the ionization and cohesive energy of transition-metal clusters

and of the ratio

Novel neutron resonance mode in d x 2 y 2-wave superconductors

{\ensuremath{\tau}}_{\ensuremath{\uparrow}}/{\ensuremath{\tau}}_{\ensuremath{\downarrow}}

The magnetic properties of small Fen-clusters

of majority and minority relaxation times for the different ferromagnetic transition metals result not only from density-of-states effects, but also from different Coulomb matrix elements M. Taking

Theory of nonlinear optical properties of small metallic spheres

{M}_{\mathrm{Fe}}g{M}_{\mathrm{Cu}}g{M}_{\mathrm{Ni}}{=M}_{\mathrm{Co}}

Tight-binding study of the structural stability of the (110) surface of the 5d-transition metals Ir, Pt and Au

we get reasonable agreement with experiments.

Time-dependent energy absorption changes during ultrafast lattice deformation

Using a new method for the solution of the FLEX-equations, which allows the determination of the self energyk(!) of the 2D Hubbard model on the real frequency axis, we calculate the doping dependence of the quasi-particle excitations of High-Tc superconductors. We obtain new results for the shadows of the Fermi surface, their dependence on the deformation of the quasi particle dispersion, an anomalous !-dependence of Imk(!) and a related violation of the Luttinger theorem. This sheds new light on the influence of short range magnetic order on the low energy excitations and its significance for photoemission experiments. 74.25.Jb,79.60.-i,71.27.+a Despite an enormous progress (1), the electronic exci- tation spectrum of the strongly correlated High-Tc su- perconductors is still far from being understood. Re- cent angular resolved photoemission measurements (2-6) for different doping concentrations demonstrate that pro- nounced deformations of the quasiparticle dispersion oc- cur. The opening of a spin density gap and the variation in spectral weight as function of k (6) reflect the strong in- fluence of the antiferromagnetic correlations on the low energy excitations. In particular, the interpretation of the shadows of the Fermi surface (FS) observed by Aebi et al. (4) in terms of antiferromagnetic correlations is un- der current debate (7). Therefore, the determination of the elementary excitations with high energy and momen- tum resolution is of extreme importance. In this Letter, we calculate the quasiparticle excitation spectrum of the one band Hubbard Hamiltonian using a new numerical method for the self consistent summa- tion of all bubble and ladder diagrams (8) (fluctuation exchange approximation) on the real frequency axis, and compare our results with recent photoemission experi- ments. Due to the high degree of numerical stability of this method, we present interesting new results for a strong deformation of the quasiparticle dispersion at the wave vector k = (π,0) upon doping, the occurrence of satellite peaks and shadows of the Fermi surface in the paramagnetic state and an unusual momentum and fre- quency dependence of the electronic self energy. All this sheds new light on the occurrence of FS-shadows w ithout long range antiferromagnetic order.

Theory for the laser-induced femtosecond phase transition of silicon and GaAS

Abstract A simple tight-binding type of electronic theory is presented for the size and structural dependence of the ionization energy I n and cohesive energy E coh ( n ) of transition-metal clusters. In agreement with recent experimental results a characteristic structure in I n is obtained for Fe n and Ni n , which results from size-dependent changes of the d-electron energies and band widths. The calculated I n and E coh ( n ) depend not only on cluster size n but also on the geometrical arrangement of the atoms. Assuming that small Fe n and Ni n clusters have bcc- and fcc-like structures, respectively, gives the best agreement with experiment.