On Ultrafast Spin Dynamics: Spin Dependent Fast Response of Hot Electrons, of Band--Structure

Different energy shifts for majority and minority electrons occur. Thus, for example in case of (laser) excited ferromagnetic metals majority and minority electrons may respond differently in time during closing the exchange splitting. Spin flip transitions of the hot electrons due to electron interactions cause quasi hybridization of the spin split states. This is also the case in itinerant ferromagnetic metals due to hopping between sites having magnetic moments pointing in direction of the magnetization (+) and opposite direction (-) and with energy levels $\varepsilon^+_{i\sigma}$ and $\varepsilon^-_{i\sigma}$. For energetic reasons the molecular field acts asymmetrically on the spins of the electrons and on spin flip transitions and thus causes different lifetimes of minority and majority electrons and spin dependent electron energy shifts. Quite general minority hot electrons in spin split states may respond faster than majority electrons at non--equilibrium. The molecular field acting on the spins delays spin flip transitions $\uparrow \rightarrow \downarrow$ and thus a response of the hot majority electrons and their energy levels. The closing of the exchange splitting in the electron spectrum of ferromagnetic transition and rare--earth metals, ferromagnetic semiconductors, spin split quantum well states in thin ferromagnetic films, etc. will reflect this. The time and spin dependent energy shifts of electrons at non--equilibrium may cause interesting behavior, in particular of magnetic tunnel junctions, spin currents etc.. In ferromagnets the moment reversal lifetime of (local) magnetic moments parallel to the global magnetization is larger than of moments pointing in opposite direction.Comment: 25pages,8 Fig

Theory for the Nonlinear Optical Response of Transition-Metals: Polarization Dependence as a Fingerprint of the Electronic Structure at Surfaces and Interfaces

We show that the nonlinear optical response reflects sensitively the electronic structure of transition-metal surfaces and interfaces. $d$ and $s$ electrons may contribute rather differently to the second harmonic generation (SHG) signal. This results from the different sensitivity of $d$ and $s$ electrons to surface and symmetry changes. Consequently, SHG for noble metals shows a by far larger dependence on the polarization of the incoming light than for transition metals like Fe, Co, Ni, in particular at lower frequencies. The theoretical results are compared with recent measurements. We conclude that the SHG yield is in addition to the nonlinear magneto-optical Kerr-effect a sensitive fingerprint of the electronic structure at surfaces and interfaces.Comment: 20 pages+ 4 postscript-formatted figures,Revtex, submitted to Phys. Rev. B 08/02/199

Nonlinear magneto-optical Kerr spectra of thin ferromagnetic iron films calculated with ab initio theory

Using a spin-polarized full-potential linear muffin-tin orbital method we present calculations of the nonlinear magneto-optical Kerr effect for thin bcc Fe films within a slab geometry. Film layer dependent contributions to the Kerr spectrum are determined. Thus, we calculate the magneto-optical Kerr spectra for the linear and nonlinear case. Our results show clearly that the Kerr spectra of thin films are characteristicly different from those at surfaces of bulk materials. In the case of linear Kerr spectra of Au/Fe(bcc)/Au(001) films our theoretical results are in good agreement with observed frequency- and thickness-dependent spectra.Comment: 13 pages, latex (revtex), 8 uuencoded postscript figure

Analysis of the elementary excitations in high-T_c cuprates: explanation of the new energy scale observed by ARPES

Using the Hubbard Hamiltonian we analyze the energy- and momentum-dependence of the elementary excitations in high-T$_c$ superconductors resulting from the coupling to spin fluctuations. As a result of the energy dependence of the self-energy $\Sigma ({\bf k}, \omega)$, characteristic features occur in the spectral density explaining the 'kink' in recent ARPES experiments. We present results for the spectral density $A({\bf k}, \omega)$ resulting from the crossover from Im $\Sigma ({\bf k}, \omega)\propto \omega$ to Im $\Sigma ({\bf k}, \omega)\propto \omega^2$, for the feedback of superconductivity on the excitations, and for the superconducting order parameter $\Delta({\bf k}, \omega)$. These results relate also to inelastic neutron scattering and tunneling experiments and shed important light on the essential ingredients a theory of the elementary excitations in the cuprates must contain.Comment: 5 pages, 4 figure, misprints correcte

Theory for Nonlinear Mie-Scattering from Spherical Metal Clusters

Using classical electrodynamics we determine the angular dependence of the light intensities radiated in second and third harmonic generation by spherical metal clusters. Forward and backward scattering is analyzed in detail. Also resonance effects in the integrated intensities are studied. Our work treats the case of intermediate cluster sizes. Thus it completes the scattering theory fo spherical clusters between Rayleigh-type analysis for small spheres and geometrical optics for spheres much larger than the wavelength for nonlinear optics. Since the particle size sensitivity of Mie-scattering is increased by nonlinearity, the results can be used to extract sizes of small particles from nonlinear optics. (submitted to Phys. Rev. B)Comment: 34 pages with 13 figures (postscript code avalaible upon request), RevTe

Theoretical analysis of Cooper-pair phase fluctuations in underdoped cuprates: a spin-fluctuation exchange study

We study Cooper-pair phase fluctuations in cuprate superconductors for a spin fluctuation pairing interaction. Using an electronic theory we calculate in particular for the underdoped cuprate superconductors the superfluid density $n_s(T)$, the superconducting transition temperature $T_c(x) \propto n_s$ below which phase coherent Cooper-pairs occur, and $T_c^{*}(x)$ where the phase incoherent Cooper-pairs disappear. Also we present results for the penetration depth $\lambda(x,T)$ and for the weak pseudogap temperature $T^{*}(x)$ at which a gap structure occurs in the spectral density. A Mei{\ss}ner effect is obtained only for $T < T_c$. We find that phase fluctuations become increasingly important in the underdoped regime and lead to a reduction of $T_c$ in good agreement with the experimental situation.Comment: 4 pages, 5 figures. One reference adde

Theory for the nonlinear optical response of a nonspherical metal cluster

Using classical electrodynamics we determine the higher harmonic radiation by a nonspherical metal cluster in form of a matrix formalism. Extending the theory for the source of the higher harmonic radiation for spherical metal clusters as introduced by \"Ostling et al. [Z. Phys. D {\bf 28}, 169 (1993)] we calculate the sources for nonspherical particles. Employing these sources we develop the nonlinear Mie theory and determine the radiated higher harmonic fields generated by the cluster. Our theory is valid for arbitrary shape and arbitrary complex refractive index for cluster sizes much smaller and comparable to the wavelength of the incident light.Comment: 6 pages, Revte

Dynamical Mean Field Theory for Perovskites

Using the Hubbard Hamiltonian for transition metal-3d and oxygen-2p states with perovskite geometry, we present a dynamical mean field theory which becomes exactin the limit of large coordination numbers or equivalently large spatial dimensions $D$. The theory is based on a new description of these systems for large $D$ using a selective treatment of different hopping processes which can not be generated by a unique scaling of the hopping element. The model is solved using a perturbational approach and an extended non-crossing approximation. We discuss the breakdown of the perturbation theory near half filling, the origin of the various 3d and 2p bands, the doping dependence of its spectral weight, and the evolution of quasi particles at the Fermi-level upon doping, leading to interesting insight into the dynamical character of the charge carriers near the metal insulator instability of transition metal oxide systems, three dimensional perovskites and other strongly correlated transition metal oxides.Comment: 9 pages, Revtex, 8 postscript figure

Theory for the Dependence of Optical Second Harmonic Generation Intensity on Non-equilibrium Electron Temperatures at Metal Surfaces

We present a theory for the electron-temperature dependence $T_{el}$ of optical second harmonic generation (SHG). Such an analysis is required to study the dynamics of metallic systems with many hot electrons not at equilibrium with the lattice. Using a tight-binding theory for the nonlinear susceptibility \cwtel and the Fresnel coefficients we present results for the SHG intensity \iwtel for a Cu monolayer. In the case of linear optical response we find that the intensity will decrease monotonously for increasing $T_{el}$. In agreement with experiment we find a frequency range where \iwtel may be enhanced or reduced depending on electron temperature. Note, \cwtel rather than the Fresnel coefficients determines essentially the temperature dependence. Our theory yields also that SHG probes effects due to hot electrons more sensitively than linear optics. We also discuss the $T_{el}$-dependence of SHG for Au and Ag.Comment: 17 pages, Revtex, 10 postscript figures, tar'ed together, compressed with 'gzip', encoded with 'uuendcode

Theory for underdoped high-T_c superconductors: effects of phase fluctuations

In underdoped cuprates, T_c is thought to be determined by Cooper pair phase fluctuations because of the small superfluid density n_s. Experimentally, T_c is found to scale with n_s. The fluctuation-exchange approximation (FLEX) in its standard form fails to predict this behavior of T_c since it does not include phase fluctuations. We therefore extend the FLEX to include them selfconsistently. We present results for T_c[n_s,x], where x is the doping.Comment: (5 pages, 3 figures included, minor changes, to appear in JLTP proceedings MOS99