Seminars Archive

Femtosecond time-resolved spectroscopy of the solid state

Abstract
Solids are characterized by their different degrees of freedom charge, lattice, and spin, which form separate contributions to the specific heat. Couping of these degrees of freedom to each other results in fundamental phenomena like superconductivity (electron-phonon coupling) or the Kondo effect (spin-dependent electron scattering). Time-resolved spectroscopy employing femtosecond laser pulses faciliates direct and independent access of these degrees of freedom and occasionally novel insight into coupling mechanisms. In this talk two examples will be presented. (a) Quasi-one dimensional structurs form charge density waves. Using time- and angle-resolved photoelectron spectroscopy we have succeeded to monitor the single and many particle excitations in the charge density wave compound TbTe3close to thermodynamic equilibrium. In case of more intense optical excitation non-equilibrium conditions are generated, which finally lead to melting of the charge density wave and facilitate an identification of the responsible mode. (b) The ferromagnet Gd stands out due to a weak spin-lattice interaction with a characteristic time constant of 40 ps, which is explained by the half-filled 4f shell with its isotropic orbital. The selective time-resolved detection of the magnetic moment of the 4f shell and the 5d valence band after optical excitation was achieved by X-ray magnetic circular dichroism and magneto-optics. We find a simultaneous demagnetization within the first picosecond. This demonstrates the importance of non-equilibrium processes for ultrafast magentization dynamics, which in Gd bring about a 40 times accelerating of spin-lattice interaction.