Optical spectroscopy and angle resolved photoemission have replaced tunneling spectroscopy as techniques that yield the best data on the self energy of the charge carriers in new superconductors. We review the most recent results of self-energy spectroscopy for several cuprate superconductors. We show that the self energy is determined by two channels of bosonic excitations, a sharp peak and a continuous background. Both channels appear to be of magnetic origin, the sharp mode is associated with the so-called 41 meV neutron resonance that appears only at low temperatures, a few tens of degrees above the superconducting transition temperature in underdoped materials, while the continuous background has a temperature independent spectral function.
Terahertz (THz) radiation is ideal for probing many different materials and processes. Photons in the THz regime have energies on the order of an meV, which is an important energy scale for many electronic processes. In this paper we will describe the use of optical rectification of 50 fs IR pulses to generate THz pulses. Using this method, spectrally broad THz pulses with durations on the order of ps can be produced. This feature allows us to obtain time and frequency resolved information about the transmission of THz radiation during transient processes. A 50 fs IR pulse is used to optically excite a material and the relaxation as a function of time can be observed with the THz probe.
We are developing a transient THz spectroscopy to study non-equilibrium processes in thin film superconducting YBa2Cu3O7-δ (YBCO). We use ultrafast optical pulses to excite the sample, breaking a fraction of the Cooper pairs responsible for the film's superconductivity. This process produces highly energetic quasiparticles which thermalize and recombine on a picosecond timescale as the superconducting state recovers. Transient THz spectroscopy allows us to follow the evolution of this process with the required resolution, while simultaneously providing valuable spectroscopic information.
The combination of lowered dimensionality and electron-electron correlations are responsible for the unusual temperature and frequency dependence of the electrical conductivity of the new superconductors. We first review the electrodynamics of two systems, U2Ru2Si2 and Sr2RuO4 where conventional Fermi liquid ideas seem to work. Here transport is by free carriers with strongly renormalized masses. On the other hand the electrodynamics of the high Tc cuprates and the organic charge transfer salts is unconventional. The high Tcs show a Drude peak with an anomalous temperature and frequency dependent scattering rate for the in-plane conductivity, while normal to the planes they are almost insulating. In the organics, the transport currents are carried by a narrow collective mode coupled to phonons.
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