Transition Spectra for a BCS Superconductor with Multiple Gaps: Model Calculations for MgB_2

We analyze the qualitative features in the transition spectra of a model superconductor with multiple energy gaps, using a simple extension of the Mattis-Bardeen expression for probes with case I and case II coherence factors. At temperature T = 0, the far infrared absorption edge is, as expected, determined by the smallest gap. However, the large thermal background may mask this edge at finite temperatures and instead the secondary absorption edges found at Delta_i+Delta_j may become most prominent. At finite T, if certain interband matrix elements are large, there may also be absorption peaks at the gap difference frequencies | Delta_i-Delta_j | . We discuss the effect of sample quality on the measured spectra and the possible relation of these predictions to the recent infrared absorption measurement on MgB_2

Comparison of s- and d-wave gap symmetry in nonequilibrium superconductivity

Recent application of ultrafast pump/probe optical techniques to superconductors has renewed interest in nonequilibrium superconductivity and the predictions that would be available for novel superconductors, such as the high-Tc cuprates. We have reexamined two of the classical models which have been used in the past to interpret nonequilibrium experiments with some success: the mu* model of Owen and Scalapino and the T* model of Parker. Predictions depend on pairing symmetry. For instance, the gap suppression due to excess quasiparticle density n in the mu* model, varies as n^{3/2} in d-wave as opposed to n for s-wave. Finally, we consider these models in the context of S-I-N tunneling and optical excitation experiments. While we confirm that recent pump/probe experiments in YBCO, as presently interpreted, are in conflict with d-wave pairing, we refute the further claim that they agree with s-wave.Comment: 14 pages, 11 figure

Multiband model for penetration depth in MgB2

The results of first principles calculations of the electronic structure and the electron-phonon interaction in MgB2 are used to study theoretically the temperature dependence and anisotropy of the magnetic field penetration depth. The effects of impurity scattering are essential for a proper description of the experimental results. We compare our results with experimental data and we argue that the two-band model describes the data rather well.Comment: submitted to Phys. Rev.

Reversible magnetization of MgB2 single crystals with a two-gap nature

We present reversible magnetization measurements on MgB2 single crystals in magnetic fields up to 2.5 T applied parallel to the crystal's c-axis. This magnetization is analyzed in terms of the Hao-Clem model, and various superconducting parameters, such as the critical fields [Hc(0) and Hc2(0)], the characteristic lengths [xi(0) and lambda(0)], and the Ginzburg-Landau parameter, kappa, are derived. The temperature dependence of the magnetic penetration depth, lambda(T), obtained from the Hao-Clem analysis could not be explained by theories assuming a single gap. Our data are well described by using a two-gap model.Comment: 20 pages, 1 table, 4 figures, will be published in Phys. Rev.

Modelling laser-based table-top THz sources : Optical rectification, propagation and electro-optic sampling

A model describing the generation of THz pulses by optical rectification and the detection of THz pulses by electro-optic sampling is presented. The model is comprehensive and mostly analytical: physical phenomena such as dispersion, group velocity mismatch, multiple reflections and diffraction are represented by one dimensional transfer functions. The model is compared with experimental results and shows good agreement with experiments. It is shown that including diffraction is crucial for retrieving the details of the THz spectrum

Terahertz Probes of Transient Conducting and Insulating Phases in Quasi-2D Electron-hole Gases

We employ ultrafast terahertz (THz) pulses to study the dynamical interplay of optically-induced excitons and unbound electron-hole pairs in GaAs/AlGaAs quantum wells. A distinct low-energy oscillator appears upon resonant excitation of heavy-hole excitons, linked to transitions between their internal degrees of freedom. Time resolving changes in the THz conductivity, we can observe dynamical transitions between conducting and insulating phases as excitons form or ionize on ultrashort timescales

Exciton dynamics studied via internal THz transitions

We employ a novel, ultrafast terahertz probe to investigate the dynamical interplay of optically-induced excitons and unbound electron-hole pairs in GaAs quantum wells. Resonant creation of heavy-hole excitons induces a new low-energy oscillator linked to transitions between the internal exciton degrees of freedom. The time resolved terahertz optical conductivity is found to be a probe well suited for studies of fundamental processes such as formation, relaxation and ionization of excitons

Exciton dynamics studied via internal THz transitions

We employ a novel, ultrafast terahertz probe to investigate the dynamical interplay of optically-induced excitons and unbound electron-hole pairs in GaAs quantum wells. Resonant creation of heavy-hole excitons induces a new low-energy oscillator linked to transitions between the internal exciton degrees of freedom. The time resolved terahertz optical conductivity is found to be a probe well suited for studies of fundamental processes such as formation, relaxation and ionization of excitons