Phonons in a conventional superconductor : Electron-phonon coupling and superconductivity-induced effects in zero and finite magnetic fields

Orateur : Frank WEBER, Lothar PINTSCHOVIUS, Rolf HEID, Winfried REICHARDT, Dmitry REZNIK, Andreas KREYSSIG, S. ROSENKRANZ

Superconductivity is one of the most intriguing phenomena in condensed matter physics and each discovery of new compounds, such as the iron-based superconductors, sparks a tremendous amount of scientific interest. To date, BCS theory [1], with the Cooper pairing mechanism provided by electron-phonon interaction λ, represents the only well-understood and experimentally verified microscopic picture of superconductivity, and systems which are successfully described by BCS theory are known as conventional superconductors. Here, inelastic neutron scattering can in principle provide all necessary information about phonon energies and line widths needed to extract λ and calculate the properties of the superconductor. In reality, however, this is very time consuming and, therefore, ab initio calculations of the lattice dynamical properties are often used to estimate λ and the corresponding superconducting transition temperature Tc [2]. We have studied the lattice dynamical properties of the conventional superconductor YNi2B2C in great detail in order to make a comprehensive comparison with ab-initio calculations [3]. In particular, we studied the complete range of phonon energies up to 160 meV using thermal neutron triple axis spectroscopy for 0 < E ≤ 70 meV [4] and the direct chopper spectrometer ARCS, SNS, for phonon energies around and above 100 meV [5]. Further, we investigated strongly distorted phonon line shapes of strong-coupling modes at T ≤ Tc [6], which we found to be a universal phenomenon also present in elemental Nb [7]. At the end I will discuss phonon measurements in the vortex phase at Bc1 < B < Bc2, which show that phonons resolve the inhomogeneous gap distribution, i.e. they act as local probes on length scales below 300 Å [8].

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