Magnetic excitations in novel superconductors: Feedback of unconventional order parameter

speaker:	Ilya Eremin MPIPKS Dresden
time:	November 17, 16:30 - 16:55

Since the original pioneering work of Bardeen, Cooper, and Schrieffer on the microscopic origin of superconductivity and the discovery of isotope effect on the superconducting transition temperature, the electron-phonon interaction has been considered for a long time as a main source of the Cooper-pair formation and, correspondingly, superconductivity. This general belief has been called in question after the discovery of the heavy-fermion and high-temperature superconductivity. There have been many scenarios as regard the non-phononic mechanisms of superconductivity like, for example, spin fluctuation-mediated Cooper-pairing or superconductivity through intra-atomic excitations and so on. At the same time, most of these proposals are at present qualitative and not quantitative and therefore, many questions have to be resolved.

One of the most interesting question concerning the non-phononic mechanisms of the Cooper-pairing is: what is the feedback effect of unconventional superconductivity on the bosonic excitations that in turn are supposed to drive the superconducting instability? For the usual 'conventional' electron-phonon mediated superconductors such a feedback effect of superconductivity on the phononic spectrum is known. In particular, below T_c superconductivity induces shifts in the frequency and the linewidth of the acoustic phonons depending on its wavevector [1]. In this project, we have analyzed the feedback effect of unconventional superconductivity on the magnetic excitations in layered cuprates and on the magnetic excitons in heavy-fermion superconductor UPd₂Al₃. In both cases, Inelastic Neutron Scattering(INS) experiments reveal the formation of a magnetic resonance mode at temperatures Tc and energies below 2Δ₀ [2,3].

[1] H.G. Shuster, Solid State Commun. 13 (1973) 1559.
[2] J. Rossat-Mignod, L.P. Regnault, C. Vettier, P. Bourges, P. Burlet, J. Bossy, Physica C 185-189 (1991) 86; S. Pailhes, Y. Sidis, P. Bourges, V. Hinkov, A. Ivanov, C. Ulrich, L.P. Regnault, and B. Keimer, Phys. Rev. Lett. 93 (2004) 167001; S.M. Hayden, H.A. Mook, P. Dai, T.G. Perring, and F. Dogan, Nature (London) 429, (2004) 531.
[3] N.K. Sato, N. Aso, K. Miyake, R. Shiina, P. Thalmeier, G. Varelogiannis, C. Geibel, F. Steglich, P. Fulde, and T. Komatsubara, Nature 410, (2001) 340.

The work dones together with:
Jun Chang, Peter Thalmeier, and Peter Fulde