University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Calendar

Thursday, January 22nd 2015
12:00 pm:
Speaker: Dmitry Reznik, University of Colorado-Boulder
Subject: Anomalous Phonons and High Temperature Superconductivity in Copper Oxides
Note: change of time and location for seminar, this week only.

It is well known that electron-phonon coupling is responsible for superconductivity in conventional superconductors, but the prevailing view is that it is not important in high temperature superconductivity. Yet, evidence that electron-phonon coupling is very strong for certain phonons in the copper oxides has been building. In particular, Cu-O bond-stretching phonons at 65-85meV in La2−xSrxCuO4 are known to show anomalously large broadening and softening near the reduced wavevector q=(0.3,0,0). Recently we systematically investigated spectral functions of these phonons by inelastic neutron and x-ray scattering and measured dispersions of electrons to which these phonons should be coupled by angle resolved photoemission (ARPES). These electronic dispersions have kinks around 70 meV that are typically attributed to coupling of electrons to a bosonic mode (which could be a phonon) that mediates superconductivity. Remarkably, we found that the kinks remain strong in the heavily overdoped region of the doping phase diagram of La2−xSrxCuO4, even when the superconductivity completely disappears. We also found that doping dependence of the magnitude of the giant phonon anomaly is very different from that of the ARPES kink, i.e., the two phenomena are not connected. In fact, while the Cu-O bond stretching phonons show giant electron-phonon effects, there are no features in the electronic dispersions of the same samples that can be attributed to these phonons. We show that these results provide indirect evidence that the phonon anomaly originates from novel collective charge excitations as opposed to interactions with electron-hole pairs. Their amplitude follows the superconducting dome so these charge modes may be important for superconductivity. I will also discuss earlier results on a copper oxide with a very high Tc, YBa2Cu3O7, where a similar phonon anomaly becomes greatly enhanced in the superconducting state.

Faculty Host: Martin Greven

The weekly calendar is also available via subscription to the physics-announce mailing list, and by RSS feed.