Dresden 2006 – scientific programme

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TT: Tiefe Temperaturen

TT 30: Superconductivity: Twenty Years High-T_c Cuprates - Recent Progress

TT 30.4: Talk

Thursday, March 30, 2006, 14:45–15:00, HSZ 02

Dispersion and geometry of spin excitations in the normal and superconducting state of twin-free YBCO(6.6) — •Vladimir Hinkov¹, Bernhard Keimer¹, P. Bourges², S. Pailhes², Y. Sidis², A. Ivanov³, and CT. Lin¹ — ¹Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany — ²Laboratoire Leon Brillouin, CEA-CNRS, France — ³Institut Laue-Langevin, Grenoble, France

There is a strong debate about the normal state (NS) and the *pseudogap* (PG) state in the high-Tc cuprates. For now, our understanding of their magnetism suffers from the lack of information about the regimes above Tc. We present an inelastic neutron scattering study (see also V. Hinkov et al, Nature 430, 650 (2004)), of the in-plane (a-b) anisotropy of spin-excitations between 26meV and 60meV in twin-free, underdoped YBCO(6.6), T_c=61 K. We identify three fundamentally different temperature regimes: the superconducting regime exhibits incommensurate peaks both along a and b with a X-shaped dispersion. Intensity shows a-b-anisotropy below the resonance peak and no anisotropy above. Heating to 70 K (PG-state), the dispersion changes to Y-shape, with a flat top, steeply dispersing signal below the resonance, which is 50% broader along a. Finally, at 290 K (NS), intensity is depleted below the resonance and is only visible at higher energies. We discuss the implication of our findings on different theoretical models proposed for the PG-regime: the stripe-model, spin-ladder models, preformed Cooper-pairs, competing order parameter. We compare our results with other techniques such as ARPES and STM, and with other systems such as the Haldane-compound YBaNiO(5).

This work is supported by the Deutsche Forschungsgemeinschaft (Research unit 538).