Eur. Phys. J. Special Topics 188, 113-129 (2010)
https://doi.org/10.1140/epjst/e2010-01301-5

Regular Article

Superconductivity and electronic liquid-crystal states in twin-free YBa₂Cu₃O_6+x studied by neutron scattering

¹ Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany
² Department of Physics and Astronomy, University of British Columbia, Vancouver, Canada V6T 1Z1
³ AMPEL, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4
⁴ Laboratoire Léon Brillouin, CEA-CNRS, CEA-Saclay, 91191 Gif-sur-Yvette, France
⁵ Institut Laue-Langevin, 6 rue Jules Horowitz, 38042 Grenoble Cedex 9, France

^a e-mail: V.Hinkov@fkf.mpg.de

Received: 14 July 2010
Revised: 12 August 2010
Published online: 11 October 2010

Abstract

In this paper, we first give a concise overview of recent experimental and theoretical work dealing with “electronic liquid-crystal states” which spontaneously break different symmetries of the CuO₂ layers of high-T_c cuprates, with an emphasis on evidence in the spin excitation spectrum. Then we describe the importance of using twin-free samples to look for evidence for fourfold symmetry breaking in the spectrum and explain the preparation procedure to obtain such samples. We present inelastic neutron scattering results for moderately underdoped YBa₂Cu₃O_6.6(T_c = 61  K) and nearly optimally doped YBa₂Cu₃O_6.85(T_c = 89  K). In YBa₂Cu₃O_6.6, the dispersion topology changes when heating above T_c from an hourglass shape with constricted, commensurate resonance peak to a “Y”-shape without resonance anomaly. This change, and the fact that the low-energy signal above T_c can be described by an incommensurate, quasi-one-dimensional distribution, indicates a competition of superconductivity with an electronic liquid-crystal state. We then show a striking analogy between the difference signal I(5  K) − I(70  K) and the downward dispersing resonance mode in YBa₂Cu₃O_6.85. We therefore argue that a resonance mode only emerges below T_c, irrespective of the doping level. We finally discuss the implications of our results for the different scenarios invoked to explain the electronic liquid-crystal state in cuprates.