Superconductivity and electronic liquid-crystal states in twin-free YBa2Cu3O6+x studied by neutron scattering

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 YBa2Cu3O6.6(T c = 61 K) and nearly optimally doped YBa2Cu3O6.85(T c = 89 K). In YBa2Cu3O6.6, 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 YBa2Cu3O6.85. 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.