Magnetic Dynamics in UnderdopedYBa2Cu3O7−x

Abstract

Polarized and unpolarized triple-axis neutron measurements were performed on an underdoped crystal of YBa 2 Cu 3 O 7−x (x = 0.4 ± 0.02, T c = 62.7 K).Our results indicate, contrary to earlier evidence, that the spin excitations in the superconducting state are essentially the same as those in the fully doped material except that the unusual 41 meV resonance is observed at 34.8 meV. The normal state spin excitations are characterized by a weakly energy-dependent continuum whose temperature dependence shows the clear signature of a superconducting gap at T c . The enhancement at the resonance is accompanied by a suppression of magnetic fluctuations at both higher and lower energies. (below which no magnetic intensity can be found) which is much less than 2∆. They also reported a complicated temperature dependence of χ ′′ (q, ω), but found no resonance coupled directly to T c .We report here the first polarized neutron scattering measurements on an underdoped single crystal of (123)

Effect of Nonmagnetic Impurities on the Magnetic Resonance Peak inYBa2Cu3O7Fong1, Bourges2, Sidis3 et al. 1999Phys. Rev. Lett.6411790View full textAdd to dashboardCite

“…The peak is also observed at lower energies in underdoped YBa 2 Cu 3 O 6+x [8,9,10]. As the existence of this peak requires d-wave superconductivity, it demonstrates that magnetic neutron scattering is a phase sensitive probe of superconductivity [4].…”

“…On the other hand, the characteristic features of the resonance peak (i.e., its sharpness in energy and its coupling to superconductivity) are obliterated. It is worth noting that in the underdoped regime, where the normal-state susceptibility is also enhanced with respect to YBa 2 Cu 3 O 7 , the resonance peak remains sharp and coupled to superconductivity [8]- [10]. This aspect thus seems to be a manifestation of a delicate coherence that is very easily disrupted by disorder.…”

Mean-field description of the phase string effect in thet−Jmodel

“…Most recently, in the underdoped Y Ba 2 Cu 3 O 6.6 , a second energy scale near ∼ 70meV has been also indicated [24] besides the earlier report of the lower energy peak near 34meV . [25] It is noted that the energy scales shown in Fig. 5 are generally doping dependent, and those energies in Y Ba 2 Cu 3 O 6.5 are expected to be relatively smaller than the corresponding peaks in Y Ba 2 Cu 3 O 6.6 .…”

Global Phase Diagram of the High-Tc Cuprates

“…The SO(5) theory unifies the AF and the SC order parameters into a single five dimensional order parameter called the superspin, and the effective quantum theory of the superspin naturally explains proximity between the AF and the SC phases in the observed phase diagram 1 . The Goldstone modes of the superspin fluctuations can be identified with the π resonance mode observed in the neutron scattering experiments 2,3,4,5,6,7,8,9,10,11,12,13,14,15,16 . This theory also predicts the AF vortex state 1,17 , which has recently been observed in a number of experiments 18,19,20,21,22,23,24,25,26 .…”