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Martin Greven - School of Physics & Astronomy, Univ. of Minnesota, Minneapolis, USA

Unusual magnetic excitations and oxygen-chain order in a model cuprate superconductor


Magnetic correlations might cause the superconductivity in the cuprates, and they are generally believed to be antiferromagnetic and to arise from the underlying copper-oxygen planes. Using neutron scattering on HgBa2CuO4+δ, we recently discovered the existence of a prominent magnetic excitation with unusual characteristics [1] : (i) the excitation appears to involve active degrees of freedom on both planar and apical oxygen ; (ii) it is strongest at the two-dimensional Brillouin zone center (q=0) ; (iii) it exhibits a weak doping dependence and dispersion, and (iv) a maximum energy of 56 meV at the antiferromagnetic point, where it meets the magnetic resonance, a well-known spin-one excitation that appears in the superconducting state and that we have shown to be universally related to the superconducting gap [2] ; (v) furthermore, unlike the resonance, the new excitation maintains its integrity in the normal state up to the pseudogap temperature (T*), and thus appears to be associated with the novel q=0 magnetic order recently identified in the pseudogap phase [3]. The novel excitation is a new candidate for the ubiquitous bosonic mode observed in the cuprates. Our results suggest a remarkably unified picture of magnetic excitations in the cuprates that has remained elusive so far, in part because most previous inelastic neutron scattering experiments focused on antiferromagnetic fluctuations.

 

In an extensive synchrotron X-ray scattering study of 20 crystals, we established the existence of an unusual oxygen-chain order in HgBa2CuO4+δ [4]. The chains are uncorrelated and stable up to high temperature. The appearance of the chain order with increasing hole concentration (p) coincides with the disappearance of the q=0 magnetic order and with the end of the Tc(p) plateau. While this chain order is unique to HgBa2CuO4+δ, we discuss the possibility that it may be a manifestation of universal changes in the copper-oxygen plane’s electronic structure.

 

[1] Y. Li, V. Balédent, G. Yu, N. Barisic, K. Hradil, R.A. Mole, Y. Sidis, P. Steffens, X. Zhao, P. Bourges, M. Greven, Nature (in review).

[2] G. Yu, Y. Li, E.M. Motoyama, M. Greven, Nature Physics 5 (2009) 873.

[3] B. Fauqué et al., Phys. Rev. Lett. 96 (2006) 197001 ; Y. Li et al., Nature 455 (2008) 372.

[4] G. Chabot-Couture, J. N. Hancock, Z. Islam, L. Lu, G. Yu, Y. Li, X. Zhao, Y. Ren, A. Mehta, M. Greven (unpublished).

 

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