* Corresponding author. E-mail address: gavrilov@hep486.pnpi.spb.ru (S. Gavrilov). Physica B 297 (2001) 75}77 Investigation of in-chain spin correlations in YBa  Cu  O  by neutron polarization analysis S. Gavrilov*, V. Plakhty, P. Burlet, J. Kulda, H. Humblot Petersburg Nuclear Physics Institute, 188300 Gatchina, Russia CEA Grenoble/DRFMC/SPSMS, 38054 Grenoble Cedex, France Institute Laue-Langevin, 38042 Grenoble Cedex 9, France Abstract Spin correlation in Cu}O chains of YBCO are studied. The results were obtained at ¹"110K by neutron polarization analysis with He "lter as analyzer. Experimental data indicate a magnetic peak at k"0.75, which corresponds to the holes concentration in the chain c"0.5. There is another anomaly at k"0.5 (c"0), which may be connected with the sample inhomogenity and some admixtures of copper oxide. The cross-section in the peak at k"0.75 is 5 mb/sr.  2001 Elsevier Science B.V. All rights reserved. Keywords: Polarization analysis; He "lter; Spin #uctuations; YBCO Magnetic properties of the oxygen-de"cient } Cu(1) } O(4) } Cu(1) } chains in YBa  Cu  O  (YBCO) is an important subject for the modern neutron scattering studies. These chains make a reservoir of the charge carriers (holes) for the superconducting CuO  planes [1]. The copper atoms in the Cu(1)O  planes of YBa  Cu  O  at x"0, being twofold coordinated by the apical oxy- gen atoms, are monovalent with the spin equal to zero. The neutral oxygen atoms incorporated into these planes from atmosphere are built into the Cu}O chain fragments (CF). They take one elec- tron from each neighbouring Cu ion oxidizing it to the Cu state with the spin S"  . Cop- per}oxygen chains in YBCO present an unique example of one-dimensional (1-D) strongly correlated fermionic system with the charge and the spin #uctuation e!ects. At x(1 chains consist of "nite CF. Su$ciently long CFs transfer the holes into the CuO  planes and are described by the formula Cu  O  O  [1]. According to X-ray spectroscopy data [2] the nominal oxygen hole concentration in long CFs at x close to 1 is c+0.3 which corresponds M/N"0.7 [3]. Purely inelastic peaks were observed [3] at the positions close to 2k  for the chains as predicted by the band structure calculations [4]. The objective of our experiment was a study of the spin #uctuations of strongly interacting fermions in the CFs. The in- chain spin #uctuations were studied theoretically in Ref. [5,6] using 1-D Emery model that includes antiferromagnetic interactions between spins and Kondo-like scattering of the oxygen holes. From these studies one can conclude that the spectrum of magnetic excitations should extend up to the E  & 1 eV. Quasielastic scattering due to magnetic 0921-4526/01/$-see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0921-4526(00)00824-3