Journal of Magnetism and Magnetic Materials 116 (1992) 336-338

North-Holland

Inelastic neutron scattering study of the spin dynamics in YBa,Cu,O,,, *

J. Rossat-Mignod a,b, L.P. Regnault a, C. Vettier c3d, P. Bourges b, P. Burlet a, J. Bossy ‘, J.Y. Henry a and G. Lapertot a a Centre d’Etudes Nu&aires de Grenoble, DRFMC/SPSMS/MDN, 85X, 38041 Grenoble Cedex, France b Laboratoire Lion Brillouin, CEN-Saclay, 91191 Gif-sur-Yvette Cedex, France ’ Institut Laue - Langevin, 156X, 38042 Grenoble Cedex, France d European Synchrotron Research Facility, BP 220, 38043 Grenoble Cedex, France

Inelastic neutron scattering experiments have been carried out on a YBa,Cu,O,.,, single crystal (T, = 91 K) to investigate

the spin dynamics as a function of temperature. Dynamical AF-correlations have been found together with an energy gap

(E, = 28 meV) in the spin excitation spectrum. The gap actually persists above T, up to 120-130 K.

1. Introduction

The knowledge of the wavevector and energy dependences of the spin excitation spectrum is of crucial importance in building up and appropri- ate theory of high T, superconductivity. We have focused our interest on the YBa,Cu,O,+, system because the availability of large single crystals (= 0.3 cm3), in which the oxygen content can be changed from 0, to 0, with good accuracy and homogeneity, has allowed to undertake inelastic neutron scattering experiments. The spin dynam- ics has been successively investigated in the five characteristic regimes of the phase diagram de- termined by neutron diffraction [l] (see fig. 1 in ref. [3]): the pure (X = 0.15) and doped (X = 0.37) AF-state [l-3], the weakly doped metallic state (X = 0.45 and 0.51) [3,4], the heavily doped metal- lic state which develops superconductivity below T, = 60 K (x = 0.69) [3,4] or T, = 90 K (x = 0.92) [3,4]. In this paper we shall report on the newly

Correspondence too: Dr. J. Rossat-Mignod, Centre d’Etudes

Nucleaires de Grenoble, DRFMC/SPSMS/MDN, 85X, 38041

Grenoble Cedex, France.

* Paper presented at the International Conference on Mag- netism (ICM ‘911, Edinburgh, Scotland, 2-6 September

1991.

obtained results for a superconducting sample with an oxygen content x = 92 (T, = 91 K). A magnetic scattering was first discovered on the triple-axis spectrometer IN8 at ILL, however more accurate results were obtained later on the high flux triple-axis spectrometer 2T at the Labo- ratoire Leon Brillouin. The single crystal was oriented with the [llO] and [OOll axes within the horizontal scattering plane.

2. Results

For the first time, a magnetic response has been identified at low temperatures, above an energy transfer of about 25 meV. Energy scans performed at various temperatures exhibit a com- plex lineshape (see fig. 1). However q-scans clearly show evidence for a magnetic scattering signal and allow us to define the nuclear contribution. The latter one can be explained by assuming three contributions on top of a flat background: a contamination from the (006) Bragg peak at 24 meV, a peaked contribution at 20 meV arising from a local mode involving oxygen atoms in Cu chains and a broad response which can be ex- plained by a damped harmonic oscillator (fiti0 =

0304-8853/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

J. Rossat-Mignod et al. / Spin dynamics in YBa,Cu.,O, 9z and inelastic neutron scattering 337

,soo - y Ba,Ch%x

Cu (llll/PG(0021

00 10 20 30 LO 50

Energy 1 meV 1

Fig. 1. Energy scans performed at Q = (l/2 l/2 5.2) for

YB~z~~~~~.Y~ at increasing temperatures. Above the back-

ground (B.G.) a nuclear contribution is indicated (0 ).

21.5 meV, r, = 26 meV) and may arise from large fluctuations of apical oxygen atoms.

The magnetic contribution, reported in fig. 2 in the form of Im[X(q, hw)}, shows many impor- tant features. At T = 5 K, first, there is no size-

able scattering below 25 meV indicating a sharp energy gap, E, = (28 f 1) meV, in the spin exci- tation spectrum. More surprising is the sharp drop of the signal above 45 meV and the exis- tence of a sharp peak at 41 meV (resolution limited) which looks like a collective excitation through the superconducting gap.

YBa2Cu306.92 a’= (l/2 l/2 5.2) = 5

g 100

2 ;

x 200

z 0

0 10 20 30 LO 50

Energy (meV 1

Fig. 2. Im{,y(q, ho)) as a function of the energy for

YBa2Cu30,,2 for increasing temperatures below and above

T, = 91 K. An energy gap (E, = 28 meV) is clearly seen in the

spin excitation spectrum.

At T = 81 K CT< T,) the magnetic scattering remains unchanged, except for a shift of the 41 meV peak to a lower energy of = 25 meV which could reflect the T-dependence of‘the supercon- ducting order parameter.

At higher temperatures the magnetic scatrer- ing is strongly depressed, however it must be emphasized that the energy scale &, = 30 meV does not change with temperature, nor does the AF-correlation length (Aq = 0.25 r.l.u., (/a = 0.84 & 0.04). The T-dependence of the low energy part of the spectrum (hw = 10 meV), reported in fig. 3, shows a sizeable scattering only above 80 K. Im x at hw = 10 meV, which is nothing than the initial shape of Im x and corresponds to

F v

100 0

__4__-13----

300 00 A’ - T.-iwr . _^_

200 / _

y Ba2 c% O6.92 500

a*

01 I dl I I 0 50 100 150 200

Temperature (K)

Fig. 3. Magnetic scattering intensity and Im x at hw = 10

meV for YBa2Cu,0,,2 as a function of temperature. The inset shows a q-scan at 10 meV and 150 K where the back-

ground has been suppressed.

338 .I. Rossat-Mignod et al. / Spin dynamics in YBa,Cu,O,,,, and inelastic neutron scattering

l/T,T, measured by NMR technique, exhibits a maximum around 120-130 K in excellent agree- ment with NMR results on the same sample [5]. Thus, this behaviour demonstrates the opening of a pseudogap in the spin excitation spectrum well above T,, as was previously found in a sample with x = 0.69 CT, = 59 K) [3]. The origin of this gap is not yet understood, it may result either from some superconductivity pairing or from the special shape of the quasi-particle Fermi surface, as shown by recent theoretical investigations [6-

81.

3. Conclusion

superconducting sample will be useful in building up the theory of high T, superconductivity.

References

[ll J. Rossat-Mignod et al., J. de Phys. 49 (1988) C8-2119. [2] J. Rossat-Mignod et al., in: Dynamics of Magnetic Fluctu-

ations in High-T, Materials, eds. G. Reiter, P. Horsh and G. Psaltakis (Plenum, New York, 1990).

[3] J. Rossat-Mignod et al., Physica B 169 (1991) 58. 141 J. Rossat-Mignod et al., Proc. 3rd High T, Conf.,

Kanazawa, Japan (July 1991). [5] C. Berthier et al., Proc. 3rd High T, Conf. Kanazawa,

Japan (July 1991). [61 N. Bulut et al., Phys. Rev. B 41 (1990) 1797. [7] T. Tanamoto, K. Kuboki and H. Fukuyama, preprint. [S] D. Grempel and M. Lavagna, preprint.

We hope that our discovery of many quite unusual aspects in the spin dynamics of the 90 K