High energy spin excitations of a high-Tc superconductor YBa2Cu3O6.6

Physica B 241-243 (1998) 856 858

ELSEVIER

High energy spin excitations of a high-Tc superconductor YBazCu3O6.6 T. N i s h i j i m a a'*, M . A r a i b, Y. E n d o h c, S.M. B e n n i n g t o n d, S. T a j i m a e, K. T o m i m o t & , A.I. R y k o v e, N. K o s h i z u k a e, Y. S h i o h a r a e aDepartment of Accelerator Science, The Graduate Universi~ jor Advanced studies', 1-10ho, Tsukuba 305, Japan b High Ener~, Accelerator Research Organization, 1-10ho, Tsukuba 305, Japan cDepartment of Physics, Tohoku University, Aoba, Sendai, 980, Japan a Rutherford Appleton Laborato~, Chilton, Didcot, Oxon, OXI 10QX, UK International Superconductivi~ Technology Center, lO-13 Shinonome l-ehome, Koto-ku, Tokyo 135, Japan

Abstract

High energy inelastic neutron scattering measurements have been performed to study the magnetic excitation spectrum on a high quality single crystal of YBazCu306. 6 (T¢ ~ 64 K) by using pulsed neutron techniques. Magnetic excitations at the antiferromagnetic zone center have been clearly observed up to 150 meV for the first time. The magnetic resonance, which has been observed at 41 meV in the fully doped material, is observed at 35 meV. © 1998 Elsevier Science B.V. All rights reserved.

Keywords. YBazCu306.6; High-Tc superconductivity; Neutron scattering; Spin fluctuation

1. Introduction

In high-To cuprates, the persistence of antiferromagnetic fluctuations in the metallic state is probably one of the most striking features. Recently, intensive experimental works on La2 xSrx CuO4 [1,2] and YBazCu3OT-x [3-5] have been carried out in order to elucidate the details of the magnetic response in the normal and metallic state. For Lal.85Sro.15CuO4, the magnetic intensity *Corresponding author. Fax: [email protected].

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around (rt,rt,0) was observed up to 280 meV [6], which is a smaller energy than in the mother compound LazCuO4. On the other hand, measurements of the magnetic excitations in YBazCu30 7 x systems have been limited to relatively low frequencies (~50meV), much smaller than the coupling energy J (=125meV) responsible for the antiferromagnetism of the mother compound YBa/Cu306. Hence in order to elucidate the details of high energy spin dynamics, we performed high energy neutron scattering measurements on a single crystal of YBa2Cu306.6 by using pulsed neutron techniques.

0921-4526/98/$19.00 ~" 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 8 5 5-7

T. Nishijima et al. / Physica B 241 243 (1998) 856 858

2. Experimental

Experiments were performed on the choppertype spectrometer MARI at the ISIS pulsed spallation neutron source of Rutherford Appleton Laboratory. The sample used in the present experiments was a single crystal of YBazCu306. 6 of 15 g, which was synthesized by the SRL-CP method [7]. The as-grown crystal was annealed at 644°C under 1 atm oxygen flow for 32 days. Susceptibility measurements revealed Tc = 6 4 K (midpoint) with a somewhat broad transition of ATe = 7 K. Neutron scattering measurements were performed in the (h h k) plane oriented to coincide with the vertical scattering plane of the spectrometer and the c*-direction was parallel to the incident beam. The large number of detectors can observe a wide energy range in the c*-(n, n, 0) space simultaneously. The experiments were done using incident energies of 130, 250, 400 meV.

3. Results

Fig. 1 shows the intensity contour map in the energy-transfer versus two-dimensional momentum-

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transfer (q2D) plane, which was measured at 23 K with an incident energy of 130 meV. It is clear that the magnetic rod caused by 2D spin fluctuations around (rt, n, 0) extends up to 80 meV. We have also confirmed that the magnetic response around (r~, rt, 0) extends up to about 150 meV by measurements done with an incident energy of 250 meV. There is a broad band of phonon scattering at 20 meV. However, the opening of the pseudo-gap can be clearly seen in the low-energy region. In order to separate the magnetic response from the observed intensity, the contamination from phonon scattering was estimated to be A + B qZo for each constant-E slice. In Fig. 2 the corrected integrated magnetic response is depicted as a function of the energy transfer. The subtracted spectra are very similar to those measured recently over a wide energy region ( < 120meV) on a triple axis spectrometer [-8]. The magnetic resonance peak is observed at 35 meV and a sudden decrease at 50 meV is due to the bilayer modulation along the (0, 0, L) direction. Fig. 3a shows the temperature dependence of the magnetic response at 35 meV. There is a sudden increase below Tc as was reported previously [4]. Fig. 3b shows the temperature dependence of the peak width (FWHM), obtained from fits to a Gaussian function of the resonance peak at 35 meV. There is a weak maximum at Tc and an abrupt decrease below it.

YBa2Cu306. 6 18.0 13.4 9.93

7.40 5.53 4.16 3.14 2.39 1.84

Ei=130meV

35

Intensity

i 30 .........

,

T..................~

~"

.!

' •

•

"

23KI

I o ,o4 I

. . . . . . . . . . . . . . . . . . . . . . .

1.43

0.90 0.74 0.62 0.53 0.46

._~ 15 ¢,0 ¢-

0.42 0.38 0.35 0.33 0.32 0.31 0.30

5.0 0.0 0

(~,rO

Momentum

T r a n s f e r q ~ ( A -1)

Fig. 1. The intensity c o n t o u r m a p at 23 K obtained using an incident neutron energy of 130 meV.

20

40

60

80

1O0

120

Energy Transfer (meV) Fig. 2. Energy dependence of the integrated magnetic intensity a r o u n d (~, n, 0) at 23 and 104 K.

858

T. Nishi/ima et al. ,' Phvsica B 241 243 (199~) 856 858

30

i

i

i

i

i

i

i

i

l l l l

I l l l

I I I l

I l l l ! l l l l

I I , I

I , J I

I J l l

0,8

iTc

Tc .

---- 25 09

.

¢-

.....i I >.,

...................

YBa2C u308 s I

......

E=35meV " I

i i'i

't

i..l ............................................................ 0.7

o6

.........

.i;ii

20

tj~ t'-

¢15

0.5

-rE

50

100

150

200

i

0.4->-

0.3

10 0

m

250

50

Temperature (K)

100

150

200

0.2 250

Temperature (K)

Fig. 3. (a) Temperature dependence of the scattering intensity at E = 35 meV. The intensity is corrected for the Bose population factor. [b) The full width at half maximum obtained by least squares [its of the peak profile.

4. D i s c u s s i o n

Although we show the c o n t o u r m a p obtained by using an incident energy of 130meV in this paper, m e a s u r e m e n t s using a higher incident energy of 250 meV show that the m a x i m u m energy of the magnetic excitations is a b o u t 150 meV. This m a x i m u m energy is much smaller than that of Lal.85Sro15 CuO4 [6]. Hence our observation seems to be contrary to the prediction of the SCR theory [9], i.e. the wider the energy range in X" (q, co), the higher the transition temperature T~. However, we should take into a c c o u n t the intensity and the correlation length in order to draw a conclusion, which will be done in a further analysis. The a b r u p t evolution of the intensity and the width is attributed to a drastic change of the

spin correlations below T~. The origin of the resonance peak is still a controversial topic. However, we should point out that a n a l o g o u s results have been also o b t a i n e d for the fully-doped c o m p o u n d [3].

References

[I] [2] [3] [4] [5] [6] [7] [8] [9]

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