Physica C 185-189 (1991) 1077-1078 North-Holland

2°5TI-NMR study on TIBa2CaCu207_ 6

Takayuki GOTO, Takeshi SHINOHARA, Takashi SATO, Satoshi NAKAJIMA A, Masae KIKUCHI, Yasuhiko SYONO and Tetsuo FUKASE

Institute for Materials Research, Tohoku University 2-1-1 Katahira, Sendal 980 Japan Hachioji Research Center A CASIO, Hachioji 192 Japan

2°STI-NMR study has been performed on the copper oxide superconductor of TIBa2CaCu2OT_ 6 with Tc=84K. The nuclear spin-lattice relaxation rate T~ 1 showed the significant deviation from the Korringa law in the normal state: T1-1 became temperature independent above 200K. This indicates that the geometrical cancellation of the antiferromagnetic fluctuation of the Cu 3d-spins does not occur at the TI site.

1. Introduction

In most copper oxide superconductors, T~ "1 of the plane

site Cupl is reported to deviate from the Korringa law which

holds for the Fermi liquid. So far, many models have been proposed in order to interpret this deviation. Among them, Hammel I and Yasuoka 2 et al claimed that CupI-T/"1 is

anomalously enhanced by the antiferromagnetic(AF) fluc- tuation of the Cu 3d-spins. The Korringa-like tempera-

ture dependence of T~ -1 at the plane site oxygen O pl in YBa2Cu307 is consistently explained by their model as a consequence of a geometrical cancellation of the AF spin fluctuation at the O pl site. This is because a pair of Cu pl, spins of which antiferromagnetically correlate, are located symmetrically around O pl in the CuO plane.

On the other hand, the location of Cu pl around the TI

site in TI2Ba2CuO6_~ (TI2201), is not symmetric, so that TI-T1 --1 is affected by the AF spin fluctuation. We have already reported that TI-T/-1 in TI2201 deviates from the

Korringa law and that the AF fluctuation remains even in the overdoped region 4,s.

For the copper oxide superconductor, TiBa2CaCu207_6 (TI1212), which we report here, the geometrical canceling

of the AF spin fluctuation at the TI site is a delicate and crucial problem, because the two Cu pl atoms on the differ-

ent CuO planes are located just vertically above and below

the TI site. Therefore, the canceling takes place only when

there exists the inter-plane AF spin correlation, as well as

the intra-plane correlation. The band calculation reports that the electronic state

of the TI-O layer in TI1212 is quite different from that in TI22017. According to ref.7, the bottom of the TI-6s band

in TI1212 lies at well above the Fermi level EF, so that

there are no TI-6s electrons contributing to the density of the states at E F, while in TI2201, the TI-6s band lies below E F, making an electron pocket around [,7 The reported large magnitude of TI-T1-1 in TI22014 supports the band

calculation. In order to investigate the electronic state of the TI-O

layer and the effect of the AF spin fluctuation, we have performed the 205TI-NMR study on TI1212.

2. Experimental The powdered sample of TI1212 for the NMR measure-

ments was prepared by the solid-state reaction of TI20 3,

Ba02, CaO and CuO as described in detail elsewhere 6. The

specimen was confirmed to be of single phase by the X-ray

diffraction measurement. The bulk Tc was determined to

be 84K by the onset of the meissner signal, measured by

the SQUID magnetometer in 20Oe. The spin-echo 2053"I-

NMR at 60.17MHz was carried out in the magnetic field

of about 3T, and in the temperature range of 5,,-300K.

The relaxation rate T1-1 was measured by the conventional

saturation-recovery method. The shift of the resonance line

was defined as the top of the resonance peak.

3. Results The significant deviation of T1-1 from the Kc,rringa law

was observed in the normal state, especially above 20OK, where T] -1 became temperature independent as shown in

Fig. 1. This striking feature has never been reported in

other hole doped copper oxide superconductors. The mag- nitude of T1-1 was about the half of that in TI22014`s

Below 200K, T~ -l decreases gradually with the tern-

0921-4534/91/503.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.

1078 7'. Goto et aL / 2°STI-NMR study on TlBazCaCuz07. s

perature dependence of T,T --- consL, and shows drastic reduction at Tc without any BCS-like hump, The tem- perature dependence below Tc was described by neither a thermal activation nor a power law.

The line shape above Tc was slightly asymmetric with a small tail at thehigherfield side. The line shift was temper- ature independent above Tc within the experimental ac- curacy: 0.29(-I-0.005)%, which is smaller than the value of TI2201(0.32%) 4. The line width was also temperature

independent above Tc: FWHM was about 40Oe.

4. Discussion NMR-Tz "1 is generally described by Moriya's relation as;

T r ' oc T E q A~Imx(q, Wo)/Wo.

where Aq is the q-dependent hyperfine coupling constant, x(q, w), the dynamic susceptibility and w0, the nuclear spin Larmor frequency 8. In the Yasuoka-lmai analysis of Ti -1 in the copper oxide superconductors 2,3, the q-sum is divided into two parts, q=O and Q, where q=( l r , lr) According to them, the former gives the Korringa-like temperature dependence T~'Z=aT above Tc for the samples showing the temperature independent Knight shift, because the Knight shift selectively probes the uniform part of X. The latter, which is enhanced by the antiferromagnetically correlated Cu 3d-spins, gives the temperature independent T~ -z at the high temperature limit where x(q,w0) obeys the Curie law.

Using this model, we tried to explain the temperature dependence of TI-T~ -1 in TI1212. Because the two Cu pl

atoms are located vertically above and below the TI site, AQ at the TI site is zero only when these two £u pl 3-d spins

on the different planes correlate antiferromagnetically. If we assume AQ=0, then, T~ "1 is expected to obey the Korringa law TlZ=aT because the line shift was temperature inde- pendent. However, this opposes our experimental result of T~-l=const above 200K. Therefore, we concluded that AQ is not zero, and that the TI-T? a is affected by the AF fluctuation of the Cu 3d-spins. The gradual reduction in

Ta -1 below 200K suggests that x(Q, w0) deviates from the Curie law and starts to saturate 3.

Furthermore, one can see that the contribution from x(q--O) to T1 "1 is zero or negligibly small, because there was no aT term in T1-1 above 200K. This vanishing of aT term may be related to the absence of the electron pocket around F in TI1212 predicted by the band calculation 7.

5. Summary The temperature dependence of TI-TI -z in TI1212 has

been studied, and the deviation from the Korringa law in the normal state was observed. It indicates that in TI1212, the intre-plane AF correlation of the Cu 3d-spins exists, but not the inter-plane correlations.

This work was partly supported by the Grant-in-Aid for Scientific Research given by the Ministry of Education, Sci- ence and Culture, Japan.

Reference 1. P. C. Hammel, M. Takigawa, R. H. Heffner, 7. Fisk and

K. C. Ott: Phys.Rev.B63(1989)1992.

2. H.Yasuoka,H.Imai,T.Shimizu: "Strong Correlation aud Superconductivity"(1988)Springer-Verlag.

3. T. Imai:J. Phys. Soc. Jpn. 59(1990)2508.

4. T.Goto, T.Shinohara, T. Sato, S. Nakajima, M. Kikuchi, Y. Syono, T. Fukase: "Advances in Superconductivity III" (1990) Springer-Verlag.

5. T. Shinohara, T. Goto, T.Sato, S. Nakajima, M. Kikuchi, Y. Syono, T. Fukase: to be published.

6. S. Nakajima, M. Kikuchi, Y. Syono, K. Nagase, T. Oku, N. Kobayashi, D Shindo and K. Hiraga: Physica £170 (1990) 443.

7. D. Jung, M.-H. Whangbo, N Herron and C.C Torradi: Physica £160(1989)381.

8. T. Moriya:J. Phys. Soc. Jpn. 18(1963)516.

2000

1500

= 1000 o

X q.I

n- 500

2°STI- NMR TI2Ba2CuO6 ~

~¢ TlgazCaCu2Oz ¢ A @ ~ . i

@00

SO Q@ 0

f. 0 0 50

i

lOO 15o 26o 300 Temperature (K)

Fig. 1 Temperature dependence of TI-T? z in TI1212 (To=84K) and in TI2201 (Tc=82K) 5,6. The solid line is a guide to eyes, indicating the asymptotic temperature dependence above 200K