Journal of Low Temperature Physics, Vol. 105, Nos. 3/4, 1996

C u - N M R Study on High-T c Cuprate

Lal.sgCal.nCu206+ ~ (La2126)

Takayuki. Goto, Takao Watanabe,* Kyoichi Kinoshita,* Azusa Matsuda,* Masafumi Sera, and Tetsuo Fukase

lnstitute for Materials Research, Tohoku University, Sendai 980-77, Japan

*NTT Basic Research Lab., Wakamiya Morinosato, Atsugi, Kanagawa 243-01, Japan

Cu-NMR spectra and the nuclear spin-lattice relaxation rate T~ -~ have been studied intensively on the bilayer type high-T c cuprate Lal.sgCa~.~lCuz06+ ~ (La2126). The resonance line shift showed a monotonic decrease with lowering temperature in the normal state, indicating that this compound belongs to the lightly-doped region. The Curie-Weiss temperature dependence of (TIT) -1 in the normal state shows that the pseudo spin-gap does not always exist in the light-doped bilayer systems.

PACS numbers: 74.72.Dn, 76.60.-k

1. INTRODUCTION

The anomalous reduction in the NMR relaxation rate (T~T) -1 at the temperature far above T c has been reported 1-4 for lightly-doped high-T c cuprates such as YBa2CuaO 8 and 60K-class YBa2Cu3Or+y. This phenomenon, so-called as the

pseudo spin-gap, is interpreted independently by Fukuyama and Nagaosa as the condensation of the spin-degree freedom in their theoretical model of the electronic phase diagram for high-T c cuprates, s'6 So far, the pseudo spin-gap has been observed only for bilayer-type cuprates, but not for single-layer materials such as La~.ssSro~sCu204. Importance of the interlayer coupling within the bilayer for the mechanism of the pseudo spin-gap has been suggested by Menke and Lee, but the detailed mechanism is not understand yet] In this article, we report the temperature dependence of the Cu-NMR relaxation and spectrum on the lightly- doped high-T c cuprate Lal.svCal.l~Cu2Or+ ~ (La2126), on which few nuclear

401

0022-2291/96/1100-0401509.50/0 �9 1996 Plenum Publishing Corporation

402 T. Goto et al.

resonance studies have been reported except for La-NQR by Sasaki e t a l . 9 We expect that the Cu-NMR relaxation study on this cuprate s should provide the answer to the fundamental question whether or not the bilayer type structure is crucial to the formation of the spin-gap.

2. E X P E R I M E N T A L

The polycrystalline sample of Lal.s9Cat.nCu206+a was synthesized by the conventional solid state reaction, fOllowed by the oxygen anneal treatment with the condition 300-atm, 1080~ for 200 hours) The hole carrier is introduced in this system by the excess oxygen and the substitution of Ca > for La 3+ site, both of which are necessary for the superconductivity. The obtained compound was powdered and mixed with epoxy to be cured in high magnetic field of 11T for the c-axis alignment. The superconducting transition temperature was determined to be 52K as an onset of the diamagnetic signal measured by a SQUID magnetometer under the magnetic field 20Oe.

Field-swept spectra of 63J65Cu-NMR as shown in Fig. 1 were obtained by the conventional spin-echo method with the aligned c-axis set perpendicular to the external field. The simultaneous determination of the Knight shift and the quadrupolar shift was performed by the conventional method with the second perturbation 10 from positions of the center transition peak measured under several magnetic fields between 10 and 15T. Typical plots of the resonance line position against various magnetic fields are shown in Fig. 2. The nuclear spin

i i i

Lal.g9Cal.11Cu206+ 5 : 160.17MHz 4.2K

63VQ i ~: :i

m a i n 25.3MHz i{ i~

. . . . i s u b ~ 32MHz ,

Ho ic ' . . . . . . ':_-Z=k_-z_-_-_-z_-_-"=_-_-_-~ . . . .

1 1'2 1'3 1'4 15 Magne t i c Field (T)

Fig. 1 63/6SCu-NMR spectrum at 4.2K. Peak positions of main and sub sites are shown by solid and dashed lines.

- - ' l ' I ' i , , ~ I - r ~ - ~ f - -

" H o • , ~ ' , - , 0.8" .~~

Ks=0.2!% ~,."~ 0.6" 30K ..: r Ks=0.17%

0 . 4 / . 4 / ~ , , ' ; ' / 4.2K

t,;';'" 63Vo=25.6(• z

0.21 Lal.89Ca1.llCu206+a

0t5 ' 012 ' 014 ' 016 ' 018 ' Hobs'2( 10-2MHz "2)

Fig. 2 Typical plot of the resonance line shift against the external field to obtain the Knight shift and quadrupolar frequency.

Cu-NMR Study on High-T c Cuprate La2126 4o3

relaxation was measured in the temperature range between 4.2K and 200K on the peak of the central transition between Iz=+�89 by the saturation-recovery method with a pulse train. The relaxation rate T~ -~ for the center transition peak was determined by fitting observed relaxation curves with the function 1 - 0.9e -rt/T' - O.le -tIT', which corresponds to the case that only the center transition for the nuclear spin I = ~ is saturated by the pulse train] ~ In order to determine T~ -1 with a high precision, we traced the time evolution of the nuclear spin magnetization until its difference from the value at the thermal equilibrium becomes less than 1%.

3. RESULTS AND DISCUSSION

The field-swept spectrum shown in Fig. 1 is explained in terms of the two Cu sites, though there exists a single crystallographic Cu site. The quadrupole frequencies of the main and sub site are 63 vQ = 25.6 and 32 MHz. The main-site, the intensity of which is dominant to the other, is safely attributed to the plane site Cu, for its 63vQ is close to what observed for other cuprates with the pyramidal configuration of oxygen atoms, such as YBCO. For the sub-site, 63 vQ = 32MHz, which was estimated from the separation between the center line and the 1st satellite line, is close to what observed for those with the octahedral configuration such as LSCO] 3 The existence of this sub-site is interpreted as the effect of the excess oxygen atoms, which have been predicted by the Madelung potential calculation 11 to locate in between the bilayer and make the local configuration

L a l . 8 9 C a o . l l C u 2 0 6 + f i ~

"~ Tc(H...0)=52 K

Moic 15K .g I z = ~ : ~ f - - ' , ~ S K

O

1 4 . 0 4 ' 1 4 . 0 6 ' 14.08' 14.1 14.12 Magnetic Field (T)

Fig. 3 Typical profiles of the center transition of 63Cu spectrum at various temperatures.

8

i.a

I I [

Lal,89Cal. 11 CU206+8

Iz=+�89 H0-14T

~;o~ , . ,~ . . T1 =l.6msec

50K " ~ 1 7 6 1 7 6 1 7 6 40K 0.01 T1=l.3msec ~ . o o,%...~.,

, I , I ,o ~ , ~"

0 1 2 3 4 Delay f rom saturat ion (msec)

Fig. 4 Typical relaxation curves with the fitted function shown by dashed c u r v e s .

404 T. Goto et al.

around the Cu site octahedral. As shown in Fig. 2, the residual shift at 4.2K of the main site was as small as

0.17%, which is nearly the same as the typical value of the orbital shift for other high-T c cuprates) ~ Therefore, the spin contribution to the Knight shift at 4.2K is almost vanishing in this system due to the singlet paring in the superconducting state. This means that we can neglect the effect of electron scattering due to impurities, for those impurities which cause the unitarity limit scattering smear out the node of the superconducting gap to produce a finite residual Knight shift at low temperatures) ~ Note that this point is important to the analysis of the nuclear spin relaxation, because the unitarity scattering may hide ~2 also the intrinsic temperature dependence in (T~T) q .

Profiles of the center line spectrum at various temperatures are shown in Fig. 3. The resonance line shift shows a monotonic decrease with decreasing temperature from 200K down to 4.2K both in the normal and superconducting state. For the temperature dependence of the quadrupolar frequency is considered to be negligibly small, as is reported 13 for LSCO, the observed temperature dependence of the spectrum in the normal state indicates the reduction in the Knight shift at low temperatures. For this behavior is characteristic to lightly-doped high-T c cuprates, we can conclude that the compound belongs to the lightly-doped region. 8

Figure 4 shows typical nuclear spin relaxation curves with the fitting function

2000, , , 20 , i t

'" "~ - ' ~ [] []

1500

(D r13

-~ 1000

~, 500 ~ D

~o. ~. DDDDDn 15 ~ D e ' m ' - ,

�9 ~ 3 10 "~ []

63 -4--- 5

r-l[] 3

C ' 5'0 ' 100 ' 150 ' 2 6 2 0 T e m p e r a t u r e (K)

Fig. 5 Temperature dependence of T( l and (T~T) q, fitted with

Curie-Weiss temperature dependence (T~T)-1r 1/(T+ 1 t0), shown by the dashed curve.

A60.27MHz Ho• Lal.89Cao.llCU206+~

Cu-NMR Study on High-T c Cuprate La2126 405

1 - 0.9e -6t/r~ - 0.1e t/r~. Observed relaxation curves obey the function fairly well above 10K, and enable us to determine T~ -1 uniquely. At 4.2K, however, there appear two components in the relaxation. The temperature dependence of thus obtained T~ -l and (T~T) -1 are shown in Fig. 5, where the value for 4.2K corresponds the faster component. The abrupt reduction around 32K is considered to be the superconducting transition under the magnetic field of 15T. The rather gradual reduction in T~ -1 below T c is explained as the effect of the nuclear spin-diffusion from normal electrons in the vortex core, as was reported by Kitaoka on YBCO) 4 The origin of the two relaxation components at 4.2K may be related to the suppression of the spin diffusion at the low temperature, though the detail is not clear at this stage.

Next, in the normal state, note that the temperature dependence obeys the Curie-Weiss law in the entire temperature range. This fact immediately brings us to the second conclusion that pseudo spin-gap does not always exist in the bilayer systems. We believe that this conclusion gives a constraint to the theoretical understanding of the mechanism of the pseudo spin-gap.

Finally, the Weiss temperature determined by the curve fitting with the function (T~T)-~ocl/(T+| is approximately O = l l0 K , which is just

comparable to 75K for LSCO(x=0.15). The observed Curie-Weiss temperature dependence of (T~T) -~ in the normal state indicates that there exists the two dimensional antiferromagnetic spin correlation, which is a hallmark for the lightly-doped high-T c cuprates such as LSCO (x =0.15)] 3 This observation is consistent with the result of the Knight shift described before. However, the magnitude of (T1T) -~ in the normal state is as small as one forth ofLSCO (x =0.15), and is even smaller than the 90K-class YBCO. In order to explain consistently these two features, the temperature dependence and the magnitude of (T~T) -~, one needs to assume that the hyperfine coupling constant of Cu site is approximately one half of that in LSCO (x~-0.15). The investigation to determine the hyperfine coupling constant by measuring the anisotropy in the relaxation rate is now under the progress.

CONCLUSION

The temperature dependence of Cu-NMR relaxation rate and spectrum have been measured for the bilayer type high-T c cuprate La2126 in the temperature range between 4.2 and 200K. The resonance line shift showed monotonic decrease with lowering temperature, indicating that this compounds belongs to the lightly-doped region. The Curie-Weiss temperature dependence of (T~T) -~ in the normal state showed that there does not exist the pseudo spin-gap in this bilayer system.

4o6 T. Goto et al.

A C K N O W L E D G E M E N T S

The authors would like to show special thanks to Prof. K. Watanabe and Dr. T. Awaji for the usage of the 11 Tesla liquid helium-free superconducting magnet, which was supported by Toray Science Foundation, and o f the 17Tesla superconducting magnet with high-homogeneity at HFLSM, IMR, Tohoku University. A part of this work was supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture.

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