LT 21 Proceedings of the 21st International Conference on Low Temperature Physics Prague, August 8-14, 1996

Part $3 - Superconductivity 2: HTS - Normal state properties, etc.

O u t - o f - p l a n e res is t iv i ty o f N i - d o p e d B i 2 S r 2 C a ( C U l . x N i x ) 2 0 8 + ~ s ingle c rys ta l s

Ryozo Yoshizaki and Hiroshi Ikeda

Institute of Applied Physics and Cryogenics Center, University of Tsukuba Tsukuba, Ibaraki 305, Japan

Ni-substitution effect for Cu on the magnitude of a pseudogap was investigated for Bi-2212 single crystals, and the decrease of the gap was observed with Ni doping as well as with increasing magnetic field.

1. INTRODUCTION As an anomalous behavior of the normal state of the

high-temperature oxide superconductors a semiconduc- tor-like profile of the out-of-plane resistivity has been studied in the relevance to the carrier confinement. Recently, the presence of the negative magnetoresis- tance (MR) was investigated for Bi-2212 single crystals with assuming a pseudogap formation below T,,,, [1]. In this paper we measured the Ni-substitution effect for Cu on the out-of-plane resistivity in order to make it clear the origin of the pseudogap.

2. EXPERIMENTS The Ni-doped single crystals were grown by a trav-

eling-solvent-floating-zone method with the Ni content up to the solubility limit (3.3%). The present samples were investigated being in the range of the optimal hole doping. The typical sample size is 4x2x0.05 mm 3 with the shortest length along the c axis. The Ni content was determined by electron-probe-microscope analysis (EPMA). The magnetic field was applied up to 17 T along the c axis in the longitudinal configuration, and a quasi-four-terminal method was employed for the out- of-plane resistivity measurement.

3. RESULTS AND DISCUSSION Figure 1 shows the temperature dependence of the

out-of-plane resistivity (p=) in zero magnetic field. Three distinguished features are observed as Ni-doping effect: (1) The steep reduction of p= occurs with main- taining the semiconductor-like profile, suggesting the decrease of the pseudogap; (2) The constant resistivity at high temperatures is also decreased with the doping, which is contrast to the linear increase of the residual in-plane resistivity against to the Ni content x for the samples from the same batch [2]; (3) The transition

temperatures are decreased with doping as summarized in Table I, which is consistent with the early results of the sintered samples.

We confirmed that the reduction of the out-of-plane resistivity was not due to the change of the ox3~gen content but to the Ni doping effect. The up-turn profile of the resistivity was analyzed with using the equation of pr which was introduced by Yan et al. assuming the presence of a pseudogap A [1]. The fit results are indicated in Fig. 1 by thin lines, and the fit parameters a, b, d and A are listed in Table I. It is noted that the heavily Ni-doped sample of x=0.024 has a possibility of the inhomogeneous Ni distribution in the crystal from the results of EPMA and the in- plane resistivity profiles. In the four parameters b is

S

4

Bi=SrCa(Cu,,Ni,)=O,. 8

I # c H:0 As grown

E u 3 C

~2 CL

x=0 x I/Z 0.006

0.010 0.015 0.024

0 ~ 0 I O0 200 300

T (K)

Fig. 1. Temperature dependence of pC for the Ni- doped Bi-2212 single crystals. The Ni content is in- creased for the data from top to bottom. The thin lines are the fit curves (see text).

Czechoslovak Journal of Physics, Vol. 46 (1996), Suppl. $3 1387

negligibly small and will be neglected below. It is no- ticed that the most effective parameter responsible for the reduction of Pc is the decrease of the pseudogap A, which varies almost linearly with the Ni content x.

TABLE I. The parameters a, b, d and A in the fit equation (see text) for the Ni-doped Bi-2212 samples with the Ni content of x at the first line. Tc is obtained from zero resistance temperature.

0 0.006 0.010 0.015 0.024

Tc ( K ) 91 85 84 82 77

a(tlcmK) 30.0 10.1 8.63 9.18 2.80

b(facm/K ) -0.0027 -0.000l 0.0005 0.00015 0.0001

d ( ftcm) 2.95 0.88 0.35 0.31 0.30

A (K) 277 239 191 147 193

When the magnetic field was applied, the out-of- plane resistivity showed negative MR at low tempera- tures as was observed for the undoped Bi-2212 single crystals [1]. The negative MR for the l%-Ni doped sample is shown in Fig. 2 at the temperatures just above To. The important features of the result are the satura- tion tendency of p= in the high field region and the increase of the saturation p= with lowering temperature, which will not be explained by a suppression of para- conductivity model [ 11.

The temperature dependence of Pc in magnetic field was also analyzed with using the above pc(T) fit equa- tion, and A was decreased whereas a was increased with increasing the field. In contrast, d was ahnost independent on the field. Taking into account of the result, the fit equation predicts the negative MR at low temperature where the contribution of d is neglected. Then we obtain pJH)=pc(0)f(H) and f(H)=(I+ct(H)) exp(-fl(H)), where ct and fJ are the positive parameters and increment functions of H when we take the field dependence of A and a into account. The profile of f(H) is consistent with the results in Fig. 2, although d is not negligible for the precise fitting.

If we assume the origin of the pseudogap is related to the antiferromagnetic (AF) spin correlation of Cu spins, doped Ni ions will disturb the correlation and result in the reduction of the pseudogap. According to

1.8

. - - , I .6 E fo G

1.4 v

c~ 1.2

I.C . . . .

0 H (T)

�9 �9 �9 �9 | " " �9 �9 I �9 " �9 �9 | " ' ' '

BiSrCa(Cu Ni )O 2 2 I-# I I | . ~S

WB//c As grown x=0.01

T=89 K

T=91 K

T=94 K

I . . . . I . . . . I = �9 �9 �9

5 10 15 20

Fig. 2. Magnetoresistance of Pc at the temperature just above T= for the l%-Ni doped sample.

this scenario, AF spin correlation is more sensitive to the Zn substitution for Cu. In Elct, the more rapid and destructive reduction of the out-of-plane resistivity was observed for tile Bi-2212 single crystals with Zn substi- tution for Cu [3 I.

4. CONCLUSIONS We mcasurcd the temperature dependence of Pc for

the Bi-2212 single crystals with Ni substitution for Cu in magnetic field and analyzed based on a model as- suming the presence of a pscudogap. The analysis showed the decrease of the pseudogap with iner~lsing Ni content and increasing magnetic field. Those fea- tures suggest the participation of spin correlation to the pseudogap formation.

REFERENCES [1] Y.F.Yan et al., Phys. Rcv. B 52 (1995) R751. [2] R.Yoshizaki ct al., submitted to MOS'96. [3] D.S.Jcon et al., Physica C 253 (1995) 102.

] 3 8 8 Czech. J. Phys. 46 (1996), Suppl. S3