ELSEVIER Physica C 263 (1996) 461-463

PHYSICA

Vortex-glass superconductivity of CuO 2 trilayers in BSCCO single crystals

S.H. Han a, y . Zhao a, G.D. Gu b,., G.J. Russell c, N. Koshizuka b a School of Materials Science and Engineering, University of New South Wales, Sydney 2052, NSW, Australia

b SRL//ISTEC. 10-13 Shinonome 1-chome, Koto-ku, Tokyo 135, Japan c School of Physics, University of New South Wales, Sydney 2052 NSW, Australia

Abstract

Superconductivity of BSCCO single crystals in which some CuO 2 trilayers are inserted in the Bi2Sr2CaCu20 s matrix is studied. The curvatures of the I -V curves for CuO 2 trilayers change from negative to positive with increasing field, and the scaling behaviour of a vortex-glass state is observed. 1 c is found to obey the relation I c =/co (1 - T/Tc) ~ with a = 1.2. Our results reveal that the vortex-glass transition exists in the system constructed with CuO 2 trilayers.

The vortex glass is a model which predicts that for the system with random pinning and collective effects of the vortex lines, a true superconducting mixed-state "vor tex glass" may exist at a tempera- ture lower than a well-defined critical temperature, the vortex-glass transition temperature, Tg [1]. Some strong evidence for the vortex-glass state has been observed in epitaxial film [2], single-crystal [3], and bulk polycrystalline samples [4] o f YBa2CU3OT_y and the BSCCO system [5]. In this paper, we report vortex-glass behaviour in a system in which some CuO 2 trilayers are inserted in the Bi2Sr2CaCu20 s matrix.

The BSCCO single crystals were prepared by a travelling-solvent floating-zone method. High-resolu- tion transmission electron microscopy (HRTEM) im- ages revealed some CuO 2 trilayers existing in the Bi-2212 matrix. The detailed results are reported elsewhere [6]. The superconducting transitions were

* Corresponding author. Fax: +81 3 3536 5717.

measured by using a SQUID magnetometer. The I - V characteristics as well as the DC resistivity were measured using the standard four-probe method. During the measurement, the temperature fluctuation was less than 0.01 K.

Fig. 1 shows the typical results of resistivity measured at various fields. There are two transition steps, one at 110 K, another at 90 K. The 110 K transition is due to the superconducting transition of the CuO 2 trilayers and the 90 K transition is due to the superconducting transition of the Bi-2212 matrix. In the present study, we focus on the CuO 2 trilayers. Therefore, only the behaviour between 90 to 110 K will be given attention. In this temperature region, the Bi-2212 matrix can be regarded as non-supercon- ducting.

A typical I - V curve taken at 102.31 K is shown in the inset of Fig. I. The original curve (line 1) includes contributions from two parts: CuO 2 trilayers and the Bi-2212 matrix. Because the Bi-2212 matrix is non-superconducting, its contribution should be a straight line (line 2). After taking away the back-

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462 S.H. Han et a l . / Physica C 263 (1996) 461-463

0.12 - 8i-system crystal

1 (H=0T) : f : 2 (H=0.025T) 3 (H=0.1T) ~ ,v_

i~ 0.04

"Z II o 20

0.00-

70 80 90 100 110 120 T(K)

Fig. I. Resistive transition under various fields. Inset: typical I - V curve before and after subtracting the background.

10 t

T = 103.91 K

100 H = 0, 70. 148.4, 249.2. 305.2 . o 392. 520.8, 5935, and / 672 Oe, respectively, f r o

. . - d O - I >.

---.1 0-z >

10 -3

1 0 -4

. . . . . . . . t , , I i I i i i t i i , i l l l

I0 ° I0 i I0 2

I(mA) Fig. 3. Log-log plot of I - V curve. Dashed line corresponds to the vortex-glass transition.

ground of the Bi-2212 matrix, the I - V curve of the CuO 2 trilayers is obtained (line 3).

Fig. 2 shows the temperature dependence of I c at H = 0. The data can be fitted as I c =/co (1 - T/T~) ~ with a = 1..2. This is different from the temperature dependence of the critical current for thin wire or thin film which has a = 3 /2 . This indicates that the assembly of CuO 2 trilayers can be regarded as a bulk system, although each CuO 2 trilayer is thin (1.8 nm) and they are separated from each other by from 1 to 40 unit cells of Bi-2212 [6].

15 o o o o o Experimental data 12

Ic(t)=l=(O)(1 - T/To)"

10 = <

o 5

I I t l , , , I . . . . , , , ,

ao l o o 12o Temperature (K)

Fig. 2. Temperature dependence of the critical current.

The log / - log V curves taken at T = 103.91 K and various fields are shown in Fig. 3. Similar curves were also taken at other temperatures. The general characteristic of the vortex-glass transition, i.e., an abrupt change of the curvature from upward to downward with decreasing field, was observed. The vortex transition point was identified as that at which the upward- and downward-curvature regions are separated. The dashed lines in Fig. 3 represent the vortex transition with Tg = 103.91 K at H = 558 Oe for the sample. It is evident that in the vortex-glass state the sample has a vanishing linear resistivity R L = E / J . In a similar way, a series of Tg under different magnetic fields was obtained, and the rela- tion between Tg and H can be fitted as H ~ (1 -T/Tg) 1"5, which sets up the equilibrium phase boundary line between the vortex glass and vortex liquid.

As predicted theoretically [1], at the transition temperature Tg, the electric field E and current density J obey a power-law behaviour as

e ~ s " , (1) with

/x= ( z + 1 ) / ( d - 1), (2)

where z is the dynamic exponent, d the dimension of the system. Another important prediction of the theory [1] is that a scaling relation E -

S.H. Hart et al. / Physica C 263 (1996) 461-463 463

1 0 ~ H = 350 0e /

/ ~.~1 0 °

;> 0-1 ."

- 2 ,0 .o , 1 @ " "

a o

o

i I I ~ L ~ H I I h , l l l i l . I , L , I h , I , , ~

1 0 ° 1 01 1 0 2 1 03 Isel

Fig. 4. Collapsed data plot of the I - V curves at H = 350 Oe.

j~d-2-z E+(j~d-1), where E+(X) is an appropriate scaling function, holds near the vortex-glass transi- tion, where ~ is the vortex-glass coherence length. According to this relation, if scaled as J ~ - jZI T - Tg ] v(d-1) and (E /J ) sc I =- ( E / J ) / I T - Tg I (z+2-a) in which v is a scaling exponent describing the behaviour of the vortex-glass coherence length ~: ~

I T - T g I, all the I - V curves should collapse onto two universal curves above and below Tg, respec- tively. Fig. 4 shows this scaling behaviour which reconfirms that the I - V curves of the sample obey the scaling behaviour of the vortex-glass transition.

From the results described above, it is evident that the vortex-glass transition does exist in the CuO 2 trilayer system. Because the coupling between CuO 2 trilayers in the present system is weaker than that in the Bi-2223 compound, our result gives strong sup- port to the conclusion of Li et al. [5] that the vortex-glass state exists in the Bi-2223 system.

References

[1] M.P.A. Fisher et al., Phys. Rev. Lett. 62 (1989) 1415; idem, Phys. Rev. B 43 (1991) 130.

[2] R.H. Koch, V. Foglietti, W.J. Gallagher, G. Koren, A. Gupta and M.P.A. Fisher, Phys. Rev. Lett. 63 (1989) 1512.

[3] P.L. Gammel, L.F. Schneeraeyer and D.J. Bishop, Phys. Rev. Lett. 66 (1991) 953.

[4] Y. Zhao et al., Phys. Rev. B 49 (1994) 6985. [5] Q. Li, H2. Wiesmann, M. Suenaga, L. Motowidiow and P.

Haldar, Phys. Rev. B 50 (1994) 4256. [6] Y. Zhao, G.D. Gu et al., Phys. Rev. B 51 (1995) 3134.