Physica C 282-287 (1997) 2331-2332

PHYSICA 8

Vortex Lattice Melting and Dimensionality Crossover in Bi,Sr,CaC~O,+,

S.H. Han”, Y. Zhao”. G.D. Gub, G.J. Russellb, Y.Y. HeC, and N. Koshizukad

“Superconductivity Research Group, School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia

bSchool of Physics, University of New South Wales, Sydney, NSW 2052, Australia

‘Cryogenic Laboratory, Chinese Academy of Sciences, Beijing 100080, P.R. China

dSuperconductivity Research Lab, ISTEC, lo-13 Shinonome l-chrome, Koto-ku, Tokyo 135, Japan.

Differential resistivity, I-V characteristics, and voltage noise have been measured to study the vortex dynamics in Bi,Sr,CaCu,O,+, single crystals. The sample shows a crossover from three to two dimension- al behaviour as the magnetic field is higher than a crossover field of 365 Oe. Below the crossover field, the sample show a vortex lattice melting in pinned vortex state as well as in the flux flow state. During the melting, a remarkable voltage noise was observed. The I-V curve measurements reveal that the irreversibility line does not overlap with the meling ling in H-T plane. The separation between these two lines is greatly enhanced when the applied field is over the crossover field.

The vortex dynamics of high-T, superconductor in the superconducting state has been the subject of much experimental [l] and theoretical [2] investigations. Of particular interest is the case of Bi,Sr,CaCu,O,+,(2212) compound, where a strong anisotropy may lead to a complex flux- vortex structures. In a layered superconductor like 2212, there may be a crossover field B,,=4+,ly2s2, where 9, is the flux quantum, y the anisotropy ratio, and s the interlayer spacing. The thermal fluctuations behaviour quasi-two- dimensionally above B,,, but produce a melting of the vortex lattice, leading to a three- dimensional vortex liquid. In this work, we investigated the melting and crossover processes by means of the transport and voltage noise measurements.

The 2212 single crystals used in this study were prepared by a travelling solvent floating-zone method. The dimensions of a typical sample are 5x2.1x0.08 mm. The T, (onset transition) was 91.89 K by a SQUID measurement at 10 Oe. The differential resistance (dV/dI) is measured by supercomposing an ac current on top of the dc current and the response at the ac frequency is monitored with a lock-in amplifier. During the

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measurement the magnetic field is perpendicular to the current.

Fig. 1 shows the differential resistance at a fixed field versus temperature for different applied dc current. The melting temperature T, can be determined with the sharp transition as &,=O.

80 85 90 95 T (K]

Figure 1. dV/dI vs T at H=72 Oe.

Above T,, the flux lines form a liquid, but a solid as temperature is below T,. So, for weak current, the flow is dissipationless. With increasing dc current, a nonzero differential resistance can be observed well below T,. This means that flowing lattice can be melted as observed in YBCO [3]. The melting line in H-T

2332 S.H. Han et al./Physica C 282-287 (1997) 2331-2332

phase diagram determined by differential resistance is consistent with that obtained by ac susceptibility measurement. It is also observed during the vortex lattice melting, a remarkable voltage noise was observed which means a plastic flow of the vortex liquid (results are not shown here).

It is interesting to note that as the applied field is higher than 365 Oe, the lattice melting process is greatly broadened (See Fig.2), showing a very strong thermal fluctuation in both superconduct- ing and normal states. This may be due to the fact that above the crossover field, the super- conducting order in the field direction is destroyed, and the flux lines are dissolved into 2D pancake vortices, which has stronger thermal fluctuations.

T w Figure 2. dV/dI-T in different field.

Figures 3 and 4 show the log-log I-V curves at 87 K in different applied field along ab-planes and c direction, respectively. The curvature of the curves changes from negative to positive with increasing the field. A line (dashed lines in Fig.3 and 4) can be identified which separate the curves of the negative curvature from those of the positive one. In this way, a boundary line (or irreversibility line) can be determined which separates the true superconducting state from the resistive state. Our results clearly show that the irreversibility line is below the vortex lattice melting line. This is consistent with our observa- tion shown in Fig.1 where we can see that a depinned vortex state can be melted. This is similar to that observed in YBCO system [3]. The difference in 2212 and YBCO is that in

0.001 ’ ’ 1 “‘I t

‘-“I (A) 0.1

Figure 3. Log-log plot of I-V curves in ab- planes.

87 K, H-O, 18, 54 99, 126. 180 Oe

0.0001 0.001 , (Ay.o1 0.1

Figure 4. The log-log plot of I-V curves in c direction.

2212, as the applied is higher than the crossover field, the separation between the melting line and the irreversibility is greatly broadened. This phenomenon has not been observed in YBCO system.

Acknowledgment The authors are greatly acknowledged the financial support from the Australian Research Council.

REFERENCES:

1. H. Pastoriza, et al., Phys. Rev. Lett., 72 (1994) 2951. 2. L.I. Glazman and A.E. Koshelev, Phys. Rev. B, 43 (1991) 2835. 3. G. D’Anna, et al., Phys. Rev. L&t., 75 (1995) 3521.