Physica C 235-240 (1994) 2961-2962

North-Holland PHYSICA 8

Quantum tunneling of vortices in an epitaxial Tl-based thin-film

A. Garcial, X. X. Zhangl, J. Tejadal, M. Manze12 and H. Bruchlos2

I Departament de Fisica Fonamental, Universitat de Barcelona, Diagonal 647. E-08028 Barcelona, Spain

* Institut ftir Physikalische Hochtechnologie, Helmholtzweg 4, D-07743 Jena, Germany

The vortex dynamics in a highly c-axis oriented epitaxial Tl,Ba$$Cu,O, (Tl-2223) thin-film has been investigated via time-relaxation of the zero-field cooled and remanent magnetization, for low (50 Qe) and moderate (500 Oe) applied magnetic fields below 15 K. A T-independent plateau has been found and explained in terms of quantum tunneling of 2D vortices in the CuO, planes. The influence of the field on the transition has been studied. A T2dependence has been found, this being an evidence of strong ohmic dissipation.

1. INTRODUCTION

Troughout the last few years, anisotropic and, especially, layered high-T, superconductors (HTSC) have provide the best ground for the experimental detection [l-3] of large, temperature independent magnetic relaxation at low temperatures. An anisotropic superconductor is a material where an anisotropic continuous Ginzburg-Landau or London theory gives an accurate description of the physics, while a layered superconductor is a material which has to be described by the discrete 2D Lawrence- Doniach model. The reason for these different approaches lies in the different strength of the coupling between the copper oxide planes, that coupling being closely related to the anisotropy of the material. Thus, the more anisotropic the material, the weaker the interaction between neighbouring planes, and hence the inability of a 3D, continous theory to explain the physics of a layered superconductor.

In the last two years we have been investigating [2] low-temperature non-thermally activated vortex motion in different Tl-based systems. To the date, these materials are the most anisotropic HTSC, so they are the extreme case within layered superconductors. Thus, in the single vortex pinning regime, for low and moderate applied magnetic fields, they are the most suitable materials for the observation of relaxation processes only involving very small, single 2D pancake vortices [4].

2. RESULTS AND DISCUSSION

In this paper, we present the detection of quantum vortex motion in a Tl-2223 thin-film, epitaxially grown on a SrTiO, single-crystal substrate. The preparation and characterization of this material has been extensively discussed elsewhere [5]. By using a commercial SQUlD equipment, we studied the low-temperature time- decay of the zero-field cooled and remanent magnetizations, Mrc and Men,, in two different magnetic fields, H,= 50 and 500 Oe.

Fig. 1. Time-logarithmic decay of the zero-field cooled magnetization with H,= 500 Oe.

For instance, figure 1 shows M, relaxation curves for H,= 500 Oe. At all temperatures, the decays appear to be logarithmic through the selected

0921.4534/94/$07.00 0 1994 - El scvier Science B.V. All rights rescrvcd. SSDI 0921-4534(94)02046-9

2962 A. Garcia et al./Physica C 235-240 (1994) 2961.-2962

time window (210 set I t I 4000 set). The same linear dependence is found for M, in Ha= 50 Oe, and M,, in both applied magnetic fields.

Fig. 2. Low-temperature dependence of the Mr, (squares) and Mrem (circles) magnetic viscosities. The applied magnetic field was Ha= 50 Oe.

The 50 Oe M, and M,, relaxation rates, normalized to the first esperimental data point M,, are plotted as a function of temperature in figure 2. Two different dependences can be detected: a linear regime, which extends from the highest temperature (- 16 K) down to roughly 3.5 K; and a constant, T- independent plateau, which stretches from 3.5 K down to the lowest working temperature. The change from one regime into another it is seen to be rather sharp, 3.5 K being the approximate crossover temperature, T,,. This peculiar behaviour of the magnetic viscosity is a consequence of the existence of a transition in the mechanism that dominates the escape of vortices from pinning sites: above 3.5 K. vortices jump over the energy barriers due to thermal activation; below that temperature, vortices do not have enough thermal energy to overcome the barrier, so they can only relax via quantum transitions under the barrier.

Figure 3 plots the 500 Oe M, and M,,, magnetic viscosities. It can be seen that when the magnetic field increases, the transition becomes gradual, and Tcr decreases (- 2.4 - 2.6 K for the M&C rate, and 2: 2.8 - 3 K for the M_ rate). These two features are theoretically predicted and calculated [6] when taking into account the influence of dissipation in the macroscopic quantum tunneling theory. This agreement suggests that the system becomes more dissipative when increasing

the applied magnetic field. Furthermore, the transition is very well fitted to a T’-dependence, for both the M, and M,m viscosities, as it was predicted in the case of strong ohmic dissipation [7].

T(K)

Fig. 3. 500 Oe M,, (triangles) and M,,,, (stars) magnetic viscosities as a function of temperature, in the low-temperature regime (1.8 K I T < 20 K).

In conclusion, we have detected low-temperature quantum effects in epitasial Tl-based thin-films, and found that increasing the applied field acts as strong ohmic dissipation on the system.

A. G. thanks the Comissionat per a Universitats i Recerca (Departament de la Presidencia, Generalitat de Catalunya) for the research grant. X. X. Z. thanks the CIRIT (Generalitat de Catalunya) for financial support. J. T. acknowledges the Fundacion Domingo Martinez.

REFERENCES

1. 2.

3. 4. 5. 6.

7.

L. Fruchter et nl., Phys. Rev. B 43 (1991) 8709. J. Tejada et crl.. Phys. Rev. B 47 (1993) 11552, and references therein; X. X. Zhang et al. (this Conference). K. Aupke et al.. Physica C 209 (1993) 255. J. Clem. Phys. Rev. B 43 (1991) 7837. M. Manzel et nf.. Physica C 201 (1992) 337. A. 0. Caldeira and A. J. Leggett, Phys. Rev. Lett. 46 (1981) 211. H. Grabert ef nl.. Phys. Rev. Lett. 52 (1984) 2193.