Science Diliman (July-December 1999) 11: 2,15-17

ABSTRACT

!\erworc/s: superconductors, resistivity, vortices, flux flow resistivity, magnetoresistance

INTRODUCTION

As expedient as high critical temperature
superconductors (HTSC) are, their properties in the
presence of an external magnetic field should inexorably
be studied since these materials would necessarily be
exposed to these fields in technological use.

An applied magnetic field penetrates a superconductor
in the mixed state. The pendration occurs in the form
of tubes, called vortices, which serve to confine the
flux. The interaction between vortices is repulsive so
that they arrange themselves as far away from each
other, as in a flux lattice in clean samples or as vortex
glass in the presence of randomly distributed pinning
centers. The pinning centers may be point defects, grain

Clarina de la Cruz', Leandro Guerra, Roland Sarmago, and Arnel Salvador
Condensed Matter Physics Laboratory, National Institute of Physics

l;niversity of the Philippines Diliman. Quezon City 1101 Philippines

Email: c1ar«(l)nip.upd.edu.ph; Fax: +632-920-5474

boundary or intra/intergranular non-superconducting
regions. On the other hand, transport currents and
thermal fluctuations can both induce flux motion, due
to the Lorentz force, and produce dissipation. This can
involve, for example, the release and transportation of
vortices to other pinning centers, and the pinning and
depinning of flux bundles. Flux flow can be viewed as
a rate of change of flux which, by faraday's law,
produces a voltage drop in the superconductor along
the direction of the transport current. The resistance
associated with this motion provides a mechanism for
heat dissipation (Poole et aI., 1995).

In this study, the contribution of these vortices were
investigated through resistivity measurements in varying
magnetic fields. This contribution was quantitatively
determined as the flux flow resitivity.

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The behavior of high-temperature superconductors in the presence of an external magnetic field is of

particular interest in light of its technological application and commercialization. In this paper, we perfonned

resistivity measurements on bulk superconducting pellets of in the presence of external

magnetic fields below 0.5T. The broadening of the transition region below Tc in the resistivity plots, was

attributed to the residual resistance imparted by flux flow in the sample. From 1-V measurements at 50 K at

fields below 0.6T, the contribution of vortices was quantitatively measured as a flux flow resistivity which

range from 0.1231 to 1.700 (m( -mm) for applied magnetic fields from 0.04T to 0.6T. The increase in the flux

flow resistivity with increasing applied field was due to the increase in the number of vortices moving in

steady state motion brought about by the interaction of the vortices with the transport current.



Fig. 2 shows that the critical current decreases with
increasing magnetic fields. This is because, the external
magnetic field destroys the superconductivity in the
sample resulting in the lowering of the amount of
supercurrent that the sample can carry.

METHODOLOGY

After the sample was cooled down to -10K, using a
closed-cycle helium cryostat, resistivity measurements
were done using a current of SOmA in different magnetic
fields below 0.5T.

The contribution of the vortices when the resistivity
measurements were done with an external magnetic
field was determined through I-V measurements with
fields below 0.6T.

RESUL 1'S AND DISCUSSION

Fig. I. The resulting resistivity plots at varying magnetic fields

below 0.5 T. Inset shows a typical resistivity plot at 0 T

16

This residual resistance termed as flux flow resistance
was quantitatively determined from:

In equation (I), Ie is the critical current which was
determined through I-V measurements done at 50 K
for varying magnetic fields.

where 0" is the sample cross-section and dL is the
separation of the voltage contacts.

de la Cruz et al.

Fig. 2. Magnetic field dependence of the transport critical current

ofa bulk Bi,Sr,CaCup8+& sample determined at 50 K

Fig. 3 shows that the flux flow resistivity increases with
respect to the magnitude of the applied magnetic field.
This increase in the resistance indicates that the vortices
are interacting with the transport current (Poole et aI.,
1995; Bishop, 1993). If the Lorentz force on the vortices
due to the transport current is large enough to unpin

The samples were bulk superconducting pellets of
which were prepared via the solid

state reaction method. The pellets were cut into
3 .Ox8.0x 1.0 mm rods. The electrical contacts used were

gold wires attached by pressed indium solder. The
transport measurements were done using the in-line
contact configuration to minimize errors from leads and
contact resistances (Ison, 1999).

From the resistivity measurements performed at varying
fields below 0.5T (Fig. 1), a broadening of the transition
region below the critical temperature was observed.
This is attributed to the residual resistance imparted by
flux flow in the sample. Also, it can be seen that the
plots coincide in the high temperature region above Te
since there are no more defined vortices in the sample
when it is in its normal state (Bishop et aI., 1993).

The flux flow resistance, , was denved as the slope
of the linear part of the corresponding I-V curves.
Knowing the dimensions of the sample, the
corresponding flux flow resistivity , was calculated
usmg:



the vortices so that they move from one pinning center
to the next, then the resistance effectively increases.
On the other hand, the granular nature of the sample
would establish that the vortices would remain pinned
due to the random orientation of the supercooducting
planes. Therefore, the increase in the resistance implies
that there is an increase in the number of vortices in
the liquid phase, present in the sample. The increase in
the flux flow resistivity corresponds to an increase in
the amOlmt of field that was able to penetrate the sample.
Since vortices are quantized, an increase in the internal
field would be due to the addition of vortices moving in
steady-state motion.

The dependence of the critical current and the flux flow
resistivity on the externally applied magnetic field, as
detennined in this investigation, conformed with previous
studies (poole et aI., 1995; Ison, 1993; Yeshurun, 1996).

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	Titles
	Science Diliman (July-December 1999) 11: 2,15-17 
	ABSTRACT 
	!\erworc/s: superconductors, resistivity, vortices, flux flow resistivity, magnetoresistance 
	INTRODUCTION 
	15 


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	METHODOLOGY 
	RESUL 1'S AND DISCUSSION 
	16 
	de la Cruz et al. 
	The samples were bulk superconducting pellets of 


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