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Abstract
Although the pairing mechanism in MgB2 is thought to be phonon mediated,
there are still many experimental results that lack appropriate explanation. For
example, there is no consensus about the magnitude of the energy gap, its
temperature dependence, and whether it has only one-gap or not. Many
techniques have been used to investigate this, like Raman spectroscopy, farinfrared
transmission, specific heat, high-resolution photoemission and tunneling.
Most tunneling data on MgB2 are obtained from mechanical junctions.
Measurements of energy gap by these junctions have many disadvantages like
the instability to temperature and field changes. On the other hand, sandwich-like
planar junctions offer a stable and reliable measurement for temperature
dependence of the energy gap, where any variation in the tunneling spectra can
be interpreted as a direct result from the sample under study.
To the best of our knowledge, we report the first energy gap temperatureand
magnetic field-dependence of MgB2/Pb planar junctions. Study of the
temperature-dependence shows that the small gap value (reported by many
groups and explained as a result of surface degradation) is a real bulk property of
MgB2. Moreover, our data is in favor of the two-gap model rather than the onegap,
multi-gap, or single anisotropic gap models. The study of magnetic field
effect on the junctions gave an estimation of the upper critical field of about 5.6 T.
The dependence of energy gap on the field has been studied as well.
Our junctions show stability against temperature changes, but ”collapsed”
when the magnetic field (applied normal to the junction barrier) is higher than 3.2
T. The irreversible structural change switched the tunnling mechanism from
quisiparticle tunneling into Josephson tunneling. Josephson I-V curves at
different temperatures have been studied and the characteristic voltages