| Budget Amount *help |
¥3,500,000 (Direct Cost: ¥3,500,000)
Fiscal Year 2005: ¥900,000 (Direct Cost: ¥900,000)
Fiscal Year 2004: ¥1,600,000 (Direct Cost: ¥1,600,000)
Fiscal Year 2003: ¥1,000,000 (Direct Cost: ¥1,000,000)
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| Research Abstract |
We have investigated the high temperature layered boride superconductor MgB_2 by electron tunneling and point contact spectroscopy.
The tunneling conductance at low temperatures shows clear multiple-gap structures, which are described by the two-gap model proposed by McMillan. This indicates that the multiple gaps are due to non-trivial origin. We call this model as "the correlated two gap" model. The specific gap values are Δ=2-2.5,5-7,10-12 meV that exist up to T_c=39K. The gap ratio 2Δ/k_BT_c=5-6 for the largest gap indicates extremely strong coupling, which is the largest one among non-copper superconductors. On the other hand, the smallest gap giving the ratio 1.2-1.5 is surprisingly smaller than the BCS one 3.5, indicating an immature gap feature from induced superconductivity. From the temperature variations, we attribute that the middle gap can be due to the proximity effect. From these analyses, it is possible to consider that the largest and smallest gaps are coming from the 2 band superconductivity arising from boron σ and π bands.
The magnetic-field dependence of the gaps makes clear distinction between the gaps, namely, the larger gap vanishes at the higher critical field. This is in contrast to the temperature dependence of these gaps showing the same critical temperature. The extrapolated critical fields of 18-23 T and 7-11 T for the largest and smallest gaps correspond to in-plane and c-axis upper critical fields, respectively. These agreements indicate that the multiple gaps are intimately related to the layered structure, and the largest gap is predominant to MgB_2.
Point- contact spectra show that the peak positions are in agreement with the phonon spectra from the neutron scattering experiments. More significantly, the high-energy boron vibration modes around 60-90 meV are suggested to play an important role in the electron-phonon interaction in MgB_2.
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