| Budget Amount *help |
¥2,200,000 (Direct Cost: ¥2,200,000)
Fiscal Year 1997: ¥900,000 (Direct Cost: ¥900,000)
Fiscal Year 1996: ¥1,300,000 (Direct Cost: ¥1,300,000)
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| Research Abstract |
Measurements of the energy gap in the Ce- and Yb- based Kondo semiconductors have been carried for the first time using break junctions. Since this technique provides the cleanest interface, it is possible to investigate the intrinsic features of this compound with very reactive surface properties. For Ce<@D23@>D2Bi<@D24@>D2Pt<@D23@>D2, the features of clear gap-edge peaks and low zero-bias leakage in dI (V) /dV resemble those of the superconducting tunneling density of states, although the mechanism of gap formation is completely different to each other. The gap magnitudes of 2DELTA (]SY.apprxeq.[) 90 meV at 4.2 K defined by the peak-to-peak separation in dI (V) /dV are much larger than the activation energy obtained from p (T) for the identical sample, but are roughly the same magnitude as that obtained from the infrared measurements. The ratio 2DELTA /k<@D2B@>D2T<@D2x@>D2 = 13 (]SY.+-。[) 2 is evaluated using T<@D2x@>D2 = 80 K,where T<@D2x@>D2 is the temperature at which the suscepti
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bility exhibits a broad maximum. This ratio agrees well with that of CeRhSb and CeNiSn, in which the gaps open below T<@D2x@>D2. These results demonstrate that the gap formation is directly connected with T<@D2x@>D2 in these Ce-based Kondo semiconductors. For YbB<@D212@>D2, very sharp gap-edge structures with quite low leakage of the zero-bias conductance are commonly observed in every junction at 4.2 K,from which the average gap value of 2DELTA (]SY.apprxeq.[) 230 meV is obtained. This is unexpectedly larger than the gap of (]SY.apprxeq.[) 12 meV from the transport measurements. Upon warming, this giant gap of (]SY.apprxeq.[) 3000 K reduces to disappear at 60 - 90 K,which is close to the T<@D2x@>D2. of this comound. This feature is similar to that of Ce-based Kondo-semiconductors. However, the ratio 2DELTA/k<@D2B@>D2T<@D2x@>D2 (]SY.apprxeq.[) 37 of YbB<@D212@>D2 is (]SY.apprxeq.[) 3 times larger than that of the cubic Ce<@D23@>D2Bi<@D24@>D2Pt<@D23@>D2 compound having almost the same T<@D2x@>D2 value, thereby indicating that the characteristic energy for the gap creation in ybB<@D212@>D2 is larger than that in Ce<@D23@>D2Bi<@D24@>D2Pt<@D23@>D2. Further, by examining the susceptibility data of YbB<@D212@>D2 and Ce<@D23@>D2Bi<@D24@>D2Pt<@D23@>D2, we find the scaling temperatures T<@D1**@>D1 for each compound, which make the ratio 2DELTA/k<@D2B@>D2T<@D1**@>D1 a common value of (]SY.apprxeq.[) 3 - 4. Less
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