| Summary: | 碩士 === 輔仁大學 === 物理學系 === 95 === The bulk of CeRu2 is a type II superconductor with TC=6.3 K. We fabricated CeRu2 nanoparticles with various sizes (7, 9, 29, 120 nm) and studied their size dependence of magnetism and heat capacity. As size decreases, the energy levels of electrons gradually split and physical properties can be influenced, this phenomenon is called as “quantum size effect”. The surface/volume ratio also increases with size reduction, the change of physical properties related to the surface atoms is called as “surface effect”.
Fit the data of heat capacity for 120 nm- CeRu2 to Kondo model and lattice phonon, the Kondo temperature TK and the fraction of magnetic ion Ce3+ nf are obtained to be 1.4 K and 6.4% respectively. As the size decreases to 9 nm, nf increases to a maximum 20.5%, and TK increases to 12 K as well. When the sample size further decreases to 7 nm, both nf and TK start to decrease to 3.9% and 5.5 K. The measurement of magnetic susceptibility shows that TC and percentage of superconducting phase decrease with size reduction. According to Anderson criterion of superconductivity in nanoparticles, if electronic energy level spacing is less than superconducting energy gapΔ, superconductivity should exist. However in the early study of CeCo2 compound, no superconductivity can be found in 11-24 nm nanoparticles although Δ~4 K is larger than electronic energy level spacing ~0.02-0.6 K. The possible scenario is that the Kondo temperature ~8 K in CeCo2 is larger than the superconducting gap magnitudes of TK.
In this study of CeRu2 , since its superconductivity energy gap is ~19 K, which is much larger than its Kondo temperature TK ~ 1.4-12 K and electronic energy level spacing ~0.1-0.8 K, so the superconductivity should still exists even for smallest 7 nm nanoparticles. The work demonstrates the competition of TK and electronic energy level spacing with Δ.
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