• Progress in Superconductivity
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• v.3 no.2
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• pp.140-145
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• 2002

An enhancement of the c-axis differential conductance around the zero-bias voltage near the superconducting transition temperature $T_{c}$ has been observed in $Au/Bi_2$$Sr_2$$CaCu_2$$O_{8+x}$ junctions. We attribute such an enhancement to the Andreev reflection (AR) between the surface Cu-O bilayer with suppressed superconductivity and the next superconducting Cu-O bilayer. The continuous evolution of the differential conductance, from gap like depression to an AR-like peak structure, around the zero-bias voltage points to weakening of the barrier strength of the nonsuperconducting layer between adjacent Cu-O bilayers as temperature approaches $T_{c}$ from below. The peak structure disappeared just below the bulk $T_{c}$ value of underdoped Bi2212 single crystals, whereas it survived up to ~1 K above $T_{c}$ in Junctions prepared on slightly overdosed crystals. According to a recently proposed theoretical consideration, a wider temperature range of the AR a bone $T_{c}$ is expected in the underdoped regime when phase-incoherent preformed pairs emerge in the pseudogap state. Our result is in contradiction to the preformed pair scenario. scenario.o.

• 한국초전도학회:학술대회논문집
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• v.11
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• pp.20-20
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• 2001

• 한국초전도학회:학술대회논문집
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• v.10
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• pp.85-85
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• 2000

The normal state of high-Tc superconducting materials has been believed to contain important clues to finding the correct mechanism of the high-Tc superconductivity. One example is the existence of pseudogap in the normal state even above Tc, as observed in various measurements such as photoemission spectroscopy and tunneling conductance. In this pseudogap regime the existence of preformed pairs only with local phase coherence has been debated. Recently Choi, Bang, and Campbell[1] have proposed the occurrence of the zero-bias conductance enhancement due to Andreev quasiparticle reflection from the preformed pairs even with the local phase coherence. In this study we examine the zero-bias enhancement of the differential conductance near or slightly above Tc, using c-axis tunneling in mesa structure of Bi2Sr20a0u208+d single crystals. In slightly overdoped samples zero-bias conductance enhancement (ZBCE) has been observed over a range of 2 K above Tc. In contrast, in underdoped samples with Tc${\sim}$72K the ZBCE appears over a range of 5-6 K above Tc, a much wider temperature range than in overdoped samples. This result may pose as positive signs of the existence of prefurmed pairs in the normal state of high- Tc superconducting materials.