Observation of the Andreev reflection in the c-axis transport of Bi2Sr2CaCu2O8+x single crystals near Tc and the search for the preformed-pair state
Hyun-Sik Chang, Hu-Jong Lee, Migaku Oda
DOI 10.1103/PhysRevB.64.134504 · Physical Review B
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Abstract
We observed an enhancement of the c-axis differential conductance around the zero bias in Au/Bi2Sr2CaCu2O8+x (Bi2212) junctions near the superconducting transition temperature Tc. We attribute the conductance enhancement to the Andreev reflection between the surface Cu-O bilayer with suppressed superconductivity and the neighboring superconducting inner bilayer. The continuous evolution from depression to an enhancement of the zero-bias differential conductance, as the temperature approaches Tc from below, points to weakening of the barrier strength of the nonsuperconducting layer between adjacent Cu-O bilayers. We observed that the conductance enhancement persisted up to a few degrees above Tc in junctions prepared on slightly overdoped Bi2212 crystals. However, no conductance enhancement was observed above Tc in underdoped crystals, although a recently proposed theoretical consideration suggests an even wider temperature range of enhanced zero-bias conductance. This seems to provide a negative perspective to the existence of the phase-incoherent preformed pairs in the pseudogap state.
Source-reported materials — not catalogue approval
| Formula | Reported Tc (K) | Pressure (GPa) | Type |
|---|---|---|---|
| Bi2Sr2CaCu2O8+x Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula. | 90 | Pressure not reported | unknown |
| Bi2Sr2CaCu2O8+x Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula. | 83 | Pressure not reported | unknown |
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