The Coexistence of Spinon Superconductivity and d-wave Superconductivity in Bi-2212 from Tunneling Measurements

Abstract

We present electron-tunneling spectroscopy of slightly overdoped Bi2212 single crystals at low temperature using a break-junction technique. Below Tc, we observe, at least, two different superconducting gaps. We show that (i) the largest superconducting gap occurs due to pairing of spinons; (ii) the second superconducting gap has the d-wave symmetry; (iii) the d-wave gap is smaller than the spinon gap, however, predominant and occurs due to pairing of fermionic excitations carrying charge; (iv) the spinon gap has either a s-wave or (s+d) mixed symmetry with the maximum located in (p, p) direction on the Fermi surface. It is possible that there exists the third superconducting gap having a g-wave symmetry, which is approximately twice smaller in magnitude than the d-wave gap. Thus, there are two different types of superconductivity and two different types of carriers in Bi2212.