Coexistence of Weak and Strong Coupling Mechanism, in an Iron-Based Superconductor FeSe 0.5 Te 0.5: Possible Signature of BCS-BEC Crossover

On the Thermodynamic Properties of FexSe0.5Te0.5 Superconducting Single Crystals: An Experimental Study

“…34,35 Recently, we measured the energy gap in the FeSe 0.5 Te 0.5 superconductor using scanning tunneling microscopy/spectroscopy (STM/STS). 29 These measurements revealed the presence of two energy gaps, which is consistent with the presence of weak and strong coupling mechanisms in the FeSe 0.5 Te 0.5 superconductor. The quality of the grown single crystals, impurities, and doping has been found to greatly affect the obtained values of the superconducting energy gap.…”

“…Several studies suggest that excess iron leads to internal stress that may affect the electron-phonon coupling and energy gap. 29,38 Moreover, calculated values of the coupling strength ratio 2Δ(0)/k B T c range between ∼2.6 to 3.1 which is within the weak coupling limit. The ratio tends to decrease with Fe content (Table II).…”

“…41 Furthermore, scanning tunneling microscopy/spectroscopy (STM/STS) were reported by Kunwar et al with two gaps at ∼1.5 and ∼5.0 meV, indicating the coexistence of weak and strong coupling mechanism. 29 The calculated values of the deviation D of the reduced thermodynamic critical field [H c /H c (0)] from parabolic behavior vs t 2 for all samples are presented in Fig. 5.…”

“…The BCS theory relates the thermodynamical critical field with several microscopic properties, such as the superconducting energy gap Δ(0), the jump in the specific heat at the transition temperature, the density of state N(0), and coupling strength −2Δ(0)/k B T c ∼3.52, 28,29 Specific heat measurements and tunneling spectroscopy have been used to evaluate the superconducting energy gap and the coupling strength. The superconducting energy gap for FeSe 1−x Te x has been investigated.…”