Abstract
Recently, Kayyalha et al. (Phys. Rev. Lett., 2019, 122, 047003) reported on the anomalous enhancement of the self-field critical currents (Ic (sf, T)) at low temperatures in Nb/BiSbTeSe2-nanoribbon/Nb Josephson junctions. The enhancement was attributed to the low-energy Andreev-bound states arising from the winding of the electronic wave function around the circumference of the topological insulator BiSbTeSe2 nanoribbon. It should be noted that identical enhancement in Ic (sf, T) and in the upper critical field (Bc2 (T)) in approximately the same reduced temperatures, were reported by several research groups in atomically thin junctions based on a variety of Dirac-cone materials (DCM) earlier. The analysis shows that in all these S/DCM/S systems, the enhancement is due to a new superconducting band opening. Taking into account that several intrinsic superconductors also exhibit the effect of new superconducting band(s) opening when sample thickness becomes thinner than the out-of-plane coherence length (c (0)), we reaffirm our previous proposal that there is a new phenomenon of additional superconducting band(s) opening in atomically thin films.
Highlights
Intrinsic superconductors can be grouped into 32 classes under “conventional”, “possibly unconventional”, and “unconventional” categories, according to the mechanism believed to give rise to superconductivity [1]
In the most recently updated Uemura plot (Figure 1), it can be seen that elemental superconductors are located for wide range of Tc/TF ≤ 0.001, while all unconventional superconductors, including both nearly-room-temperature superconductors of H3S [4] and LaH10 [5], are located within a narrow band of 0.01 ≤ Tc/TF ≤ 0.05
It should be noted that several researchers and research groups have proposed different mechanisms for high-temperature superconductivity in cuprates, the first two-band BCS superconductor (MgB2), pnictides, and hydrogen-rich superconductors, for which we refer the reader to original papers and comprehensive reviews [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31]
Summary
Intrinsic superconductors can be grouped into 32 classes under “conventional”, “possibly unconventional”, and “unconventional” categories, according to the mechanism believed to give rise to superconductivity [1]. It should be mentioned that Hardy et al [10] in 1993 (seven years after the discovery of high-temperature superconductivity in cuprates by Bednoltz and Mueller [11]) were the first to experimentally find that YBa2Cu2O7-x has nodal superconducting gap. This experimental result was used to propose d-wave superconducting gap symmetry in HTS cuprates by Won and Maki [12]. It should be noted that several researchers and research groups (over last 33 years) have proposed different mechanisms for high-temperature superconductivity in cuprates, the first two-band BCS superconductor (MgB2), pnictides, and hydrogen-rich superconductors, for which we refer the reader to original papers and comprehensive reviews [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31]
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