On the origin of critical temperature enhancement in atomically thin superconductors

Abstract

Recent experiments showed that thinning gallium, iron selenide and 2H tantalum disulfide to single/several monoatomic layer(s) enhances their superconducting critical temperatures.Here, we characterize these superconductors by extracting the absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the transition temperature from their self-field critical currents. Our central finding is that the enhancement in transition temperature for these materials arises from the opening of an additional superconducting gap, while retaining a largely unchanged "bulk" superconducting gap. Literature data reveals that ultrathin niobium films similarly develop a second 2 superconducting gap. Based on the available data, it seems that, for type-II superconductors, a new superconducting band appears when the film thickness becomes smaller than the out-ofplane coherence length. The same mechanism may also be the cause of enhanced interface superconductivity.

Classifying superconductivity in an infinite-layer nickelate Nd0.8Sr0.2NiO2

“…Deduced (0) = 740 ± 3 nm is similar to (0) = 690-850 nm measured for samples possessing maximal Tc values for cuprate counterpart La1-xSrxCuO2 [37]. By utilizing deduced (0) value the Ginzburg-Landau parameter = λ(0) (0) = 130 which is similar to La1-xSrxCuO2 [33,37] and this value is at the upper-level range for other cuprates and unconventional superconductors [15,23,24,26,[38][39][40][41][42][43]. Table I.…”

“…There is an alternative way to deduce (0), C/C, Tc and 2Δ(0) by the fit of experimental self-field critical current density, Jc(sf,T), to universal equation, which is for thin-film superconductors reduced to simple form [23,24]:…”

“…where ( ) = ; polar configuration (15) ( ) = √1 − 2 ; axial configuration (16) More details about the Jc(sf,T) analysis for p-wave symmetry can be found elsewhere [26,27]. This approach is recently applied for wide range of thin film unconventional superconductors [23,24,[26][27][28][29][30][31][32].…”

Classifying Induced Superconductivity in Atomically Thin Dirac-Cone Materials

“…We have to note that several intrinsic superconductors exhibit multiple-band superconducting gapping [50,52] and the enhancement of the transition temperature [52][53][54][55][56][57][58][59] when the condition of 2b < c(0) [52] is satisfied. The first discovered material in this class of superconductors is atomically thin FeSe [53][54][55] in which 13-fold increase (i.e., 100 K vs 7.5 K) was experimentally registered to date.…”

“…Another milestone experimental finding in this field was reported by Liao et al [9] who observed the effect of new superconducting band opening and Tc enhancement in few layer stanene (which is the closest counterpart of graphene) by tuning the films thicknesses. To date, maximal Tc increase due to the effect [52], stands with another single-atomic layer superconductor, Td-MoTe2, for which Rhodes et al [59] reported 30-fold Tc increase when samples were thinning down to single atomic layer.…”