On the origin of critical temperature enhancement in atomically thin superconductors
E. F. Talantsev (Victoria University of Wellington)
W. P. Crump (Victoria University of Wellington)
J.O. Island (TU Delft - QN/van der Zant Lab, University of California, Kavli institute of nanoscience Delft)
Ying Xing (Peking University, China University of Petroleum - Beijing, Collaborative Innovation Center of Quantum Matter)
Yi Sun (Collaborative Innovation Center of Quantum Matter, Peking University)
Jian Wang (Collaborative Innovation Center of Quantum Matter, Peking University)
J. L. Tallon (Victoria University of Wellington, MacDiarmid Institute for Advanced Materials and Nanotechnology)
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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 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-of-plane coherence length. The same mechanism may also be the cause of enhanced interface superconductivity.
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