Session 4-4

Abstract

Understanding size effects in low-dimensional superconductors is essential for the scalability of superconducting quantum circuits. When the thickness of a superconductor falls below the coherence length, its transition temperature (Tc) can be suppressed by thermal fluctuations, disorder, and size-dependent changes in electronic and phonon states. The latter effects are particularly relevant in two-dimensional materials with layer-sensitive electronic structures [1].

We investigated graphite intercalation compounds (GICs) as model systems. These materials, formed by inserting alkali or alkaline earth metals between graphene layers, exhibit superconductivity through interactions between interlayer electrons and carbon atoms. This interaction depends on the spatial distribution of interlayer electrons, which is known to be reduced in bilayer GICs [2]. Among GICs, CaC₆ shows the highest bulk Tc (11 K), which drops to ~3 K in bilayer form [3,4]. KC₈, by contrast, has a bulk Tc of 0.13 K but was previously reported to exceed 3 K in bilayers, suggesting an opposite size effect [5]. To clarify how the choice of intercalant influences these trends, we examined the layer-dependent Tc and electronic structure of Ca- and K-intercalated graphene.

Transport measurements confirmed a positive size effect in CaC₆: Tc increased with the number of layers. In KC₈, however, no superconductivity was observed above 2.7 K in 2–6 layer samples, contradicting earlier reports of a strong negative size effect. ARPES measurements revealed that interlayer electron density in CaC₆ increases with thickness, enabling greater out-of-plane wavefunction delocalization and enhanced interaction with graphene layers, explaining its rising Tc. In contrast, KC₈ showed no thickness dependence in electron density, consistent with its limited Tc response. These results indicate that size effects in GICs are strongly element-dependent, primarily due to differences in interlayer spacing: smaller in CaC₆, which promotes 3D delocalization; larger in KC₈, which restricts it.