Department of Physics, Nagoya University, Nagoya 464-8602, Japan1
Toyota Technological Institute, Nagoya 468-8511, Japan2
Department of Physics, Tohoku University, Sendai 980-8578, Japan3
Toyota Physical and Chemical Research Institute, Aichi 480-1192, Japan4
Owing to its unique geometry, quasicrystal is expected to have an electronic state called “critical state” that is neither extended nor localized. This unique eigenstate emerges as a result of the competition between the broken translational invariance and the self-similarity of quasiperiodic structure. Besides extensive studies, the electronic state of quasicrystals is veiled in mystery. For example, no long-range-ordered state of electronic origin was established before our discovery of superconductivity in a quasicrystal at 2018 [1].
In the presentation, we review the experimental results of bulk superconductivity in Al-Zn-Mg quasicrystal, i.e., zero resistance, Meissner effect, and heat capacity jump at the transition temperature Tc [1]. The geometric structure of the Al-Zn-Mg alloy depends on the ratio of the three constituent elements. When reducing the Al content while keeping the Mg content almost constant, the alloy remains to be an approximant crystal up to 15% Al, above which it suddenly transforms into a quasicrystal [1,2].
Interestingly, all the approximant crystals show bulk superconductivity at low temperatures; Tc decreases gradually from 0.8 to 0.2 K with reducing the Al content, and drops to 0.05 K at the critical concentration where the quasicrystal is formed.
This observation of superconductivity raises a fundamental question of whether the emerging superconductivity is in weak- or strong-coupling state. From the temperature dependences of the thermodynamic properties, we suggested the formation of weak-coupling, spatially extended pairs. However, this does not necessarily mean that the superconducting state of the quasicrystal is the same as that of periodic crystals. According to a theoretical study [3], the paring state of quasicrystal is expected to differ from the conventional Cooper pairing of the BCS theory. This novel superconductivity, which we name “fractal superconductivity”, may open the door to a new type of superconductivity.
[1] K. Kamiya, T. Takeuchi, N. Kabeya, N. Wada, T. Ishimasa, A. Ochiai, K. Deguchi, K. Imura, N. K. Sato, Nat. Commun. 9, (2018) 154.
[2] T. Takeuchi and U. Mizutani, Phys. Rev. B 52, (1995) 9300.
[3] S. Sakai, N. Takemori, A. Koga, R. Arita, Phys. Rev. B 95, (2017) 024509.
Keywords: quasicrystal, approximant, fractal superconductivity