Abstract Body

Recently, van der Waals (vdW) layered materials have been studied extensively owing to their unique physical properties. The vdW materials that have been the subject of research are all “crystals” with periodicity. On the other hand, a vdW layered “quasicrystal (QC)”, dodecagonal quasicrystalline (dd-QC) phase in the Ta–Te system, was reported in 1998 by Conrad et al. [1]. To date, no physical properties of the vdW layered QC have been reported even for the bulk.

In this study, we prepared a single-phase Ta–Te vdW layered dd-QC and measured the temperature dependence of electrical resistivity, magnetic susceptibility, and specific heat in order to clarify its physical properties.

Samples were prepared by reaction sintering. TaTe₂ and Ta powders were mixed at a mol ratio of 1:3 and compacted with a trace amount of iodine inside. Then, the mixture was sealed in an evacuated quartz tube, followed by heat treatment at 1000 °C for 6 days. In Figure 1(a), the electron diffraction pattern of the synthesized sample shows an arrangement of sharp spots with dodecagonal symmetry, verifying the formation of a dd-QC phase. Powder X-ray diffraction measurements confirmed that the sample is a single phase of vdW layered quasicrystal. Figure 1(b) shows the results of electrical resistivity measurement, which exhibits zero resistivity below T_c ≈1 K. In addition, we also observed large diamagnetism and a jump in specific heat. These results confirm the bulk superconductivity of the synthesized Ta–Te vdW layered dd-QC with T_c ≈1 K [2]. It is the second example of superconductivity in QCs and the first in a thermodynamically stable QCs. Superconductivity in a thermodynamically metastable icosahedral QC of Al–Zn–Mg with T_c of ~ 0.05 K has been discovered in 2018 [3]. Recent theoretical studies have revealed that quasicrystalline superconductors exhibit several unconventional behaviors that are typically not observed in other known superconductors in periodic and disordered systems, thus opening a new field in the research of superconductivity. In order to experimentally verify such unconventional features, superconducting QCs with higher transition temperature are expected. Thus, the Ta–Te vdW layered dd-QC, with T_c ≈1 K, can promote new research on superconducting properties of QCs. Moreover, it paves the way toward new frontiers of vdW layered QCs.

Then, the magnetic field dependence of resistance at various temperatures below T_c was measured. The temperature dependence of the upper critical field H_c2 exhibited a linear increase from T_c to 0.04 K [4]. The experimental data yielded an extrapolated H_c2 at 0 K of 42.1 kOe, which is more than twice the Pauli limit H_P(0) of 18 kOe. Moreover, the experimental H_c2 surpasses the calculated curve based on the standard Werthamer–Helfand–Hohenberg theory with the dirty limit at low temperatures. Considering Ta and Te are heavy atoms, it is suggested that spin–orbit interactions may play an important role in enhancing H_c2 in Ta–Te vdW layered dd-QC.

Specific heat measurements were carried out on the (Ta,Cu)–Te ternary vdW layered dd-QC system to analyze the temperature dependence of the electronic specific heat of the superconducting state. The results of the specific heat measurements showed a large jump corresponding to the superconducting transition at 0.91 K. The temperature dependence of the electronic component of the specific heat fits well the full-gap model.

References

[1] M. Conrad, et.al., Angew. Chem. Int. Ed. 37, 1383 (1998).

[2] Y. Tokumoto, et.al, Nat. Commun. 15, 1529 (2024).

[3] K. Kamiya, et. al., Nat. Commun. 9, 154 (2018).

[4] T. Terashima, et. al., npj Quantum Mater. 9, 56 (2024).

Acknowledgment

This study was supported by JSPS KAKENHI (Grant Number JP23K04355) and Tokuyama Science Foundation and Murata Science and Education Foundation.

Figure 1. (a) Electron diffraction pattern of the synthesized sample. (b) Temperature dependence of the electrical resistivity normalized by the value at T = 300 K, i.e., ρ = ρ_{300 K}.

Keywords: van der Waals layered materials, quasicrystals, transition-metal chalcogenides