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

Abstract Body

Quasicrystals are materials with aperiodicity and long-range order. Due to the lack of periodicity, the conventional picture of Cooper pairs near the Fermi surface is not available. Nevertheless, some BCS-like superconducting quasicrystals have been experimentally discovered [1, 2]. In these superconductors, superconducting order parameters are non-uniformly distributed. In addition, quasicrystals have so-called confined states that are electronic states strictly localized at the certain sites. They are macroscopically degenerate and bring sharp peaks to the density of states.

We show that the non-uniformity and confined states can stabilize the gapless superconducting phase where the bulk quasiparticle excitation gap becomes infinitesimal in Ammann-Beenker quasicrystals under a magnetic field at half filling, unlike at low filling [3]. When the Rashba spin-orbit coupling is present, the quasicrystalline gapless superconductor can be topologically nontrivial with Majorana zero-energy edge modes, which is characterized by a nonzero pseudospectrum index given by the spectral localizer [4].

References

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

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

[3] M. Hori, T. Sugimoto, T. Tohyama, and K. Tanaka, arXiv:2401.06355.

[4] A. Cerjan and T. A. Loring, Phys. Rev. B 106, 064109 (2022).

Acknowledgment

This work is supported by JST SPRING, Grant Number JPMJSP2151 and JSPS KAKENHI (Grant No. JP23K13033 and No. JP24K00586).

Figure 1. Probability amplitude of near-zero-energy modes in topologically (a) trivial, and (b) nontrivial phase.

Keywords: Gapless superconductivity, Topological superconductivity, Quasicrystals, Ammann-Beenker tiling

Abstract Body

Weyl superconductivity (WSC) is a three-dimensional (3D) topological superconducting phase that possesses point nodes with a linear dispersion in 3D momentum space [1]. The point nodes are named Weyl nodes because their dispersion is effectively described by the Weyl equation. Weyl nodes are stable due to topological protection by translational symmetry. The nontrivial topology also leads to surface Majorana arc states. Meanwhile, quasicrystals are materials without translational symmetry. Since 3D quasicrystals do not have 3D momentum space, a theory for WSC in quasicrystals is yet to be established.

In this presentation, we discuss the possibility of quasicrystalline WSC as a topological superconducting phase in layered quasicrystals [2], which are periodic only in the stacking direction. The band structure of quasicrystalline WSC has gapless nodes protected topologically, which we call quasicrystalline Weyl nodes. In a similar way to the characterization of conventional WSC, quasicrystalline Weyl nodes are defined by a change in a real-space topological invariant in one-dimensional momentum space [3]. We demonstrate that quasicrystalline WSC can be realized by using layered quasicrystalline topological superconductors. Furthermore, we show that quasicrystalline Weyl superconductors exhibit Majorana arcs on their surfaces.

References

[1] T. Meng and L. Balents, Phys. Rev. B 86, 054504 (2012).

[2] M. Hori, R. Okugawa, K. Tanaka, and T. Tohyama, Phys. Rev. Resarch 6, 033088 (2024).

[3] A. G. e Fonseca, T. Christensen, J. D. Joannopoulos, and M. Soljacic, Phys. Rev. B 108, L121109 (2023).

Keywords: Topological superconductivity, Quasicrystal, Weyl nodes, Majorana modes

Abstract Body

The discovery of infinite-layer nickelate superconductivity provides a new pathway to realize cuprate analog systems [1]. We first analyzed this compound using a combination of density functional theory and dynamical mean field theory (DFT+DMFT). The results show that the system can be described by a single-band Hubbard model with an additional electron reservoir at least around the superconducting doping region. We then calculated the critical temperature of this simplest model using the dynamical vertex approximation. We obtained a Tc-dome structure centered around 20% Sr-doping [2], which agrees with subsequent experiments. Building on the successful description of the experimental phase diagram, we conducted a comprehensive study of the superconducting instability in the single-band Hubbard model to search for sweet spots for high Tc. Combined with first-principles calculations, we propose that palladates would be a possible alternative to nickelates for optimizing model parameters and achieving a higher Tc in practice [3].

References

[1] D. Li, K. Lee, B.Y. Wang, M. Osada, S. Crossley, H.R. Lee, Y. Cui, Y. Hikita, H.Y. Hwang, Nature 572, 624–627 (2019).

[2] M. Kitatani, L. Si, O. Janson, R. Arita, Z. Zhong, and K. Held, npj Quantum Mater. 5, 59 (2020).

[3] M. Kitatani, L. Si, P. Worm, J. M. Tomczak, R. Arita, and K. Held, Phys. Rev. Lett. 130, 166002 (2023).

Keywords: unconventional superconductivity, nickelates, palladates, dynamical mean field theory, dynamical vertex approximation

Abstract Body

Following the copper and iron ages, the discovery of superconductivity in infinite layer nickelates _RE_NiO₂ (RE =Nd, La, etc.) [1] has kicked-off the “nickel age”[2] of unconventional superconductivity. Indeed, last year, superconductivity with a maximum T_c ~ 80K was discovered in a bilayer Ruddlesden-Popper nickelate La₃Ni₂O₇[3] under high pressure, and this was followed by the discovery of superconductivity with T_c = 20 ~ 30K in a trilayer La₄Ni₃O₁₀ also under high pressure [4], which was further confirmed by other experiments [5]. It is worth mentioning that for the bilayer nickelate La₃Ni₂O₇ two of the present authors discussed the possibility of superconductivity in this material even before its experimental discovery [6], and for the trilayer La₄Ni₃O₁₀ the structural transition to tetragonal symmetry under pressure as well as the occurrence of superconductivity with a T_c ~ ~comparable to those of the low T_c cuprates was predicted theoretically by the present authors [4].

As for the infinite layer nickelates, several authors, including us, have theoretically proposed a d-wave pairing scenario, since the electron configuration is considered to be close to d⁹ as in the cuprates [7-9]. Nonetheless, in a previous study, two of the present authors proposed a possibility of s±-wave superconductivity with an even higher Tc than for the d-wave pairing [10]. Here, s±-wave means that the sign of the superconducting gap function between 3d_x2-y2 and other four 3d orbitals are reversed. In Ref. [10], the existence of residual hydrogen was assumed so that the electron configuration is closer to d⁸.  When such an electron configuration is combined with a large energy level offset ΔE between 3d_x2-y2 orbitals and other 3d (especially 3d_3z2-r2 ) orbitals, as in the infinite-layer case, where there are no apical oxygens, a possibility of high T_c s±-wave superconductivity arises owing to a situation where the four 3d bands are incipient, namely, they lie close to, but does not intersect, the Fermi level,. The basic idea of the high T_c mechanism originates from the equivalence between two-orbital and bilayer Hubbard models [11, 12], where the interorbital energy level offset ΔE in the former is transformed to the interlayer hopping t_⊥ in the latter.

In the present study, we theoretically propose an alternative possibility of achieving s±-wave high T_c~  ~superconductivity in the infinite layer nickelates through realization of electron configuration close to d⁸. Namely, we consider doping large amount of holes in, e.g., LaNiO₂ by largely substituting the rare earth elements (La in this case) with alkaline-earth elements (such as Sr or Ba). The end materials SrNiO₂ and BaNiO₂ are known to exhibit orthorhombic crystal structures, but if the tetragonal structure can be achieved in thin films grown on substrates having tetragonal symmetry, there may be a chance of realizing the above mentioned scenario and hence high T_c s±-wave superconductivity.

In the actual calculation, we assume tetragonal symmetry of the lattice with the lattice constants fixed at those of SrTiO₃ LSAT, and LaAlO₃ substrates, and perform first principles band calculation, taking into account the effect of partial substitution through virtual crystal approximation.  Interestingly, our phonon calculations suggest that the P4/mmm symmetry as in LaNiO₂ is dynamically stable for La_1-xSr_xNiO₂ for the entire range of x and on all three substrates. Based on the obtained electronic band structure, we construct a five orbital model (including all Ni 3d orbitals). We also estimate the intraorbital and interorbital interaction values using constrained RPA. We apply the fluctuation exchange approximation to the models to take into account the effect of the spin fluctuations, and solve the linearized Eliashberg equation at a fixed temperature of T=0.01eV. The obtained eigenvalue of the linearized equation serves as a measure for the expected T_c which indeed suggests a possibility of high T_c superconductivity in the heavily hole-doped regime, especially for the substrates with small lattice constants.

References

[1] D. Li et al., Nature (London) 572, 624 (2019).

[2] M. R. Norman, Physics 13, 85 (2020).

[3] H. Sun et al., Nature 621, 493 (2023).

[4] H. Sakakibara et al., Physical Review B 109, 144511 (2024).

[5] Y. Zhu et al., arXiv:2311.07353.

[6] M. Nakata et al., Phys. Rev. B 95, 214509 (2017).

[7] H. Sakakibara et al., Phys. Rev. Lett. 125, 077003 (2020).

[8] X. Wu et al., Phys. Rev.B 101, 060504 (2020).

[9] M. Kitatani et al., npj Quantum Materials 5, 59 (2020).

[10] N. Kitamine et al.,Phys. Rev. Res. 2, 042032(R) (2020).

[11] H. Shinaoka et al., Phys. Rev. B 92, 195126 (2015).

[12] K. Yamazaki et al., Phys. Rev. Res. 2, 033356 (2020).

Acknowledgment

We are supported by JSPS KAKENHI Grant No. JP22K03512 (H. S.), JP22K04907 (K. K.), JP24K01333. The computing resource is supported by the supercomputer system (system-B) in the Institute for Solid State Physics, the University of Tokyo, and the supercomputer of Academic Center for Computing and Media Studies (ACCMS), Kyoto University.

Abstract Body

The discovery of superconductivity in cuprates with a critical temperature above 30 K has sparked extensive research into this family of materials. Not long after, nickelate compounds with Ni1+ ions were proposed to be the “cousin” of cuprates due to their similar electronic structures. This naturally led us to believe that the nickelates might also possess high-Tc superconductivity. Decades of research followed in search of superconducting nickelates, and in 2019, superconductivity was finally discovered in the nickelate thin films with an infinite-layer structure, Nd0.8Sr0.2NiO2, which requires complicated sample growth methodologies [1-3]. Since then, the nickelate superconductors have attracted attention and fueled lots of investigations, not to mention the boost by the latest discovery of bulk superconducting nickelates under pressure. In this talk, the recent advances in different nickelate superconductor families will be introduced. Particularly intriguing is the magnetotransport data, which has revealed a puzzling violation of the Pauli limit in the upper critical field of La-based nickelates [4]. To shed light on this phenomenon, we have meticulously measured the temperature dependence of self-field critical current density (Jc) and analyzed the data within a theoretical framework of superconductors in the thin-film limit [5], seeking clues on the possible SC pairing mechanism. Furthermore, the in-plane magnetic field angular dependence of Jc in different nickelate compounds will be discussed. These samples have demonstrated different degrees of unexpected C2 rotational symmetry in addition to the anticipated C4 symmetry dictated by the crystal symmetry [6,7]. These remarkable features exhibited by infinite-layer nickelate superconductors has urged the continued explorations in order to reveal the whole picture of SC in these systems. This work is in collaboration with Km Rubi, M. Pierre, Z. T. Zhang, T. Heil, J. Deuschle, P. Nandi, S. K. Sudheesh, Z. S. Lim, Z. Y. Luo, M. Nardone, A. Zitouni, P. A. van Aken, M. Goiran, Maxime Leroux, S. K. Goh, W. Escoffier, Changjian Li, and Neil Harrison.

References

[1] D. Li et al., Nature 572, 624-627 (2019)

[2] S. Zeng et al., Phs. Rev. Lett. 125, 147003 (2020)

[3] S. Zeng et al., Sci. Adv. 8, eabl9927 (2022)

[4] L. E. Chow, KYY et al., arXiv:2301.07606 (2023)

[5] E. F. Talantsev & J. L. Tallon, Nat. Commun. 6, 7820 (2015)

[6] L. E. Chow et al., arXiv:2201.10038 (2022)

[7] L. E. Chow, Km Rubi, KYY et al., arXiv:2301.07606 (2023)

Figure 1. Field-temperature phase diagrams of infinite-layer nickelates, indicating violation of Pauli-limit in La-based nickelates. The x- and y-axes are normalized to the Tc for easier comparisons between different systems. Figure adapted from [4].

Keywords: Infinite layer nickelates, superconductivity, electronic transport