The hexagonal pnictide CaAgP is a promising candidate for a topological line-node semimetal. This compound was synthesized to be the ZrNiAl-type crystal structure. Subsequently, it was demonstrated that the material possesses a topologically nontrivial band structure, by using first-principles calculations [1]. It is also reported that surface states are expected to exist in the nodal ring, resulting in drumhead dispersion along the (001) plane. The bottom of the drumhead surface can be regarded as a flat band.
Recently, an experimental study has revealed that the surface states of Pd-doped CaAgP (CaAg1-xPdxP) exhibit unconventional superconductivity around 1.5 K [2]. They also carried out the soft point contact experimental study, that is known to be surface-sensitive measurement, and observed that the so-called bell-shaped conductance spectra, that is a manifestation of the unconventional SC.
In contrast, a recent angle-resolved photoemission spectroscopy (ARPES) measurement has asserted that the bulk band gap and surface states in CaAgP are topologically trivial, as evidenced by a comparison with first-principles calculations [3]. Furthermore, they pointed out that the conventional LDA/GGA approach is incapable of reproducing the ARPES results, while a DFT calculation based on the Heyd-Scuseria-Ernzerhof (HSE) hybrid exchange-correlation functional [4] may give an accurate description of the topological characteristics of the bands in proximity to EF within this system. This observation implies that there is still room for consideration in the theoretical predictions for topological materials. In addition, the nature of superconductivity and the bell-shaped conductance spectra observed in CaAg1-xPdxP should be re-examined in light of these findings.
Motivated by these backgrounds, we theoretically study the superconducting state of CaAg1-xPdxP systems based on first-principles calculations and the Eliashberg theory. First, we obtain the electronic state consistent with the ARPES measurements by using the so-called Yukawa-screened Perdew-Burke-Ernzerhof (YS-PBE) exchange functional [5] implemented in the WIEN2k code [6], which is known to yield similar results to the HSE. Then we construct an effective tight-binding Hamiltonian for the system using the Wannier 90 code [7]. We solve the linearized Eliashberg equation for several superconducting pairing states to elucidate the nature of the superconducting state realized in CaAg1-xPdxP. We will also present the theoretical tunneling spectra assuming the obtained plausible superconducting state.
[1] A. Yamakage et al., J. Phys. Soc. Jpn. 85, 013708 (2016).
[2] R. Yano et al., Nat. Commun. 14, 6817 (2023).
[3] N. Xu et al., Phys. Rev. B 97, 161111(R) (2018).
[4] J. Heyd et al., J. Chem. Phys. 118, 8207 (2003).
[4] F. Tran and P. Blaha, Phys. Rev. B 83, 235118 (2011).
[5] P. Blaha et al., WIEN2k, An Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties (Tech. Univ. Wien, Vienna, 2001).
[6] G. Pizzi et al., J. Phys.: Condens. Matter 32, 165902 (2020).
Keywords: Topological superconductivity, First-principles calculations, Eliashberg theory