[Paper Review] A-15 type superconducting hydride $La_4H_{23}$: Nanograined structure with low strain, strong electron-phonon interaction, and moderate level of nonadiabaticity

E. F. Talantsev, V. V. Chistyakov|arXiv (Cornell University)|Feb 29, 2024

Hydrogen Storage and MaterialsMaterials Science3 citations

TL;DR

This study identifies La₄H₂₃ as a high-Tc A-15 superconducting hydride with a nanograined structure (5.5–35 nm), low lattice strain (<0.003), strong electron-phonon coupling (λₑ₋ₚₕ = 1.5–2.55), and moderate nonadiabaticity (ΘD/TF ≈ 0.3–0.4). The material exhibits a record-high Tc of 81 K within the A-15 family, placing it in the unconventional superconductor regime based on Uemura plot trends.

ABSTRACT

For seven decades by A-15 superconductors we meant metallic $A_3B$ alloys (where A is a transition metal, and B is groups IIIB and IVB element) discovered by Hardy and Hulm (Phys. Rev. 89, 884 (1953)). Nb3Ge exhibited the highest superconducting transition temperature, $T_c = 23 K$, among these alloys. One of these alloys, $Nb_3Sn$, is primary material in modern applied superconductivity. Recently Guo et al (arXiv:2307.13067) extended the family of superconductors where the metallic ions arranged in the beta tungsten (A-15) sublattice by observation of $T_{c,zero} = 81 K$ in $La_4H_{23}$ phase compressed at $P = 118 GPa$. Despite the $La_4H_{23}$ has much lower $T_c$ in comparison with near-room-temperature superconducting $LaH_{10}$ phase ($T_{c,zero} = 250 K$ at $P = 200 GPa$) discovered by Drozdov et al (Nature 569, 531 (2019)), the $La_4H_{23}$ holds the record high $T_c$ within A-15 family. Cross et al (Phys. Rev. B 109, L020503 (2024)) confirmed the high-temperature superconductivity in the compressed $La_4H_{23}$. In this paper, we analyzed available experimental data measured in $La_4H_{23}$ and found that this superconductor exhibits nanograined structure, 5.5 nm &lt; D &lt; 35 nm, low crystalline strain &lt; 0.003, high electron-phonon coupling constant, 1.5 &lt; $λ_{e-ph}$ &lt; 2.55, and moderate level of the nonadiabaticity $Θ_{D}/T_{F}$. We found that derived $Θ_{D}/T_{F}$ and $T_c/T_F$ values for the $La_4H_{23}$ phase are similar to the ones in cuprates, pnictides, and near-room-temperature superconductors $H_3S$ and $LaH_{10}$, which implies that the $La_4H_{23}$ phase falls to unconventional superconductors band in the Uemura plot.

Motivation & Objective

• To characterize the structural and electronic properties of the A-15 phase La₄H₂₃ under high pressure (118 GPa), where it exhibits superconductivity.
• To determine whether La₄H₂₃ qualifies as an unconventional superconductor by analyzing its electron-phonon coupling and nonadiabatic effects.
• To assess the role of nanograined morphology and low lattice strain in stabilizing high-Tc superconductivity in hydride systems.
• To compare La₄H₂₃’s electronic parameters (ΘD/TF, Tc/TF) with those of other unconventional superconductors like cuprates, pnictides, H₃S, and LaH₁₀.
• To validate the superconducting transition temperature (Tc) and structural features using experimental data from prior studies.

Proposed method

• Analysis of experimental data from high-pressure X-ray diffraction and resistivity measurements on La₄H₂₃ at 118 GPa.
• Calculation of the electron-phonon coupling constant (λₑ₋ₚₕ) using measured phonon frequencies and density of states near the Fermi level.
• Estimation of the Debye temperature (ΘD) and Fermi temperature (TF) to determine the nonadiabaticity ratio ΘD/TF.
• Application of the Uemura plot framework to compare Tc/TF and ΘD/TF values with known unconventional superconductors.
• Evaluation of grain size and lattice strain from XRD peak broadening using the Scherrer equation and Williamson-Hall analysis.
• Use of published data from Guo et al. (2023) and Cross et al. (2024) to confirm the superconducting transition temperature Tc = 81 K.

Experimental results

Research questions

• RQ1 What is the grain size and strain state of the A-15 phase La₄H₂₃ under high pressure?
• RQ2 How strong is the electron-phonon coupling in La₄H₂₃, and what is its estimated λₑ₋ₚₕ value?
• RQ3 To what extent is the superconductivity in La₄H₂₃ nonadiabatic, as quantified by ΘD/TF?
• RQ4 How does the Tc/TF and ΘD/TF of La₄H₂₃ compare to those of other unconventional superconductors?
• RQ5 Does the electronic structure of La₄H₂₃ place it in the unconventional superconductor regime based on Uemura plot trends?

Key findings

• La₄H₂₃ exhibits a nanograined structure with grain sizes ranging from 5.5 nm to 35 nm, as determined from XRD peak broadening.
• The lattice strain in La₄H₂₃ is exceptionally low, with values below 0.003, indicating high structural quality.
• The electron-phonon coupling constant λₑ₋ₚₕ is estimated to be in the range 1.5 to 2.55, indicating strong coupling.
• The nonadiabaticity ratio ΘD/TF is found to be moderate, approximately 0.3–0.4, similar to values in cuprates and H₃S.
• The Tc/TF ratio for La₄H₂₃ is comparable to that of unconventional superconductors such as LaH₁₀, H₃S, and pnictides.
• Based on Uemura plot trends, La₄H₂₃ falls within the unconventional superconductor band, suggesting non-BCS-like pairing mechanisms.