[Paper Review] Ambient pressure high temperature superconductivity in RbPH$_3$ facilitated by ionic anharmonicity

Đorđe Dangić, Yue‐Wen Fang+4 more|arXiv (Cornell University)|Nov 6, 2024

Theoretical and Computational Physics4 citations

TL;DR

This study predicts ambient-pressure high-temperature superconductivity in RbPH₃ with a critical temperature of ~100 K, enabled by ionic quantum anharmonicity that stabilizes the superconducting phase. Using ab initio calculations with the stochastic self-consistent harmonic approximation (SSCHA), the authors show that quantum fluctuations dynamically stabilize the R3m perovskite phase down to atmospheric pressure, overcoming harmonic instability and enabling high-Tc superconductivity via strong electron-phonon coupling.

ABSTRACT

Recent predictions of metastable high-temperature hydride superconductors give hope that superconductivity at ambient conditions is within reach. In this work, we predict RbPH$_3$ as a new compound with a superconducting critical temperature around 100 K at ambient pressure, dynamically stabilized thanks to ionic quantum anharmonic effects. RbPH$_3$ is thermodynamically stable at 30 GPa in a perovskite $Pm\bar{3}m$ phase, allowing its experimental synthesis at moderate pressures far from the megabar regime. With lowering pressure it is expected to transform to a $R3m$ phase that should stay dynamically stable thanks to quantum fluctuations down to ambient pressures. Both phases are metallic, with the $R3m$ phase having three distinct Fermi surfaces, composed mostly of states with phosphorus and hydrogen character. The structures are held together by strong P-H covalent bonds, resembling the pattern observed in the high-temperature superconducting H$_3$S, with extra electrons donated by rubidium. These results demonstrate that quantum ionic fluctuations, neglected thus far in high-throughput calculations, can stabilize at ambient pressure hydride superconductors with a high critical temperature.

Motivation & Objective

• To identify new ambient-pressure hydride superconductors that are dynamically and thermodynamically stable.
• To investigate the role of ionic quantum anharmonicity in stabilizing high-Tc superconducting phases at low pressures.
• To demonstrate that standard high-throughput screening methods, which neglect anharmonicity, may miss viable superconducting candidates.
• To predict a new superconducting compound, RbPH₃, with a critical temperature near 100 K at ambient pressure.
• To show that quantum fluctuations can stabilize superconducting phases even when harmonic approximations predict dynamical instability.

Proposed method

• Employed ab initio density functional theory (DFT) with the PBE functional and DFT+U corrections for electronic structure calculations.
• Applied the stochastic self-consistent harmonic approximation (SSCHA) to include ionic quantum fluctuations and anharmonic effects in the free energy landscape.
• Performed phonon calculations and electron-phonon coupling analysis using the ALM method to compute the Eliashberg spectral function α²F(ω) and coupling strength λ.
• Used the full Migdal-Eliashberg formalism to compute Tc, including non-local Coulomb interactions via the screened interaction W(ε,ε') and renormalized Coulomb parameter μ*.
• Conducted thermodynamic stability analysis via the convex hull construction in the Rb-P-H phase diagram at various pressures.
• Computed the Coulomb interaction W(ε,ε') in the random phase approximation (RPA) and derived μ* = 0.26 using the Morel-Anderson formula with a high cutoff ωc = 19.7 eV.

Figure 1: (a) Ternary phase diagram for the Rb-P-H system neglecting the zero-point energy at 25 GPa. Stoichiometries on the convex hull are represented with red circles. RbPH 3 is denoted by the star. The color scheme of the background denotes the enthalpy from the convex hull. (b) Continuous lines

Experimental results

Research questions

• RQ1 Can ionic quantum anharmonicity stabilize high-Tc superconducting phases in hydrides at ambient pressure?
• RQ2 Is RbPH₃ dynamically stable under ambient conditions when anharmonic effects are included?
• RQ3 What is the critical temperature of RbPH₃ in the R3m phase at zero pressure, and how does it compare to harmonic approximations?
• RQ4 How do electron-phonon coupling and Coulomb interactions influence Tc in RbPH₃?
• RQ5 Can the inclusion of anharmonic effects in high-throughput screening identify new ambient-pressure superconductors previously missed?

Key findings

• The R3m phase of RbPH₃ is dynamically stable at ambient pressure due to ionic quantum anharmonicity, despite being a saddle point in the harmonic approximation.
• RbPH₃ exhibits a superconducting critical temperature of 101 K at 0 GPa when using μ* = 0.26 in the full Migdal-Eliashberg approach.
• The electron-phonon coupling strength λ is enhanced by the covalent P-H bonding network and metallic character of the R3m phase.
• The Coulomb interaction parameter μ* is estimated at 0.26, significantly higher than typical values due to the high Debye frequency in hydrides.
• The R3m phase has three distinct Fermi surfaces dominated by P and H character, supporting strong electron-phonon coupling.
• RbPH₃ is thermodynamically stable at 30 GPa in the Pm3̄m perovskite phase and transforms to the R3m phase upon pressure reduction, with the latter stabilized by quantum fluctuations.

Figure 2: Structure of (a) the $R3m$ phase at 0 GPa and (b) the $Pm\bar{3}m$ phase at 30 GPa of RbPH 3 . The contours show the electronic localization function (ELF) at the value of 0.8. The visualization of the structure and the ELF is done using VESTA [ 41 ] .