Strong anharmonic and quantum effects in Pm3¯n AlH3 under high pressure: A first-principles study

Pugeng Hou,Francesco Belli,Ion Errea,Raffaello Bianco

doi:10.1103/physrevb.103.134305

Abstract

Motivated by the absence of experimental superconductivity in the metallic Pm-3n phase of AlH3 despite the predictions, we reanalyze its vibrational and superconducting properties at pressures above 99 GPa making use of first-principles techniques. In our calculations based on the self-consistent harmonic approximation method that treats anharmonicity beyond perturbation theory, we predict a strong anharmonic correction to the phonon spectra and demonstrate that the superconducting critical temperatures predicted in previous calculations based on the harmonic approximation are strongly suppressed by anharmonicity. The electron-phonon coupling concentrates on the lowest-energy hydrogen-character optical modes at the X point of the Brillouin zone. As a consequence of the strong anharmonic enhancement of their frequency, the electron-phonon coupling is suppressed by at least a 30%. The suppression in {\lambda} makes Tc smaller than 4.2 K above 120 GPa, which is well consistent with the experimental evidence. Our results underline that metal hydrides with hydrogen atoms in interstitial sites are subject to huge anharmonic effects.

Highlights

IntroductionMotivated by the quest for metallic and superconducting hydrogen at very high pressures [1], a combination of first-principles structural predictions and calculations of the electron-phonon interaction has led in the last years to the prediction of many superconducting hydrides with high values of the superconducting critical temperature Tc [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]
In our calculations based on the self-consistent harmonic approximation method that treats anharmonicity beyond perturbation theory, we predict a strong anharmonic correction to the phonon spectra and demonstrate that the superconducting critical temperatures predicted in previous calculations based on the harmonic approximation are strongly suppressed by anharmonicity
For the same lattice parameter, the pressure obtained from the classical calculation based on the Born-Oppenheimer energy surface (BOES) is always about 10 GPa lower than the quantum result obtained with the stochastic selfconsistent harmonic approximation (SSCHA)

Summary

Introduction

Motivated by the quest for metallic and superconducting hydrogen at very high pressures [1], a combination of first-principles structural predictions and calculations of the electron-phonon interaction has led in the last years to the prediction of many superconducting hydrides with high values of the superconducting critical temperature Tc [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]. Critical temperatures above 200 K have been observed in sulfur [25], lanthanum [26,27], and yttrium [28,29,30] superhydrides at pressures exceeding 100 GPa. A mixture of C-H-S has reached room-temperature superconductivity at pressures above 250 GPa [31], showing that there is lots of room for further increase in Tc among ternary compounds [19]. The discoveries of high-Tc superconductivity in sulfur, lanthanum, and yttrium hydrides had been anticipated by ab initio calculations [14,15,17,32]

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Results

Conclusion