Anharmonic phonons and second-order phase-transitions

Anharmonic effects can generally be treated within perturbation theory. Such an approach breaks down when the harmonic solution is dynamically unstable or when the anharmonic corrections of the phonon energies are larger than the harmonic frequencies themselves. This situation occurs near lattice-related second-order phase-transitions such as charge-density-wave (CDW) or ferroelectric instabilities or in H-containing materials, where the large zero-point motion of the protons results in a violation of the harmonic approximation. Interestingly, even in these cases, phonons can be observed, measured, and used to model transport properties. In order to treat such cases, we developed a stochastic implementation of the self-consistent harmonic approximation valid to treat anharmonicity in the nonperturbative regime and to obtain, from first-principles, the structural, thermodynamic and vibrational properties of strongly anharmonic systems [1,2]. I will present applications to the ferroelectric transitions in SnTe [2], to the CWD transitions in NbS2 and NbSe2 (in bulk and monolayer) and to the hydrogen-bond symmetrization transition in the superconducting hydrogen sulfide system [3], that exhibits the highest Tc reported for any superconductor so far. In all cases we are able to predict the transition temperature (pressure) and the evolution of phonons with temperature (pressure).

[1] I. Errea, M. Calandra, F. Mauri, Phys. Rev. Lett. 111, 177002 (2013) and Phys. Rev. B 89, 064302 (2014)

[2] R. Bianco, I. Errea, L. Paulatto, M. Calandra, F. Mauri, Phys. Rev. B 96, 014111 (2017)

[3] I. Errea, M. Calandra, C. J. Pickard, J. R. Nelson, R. J. Needs, Y. Li, H. Liu, Y. Zhang, Y. Ma, F. Mauri, Nature 532, 81 (2016)

Bio: Professor Francesco Mauri is a Full Professor at the Sapienza University. Following a master’s degree in physics from the University of Pisa and Scuola Normale Superiore, he obtained his PhD from Université de Geneva, and from Sissa in Trieste in 2000. Following postdoctoral stays as Miller Fellow at University of California, Berkeley, he obtained a Full professorship at the Université Pierre et Marie Curie in Paris in 1998. He moved to Rome at Sapienza in 2015. He is an expert in the description of physical properties of materials by first-principles electronic structure methods, with emphasis on the simulation of spectroscopy data (NMR, EPR, Raman, IR, Xanes), on the electron-phonon and phonon-phonon interactions, on the thermal and electronic transport, on superconductivity, on graphene and 2D materials.