Structural phase transitions from first-principles calculations

Sep 11, 2019, 8:30 AM
Keynote Keynote talk


Ion Errea (University of the Basque Country)


Describing atomic vibrations from first-principles accurately is of paramount importance to
understand the thermodynamic and transport properties of solids, especially to understand
structural phase transitions. Phonon dispersions are routinely calculated within the harmonic
approximation, and transport properties can be studied by estimating the electron-phonon
and phonon-phonon interactions within perturbation theory. Nevertheless, whenever the
amplitude of the atomic displacements largely exceeds the range in which the harmonic
potential is valid, the harmonic approximation completely fails without allowing a
perturbative expansion. This clearly hinders any ab initio calculation of structural phase
transitions in these situations
The stochastic self-consistent harmonic approximation (SSCHA) that we have developed
[1,2,3,4] offers an efficient method to calculate vibrational properties of solids even when the
harmonic approximation completely collapses. The method is variational and takes into
account quantum and thermal effects rigorously. With our recent developments on the
SSCHA method [3], we show how phonon frequencies should be defined from the second
derivative of the free energy, which allows calculating the transition temperature of structural
second-order phase transitions. Moreover, the new developments [3] allow calculating thirdorder anharmonic force-constants, which determine thermal properties, beyond the
perturbative limit.
In this lecture we will present the method and several applications of it in superconducting
hydrides, charge-density-wave systems, and thermoelectric materials.
[1] I. Errea, M. Calandra, and F. Mauri, Phys. Rev. Lett. 111 (2013) 177002.
[2] I. Errea, M. Calandra, and F. Mauri, Phys. Rev. B 89 (2014) 064302.
[3] R. Bianco, I. Errea, L. Paulatto, M. Calandra, and F. Mauri, Phys. Rev. B 96 (2017)
[4] L. Monacelli, I. Errea, M. Calandra, and F. Mauri, Phys. Rev. B 98 (2018) 024106.

Topic 6. Theoretical and experimental methods

Primary author

Ion Errea (University of the Basque Country)

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