Session F20: Electrons, Phonons, Electron-Phonon Scattering, and Phononics III

Live

In this work, the temperature renormalization of the bulk band structure of a topological crystalline insulator, SnTe [1], is calculated from first principles [2], including the effect of thermal expansion on the electron-phonon self-energy. We show that thermal expansion and electron-phonon interaction tend to decrease the band gap and drive SnTe closer to the phase transition to a normal insulating phase, in contrast to electron-electron interaction. We find that the band gap renormalization due to electron-phonon interaction exhibits a nonlinear dependence on temperature as the material approaches the topological phase transition, while the lifetimes of the conduction band states near the band edge show a nonmonotonic behavior with temperature. Our results show that electron-phonon interaction in topological materials is considerably affected by the thermal variations of their bulk electronic states. These effects should have important implications on bulk electronic and thermoelectric transport in SnTe and other topological insulators.


[1] T. H. Hsieh et al., Nat. Commun. 3, 982 (2012).
[2] J. D. Querales-Flores, P. Aguado-Puente, D. Dangić et al., Phys. Rev. B 101, 235206 (2020).   

Live

Nonlinear phononics provides an avenue for ultrafast control of crystal structure and properties. Anharmonic coupling between IR and Raman phonons in the lattice potential opens pathways for light to induce sizable unidirectional excitations of Raman phonons, altering the crystal structure and therefore its properties. The magnitude of the change in structure and properties is controlled by a combination of key intrinsic microscopic quantities. Strategies for tuning these microscopic quantities, and for engineering an enhanced response, are currently unknown.

Using theory and first-principles calculations, we investigate how the nonlinear phononics response evolves with pressure and strain in SrTiO₃ and LaAlO₃. Specifically, we track how the strength of the anharmonic coupling between, and force constants of, the IR and Raman modes, and the strength of the coupling between the light pulse and the excited IR mode evolve with pressure and epitaxial strain. Our results suggest that the amplitude of key structural distortions at equilibrium determines the strength of dynamical anharmonicity in the optically excited material, and more generally, that the nonlinear phononics response can be tailored via pressure and epitaxial strain.   

Live

We group materials into five symmetry classes and determine in which of these classes will phonons generically carry angular momentum. In some classes of materials, phonons acquire angular momentum via the forces induced by relative displacements of atoms out of their equilibrium positions. However, for other materials, such as ferromagnetic iron, phonon angular momentum arises from the forces induced by relative velocities of atoms. These latter effects are driven by the spin-orbit interaction.   

Live

We studied the angular momentum of phonons in BaTiO3 from first principles. We find that in the tetragonal phase of BaTiO3 the average phonon angular momentum is significantly larger in directions perpendicular to polarization than parallel. Furthermore, there is additional anisotropy within the plane perpendicular to polarization. We analyze the interatomic force constants and build a simple effective model to reveal the origin of the anisotropies. Finally, we suggest experiments using BaTiO3 as an electric field controllable phonon angular momentum reservoir.   

Live

The need for an accurate description of the vibrational properties of 1D materials is strongly motivated by the growing interest in low-dimensionality systems - semiconductor nanowires in particular - with vibrational spectroscopies probing accurately their properties.
In 3D polar materials, long-wavelength phonons (exactly those probed by IR and Raman spectroscopies) undergo a frequency splitting in their longitudinal and transverse optical modes which depends on the Born effective charges and the dielectric response. This splitting is driven by the need to build up an electrostatic energy density for longitudinal optical phonons. In 2D the splitting has been shown to depend upon the phonon wavevector and to vanish at small momenta [1]; here, we show that it also vanishes in 1D, but with a different asymptotic behavior. We develop an analytical model to describe the relevant physics, and compare it with first-principles simulations in realistic systems as a function of the nanowire diameter. The present work not only provides useful insight into the vibrational physics of nanowires but also a ready-to-use tool for the experimental community to encourage further studies.

[1] T. Sohier, M. Gibertini, M. Calandra, F. Mauri, and N. Marzari, Nano Lett. 17, 3758 (2017).   

Live

The angular momentum of static vibrational modes in solids manifests, among others, at the Brillouin zone corners of non-centrosymmetric crystal lattices and in ionic crystals in presence of an external magnetic field.
In this work, we show that also in molecules, under certain conditions, the angular momentum of vibrational modes can be nonzero. In particular we enforce the dynamical scheme of linear response to assess the vibrational properties of molecules.
In this framework we prove that an intrinsic angular momentum can emerge in the optical modes of non-collinear magnetic clusters as a result of the electron-phonon interaction.
These vibrational modes are suitable for detection in polarized Raman spectroscopy.   

Live

In the prototypical Peierls transition system, a half-filled 1-dimensional chain of hydrogen atoms, the metallic configuration in which all nuclei are equally distant has been shown to be unstable to ionic displacements, leading to an insulating, dimerized ground state composed of H2 pairs. Predicting Peierls transitions in systems is of great interest due to they emerge as a consequence of the SSH model of electron-phonon coupling and due to their association with charge density waves . In this talk, we focus on the ability of predicting the presence of such instabilities purely through Density Functional Theory (DFT) calculations. DFT results directly indicative of Peierls instabilities, such as pronounced Kohn anomalies in the phonon spectra and potential energy surfaces, are presented for a selection of systems (H, C, Na). Furthermore, the influence of the electron filling level on the geometry of the ground state unit cell is explored; a trimerized structure is found to be energetically favorable at third-filling.   

Live

Thermoelectric materials enable direct conversion of waste heat into electrical energy. The conversion efficiency is inversely proportional to the thermal conductivity, which is generally dominated by phonons in semiconductors. Zintl compounds AMg₂X₂ constitute a class of new thermoelectric compounds with excellent thermoelectric performance in n-type Mg₃(Sb,Bi)₂ alloys, with zT values up to 1.6 reported so far. Mg₃Sb₂ exhibits very low lattice thermal conductivity (~1-1.5 W/m/K at 300K), comparable with PbTe and Bi₂Te₃, despite a much lighter average ionic mass. We report on neutron scattering and first-principles studies of the lattice dynamics of AMg₂X₂. Inelastic neutron scattering measurements provided the temperature dependence of the phonon density of states (DOS). Extra peaks and overall softer phonons were found at low frequency in Mg₃Sb₂ and Mg₃Bi₂ compared to CaMg₂X₂ or YbMg₂X₂. Combined with simulations, we highlight the importance of a specific soft Mg-X chemical bond that suppresses phonon group velocities and drastically enlarges the scattering phase-space, enabling the threefold suppression in thermal conductivity.   

Live

We have recently shown that soft transverse optical (TO) phonons play the key role in the high thermoelectric figure of merit (ZT) of PbTe: they strongly suppress the lattice thermal conductivity [1], but do not degrade the electronic transport properties [2]. Using first principles calculations, here we investigate how driving PbTe closer to the soft-mode phase transition via lattice expansion affects the thermoelectric properties and ZT of n-type PbTe. We find that the very soft TO phonons do not deteriorate the electronic thermoelectric properties of PbTe. Scattering between electrons and soft phonons remains weak even very near the phase transition as a result of symmetry restrictions. In contrast, the phonon softening considerably reduces the lattice thermal conductivity of expanded PbTe and doubles the ZT with respect to the equilibrium structure. Our results indicate that the ZT of materials with soft phonon modes that interact strongly with other phonons but weakly with conducting electronic states could be significantly increased by bringing these materials closer to the soft-mode phase transition.

[1] R. M. Murphy et al, Phys. Rev. B 93, 104304 (2016)

[2] J. Cao et al, Phys. Rev. B 98, 205202 (2018)   

Live

Using a combination of inelastic neutron/x-ray scattering and first-principles simulations, we investigate anharmonic effects on phonons across phase transitions. This presentation will focus on transitions potentially associated with soft mode condensation. In particular, we will highlight results on phonons in SnS and SnSe across the high-temperature Pnma-Cmcm transition, in VO2 across the rutile-M1 metal-insulator transition, and FeS across its magnetic and structural transitions. The effects of phonon anharmonicity on thermal transport and thermodynamics will be discussed.   

Live

Iron (Fe) is a polymorph with a great variety of applications. At the lowest pressures and temperatures it has the bcc structure whereas at high temperatures it crystallizes in the fcc structure. The bcc phase is ferromagnetic below its Curie temperature of 1043 K and it remains in the bcc phase while paramagnetic before finally transforming to fcc at 1183 K. We performed density functional theory (DFT) molecular dynamics calculations in the Born-Oppenheimer approximation of ferromagnetic and paramagnetic bcc iron at temperatures close to the Curie temperature. Effective interatomic forces constants up to fifth nearest-neighbors were calculated from the dynamics calculations and used to generate phonon dispersions and density of states curves. A softening of the phonons in the Γ to N direction is observed, and we discuss this and other trends.   

Live

Structural chirality plays an important role in determining various physical properties of materials and their potential functionalities. However, little consideration has been given to chirality-driven lattice and thermal properties. Here, the role of chirality in describing non-trivial topological vibrational properties of materials will be discussed. A comparison of the vibrational properties of materials with and without structural chirality will be developed and will be supported by corresponding inelastic neutron scattering measurements. The non-trivial topological features will be illustrated with the help of Wannier charge centers and non-zero Chern numbers.   

Live

Multi-valley structure is known as one of the favorable band structures for enhancing thermoelectric efficiency. N-type TiS₂, also has electronic structure with multi-valley character, and its power factor is relatively high: ~40 μW/cmK² at room temperature. In previous experimental studies, the electrical resistivity of TiS₂ has been found to exhibit strong temperature dependence of ~T². One of the previous studies indicated that inter-valley scattering among conduction band valleys may be related to the peculiar temperature dependence of the electrical resistivity [1].
In the present study, we perform DFT calculations and calculate the electronic transport properties by considering the electron-phonon scattering effect using the EPW code [2]. As a target material in this study, we consider not TiS₂, but instead its analogous compound ZrS₂. One of the results suggests that inter-valley scattering has a larger contribution to electrical resistivity than intra-valley scattering. In this presentation, we will discuss the role of the inter-valley scattering played in the electronic transport properties of ZrS₂.

[1] P. C. Klipstein et al., J. Phys. C: Solid State Phys. 14, 4067 (1981).
[2] S. Ponce, E. R. Margine and F. Giustino, Phys. Rev. B 97, 121201 (2018).