Research

Novel theories

Electron-phonon coupling

I created and assessed a consistent theoretical framework for computing the zero-point motion renormalization (ZPR) within the harmonic adiabatic approximation including terms beyond the rigid-ion approximation [8]. I showed that the adiabatic approximation breaks down in polar materials and derived models to extrapolate the ZPR to infinite momentum samplings [10]. Together with Prof. Antonius, we showed that many-body effects increased by as much as 50% the electron-phonon coupling (EPC) in diamond using a combination of perturbation theory and finite differences [7]. We also investigated the effect of dynamical EPC and anharmonicity in diamond, BN, LiF and MgO, presenting for the first time, their full ab-initio electron-phonon spectral function [11].

Carrier transport

I worked on a consistent general theory for carrier transport, starting from a many-body quantum mechanical framework and making the link with popular formalism and common approximations, including the response to electric and magnetic fields [31]. I also developed the capability of computing electron and hole mobility in semiconductors within the linearized Boltzmann transport equation and applied it to study the transport properties of strained GaN [29,30] and halide perovskites [24,25]. In those papers, I showed that the mobility crucially depends on accurate carrier effective masses. Interestingly, I have also worked on a new method for computing these using perturbation theory [14]. Both approaches could be combined to improve mobility predictions.



Computed carrier mobility in Methylammonium lead triiodide from Ref. [25].

Applications

Rare-earth doped phosphors

I collaborated with Mitsubishi Chemical Corporation (Japan) to understand the puzzling thermal quenching behavior of two Europium doped Barium silicate oxynitrides used in white LEDs. The two materials, despite having similar crystal structures, were showing drastically different emission intensity profiles with increasing temperature. We predicted the emission center positions and showed that thermal quenching appears only in one of the crystals due to auto-ionization of the excited electron into the delocalized conduction band continuum [12]. We later extended this methodology to the systematic study of many Ce- and Eu-doped materials [13,18,20,32].

Phonon-limited superconductivity

We used the EPW code to study the superconducting properties of 2H-NbS2 using the phonon-limited anisotropic Migdal-Eliashberg theory including many-body Coulomb interaction and showed that the system was on the verge of a charge density wave instability [19].



Superconducting gap in MgB2 from [16].

Halide perovskites

In collaboration with Prof. Saidi and Dr Montserrat, we showed that the high-order terms in the electron-phonon expansion terms were crucial in perovskites (used for solar cells applications) by directly comparing three different methods [17]. While at Oxford I also collaborated with Dr M. Schlipf to study the electron-phonon and carrier mobility of halide perovskites. We developed a multi-phonon polar model to predict scattering rates and mass-renormalization parameters [24], and demonstrated that the lower mobility in halide perovskite compared to simple semiconductors was due to forbidden piezoacoustic coupling and softer phonon modes [25]. The existence and position of three predicted dominant phonon modes was later confirmed experimentally [Ref].

Nitrides and oxydes

I studied experimentally (internship in the LumiLab group, Ghent University) and theoretically the optical and absorption properties of Eu-doped oxides and sulphides [3]. I solved why the hole mobility was orders of magnitude lower than the electron mobility in wurtzite GaN and proposed a way to triple the hole mobility using strain engineering. We are now collaborating with the experimental group of Prof. Jena to realize a CMOS device based on this concept and this led to the filing of a patent [1]. This work was selected as Editors’ Suggestion of Physical Review Letters [30] and Physical Review B [29].

Numerical implementations

I am the lead developer of the Electron-Phonon Wannier (EPW) software package [16] and I am/was also actively involved in the development of the Quantum ESPRESSO (QE) [21], Wannier90 [33] and Abinit [15] codes. I led an extensive verification and validation study of the theory and software implementations of the temperature-dependence of optical properties in semiconductors. This allowed me to collaborate with Dr Marini (initiator of the Yambo code) and Prof. Côté. Together, we assessed and improved the agreement between independent first-principles codes [5]. To code in a professional and sustainable way that would ensure continuous integration and stability of the code, I took the initiative to create an automatic computer test-farm composed of many compilers/libraries and various hardware.

This project was highly appreciated by the community and this newly-developed test-farm is now the official one for the QE, Wannier90, SternheimerGW and EPW codes. The EPW website offers tutorials and YouTube videos that I created to ease the learning curve of new users. As a result, many scientists have already shown interest in the software and the forum that I started four years ago (epwforum.uk) has over 420 active members.