Most recent publications
 | Hyungjun Lee, Samuel Poncé, Kyle Bushick, Samad Hajinazar, Jon Lafuente-Bartolome, Joshua Leveillee, Chao Lian, Francesco Macheda, Hari Paudyal, Weng Hong Sio, Marios Zacharias, Xiao Zhang, Nicola Bonini, Emmanouil Kioupakis, Elena R Margine, Feliciano Giustino Electron-phonon physics from first principles using the EPW code Journal Article In: npj Computational Materials, vol. 8, no. 156, 2023. @article{Lee2023,EPW is an open-source software for ab initio calculations of electron–phonon interactions and related materials properties. The code combines density functional perturbation theory and maximally localized Wannier functions to efficiently compute electron–phonon coupling matrix elements, and to perform predictive calculations of temperature-dependent properties and phonon-assisted quantum processes in bulk solids and low-dimensional materials. Here, we report on significant developments in the code since 2016, namely: a transport module for the calculation of charge carrier mobility under electric and magnetic fields using the Boltzmann transport equation; a superconductivity module for calculations of phonon-mediated superconductors using the anisotropic multi-band Eliashberg theory; an optics module for calculations of phonon-assisted indirect transitions; a module for the calculation of small and large polarons without supercells; and a module for calculating band structure renormalization and temperature-dependent optical spectra using the special displacement method. For each capability, we outline the methodology and implementation and provide example calculations. |
 | Marios Zacharias, George Volonakis, Feliciano Giustino, Jacky Even Anharmonic lattice dynamics via the special displacement method Journal Article In: Physical Review B, vol. 108, iss. 3, pp. 035155, 2023. @article{Zacharias2023,On the basis of the self-consistent phonon theory and the special displacement method, we develop an approach for the treatment of anharmonicity in solids. We show that this approach enables the efficient calculation of temperature-dependent anharmonic phonon dispersions, requiring very few steps to achieve minimization of the system's free energy. We demonstrate this methodology in the regime of strongly anharmonic materials, which exhibit a multiwell potential energy surface, like cubic SrTiO3, CsPbBr3, CsPbI3, CsSnI3, and Zr. Our results are in good agreement with experiments and previous first-principles studies relying on stochastic nonperturbative and molecular dynamics simulations. We achieve a very robust workflow by using harmonic phonons of the polymorphous ground state as the starting point and an iterative mixing scheme of the dynamical matrix. We also suggest that the phonons of the polymorphous ground state might provide an excellent starting approximation to explore anharmonicity. Given the simplicity, efficiency, and stability of the present treatment to anharmonicity, it is especially suitable for use with any electronic structure code and for investigating electron-phonon couplings in strongly anharmonic systems. |
 | Joshua Leveillee, Xiao Zhang, Emmanouil Kioupakis, Feliciano Giustino Ab initio calculation of carrier mobility in semiconductors including ionized-impurity scattering Journal Article In: Physical Review B, vol. 107, iss. 12, pp. 125207, 2023. @article{Leveillee2023,The past decade has seen the emergence of ab initio computational methods for calculating phonon-limited carrier mobilities in semiconductors with predictive accuracy. More realistic calculations ought to take into account additional scattering mechanisms such as, for example, impurity and grain-boundary scattering. In this paper, we investigate the effect of ionized-impurity scattering on the carrier mobility. We model the analytical impurity potential parameterized from first principles by a collection of randomly distributed Coulomb scattering centers, and we include this relaxation channel into the ab initio Boltzmann transport equation, as implemented in the EPW code. We demonstrate this methodology by considering silicon, silicon carbide, and gallium phosphide, for which detailed experimental data are available. Our calculations agree well with experiments over a broad range of temperatures and impurity concentrations. For each compound investigated here, we compare the relative importance of electron-phonon scattering and ionized-impurity scattering, and we critically assess the reliability of Matthiessen's rule. We also show that an accurate description of dielectric screening and carrier effective masses can improve quantitative agreement with experiments. |
 | Weng Hong Sio, Feliciano Giustino Polarons in two-dimensional atomic crystals Journal Article In: Nature Physics, vol. 19, pp. 629-636, 2023. @article{Sio2023,Polarons are quasiparticles that emerge from the interaction of fermionic particles with bosonic fields. In crystalline solids, polarons form when electrons and holes become dressed by lattice vibrations. While experimental signatures of polarons in bulk three-dimensional materials abound–, only rarely have polarons been observed in two-dimensional atomic crystals. Here, we shed light on this asymmetry by developing a quantitative ab initio theory of polarons in atomically thin crystals. Using this conceptual framework, we determine the real-space structure of the recently observed hole polaron in hexagonal boron nitride, discover a critical condition for the existence of polarons in two-dimensional crystals and establish the key materials descriptors and the universal laws that underpin polaron physics in two dimensions. |
 | Jon Lafuente-Bartolome, Chao Lian, Weng Hong Sio, Idoia G Gurtubay, Asier Eiguren, Feliciano Giustino Unified approach to polarons and phonon-induced band structure renormalization Journal Article In: Physical Review Letters, vol. 129, iss. 7, pp. 076402, 2022. @article{Lafuente-Bartolome2022b,Ab initio calculations of the phonon-induced band structure renormalization are currently based on the perturbative Allen-Heine theory and its many-body generalizations. These approaches are unsuitable to describe materials where electrons form localized polarons. Here, we develop a self-consistent, many-body Green’s function theory of band structure renormalization that incorporates localization and self-trapping. We show that the present approach reduces to the Allen-Heine theory in the weak-coupling limit, and to total energy calculations of self-trapped polarons in the strong-coupling limit. To demonstrate this methodology, we reproduce the path-integral results of Feynman and diagrammatic Monte Carlo calculations for the Fröhlich model at all couplings, and we calculate the zero point renormalization of the band gap of an ionic insulator including polaronic effects. |
 | Weng Hong Sio, Feliciano Giustino Unified ab initio description of Fröhlich electron-phonon interactions in two-dimensional and three-dimensional materials Journal Article In: Physical Review B, vol. 105, iss. 11, pp. 115414, 2022. @article{Sio2022,Ab initio calculations of electron-phonon interactions including the polar Fröhlich coupling have advanced considerably in recent years. The Fröhlich electron-phonon matrix element is by now well understood in the case of bulk three-dimensional (3D) materials. In the case of two-dimensional (2D) materials, the standard procedure to include Fröhlich coupling is to employ Coulomb truncation, so as to eliminate artificial interactions between periodic images of the 2D layer. While these techniques are well established, the transition of the Fröhlich coupling from three to two dimensions has not been investigated. Furthermore, it remains unclear what error one makes when describing 2D systems using the standard bulk formalism in a periodic supercell geometry. Here we generalize previous work on the ab initio Fröhlich electron-phonon matrix element in bulk materials by investigating the electrostatic potential of atomic dipoles in a periodic supercell consisting of a 2D material and a continuum dielectric slab. We obtain a unified expression for the matrix element, which reduces to the existing formulas for three-dimensional and 2D systems when the interlayer separation tends to zero or infinity, respectively. This expression enables an accurate description of the Fröhlich matrix element in 2D systems without resorting to Coulomb truncation. We validate our approach by direct ab initio density-functional perturbation theory calculations for monolayer BN and MoS2, and we provide a simple expression for the 2D Fröhlich matrix element that can be used in model Hamiltonian approaches. The formalism outlined in this work may find applications in calculations of polarons, quasiparticle renormalization, transport coefficients, and superconductivity, in 2D and quasi-2D materials. |
 | Nikolaus Kandolf, Carla Verdi, Feliciano Giustino Many-body Green's function approaches to the doped Fröhlich solid: Exact solutions and anomalous mass enhancement Journal Article In: Physical Review B, vol. 105, iss. 8, pp. 085148, 2022. @article{10.1103/PhysRevB.105.085148,In polar semiconductors and insulators, the Fröhlich interaction between electrons and long-wavelength longitudinal optical phonons induces a many-body renormalization of the carrier effective masses and the appearance of characteristic phonon sidebands in the spectral function, commonly dubbed “polaron satellites.” The simplest model that captures these effects is the Fröhlich model, whereby electrons in a parabolic band interact with a dispersionless longitudinal optical phonon. The Fröhlich model has been employed in a number of seminal papers, from early perturbation-theory approaches to modern diagrammatic Monte Carlo calculations. One limitation of this model is that it focuses on undoped systems, thus ignoring carrier screening and Pauli blocking effects that are present in real experiments on doped samples. To overcome this limitation, we here extend the Fröhlich model to the case of doped systems, and we provide exact solutions for the electron spectral function, mass enhancement, and polaron satellites. We perform the analysis using two approaches, namely, Dyson's equation with the Fan-Migdal self-energy, and the second-order cumulant expansion. We find that these two approaches provide qualitatively different results. In particular, Dyson's approach yields better quasiparticle masses and worse satellites, while the cumulant approach provides better satellite structures, at the price of worse quasiparticle masses. Both approaches yield an anomalous enhancement of the electron effective mass at finite doping levels, which in turn leads to a breakdown of the quasiparticle picture in a significant portion of the phase diagram. |
 | VA Ha, H Lee, F Giustino CeTaN3 and CeNbN3: Prospective Nitride Perovskites with Optimal Photovoltaic Band Gaps Journal Article In: Chemistry of Materials, vol. 34, iss. 5, pp. 2107-2122, 2022. @article{Ha2022,Perovskites constitute an exceptionally tunable materials familywith diverse applications in electronics, optoelectronics, energy, and quantumtechnologies. Out of the thousands of known perovskites, the majority ofcompounds are oxides, halides, and chalcogenides. In contrast, only two nitrideperovskites are currently known. In this work we perform a thoroughab initiocomputational screening of possible nitride perovskites, and we identify two newcompounds, CeNbN3and CeTaN3, with band gaps in the near-infrared to visiblerange, depending on temperature. In their room-temperature orthorhombic phase,we predict that both compounds exhibit direct or quasidirect band gaps in therange 1.1−2.0 eV, with thePnmaphases matching the Shockley−Queisser limit for photovoltaic energy conversion efficiency. Thesecompounds are also predicted to be strong light absorbers, with absorption coefficients surpassing those of high-performancesemiconductors such as GaAs and CH3NH3PbI3. The presentfindings reveal a potentially new class of nitride semiconductors withpromise for electronics, optoelectronics, and light harvesting and for integration with existing nitride-based lighting technology. |