# Research Overview

**We work at the intersection of materials and physics**

This is an amazing time to study the physics of materials. Scientists are developing increasingly complex materials − ultrathin semiconductors and oxides for electronics, perovskites for solar energy, qubits and quantum materials for quantum technologies, among others. Meanwhile, new experiments can probe materials at ever shorter timescales, unraveling their atomic and electronic motions. Empirical models developed decades ago, and still widely used, are inadequate to understand these novel materials and nonequilibrium physics.

With this motivation, our research develops numerical quantum mechanical calculations that can accurately predict the microscopic dynamics in materials [1]. We focus on “first-principles” approaches, which take as input only the atomic structure of the material, and aim to make quantitative predictions without using empirical parameters or experimental inputs. These methods provide an insight, resolution, and quantitative detail beyond the reach of experiments, and enable the interpretation of advanced measurements and spectroscopies.

**Our research spans several aspects of electron dynamics in materials:**

**Electronic interactions:**We devevelop accurate calculations of electronic interactions in materials, focusing on the interactions between electrons and phonons (atomic vibrations), which govern wide-ranging phenomena in solids, including transport, nonequilibrium dynamics, superconductivity, and phase transitions. We have shown how to address such electron-phonon (e-ph) interactions in materials with spin-orbit coupling [2], piezoelectricity [3, 4], strong e-ph coupling and polarons [5–8], and strong electronic [9] and electron-hole [10] correlations. These advances have made precise calculations of e-ph interactions possible in a wide range of materials of fundamental and technological interest.**Electron transport:**We investigate charge transport in novel semiconductors and oxides, both to advance microscopic understanding of transport phenomena and to generate quantitative tools to study novel electronic materials. Our work showed the first parameter-free calculations of transport in polar [11] and organic semiconductors [6, 12]; explained the origin of the electron mobility in complex oxides, shedding light on the role of anharmonic phonons [13] and polarons [5, 14]; developed accurate calculations of the interactions between electrons and crystallographic defects and the related low-temperature transport [15–17]; and advanced modeling of transport in magnetic and high electric fields [18, 19]. These studies have greatly expanded the depth and scope of first-principles quantum mechanical calculatulations of transport.**Electron spin dynamics:**A microscopic understanding of spin dynamics is essential to advancing quantum technologies. We are developing theory and computational methods that can more precisely characterize electron spin relaxation and decoherence due to phonons (atomic vibrations), which set an intrinsic limit to the performance of spin-based quantum devices. We have demonstrated accurate predictions of spin relaxation and derived a unified approach to describe phonon-induced spin dynamics [20–22]. We are building on these advances to study spin dynamics in quantum materials and quantum devices.**Nonequilibrium dynamics:**We study materials excited out of equilibrium to characterize ultrafast phenomena and the associated time-domain spectroscopies. We demonstrated a method to model excited electrons in materials from first principles [23–25], and later extended this early approach to study nonequilibrium electron and structural dynamics with atomic detail [10, 26, 27] and simulate ultrafast spectroscopies [27]. Using these tools, we discovered an asymmetry between the cooling rates of excited electron and hole carriers in gallium nitride (GaN) which explained the main source of energy loss in GaN-based LEDs [26]. We have also developed predictions of light-emission and radiative processes, both in bulk and two-dimensional in semiconductors [10, 28–32]. These studies create new tools to understand excited materials and their nonequilibrium physics.-
**Software development:**Our group develops open source software to share new theoretical tools and computational workflows with the community. We develop PERTURBO, an open source code with hundreds of users enabling quantitative studies of electron interactions and dynamics in materials. PERTURBO equips the scientific community with new theoretical tools and efficient algorithms to study complex materials and nonequilibrium physics.

**Our research is highly interdisciplinary**

Our studies span physics, materials science, chemistry, and engineering. We seek to break new grounds in materials physics to addresses questions with broad societal impact:

- Can we use computers to understand matter with ultimate space and time resolutions?
- By understanding electronic motion at the atomic scale, can we aid the design of a new generation of materials and devices?
- What limits the performance of materials and devices for electronics, energy and computing?

## References

- M. Bernardi, First-principles dynamics of electrons and phonons, Eur. Phys. J. B 89, 239 (2016).
- J.-J. Zhou, J. Park, I.-T. Lu, I. Maliyov, X. Tong, and M. Bernardi, PERTURBO: A software package for ab initio electron-phonon interactions, charge transport and ultrafast dynamics, Comput. Phys. Commun. 264, 107970 (2021). The code can be downloaded at perturbo-code.github.io.
- V. A. Jhalani, J.-J. Zhou, J. Park, C. E. Dreyer, and M. Bernardi, Piezoelectric electron-phonon interaction from ab initio dynamical quadrupoles: Impact on charge transport in wurtzite GaN, Phys. Rev. Lett. 125, 136602 (2020).
- J. Park, J.-J. Zhou, V. A. Jhalani, C. E. Dreyer, and M. Bernardi, Long-range quadrupole electronphonon interaction from first principles, Phys. Rev. B 102, 125203 (2020).
- J.-J. Zhou and M. Bernardi, Predicting charge transport in the presence of polarons: The beyondquasiparticle regime in SrTiO3, Phys. Rev. Research 1, 033138 (2019).
- B. K. Chang, J.-J. Zhou, N.-E. Lee, and M. Bernardi, Intermediate polaronic charge transport in organic crystals from a many-body first principles approach, npj Comput. Mater. 8, 63 (2022).
- N.-E. Lee, H.-Y. Chen, J.-J. Zhou, and M. Bernardi, Facile ab initio approach for self-localized polarons from canonical transformations, Phys. Rev. Materials 5, 063805 (2021).
- Y. Luo, B. K. Chang, and M. Bernardi, Comparison of the canonical transformation and energy functional formalisms for ab initio calculations of self-localized polarons, Phys. Rev. B 105, 155132 (2022).
- J.-J. Zhou, J. Park, I. Timrov, A. Floris, M. Cococcioni, N. Marzari, and M. Bernardi, Ab initio electron-phonon interactions in correlated electron systems, Phys. Rev. Lett. 127, 126404 (2021).
- H.-Y. Chen, D. Sangalli, and M. Bernardi, Exciton-phonon interaction and relaxation times from first principles, Phys. Rev. Lett. 125, 107401 (2020).
- J.-J. Zhou and M. Bernardi, Ab initio electron mobility and polar phonon scattering in GaAs, Phys.Rev. B 94, 201201(R) (2016).
- N.-E. Lee, J.-J. Zhou, L. A. Agapito, and M. Bernardi, Charge transport in organic molecular semiconductors from first principles: The bandlike hole mobility in a naphthalene crystal, Phys. Rev. B 97, 115203 (2018).
- J.-J. Zhou, O. Hellman, and M. Bernardi, Electron-phonon scattering in the presence of soft modes and electron mobility in SrTiO3 perovskite from first principles, Phys. Rev. Lett. 121, 226603 (2018).
- T. Truttmann, J.-J. Zhou, I.-T. Lu, A. Rajapitamahuni, F. Liu, T. Mates, M. Bernardi, and B. Jalan, Combined experimental-theoretical study of electron mobility-limiting mechanisms in SrSnO3, Commun. Phys. 4, 241 (2021).
- I.-T. Lu, J.-J. Zhou, and M. Bernardi, Efficient ab initio calculations of electron-defect scattering and defect-limited carrier mobility, Phys. Rev. Mater. 3, 033804 (2019).
- I.-T. Lu, J. Park, J.-J. Zhou, and M. Bernardi, Ab initio electron-defect interactions using Wannier functions, npj Comput. Mater. 6, 17 (2020).
- I.-T. Lu, J.-J. Zhou, J. Park, and M. Bernardi, First-principles ionized-impurity scattering and charge transport in doped materials, Phys. Rev. Materials 6, L010801 (2022).
- I. Maliyov, J. Park, and M. Bernardi, Ab initio electron dynamics in high electric fields: Accurate prediction of velocity-field curves, Phys. Rev. B 104, L100303 (2021).
- D. C. Desai, B. Zviazhynski, J.-J. Zhou, and M. Bernardi, Magnetotransport in semiconductors and two-dimensional materials from first principles, Phys. Rev. B 103, L161103 (2021).
- J. Park, J.-J. Zhou, and M. Bernardi, Spin-phonon relaxation times in centrosymmetric materials from first principles, Phys. Rev. B 101, 045202 (2020).
- J. Park, J.-J. Zhou, and M. Bernardi, Predicting phonon-induced spin decoherence from first principles: Colossal spin renormalization in condensed matter, Phys. Rev. Lett. (Accepted) (2022).
- J. Park, J.-J. Zhou, and M. Bernardi, Many-body theory of phonon-induced spin relaxation and decoherence (Submitted), arXiv 2208.09575 (2022).
- M. Bernardi, D. Vigil-Fowler, J. Lischner, J. B. Neaton, and S. G. Louie, Ab initio study of hot carriers in the first picosecond after sunlight absorption in silicon, Phys. Rev. Lett. 112, 257402 (2014).
- M. Bernardi, D. Vigil-Fowler, C. S. Ong, J. B. Neaton, and S. G. Louie, Ab initio study of hot electrons in GaAs, PNAS 112, 5291 (2015).
- M. Bernardi, J. Mustafa, J. B. Neaton, and S. G. Louie, Theory and computation of hot carriers generated by surface plasmon polaritons in noble metals, Nat. Commun. 6, 7044 (2015).
- V. A. Jhalani, J.-J. Zhou, and M. Bernardi, Ultrafast hot carrier dynamics in GaN and its impact on the efficiency droop, Nano Lett. 17, 5012 (2017).
- X. Tong and M. Bernardi, Toward precise simulations of the coupled ultrafast dynamics of electrons and atomic vibrations in materials, Phys. Rev. Research 3, 023072 (2021).
- M. Palummo, M. Bernardi, and J. C. Grossman, Exciton radiative lifetimes in two-dimensional transition metal dichalcogenides, Nano Lett. 15, 2794 (2015).
- H.-Y. Chen, M. Palummo, D. Sangalli, and M. Bernardi, Theory and ab initio computation of the anisotropic light emission in monolayer transition metal dichalcogenides, Nano Lett. 18, 3839 (2018).
- H.-Y. Chen, V. A. Jhalani, M. Palummo, and M. Bernardi, Ab initio calculations of exciton radiative lifetimes in bulk crystals, nanostructures and molecules, Phys. Rev. B 100, 075135 (2019).
- V. A. Jhalani, H.-Y. Chen, M. Palummo, and M. Bernardi, Precise radiative lifetimes in bulk crystals from first principles: the case of wurtzite gallium nitride, J. Phys. Condens. Matter 32, 084001 (2020).
- S. Gao, H.-Y. Chen, and M. Bernardi, Radiative properties of quantum emitters in boron nitride from excited state calculations and bayesian analysis, npj Comput. Mater. 7, 85 (2021).