Session G43: DCOMP Award Symposium

Rahman Prize (2022): The universal language of quantum simulations

We discuss studies of  materials for sustainable energy sources and  quantum technologies carried out using ab initio simulations.  We show that, despite  several approximations to the basic equations of quantum mechanics and the computational challenges  in describing realistic size and time scales,  insightful predictions can be made that are not only corroborated by  experiments but inspire new ones.   

Nicholas Metropolis Award for Outstanding Doctoral Thesis Work in Computational Physics (2022): Surface processes in ion-irradiated materials from first principles

First-principles modeling offers exciting prospects for computational design of not only materials, but also materials imaging and processing techniques. Focused ion beams are a particularly versatile tool: depending on beam parameters, energetic ions can allow nondestructive high-resolution imaging or intentional modification of atomic structure. While ion-irradiated bulk materials are fairly well studied, applications of ion irradiation often involve poorly understood surface processes like pre-equilibrium energy transfer and electron emission. The extreme case of 2D materials is especially compelling because of their departures from bulk properties and myriad applications relying on precise control of nanostructure. First-principles modeling can help elucidate important surface physics and predict optimal beam parameters for desired outcomes. Here, we first present methodological advances for simulating ion-irradiated surfaces and 2D materials using real-time time-dependent density functional theory. Then, we apply our extended framework to examine a variety of surface effects in ion-irradiated aluminum sheets and graphene. We uncover important differences in the energy deposited by energetic ions in these systems compared to bulk counterparts, including contributions from surface plasmon excitations and dissipation through electron emission. We also find interesting projectile charge state dynamics and that electron emission plummets below a threshold ion velocity. Finally, we offer insights into the early stages of ion-induced defect formation mechanisms. This work elevates our understanding of the basic physics of ion-irradiated surfaces and 2D materials and advances the ability to design ion beam techniques for specific applications.   

Electronic response to high-velocity nuclei through matter from first principles

An atomic nucleus travelling through condensed matter at a velocity comparable to that of valence electrons produces significant electronic excitation in the scale of eV to keV per Angstrom along the trajectory, which is key for the understanding of many radiation damage processes. The nucleus produces a strong perturbation, giving rise to nanoscopic, far from equilibrium non-adiabatic processes hard to describe beyond linear response. Dynamical simulations from first principles using real-time time-dependent density-functional theory are being used to simulate them, achieving remarkable success. Important challenges remain, however, both theoretically and experimentally, towards understanding such a fundamental problem in non-equilibrium quantum physics. Building from the ground up, the focus is now on the characterisation of stationary states in time-periodic situations for constant velocity nuclei along space periodic directions in crystals, a model situation that allows looking at (stroboscopically) stationary states in terms of Floquet modes.   

Phase diagram, metallization, black and shiny reflections of Hydrogen in the 3-7 Mbar range by quantum Monte-Carlo with anharmonic zero-point fluctuations

The interplay between electron correlation and nuclear quantum effects makes our understanding of elemental hydrogen a formidable challenge. Here, we present the phase diagram of hydrogen at low temperatures in high-pressure 300-700 GPa range, by accounting for highly accurate electronic and nuclear enthalpies. We evaluated internal electronic energies by diffusion quantum Monte Carlo, an nuclear quantum motion and anharmonicity by the stochastic self-consistent harmonic approximation [1]. Our results [2] show that the long-sought atomic shiny metallic hydrogen (Cs-IV), predicted to host room-temperature superconductivity, forms at 577(10) GPa. Indeed, anharmonicity pushes the stability of this phase towards pressures much larger than previous theoretical estimates or attained experimental values. Before atomization we predict a metal-insulator transition in the molecular phase III (C2/c-14) at 380 due to band overlap [3], in agreement with transport data [4]. This state is an unusual, non-reflecting, black metal, transparent in the infrared, as observed in [5]. At 412(40) GPa we predict a transition to phase VI (Cmca-12), another non-reflecting, black, metallic structure that is still molecular, but in contrast to phase III, totally opaque in the IR, as observed in [5]. We suggest that the III-VI boundary could be also detected by conductivity measurements, since we predict a significant drop of resistivity in correspondence of this phase transition [2].

[1] L. Monacelli, R. Bianco, M. Cherubini, M. Calandra, I. Errea, F. Mauri, Journal of Physics: Condensed Matter 33, 363001 (2021)

[2] L. Monacelli, M. Casula, K. Nakno, S. Sorella, F. Mauri, to be published

[3] L. Monacelli, I. Errea, M. Calandra, F. Mauri, Nat. Phys. 15, 1246–1249 (2019)

[4] M. I. Eremets, A. P. Drozdov, P.P. Kong, H. Wang, Nat. Phys. 17, 163 (2021)

[5] P. Loubeyre, F. Occelli, P. Dumas, Nature 577, 631 (2020)   

First-Principles Simulations of Heterogeneous Interfaces for the Water-Energy Nexus

Understanding physicochemical processes at heterogeneous interfaces is critical in a wide range of emerging technologies, from energy storage and conversion to water treatment. In this talk, I will review our recent activities in the simulation of electrochemical interfaces using first-principles approaches. I will discuss how first-principles simulations have been applied to elucidate chemical evolutions and degradation mechanism of materials at the interface with aqueous electrolytes. In addition, I will show how first-principles calculations have been combined with implicit solvation models to understand electric double layer effects in porous materials. Finally, I will discuss how first-principles simulations are combined with multi modal characterization techniques to provide mechanistic understandings of ion transport and selectivity in sub-nm nanopores. Our results demonstrate the complex synergy between interfacial chemistry, solvation effects and applied potentials, leading to behavior that qualitatively deviates from conventional models.