Session F58: Electrons, Phonons, Electron-Phonon Scattering, and Phononics III

Effects of Spin-Orbit Coupling and Thermal Lattice Expansion on the Phonon-limited Resistivity of Pb from First Principles

In this presentation, we will report the results of calculations on the phonon-limited temperature-dependent electrical resistivity of bulk Pb. Recent developments in computational techniques allow us to make quantitative comparisons between ab initio results of resistivity and experimental data. We use the approach of Wannier interpolation implemented in the EPW code to perform these calculations. The resistivity is computed by iteratively solving the self-consistent linearized Boltzmann Transport Equation. Comparisons are made with the SERTA formulation of the resistivity. We observe that the lattice thermal expansion must be included to better compare to experimental data. We find that including spin-orbit coupling (SOC) affects the results considerably. A softening of the phonon modes at the X point of the Brillouin zone is accentuated with SOC and with the increase of the unit cell volume. That volume change is obtained using the harmonic approximation of the free energy. Here again, we show the effect of including the SOC on the results obtained. Furthermore, we show that the temperature effects on the electronic free energy are negligible compared to the phonon-free energy.   

Ab-Initio calculated Low field transport properties in Supercells and its application to Alloys

To investigate the transport properties in random alloys, it is important to capture the alloy disorder using supercells. In case of supercells, the self-image interaction error between the impurities is reduced and translational symmetry is explicitly imposed over larger length scales which traditional methods like Virtual Crystal Approximation fails to capture. In this work, we have investigated the polar optical phonon (POP) limited transport properties from first-principles calculations using supercells without unfolding the phonon dispersion. First, the phonon properties of GaN supercells are calculated and verified by comparing with the primitive cell. To validate our methods of supercell based mobility calculations, the Boltzmann Transport Equation is solved using Rode’s method to compare the POP and IIP limited mobility in the 4 atom GaN primitive cell and 12,40,32 atom GaN supercell. For random Al_xGa_1-xN alloy systems, the phonon dispersion properties are calculated using Density Functional Perturbation Theory on a 40-atom random Al_xGa_1-xN system. Solving the Boltzmann Transport Equation iteratively our calculations predict a room temperature mobility of 189 cm²/V-s at x=0.25 and 149 cm²/V-s at x=0.5 Al fractions at n=1e18cm^-3 electron concentration which show that along with alloy scattering, electron-phonon scattering also plays an important role at room temperature. This technique opens the path for calculating phonon limited transport properties in random alloy systems.   

Strain Engineering of GaN for Hole Mobility Optimization: A comprehensive approach

Gallium nitride (GaN) is a versatile wide-band gap semiconductor vital for applications in power electronics, radio-frequency devices, and rapid switching transistors. However, its p-channel devices face a challenge due to low hole mobility, obstructing its integration into next-generation technologies. Various strain states have been explored individually to enhance hole mobility, but a comprehensive assessment of their collective impact is lacking. In this study, we establish a linear tensor equation that correlates hole mobility and applied strain in GaN. Our approach leverages ab initio Boltzmann transport equations, accounting for electron-phonon scattering and quasiparticle energy corrections to derive hole mobilities. We use this knowledge to identify a set of optimal strain conditions, maximizing GaN hole mobility. Moreover, our methodology offers a general framework for first-principles-based material property engineering through strain manipulation.   

First-principles transport including magnetic and spin-orbit effects

Topology is the next frontier in materials science, opening possibilities for ultra low power devices, exquisite sensing capabilities, and synergies with quantum computing through the protection of state coherence.

We will showcase recent advances in the theory and applications of first-principles transport calculations, to include magnetism and spin-orbit interactions, and what will be needed to tackle the most complex topological materials, which contain both effects. Using spinor wave functions and SOC naturally incorporates spin-flip processes in electron-phonon scattering. Comparisons to experiments on "simple" 3d ferromagnetic metals require the inclusion of electron-magnon scattering, both in resistivity and in magnon drag. 

As a further step into topology, we have calculated the first-principles conductivity of Weyl semi-metals, in particular TaAs for which our calculations are in good agreement with available experiments. Finally, magnetoelectrically-active 2D Nickel Iodide can host topological magnetic states, and we find that it shows strong sensitivity to pressure. Combining experimental characterization with first and second principles simulations, we determine the pressure dependency (up to 20 GPa) of the electronic band structure, magnetic phase transition, and spinwave dispersion.

- X Ma et al. New J Phys 25, 043022 (2023)

- G Allemand and MJ Verstraete, unpublished (2023)

- J Kapeghian et al. arxiv.org/abs/2306.04729

- C Occhialini et al. arxiv.org/abs/2306.11720

High mobility 2D semiconductors: A computational search via the ab initio Boltzmann transport equation

Higher integration density of transistor on chip requires the reduction of their channel lengths. However, short-channel effect prevents transistor channel from being shrunk down to the scale of a few nanometers. Two-dimensional materials offer a potential avenue to overcome this bottleneck. This materials family possesses dangling-bond-free surfaces and can be used as transistor channels down to the sub-nanometer scale in their monolayer limit. However, most known 2D semiconductors exhibit low carrier mobility, as a result of their high density of states at band edges. In this work, we establish a high-throughput computing strategy to search for 2D semiconductors with high carrier mobility. Starting from available 2D materials database, we identify promising high-mobility materials by evaluating the conductivity effective mass and by solving the ab initio Boltzmann transport equation.   

Calculations of the nonlinear Hall effect with first-principles electron-phonon collisions and Berry curvature

In certain topological materials, the application of an electric field results in a finite second-order transverse Hall voltage, a phenomenon commonly known as the nonlinear Hall effect (NLHE). This recently discovered effect has gathered significant attention, primarily because it does not require the breaking of time-reversal symmetry, unlike conventional Hall effects. First-principles calculations have shown that intrinsic NLHE originates from the presence of a finite Berry-curvature dipole. However, accurately computing NLHE requires knowledge of both band topology and the electronic scattering mechanisms. In this talk, we combine first-principles Berry curvature and electron-phonon scattering calculations to achieve a quantitative description of nonlinear Hall transport. We solve the Boltzmann equation including both Berry curvature and e-ph collisions, and apply our method to two prototypical systems – monolayer WSe₂ and bilayer WTe₂ – to demonstrate quantitative predictions of NLHE.    

Coupled electron-phonon transport and viscous thermoelectric equations

Non-diffusive, hydrodynamic-like transport of charge or heat has been observed in several materials, and recent pioneering experiments have proposed the possible emergence of electron-phonon bifluids. We introduce a first-principles computational framework to investigate these phenomena, showing that the macroscopic viscosity of electrons-phonon bifluids is microscopically determined by composite 'relaxon' electron-phonon excitations, and these excitations also describe electron-phonon drag effects on standard thermoelectric transport coefficients.

We demonstrate that these composite excitations emerge from the microscopic coupled electron-phonon Boltzmann transport equation, and show how the latter can be coarse-grained into a set of mesoscopic Viscous Thermoelectric Equations (VTE). The VTE unify the established hydrodynamic equation for electrons derived by Gurzhi [Sov. Phys. Usp. 11 (2) 1968], and the recently developed Viscous Heat Equations for phonons [Phys. Rev. X 10 (2022)], while also covering the mixed electron-phonon bifluid regime.

Using the Phoebe code (https://github.com/mir-group/phoebe), we apply this framework to predict phonon drag effects and to investigate the possibility of coupled electron-phonon hydrodynamics in several experimentally-reported materials.   

Electron-phonon Drag Effect on Thermal Conductivity in Two-dimensional Materials

Electron-phonon drag refers to the momentum exchange between nonequilibrium phonons and electrons, and its effect on electronic transport properties, including the Seebeck coefficient and mobility, has been widely studied. However, a systematic study of the impact of nonequilibrium electrons on thermal transport properties, i.e., how extra momentum flow from nonequilibrium electrons to phonons affects thermal conductivity, is still lacking. In this work, by solving the fully coupled electron and phonon Boltzmann transport equations with ab initio scattering parameters, we capture the nonnegligible effect of electron drag on thermal conductivity in two-dimensional (2D) materials. We find that the electron drag effect can significantly increase thermal conductivity. Our study advances the understanding of the effect of nonequilibrium carriers on thermal transport and gives new insights into the nature of coupled electron-phonon transport in 2D semiconductors. This work is based on research supported by the U.S. Air Force Office of Scientific Research under award number FA9550-22-1-0468 and the National Science Foundation (NSF) under award number CBET-1846927. Y.Q. also acknowledges the support from the Graduate Traineeship Program of the NSF Quantum Foundry via the Q-AMASE-i program under award number DMR-1906325 at the University of California, Santa Barbara (UCSB).   

The Anharmonic LAttice DYNamics (ALADYN) suite of codes and thermal properties of materials from first-principles data

We will present our work on the development of an anharmonic lattice dynamics model and its related codes.

The first code, FOrce Constants EXtraction (FOCEX) is able to extract from a force-displacement data anharmonic force constants up to rank 4. From them, a second code, THERMACOND, calculates the phonon dispersion and solves the Boltzmann equation to calculate the lattice THERMAl CONDuctivity beyond the relaxation time approximation.

Results on simple crystals such as FCC Ge and RbBr, and more complex structures such as the Pnma phase of GeSe, will be illustrated. A third code, ANharmonic FOrce MOlecular Dynamics (ANFOMOD) allows the user to use this force field to perform molecular dynamics and calculate for instance the thermal conductivity by using the Green-Kubo formula.

Finally, at high temperatures, lattice dynamical properties are described using the Self-COnsistent Phonon theory, SCOP. This code calculates, at a given temperature, the effective harmonic force constants, the related phonons and the equilibrium, eventually deformed, crystal structure and the relaxed atomic positions. It is therefore able to predict structural phase transitions in addition to thermal expansion.

These codes are now made available to the public under the GPL-3.0 licence, and can be freely downloaded from our github page.

https://github.com/KeivanS/Anharmonic-lattice-dynamics

The documentation and tutorials can be found at:

https://aladyn.readthedocs.io/en/latest/run.html   

Neural network assisted solution of the Peierls-Boltzmann Equation for phonon transport in semiconductors and insulators

Unusual heat flow phenomena, such as the hydrodynamic phonon transport, arise out of dissipation-free strong coupling among phonons in crystalline solids, and find applications in thermal cloaking and shielding of semiconductor devices. The Peierls-Boltzmann equation (PBE) governs the coupled dynamics, transport and equilibriation of phonons in these systems and its predictive first principles solution including three- and four-phonon scattering processes, which could serve as a search tool for new materials, has been computationally very expensive, due to its high dimensionality and the need to resolve highly localized, temporally evolving interactions among phonons. To overcome this issue, we present a neural network scheme to solve the PBE, which enables rapid convergence of its steady-state solution and allows for computationally efficient high temporal resolution of localized interactions among phonons under transient transport conditions. We show that, even for materials with complex crystal geometries such as bulk MoS₂, graphite, w-GaN and hBN, the neural network scheme significantly outperforms the conventional iterative solution of the PBE under both steady-state and transient conditions. Our findings highlight the computational advantages of this neural network-based first principles solver for the PBE, which can significantly impact efficient computational search for new materials with exceptional thermal transport properties.   

Topological entropy controls thermal conductivity in disordered carbon polymorphs

The structural and thermal properties of disordered carbon play a pivotal role in many diverse energy technologies: nanoporous carbon finds applications in batteries and supercapacitors; (defective) graphite serves as a moderator in nuclear reactors, where neutron irradiation causes structural changes and material's aging. Although experiments indicate a strong dependence of thermal conductivity on structural disorder, the fundamental relationship between atomistic structure and macroscopic conductivity remains unclear. Here we address this challenge, using the Wigner formulation of thermal transport, quantum-accurate machine-learning potentials, and real-space topological descriptors to shed light on how disorder in the atomic bond network affects thermal conductivity. We introduce a descriptor – topological entropy – which quantitatively captures the variability of local coordination environments, and we show that it correlates with thermal conductivity at a given density. Our research establishes disorder in the atomic bond topology as a fundamental degree of freedom to control and engineer thermal conductivity, calling for studies on how to practically control the local bonding topology. Finally, our findings also suggest the possibility to probe atomistic structural properties using thermal-conductivity measurements.   

Large Bandgap Renormalization in Cu2Se: Liquid-like Behavior and Electro-Phonon Coupling Effects at Finite Temperatures

ß-Cu₂Se is one of the most promising thermoelectric materials due to its abundance, low cost and toxicity yet high figure of merit. The reason lies in superionic Cu vibrations, which creates the phonon-liquid electron-crystal effect, inhibiting heat transport while maintaining high electrical conductivity. However, accurate characterization of the electronic structure  remains a challenge due to the strong effects of polymorphism (local disorder), electron-phonon coupling and phonon anharmonicity. DFT calculations on the high symmetry structure yields semi-metallic behavior. In this work, we address the problem by treating the effects of symmetry breaking and high-temperature anharmonic vibrations utilizing the anharmonic special displacement method (ASDM). We determined the ground-state polymorphous structure following the recipe in ASDM and obtained a converged bandgap of 0.9 eV with a 4x4x4 supercell, which is in excellent agreement with experimental values. Further, we applied ASDM starting from the polymorphous structure and investigated the bandgap renormalization as a function of temperature, and found that increasing temperature reduces the bandgap by 0.06 eV. We layout a framework to elucidate how anharmonicity impacts the electronic properties and the thermoelectric performance of Cu₂Se.