Session G58: Electrons, Phonons, Electron-Phonon Scattering, and Phononics IV

Correlation-enhanced electron-phonon interaction in oxide superconductors from GW perturbation theory

Accurate and practical ab initio treatment of electron-phonon (e-ph) coupling is essential to the understanding of many condensed-matter phenomena. In this talk, I will present a recently developed ab initio linear-response method named GW perturbation theory (GWPT) that computes the e-ph interaction with the inclusion of the GW nonlocal, energy-dependent self-energy effects. GWPT goes beyond the commonly used density-functional perturbation theory (DFPT), which becomes inadequate in some materials when correlation effects are non-negligible. We demonstrate the GWPT method by showing that the e-ph coupling in Ba_1-xK_xBiO₃ is significantly enhanced by many-electron correlation, strong enough to explain its high superconducting Tc of 32 K. Furthermore, GW-level anisotropic Eliashberg equation calculations suggest that infinite-layer nickelate superconductor Nd_1-xSr_xNiO₂ may host a strong phonon-mediated s-wave two-gap superconductivity. I will also present studies on the e-ph coupling in cuprates and discuss new understanding in phenomena such as the ubiquitous 70-meV nodal dispersion kink.   

Compression for GW perturbation theory calculations via tensor rank decomposition

GW perturbation theory (GWPT) [1] is an ab initio linear-response method to compute electron-phonon (e-ph) coupling matrix elements that include many-electron self-energy effects at the GW level. However, large-scale or systematic studies using GWPT are highly demanding due to the computational expense of calculating the GW self-energy corrections to the numerous e-ph matrix elements that are needed in physical studies. In this talk, we present a method to reduce the cost of GWPT based on tensor rank decomposition, a widely used compression technique for high-dimensional data. This method allows us to obtain properties such as the e-ph coupling constant λ to high accuracy for much lower computational cost, as the full GW self-energy correction matrix is replaced by a low-rank approximation requiring only a small fraction of all terms to be computed.

[1] Li, Antonius, Wu, da Jornada, and Louie, Phys. Rev. Lett. 122, 186402 (2019).   

First-principles study of phonon-mediated superconductivity beyond the Migdal approximation

The Migdal-Eliashberg theory has been remarkably successful in explaining phonon-mediated superconductivity in materials under the adiabatic limit, where the ratio of the phonon frequency to the electronic Fermi energy is much smaller than unity. The breakdown of the adiabatic limit has been pointed out in various systems such as high-T_c, flat band, and low-carrier density superconductors. To address the impact of nonadiabatic effects in this class of materials, we recently implemented first-order vertex corrections for electron-phonon interactions within the Eliashberg formalism in the EPW code. In this talk, we highlight the significance of nonadiabaticity in predicting the transition temperature of representative high-T_c superconductors.   

Application of Intermediate Representation basis in Migdal-Eliashberg calculations for low-Tc superconductors

Over the past few years, the anisotropic Migdal-Eliashberg formalism has been successfully used to compute the superconducting properties from low- to high-temperature superconductors. However, computing the superconducting gap function at very low temperatures is extremely difficult as a large number of sampling points are required to convergence the summation over the Matsubara frequency on the imaginary axis. To overcome this problem, we have recently implemented the intermediate Representation (IR) basis method [1] in the EPW code [2] which enables the reduction in the number of sampling points for the Matsubara frequency summation. This methodology also allows one to accurately evaluate the Coulomb retardation effects by taking into account in the Migdal-Eliashberg equations the contributions coming from the high energy bands. In this talk, I will show the performance of this approach in the case of representative low-temperature superconductors.

[1] M. Wallerberger, S. Badr, S. Hoshino et al., Software X 21 101266 (2023).

[2] H. Lee, S. Poncé, K. Bushick et al., npj Comput. Mater. 9, 156 (2023).   

Origin of CDW in Kagome AV3Sb5

Recently, the kagome superconductor family AV₃Sb₅ (A = K, Rb, Cs) has been heavily investigated due to its interesting lattice and the occurrence of a charge density wave (CDW). Especially, the origin of the instability and its connection to superconductivity is intensely discussed. Among the most common proposed explanations for the CDW is Fermi surface nesting, influence of van-Hove-singularities and matrix element effects [1,2,3]. However, a clear cut identification is still outstanding.

By utilizing phonon fluctuation diagnostics [4] we try to pinpoint the electronic processes behind phonon softening​. This allows us to disentangle fermiology and matrix-element effects and thereby enhance the understanding of the CDW origin. 

[1] M. Kang, et al., Nat. Phys. 18, 301–308 (2022)

[2] X. Wu, et al., Phys. Rev. Lett. 127, 177001

[3] H. Luo, et al., Nat Commun 13, 273 (2022)

[4] J. Berges et al., Phys. Rev. B 101, 155107 (2020)​   

Lithium doping effects on phonon and electron-phonon properties of Bi2Sr2CaCu2O8

Cuprate metal oxides are known for their unconventional high-temperature superconductivity, the details of which remain a subject of ongoing research and discussion. Within the established BCS theory and its generalization made by Eliashberg, the introduction of high-frequency phonons, e.g. by doping light elements like lithium, can lead to higher superconducting transition temperature. Drawing on these ideas, our study explores the effects of introducing light elements into cuprates and focuses on their impact on phonon modes and electron-phonon interactions. Using density functional theory, we’ve selected Bi₂Sr₂CaCu₂O₈ (BSCCO, Bi-2212) as a prototypical cuprate and investigated how lithium doping affects its phononic and electron-phonon properties. We present an analysis on atom, mode, and layer-resolved electron-phonon coupling for Li-Bi substitution.   

First-principles analysis of spin relaxation in germanium

Germanium has attracted significant interest for use in quantum technologies owing to its favorable properties such as strong spin-orbit coupling and relatively long spin relaxation times (SRTs). While previous theoretical studies of spin relaxation in Ge have relied on semiempirical models, recent advances have enabled accurate first-principles calculations of SRTs in semiconductors [1,2]. Here we present a first-principles study of spin relaxation in bulk Ge for both electron and hole carriers. We show predictions of T1 SRTs in quantitative agreement with experiment in the 50-350 K temperature range, and analyze the phonon mode- and valley-dependent scattering mechanisms responsible for spin relaxation via the Elliott-Yafet mechanism. Our calculations employ hybrid functionals to describe the electronic structure and obtain accurate e-ph interactions validated by predicting charge transport properties (mobility and velocity-field curves) in close agreement with experiments. The atomistic details obtained with our first-principles approach provide valuable information for designing Ge-based quantum materials and devices.   

Magnetic Field Dependence of Spin-Phonon Relaxation and Dephasing from First Principles

Spintronic devices require materials with long spin lifetimes, diffusion lengths and strong spin-orbit coupling (SOC). Strong SOC usually leads to significant k-dependent deviations of the electron g-factor from its free electron value, which in the presence of magnetic fields, leads to strong dephasing in the spin dynamics. We demonstrate real-time first-principles calculations of T₁, T₂, and T₂* lifetimes using a density-matrix dynamics approach. T₂, which represents the spin coherence time, is calculated by simulating a real-time Hahn spin-echo measurement in order to separate the effects of spin dephasing from the overall relaxation time. We predict the magnetic field dependence of each of these spin relaxation lifetimes for crystalline materials with varying complexity and SOC strength, ranging from Si to perovskite CsPbBr₃. We show the transition from relaxation-dominated dynamics with T₂ ~ T₂* at low magnetic fields to dephasing-dominated dynamics T₂ » T₂* at high magnetic fields, even for intrinsic spin-phonon relaxation due to g-factor fluctuations.   

Phonon-induced spin-orbit torques from first principles

Understanding the transfer of electron spin angular momentum to the environment is paramount for the development of quantum devices. Phonons induce a dynamical magnetic field through relativistic spin-orbit coupling, applying an intrinsic torque to the electron spin angular momentum. A microscopic quantification of these torques remains an open challenge. Here we show a rigorous framework for computing phonon-induced spin-orbit torques from first principles. We apply this method to prototypical materials in each of the Laue point groups and analyze how different phonon modes contribute to the spin torque in each point group. Our results highlight fundamental differences between the same-spin / spin-flip electron-phonon matrix elements and phonon-induced spin torques, shedding light on how phonons absorb the electron spin angular momentum during relaxation. Extensions to include orbital angular momentum transfer will be described. Our work enables quantitative analysis of spin angular momentum transfer between electron spins and lattice vibrations, enabling the description of spin Hall effects and related physics in materials and devices.   

Quantum vibronic effects on the electronic properties of solid-state spin defects

We present a study of the electronic properties of solid-state spin defects to unravel the impact of quantum nuclear vibrations on vertical excitation energies. Our analysis focused on the negatively charged nitrogen vacancy center in diamond as a prototypical example. Electronic properties were computed using DFT, and nuclear vibrations were determined using stochastic methods,¹ which were validated against first-principles molecular dynamics with a quantum thermostat (QT-FPMD).² We found a significant dynamic Jahn-Teller splitting of the doubly degenerate single-particle levels within the diamond's band gap, even at 0 K, with a magnitude exceeding 180 meV.  This pronounced splitting leads to substantial renormalizations of the defect levels and consequently, of the vertical excitation energies of the doubly degenerate singlet and triplet excited states. Our study underscores the pressing need to incorporate quantum vibronic effects in first-principles calculations of spin defects, especially when comparing computed vertical excitation energies with experimental data. We utilized several computational tools, including PyEPFD (https://pyepfd.readthedocs.io/) and i-PI (http://ipi-code.org/) for stochastic and QT-FPMD simulations, respectively, Qbox (http://qboxcode.org/) to compute DFT forces and the WEST code (https://west-code.org/) to compute  TDDFT vertical excitation energies.

 

A. Kundu et al, ¹JCTC 2023, ² PRM. 2021 & PNAS 2022.

Ab initio spatio-temporal spin transport in a density-matrix formalism

Ab initio design of materials and devices for spintronics requires simulation of spin dynamics and spatial transport in realistic device geometries, accounting for both coherent and incoherent processes. Combining density-matrix quantum dynamics with semi-classical spatial transport within the Wigner function formalism, we develop a computational framework for simulating spatio-temporal spin dynamics and transport. This framework accounts for electron-phonon, electron-electron, and electron-impurity scatterings at device length scales using a Lindbladian formalism applied to ab initio density matrices. The Wigner function formalism allows resolution of physical properties such as spin in phase space, with real and reciprocal space resolution, which also provides significant opportunities for parallelization that we leverage for an efficient GPU-enabled implementation targeting exascale computing resources. Using this approach, we showcase dephasing effects due to transport in spintronic devices, investigating the impact of spin textures including Rashba and persistent spin helix textures on the spin mixing in reciprocal space.   

First-principles electron-electron scattering simulations in warm dense matter

Obtaining accurate descriptions of the scattering processes in the warm dense matter (WDM) regime will result in better predictions of electrical and thermal conductivities, which are an essential part in the modeling and design of inertial confinement fusion (ICF) experiments. In our approach, we use first-principles calculations to study isochorically heated solid-density beryllium and hydrogen which is closer to plasma conditions, both materials are commonly found in ICF experiments. We go beyond the commonly used Kubo-Greenwood formalism, and instead we utilize the GW approximation to study electron-electron (e-e) scattering through the relationship between the electron self-energy and the e-e scattering rate. We compare e-e scattering rates using two methods: one in which we fit the full-frequency GW self-energies to the Landau theory of the Fermi liquid to approximate energy-dependent lifetimes. Since this model is calculated within the zero-temperature formalism, we model a temperature-dependence by averaging the energy-dependent lifetimes over Fermi occupations and the density of states to obtain an average e-e- scattering rate. The second approach utilizing a low-scaling GW calculation which uses a compressed Matsubara-frequency grid to determine state-dependent lifetimes which have an explicit temperature dependence. Our results will lead to the improvement of models used to generate data for simulations of ICF experiments.