First-principles prediction of spin relaxation, dephasing, and transport from density-matrix dynamics

Abstract

Improved methods for manipulating and detecting spin in materials are needed for advancing the state of the art in spintronics, quantum computing, and quantum information science. Modeling spin properties from first principles allows us to find materials with desirable properties such as long spin lifetimes and strong spin orbit coupling. This work develops new methods for modeling spin dynamics and transport from first principles which are highly versatile and successfully predict properties for a wide range of systems and phenomena. Firstly, we examine models for spin dynamics, particularly spin lifetimes. The electron spin decay lifetime in materials can be characterized by relaxation (T1) and reversible (T2) and irreversible decoherence processes (T2*). Their interplay leads to a complex dependence of spin lifetimes on the direction and magnitude of magnetic fields, relevant for spintronics and quantum information applications. Here, we use real-time first-principles density matrix dynamics simulations to directly simulate Hahn echo measurements, disentangle dephasing from decoherence, and predict T1, T2 and T2* spin lifetimes. We show that g-factor fluctuations lead to non-trivial magnetic field dependence of each of these lifetimes in inversion-symmetric crystals of CsPbBr3 and silicon, even when only intrinsic spin-phonon scattering is present. Most importantly, fluctuations in the off-diagonal components of the g-tensor lead to a strong magnetic field dependence of even the T1 lifetime in silicon. Our calculations elucidate the detailed role of anisotropic g-factors in determining the spin dynamics even in simple low spin-orbit coupling materials such as silicon. In addition to spin dynamics, we also model spin transport. We introduce a computational framework for first-principles density matrix transport within the Wigner function formalism to predict transport of quantum-mechanical degrees of freedom such as spin over long time and length scales. This framework facilitates simulation of spin dynamics and transport from first principles, while accounting for electron-phonon scattering at device length scales. We demonstrate this framework to elucidate the impact of various spin-orbit field profiles, such as Rashba and persistent spin helix, on coherent spin transport in several materials. Using graphene under an electric field as an example to illustrate the impact of electron-phonon scattering on incoherent transport, we show how the transport changes with the strength of scattering. We identify three distinct regimes of incoherent spin transport corresponding to the free induction decay, Dyakonov-Perel and Elliott-Yafet regimes of spin relaxation. In particular, we show that the spin diffusion length is insensitive to the strength of scattering within the Dyakonov-Perel regime. Extending this transport framework to account for electromagnetic field contributions is a nontrivial problem, since it requires modeling terms with a momentum space derivative in addition to a spatial derivative. We derive the appropriate terms for modeling these contributions within the relevant limits and show good agreement with other known first-principles methods. Another important factor to consider in realistic materials is temperature dependence. Spin dynamics is sensitive to electron-phonon interactions, so we need a highly accurate estimate of phonon behavior. At high temperatures, the harmonic approximation for estimating phonons becomes inadequate. We develop methods for calculating temperature dependent phonons more robustly, making sure to integrate contributions from both classical and quantum statistics. In addition to spin dynamics, we also develop novel methods for modelling spatial transport. We develop a framework for real time spatio-temporal spin transport in arbitrary 2D geometries. Using these methods, we estimate spin diffusion length and model coherent spin transport in materials with a variety of spin textures. We also examine electromagnetic field dependence and show that our real time transport calculations accurately estimate material properties. Overall, these frameworks are highly general, i.e. not constrained to any specific kinds of Hamiltonians, and accurately account for the electron-electron and electron-phonon interactions which dominate spin properties. In the future, these methods can be used to study a large set of materials, realistic or hypothetical.