Session E62: Dealing With Spin-Orbit Coupling

Spin-orbit-proximitized ferromagnetic metals in spintronic phenomena: A first-principles Green’s function approach

The talk reviews first-principles Green’s function methodology to obtain spectral function and spin texture at an arbitrary plane around interfaces within multilayers of different materials [1]. When applied to ultrathin ferromagnetic layer in contact with a material with strong spin-orbit coupling-such as topological insulators, heavy metals, monolayer transition metal dichalcogenides and Weyl semimetals (all of which have been utilized very recently to develop novel spintronic devices and they will be discussed in the talk)-this methodology reveals dramatic modification of ferromagnet electron and spin structures which acquire properties of the adjacent layer due to hybridization of their wave functions. In particular, such spin-orbit-proximitized ferromagnetic layer acquires spin textures that are noncollinear to its own magnetization. Passing unpolarized charge current parallel to the interface generates nonequilibrium spin density [2] in the presence of such textures, which becomes resource for spintronic devices operated by spin-orbit torques [3]. On the other hand, passing spin-polarized charge current or pure spin current perpendicular to the interface leads to spin memory loss due to the same spin textures [4] (even in the absence of disorder, intermixing and magnons at the interface) which adversely affects spintronic device operation. The talk will explain how to model spin-orbit torques and spin memory loss by employing first-principles Green’s functions to construct relevant nonequilibrium density matrices [3,4].

References:

[1] J. M. Marmolejo-Tejada, K. Dolui, P. Lazić, P.-H. Chang, S. Smidstrup, D. Stradi, K. Stokbro, and B. K. Nikolić, Nano Lett. 17, 5626 (2017).
[2] P.-H. Chang, T. Markussen, S. Smidstrup, K. Stokbro, and B. K. Nikolić, Phys. Rev. B 92, 201406(R) (2015).
[3] B. K. Nikolić, K. Dolui, M. Petrović, P. Plecháč, T. Markussen, and K. Stokbro, https://arxiv.org/abs/1801.05793.
[4] K. Dolui and B. K. Nikolić, Phys. Rev. B 96, 220403 (2017).   

Comprehensive modeling of band gaps and absorption spectra of complex semiconductors for solar applications

The performance of photoabsorbing materials is closely related to their band gaps and absorption spectra. In the fields of solar cells and photocatalysis, search for novel materials is ongoing. To identify optimal compounds, a modelling approach yielding accurate predictions for their electronic and optical properties is highly desirable. We recently applied the quasiparticle self-consistent QSGW method, efficiently accounting for the vertex correction, to study the electronic structure of halide perovskites and complex oxides. In the calculations, we included effects such as spin-orbit coupling, electron-hole interaction, magnetic ordering, nuclear quantum motions, and thermal vibrations. I will show that when an accurate electronic structure method is applied, and the significant effects are correctly accounted for, the band gaps and absorption spectra of complex materials can be reliably determined.   

Carrier recombination in materials with strong spin-orbit coupling

Materials that have large spin-orbit coupling and no inversion symmetry can display interesting spin texture. The hybrid perovskites, which are receiving a lot of attention for photovoltaic applications, are prominent examples in which the spin texture exhibits unexpected patterns. Based on model calculations, it has been suggested that due to mismatched spins the radiative recombination is severely suppressed. We have performed first-principles calculations to compute the spin texture in the prototypical hybrid perovskite, CH₃NH₃PbI₃. We find that the spin texture is dynamically evolving in various patterns with the rotation of the organic molecule [1]. However, the spin-orbit coupling always leads to spin-allowed optical transitions and the momentum splitting affects the radiative recombination coefficient by less than a factor of two [2]. The spin-orbit coupling does, however, significantly enhance the Auger coefficient due to a coincidental resonance between the band gap and interband transitions to spin-orbit-split conduction bands [3]. These insights demonstrate the importance of accurate treatment of spin-orbit coupling in describing materials properties, and also offer new computational approaches to quantitatively calculate the spin texture and its impact on carrier recombination.
[1] X. Zhang, J.-X. Shen, and C. G. Van de Walle, J. Phys. Chem. Lett. 9, 2903 (2018).
[2] X. Zhang, J.-X. Shen, W. Wang, and C. G. Van de Walle, ACS Energy Lett. 3, 2329 (2018).
[3] J.-X. Shen, X. Zhang, S. Das, and E. Kioupakis, C. G. Van de Walle, Adv. Energy Mater. 10, 1801027 (2018).   

The role of temperature in spin-orbit materials

The spin-orbit interaction underlies multiple physical phenomena, including topological order and the spin splitting of bands in inversion asymmetric crystals, which could be harnessed in novel technologies including spintronics or quantum computers. However, for any practical device, the spin-orbit driven properties need to survive all the way to room temperature, and therefore understanding the role of electron-phonon coupling and thermal expansion in spin-orbit materials becomes central.

In this talk, I will describe our work exploring the role of temperature in spin-orbit materials. I will describe the temperature dependence of the bulk Rashba splitting in the bismuth tellurohalides [1], temperature induced topological transitions in the bismuth selenide family [2], and structural stabilization driven by spin-orbit coupling in superconducting In₅Bi₃.

[1] Phys. Rev. Materials 1, 054201 (2017)
[2] Phys. Rev. Lett. 117, 226801 (2016)
  

Impact of spin-orbit coupling on the magnetic phase diagram of the iron pnictides

Experimental research over the past few years has shown the presence of sizeable spin-orbit coupling (SOC) in the iron pnictides [1]. Here, we focus on the impact of SOC on the magnetic phase diagram of the iron pnictides and in particular the implications for competing magnetic states and quantum criticality. Magnetism in the pnictides occurs in three distinct types. The prevalent magnetic phase is the C₂ symmetric stripe magnetism, with moments parallel to the ordering vector. Additionally, there are two tetragonal, or C₄, magnetic phases, one with out-of-plane moments observed close to optimal doping in several compounds [2], and another with in-plane moments forming a spin-vortex structure [3]. The intimate relation between the ordering vectors and the moment direction is a direct consequence of SOC. Interestingly, a proliferation of different magnetic phases in close proximity in the phase diagram is observed in different pnictide materials as the putative magnetic quantum critical point is approached, e.g. by changing doping or pressure. We demonstrate, using a renormalization group approach, that such a behavior is a natural consequence of the interplay between magnetic quantum fluctuations and SOC. Formally, this is due to the emergence of a Gaussian fixed point leading to an enhanced magnetic degeneracy. This leads to an increase in the phase space of fluctuations which can enhance the transition temperature of superconductivity. Furthermore, we show how a rich landscape of magnetic phases emerges as a result of frustration between spin-anisotropic and spin-isotropic interactions. These novel phases consist of admixtures of the known C₂ and C₄ orders, and provide possible candidates for experimental observations [4].

[1] Borisenko et al, Nat. Phys. 12, 311 (2016).
[2] Allred et al, Nat. Phys. 12, 493 (2016).
[3] Meier et al, npj Quant. Mat. 3, 5 (2018).
[4] Wang et al, PRB 93, 014514 (2016).