Collinear Rashba-Edelstein effect in non-magnetic chiral materials

Efficient generation and manipulation of spin signals in a given material without invoking external magnetism remain one of the challenges in spintronics. The spin Hall effect (SHE) and Rashba-Edelstein effect (REE) are well-known mechanisms to electrically generate spin accumulation in materials with strong spin-orbit coupling (SOC), but the exact role of the strength and type of SOC, especially in crystals with low symmetry, has yet to be explained. In this study, we investigate REE in two different families of non-magnetic chiral materials, elemental semiconductors (Te and Se) and semimetallic disilicides (TaSi$_2$ and NbSi$_2$), using an approach based on density functional theory (DFT). By analyzing spin textures across the full Brillouin zones and comparing them with REE magnitudes calculated as a function of chemical potential, we link specific features in the electronic structure with the efficiency of the induced spin accumulation. Our findings show that magnitudes of REE can be increased by: (i) the presence of purely radial (Weyl-type) spin texture manifesting as the parallel spin-momentum locking, (ii) high spin polarization of bands along one specific crystallographic direction, (iii) low band velocities. By comparing materials possessing the same crystal structures, but different strengths of SOC, we conclude that larger SOC may indirectly contribute to the enhancement of REE. It yields greater spin-splitting of bands along specific crystallographic directions, which prevents canceling the contributions from the oppositely spin-polarized bands over wider energy regions and helps maintain larger REE magnitudes. We believe that these results will be useful for designing spintronics devices and may aid further computational studies searching for efficient REE in materials with different symmetries and SOC strengths

Analogs of Rashba-Edelstein effect from density functional theory

Studies of structure-property relationships in spintronics are essential for the design of materials that can fill specific roles in devices. For example, materials with low symmetry allow unconventional configurations of charge-to-spin conversion which can be used to generate efficient spin-orbit torques. Here, we explore the relationship between crystal symmetry and geometry of the Rashba-Edelstein effect (REE) that causes spin accumulation in response to an applied electric current. Based on a symmetry analysis performed for 230 crystallographic space groups, we identify classes of materials that can host conventional or collinear REE. Although transverse spin accumulation is commonly associated with the so-called 'Rashba materials', we show that the presence of specific spin texture does not easily translate to the configuration of REE. More specifically, bulk crystals may simultaneously host different types of spin-orbit fields, depending on the crystallographic point group and the symmetry of the specific $k$-vector, which, averaged over the Brillouin zone, determine the direction and magnitude of the induced spin accumulation. To explore the connection between crystal symmetry, spin texture, and the magnitude of REE, we perform first-principles calculations for representative materials with different symmetries. We believe that our results will be helpful for further computational and experimental studies, as well as the design of spintronics devices.Comment: 10 pages, 5 figure

Advanced modeling of materials with PAOFLOW 2.0:New features and software design

Recent research in materials science opens exciting perspectives to design novel quantum materials and devices, but it calls for quantitative predictions of properties which are not accessible in standard first principles packages. PAOFLOW, is a software tool that constructs tight-binding Hamiltonians from self consistent electronic wavefunctions by projecting onto a set of atomic orbitals. The electronic structure provides numerous materials properties that otherwise would have to be calculated via phenomenological models. In this paper, we describe recent re-design of the code as well as the new features and improvements in performance. In particular, we have implemented symmetry operations for unfolding equivalent k-points, which drastically reduces the runtime requirements of first principles calculations, and we have provided internal routines of projections onto atomic orbitals enabling generation of real space atomic orbitals. Moreover, we have included models for non-constant relaxation time in electronic transport calculations, doubling the real space dimensions of the Hamiltonian as well as the construction of Hamiltonians directly from analytical models. Importantly, PAOFLOW has been now converted into a Python package, and is streamlined for use directly within other Python codes. The new object oriented design treats PAOFLOW's computational routines as class methods, providing an API for explicit control of each calculation.</p

Relaxation time approximations in PAOFLOW 2.0

Regardless of its success, the constant relaxation time approximation has limited validity. Temperature and energy dependent effects are important to match experimental trends even in simple situations. We present the implementation of relaxation time approximation models in the calculation of Boltzmann transport in PAOFLOW 2.0 and apply those to model band-structures. In addition, using a self-consistent fitting of the model parameters to experimental conductivity data, we provide a flexible tool to extract scattering rates with high accuracy. We illustrate the approximations using simple models and then apply the method to GaAs, Si, Mg3Sb2, and CoSb3.Comment: 20 pages, 7 figure

Long-range current-induced spin accumulation in chiral crystals

Chiral materials, similarly to human hands, have distinguishable right-handed and left-handed enantiomers which may behave differently in response to external stimuli. Here, we use for the first time an approach based on the density functional theory (DFT)+PAOFLOW calculations to quantitatively estimate the so-called collinear Rashba–Edelstein effect (REE) that generates spin accumulation parallel to charge current and can manifest as chirality-dependent charge-to-spin conversion in chiral crystals. Importantly, we reveal that the spin accumulation induced in the bulk by an electric current is intrinsically protected by the quasi-persistent spin helix arising from the crystal symmetries present in chiral systems with the Weyl spin–orbit coupling. In contrast to conventional REE, spin transport can be preserved over large distances, in agreement with the recent observations for some chiral materials. This allows, for example, the generation of spin currents from spin accumulation, opening novel routes for the design of solid-state spintronics devices

Quasi-two-dimensional Fermi surface of superconducting line-nodal metal CaSb2

We report on the Fermi surfaces and superconducting parameters of CaSb₂ single crystals (superconducting below Tc ~ 1.8 K) grown by the self-flux method. The frequency of de Haas–van Alphen and Shubnikov–de Haas oscillations evidences a quasi-two-dimensional (quasi-2D) Fermi surface, consistent with one of the Fermi surfaces forming Dirac lines predicted by first-principles calculations. Measurements in the superconducting state reveal that CaSb₂ is close to a type-I superconductor with the Ginzburg-Landau parameter of around unity. The temperature dependence of the upper critical field Hc₂ is well described by a model considering two superconducting bands, and the enhancement of the effective mass estimated from Hc₂(0K) is consistent with the quasi-2D band observed by the quantum oscillations. Our results indicate that a quasi-2D band forming Dirac lines contributes to the superconductivity in CaSb₂