9 research outputs found
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 and NbSi), 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 -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