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Register a new userEffective spin-orbit models using correlated first-principles wave functions
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Diffusion Monte Carlo is one of the most accurate scalable many-body methods for solid state systems. However, to date, spin-orbit interactions have not been incorporated into these calcualtions at a first-principles level; only having been applied to small systems. In this technique, we use explicitly correlated first-principles quantum Monte Carlo calculations to derive an effective spin-orbit model Hamiltonian. The simplified model Hamiltonian is then solved to obtain the energetics of the system. To demonstrate this method, benchmark studies are performed in main-group atoms and monolayer tungsten disulfide, where high accuracy is obtained.
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We compute spin-orbit torques (SOTs) in strained PtMnSb from first principles. We consider both tetragonal strain and shear strain. We find a strong linear dependence of the field-like SOTs on these strains, while the antidamping SOT is only moderately sensitive to shear strain and even insensitive to tetragonal strain. We also study the dependence of the SOT on the magnetization direction. In order to obtain analytical expressions suitable for fitting our numerical textit{ab-initio} results we derive a general expansion of the SOT in terms of all response tensors that are allowed by crystal symmetry. Our expansion includes also higher-order terms beyond the usually considered lowest order. We find that the dependence on the strain is much smaller for the higher-order terms than for the lowest order terms. In order to judge the sensitivity of the SOT on the exchange correlation potential we compute the SOT in both GGA and LDA. We find that the higher-order terms depend significantly on the exchange-correlation potential, while the lowest order terms are insensitive to it. Since the higher-order terms are small in comparison to the lowest order terms the total SOT is insensitive to the exchange correlation potential in strained PtMnSb.
The electronic band structure of graphene in the presence of spin-orbit coupling and transverse electric field is investigated from first principles using the linearized augmented plane-wave method. The spin-orbit coupling opens a gap at the $K(K)$-point of the magnitude of 24 $mu$eV (0.28 K). This intrinsic splitting comes 96% from the usually neglected $d$ and higher orbitals. The electric field induces an additional (extrinsic) Bychkov-Rashba-type splitting of 10 $mu$eV (0.11 K) per V/nm, coming from the $sigma$-$pi$ mixing. A mini-ripple configuration with every other atom is shifted out of the sheet by less than 1% differs little from the intrinsic case.
Localized Wannier functions provide an efficient and intuitive framework to compute electric polarization from first-principles. They can also be used to represent the electronic systems at fixed electric field and to determine dielectric properties of insulating materials. Here we develop a Wannier-function-based formalism to perform first-principles calculations at fixed polarization. Such an approach allows to extract the polarization-energy landscape of a crystal and thus supports the theoretical investigation of polar materials. To facilitate the calculations, we implement a quasi-Newton method that simultaneously relaxes the internal coordinates and adjusts the electric field in crystals at fixed polarization. The method is applied to study the ferroelectric behavior of $mathrm{BaTiO_3}$ and $mathrm{PbTiO_3}$ in tetragonal phases. The physical processes driving the ferroelectricity of both compounds are examined thanks to the localized orbital picture offered by Wannier functions. Hence, changes in chemical bonding under ferroelectric distortion can be accurately visualized. The difference in the ferroelectric properties of $mathrm{BaTiO_3}$ and $mathrm{PbTiO_3}$ is highlighted. It can be traced back to the peculiarities of their electronic structures.
We have systematically investigated substrate-strain effects on the electronic structures of two representative Sr-iridates, a correlated-insulator Sr$_2$IrO$_4$ and a metal SrIrO$_3$. Optical conductivities obtained by the emph{ab initio} electronic structure calculations reveal that the tensile strain shifts the optical peak positions to higher energy side with altered intensities, suggesting the enhancement of the electronic correlation and spin-orbit coupling (SOC) strength in Sr-iridates. The response of the electronic structure upon tensile strain is found to be highly correlated with the direction of magnetic moment, the octahedral connectivity, and the SOC strength, which cooperatively determine the robustness of $J_{eff}$=1/2 ground states. Optical responses are analyzed also with microscopic model calculation and compared with corresponding experiments. In the case of SrIrO$_3$, the evolution of the electronic structure near the Fermi level shows high tunability of hole bands, as suggested by previous experiments.
Element-specific orbital magnetic moments and their anisotropies in perpendicularly magnetised Co/Pd multilayers are investigated using Co L-edge and Pd M-edge angle-dependent x-ray magnetic circular dichroism. We show that the orbital magnetic moments in Co are anisotropic, whereas those in Pd are isotropic. The first-principles density-functional-theory calculations also suggest that the Co/Pd interfacial orbital magnetic moments in Co are anisotropic and contribute to the perpendicular magnetic anisotropy (PMA), and that the isotropic ones in Pd manipulates the Co orbitals at the interface through proximity effects. Orbital-resolved anatomy of Co/Pd interfaces reveals that the orbital moment anisotropy in Co and spin-flipped transition related to the magnetic dipoles in Pd are essential for the appearance of PMA.
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