Optimizing Tc in the (Mn,Cr,Ga)As and (Mn,Ga)(As,P) Ternary Alloys

We explore two possible ways to enhance the critical temperature $T_c$ in the dilute magnetic semiconductor Mn$_{0.08}$Ga$_{0.92}$As. Within the context of the double-exchange and RKKY pictures, the ternary alloys Mn$_{x}$Cr$_{0.08-x}$Ga$_{0.92}$As and Mn$_{0.08}$Ga$_{0.92}$As$_y$P$_{1-y}$ might be expected to have $T_c$ higher than the pseudobinary Mn$_{0.08}$Ga$_{0.92}$As. To test whether the expectations from model pictures are confirmed, we employ linear response theory within the local-density approximation to search for theoretically higher critical temperatures in these ternary alloys. Our results show that neither co-doping Mn with Cr, nor alloying As with P improves $T_c$. Alloying with Cr is found to be deleterious to the $T_c$. Mn$_{0.08}$Ga$_{0.92}$As$_y$P$_{1-y}$ shows almost linear dependence of $T_c$ on $y$.Comment: 10 pages, 5 figure

Many-body effects in iron pnictides and chalcogenides -- non-local vs dynamic origin of effective masses

We apply the quasi-particle self-consistent GW (QSGW) approximation to some of the iron pnictide and chalcogenide superconductors. We compute Fermi surfaces and density of states, and find excellent agreement with experiment, substantially improving over standard band-structure methods. Analyzing the QSGW self-energy we discuss non-local and dynamic contributions to effective masses. We present evidence that the two contributions are mostly separable, since the quasi-particle weight is found to be essentially independent of momentum. The main effect of non locality is captured by the static but non-local QSGW effective potential. Moreover, these non-local self-energy corrections, absent in e.g. dynamical mean field theory (DMFT), can be relatively large. We show, on the other hand, that QSGW only partially accounts for dynamic renormalizations at low energies. These findings suggest that QSGW combined with DMFT will capture most of the many-body physics in the iron pnictides and chalcogenides.Comment: 4+ pages, 3 figure

Theory of spin loss at metallic interfaces

Interfacial spin-flip scattering plays an important role in magnetoelectronic devices. Spin loss at metallic interfaces is usually quantified by matching the magnetoresistance data for multilayers to the Valet-Fert model, while treating each interface as a fictitious bulk layer whose thickness is $\delta$ times the spin-diffusion length. By employing the properly generalized circuit theory and the scattering matrix approaches, we derive the relation of the parameter $\delta$ to the spin-flip transmission and reflection probabilities at an individual interface. It is found that $\delta$ is proportional to the square root of the probability of spin-flip scattering. We calculate the spin-flip transmission probability for flat and rough Cu/Pd interfaces using the Landauer-B\"uttiker method based on the first-principles electronic structure and find $\delta$ in reasonable agreement with experiment.Comment: 5 pages + supplementary material, 3 figures, version accepted in Phys. Rev. Let

Ab initio transport calculations: from normal to superconducting current

Applying the Bogoliubov-de Gennes equations with density-functional theory, it is possible to formulate first-principles description of current-phase relationships in superconducting/normal (magnetic)/superconducting trilayers. Such structures are the basis for the superconducting analog of Magnetoresistive random access memory devices (JMRAM). In a recent paper [1] we presented results from the first attempt to formulate such a theory, applied to the Nb/Ni/Nb trilayers. In the present work we provide computational details, explaining how to construct key ingredient (scattering matrices $S_N$) in a framework of linear muffin-tin orbitals (LMTO).Comment: Proceeding for the Spintronics XVI - SPIE 2023 conferenc

Quasiparticle Self-Consistent GW Theory

In past decades the scientific community has been looking for a reliable first-principles method to predict the electronic structure of solids with high accuracy. Here we present an approach which we call the quasiparticle self-consistent GW approximation (QpscGW). It is based on a kind of self-consistent perturbation theory, where the self-consistency is constructed to minimize the perturbation. We apply it to selections from different classes of materials, including alkali metals, semiconductors, wide band gap insulators, transition metals, transition metal oxides, magnetic insulators, and rare earth compounds. Apart some mild exceptions, the properties are very well described, particularly in weakly correlated cases. Self-consistency dramatically improves agreement with experiment, and is sometimes essential. Discrepancies with experiment are systematic, and can be explained in terms of approximations made.Comment: 12 pages, 3 figure

Role of Disorder in Mn:GaAs, Cr:GaAs, and Cr:GaN

We present calculations of magnetic exchange interactions and critical temperature T_c in Mn:GaAs, Cr:GaAs and Cr:GaN. The local spin density approximation is combined with a linear-response technique to map the magnetic energy onto a Heisenberg hamiltonion, but no significant further approximations are made. Special quasi-random structures in large unit cells are used to accurately model the disorder. T_c is computed using both a spin-dynamics approach and the cluster variation method developed for the classical Heisenberg model. We show the following: (i) configurational disorder results in large dispersions in the pairwise exchange interactions; (ii) the disorder strongly reduces T_c; (iii) clustering in the magnetic atoms, whose tendency is predicted from total-energy considerations, further reduces T_c. Additionally the exchange interactions J(R) are found to decay exponentially with distance R^3 on average; and the mean-field approximation is found to be a very poor predictor of T_c, particularly when J(R) decays rapidly. Finally the effect of spin-orbit coupling on T_c is considered. With all these factors taken into account, T_c is reasonably predicted by the local spin-density approximation in MnGaAs without the need to invoke compensation by donor impurities.Comment: 10 pages, 3 figure

First-principles analysis of spin-disorder resistivity of Fe and Ni

Spin-disorder resistivity of Fe and Ni and its temperature dependence are analyzed using noncollinear density functional calculations within the supercell method. Different models of thermal spin disorder are considered, including the mean-field approximation and the nearest-neighbor Heisenberg model. Spin-disorder resistivity is found to depend weakly on magnetic short-range order. If the local moments are kept frozen at their zero-temperature values, very good agreement with experiment is obtained for Fe, but for Ni the resistivity at elevated temperatures is significantly overestimated. Agreement with experiment for Fe is improved if the local moments are iterated to self-consistency. The overestimation of the resistivity for paramagnetic Ni is attributed to the reduction of the local moments down to 0.35 Bohr magnetons. Overall, the results suggest that low-energy spin fluctuations in Fe and Ni are better viewed as classical rotations of local moments rather than quantized spin fluctuations that would require an (S+1)/S correction.Comment: 10 pages (RevTeX), 6 eps figure