We report the results of our calculations of the lattice parameters and fundamental band gaps in the rocksalt structured quasibinary random alloy aluminum scandium nitride (AlxSc1-xN) in the range of 0 x 1. AlxSc1-xN is a random alloy with very promising properties for the introduction of novel functionalities in (opto)electronic devices. The lattice parameters for the cubic rocksalt AlxSc1-xN deviate positively from Vegard's law with a small bowing. The fundamental band gap shows a more complex behavior and contrary to previous reports, the fundamental band gap in rocksalt AlxSc1-xN is indirect. Furthermore, rocksalt AlxSc1-xN allows band -gap engineering in the range from 1.4 to 7.1 eV.

The cubic phase of the ternary InxGa1-xN alloy has attracted significant interest as a material for next- generation high-performance electronic and optoelectronic applications. It has recently been demonstrated that epitaxial growth techniques can overcome the miscibility gap in this alloy system, thereby revealing the existence of CuPt-type ordering around x approximate to 0.5 [ACS Appl. Mater. Interfaces 15 , 39513 (2023)]. We present a comprehensive theoretical analysis of the structural and electronic properties of the (In, Ga)N alloy in the zinc-blende structure. This study covers both pure phases of InN and GaN, as well as random alloys of InxGa1-xN. Furthermore, it examines the composition of In 0.5 Ga 0.5 N in the ordered phases with CuPt-type ordering and chalcopyrite-type ordering. We establish that the investigated structures are metastable. The results presented include the lattice parameters, elastic constants, and the phononic and electronic band structures. We compare the random phase with the ordered structures and report how the structure affects the properties. This results facilitate the differentiation between the phases in experimental settings.

Disorder effects in alloys are usually modeled by averaging various supercell calculations considering different positions of the alloy atoms. This approach, however, is only possible as long as the portion of the individual components of the alloy is sufficiently large. Herein, we present anab initiostudy considering the lithium insertion material Li1-x[Ni0.33Co0.33Mn0.33]O2as model system to demonstrate the power of the coherent potential approximation within the Korringa-Kohn-Rostoker Green's function method. This approach enables the description of disorder effects within alloy systems of any composition. It is applied in this study to describe the (de-)intercalation of arbitrary amounts of lithium from the cathode active material. Moreover, we highlight that using either fully optimized structures or experimental lattice parameters and atomic positions both lead to comparable results. Our findings suggest that this approach is also suitable for modeling the electronic structure of state-of-the-art materials such as high-nickel alloys.

Understanding electronic transport properties is important for designing devices for applications. Many studies rely on the semiclassical Boltzmann approach within the relaxation time approximation. This method delivers a graphic physical picture of the scattering process, but in some cases it lacks full quantum-mechanical effects. Here, we use a non-equilibrium Green's function Korringa-Kohn-Rostoker (KKR) method with phase-breaking scattering via virtual Bilttiker terminals as a fully quantum mechanical approach to transport phenomena. With this, we assess the validity of the relation of the self-energy Sigma to the scattering time tau, often used in literature in the case of constant relaxation time approximation. We argue that the scattering time does not affect the thermopower in the Boltzmann approach and thus should take no effect either on the thermopower calculated via the Keldysh approach. We find a nearly linear relation for the transmission function T-s(E-F, Sigma) of free electrons and Cu with respect to 1/Sigma. However, we find that this is not the case for Pd. We attribute this to neighboring states contributing due to the additional broadening via the self-energy Sigma. These findings suggest that a simple identification of scattering time and self-energy is not sufficient. Finally, we discuss the benefits and limits of the application of the virtual terminal approach.

For the technologically relevant spin Hall effect, most theoretical approaches rely on the evaluation of the spin-conductivity tensor. In contrast, for most experimental configurations the generation of spin accumulation at interfaces and surfaces is the relevant quantity. Here, we directly calculate the accumulation of spins due to the spin Hall effect at the surface of a thin metallic layer, making quantitative predictions for different materials. Two distinct limits are considered, both relying on a fully relativistic Korringa-Kohn-Rostoker density functional theory method. In the semiclassical approach, we use the Boltzmann transport formalism and compare it directly with a fully quantum mechanical nonequilibrium Keldysh formalism. Restricting the calculations to the spin-Hall-induced, odd-in-spatial-inversion, contribution in the limit of the relaxation time approximation, we find good agreement between the two methods, where deviations can be attributed to the complexity of Fermi surfaces. Finally, we compare our results with experimental values of the spin accumulation at surfaces as well as the Hall angle and find good agreement for the trend across the considered elements.

Since there are still research interests in the physical properties of quasi-binary thermoelectric Mg2X1-xYx alloys, with X, Y = Si, Ge, Sn, we present an ab initio analysis that yields the relative formation energy and effective masses of the conduction bands, in the whole compositional range x. We base our calculations on the full-relativistic Korringa, Kohn and Rostocker (KKR) Green's functions formalism within the coherent potential approximation (CPA). Formation energies, measured relative to the end Mg2X compounds, show no excess energy for the Mg2Si-Mg2Ge substitution thus indicating a complete solubility. In contrast, concave and asymmetric formation energies for intermediate compositions in the Mg2X-Mg2Sn alloys manifest a miscibility gap. With this basis, we compute and discuss the crossing of the conduction bands observed in n-type Mg2X1-xSnx materials. We present direction- and band-dependent effective masses using a generalized single parabolic band effective mass approximation to discuss anisotropic effects, to interpret available experimental and theoretical data, and to predict intermediate and not yet published transport parameters on these alloys.

Magnetite nanoparticles (particle diameter d(NP) = 20 nm) were arranged into chains of cylinderlike entities of fixed radius R with constant spacings D between neighboring entities. For this purpose, chains of circular openings were defined in a 250-nm-thick electron-sensitive resist on a Si substrate by electron beam lithography. These patterns were subsequently filled with the magnetite nanoparticles using a variant of the meniscus force deposition method. To study the dipolar magnetic interaction between the spherical magnetite particles within the cylinders as well as that between cylinders, three series of chain arrangements were prepared, each with another constant average cylinder radius (R = 360, 240, and 160 nm). The three samples of each series differ in terms of their D values, which vary between 700 nm (no intercylinder coupling) and 50 nm (magnetic coupling between cylinders). Angle-dependent ferromagnetic resonance (FMR) measurements revealed that for large R and D only one broad resonance appears, while for R = 240 and 160 nm two resonances are present. At short D, an angular dependence of the resonances induced by the coupling between the cylinders is clearly visible. Furthermore, the amplitude of the main resonance decreases, and side bands occur when the cylinders of the chain are hollow, i.e., when some nanoparticles are removed from the center of each cylinderlike entity. The dynamics of the coupled magnetic dipoles of the magnetite particles and its impact on the FMR spectra of the samples, i.e., associating the different resonances to characteristic collective oscillations of the magnetic moments within the ordered arrangement of magnetite nanoparticles, can be understood using micromagnetic simulations based on a numerical solution of the Landau-Lifschitz-Gilbert equation.

In our proof-of-principle study we examine the influence of skyrmions on magnetoresistive transport. In particular, we show that magnetic tunnel junctions are a technologically appealing and promising way for electrical detection of noncollinear magnetic structures. The calculated effect is shown to originate from scattering between different k states and cannot be identified through densities of states alone. Our results suggest that the detection efficiency strongly depends on the utilized materials.

Spin caloric transport refers to the coupling of heat with spin transport. Its applications primarily concern the generation of spin currents and control of magnetisation by temperature gradients for information technology, known by the synonym spin caloritronics. Within the framework of ab initio theory, new tools are being developed to provide an additional understanding of these phenomena in realistic materials, accounting for the complexity of the electronic structure without adjustable parameters. Here, we review this progress, summarising the principles of the density-functional-based approaches in the field and presenting a number of application highlights. Our discussion includes the three most frequently employed approaches to the problem, namely the Kubo, Boltzmann, and Landauer–Büttiker methods. These are showcased in specific examples that span, on the one hand, a wide range of materials, such as bulk metallic alloys, nano-structured metallic and tunnel junctions, or magnetic overlayers on heavy metals, and, on the other hand, a wide range of effects, such as the spin-Seebeck, magneto-Seebeck, and spin-Nernst effects, spin disorder, and the thermal spin-transfer and thermal spin–orbit torques.

The interplay of charge, spin and heat transport is investigated in the fascinating research field of spin caloritronics, the marriage of spintronics and thermoelectrics. Here, many new spin-dependent thermal transport phenomena in magnetic nanostructures have been explored in the recent years. One of them is the tunnel magneto-Seebeck (TMS) effect in magnetic tunnel junctions (MTJs) that has large potential for future nanoelectronic devices, such as nanostructured sensors for three-dimentional thermal gradients, or scanning tunneling microscopes driven by temperature differences. The TMS describes the dependence of the MTJ's thermopower on its magnetic cofiguration when a thermal gradient is applied. In this review, we highlight the successful way from first observation of the TMS in 2011 to current ongoing developments in this research area. We emphasize on different heating techniques, material designs, applications, and additional physical aspects such as the role of the thermal conductivity of the barrier material. We further demonstrate the efficient interplay between ab initio calculations and experiments within this field, as this has led, e.g., to the detection of large TMS ratios in MTJs with half-metallic Heusler electrodes.

Mg2X1-xYx thermoelectric materials and their pseudobinary related alloys, for X, Y = Si,Ge,Sn, have been the subject of intense research activity. Studies have revealed their electronic nature, but important properties are still unclear or missing in the case of intermediate solid solutions. Within the CPA-KKR formalism, in the full-relativistic description, we observe stronger deviations from Vegard's law as the lattice constant difference increases. We compute the Bloch spectral density function to map a local-parabolic band structure in the alloys, whenever the disorder-induced broadening is small. We trace nonlinear trends for the indirect and direct band gaps, spin-orbit coupling and the crossing between the low-lying conduction bands observed in the Mg2X1-3Snx systems. Our computations show that the broadening of the heavy- and light-hole bands is the smallest, but, in either case, we discuss the trends and anisotropy effects of band- and direction-dependent effective masses.

Rectangular assemblies with different aspect ratios were prepared with spherical magnetite nanoparticles (diameter d = 20 nm) on lithographically patterned substrates using a variant of the meniscus force deposition method. The aspect ratio (width:length) of the rectangular assemblies was varied from very low (1:1) to very high (1:1000) values. Angle-dependent ferromagnetic resonance measurements were performed to study the influence of the shape anisotropy on the magnetic properties. Using an analytical model based on the Smit-Suhl formalism for single-domain magnets, the demagnetizing factors were determined. The analysis of the resonance signals shows that, for small aspect ratios, the ratio of the in-plane demagnetization factors is inversely proportional to the corresponding ratio of width:length; that is, the assemblies behave like a single ferromagnet due to dipolar magnetic coupling between the particles. At larger aspect ratios a more complicated behavior is observed which indicates the formation of a multidomain-like structure inside the assemblies caused by geometrical inhomogeneities in the filling of the assemblies. Micromagnetic simulations of the magnetic properties of the assemblies support this assumption qualitatively and suggest that the formation of these inhomogeneities can be controlled by the fabrication process. The results provide insights into the collective magnetic behavior of nanoparticle assemblies, highlighting the high degree of tunability of the magnetic properties of such assemblies, which makes them promising building blocks for future magnetic devices.

We find an unusual angular dependence of the tunneling magneto-Seebeck effect (TMS). The conductance shows normally a cosine-dependence with the angle between the magnetizations of the two ferromagnetic leads. In contrast, the angular dependence of the TMS depends strongly on the tunneling magneto resistance (TMR) ratio. For small TMR ratios we obtain also a cosine-dependence whereas for very large TMR ratios the angular dependence approaches a step-like function.

We present a method to calculate the electron-phonon induced resistivity of metals in scattering-time approximation based on the nonequilibrium Green's function formalism. The general theory as well as its implementation in a density-functional theory based Korringa-Kohn-Rostoker code are described and subsequently verified by studying copper as a test system. We model the thermal expansion by fitting a Debye-Gruneisen curve to experimental data. Both the electronic and vibrational structures are discussed for different temperatures, and employing a Wannier interpolation of these quantities we evaluate the scattering time by integrating the electron linewidth on a triangulation of the Fermi surface. Based thereupon, the temperature-dependent resistivity is calculated and found to be in good agreement with experiment. We show that the effect of thermal expansion has to be considered in the whole calculation regime. Further, for low temperatures, an accurate sampling of the Fermi surface becomes important.

Topological insulators are candidates to open up a novel route in spin based electronics. Different to traditional ferromagnetic materials, where the carrier spin-polarization and magnetization are based on the exchange interaction, the spin properties in topological insulators are based on the coupling of spin- and orbit interaction connected to its momentum. Specific ways to control the spin-polarization with light have been demonstrated: the energy momentum landscape of the Dirac cone provides spin-momentum locking of the charge current and its spin. We investigate a spin-related signal present only during the laser excitation studying real and imaginary part of the complex Kerr angle by disentangling spin and lattice contributions. This coherent signal is only present at the time of the pump-pulses' light field and can be described in terms of a Raman coherence time. The Raman transition involves states at the bottom edge of the conduction band. We demonstrate a coherent femtosecond control of spin-polarization for electronic states at around the Dirac cone.

In the field of spin caloritronics, spin-dependent transport phenomena are observed in a number of current experiments where a temperature gradient across a nanostructured interface is applied. The interpretation of these experiments is not clear as both phonons and electrons may contribute to thermal transport. Therefore, it still remains an open question how the temperature drop across a magnetic nanostructured interface arises microscopically. We answer this question for the case of a magnetic tunnel junction (MTJ) where the tunneling magneto-Seebeck effect occurs. Our explanation may be extended to other types of nanostructured interfaces. We explicitly calculate phonon and electron thermal conductance across Fe/MgO/Fe MTJs in an ab initio approach using a Green function method. Furthermore, we are able to calculate the electron and phonon temperature profile across the Fe/MgO/Fe MTJ by estimating the electron-phonon interaction in the Fe leads. Our results show that there is an electron-phonon temperature imbalance at the Fe-MgO interfaces. As a consequence, a revision of the interpretation of current experimental measurements may be necessary.

Thermoelectric effects in magnetic tunnel junctions are promising to serve as the basis for logic devices or memories in a ”green” information technology. However, up to now the readout contrast achieved with Seebeck effects was magnitudes smaller compared to the well-established tunnel magnetoresistance effect. Here, we resolve this problem by demonstrating that the tunnel magneto-Seebeck effect (TMS) in CoFeB/MgO/CoFeB tunnel junctions can be switched on to a logic “1” state and off to “0” by simply changing the magnetic state of the CoFeB electrodes. This new functionality is achieved by combining a thermal gradient and an electric field. Our results show that the signal crosses zero and can be adjusted by tuning a bias voltage that is applied between the electrodes of the junction; hence, the name of the effect is bias-enhanced tunnel magneto-Seebeck effect (bTMS). Via the spin- and energy-dependent transmission of electrons in the junction, the bTMS effect can be configured using the bias voltage with much higher control than the tunnel magnetoresistance and even completely suppressed for only one magnetic configuration. Moreover, our measurements are a step towards the experimental realization of high TMS ratios without additional bias voltage, which are predicted for specific Co-Fe compositions.