We perform first-principles calculations based on density functional theory of the spin-resolved conductance of poly(vinylidene fluoride)- (PVDF) based multiferroic tunnel junctions (MFTJs). We consider Co/PVDF/O/Co (0001) MFTJs with one oxidized interface, representing the different experimental growth conditions for the two interfaces. We demonstrate that this natural asymmetry leads to multiple resistance states associated with different magnetization configurations of the electrodes and ferroelectric polarization orientations of the barrier. Our results indicate very high tunability of the tunneling magnetoresistance and electroresistance effects, which could be useful for logic and memory applications.

Topological insulators are very interesting from a fundamental point of view, and their unique properties may be useful for electronic and spintronic device applications. From the point of view of applications it is important to understand the decay behavior of carriers injected in the band gap of the topological insulator, which is determined by its complex band structure (CBS). Using first-principles calculations, we investigate the dispersion and symmetry of the complex bands of Bi ₂ Se ₃ family of three-dimensional topological insulators. We compare the CBS of a band insulator and a topological insulator and follow the CBS evolution in both when the spin–orbit interaction is turned on. We find significant differences in the CBS linked to the topological band structure. In particular, our results demonstrate that the evanescent states in Bi ₂ Se ₃ are non-trivially complex, i.e. contain both the real and imaginary contributions. This explains quantitatively the oscillatory behavior of the band gap obtained from Bi ₂ Se ₃ (0 0 0 1) slab calculations.

We derive expressions for both parallel and perpendicular components of spin transfer torque (STT) in magnetic tunnel junctions (MTJs), which have several important advantages over the currently available expressions: First they are derived in a more realistic approximation, resulting in excellent agreement with exact results even in the presence of resonant tunneling. Second, we show that they can be expressed in terms of the scattering matrix elements, which gives them a clear physical interpretation. Third, they are given entirely in terms of collinear quantities, which are readily available in existing transport codes. We use these expressions to investigate STT behavior in MTJs with asymmetric barriers at finite bias. The results show that lowering the barrier height in the bulk does not qualitatively change the behavior of STT. The absolute STT increases on account of the overall increase of the barrier transparency; however, the STT efficiency remains in the same range. At the same time, modifications of the interfaces can qualitatively change STT behavior. Thus, interface engineering can be used to control the bias dependence of STT and optimize the performance of STT-based devices.

One of the main challenges for the future hydrogen economy is finding a safe and efficient way to store hydrogen. Materials with large surface areas, like carbon nanotubes and their analogues boron-nitride nanotubes, are being studied as potential candidates for this purpose. We perform density functional theory (DFT) and dispersion-corrected DFT (DFT-D) calculations of the adsorption of molecular hydrogen on graphene and boron-nitride sheets and compare the results against Møller-Plesset perturbation theory (MP2 and MP2.5). Our results indicate that DFT underestimates the binding energies, while DFT-D gives a very good agreement with the higher-order theory. Within DFT-D, we show that the binding energy of molecular hydrogen to the outer walls of carbon nanotubes is more than 40% larger than that of boron-nitride nanotubes.