Julian P. Velev

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.

The measurement of transverse currents through nanopores holds great promise for rapid DNA sequencing, however, the technology is error prone. In this work, a new procedure is developed to overcome the intrinsic structural noise by making use of correlations of currents between neighboring bases.

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.

The transport properties of magnetic tunnel junctions (MTJs) are very sensitive to interface modifications. In this work we investigate both experimentally and theoretically the effect of asymmetric barrier modifications on the bias dependence of tunneling magnetoresistance (TMR) in single crystal Fe/MgO-based MTJs with (i) one crystalline and one rough interface and (ii) with a monolayer of O deposited at the crystalline interface. In both cases we observe an asymmetric bias dependence of TMR and a reversal of its sign at large bias. We propose a general model to explain the bias dependence in these and similar systems reported earlier. The model predicts the existence of two distinct TMR regimes: (i) tunneling regime when the interface is modified with layers of a different insulator and (ii) resonant regime when thin metallic layers are inserted at the interface. We demonstrate that in the tunneling regime negative TMR is due to the high voltage which overcomes the exchange splitting in the electrodes, while the asymmetric bias dependence of TMR is due to the interface transmission probabilities. In the resonant regime inversion of TMR could happen at zero voltage depending on the alignment of the resonance levels with the Fermi surfaces of the electrodes. Moreover, the model predicts a regime in which TMR has different sign at positive and negative bias suggesting possibilities of combining memory with logic functions.

Accounting for the effects of disorder on the transport properties of electronic devices is indispensable for comparison with experiment. However, theoretical treatment of disorder presents essential difficulty because the disorder breaks the periodicity of the system. The coherent potential approximation (CPA) solves this problem by replacing the disordered medium with a periodic effective medium. However, calculating the electron current within CPA requires summing scattering diagrams to infinite order called vertex corrections. In this work we reformulate CPA for nonequilibrium electron transport. This approach, based on the nonequilibrium Green's function formalism, greatly simplifies the treatment of disordered transport by eliminating the vertex corrections.

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.

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.

Organic materials are promising for applications in spintronics due to their long spin-relaxation times in addition to their chemical flexibility and relatively low production costs. Most studies of organic materials for spintronics focus on nonpolar dielectrics or semiconductors, serving as passive elements in spin transport devices. Here, we demonstrate that employing organic ferroelectrics, such as poly(vinylidene fluoride) (PVDF), as barriers in magnetic tunnel junctions (MTJs) allows new functionality in controlling the tunneling spin polarization via the ferroelectric polarization of the barrier. Using first-principles methods based on density functional theory we investigate the spin-resolved conductance of Co/PVDF/Co and Co/PVDF/Fe/Co MTJs as model systems. We show that these tunnel junctions exhibit multiple resistance states associated with different magnetization configurations of the electrodes and ferroelectric polarization orientations of the barrier. Our results indicate that organic ferroelectrics may open a new and promising route in organic spintronics with implications for low-power electronics and nonvolatile data storage.

Magnetic tunnel junctions (MTJs), composed of two ferromagnetic electrodes separated by a thin insulating barrier layer, are currently used in spintronic devices, such as magnetic sensors and magnetic random access memories. Recently, driven by demonstrations of ferroelectricity at the nanoscale, thin-film ferroelectric barriers were proposed to extend the functionality of MTJs. Due to the sensitivity of conductance to the magnetization alignment of the electrodes (tunneling magnetoresistance) and the polarization orientation in the ferroelectric barrier (tunneling electroresistance), these multiferroic tunnel junctions (MFTJs) may serve as four-state resistance devices. On the basis of first-principles calculations, we demonstrate four resistance states in SrRuO₃/BaTiO₃/SrRuO₃ MFTJs with asymmetric interfaces. We find that the resistance of such a MFTJ is significantly changed when the electric polarization of the barrier is reversed and/or when the magnetizations of the electrodes are switched from parallel to antiparallel. These results reveal the exciting prospects of MFTJs for application as multifunctional spintronic devices.