Thin films of a three-dimensional topological insulator in a strong magnetic field: a microscopic study

The response of thin films of Bi$_2$Se$_3$ to a strong perpendicular magnetic field is investigated by performing magnetic bandstructure calculations for a realistic multi-band tight-binding model. Several crucial features of Landau quantization in a realistic three-dimensional topological insulator are revealed. The $n=0$ Landau level is absent in ultra-thin films, in agreement with experiment. In films with a crossover thickness of five quintuple layers, there is a signature of the $n=0$ level, whose overall trend as a function of magnetic field matches the established low-energy effective-model result. Importantly, we find a field-dependent splitting and a strong spin-polarization of the $n=0$ level which can be measured experimentally at reasonable field strengths. Our calculations show mixing between the surface and bulk Landau levels which causes the character of levels to evolve with magnetic field.Comment: 5 pages, 4 figure

Electronic structure and magnetic properties of Mn and Fe impurities near the GaAs (110) surface

Combining density functional theory calculations and microscopic tight-binding models, we investigate theoretically the electronic and magnetic properties of individual substitutional transition-metal impurities (Mn and Fe) positioned in the vicinity of the (110) surface of GaAs. For the case of the [Mn2+]0 plus acceptor-hole (h) complex, the results of a tight-binding model including explicitly the impurity delectrons are in good agreement with approaches that treat the spin of the impurity as an effective classical vector. For the case of Fe, where both the neutral isoelectronic [Fe3+]0 and the ionized [Fe2+]− states are relevant to address scanning tunneling microscopy (STM) experiments, the inclusion of d orbitals is essential. We find that the in-gap electronic structure of Fe impurities is significantly modified by surface effects. For the neutral acceptor state [Fe2+,h]0, the magnetic-anisotropy dependence on the impurity sublayer resembles the case of [Mn2+,h]0. In contrast, for [Fe3+]0 electronic configuration the magnetic anisotropy behaves differently and it is considerably smaller. For this state we predict that it is possible to manipulate the Fe moment, e.g., by an external magnetic field, with detectable consequences in the local density of states probed by STM

Electrical control of spin dynamics in finite one-dimensional systems

We investigate the possibility of the electrical control of spin transfer in monoatomic chains incorporating spin-impurities. Our theoretical framework is the mixed quantum-classical (Ehrenfest) description of the spin dynamics, in the spirit of the s-d-model, where the itinerant electrons are described by a tight-binding model while localized spins are treated classically. Our main focus is on the dynamical exchange interaction between two well-separated spins. This can be quantified by the transfer of excitations in the form of transverse spin oscillations. We systematically study the effect of an electrostatic gate bias V_g on the interconnecting channel and we map out the long-range dynamical spin transfer as a function of V_g. We identify regions of V_g giving rise to significant amplification of the spin transmission at low frequencies and relate this to the electronic structure of the channel.Comment: 9 pages, 11 figure