Gap-like feature observed in the non-magnetic topological insulators.

Non-magnetic gap at the Dirac point of topological insulators remains an open question in the field. Here, we present angle-resolved photoemission spectroscopy experiments performed on Cr-doped Bi2Se3 and showed that the Dirac point is progressively buried by the bulk bands and a low spectral weight region in the vicinity of the Dirac point appears. These two mechanisms lead to spectral weight suppression region being mistakenly identified as an energy gap in earlier studies. We further calculated the band structure and found that the original Dirac point splits into two nodes due to the impurity resonant states and the energy separation between the nodes is the low density of state region which appears to be like an energy gap in potoemission experiments. We supported our arguments by presenting photoemission experiments carried out with on- and off- resonant photon energies. Our observation resolves the widely debated questions of apparent energy gap opening at the Dirac point without long range ferromagnetic order in topological insulators.

Prolonged duration of nonequilibrated Dirac fermions in neutral topological insulators.

Topological insulators (TIs) possess spin-polarized Dirac fermions on their surface but their unique properties are often masked by residual carriers in the bulk. Recently, (Sb1-x Bi x )2Te3 was introduced as a non-metallic TI whose carrier type can be tuned from n to p across the charge neutrality point. By using time- and angle-resolved photoemission spectroscopy, we investigate the ultrafast carrier dynamics in the series of (Sb1-x Bi x )2Te3. The Dirac electronic recovery of ∼10 ps at most in the bulk-metallic regime elongated to >400 ps when the charge neutrality point was approached. The prolonged nonequilibration is attributed to the closeness of the Fermi level to the Dirac point and to the high insulation of the bulk. We also discuss the feasibility of observing excitonic instability of (Sb1-x Bi x )2Te3.

Trend of the magnetic anisotropy for individual Mn dopants near the (1 1 0) GaAs surface.

Using a microscopic finite-cluster tight-binding model, we investigate the trend of the magnetic anisotropy energy as a function of the cluster size for an individual Mn impurity positioned in the vicinity of the (1 1 0) GaAs surface. We present results of calculations for large cluster sizes containing approximately 10(4) atoms, which have not been investigated so far. Our calculations demonstrate that the anisotropy energy of a Mn dopant in bulk GaAs, found to be non-zero in previous tight-binding calculations, is purely a finite size effect that vanishes with inverse cluster size. In contrast to this, we find that the splitting of the three in-gap Mn acceptor energy levels converges to a finite value in the limit of the infinite cluster size. For a Mn in bulk GaAs this feature is related to the nature of the mean-field treatment of the coupling between the impurity and its nearest neighbor atoms. We also calculate the trend of the anisotropy energy in the sublayers as the Mn dopant is moved away from the surface towards the center of the cluster. Here the use of large cluster sizes allows us to position the impurity in deeper sublayers below the surface, compared to previous calculations. In particular, we show that the anisotropy energy increases up to the fifth sublayer and then decreases as the impurity is moved further away from the surface, approaching its bulk value. The present study provides important insights for experimental control and manipulation of the electronic and magnetic properties of individual Mn dopants at the semiconductor surface by means of advanced scanning tunneling microscopy techniques.

Time-dependent electron transport through a strongly correlated quantum dot: multiple-probe open-boundary conditions approach.

We present a time-dependent study of electron transport through a strongly correlated quantum dot, which combines adiabatic lattice density functional theory in the Bethe ansatz local-density approximation (BALDA) to the Hubbard model, with the multiple-probe battery method for open-boundary simulations in the time domain. In agreement with the recently proposed dynamical picture of Coulomb blockade, a characteristic driven regime, defined by regular current oscillations, is demonstrated for a certain range of bias voltages. We further investigate the effects of systematically improving the approximation for the electron-electron interaction at the dot site (going from non-interacting, through Hartree-only to adiabatic BALDA) on the transmission spectrum and the I-V characteristics. In particular, a negative differential conductance is obtained at large bias voltages and large Coulomb interaction strengths. This is attributed to the combined effect of the electron-electron interaction at the dot and the finite bandwidth of the electrodes.