We propose a realistic topological p?n junction (TPNJ) by matching two Bi₂Se₃ (0001) slabs with opposite arrangements of planar twin boundary defects. The atomistic modeling of such a device leads to dislocation defects in the hexagonal lattice in several quintuple layers. Nevertheless, total energy calculations reveal that the interface relaxes, yielding a smooth geometrical transition that preserves the nearest-neighbors fcc-type geometry throughout these defect layers. The electronic, magnetic, and transport properties of the junction have then been calculated at the ab initio level under open boundary conditions, i.e., employing a thin-film geometry that is infinite along the electron transport direction. Indeed, a p?n junction is obtained with a built-in potential as large as 350 meV. The calculations further reveal the spin texture across the interface with unprecedented detail. As the main result, we obtain non-negligible transmission probabilities around the Γ point, which involve an electron spin-flip process while crossing the interface.

Cl-functionalized scanning tunneling microscopy (STM) tips are fabricated by modifying a tungsten STM tip in situ on islands of ultrathin NaCl(100) films on Au(111) surfaces. The functionalized tips are used to achieve clear atomicresolution imaging of NaCl(100) islands. In comparison with bare metal tips, the chemically modified tips yield drastically enhanced spatial resolution as well as contrast reversal in STM topographs, implying that Na atoms, rather than Cl atoms, are imaged as protrusions. STM simulations based on a Green's function formalism reveal that the experimentally observed contrast reversal in the STM topographs is due to the highly localized character of the Cl-p_z states at the tip apex. An additional remarkable characteristic of the modified tips is that in dI/dV maps, a Na atom appears as a ring with a diameter that depends crucially on the tip-sample distance.