Program Highlights

By investigating a series of solar cells with identical device architectures, but with interfacial segregations that are reversed in the active layers, Kahn and Loo et al. [1] found that the device performance of bulk-heterojunction polymer solar cells is, in fact, independent of interfacial segregation of active layers, contrary to a widespread assumption.

To date, much of the information on topological insulators has come from photoemission experiments and scanning tunneling microscopy. Now a team at Princeton has shown that important information may also be gleaned by isolating the small electrical current carried by the surface electrons.

A single electron spin in an external magnetic field forms a two-level system that can be used to create a spin qubit. However, achieving fast single spin rotations, as would be required to control a spin qubit, is a major challenge. It is difficult to drive spin rotations on timescales that are faster than the spin dephasing time and to individually address a single spin on the nanometer scale. We have developed a new method for quantum control of single spins that does not involve conventional electron spin resonance (ESR).

In an ordinary insulator, such as diamond, the occupied electronic states are separated from unoccupied states by a large energy “gap”. The gap prevents current flow when an electric field is applied. Recent research has uncovered a new class of insulators, called topological insulators, in which electrons can bypass the energy gap by moving in surface states. The energy vs. momentum dispersion of these unusual surface states are Dirac-like. They exhibit unusual topological properties which may be important for quantum computing. Previously, a group at Princeton led by Hasan and Cava detected these unusual surface states in the bismuth alloy Bi_1-xSb_x using angle-resolved photoemission spectroscopy (ARPES). In a new breakthrough, Hasan and Cava have now identified a second Topological Insulator, Bi₂Se₃. Compared with Bi_1-xSb_x, the surface states of Bi₂Se₃ are far easier to investigate. Bi₂Se₃ has only 1 Dirac state on the surface, which greatly simplifies the analysis of its measured properties. Its dispersion, shown as the red curves SS in the figure (right panel), displays clearly the distinctive champagne-glass profile of Dirac electrons. In addition, the energy gap is much larger, which allows the topological states to be studied even at room temperature. The new material should greatly facilitate research on this new class of materials.

Left: The photoemission intensity at the Fermi level reveals a single circular feature formed by the topological surface state.
Right: The underlying surface band dispersion of Bi₂Se₃ reveals a Dirac cone as well as a single Fermi level crossing confirming that it is a strong topological insulator