Surfacestate

Dirac point

Fermi levelBi2Se3

GaAs

Bi2Se3

Formation of new quantum states by molecular beam epitaxy: an emerging frontier in spintronics and quantum computation

Background:During the past two years, studies involving topology have led to predictions of entirely new quantum states of matter, such as charge carriers with zero mass and new quantum quasiparticles known as Majorana fermions. Since these novel concepts emerge from topological properties of a solid, they can be designed and manipulated by controlling the surface of a solid or by designing the interfaces within a multilayer structure. Molecular beam epitaxy (MBE) is ideally suited to achieve these goals.Description of achievement and methodology:Our team has focused on two approaches, both involving the manipulation of topological effects by MBE: on creating entirely new materials with topological properties that are protected by time reversal symmetry (topological insulators, TIs); and on forming new combinations of traditional materials, in which the interfaces between these materials results in entirely new effects (in our case, in the formation of new quasiparticles referred to as Majorana fermions).Topological insulators: We have successfully grown high-quality specimens of the canonical topological insulators Bi2Se3 and Bi2Te3. The crystalline graphite-like structure of this system is shown in the transmission electron micrograph (a) on the right. The existence of the so-called Dirac cone is demonstrated by the angle-resolved photo-emission image (b), attesting to the existence of massless electrons, which have their spin locked at right-angle to their momentum (spin-momentum locking). Majorana fermions: By interfacing MBE-grown nanolayers of narrow-gap semiconductor InSb with a superconducting Nb nanowire shown in Fig. (c), we have also succeeded in observing clear signatures of Majorana fermions, elusive quasiparticles which could be used as building blocks of topological quantum computers. This Majorana fermion signature comes in the form of a discontinuity in the progression of Shapiro steps in the Josephson effect shown by the oval in Fig. (d) in the presence of a moderately strong magnetic field.

(a) (b)

(c) (d)

Margaret Dobrowolska, University of Notre Dame, DMR 1005851

DMR 1005851: Electron spin Effects in Semiconductor Nanostructures

Margaret Dobrowolska, University of Notre Dame, DMR 1005851

Education:

Our program typically involves three graduate students and two undergraduate students each year. These students tare exposed to a wide range of materials fabrication and characterization techniques, thus preparing them for the US manpower needs in the area of semiconductor science and technology. We also conduct a broad program of international scientific exchanges. Currently we are hosting Prof. Sanghoon Lee from Korea University on his sabbatical. Moreover, Dr. K. Dziatkowski of Warsaw University and Prof. T. Wojtowicz of the Polish Academy of Sciences will be our guests on extended visits to conduct research in our group.

Societal Impact:

Our group has continually acted as a resource of materials for other groups. We are currently interacting with at least 15 other institutions around the world by providing them with magnetic semiconductor specimens for their research; and at least ten graduate students in institutions other than Notre Dame are currently doing their doctoral research on materials which we have provided. The understanding of these materials obtained in our laboratory is thus automatically of benefit to our collaborators, whose research depends not only on the specimens that we provide, but on the intellectual input from our group in the form of characterization and general understanding of the properties of these materials.