White Paper: Tilted Elliptical Dirac Cones at a Half-metal Surface

The Department of Chemistry at the University of Cambridge recently published a paper in Physical Review B. The article is titled, Tilted Elliptical Dirac Cones at a Half-metal Surface. The paper, written by Dr. Stephen Jenkins, highlights the novel electronic properties of an alloy. The half-metallic alloy NiMnSb exhibits properties previously only observed in graphene and topological insulators. Electrons confined near the surfaces and interfaces of NiMnSb are constrained to move according to a conical energy-momentum relationship (the so-called “Dirac cone”) and have no effective mass, meaning these surfaces exhibit the exotic properties associated with effectively mass-less electrons. This includes properties which could allow us to manipulate electrons and “holes”, which presents new opportunities in semiconductor electronics.

The close proximity of two useful properties (the surface properties associated with the Dirac cones and the half-metal properties of the material underneath) could enable the design of a new kind of device that uses both. This has implications for the development of spintronic devices, which use electron spin to carry information. These devices would be much faster than ordinary devices, better suited to interfacing with magnetic storage media, and of a greater capacity and speed than magnetic storage media, meaning they could eventually replace them.

The problems previously encountered in spintronics were related to the tendency of the surfaces and interfaces of half-metals to not act with half-metal properties. Rather, these surfaces conduct like ordinary metals, allowing them to effectively short circuit the half metal that we wish to utilise. Whilst this new discovery does not directly overcome these problems in NiMnSb, it may offer a new way around them, allowing us to use the spintronic potential of NiMnSb without subjecting it to many complicated further treatments.

Tilted Elliptical Dirac Cones at a Half-metal Surface

The surface electronic structure of the half-metallic alloy NiMnSb is investigated in detail, revealing the presence of Dirac cones in the minority-spin states. Surface-localized carriers of suitable energy will consequently behave as massless Dirac fermions, a remarkable situation for a half-metallic material and potentially of great significance in future spintronic device applications. Unlike related features observed previously in graphene and at the surfaces of topological insulators, the Dirac cones described here are of elliptical cross section and are tilted with respect to the energy axis. Such unique properties are permitted due to the lower symmetry in the present case.

More information: Tilted Elliptical Dirac Cones at a Half-metal Surface (pdf)