Ternary intermetallic Heusler compounds, originally discovered by a German mining engineer and chemist in 1903, may show exotic topological insulator behaviour unknown to science just five years ago. Heusler alloys are described at wikipedia.
A Heusler alloy is a ferromagnetic metal alloy based on a Heusler phase. Heusler phases are intermetallics with particular composition and face-centered cubic crystal structure. They are ferromagnetic—even though the constituting elements are not—as a result of the double-exchange mechanism between neighboring magnetic ions.
It is a historical fact that most of the key discoveries in condensed-matter physics and materials science have been made by experimentalists (think superfluidity, superconductivity, the quantum Hall effect, to name but a few). From time to time, theorists make paradigm-shifting predictions that lead to fundamentally new phenomena.
(Johannes Gutenberg University of Mainz) However, this was not the case with topological insulators. In 2006, Professor Zhang of Stanford predicted that a new quantum state of matter would be identified in nanostructures of the familiar semiconductor mercury cadmium telluride (HgTe). One year later, this was confirmed in experiments carried out by the Würzburg team led by Professor Laurens Molenkamp. Completely new mathematical concepts are required to understand the physical aspects of what has been discovered.
The new work from Nature Materials on Heusler materials are paradigm shifting. Calculations have uncovered a new quantum state of matter in Heusler compounds, which opens up previously unimagined usage possibilities. Heusler compounds can behave like topological insulators (TI).
For almost five years now, TIs have been a hot topic in the field of solid state and material physics. Characteristic of topological insulators is the fact that the materials are actually insulators or semiconductors, although their surfaces or interfaces are made from metal – but not ordinary metal. Like superconductors, the electrons on the surfaces or interfaces do not interact with their environment – they are in a new quantum state. In contrast with superconductors, topological insulators have two non-interacting currents, one for each spin direction. These two spin currents, which are not affected by defects or impurities in the material, can be employed in the futuristic electronics field of ‘spintronics’ and for processing information in quantum computers.
It is now supposed that Heusler materials may have the same capabilities. Heusler compounds are made up of three elements, which often have semiconductor or magnetic properties. This compound class was discovered by Fritz Heusler back around 1900. One special feature of these compounds is that they exhibit characteristics other than those that might be expected in view of the elements of which they are composed. The first Heusler compound, for example, was made from the non-magnetic elements copper, manganese, and aluminium. Yet, Cu2MnAl acts as a ferromagnet, even at room temperature. On the other hand, a semiconductor can result when three metals are combined. New semiconductors can be designed in the class of Heusler materials with regard to the field of renewable energies; they can be used in solar cells or in thermoelectric applications, for converting heat into electricity. Mainz is internationally renowned as a major location for the design and synthesis of Heusler materials. Important discoveries with regard to Heusler compounds, their properties, and their uses in a wide range of potential applications have been made in Mainz.
The news that Heusler materials are now being considered as possible topological insulators has met with excitement all over the world. “There are two reasons for this,” explains Professor Felser. “On the one hand, this large material class with over 1,000 known representatives contains more than 50 compounds that bear the hallmark of TIs. And on the other hand, it is now possible to design completely new physical effects. As the materials are made up of three elements, they can offer a range of other interesting features in addition to the topological quantum state.” It is now possible to combine two quantum states such as superconductivity and topological surface effects. This paves the way for completely new and as yet undiscovered characteristics, some of which have already been predicted. “It was previously not considered possible to combine all these possibilities in just one material,” explains Professor Felser.
Recently the quantum spin Hall effect was theoretically predicted and experimentally realized in quantum wells based on the binary semiconductor HgTe. The quantum spin Hall state and topological insulators are new states of quantum matter interesting for both fundamental condensed-matter physics and material science. Many Heusler compounds with C1b structure are ternary semiconductors that are structurally and electronically related to the binary semiconductors. The diversity of Heusler materials opens wide possibilities for tuning the bandgap and setting the desired band inversion by choosing compounds with appropriate hybridization strength (by the lattice parameter) and magnitude of spin–orbit coupling (by the atomic charge). Based on first-principle calculations we demonstrate that around 50 Heusler compounds show band inversion similar to that of HgTe. The topological state in these zero-gap semiconductors can be created by applying strain or by designing an appropriate quantum-well structure, similar to the case of HgTe. Many of these ternary zero-gap semiconductors (LnAuPb, LnPdBi, LnPtSb and LnPtBi) contain the rare-earth element Ln, which can realize additional properties ranging from superconductivity (for example LaPtBi) to magnetism (for example GdPtBi) and heavy fermion behaviour (for example YbPtBi). These properties can open new research directions in realizing the quantized anomalous Hall effect and topological superconductors.
Recent discovery of spin-polarized single-Dirac-cone insulators whose variants can host magnetism and superconductivity, has generated widespread research activity in condensed-matter and materials-physics communities. Some of the most interesting topological phenomena, however, require topological insulators to be placed in multiply connected, highly constrained geometries with magnets and superconductors all of which thus require a large number of functional variants with materials design flexibility as well as electronic, magnetic and superconducting tunability. Given the optimum materials, topological properties open up new vistas in spintronics, quantum computing and fundamental physics. We have extended the search for topological insulators from the binary Bi-based series to the ternary thermoelectric Heusler compounds. Here we show that, although a large majority of the well-known Heuslers such as TiNiSn and LuNiBi are rather topologically trivial, the distorted LnPtSb-type (such as LnPtBi or LnPdBi, Ln=fn lanthanides) compounds belonging to the half-Heusler subclass harbour Z2=−1 topological insulator parent states, where Z2 is the band purity product index. Our results suggest that half-Heuslers provide a new platform for deriving a host of topologically exotic compounds and their nanoscale or thin-film device versions through the inherent flexibility of their lattice parameter, spin–orbit strength and magnetic moment tunability paving the way for the realization of multifunctional topological devices.
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