Gaining A Better Understanding of Electronic Band Topologies Through Dirac Matter Materials

April 22, 2020 by Luke James

Physicists at ETH Zurich have established a microscopic mechanism for a material that integrates quantum magnetism and topological electronic properties, connecting magnetism and electronic-band topology.

These materials, known as dirac materials, have attracted a great deal of interest from researchers, specifically for the quantum many-body physics that are likely to unfold in them and also for potential applications in electronics.

Published in the journal Physical Review Letters, the team’s research is described by Run Yang, a postdoc, and Matteo Corasaniti, Ph.D. student from the Optical Spectroscopy group of Professor Leonardo Degiorgi at the Laboratory for Solid State Physics. The research project was carried out in conjunction with collaborators at the Chinese Academy of Sciences and Brookhaven National Laboratory.


A Material for Tomorrow’s Electronics

Dirac matter is a term that refers to a class of condensed matter systems that can be described by the Dirac equation, a relativistic equation describing the behavior of a massive charged particle. Essentially, it is the Schrödinger wave equation with relativity thrown in. Dirac matter materials feature unusual properties, such as electrons in them that act as if they have no mass, and include graphene, topological insulators, transition metal dichalcogenides, and Dirac semimetals.


The Potential of Nanomaterials in Next-Gen Electronics 

These Dirac materials are a class of complex functional nanomaterials that are expected to have beneficial applications in the next generation of electronics. Their properties, which are not found in current electronic materials, occur due to strong links between spin, charge, and structure which theoretically enable the development of magnetic or superconductive composites. 

Although Dirac matter and other topological materials are some of today’s most heavily researched condensed-matter systems, there are only a few examples where the electronic bands’ topology has been linked in a well-defined way to the materials’ magnetic properties.

One of these examples is that of CaMnBi2, a material where the link between magnetism and topological electronic states has been found. In their research, the ETH Zurich physicists reportedly offer proof that a “mild nudge” on spin canting in this material can cause considerable variations in its electronic band structure. 


A diagram of antiferromagnetic spins.

Diagram of the Antiferromagnetic and canted-antiferromagnetic order. The spins are canted relative to the easy c-axis, leading to a ferromagnetic contribution in the plane orthogonal to the axis represented by green arrows. Image used courtesy for ETH Zurich


Exploring CaMnBi2’s Properties

The material and its associated compound SrMnBi2 have garnered considerable interest because they exhibit quantum magnetism. At room temperature and below, the manganese ions in these compounds are antiferromagnetically ordered, and they also host dirac electrons simultaneously. 

The fact that an interaction occurs between the two characteristics has long been assumed, and what’s significant is that an unanticipated “bump” occurs in the conduction properties of these materials at around 50 K. However, the accurate nature of this anomaly has not been understood until now. 


The Torque Magnetometry Method 

In previous studies involving CaMnBi2’s, where the ETH researchers explored its optical properties, Corasaniti, Yang, and colleagues had already established a connection to the material’s electronic characteristics.

Specifically, the researchers exploited the fact that the bump-like anomaly that is seen in the transport properties can be changed in temperature by substituting a fraction of the calcium atoms with sodium atoms.

In a bid to understand the minute origins of the observed behavior, the research team analyzed samples with varying sodium dopings via torque magnetometry. Using this method, the torque on a magnetic sample can be quantified by exposing it to an appropriately powerful field, just like how a compass’ needle aligns with the Earth’s magnetic field. The researchers claim that this method was successful in exposing the origins of the anomaly.


Linking Magnetic and Electronic Properties

When performing their magnetic-torque experiments, the physicists found that at lower temperatures where the anomaly exists, magnetic behavior is no longer similar to what one may expect for an antiferromagnet. Here, a ferromagnetic component emerged that can be elucidated by a projection of magnetic moments onto the plane orthogonal to the easy spin c-axis of the initial antiferromagnetic order. This is known as spin-canting and is caused by a super-exchange mechanism.