Discover Top Posts Tagged with #dirac materials

A Textbook Dirac Semimetal

Researchers have synthesized a chalcogenide compound that has the electronic structure of an ideal Dirac semimetal—which could facilitate the study of this exotic class of materials.

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Dirac semimetals are exotic quantum materials regarded as three-dimensional analogs of graphene: in such semimetals, as in graphene, the electrons behave as fermions with zero mass, leading to unique optical, magnetic, and transport properties. Only a handful of such systems, however, have been uncovered. A challenge for experimentalists lies in the often complicated electronic band structure of these materials, which makes it hard to disentangle effects due to the zero-mass relativistic electrons from those due to trivial electronic states. Now a team led by David Santos-Cottin of the University of Fribourg in Switzerland has demonstrated that a chalcogenide compound, TlBiSSe, can be synthesized with an extremely clean and tunable band structure that is characteristic of a Dirac semimetal [1]. The result suggests that TlBiSSe might be an ideal material platform for exploring relativistic-electron phenomena in solids, says Santos-Cottin.

Dirac Electrons Come Back to Life in Magic-Angle Graphene – Unusual Breaking of Symmetry

A new symmetry-broken parent state discovered in twisted bilayer graphene.

In 2018 it was discovered that two layers of graphene twisted one with respect to the other by a “magic” angle show a variety of interesting quantum phases, including superconductivity, magnetism, and insulating behaviors. Now a team of researchers from the Weizmann Institute of Science led by Prof. Shahal Ilani of the Condensed Matter Physics Department, in collaboration with Prof. Pablo Jarillo-Herrero’s group at MIT, have discovered that these quantum phases descend from a previously unknown high-energy “parent state,” with an unusual breaking of symmetry.

Graphene is a flat crystal of carbon, just one atom thick. When two sheets of this material are placed on top of each other, misaligned at small angle, a periodic “moiré” pattern appears. This pattern provides an artificial lattice for the electrons in the material. In this twisted bilayer system the electrons come in four “flavors”: spins “up” or “down,” combined with two “valleys” that originate in the graphene’s hexagonal lattice. As a result, each moiré site can hold up to four electrons, one of each flavor.

When Dirac meets frustrated magnetism

The fields of condensed matter physics and materials science are intimately linked because new physics is often discovered in materials with special arrangements of atoms. Crystals, which have repeating units of atoms in space, can have special patterns which result in exotic physical properties. Particularly exciting are materials which host multiple types of exotic properties because they give scientists the opportunity to study how those properties interact with and influence each other. The combinations can give rise to unexpected phenomena and fuel years of basic and technological research.

In a new study published in Science Advances this week, an international team of scientists from the USA, Columbia, Czech Republic, England, and led by Dr. Mazhar N. Ali at the Max Planck Institute of Microstructure Physics in Germany, has shown that a new material, KV3Sb5, has a never-seen-before combination of properties that results in one of the largest anomalous Hall effects (AHEs) ever observed; 15,500 siemens per centimeter at 2 Kelvin.

Discovered in the lab of co-author Prof. Tyrel McQueen at Johns Hopkins University, KV3Sb5 combines four properties into one material: Dirac physics, metallic frustrated magnetism, 2-D exfoliability (like graphene), and chemical stability.

A twist connecting magnetism and electronic-band topology

Combined optical and torque measurements establish the microscopic mechanism linking magnetism and electronic-band topology in a Dirac material

Dirac matter is an intriguing class of materials with rather peculiar properties: electrons in these materials behave as if they had no mass. The most prominent Dirac material is graphene, but further members have been discovered during the past 15 years or so. Each one of them serves as a rich playground for exploring 'exotic' electronic behaviours, some of which hold the promise to enable novel components for electronics. However, even if Dirac matter and other so-called topological materials -- in which electrons behave in similarly unexpected ways -- are among the currently most intensively studied condensed-matter systems, there are only very few examples where the topology of the electronic bands is connected in a well-defined manner to the magnetic properties of the materials. One material in which such interplay between topological electronic states and magnetism has been observed is CaMnBi2, but the mechanism connecting the two remained unclear. Writing in Physical Review Letters, postdoc Run Yang and PhD student Matteo Corasaniti from the Optical Spectroscopy group of Prof. Leonardo Degiorgi at the Laboratory for Solid State Physics of ETH Zurich, working with colleagues at Brookhaven National Laboratory (US) and the Chinese Academy of Sciences in Beijing, now report a comprehensive study in which they provide clear evidence that it is a slight nudge on the magnetic moments, known as spin canting, that provokes substantial changes in the electronic band structure.

Compass points to the right direction on a bumpy road

CaMnBi2 and the related compound SrMnBi2 have recently attracted attention as they display quantum magnetism -- the manganese ions are antiferromagnetically ordered at around room temperature and below -- and at the same time they host Dirac electrons. That there is interplay between the two properties has been suspected for some while, not least as at ~50 K there appears an unexpected 'bump' in the conduction properties at these materials. But the precise nature of this anomaly was still poorly understood until now.

Electronically-smooth '3-D graphene': A bright future for trisodium bismuthide: Electronically-smooth nature of trisodium bismuthide makes it a viable alternative to graphene/h-BN

Researchers have found that the topological material trisodium bismuthide (Na3Bi) can be manufactured to be as 'electronically smooth' as the highest-quality graphene-based alternative, while maintaining graphene's high electron mobility.

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Na3Bi is a Topological Dirac Semimetal (TDS), considered a 3D equivalent of graphene in that it shows the same extraordinarily high electron mobility.

In graphene, as in a TDS, electrons move at constant velocity, independent of their energy.

This high electron mobility is highly desirable in materials investigated for fast-switching electronics. The flow of electrons in graphene can be, theoretically, 100 times as fast as in silicon.

However in practice there are limitations to graphene's remarkable electron mobility, driven by the material's two-dimensional nature.