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John O | March 2018

Physicists discover new quantum electronic material, kagome metal

by josh perry, editor


scientists from the massachusetts institute of technology (mit) and harvard university, both in cambridge, mass., and the lawrence berkeley national laboratory in berkeley, calif. have for the first time produced a kagome metal, which is an electrically-conducting crystal made of layers of iron and tin atoms.


an illustration depicting a kagome metal — an electrically conducting crystal, made from layers of iron and tin atoms, with each atomic layer arranged in the repeating pattern of a kagome lattice. (images by felice frankel; illustration overlays by chelsea turner)


according to a report from mit, the atomic layers of the new metal are arranged into a kagome lattice, which is related to a japanese basket-weaving pattern that has “highly-symmetrical, interlaced, corner-sharing triangles.”


when the researchers flowed current through the kagome layers of the crystal, the atoms produced quantum-like behaviors in the current, causing electrons to veer or bend back into the lattice.


“this behavior is a three-dimensional cousin of the so-called quantum hall effect, in which electrons flowing through a two-dimensional material will exhibit a ‘chiral, topological state,’ in which they bend into tight, circular paths and flow along edges without losing energy,” the article explained.


to measure the energy spectrum of the material, researchers used a modern version of the photoelectric effect.


“the spectra revealed that electrons flow through the crystal in a way that suggests the originally massless electrons gained a relativistic mass, similar to particles known as massive dirac fermions,” the report added. “theoretically, this is explained by the presence of the lattice’s constituent iron and tin atoms. the former are magnetic and give rise to a ‘handedness,’ or chirality. the latter possess a heavier nuclear charge, producing a large local electric field. as an external current flows by, it senses the tin’s field not as an electric field but as a magnetic one, and bends away.”


in order to create crystals that could be worked on at room temperature, researchers ground iron and tin together and heated the powder in a furnace at 750°c. after the kagome pattern was formed, the crystals were put into an ice bath to keep them stable.


“looking further, the team is now investigating ways to stabilize other more highly two-dimensional kagome lattice structures,” the article continued. “such materials, if they can be synthesized, could be used to explore not only devices with zero energy loss, such as dissipationless power lines, but also applications toward quantum computing.”


the research was recently published in nature. the abstract read:


“the kagome lattice is a two-dimensional network of corner-sharing triangles that is known to host exotic quantum magnetic states. theoretical work has predicted that kagome lattices may also host dirac electronic states that could lead to topological and chern insulating phases, but these states have so far not been detected in experiments.


“here we study the d-electron kagome metal fe3sn2, which is designed to support bulk massive dirac fermions in the presence of ferromagnetic order. we observe a temperature-independent intrinsic anomalous hall conductivity that persists above room temperature, which is suggestive of prominent berry curvature from the time-reversal-symmetry-breaking electronic bands of the kagome plane.


“using angle-resolved photoemission spectroscopy, we observe a pair of quasi-two-dimensional dirac cones near the fermi level with a mass gap of 30 millielectronvolts, which correspond to massive dirac fermions that generate berry-curvature-induced hall conductivity.


“we show that this behaviour is a consequence of the underlying symmetry properties of the bilayer kagome lattice in the ferromagnetic state and the atomic spin–orbit coupling. this work provides evidence for a ferromagnetic kagome metal and an example of emergent topological electronic properties in a correlated electron system.


“our results provide insight into the recent discoveries of exotic electronic behaviour in kagome-lattice antiferromagnets and may enable lattice-model realizations of fractional topological quantum states.”

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