Quantum states evolve in kagome magnet

Working with a quantum material known as a kagome magnet, researchers at Boston College and Renmin University in Beijing, China, have directly measured how individual electronic quantum states in this novel material respond to external magnetic fields by shifting energy in an unusual manner. The researchers report their findings in a paper in Nature Physics.

The measurements generated by the project are the first of their kind to directly measure the momentum-resolved, field-induced evolution of these quantum states. According to Ilija Zeljkovic, an associate professor of physics at Boston College and a lead co-author of the paper, the findings offer the first experimental demonstration of theoretical predictions about how electronic band structure can change in a kagome magnet, specifically bulk single crystals of yttrium manganese tin (YMn₆Sn₆).

“When a magnetic field is applied to a material, electronic band structure – which is a collection of quantum states that electrons in solids can occupy – can change in unusual ways,” Zeljkovic said. “These changes have thus far been inferred from theoretical calculations or accessed indirectly from field-induced changes in macroscopic measurable properties. Direct measurement of field-induced changes to the electronic band structure has been difficult to measure.”

The team overcame the experimental challenges of studying the quantum states in a kagome magnet by utilizing spectroscopic-imaging scanning tunneling microscopy. Kagome magnets like YMn₆Sn₆ are so named because they possess magnetic structure and an atomic lattice that resembles Japanese 'kagome' weaved baskets. Kagome magnets harbor so-called Dirac fermions, quasiparticles characterized by zero mass and a linear energy-momentum dispersion in electronic band structure resembling relativistic particles.

Theoretical physicists like Zeljkovic’s colleague and co-author Ziqiang Wang, a professor of physics at Boston College, have mathematically shown that Dirac fermions may evolve – from the standpoint of energy and momentum – in response to a magnetic field. The researchers set out to test those predictions.

They found that the quantum states associated with Dirac fermions respond strongly to a magnetic field, shifting to higher energies regardless of the direction of the field.

“Interestingly, they exhibit a momentum-dependent shift – for a set magnetic field, quantum states near the Dirac point shift the most; the shift becomes progressively smaller away from the Dirac point,” Zeljkovic said. The Dirac point is a point in energy-momentum space where conduction and valence bands touch.

Zeljkovic said the expectation was that the system without a magnetic field would host massless – or zero mass – Dirac fermions based on the orientation of spins lying primarily in-plane. Instead, the team made the surprising observation that Dirac fermions in YMn₆Sn₆ at zero magnetic field have finite mass. Why this occurred will be a question for theoreticians to explore further.

From an experimental standpoint, Zeljkovic said there are many additional questions to resolve based on these findings. Specifically, there are multiple competing effects that can lead to a momentum-dependent band evolution, involving electron spin and orbital degrees of freedom.

Orbital magnetism, a property that has recently generated attention and excitement among researchers studying ‘twisted’ van der Waals structures, is one of the extremely exciting possibilities.

“Our future experiments will focus on disentangling different contributions and examining orbital magnetism in this and related kagome magnets,” Zeljkovic said.

This story is adapted from material from Boston College, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.