The direct detection of a topological phase transition through a sign change in the Berry curvature dipole

Ingrid Fadelli is a writer for the website Phys.org.

The Berry and Chern numbers are important qualities of a quantum mechanical origin. The properties of certain materials are determined by these two elements.

Many studies attempt to determine how the Berry curvature and Chern number affect the properties of materials, but detecting them in an experimental setting can be very difficult. It is a promising platform to manipulate the Berry curvature and valley Chern numbers of flat bands and thus to study their effects.

More than three years have passed since researchers at the Indian Institute of Technology and the Jawaharlal Nehru Centre for Advanced Scientific Research began examining the properties of twisted double bilayer Graphene. They were able to detect a transition in a superlattice by controlling the sign change.

The paper focuses on twisted double bilayer Graphene. The researchers used strategies to detect the Berry curvature in one of their previous studies.

Subhajit Sinha, one of the researchers who carried out the study, said that before they started working on the project, the group was looking into different Hall contributions. He wrote to us on the eve of Christmas in 2020. When one of our twisted double bilayer Graphene samples was cold in a cryostat, we decided to collect measurements on it and see if we got anything. Maybe some stars were aligned because we measured some signal.

The team was able to determine with a high degree of certainty that they had measured the non- linear quantum Hall effect in their twisted double bilayer Graphene sample. They collaborated with the research group to show that they had seen a transition.

Prof. Mandar's group measured the Hall voltage in their experiment. The Hall-bar measurement can be used to drive this voltage.

When an external magnetic field is applied to the sample's plane, the Hall voltage can develop. The person explained. It was shown in the work of Sodemann and Fu that one can have a Hall voltage in the absence of a magnetic field.

The "Berry curvature dipole" is a result of a nonzero Berry curvature and small amounts of strain in twisted bilayer Graphene. A measurement that scales with the current applied to a sample.

The Hall voltages were measured at twice the frequencies of the applied current. We used a scaling analysis to find a sign change in the Berry curvature dipole.

It's difficult to detect topological phase transitions. There are many theoretical and experimental studies that suggest a transition in the top of the twisted double bilayer Graphene. In an experimental setting, the recent work by the team shows a direct observation of the phase transition.

The Berry curvature dipole was used to detect the topological transition. The handle gives us an experimental way of probing the band geometric physics.

The findings gathered by this team of researchers could have important implications for the study of phase transitions in twisted double bilayer Graphene. The methods they used can help detect transitions in other materials.

An immediate future direction for our work could be using our technique to map out the phase transition as a function of twist angle or stacking order We hope our method will be used in other 2D or even 3D materials to demonstrate similar phases. The research interest in the effects of Hall effects is growing due to its many advantages, one of which is probing the band geometric and topological properties of material. We will have to watch as the effects unfold.

More information: Subhajit Sinha et al, Berry curvature dipole senses topological transition in a moiré superlattice, Nature Physics (2022). DOI: 10.1038/s41567-022-01606-y Journal information: Nature Physics

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