Physicists reveal nonlinear transport induced by quantum geometry in planar altermagnets

In recent years, many physicists and materials scientists have studied a new class of magnetic materials known as altermagnets. These materials exhibit a unique type of magnetism that differs from both conventional ferromagnetism and antiferromagnetism, marked by electrons whose spin varies depending on their momentum.

This unique magnetism makes altermagnets very promising for the development of new spintronics and electronic devices. It also opens new possibilities for the study of topological materials (i.e. systems with unique electronic properties arising from the topology of their electronic structure).

Researchers at Stony Brook University conducted a study aimed at better understanding the nonlinear response of planar altermagnets. Their article, published in Physical Examination Lettersreports the observation of a nonlinear response in these materials derived from their quantum geometry.

“Recently, two experiments confirmed the predicted role of quantum geometry in the second-order response of conventional PT symmetric antiferromagnets,” Sayed Ali Akbar Ghorashi, co-author of the paper, told Phys.org.

“In these materials, due to the combination of parity (P) and time reversal (T) symmetries, the Berry curvature (the imaginary component of the quantum geometric tensor) disappears, and it is shown that the response of the second order is governed by the quantum metric (the real component of the quantum geometric tensor).”

Altermagnets do not have combined PT symmetry. As a result, the influence of quantum geometry on the nonlinear response reported in these materials has remained elusive.

“The goal of our work was to derive the nonlinear response of the altermagnets and distinguish the contributions from Berry curvature and quantum metric,” Ghorashi said. “Our findings turned out to be more dramatic than expected.”

Ghorashi and his colleagues originally aimed to study the nonlinear response of altermagnets and the factors behind this response. To do this, they first calculated all contributions to the nonlinear response of the altermagnets up to third order in the electric field, using semi-classical Boltzmann theory.

“We discovered the quantum geometric origin of each term, order by order in diffusion time,” Ghorashi said. “Then, for each planar alter-magnet, we used symmetry to determine which contributions survive in the longitudinal and Hall components of the third-order conductivity.”

The calculations and analyzes carried out by the researchers gave surprising and instructive results. More specifically, they identified nonlinear responses in planar altermagnets induced by the quantum geometry of the materials.

“Remarkably, due to the reversal symmetry, altermagnets have a second-order response that disappears,” Ghorashi explained.

“Therefore, to our knowledge, they constitute the first class of materials where the third-order response is the main nonlinear response. Furthermore, we have shown that this response is giant due to the significant spin splitting in these materials Additionally, the weak spin-orbit coupling (compared to the magnetic exchange term) of altermagnets also appears in their nonlinear response, providing a new transport characterization for this new class of materials, which was previously limited to. the search for linear anomalous Hall conductivity.

The results of this recent study open up new possibilities for the study of altermagnets and their unique properties. Most notably, it reveals distinctive features of nonlinear transport in this newly discovered class of materials, which could guide future experiments aimed at examining them in more detail and delineating various aspects of their quantum geometry.

“One of our immediate future research directions will be to go beyond the relaxation time approximation and study the effect of disorder which has already been shown to enrich the physics of symmetric antiferromagnets PT,” Ghorashi added.

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