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For many years, physics studies focused on two main types of magnetism, namely ferromagnetism and antiferromagnetism. The first type entails the alignment of electron spins in the same direction, while the latter entails the alignment of electron spins in alternating, opposite directions.

Yet recent studies have discovered a new kind of magnetism, referred to as altermagnetism, which does not fit into either of the previously identified categories. Altermagnetism is characterized by the breaking of time-reversal symmetry (i.e., the symmetry of physical laws when time is reversed) and spin-split band structures, in materials that retain a zero net magnetization.

Researchers at the Chinese Academy of Sciences and other institutes in China recently uncovered a new material that exhibits altermagnetism at room temperature, namely KV₂Se₂O. Their findings, in Nature Â鶹ÒùÔºics, highlight the promise of KVâ‚‚Seâ‚‚O both for the study of altermagnetism and for the development of spintronic devices.

"KV₂Se₂O belongs to a broad family of compounds containing the [T₂Q₂O]^2- layers, where T denotes a 3d transition metal and Q represents S, Se, As, Sb, or Bi," Tian Qian, senior author of the paper, told Â鶹ÒùÔº. "This family exhibits a variety of many-body phenomena, including superconductivity, Mott insulating state, charge or spin density wave (CDW/SDW)."

The initial objective of the recent study by Qian and his colleagues was to further explore the origin of the unconventional superconductivity observed in KV₂Se₂O, which is known to be associated with a SDW-like transition at temperatures around 100 K.

To achieve this, the researchers synthesized high-quality single crystals of KV₂Se₂O and then collected various measurements (i.e., resistivity, magnetic susceptibility, specific heat, ARPES, NMR and STM measurements), while also performing band structure calculations.

"NMR measurements indicate that V atoms establish a long-range magnetic order above room temperature, with spins aligned antiparallel along the c axis," explained Qian.

"This collinear compensated magnetic order is essential for altermagnets. ARPES experiments reveal a metallic electronic band structure. By comparing with band calculations under different magnetic configurations, we find that the calculated band structure of the altermagnetic order closely matches the ARPES results."

The researchers also collected spin-resolved ARPES measurements on the KV₂Se₂O and observed a momentum-dependent spin polarization with a d-wave symmetry. While they did not detect superconductivity in the material, they thus found that its band structure exhibits altermagnetic spin splitting with a magnetic ordering temperature well above room temperature.

"The most notable finding of our study is the discovery of a metallic altermagnet with d-wave spin splitting at room temperature," said Qian.

"Due to its highly anisotropic C₂-symmetry spin-polarized Fermi surfaces, this material is expected to generate highly polarized electric current and giant spin current, making it a promising platform for high-performance spintronic devices."

The new metallic room-temperature altermagnet uncovered by Qian and his colleagues could be a promising platform for exploring the many-body effects associated with altermagnetism. In the future, it could also potentially prove valuable for the development of quantum technologies and spintronic devices.

"Our future research aims to explore novel physics arising from the interplay between altermagnetism and other quantum states of matter," added Qian.

"For instance, the V₂O plane has an anti-CuO₂ structure, making the d-wave altermagnet KV₂Se₂O structurally compatible with d-wave cuprate superconductors. This compatibility opens new avenues for investigating interfacial physics between unconventional superconductors and altermagnets."

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