One of the intriguing phenomena in the field of condensed matter physics is the charge density wave or CDW. It is a periodic modulation of electronic charge and atomic positions in materials, similar to the formation of intricate patterns, like waves. This state is typically induced by the Peierls mechanism, driven by Fermi surface nesting that causes instability, triggering the wave pattern. It is commonly found in quasi-one-dimensional organic conductors and transition-metal compounds (TMCs).

But some TMCs with more isotropic two- and three-dimensional electronic structures exhibit CDW-like transitions, even though they lack conventional Fermi surface nesting. Recent studies suggest that in such TMCs, unique charge-orbital states may play a key role in driving CDW-like transitions. As a result, these materials are gaining considerable attention, particularly with the development of transition-metal dichalcogenides as promising post-graphene materials.

In light of these advancements, a new Special Topics edition of the Journal of the Physical Society of Japan features articles exploring TMCs with novel charge-orbital states.

Zhong, Yin, and Nakayama investigated the electronic states of the Kagome superconductor AV₃Sb₅ using angle-resolved photoemission spectroscopy (ARPES), revealing insights into its unconventional CDW order and superconductivity. They demonstrated the connection between CDW and van Hove singularity, and the significant influence of electronic correlations on the unique electronic states of this material.

Haraguchi and Yoshimura explored the Van der Waals magnet Nb₃Cl₈. Interestingly, they observed a unique trimerization, where charge disproportionation drives a transition from a paramagnetic to a non-magnetic state.

Mitsuishi and Ishizaka reviewed an exotic charge-orbital ordered state in VTe₂ using ARPES, focusing on the formation of localized molecular-like orbital bonds that create unique flat bands in the momentum space and the manner in which they affect Fermi surface anisotropy.

Katayama and Kojima investigated trimerization in the layered LiVX₂, caused by molecular orbital formation and its precursory fluctuations. They also highlighted its unique short-range zig-zag chain ordering that forms just before trimerization.

Hwang and Mo described the novel charge orders and the emergence of superconductivity in 1T-IrTe₂, identifying differences in its bulk, surface, and monolayer forms, and emphasizing the importance of strong interlayer coupling.

Okamoto studied the electronic properties of β-pyrochlore oxide CsW₂O₆, identifying multiple factors that contribute to a self-organization transition to a non-magnetic insulating state. These properties were also compared with other related materials.

In summary, these studies highlight the immense potential of charge-orbital ordered TMCs in the development of advanced electrical, magnetic, and optical devices that can significantly enrich our lives.

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