Recent advances in quantum materials have shifted the focus from charge transport to a broader range of electronic degrees of freedom. In this Focus issue, we highlight different facets of electronic degrees of freedom and their potential applications.
Electrons possess multiple degrees of freedom, including charge, spin and orbital motion. In solids, their coupling to crystal symmetry, many-body interactions and other structural factors can generate emergent physical behaviour. Therefore, determining which degree of freedom dominates a given response is important for understanding the underlying physics and for designing functional quantum materials.
Credit: Natali Art/Alamy
Orbital angular momentum arises from the orbital motion of electrons in atoms. It couples naturally to crystal symmetry because it derives from the spatial structure of electronic wavefunctions. This makes orbital angular momentum relevant not only at the atomic level, but also for the electronic behaviour of crystalline materials. Although recent experimental studies1,2 have shown distinct phenomena associated solely with orbital angular momentum, it remains challenging to directly isolate its contribution to the electronic properties. Techniques that can visualize orbital angular momentum textures in crystals are therefore essential. In their Perspective, Dongjin Oh and colleagues discuss how polarization-dependent angle-resolved photoemission spectroscopy serves as an experimental tool to probe orbital angular momentum textures in momentum space.
In analogy to spins in spintronics, orbitronics uses orbital angular momentum as an information carrier and processes information through orbital currents. However, the field faces two major bottlenecks, namely the lack of experimental approaches to directly isolate orbital effects as well as the absence of efficient methods for converting charge currents into orbital angular momentum currents and vice versa. Writing in a Perspective, Shunsuke Fukami and colleagues provide an overview of recent experimental and theoretical advances in orbitronics and outline a strategy for the successful implementation of magnetoresistive random access memories based on orbital torque.
The spin degrees of freedom of electrons are fundamental to the magnetic behaviour of materials. Recently, altermagnets3,4 have attracted considerable attention as a distinct type of magnetic order that combines zero net magnetization with spin-split electronic states. As they have features of ferromagnetic and antiferromagnetic materials, altermagnets offer potential for robust spintronic functionality while avoiding drawbacks, such as stray fields in ferromagnets and the absence of spin-polarized currents in antiferromagnets.
In a Review, Tomas Jungwirth and colleagues discuss the emerging concepts in altermagnetic spintronics and how their integration with ferroelectric or superconducting materials may foster the development of next-generation spintronic devices.
Recently, an emerging electronic order known as loop current order has gained popularity in the condensed matter research community. In loop current order, electrons form coherent current loops within the lattice net, producing an electronic current that does not result in a net non-dissipative (transport) current. However, their experimental detection remains controversial. Owing to their geometric frustration and complex electronic structure, Kagome metals offer a setting for the realization of such a hidden order phase. In their Perspective, Rafael Fernandes and colleagues focus on the phenomenological theory and symmetry aspects of loop current order and present an overview of the current experimental status in kagome metals.
The charge degree of freedom of electrons is a key ingredient in the emergence of complex polarization textures in ferroelectric materials with non-trivial topology. Such polarization configurations are governed by a delicate balance between competing interactions, including electrostatic forces, local strain and interfacial energies. In this way, energy minimization enables the formation of polar textures, such as vortices. Combined with their topological protection and sub-nanometre size, such polar textures hold strong promise for future high-density memory devices. In a Review article, Zijian Hong and colleagues discuss how different topological textures in polar systems can be classified and explain their formation mechanisms.
Taken together, this Focus issue showcases the rich physics of quantum materials that arises from selectively activating and coupling different electronic degrees of freedom. Orbital dynamics, spin arrangements, charge distributions and emergent current patterns each provide distinct routes to functionality beyond mere conventional electronic transport.