Orbitronics: Orbital Currents in Solids

TL;DR

This paper reviews recent advances in orbitronics, focusing on the generation, transport, and potential applications of orbital currents in solids, highlighting their role in spintronics and magnetization dynamics.

Contribution

It provides a comprehensive overview of physical mechanisms, candidate materials, experimental methods, and the significance of orbital currents in solid-state physics and device applications.

Findings

Orbital currents can be generated and transported despite orbital quenching.

Orbital currents influence phenomena like spin Hall and valley Hall effects.

Experimental techniques for detecting and quantifying orbital currents are discussed.

Abstract

In solids, electronic Bloch states are formed by atomic orbitals. While it is natural to expect that orbital composition and information about Bloch states can be manipulated and transported, in analogy to the spin degree of freedom extensively studied in past decades, it has been assumed that orbital quenching by the crystal field prevents significant dynamics of orbital degrees of freedom. However, recent studies reveal that an orbital current, given by the flow of electrons with a finite orbital angular momentum, can be electrically generated and transported in wide classes of materials despite the effect of orbital quenching in the ground state. Orbital currents also play a fundamental role in the mechanisms of other transport phenomena such as spin Hall effect and valley Hall effect. Most importantly, it has been proposed that orbital currents can be used to induce magnetization dynamics, which is one of the most pivotal and explored aspects of magnetism. Here, we give an overview of recent progress and the current status of research on orbital currents. We review proposed physical mechanisms for generating orbital currents and discuss candidate materials where orbital currents are manifest. We review recent experiments on orbital current generation and transport and discuss various experimental methods to quantify this elusive object at the heart of $orbitronics$ $-$ an area which exploits the orbital degree of freedom as an information carrier in solid-state devices.