Research

Our lab’s primary research focus is orbitronics—a rapidly emerging field that aims to harness the orbital degree of freedom of electrons as a novel carrier of information. By exploiting the quantum nature of electron orbitals, orbitronics offers a promising route to overcome the fundamental limitations of conventional charge-based electronics.

Traditionally, the role of orbital angular momentum in solids was considered negligible due to orbital quenching, a concept widely taught in textbooks. However, recent breakthroughs have overturned this assumption. It is now understood that under non-equilibrium conditions, the orbital degree of freedom exhibits nontrivial quantum dynamics, enabling the generation and transport of orbital angular momentum without dissipation (figure on the left).

Beyond charge and spin, orbital moments and orbital currents interact with a rich variety of material orders and excitations, including magnetism, lattice vibrations, and topology. These interactions open new avenues for orbital-based quantum devices, with potential applications in energy-efficient information technologies, spin-orbitronics, and quantum sensing.

Microscopic Origins of Orbitronic Phenomena

A primary mission of GO-Lab is to push the frontier of orbitronics by predicting new phenomena and revealing their microscopic mechanisms at a fundamental level. Our goal is to clarify the microscopic origins of:

1. How orbital angular momentum is generated in non-equilibrium and how it undergoes coherent quantum dynamics,

2. How non-equilibrium orbital angular momentum is transported and how it relaxes or loses quantum coherence,

3. How non-equilibrium orbital angular momentum interacts with other degrees of freedom and exchanges angular momentum, leading to coupled dynamics.

Orbital Currents for Spintronics

In 2020, we made a theoretical prediction with a strong implication on spintronics: Non-equilibrium orbital angular momentum can interact with local moments, which can be used for controlling magnetization in spintronic devices. Now denoted by orbital torque, it offers a complementary route for enhancing the efficiency of spintronic devices. This principle can be, for instance, applied in the Magnetic Random Access Memory (MRAM). The orbital torque has been confirmed in experiments, independently from various teams over the world. 

Novel Materials for Orbitronics

Many orbitronic phenomena often require non-trivial symmetry and topology of orbital textures. Therefore, to bridge our theoretical prediction with experiments, which are performed on real materials, GO-Lab aims to find novel materials hosting new orbitronic phenomena. Currently, we focus on:

• Topological matters

• Chiral crystals

• Van der Waals materials and their heterostructures

• Adsorbed surfaces and intercalated interfaces

• Transition metal heterostructures

Density Functional Theory (DFT)

To describe the complex electronic structure of real materials, we employ the DFT—a quantum mechanical framework in which the many-body interactions between electrons are effectively described by a single electron moving in an exchange-correlation potential. DFT has been remarkably successful in capturing the quantum nature of materials, and has become an essential tool in understanding and predicting the properties of metals, semiconductors, topological matters, and two-dimensional materials in wide range of phases.

Among various implementations of DFT, we primarily use the Full-Potential Linearized Augmented Plane Wave (FLAPW) method, which does not impose any shape assumptions on the density or potential. We use the FLEUR code and maintains a close collaboration with code developers in Jülich, Germany, with the shared goal of pushing the boundaries of DFT for next-generation quantum materials research.

Wannier functions

Beyond standard DFT methods, one of our group's core expertise is employing Wannier functions, which are real-space localized state conjugate to extended Bloch states. 

Wannier functions can be obtained by carefully projecting target states (which depend on our purpose) to a set of trial states and iteratively minimize their spreads, which can be done by the WANNIER90 code, for example.

One of the advantages of employing Wannier functions is that computational cost can be reduced significantly. Therefore, the method is suitable for evaluating physical properties, which sensitively depend on the Fermi surface topology and band hybridizations (conductivity, Berry curvature, susceptibility, etc.), accurately on a fine k-mesh—in the scheme known as Wannier interpolation.

Importantly, Wannier function description can bridge top-down DFT calculations with bottom-up theoretical models. Thus, it is ideal for analyzing microscopic mechanisms in detail by theory. The Wannier-based model is also often a starting point for more advanced methods incorporating many-body interactions.

GO-Lab has developed the ORBITRANS code, which is capable of computing electronic structure properties (orbital/spin textures, Berry curvature, quantum metric, etc.) and transport coefficients (orbital/spin Hall effects, Edelstein effects, current-induced torques, etc.), which has proven its power in predicting new orbitronic phenomena and material candidates for experimental observation.