Gdgai Research Reveals Stable Phonon Dispersion and 5d/4p Electronic Bands

Researchers are increasingly focused on understanding the intriguing properties of van der Waals materials, and a new study delves into the behaviour of the magnetic compound GdGaI. Tatsuya Kaneko, Ryota Mizuno, and Shu Kamiyama, all from the Department of Physics at The University of Osaka, alongside Hideo Miyamoto and Masayuki Ochi et al., have used first-principles calculations to map the electronic band structure, phonon dispersion, and a unique magnetic state within this material. Their work demonstrates the absence of phonon-driven phase transitions and reveals a complex magnetic order characterised by three q vectors, potentially driven by Coulomb interactions and Kondo coupling, offering valuable insight into the interplay between electronic and magnetic behaviour in this promising class of materials.

Phonon Stability and Electronic Structure of Magnetic GdGaI are presented

Scientists have theoretically investigated the physical properties of the magnetic van der Waals material GdGaI, revealing insights into its structural stability and electronic behaviour. Using first-principles calculations, the research team computed the phonon dispersion of GdGaI and demonstrated the absence of imaginary phonons, strongly suggesting that phase transitions driven by lattice vibrations are unlikely within this material.
This finding establishes a foundation for understanding the inherent stability of GdGaI under varying conditions. The study further unveils the composition of electronic bands near the Fermi energy, identifying contributions from Gd 5d and Ga 4p orbitals. To explore the magnetic structure, researchers constructed a tight-binding model incorporating these Gd 5d and Ga 4p orbitals, allowing for detailed analysis of electron interactions.

This model was then extended to include Kondo coupling between electrons in the Gd 5d orbitals and localized spins in the Gd 4f orbitals, presenting a modified band structure reflecting a magnetic order characterised by three q vectors connecting the valence and conduction bands. Experiments have previously shown a magnetic phase transition near 30 K in GdGaI, with nuclear magnetic resonance data suggesting a triple-q magnetic order with scalar spin chirality.

The current work builds upon these observations by discussing the origin of this spin order based on the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism. Researchers propose that Coulomb interactions acting on electrons near the Fermi level play a crucial role in the ordering of localized spins, providing a theoretical framework for understanding the magnetic behaviour of GdGaI.

This research establishes a comprehensive understanding of the electronic, structural, and magnetic properties of GdGaI, potentially paving the way for the development of novel spintronic materials and devices. The detailed analysis of band structure, phonon dispersion, and magnetic ordering provides a crucial foundation for future investigations into vdW magnets and their potential applications in advanced technologies.

Computational determination of electronic and magnetic properties in layered GdGaI is presented

Scientists investigated the physical properties of the magnetic van der Waals material GdGaI using first-principles calculations. They computed the phonon dispersion of GdGaI and confirmed the absence of imaginary phonons, indicating a stable crystal structure unlikely to undergo phonon-driven phase transitions.

Band calculations revealed that electronic bands near the Fermi energy are primarily composed of Gd 5d and Ga 4p orbitals. To further explore the magnetic structure, researchers constructed a tight-binding model incorporating Gd 5d and Ga 4p orbitals. This model included Kondo coupling between electrons in Gd 5d and localized spins in Gd 4f, modifying the band structure to reflect a magnetic order characterized by three q vectors connecting the valence and conduction bands.

The team then discussed the origin of this spin order, suggesting that Coulomb interactions acting on electrons near the Fermi level contribute to the ordering of localized spins, alongside the Ruderman-Kittel-Kasuya-Yosida mechanism. First-principles calculations were performed using the projector-augmented wave method as implemented in the Vienna ab initio simulation package, VASP.

The Heyd-Scuseria-Ernzerhof hybrid functional with a range-separation parameter μ = 0.15 Å−1 was employed to obtain an accurate band structure, with the generalized gradient approximation using the Perdew-Burke-Ernzerhof parametrization used for comparison. Open-core treatment was applied to Gd-4f localized states, utilizing [Xe 4f 7]-, [Ar]-, and [Kr 4d10]-core PAW potentials for Gd, Ga, and I atoms, respectively.

Spin-orbit coupling was omitted for simplicity. Calculations utilized a plane-wave cutoff energy of 400 eV and an 8 × 8 × 3 k mesh. The experimental crystal structure was adopted for DFT calculations, except for phonon calculations where atomic coordinates were optimized until the Hellmann, Feynman force for each atom was less than 10−4 eV Å−1.

A 3 × 3 × 2 q mesh was used in frozen phonon calculations implemented in the PHONOPY package, and all calculations were performed for both PBE and HSE functionals. The resulting phonon dispersion, presented in Figure 2(b), confirmed the structural stability of GdGaI, while the electronic band structure in Figure 2(c) revealed a semimetallic character with slight overlap between the valence band top and conduction band bottom.

Electronic Structure and Magnetic Ordering in GdGaI are discussed

Scientists investigated the physical properties of the magnetic van der Waals material GdGaI using first-principles calculations. Phonon dispersion calculations revealed no imaginary phonons, indicating that phonon-driven phase transitions are unlikely to occur within the material’s crystal structure.

The electronic band calculation showed that bands near the Fermi energy are primarily composed of Gd 5d and Ga 4p orbitals. Researchers constructed a tight-binding model incorporating Gd 5d and Ga 4p orbitals to examine the magnetic structure. Kondo coupling was introduced between electrons in Gd 5d and localized spins in Gd 4f, resulting in a modified band structure characterized by three q vectors connecting the valence and conduction bands.

The origin of this spin order is believed to be linked to the Ruderman-Kittel-Kasuya-Yosida interaction, with Coulomb interactions on electrons near the Fermi level potentially contributing to the ordering of localized spins. Experiments using the HSE hybrid functional with a range-separation parameter μ = 0.15Å⁻¹ demonstrated a stable crystal structure, confirmed by the absence of imaginary phonons in the phonon dispersion.

The electronic band structure revealed a semimetallic character, with the valence band top and conduction band bottom slightly overlapping at various k points. Specifically, the hole pocket is located around the A point, while the electron pocket resides along the M, L line. Data shows that the Gd 5d and Ga 4p orbitals dominate the electronic bands near the Fermi energy EF.

Contributions from the I 5p and Ga 4s orbitals are less significant in this energy region. Analysis of orbital weights revealed that the Ga 4p orbitals contribute to the formation of three valence bands near EF, with the highest valence band exhibiting substantial Gd 5d orbital weight around the Γ and A points, suggesting strong d-p hybridization. The honeycomb network of Ga sites results in Dirac-like dispersions around −1.5 eV and −7.5 eV below EF, formed by the Ga 4p and 4s orbitals.

GdGaI magnetism arises from interplay between band structure and Kondo interactions, leading to a complex magnetic ground state

Scientists have theoretically investigated the physical properties of the magnetic van der Waals material GdGaI, employing first-principles calculations to determine its phonon dispersion and electronic band structure. These calculations revealed no imaginary phonons, indicating a stable crystal structure unlikely to undergo phonon-driven phase transitions.

The electronic bands near the Fermi energy were found to be primarily composed of Gd 5d and Ga 4p orbitals, forming the basis for a subsequent tight-binding model. Researchers constructed a tight-binding model incorporating Gd 5d and Ga 4p orbitals, introducing Kondo coupling to account for interactions between electrons and localized spins in Gd 4f orbitals.

This model predicted a magnetic order characterized by three q vectors connecting the valence and conduction bands, with the origin of this spin order potentially linked to the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism and Coulomb interactions near the Fermi level. The study utilized the HSE hybrid functional within density functional theory, alongside the PBE parametrization for comparison, and employed specific PAW potentials for each element.

This work establishes a foundational understanding of the electronic and magnetic behaviour of GdGaI, suggesting its potential as a stable magnetic material. The absence of imaginary phonons confirms structural stability, while the identified electronic band characteristics and magnetic ordering mechanisms provide insights into its magnetic properties.

The authors acknowledge limitations including the exclusion of spin-orbit coupling for simplicity and note that further investigation is needed to fully elucidate the interplay between Kondo coupling, RKKY interactions, and Coulomb effects. Future research could focus on incorporating spin-orbit coupling and exploring the material’s behaviour under varying conditions to confirm these theoretical predictions and assess its suitability for spintronic applications.