Time-Reversal-Breaking Weyl Fermions in Magnetic Heusler Alloys (1603.00479v2)

Abstract: Weyl fermions have recently been observed in several time-reversal-invariant semimetals and photonics materials with broken inversion symmetry. These systems are expected to have exotic transport properties such as the chiral anomaly. However, most discovered Weyl materials possess a substantial number of Weyl nodes close to the Fermi level that give rise to complicated transport properties. Here we predict, for the first time, a new family of Weyl systems defined by broken time-reversal symmetry, namely, Co-based magnetic Heusler materials XCo2Z (X = IVB or VB; Z = IVA or IIIA). To search for Weyl fermions in the centrosymmetric magnetic systems, we recall an easy and practical inversion invariant, which has been calculated to be -1, guaranteeing the existence of an odd number of pairs of Weyl fermions. These materials exhibit, when alloyed, only two Weyl nodes at the Fermi level - the minimum number possible in a condensed matter system. The Weyl nodes are protected by the rotational symmetry along the magnetic axis and separated by a large distance (of order 2$\pi$) in the Brillouin zone. The corresponding Fermi arcs have been calculated as well. This discovery provides a realistic and promising platform for manipulating and studying the magnetic Weyl physics in experiments.

• The paper introduces a theoretical prediction of two minimal Weyl nodes in magnetic Co-based Heusler alloys using DFT and SOC.
• It employs first-principles calculations with magnetization along [110] to elucidate transport phenomena linked to the chiral anomaly.
• The study outlines a doping strategy with Nb to tune Weyl nodes to the Fermi level, thereby advancing experimental detection in spintronic applications.

Time-Reversal-Breaking Weyl Fermions in Magnetic Heusler Alloys

The paper presented delineates the theoretical prediction of Weyl systems characterized by broken time-reversal symmetry, particularly focusing on Co-based magnetic Heusler materials with the formula XCo2Z (where X = IVB or VB, Z = IVA or IIIA). This paper enriches our understanding of Weyl fermions by extending their realization to magnetic systems, unveiling their potential applicability in spintronic devices.

Theoretical Framework

In condensed matter physics, Weyl fermions manifest as unremovable band crossings in certain materials, which have garnered substantial interest following experimental discoveries in non-centrosymmetric materials. In contrast, this work ventures into the domain of time-reversal symmetry-broken systems, utilizing Heusler alloys that exhibit an inherent magnetic ordering suitable for the realization and exploration of Weyl fermions.

The authors apply first-principles calculations using Density Functional Theory (DFT) and the Generalized Gradient Approximation (GGA), adhering to computational methodologies that incorporate the effects of spin-orbit coupling (SOC) and magnetic orientation. They focus particularly on configurations with magnetization along the [110] direction.

Novel Predictions and Implications

The paper unveils a specific characteristic of these Heusler materials: the minimal Weyl node count, which simplifies experimental detection and the analysis of transport phenomena. Notably, the presence of only two Weyl nodes—protected by rotational symmetry—at the Fermi level in the Brillouin zone is emphasized. The calculated large separation between these nodes in momentum space and their Fermi arcs corroborates the distinctive nature of these materials and their transport properties.

Contrary to previous Weyl systems characterized by an abundance of nodes, this minimalist structure permits more straightforward observations of phenomena such as negative longitudinal magnetoresistance, attributed to the chiral anomaly. The clarity in the signature and behavior of these Weyl nodes underscores the potential for enhanced experimental investigations into the Weyl physics domain within these alloys.

Experimental Prospects

The authors also outline practical strategies to optimize the positioning of Weyl nodes in these materials. By alloying, specifically doping ZrCo2Sn with Nb, they provide a pathway to shift these nodes to the Fermi level—thereby making experimental detection and measurement feasible.

Magnetic Heusler compounds, with room-temperature Curie points, offer an advantageous platform for the exploration of magnetic Weyl fermion phenomena, fostering possible applications in spintronics due to their half-metallic ferromagnetism. Furthermore, the tunability of these materials, through variations in magnetization direction or substitutional alloying, opens avenues for further research, including potential applications in quantum and spin-based devices.

Future Directions

This research emphasizes the exploitation of Co-based Heusler alloys as a class of materials holding promise for experimental realizations of Weyl nodes with magnetic order. While the paper achieves a theoretical foundation, the successful synthesis of these materials and the subsequent experimental validation of theoretical predictions will be crucial in advancing the field.

Future research might explore other topological phenomena within this family, including nodal lines and further topological electronic phases influenced by varied magnetic configurations. As the paper suggests, the integration of coherent potential approximation methodologies to account for disorder effects could enhance the understanding of these systems' robustness under different conditions.

In conclusion, this paper posits a realistic and promising framework for magnetic Weyl physics exploration, potentially extending the landscape of condensed matter research significantly. The Co-based Heusler alloys emerge as notable candidates in the quest for new quantum materials with multifaceted technological applications.