We investigate topological features of electronic structures which produce large anomalous Hall effect in the noncollinear antiferromagnetic metallic states of antiperovskite manganese nitrides by first-principles calculations. We first predict the stable magnetic structures of these compounds to be noncollinear antiferromagnetic structures characterized by either T1g or T2g irreducible representation by evaluating the total energy for all of the magnetic structures classified according to the symmetry and multipole moments. The topology analysis is next performed for the Wannier tight-binding models obtained from the first-principles band structures. Our results reveal the small Berry curvature induced through the coupling between occupied and unoccupied states with the spin-orbit coupling, which is widely spread around the Fermi surface in the Brillouin zone, dominantly contributes after the k-space integration to the anomalous Hall conductivity, while the local divergent Berry curvature around Weyl points has a rather small contribution to the anomalous Hall conductivity.
Cluster multipole theory
Multipole expansion for magnetic structures: A generation scheme for symmetry-adapted orthonormal basis set in crystallographic point group
We propose a systematic method to generate a complete orthonormal basis set of multipole expansion for magnetic structures in arbitrary crystal structure. The key idea is the introduction of a virtual atomic cluster of a target crystal on which we can clearly define the magnetic configurations corresponding to symmetry-adapted multipole moments. The magnetic configurations are then mapped onto the crystal so as to preserve the magnetic point group of the multipole moments, leading to the magnetic structures classified according to the irreducible representations of the crystallographic point group. We apply the present scheme to pyrochlore and hexagonal ABO3 crystal structures and demonstrate that the multipole expansion is useful to investigate the macroscopic responses of antiferromagnets.
First-principles study for multipole order phase
First-principles theory of magnetic multipoles in condensed matter systems
The multipole concept, which characterizes the spacial distribution of scalar and vector objects by their angular dependence, has already become widely used in various areas of physics. In recent years it has become employed to systematically classify the anisotropic distribution of electrons and magnetization around atoms in solid state materials. This has been fuelled by the discovery of several physical phenomena that exhibit unusual higher rank multipole moments, beyond that of the conventional degrees of freedom as charge and magnetic dipole moment. Moreover, the higher rank electric/magnetic multipole moments have been suggested as promising order parameters in exotic hidden order phases. While the experimental investigations of such anomalous phases have provided encouraging observations of multipolar order, theoretical approaches have developed at a slower pace. In particular, a materials’ specific theory has been missing. The multipole concept has furthermore been recognized as the key quantity which characterizes the resultant configuration of magnetic moments in a cluster of atomic moments. This cluster multipole moment has then been introduced as macroscopic order parameter for a noncollinear antiferromagnetic structure in crystals that can explain unusual physical phenomena whose appearance is determined by the magnetic point group symmetry. It is the purpose of this review to discuss the recent developments in the first-principles theory investigating multipolar degrees of freedom in condensed matter systems. These recent developments exemplify that ab initio electronic structure calculations can unveil detailed insight in the mechanism of physical phenomena caused by the unconventional, multipole degree of freedom.
Anomalous Hall effect in antiferromagnets
Cluster multipole theory for anomalous Hall effect in antiferromagnets
We introduce a cluster extension of multipole moments to discuss the anomalous Hall effect (AHE) in both ferromagnetic (FM) and antiferromagnetic (AFM) states in a unified framework. We first derive general symmetry requirements for the AHE in the presence or absence of the spin-orbit coupling, by considering the symmetry of the Berry curvature in k space. The cluster multipole (CMP) moments are then defined to quantify the macroscopic magnetization in non-collinear AFM states, as a natural generalization of the magnetization in FM states. We identify the macroscopic CMP order which induces the AHE. The theoretical framework is applied to the non-collinear AFM states of Mn3Ir, for which an AHE was predicted in a first-principles calculation, and Mn3Z (Z=Sn, Ge), for which a large AHE was recently discovered experimentally. We further compare the AHE in Mn3Z and bcc Fe in terms of the CMP. We show that the AHE in Mn3Z is characterized with the magnetization of a cluster octupole moment in the same manner as that in bcc Fe characterized with the magnetization of the dipole moment.
First-principles study for heavy fermion systems
Change of Fermi surface topology in CeRu2Si2 studied by LSDA+U method
The detailed electronic structures of CeRu2Si2 have been investigated for a nonmagnetic state and a magnetic state under magnetic fields by an LSDA+U method. It is found that the Si position in the crystal structure is an essential parameter to reproduce precise topology of the experimentally determined Fermi surfaces of non- f reference LaRu2Si2 . As for CeRu2Si2 , the LDA calculation fails to predict the nonmagnetic ground state, which is experimentally reported to be mainly occupied by | j =5/2, j z =±5/2> orbitals, and the Fermi surfaces. The nonmagnetic case solution of LSDA+ U method greatly improves the electronic structure in the nonmagnetic CeRu2Si2 . An LSDA+U method is also applied to investigate the electronic structure under applied magnetic fields, then the change of the so-called large Fermi surfaces to small Fermi surfaces is successfully described.
Emergent Loop-Nodal s±-Wave Superconductivity in CeCu2Si2: Similarities to the Iron-Based Superconductors
Heavy-fermion superconductors are prime candidates for novel electron-pairing states due to the spin-orbital coupled degrees of freedom and electron correlations. Superconductivity in CeCu2Si2 discovered in 1979, which is a prototype of unconventional (non-BCS) superconductors in strongly correlated electron systems, still remains unsolved. Here we provide the first report of superconductivity based on the advanced first-principles theoretical approach. We find that the promising candidate is an s±-wave state with loop-shaped nodes on the Fermi surface, different from the widely expected line-nodal d-wave state. The dominant pairing glue is magnetic but high-rank octupole fluctuations. This system shares the importance of multiorbital degrees of freedom with the iron-based superconductors. Our findings reveal not only the long-standing puzzle in this material, but also urge us to reconsider the pairing states and mechanisms in all heavy-fermion superconductors.
First-principles study of magnetic properties in Fe-ladder compound BaFe2S3
We study the magnetic, structural, and electronic properties of the recently discovered iron-based superconductor BaFe2S3 based on density functional theory with the generalized gradient approximation. The calculations show that the magnetic alignment in which the spins are coupled ferromagnetically along the rung and antiferromagnetically along the leg is the most stable in the possible magnetic structure within an Fe ladder and is further stabilized with the periodicity characterized by the wave vector Q=(π,π,0), leading to the experimentally observed magnetic ground state. The magnetic exchange interaction between the Fe ladders creates a tiny energy gap, the size of which is in excellent agreement with the experiments. Applied pressure suppresses the energy gap and leads to an insulator-metal transition. Finally, we also discuss what type of orbitals can play crucial roles on the magnetic and insulator-metal transition.
Multipole order and global/site symmetry in the hidden order phase of URu2Si2
On the basis of group theory and first-principles calculations, we investigate high-rank multipole orderings in URu2Si2, which have been proposed as a genuine primary order parameter in the hidden-order phase below 17.5 K. We apply Shubnikov group theory to the multipole ordered states characterized by the wave vector Q0=(0,0,1) and specify the global/site symmetry and the secondary order parameters, such as induced dipole moments and change in charge distribution. We find that such antiferroic magnetic multipole orderings are particularly advantageous to conceal the primary order parameter due to preserving high symmetry in charge distribution. Experimental observations of the induced low-rank multipoles, which are explicitly classified in this paper, will be key pieces to understand the puzzling hidden-order phase.