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Theoretical Investigation on the Origins of Novel Properties in Bi/Sb Square Nets: Structural, Electrical and Magnetic Analysis

Theoretical Investigation on the Origins of Novel Properties in Bi/Sb Square Nets: Structural, Electrical and Magnetic Analysis
Farhan, Muhammad Arshad
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This thesis is dedicated to the study of Bi/Sb square net compounds. These materials are reported to show exotic properties like Dirac fermions, Kondo lattice behavior and metal to insulator transition to name the few. First we report the observation of highly anisotropic Dirac fermions in a Bi square net of SrMnBi2, based on a first-principles calculation, angle-resolved photoemission spectroscopy, and quantum oscillations for high-quality single crystals. The collaborative work shows that the Dirac dispersion is generally induced in the (SrBi)+ layer containing a double-sized Bi square net. Theoretical study along with experimental results reveals that, in contrast to the commonly observed isotropic Dirac cone, the Dirac cone in SrMnBi2 is highly anisotropic with a large momentum-dependent disparity of Fermi velocities of ~8. These findings demonstrate that a Bi square net, a common building block of various layered pnictides, provides a new platform that hosts highly anisotropic Dirac fermions. For thorough investigation of these materials, low-energy electronic structures in AMnBi2 (AE=alkaline earths) are investigated using a first-principles calculation and a tight binding method. An anisotropic Dirac dispersion is induced by the checkerboard arrangement of AE atoms above and below the Bi square net in AMnBi2. It is observed that SrMnBi2 and CaMnBi2 have a different kind of Dirac dispersion due to the different stacking of nearby AE layers, where each Sr (Ca) of one side appears at the coincident (staggered) xy position of the same element at the other side. . Investigation of electronic structure suggested that electronic states from (MnBi) layers have energy levels well separated from Fermi level, so most electronic states near the Fermi level are from (SrBi) layers or more specifically from the Bi square net where a linear energy-momentum dispersion is realized to give Dirac fermions. Using the tight binding analysis, we report the chirality of the anisotropic Dirac electrons as well as the sizable spin-orbit coupling effect in the Bi square net. We suggest that the Bi square net provides a platform for the interplay between anisotropic Dirac electrons and the neighboring environment such as magnetism and structural changes. A significantly large spin-orbit coupling (SOC) in the Bi square net opens an energy gap with parabolic dispersion. The Dirac fermions therefore become massive making it hard to observe interesting phenomena related to massless Dirac fermions as observed in graphene. So, we investigated similar compound with lesser SOC. Therefore, in the final part, the Dirac fermions of Sb square net in AEMnSb2 (AE= Sr, Ba) are investigated by using first-principles calculation. BaMnSb2 contains Sb square net layers with a coincident stacking of Ba atoms, exhibiting Dirac fermions behavior. On the other hand, SrMnSb2 has a staggered stacking of Sr atoms with the distorted zig-zag chains of Sb atoms. The behavior is found consistent with their bi cousins. Application of hydrostatic pressure on SrMnSb2 however induces a structural change from staggered to coincident arrangement of AE ions accompanying a transition from insulator to a metal containing Dirac fermions. The structural investigations show that stacking type of cation and orthorhombic distortion of Sb layers are the main factors to decide the crystal symmetry of the material. Hence, we propose that the Dirac fermions can be obtained by controlling the size of cation and the volume of AEMnSb2 compounds.
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