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In this work we study a low-cost two-dimensional ferromagnetic semiconductor with possible applications in biomedicine, solar cells, spintronics, and energy and hydrogen storage. From first principle calculations we describe the unique electronic, transport, optical, and magnetic properties of a π-conjugated micropore polymer (CMP) with three iron atoms placed in the middle of an isolated pore locally resembling heme complexes. This material exhibits strong Fe-localized d z2 bands. The bandgap is direct and equal to 0.28 eV. The valence band is doubly degenerate at the Γ -point and for larger k -wavevectors the HOMO band becomes flat with low contribution to charge mobility. The absorption coefficient is roughly isotropic. The conductivity is also isotropic with the nonzero contribution in the energy range 0.3–8 eV. The xy -component of the imaginary part of the dielectric tensor determines the magneto-optical Faraday and Kerr rotation. Nonvanishing rotation is observed in the interval of 0.5–5.0 eV. This material is found to be a ferromagnet of an Ising type with long-range exchange interactions with a very high magnetic moment per unit cell, m = 6 μ B . The exchange integral is calculated by two independent methods: (a) from the energy difference between ferromagnetic and antiferromagnetic states and (b) from a magnon dispersion curve. In the former case J nn = 27 μeV. In the latter case the magnon dispersion is fitted by the Ising model with the nearest and next-nearest neighbor spin interactions. From these estimations we find that J nn = 19.5 μeV and J nnn = −3 μeV. Despite the different nature of the calculations, the exchange integrals are only within 28% difference.
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This content will become publicly available on May 7, 2026
Optical spectra of complex aggregates and crystals: Vibronic band structure and Davydov splitting
A thorough understanding of the nature and pattern of intermolecular interactions in molecular aggregates and crystals is a prerequisite for the design of the next generation of functional materials. In systems with multiple, symmetrically equivalent molecules per unit cell, each excited state of the isolated molecule splits into several Davydov components that appear in the absorption spectra in up to three orthogonally polarized transitions. In this work, a Frenkel–Holstein Hamiltonian is adopted to simulate the vibronic structure of the Davydov components in aggregates and crystals with up to four molecules per unit cell, where electrostatic intermolecular interactions define either 1D or 2D structures. Analysis shows that vibronic signatures report directly on the electronic couplings that contribute to the Davydov splitting and the exciton band shapes. Specifically, the vibronic signature of a given Davydov component is solely determined by its free excitonic shift. For crystals with two molecules per unit cell, the lower and upper Davydov components can each exhibit J-like or H-like behavior, resulting in JJ, JH, and HH components in order of increasing energy, all with unique vibronic signatures. Under certain conditions, null points can exist in either band, leading to a monomer-like absorption spectrum for the corresponding Davydov component. In crystals with four symmetrically equivalent molecules per unit cell, the J- or H-nature of the three orthogonally polarized Davydov components results in four possible combinations, JJJ, JJH, JHH, and HHH in order of increasing energy, all readily identified through vibronic signatures.
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- Award ID(s):
- 2221923
- PAR ID:
- 10588381
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 162
- Issue:
- 17
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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