@article{ohanyanFlatBandsJ_1J_2J_32026,

title = {Flat bands in the J_{1}-J_{2}-J_{3} XXZ sawtooth chain},

author = {V. Ohanyan and J. Richter and M. Sekania and L. Giambattista and A. Andreanov and M. Kollar},

url = {https://link.aps.org/doi/10.1103/byv6-h2m2},

doi = {10.1103/byv6-h2m2},

year  = {2026},

date = {2026-06-04},

urldate = {2026-06-01},

journal = {Phys. Rev. B},

volume = {113},

number = {21},

pages = {214418},

abstract = {We consider a generalization of the XXZ model on the sawtooth spin chain with Dzyaloshinskii-Moriya interactions in which all exchange constants (symmetric, antisymmetric, and axial anisotropy) are different for the three different bonds of each triangle. We derive and resolve algebraic constraints on the exchange constants ensuring the appearance of a flat band in the one-magnon spectrum. The properties of the corresponding flat magnon bands and localized magnon states are analyzed. We further construct the mapping of the flat-band conditions for the Dzyaloshinskii-Moriya constants onto the Katsura-Nagaosa-Balatsky parameters. Based on the mapping, the possibility of the electric-field-driven flat bands with the aid of the magnetoelectric coupling is examined.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{schilberthOpticalPhononsTesting2025,

title = {Optical phonons as a testing ground for spin group symmetries},

author = {F. Schilberth and M. Kondákor and D. Ukolov and A. Pawbake and K. Vasin and O. Ercem and L. Prodan and V. Tsurkan and A. A. Tsirlin and C. Faugeras and P. Lemmens and K. Penc and I. Kézsmárki and S. Bordács and J. Deisenhofer},

url = {https://doi.org/10.1038/s41535-026-00857-9},

doi = {10.1038/s41535-026-00857-9},

year  = {2026},

date = {2026-02-17},

urldate = {2026-02-17},

journal = {npj Quantum Mater.},

volume = {11},

pages = {26},

abstract = {Lattice vibrations are highly sensitive to crystal symmetries and their changes across phase transitions. The latter can modify irreducible (co)representations and corresponding infrared and Raman selection rules of phonons. This concept is established for relativistic magnetic point groups, simultaneously transforming spatial and spin coordinates. However, in altermagnets described by non-relativistic spin groups with disjunct symmetry operations for both vector spaces, the phonon selection rules have remained unexplored. Here, we present a detailed study of the infrared- and Raman-active modes in the collinear antiferromagnet and altermagnet candidate Co2Mo3O8. Comparing to ab initio calculations accurately capturing the eigenfrequencies, we identify all expected phonon modes at room temperature and deduce their selection rules using both symmetry approaches. Importantly, we observe the change of selection rules upon antiferromagnetic ordering, agreeing with the relativistic symmetry approach, while the spin group formalism predicts no changes. Therefore, optical phonons sensing the symmetry of the magnetic order can reveal if relevant magnon-phonon coupling is compatible with the spin-group approach or not.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{ohanyanElectricfieldDrivenFlat2026,

title = {Electric-field driven flat bands in the distorted sawtooth chain via the Katsura–Nagaosa–Balatsky mechanism},

author = {V. Ohanyan and L. Amiraghyan and M. Sekania and M. Kollar},

url = {https://doi.org/10.1515/zna-2025-0381},

doi = {doi:10.1515/zna-2025-0381},

year  = {2026},

date = {2026-02-10},

urldate = {2026-02-01},

journal = {Z. Naturforsch. A},

volume = {81},

number = {4},

pages = {289},

abstract = {We investigate flat magnonic bands in a generalized sawtooth-chain model in which three sets of exchange parameters (symmetric Heisenberg exchange, axial Ising anisotropy, and antisymmetric Dzyaloshinskii–Moriya (DM) exchange) are assigned independently to each side of the triangular plaquette. If the effective Dzyaloshinskii–Moriya (DM) interaction parameters are generated via the Katsura–Nagaosa–Balatsky (KNB) mechanism of magnetoelectricity, they become explicit functions of the electric-field magnitude and direction, as well as of the lattice geometry, which in the present case is characterized by two bond angles. We focus on the situation in which these two angles are unequal, corresponding to a distortion of the triangular plaquette. Several electric-field–induced flat-band scenarios in the distorted sawtooth chain are analyzed, and expressions are derived for the electric-field strength required to drive the one-magnon excitations into a flat-band regime when the field is aligned along the lattice bonds. The saturation field and its dependence on the distortion angle are also examined. Finally, we establish a mapping between the flat-band solutions for a general DM interaction and its specific KNB-induced form.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{kamenskyi_terahertz_2024,

title = {Terahertz Resonant Emission by Optically Excited Infrared-Active Shear Phonons in KY(MoO_{4})_{2}},

author = {D. Kamenskyi and K. Vasin and L. Prodan and K. Kutko and V. Khrustalyov and S. G. Pavlov and H. -W. Hübers},

url = {https://advanced.onlinelibrary.wiley.com/doi/abs/10.1002/advs.202407028},

doi = {10.1002/advs.202407028},

year  = {2024},

date = {2024-11-13},

urldate = {2024-11-01},

journal = {Adv. Sci.},

volume = {12},

number = {2},

pages = {2407028},

abstract = {Abstract The generation of monochromatic electromagnetic radiation in the terahertz (THz) frequency range has remained a challenging task for many decades. Here, the emission of monochromatic sub-THz radiation by optical phonons in the dielectric material KY(MoO4)2 is demonstrated. The layered crystal structure of KY(MoO4)2 causes infrared-active shear lattice vibrations to have energies below 3.7 meV, corresponding to frequencies lower than 900 GHz where solid-state-based monochromatic radiation sources are rare. Directly excited by a 5 ps long broadband THz pulse, infrared-active optical vibrations in KY(MoO4)2 re-emit narrowband sub-THz radiation as a time-varying dipole for tens of picoseconds, which is exceptionally long for oscillators with frequencies below 1 THz. Such a long coherent emission allows for the detection of more than 50 periods of radiation with frequencies of 568 and 860 GHz. The remarkably long decay time together with the chemical stability of the employed material suggests a variety of possible applications in THz technology.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{vasin_optical_2024,

title = {Optical magnetoelectric effect in the polar honeycomb antiferromagnet Fe_{2}Mo_{3}O_{8}},

author = {K. V. Vasin and A. Strinić and F. Schilberth and S. Reschke and L. Prodan and V. Tsurkan and A. R. Nurmukhametov and M. V. Eremin and I. Kézsmárki and J. Deisenhofer},

doi = {10.1103/PhysRevB.110.054401},

year  = {2024},

date = {2024-08-01},

urldate = {2024-08-01},

journal = {Phys. Rev. B},

volume = {110},

number = {5},

pages = {054401},

abstract = {The lack of both time-reversal and spatial inversion symmetry in polar magnets is a prerequisite for the occurrence of optical magnetoelectric effects such as nonreciprocal directional dichroism, i.e., a difference in refractive index and absorption for two counter-propagating electromagnetic waves, which has the potential for the realization of optical diodes. In particular, antiferromagnetic materials with magnetic excitations in the THz range such as Fe2⁢Mo3⁢O8 are promising candidates for next-generation spintronic applications. In a combined experimental and theoretical effort we investigated the THz excitations of the polar honeycomb antiferromagnet Fe2⁢Mo3⁢O8 in external magnetic fields and their nonreciprocal directional dichroism, together with the temperature dependence of the electronic transitions in the mid- and near-infrared frequency range. Using an advanced single-ion approach for the Fe ions, we are able to describe optical excitations from the THz to the near-infrared frequency range quantitatively and model the observed nonreciprocal directional dichroism in the THz regime successfully.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{ghara_magnetization_2023,

title = {Magnetization reversal through an antiferromagnetic state},

author = {S. Ghara and E. Barts and K. V. Vasin and D. Kamenskyi and L. Prodan and V. Tsurkan and I. Kézsmárki and M. Mostovoy and J. Deisenhofer},

doi = {10.1038/s41467-023-40722-y},

year  = {2023},

date = {2023-08-01},

urldate = {2023-08-01},

journal = {Nat. Commun.},

volume = {14},

number = {1},

pages = {5174},

abstract = {Magnetization reversal in ferro- and ferrimagnets is a well-known archetype of non-equilibrium processes, where the volume fractions of the oppositely magnetized domains vary and perfectly compensate each other at the coercive magnetic field. Here, we report on a fundamentally new pathway for magnetization reversal that is mediated by an antiferromagnetic state. Consequently, an atomic-scale compensation of the magnetization is realized at the coercive field, instead of the mesoscopic or macroscopic domain cancellation in canonical reversal processes. We demonstrate this unusual magnetization reversal on the Zn-doped polar magnet Fe2Mo3O8. Hidden behind the conventional ferrimagnetic hysteresis loop, the surprising emergence of the antiferromagnetic phase at the coercive fields is disclosed by a sharp peak in the field-dependence of the electric polarization. In addition, at the magnetization reversal our THz spectroscopy studies reveal the reappearance of the magnon mode that is only present in the pristine antiferromagnetic state. According to our microscopic calculations, this unusual process is governed by the dominant intralayer coupling, strong easy-axis anisotropy and spin fluctuations, which result in a complex interplay between the ferrimagnetic and antiferromagnetic phases. Such antiferro-state-mediated reversal processes offer novel concepts for magnetization control, and may also emerge for other ferroic orders.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}