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The Ising chain in a transverse field is a paradigmatic model for a host of physical phenomena, including spontaneous symmetry breaking, quantum criticality and duality. Although the quasi-one-dimensional ferromagnet CoNb2O6 has been regarded as the Ising chain’s best material realization, it exhibits substantial deviations from ideality. By combining terahertz spectroscopy and calculations, we show that CoNb2O6 is in fact described by a different model with bond-dependent interactions, which we call the ‘twisted Kitaev chain’, as these interactions are similar to those of the honeycomb Kitaev spin liquid.
“Duality and domain wall dynamics in a twisted Kitaev chain,” C. M. Morris, Nisheeta Desai, J. Viirok, D. Huvonen, U. Nagel, T. Room, J.W. Krizan, R. J. Cava, T. M. McQueen, S. M. Koohpayeh, Ribhu K. Kaul, and N. P. Armitage, Nature Physics, (2021).
A detailed example of rare earth magnetism mediated by topologically protected Weyl fermions is provided. Introducing NdAlSi, which lacks inversion symmetry, the unique physical properties that may arise from the interplay between electronic interactions and topology are identified.
“Incommensurate magnetism mediated by Weyl fermions in NdAlSi” Jonathan Gaudet, Hung-Yu Yang, Santu Baidya, Baozhu Lu, Guangyong Xu, Yang Zhao, Jose A. Rodriguez, Christina M. Hoffmann, David E. Graf, Darius H. Torchinsky, Predrag Nikolić, David Vanderbilt, Fazel Tafti, and Collin L. Broholm, arXiv:2012.12970, (2020).
We have measured Fermi-energy-dependent quantum transport in electrostatically-gated thin films of the Dirac semimetal Cd3As2 interfaced with a ferromagnet.
Preprint of publication is forthcoming and will be linked here when available.
Using time-domain THz spectroscopy and polarimetry, we have measured the magneto-optical response of the Weyl AF Mn3Sn. A large anomalous Hall conductivity |σxy|~20 Ω- 1cm-1 at THz frequencies is clearly observed as polarization rotation. Gapping of the Fermi surfaces is apparent due to translation symmetry breaking in the helical phase.
“Terahertz conductivity of the magnetic Weyl semimetal Mn3Sn films,” B. Cheng, Y. Wang, D. Barbalas, T. Higo, S. Nakatsuji, and N. P. Armitage, Applied Physics Letters, 115, 012405 (2019).
“Room-temperature terahertz anomalous Hall effect in Weyl antiferromagnet Mn3Sn thin films,” T. Matsuda, N. Kanda, T. Higo, N. P. Armitage, S. Nakatsuji, and R. Matsunaga, Nature Communications 11, 909 (2020).
The asymmetric magnetic interactions that control the low energy excitations and the skyrmionic spin texture of Cu2OSeO3 were determined using inelastic neutron scattering and a coarse-grained description of this chiral breathing pyrochlore ferrimagnet.
“Low energy magnons in the chiral ferrimagnet Cu2OSeO3: a coarse-grained approach,” Yi Luo, G. G. Marcus, B. A. Trump, J. Kindervater, M. B. Stone, J. A. Rodriguez-Rivera, Yiming Qiu, T. M. McQueen, O. Tchernyshyov, and C. Broholm, Phys. Rev. B 101, 144411 (2020).
8. Anti-chiral spin order its Goldstone modes and their hybridization with phonons in the topological semimetal Mn3Ge
The anti-chiral spin structure of Mn3Ge and its low energy excitations were determined using neutron scattering techniques. We developed a field theory of long-wavelength spin waves for Mn3Ge and establish a spin-Hamiltonian model, which accounts for the spin order and low energy excitations. We identify two magneto-elastic modes that indicate an intimate relationship between strain, magnetism and transport in Mn3Ge.
“Antichiral spin order its Goldstone modes and their hybridization with phonons in the topological semimetal Mn3Ge,” Y. Chen, J. Gaudet, S. Dasgupta, G. G. Marcus, J. Lin, T. Chen, T. Tomita, M. Ikhlas, Y. Zhao, W. C. Chen, M. B. Stone, O. Tchernyshyov, S. Nakatsuji, C. Broholm, Phys. Rev. B 102, 054403, (2020).
A large class of topological insulators (TIs) is characterized by the presence of a quantized magneto-electric (ME) “axion” coupling. We have developed a detailed theory connecting this axion coupling to features of the hybrid Wannier (HW) representation, which has emerged as one of the most important theoretical tools for understanding TIs.
“Axion coupling in the hybrid Wannier representation,” N. Varnava, I. Souza, D, Vanderbilt, Phys. Rev. B 101, 155130 (2020).
6. Dirac Fermions and Possible Weak Antilocalization in LaCuSb2
We predicted and then experimentally showed that LaCuSb2, previously reported to be superconducting, hosts Dirac fermions with an effective mass of 0.06 me.
“Dirac fermions and possible weakantilocalization in LaCuSb2,” J. R. Chamorro, A. Topp, Y. Fang, M. J. Winiarski, C. R. Ast, M. Krivenkov, A. Varykhalov, B. J.Ramshaw, L. M. Schoop, and T. M. McQueen, APL Mater. 7, 121108, (2019).
Using time-domain THz spectroscopy, we measure the low-energy EM response of the spin-1 ferromagnetic spin chain NiNb2O6 as a function of temperature and external magnetic field. Prominent magnetic excitations are seen at low T. As we warm the system, we unexpectedly observe a T dependent renormalization of the lowest energy spin-excitation, which has a strong dependence on field direction. Using theoretical arguments, exact diagonalizations and finite temperature dynamical Lanczos calculations, we connect this renormalization to a picture of magnon-magnon interactions.
“Tunable Magnon Interactions in a Ferromagnetic Spin-1 Chain,” P. Chauhan, F. Mahmood, H. J. Changlani, S. M. Koohpayeh, and N. P. Armitage, Phys. Rev. Lett. 124, 037203 (2020).
4. Antiferromagnetic Domain Walls in Ferromagnetic Yb2Ti2O7
We show that ferromagnetic and antiferromagnetic states are nearly degenerate in the frustrated quantum magnet Yb2Ti2O7. The long range nature of dipole interactions consequently induce a mixed FM/AFM state at low fields where AFM slabs serve as domain walls. Read the article commentary by Sylvain Petit in PNAS.
“Multiphase Magnetism in Yb2Ti2O7,” A. Scheie, J. Kindervater, S. Zhang, H.J. Changlani, G. Sala, G. Ehlers, A. Heinemann, G. S. Tucker, S.M. Koohpayeh, C. Broholm, Proceedings of the National Academy of Sciences, 202008791, (2020).
Yi Li’s group’s work (supported by IQM funding) about itinerant ferromagnetism is featured by Quanta Magazine in an article titled “A Child’s Puzzle Has Helped Unlock the Secrets of Magnetism.”
“Exact results on itinerant ferromagnetism and the 15-puzzle problem,” E. Bobrow, K. Stubis, Y. Li, Phys. Rev. B. 98, 10.1103 (2018).
2. New trimer-based spin liquid candidates
A previously unreported family of materials has been discovered with unconventional magnetic properties that are consistent the realization of the quantum spin liquid. The materials consist of triangular lattices of magneticntrimer molecules of the form Rh3O12 and Ir3O12 with 4d and 5d electron based magnetism.
“Trimer-based spin liquid candidate Ba4NbIr3O12,” Nguyen, L. T., Cava, R. J., Phys. Rev. Materials, 3 (1), 014412, (2019).
1. IQM study of Quantum Spin Fluctuations Highlighted by DOE
A recent IQM study of quantum spin fluctuations in Pr2Zr2O7 that was published in Physical Review Letter was highlighted by DOE. This was the thesis work of Jiajia Wen who is now at SLAC.
“Disordered route to the Coulomb quantum spin liquid: Random transverse fields on spin ice in Pr2Zr2O7,” J. J. Wen, S. M. Koohpayeh, K. A. Ross, B. A. Trump, T. M. McQueen, K. Kimura, S. Nakatsuji, Y. Qiu, D. M. Pajerowski, J. R. D. Copley, and C. L. Broholm, Phys. Rev. Lett. 118, 107206 (2017); arXiv:1609.08551.