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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).
7. 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, arXiv:2001.09495, (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).
5. 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).
3. 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.
“Exact results on itinerant ferromagnetism and the 15-puzzle problem,” E. Bobrow, K. Stubis, Y. Li, Phys. Rev. B. 98, 10.1103 (2018).
1. IQM study of Quantum Spin Fluctuations Highlighted by DOE
“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.