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### 13. Duality and domain wall dynamics in a twisted Kitaev chain

### April 2021

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).

### 12. Incomme**nsurate magnetism mediated by Weyl fermions**

### December 2020

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).

### 11. Interfacing a Dirac semimetal with magnetism

### July 2020

We have measured Fermi-energy-dependent quantum transport in electrostatically-gated thin films of the Dirac semimetal Cd_{3}As_{2} interfaced with a ferromagnet.

Preprint of publication is forthcoming and will be linked here when available.

### 10. Room-temperature THz anomalous Hall effect in Weyl antiferromagnet Mn_{3}Sn films

### May 2020

Using time-domain THz spectroscopy and polarimetry, we have measured the magneto-optical response of the Weyl AF Mn_{3}Sn. 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 Mn _{3}Sn 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 Mn _{3}Sn thin films,” **T. Matsuda, N. Kanda, T. Higo, N. P. Armitage, S. Nakatsuji, and R. Matsunaga, Nature Communications 11, 909 (2020).

### 9. Magnons from molecular spins in chiral Cu_{2}0Se0_{3}

### March 2020

The asymmetric magnetic interactions that control the low energy excitations and the skyrmionic spin texture of Cu_{2}OSeO_{3} were determined using inelastic neutron scattering and a coarse-grained description of this chiral breathing pyrochlore ferrimagnet.

**“Low energy magnons in the chiral ferrimagnet**** ****Cu**_{2}**OSeO**_{3}**: 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 Mn**_{3}**Ge**

### January 2020

The anti-chiral spin structure of Mn_{3}Ge and its low energy excitations were determined using neutron scattering techniques. We developed a field theory of long-wavelength spin waves for Mn_{3}Ge 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 Mn_{3}Ge.

** “Antichiral spin order its Goldstone modes and their hybridization with phonons in the topological semimetal Mn_{3}Ge,” **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).

### 7. Axion Magnetoelectric Coupling in the Hybrid Wannier Representation

January 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 LaCuSb_{2}

December 2019

We predicted and then experimentally showed that LaCuSb_{2}, previously reported to be superconducting, hosts Dirac fermions with an effective mass of 0.06 m_{e}.

**“Dirac fermions and possible weakantilocalization in LaCuSb _{2},” **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).

### 5. Probing Magnon Dynamics and Interactions in a Ferromagnetic Spin-1 Chain

December 2019

Using time-domain THz spectroscopy, we measure the low-energy EM response of the spin-1 ferromagnetic spin chain NiNb_{2}O_{6} 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 Yb**_{2}Ti_{2}O_{7}

December 2019

_{2}Ti

_{2}O

_{7}

We show that ferromagnetic and antiferromagnetic states are nearly degenerate in the frustrated quantum magnet Yb_{2}Ti_{2}O_{7}. 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 Yb_{2}Ti_{2}O_{7},” 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).

### 3. Yi Li’s Work on Itinerant Ferromagnetism Featured in Quanta Magazine

January 2019

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

December 2018

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 Rh_{3}O_{12} and Ir_{3}O_{12} with 4d and 5d electron based magnetism.

**“Trimer-based spin liquid candidate Ba _{4}NbIr_{3}O_{12},”** Nguyen, L. T., Cava, R. J., Phys. Rev. Materials, 3 (1), 014412, (2019).

### 1. IQM study of Quantum Spin Fluctuations Highlighted by DOE

December 2017

A recent IQM study of quantum spin fluctuations in Pr_{2}Zr2O_{7} 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 Pr _{2}Zr_{2}O_{7},”** 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.