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6. Axion Magnetoelectric Coupling in the Hybrid Wannier Representation
January 2020

wannier graphsA 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, arXiv:1912.11887, (2020).

5. Dirac Fermions and Possible Weak Antilocalization in LaCuSb2
December 2019

mock up of diracIQM_NiNb2O6_Highlight_November_2019 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).

4. Probing Magnon Dynamics and Interactions in a Ferromagnetic Spin-1 Chain
December 2019

spin-1-chartsUsing 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.

“Probing magnon dynamics and interactions in a ferromagnetic spin-1 chain,” P. Chauhan, F. Mahmood, H. J. Changlani, S. M. Koohpayeh, N. P. Armitage, arXiv:1907.06542, (2020).

3. Antiferromagnetic Domain Walls in Ferromagnetic Yb2Ti2O7
December 2019

Pechan chartWe 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.

“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, arXiv:1912.04913, (2019).

2. 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).

1. IQM study of Quantum Spin Fluctuations Highlighted by DOE
December 2017

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.