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, arXiv:1912.11887, (2020).
5. Dirac Fermions and Possible Weak Antilocalization in LaCuSb2
“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.
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.
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
“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.