Contributor info: Prof. N. Peter Armitage

A. Alexandradinata, N. P. Armitage, Andrey Baydin, Wenli Bi, Yue Cao, Hitesh J. Changlani, Eli Chertkov, Eduardo H. da Silva Neto, Luca Delacretaz, Ismail El Baggari, G. M. Ferguson, William J. Gannon, Sayed Ali Akbar Ghorashi, Berit H. Goodge, Olga Goulko, G. Grissonnanche, Alannah Hallas, Ian M. Hayes, Yu He, Edwin W. Huang, Anshul Kogar, Divine Kumah, Jong Yeon Lee, A. Legros, Fahad Mahmood, Yulia Maximenko, Nick Pellatz, Hryhoriy Polshyn, Tarapada Sarkar, Allen Scheie, Kyle L. Seyler, Zhenzhong Shi, Brian Skinner, Lucia Steinke, K. Thirunavukkuarasu, Thaís Victa Trevisan, Michael Vogl, Pavel A. Volkov, Yao Wang, Yishu Wang, Di Wei, Kaya Wei, Shuolong Yang, Xian Zhang, Ya-Hui Zhang, Liuyan Zhao, Alfred Zong

SciPost Phys. Comm. Rep. 8 (2025) · published 25 June 2025 | Toggle abstract · pdf

A central problem in modern condensed matter physics is the understanding of materials with strong electron correlations. Despite extensive work, the essential physics of many of these systems is not understood and there is very little ability to make predictions in this class of materials. In this manuscript we share our personal views on the major open problems in the field of correlated electron systems. We discuss some possible routes to make progress in this rich and fascinating field. This manuscript is the result of the vigorous discussions and deliberations that took place at Johns Hopkins University during a three-day workshop January 27, 28, and 29, 2020 that brought together six senior scientists and 46 more junior scientists. Our hope, is that the topics we have presented will provide inspiration for others working in this field and motivation for the idea that significant progress can be made on very hard problems if we focus our collective energies.

N. P. Armitage, Liang Wu

SciPost Phys. 6, 046 (2019) · published 17 April 2019 | Toggle abstract · pdf

It has been proposed that topological insulators can be best characterized not as surface conductors, but as bulk magnetoelectrics that -- under the right conditions-- have a universal quantized magnetoelectric response coefficient $e^2/2h$. However, it is not clear to what extent these conditions are achievable in real materials that can have disorder, finite chemical potential, residual dissipation, and even inversion symmetry. This has led to some confusion and misconceptions. The primary goal of this work is to illustrate exactly under what real life scenarios and in what context topological insulators can be described as magnetoelectrics. We explore analogies of the 3D magnetoelectric response to electric polarization in 1D in detail, the formal vs. effective polarization and magnetoelectric susceptibility, the 1/2 quantized surface quantum Hall effect, the multivalued nature of the magnetoelectric susceptibility, the role of inversion symmetry, the effects of dissipation, and the necessity for finite frequency measurements. We present these issues from the perspective of experimentalists who have struggled to take the beautiful theoretical ideas and to try to measure their (sometimes subtle) physical consequences in messy real material systems.