Editing Axion (section)

=== Analogous effect for topological insulators ===
A term analogous to the one that would be added to [[Maxwell's equations]] to account for axions<ref>{{cite journal |last=Wilczek |first=Frank |title=Two applications of axion electrodynamics |journal=Physical Review Letters |date=4 May 1987 |volume=58 |issue=18 |pages=1799–1802 |doi=10.1103/PhysRevLett.58.1799 |pmid=10034541 |bibcode=1987PhRvL..58.1799W}}</ref> also appears in recent (2008) theoretical models for [[topological insulators]] giving an effective axion description of the electrodynamics of these materials.<ref>{{cite journal |last1=Qi |first1=Xiao-Liang |last2=Hughes |first2=Taylor L. |last3=Zhang |first3=Shou-Cheng |title=Topological field theory of time-reversal invariant insulators |date=24 November 2008 |journal=Physical Review&nbsp;B |volume=78 |issue=19 |page=195424 |doi=10.1103/PhysRevB.78.195424 |bibcode=2008PhRvB..78s5424Q |arxiv=0802.3537|s2cid=117659977 }}</ref>

This term leads to several interesting predicted properties including a quantized [[magnetoelectric effect]].<ref name="Franz 36">{{cite journal |doi=10.1103/Physics.1.36 |volume=1 |page=36 |last=Franz |first=Marcel |title=High-energy physics in a new guise |journal=Physics |date=24 November 2008 |bibcode=2008PhyOJ...1...36F |doi-access=free}}</ref> Evidence for this effect has been given in [[N. Peter Armitage|THz spectroscopy experiments]] performed at the [[Johns Hopkins University]] on quantum regime thin film topological insulators developed at [[Rutgers University]].<ref>{{cite journal |last1=Wu |first1=Liang |last2=Salehi |first2=M. |last3=Koirala |first3=N. |last4=Moon |first4=J. |last5=Oh |first5=S. |last6=Armitage |first6=N. P. |date=2 December 2016 |title=Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator |journal=Science |volume=354 |issue=6316 |pages=1124–1127 |arxiv=1603.04317 |bibcode=2016Sci...354.1124W |doi=10.1126/science.aaf5541 |pmid=27934759 |s2cid=25311729 }}</ref>

In 2019, a team at the [[Max Planck Institute for Chemical Physics of Solids]] published their detection of an [[Magnetic topological insulator#Axion coupling|axion insulator]] phase of a [[Weyl semimetal]] material.<ref>
{{cite journal
 |first1=J.    |last1=Gooth      |first2=B.   |last2=Bradlyn
 |first3=S.    |last3=Honnali    |first4=C.   |last4=Schindler
 |first5=N.    |last5=Kumar      |first6=J.   |last6=Noky
 |first7=Y.    |last7=Qi         |first8=C.   |last8=Shekhar
 |first9=Y.    |last9=Sun        |first10=Z.  |last10=Wang
 |first11=B. A.|last11=Bernevig  |first12=C.  |last12=Felser 
 |display-authors=6
 |date=7 October 2019
 |title=Axionic charge-density wave in the Weyl semimetal (TaSe<sub>4</sub>)<sub>2</sub>I
 |journal=Nature
 |volume=575  |issue=7782   |pages=315–319
 |doi=10.1038/s41586-019-1630-4  |pmid=31590178    |arxiv=1906.04510
 |bibcode=2019Natur.575..315G    |s2cid=184487056
}}</ref> In the axion insulator phase, the material has an axion-like [[quasiparticle]] – an excitation of electrons that behave together as an axion – and its discovery demonstrates the consistency of axion electrodynamics as a description of the interaction of axion-like particles with electromagnetic fields. In this way, the discovery of axion-like quasiparticles in axion insulators provides motivation to use axion electrodynamics to search for the axion itself.<ref>
{{cite web
 |last=Fore
 |first=Meredith
 |date=22 November 2019
 |title=Physicists have finally seen traces of a long-sought particle. Here's why that's a Big Deal.
 |website=Live Science
 |publisher=Future US, Inc.
 |url=https://www.livescience.com/axion-found-in-weyl-semimetal.html
 |access-date=25 February 2020
}}</ref>