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UID:UW-Physics-Event-2996
DTSTART:20130509T150000Z
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DTSTAMP:20260419T225527Z
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LOCATION:5310 Chamberlin
SUMMARY:Magnetization dynamics and semi-metal-insulator transition on 
 the surface of a topological insulator\, R. G. Herb Condensed Matter S
 eminar\, Ilya Eremin\, Ruhr University Bochum
DESCRIPTION:Due to its unique properties\, topological insulators (TI)
  are likely to play a major role as a component material in different 
 types of heterostructures. For instance\, with a view towards spintron
 ics applications\, heterostructures involving ferromagnetic (FM) mater
 ials and topological insulators are extensively studied both theoretic
 ally and experimentally at present.<br>
\n<br>
\nA thin film of ferrom
 agnetically ordered material proximate to the surface of a three-dimen
 sional topological insulator explicitly breaks the time-reversal symme
 try of the surface states. For an out-of-plane ferromagnetic order par
 ameter on the surface\, parity is also broken\, since the Dirac fermio
 ns become massive. This leads in turn to the generation of a Chern-Sim
 ons term by quantum fluctuations. On the other hand\, for an in-plane 
 magnetization the surface states remain metallic. <br>
\n<br>
\nWe con
 sider a theory for a two-dimensional interacting conduction electron s
 ystem with strong spin-orbit coupling on the interface between a topol
 ogical insulator and the magnetic (ferromagnetic or antiferromagnetic)
  layer. For the ferromagnetic case we derive the Landau-Lifshitz equat
 ion\, which features a contribution proportional to a fluctuation-indu
 ced electric field obtained by computing the topological (Chern-Simons
 ) contribution from the vacuum polarization. We also show that fermion
 ic quantum fluctuations reduce the critical temperature T*c at the int
 erface relative to the critical temperature Tc of the bulk\, so that i
 n the interval T*c ≤T<Tc it is possible to have a coexistence of gap
 less Dirac fermions at the interface with a ferromagnetically ordered 
 layer. For the case of an antiferromagnetic layer on a topological ins
 ulator substrate\, we show that a second-order quantum phase transitio
 n occurs at the interface\, and compute the corresponding critical exp
 onents. In particular\, we show that the electrons at the interface ac
 quire an anomalous dimension at criticality. The critical behavior of 
 the Néel order parameter is anisotropic and features large anomalous 
 dimensions for both the longitudinal and transversal fluctuations. <br
 >
\n<br>
\nIn addition we study the possibility of spontaneous breakin
 g of parity due to a dynamical gap generation on the surface. We find 
 that in the absence of interaction between the fermions there is no sp
 ontaneous gap generation. In the presence of a local\, Hubbard-like\, 
 interaction of strength g\, a gap and a Chern-Simons term are generate
 d for g larger than some critical value provided the number of Dirac f
 ermions\, N is odd. For an even number of Dirac fermions the masses ar
 e generated in pairs having opposite signs\, and no Chern-Simons term 
 is generated. Our result offers a possible explanation to recent exper
 iments showing a gap opening even when the topological insulator is pr
 oximate to a planar ferromagnet.<br>
\n<br>
\n[1] F.S. Noguiera and Il
 ya Eremin\, Phys. Rev. Lett. 109\, 237203 (2012)<br>
\n[2] F.S. Noguie
 ra and Ilya Eremin\, arXiv:1304.2933 (unpublished).<br>
\n
URL:https://www.physics.wisc.edu/events/?id=2996
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