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Axion response in gapless systems.

Doron L Bergman1

  • 1Physics Department, California Institute of Technology, MC 114-36, 1200 East California Boulevard, Pasadena, California 91125, USA.

Physical Review Letters
|November 24, 2011
PubMed
Summary
This summary is machine-generated.

Topological insulators exhibit axion response, but metallic states disrupt this. This study finds a non-quantized magnetoelectric coupling persists even with bulk Fermi surfaces, extending to higher dimensions.

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Area of Science:

  • Condensed matter physics
  • Materials science
  • Quantum mechanics

Background:

  • Strong topological insulators (TIs) are predicted to exhibit a quantized magnetoelectric response, known as the axion response.
  • Many candidate TI materials exhibit metallic behavior with bulk Fermi surfaces, complicating their topological properties.
  • Previous work suggests surface states of TIs can persist even when the bulk band gap closes.

Purpose of the Study:

  • To investigate the magnetoelectric response in topological insulators when the bulk band gap is closed and metallic Fermi surfaces are present.
  • To explore the generalization of quantized transport coefficients in gapped systems to non-quantized ones in gapless systems.
  • To understand the relationship between the 3D magnetoelectric response and higher-dimensional analogs of the anomalous Hall effect.

Main Methods:

  • Theoretical analysis based on the persistence of surface states in the presence of bulk Fermi surfaces.
  • Investigation of Chern forms and their relation to quantized transport coefficients.
  • Extension of concepts from the intrinsic anomalous Hall effect to higher dimensions.

Main Results:

  • A non-quantized magnetoelectric coupling remains in 3D topological insulators when a bulk Fermi surface opens.
  • Higher-dimensional analogs of the intrinsic anomalous Hall effect are identified.
  • Quantized transport coefficients, typically defined by Chern forms in gapped systems, become non-quantized when the gap closes.

Conclusions:

  • The axion response in topological insulators is robust against the closure of the bulk band gap, transforming into a non-quantized magnetoelectric coupling.
  • The findings provide a broader understanding of topological transport phenomena beyond idealized gapped systems.
  • The study highlights the connection between 3D magnetoelectric effects and 4D anomalous Hall effect analogs, offering new perspectives on topological quantum matter.