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Paper   IPM / Physic / 13870
School of Physics
  Title:   Gapless chiral excitons in thin films of topological insulators
  Author(s):  S.A. Jafari
  Status:   Published
  Journal: EPL
  Vol.:  107
  Year:  2014
  Pages:   20005
  Supported by:  IPM
In a nanoscopic thin film of a strong topological insulator (TI) the Coulomb interaction in the channel that exchanges the two electrons with the same chirality in two different planes of the slab takes advantage of the minus sign resulting from such “exchange” and gives rise to a bound state between the positive-energy states in one surface and the negative-energy states in the opposite surface. Therefore particle and hole pairs in the undoped Dirac cone of the TI thin film form an inter-surface spin-singlet state that lies below the continuum of free particle-hole pairs. This mode is similar to the excitons of semiconductors, albeit formed between the electron and hole pairs from two different two-dimensional surfaces. For low momenta the dispersion relation characterizing this collective mode is linear. Experimental comparison of two slabs with different thicknesses can capture the exponential dependence of the present effect on the slab thickness.

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