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Paper IPM / P / 13870  


Abstract:  
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 âexchangeâ and gives rise to a
bound state between the positiveenergy states in one surface and the negativeenergy states in the
opposite surface. Therefore particle and hole pairs in the undoped Dirac cone of the TI thin film
form an intersurface spinsinglet state that lies below the continuum of free particlehole pairs.
This mode is similar to the excitons of semiconductors, albeit formed between the electron and
hole pairs from two different twodimensional 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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