Three-dimensional dispersion induced by extreme tensile strain in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">La</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">Sr</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:mi mathvariant="normal">Cu</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>films

The electronic band structure probed by angle-resolved photoemission spectroscopy on thin epitaxial ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ films under extreme tensile strain shows anomalous features compatible with $c$-axis dispersion. This result is in striking contrast with the usual quasi-two-dimensional (2D) dispersion observed up to now in most superconducting cuprates, including relaxed and compressively strained ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ films grown under the same conditions. The data were analyzed using a 3D tight-binding dispersion for a body-centered-tetragonal lattice. We relate the enhancement of the $c$-axis dispersion to the significant displacement of the apical oxygen induced by epitaxial strain.

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