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We have performed ellipsometric measurements of the far-infrared c-axisdielectric response of underdoped YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ singlecrystals. Here we report a detailed analysis of the temperature-dependentrenormalization of the oxygen bending phonon mode at 320 cm$^{-1}$ and theformation of the additional absorption peak around 400-500 cm$^{-1}$. For astrongly underdoped YBa$_{2}$Cu$_{3}$O$_{6.5}$ crystal with T$_{c}$=52 K wefind that, in agreement with previous reports based on conventional reflectionmeasurements, the gradual onset of both features occurs well above T$_{c}$ atT*$\sim $150 K. Contrary to some of these reports, however, our data establishthat the phonon anomaly and the formation of the additional peak exhibit verypronounced and steep changes right at T$_{c}$. For a less underdopedYBa$_{2}$Cu$_{3}$O$_{6.75}$ crystal with T$_{c}$=80 K, the onset temperature ofthe phonon anomaly almost coincides with T$_{c}$. Also in contrast to someprevious reports, we find for both crystals that a sizeable fraction of thespectral weight of the additional absorption peak cannot be accounted for bythe spectral-weight loss of the phonon modes but instead arises from aredistribution of the electronic continuum. Our ellipsometric data areconsistent with a model where the bilayer cuprate compounds are treated as asuperlattice of intra- and inter-bilayer Josephson-junctions

physical review. b, condensed matter

Anomaly of oxygen bond-bending mode at<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>320</mml:mn><mml:mi> </mml:mi><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">cm</mml:mi></mml:mrow><mml:mrow><mml:mi>−</mml:mi><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>and additional absorption peak in the<i>c</i>-axis infrared conductivity of underdoped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:…

Anomaly of oxygen bond-bending mode at320 cm−1and additional absorption peak in thec-axis infrared conductivity of underdoped