Infrared absorption in superconducting MgB2

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Infrared absorption measurements in the range of 125 to 700 cm(-1) have been carried out as a function of temperature upto 5 K in MgB2. The absorption spectrum is characterised by a broad band centred at 485 cm⁻¹, with shoulders at 333 and 387 cm⁻¹. Studies on the temperature dependence of absorption, indicate that these modes initially harden with the lowering of temperature, and this trend is arrested at 100 K, below which they soften. Further, in the case of the mode at 333 cm⁻¹, there is a distinct softening associated with the superconducting transition at 39 K. The implications of these experimental results in the context of superconductivity in MgB2 are discussed.