EVIDENCE FOR A SCALING OF THE SUPERCONDUCTING GAP WITH T(C) IN PRXY1-XBA2CU4O8

We report a far-infrared reflectivity study of the double-chain compou nd PrxY1-xBa2Cu4O8 (0 less-than-or-equal-to x less-than-or-equal-to 0. 8) with superconducting transition temperatures from 82 to 8 K. The te mperature dependence of z-polarized TO-phonon frequencies and linewidt hs was analyzed in detail in the range 10 - 300 K. For undoped YBa2Cu4 O8 and alloys with x less-than-or-equal-to 0.35 the superconductivity related gap is situated between approximately 300 and approximately 50 0 cm-1, as evidenced by self-energy effects of the infrared-active mod es at those frequencies. For higher Pr-contents (x greater-than-or-equ al-to 0.4) a drastic change of the temperature behavior below T(c) of the 300 cm-1 mode is observed, indicating a shift of the gap across th e phonon frequency to lower energies. Under the assumption that the ga p value 2DELTA0 scales linearly with the transition temperature T(c), superconducting gap of undoped YBa2Cu4O8 was estimated to be 2DELTA0 a lmost-equal-to (375 +/- 10) cm-1, i.e. (6.2 +/- 0.2) kT(c), nearly twi ce as much as expected in BCS theory.