TEMPERATURE DEPENDENCES OF THE 340-CM-1, 440-CM-1, AND 500-CM-1 RAMAN MODES OF YBA2CU3OY FOR 6.7-LESS-THAN-OR-EQUAL-TO-Y-LESS-THAN-OR-EQUAL-TO-7.0

The temperature dependences of the linewidths and frequencies of the 340-, 440-, and 500-cm-1 Raman-active phonons have been measured in lightly twinned single crystals of YBa2Cu3Oy with different oxygen concentrations y. The observed superconductivity-induced changes in the linewidths and frequencies that occur for temperatures below the critical temperature have been found to depend on the oxygen concentration within the range 6.7 less than or similar to y less than or similar to 7. 0, with the most sensitive dependence occurring in the 340-cm-1 phonon-linewidth variations near y almost-equal-to 7.0. The linewidth and frequency dependences of the 340-cm-1 B1g mode for T < T(c) suggest that the redistribution of electronic states below T(c) is very sensitive to the oxygen or hole concentration within this range. Estimates for the superconducting energy gap 2DELTA have been obtained from comparisons of the temperature dependence of the 340-cm-1 mode with strong-coupling theory and these indicate that 2DELTA increases as the oxygen or hole concentration y decreases from y = 7.0 where 2DELTA almost equal to 5.4 kT(c). The linewidth and frequency of the 440-cm-1 A(g) mode are, however, relatively insensitive to variations in y and it is suggested that the 440- and 500-cm phonons couple to a different set of electrons than does the 340-cm-1 mode.