Superconductivity at 45 K in Rb/Tl Codoped C 60 and C 60 /C 70 Mixtures

• DOI: 10.1126/science.254.5033.826
• Publication Date: 2006-10-05T22:10:16Z
• Authors: Zafar Iqbal; Ray H. Baughman; B. L. Ramakrishna; Sandeep Khare; N. Sanjeeva Murthy; Hans J. Bornemann; Donald E. Morris

The appearance of superconductivity at relatively high temperatures in alkali metal—doped C 60 fullerene provides the challenge to both understand the nature and origin of the superconductivity and to determine the upper limit of the superconducting transition temperature ( T c ). Towards the latter goal, it is shown that doping with potassium-thallium and rubidium-thallium alloys in the 400 to 430°C temperature range increases the T c of C 60 /C 70 mixtures to 25.6 K and above 45 K, respectively. Similar increases in T c were also observed upon analogous doping of pure C 60 . Partial substitution of potassium with thallium in interstitial sites between C 60 molecules is suggested by larger observed unit cell parameters than for the K 3 C 60 and K 4 C 60 phases. Contrary to previous results for C 60 doped with different alkali metals, such expansion does not alone account for the changes in critical temperature.