PhysicaC 185-189 (1991) 1753-1754 North-Hogand SUPERCONDUCTING GAP AND MAGNETIC FIELD EFFECTS IN Bi2Sr2CaCu208 G. MARTINEZ*, L.C. BRUNEL', S.G. LOUIE ÷, S. LABDI § and H. RAFFY § *SNCI/CNRS, 25 Avenue des Martyrs 38042 Grenoble Cedex, France +Department of Physics, University of California, Berkeley, CA 94720, USA §Laboratoire de Physique des Solides, B~t.510, UPS, 91405 Orsay, France Superconducting properties of Bi2SrzCaCu208 films have been measured using the infra-red spectros- copy technique at fixed frequency. It is possible with this technique to determine the variation of the order parameter as a function of temperature and the critical magnetic field as a function of temperature. This last result together with the BCS theory allows a determination of the Fermi ve- locity in the a-b plane of the structure. The determination of the superconducting gap in high T¢ superconductors is still a matter of intensive debate. Depending on the technique used, the observed features which have been at- tributed to the gap are not located at the same energy. Though it is not necessary that all techniques should provide the same gap value in strongly anisotropie systems, it is at least important with a given technique to get relia- ble results which may be checked against other superconducting properties like the critical temperature T e and the magnetic field effects. We have developed a technique of optical inves- tigation for different fixed IR laser energies ~. We have shown that when scanning the tempe- rature T at zero magnetic field and measuring the reflectivity of a Bi~SriCaCuiO s film that is strongly textured with the c-axis perpendi- cular to the MgO substrate, a discontinuity ap- pears which can be related to the order parame- ter given by 1~0J= 2~. The gap curve 2A versus T is shown in Fig.l and corresponds to a value of 2A(O)/k~T e ~ 3.4. Alternatively, at fixed tem- perature T lower than To, the reflectivity as a function of magnetic field B also exhibits a discontinuity. The different curves relating the observed "critical" B to T for a given la- ser energy are plotted in Fig.2. From these re- suits, it is possible to derive the variations of 2~ versus T at fixed B, which are also re- L ~ I i , I I 25 ---~ ..... ~.. 20- B=2T "O,~ > 15- 6T " QJ B -- \ E \ v "o~ q >., B=14T. , u', 10 _ o, o 6 r ~ ,, I QJ ~ t I C l l I LLI l t i 5- o 6 ¢ I I I i 6 6 Oi , 1 J ' It 0 20 40 I I [ ...... - IL I ! | I ! | I I ! I ! I ! ! ! o ! O 11 1 I .... 60 80 100 T(K) FIGURE i 2~ versus temperature at zero magnetic field (black dots) and for different values of the magnetic field (open dots). Dashed lines are guides for the eye. ported in Fig.l. One observes that for non zero magnetic field, the variation of 2~(B) with T is very sharp near Te(B) giving a very good determina- tion of the critical magnetic field. Indeed the curve at lower frequencies in Fig.2 look very 0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.