OCCURRENCE OF VANHOVE SINGULARITIES IN YBA2CU4O8 AND YBA2CU3O6.9

We report seeing a saddle point in the energy band, known as a van Hov e singularity. We measure an exact position of the singularity in both YBa2Cu4O8 (Y124) and YBa2Cu3O6.9 (Y123). In Y123 the van Hove singula rity is at binding energy of less then 10 meV, closer ot E(F), than in Y124, which has a singularity at 19 meV. The singularity has and exte nded character along the GAMMA-Y direction. Another clearly resolved b and exhibits a saddle point singularity at binding energy of 115 meV. Similar singularities are observed in the vicinity of the X point, ext ended along the GAMMA-X direction. In order to study these features th e energy resolution of the system was improved to better than 10 meV.