MODULATIONS IN THE CUO CHAIN LAYER OF YBA2CU3O7-DELTA - CHARGE-DENSITY WAVES

We have performed reversed-bias scanning tunneling microscopy at 20 K on the CuO chain layer of cold-cleaved single crystals of YBa2Cu3O7-de lta. We find 1.3-nm corrugations which change sign under bias polarity reversal. This behavior and the 1.3-nm wavelength are in agreement wi th the hypothesis that the one-dimensional CuO chains undergo a charge density wave transition. This is the first real-space evidence of suc h a transition in YBa2Cu3O7-delta. We also explore the possibility tha t the corrugations are caused by Friedel oscillations or bipolaron sca ttering. Several-nm depressions along the CuO chains transform into br oad swells and depressions as the bias voltage is increased.