Strings with negative stiffness and hyperfine structure

We propose a new string model by adding a higher-order gradient term to the rigid string, so that the stiffness can be positive or negative without loosing stability. In the large-D approximation, the model has three phases, one of which with a new type of generalized "antiferromagnetic" orientational correlations. We find an infrared-stable fixed point describing world sheets with vanishing tension and Hausdorff dimension D-H = 2. Crumpling is prevented by the new term which suppresses configurations with rapidly changing extrinsic curvature.