PREDICTED STRUCTURES AND STABILITIES OF THE SURFACE-A GROOVES AND DOUBLE BILAYER HEIGHT STEPS ON THE GAAS(001)-2X4 SURFACE

''A-type'' surface steps on GaAs(001) have edges parallel to the direc tion of the As-As dimers. We identify here two series of A step struct ures on this surface: the ''groove structures'' and the ''double bilay er height steps''. Both can be thought as being obtained by cleaving t he flat 2 x 4 surfaces into two bilayer height step segments and reass embling these segments into step structures. The relative stabilities of these two step structures and their stabilities with respect to oth er low energy A step structures are examined using our newly developed method of ''linear combination of structural motifs''. We find that ( i) for a given separation S (in units of surface atomic spacing a(s)) between the two step segments the grooves are more stable than the dou ble bilayer steps; (ii) the S = 1 A groove is predicted to be the most stable A groove structure, whereas (iii) the S = O double A step is p redicted to be the most stable A step structure over a wide range of t he Ga chemical potential.