Prediction of bias-voltage-dependent corrugation reversal for STM images of bcc (110) surfaces: W(110), Ta(110), and Fe(110)

We predict a bias-voltage dependent corrugation reversal for scanning tunne ling microscopy (STM) images with atomic resolution of bcc-(110) transition metal surfaces: Atoms which appear usually on STM images of metal surfaces as protrusions, may appear on these images anticorrugated, e.g., as hollow sites and vice versa hollow sites may appear as atoms. This makes the abso lute determination of atom sites by STM unreliable. We investigate the imag e-reversal in detail for the W(110) surface and explain its origin on the b asis of the electronic structure. We found, the image is determined by a co mpetition between surface resonance states with d(xz) and d(z)2 character c ontributing to a direct image of atomic sites and surface-state bands aroun d the (S) over bar point of the two-dimensional Brillouin zone with pd bond ing character and bonding charge between the surface atoms. The surface sta tes contribute to anticorrugated images. Finally the image depends on the b ias voltage. For W(110) and positive bias voltages the surface resonances d ominate over the surface states leading to a direct image of atomic sites. For negative voltages below a critical value of -0.4 V calculations show a reversed image, i.e., anticorrugation. The critical bias voltage depends sl ightly on the tip-sample separation. For bias voltages around the critical value corrugating and anticorrugating contributions to the STM image compen sate each other, the corrugation amplitudes become extremely small, the ato mic resolution disappears, which is consistent with the experimental diffic ulties in achieving atomic resolution on W(110) for negative voltages and s tripelike images are predicted. For positive bias voltages we found a good agreement between the theoretical results and our measured STM images. The competition between surface resonances and surface states is a quite genera l mechanism and anticorrugation is expected to occur on (110) surfaces of o ther bcc transition-metals [i.e., Nb(110), Mo(110), Ta(110)]. We demonstrat ed this explicitly for Tall (110), anticorrugation occurs practically over the entire bias-voltage range available by a STM and an image reversal from an anticorrugated to a corrugated image is predicted for 1.3 V. For magnet ic surfaces the image reversal may occur twice, once for majority and once for minority states. For Fe(110) we show that the minority spin channel con trols the STM image. We predict a direct image for majority states and an a nticorrugated image for minority states below a bias voltage of 0.7 V, and we predict only one image reversal at about 0.4 V for an ordinary non-spin- polarized STM. Employing the full-potential linearized augmented plane wave method in film geometry, the electronic structure is determined by first p rinciples calculations within the framework of the density functional theor y in the local (spin) density approximation. The STM analysis is carried ou t within the s-orbital tip-model of Tersoff and Hamann. An efficient analys is of the corrugation amplitude in terms of two-dimensional star coefficien ts of the vacuum density of states is presented. The tip-sample distance de pendent kit-point selection to the tunneling current is analyzed. The enhan cement of the corrugation amplitude due to p(z)- and d(z)2-type tip-orbital s are determined. We show that the enhancement factors calculated are close to the analytical factors given by Chen. [S0163-1829(98)04347-1].