Atomic Row Doubling in the STM Images of Cu(014)-O Obtained with MnNi Tips

Abstract

We report on scanning tunneling microscopy studies of the Cu(014)-O surface using MnNi tips. Remarkably, the results show a regular apparent doubling of surface atomic rows in the f110g direction. A qualitative explanation of this feature based on tight binding and density functional theory calculations of the electronic structure of the tip is presented. Double imaging of the same atom by different legs of d yz orbital could be the reason for the observed doubling. The orientation of the orbital is determined by the O-Mn crystal field. DOI: 10.1103/PhysRevLett.98.206101 PACS numbers: 68.37.Ef, 71.15.Mb, 73.20.At, 73.40.Gk Using scanning tunneling microscope (STM) tips of different materials makes it possible to obtain different information in atomically resolved images [1][2][3]. One of the first explanations of the atomic features in STM images of metals was given by Chen [4,5], who said that atomic resolution is most probably due to d 3z 2 ÿr 2 and p z tip states. The tunneling matrix elements for these states are proportional to the z derivative of the surface atom wave function at the center of the tip apex atom [5]. Therefore, it was predicted that the best candidates for STM tips are d-band metals and semiconductors with p z dangling bonds. When tip states with angular moment component along the surface normal m Þ 0 dominate near E F , enhanced but inverted corrugations are predicted [6]. A more recent coupled tip-surface system analysis [7] also demonstrated that electron states with m Þ 0 should give significantly larger corrugations compared to the m 0 states, but the relative contributions of different orbitals can show strong dependence on the tip-surface distance. The effects of the tip electronic structure were recently observed in ultrahigh resolution atomic force microscopy experiments [8,9]. Similar effects may play a significant role in dynamic STM experiments on Si(111) where subatomic features could be explained by scattering of electrons into double dangling bonds of the silicon tip atoms [10]. The use of dynamic STM mode for atomic orbital imaging [10] was explained by the crucial importance of having small tipsurface separations (3-5 Å ), which could be achieved reproducibly without tip breaking due to lower lateral forces with an oscillating probe.In this Letter, we present new STM data on Cu(014)-O obtained by employing a MnNi probe in constant current mode (Fig. 1). The rationale for using tips of this material is that a Mn atom at the apex could collect electrons via the 3d orbitals. We were specifically looking for unusual atomic-scale effects, e.g., multiple imaging of each atom, to address the question: Can an atomically sharp tip behave as a multiple tip with subatomic separation between the tip apexes? The surface of Cu(014)-O was chosen as it is well understood on the atomic scale. Furthermore, it contains both metal and oxygen ions [1], which is likely to produce a strong electric crystal field that could modify the 3d orbitals of a tip apex atom placed in this field.The exp...

Atomically resolved STM imaging with a diamond tip: simulation and experiment

“…Therefore the non-perturbative approach is required to find the dependence of the tunneling current from the tip-surface distance. It has also been demonstrated recently that the orbital structure of a metallic probe can induce unusual electronic effects on a subatomic scale [8,9,[18][19][20], which can complicate the explanation of experimental atomically resolved images. For this reason light element-terminated probes with spatially localized atomic orbitals at the apex which have a 2 minimal number of electron states involved in the tunneling can provide enhanced spatial resolution in STM experiments [21,22] and simplify the interpretation of atomically resolved STM data.…”

“…It is well known that the electronic structure of the tip apex atom plays a key role in STM tunneling [1][2][3]. Particularly, for some tipsample systems an agreement between experiment and simulation could only be obtained by including the fine tip electronic structure in calculations [4][5][6][7][8][9]. It is known that the electronic structure of a realistic tip is usually determined by a mixture of s-, p-and dstates while the local density of states (LDOS) near the Fermi level can be highly dependent on the energy of tunneling electrons [3].…”

High Resolution STM Imaging

“…However, even the simplest situations shown in Fig. 15.2 can be realized in STM experiments [26,34].…”

Asymmetry effects in atomically resolved STM images of Cu(014)-O and W(100)-O surfaces measured with MnNi tips

“…Our experiments showed that eight features along the ͓110͔ direction could be resolved for certain tunneling conditions at different scan rates and fast scan axis directions. 37 The splitting of atomic features along the ͓110͔ direction was observed less often than along the ͓110͔ one and was not so well resolved. Typically, the images with subtle subatomic structure appeared after a series of bias voltage pulses ͑4 -4.5 V͒ producing a suitable MnNi apex.…”