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A Low-Dimensional Metallic State on Au-Adsorbed Ge(001) Surface

Komori Group

Surface electronic states on bulk semiconductors are suitable for the study of low-dimensional electronic properties because the metallic state in the bulk band gap can provide an ideal low-dimensional metal. Moreover, we can expect to study one-dimensional (1D) surface electronic properties such as Tomonaga-Lüttinger liquid (TLL) and Peierls instability when the surface atomic structure is highly anisotropic. As one of the surfaces with a 1D atomic structure, the Au-adsorbed Ge(001) surface has recently attracted much attention and its electronic and atomic structures have been studied in detail using scanning tunneling microscopy/spectroscopy (STM/STS), and angle-resolved photoemission spectroscopy (ARPES). An interesting STS result of this system was that the surface local density of states (LDOS) depends on the energy E as (E - EF)α with α = 0.5 [1]. This was attributed to TLL while the metallic surface state has an anisotropic two-dimensional (2D) dispersion in the ARPES study [2]. We have studied this surface state by using high-resolution STS to understand the origins of these interesting features [3].

Fig. 1. (a-c) STM images of the Au-adsorbed Ge(001) surface at 78 K for three sample-bias voltages (Vb) shown in the figures. One-dimensional chain structure can be always observed while the image largely depends on Vb. The top of the chain structure consists of a zig-zag part (yellow) and a chevron part (blue) at Vb = -1 V. The both structures are not clear at Vb = 1 V, and the zig-zag structure turns to a double row at Vb = - 200 mV. (d) Spatially-averaged STS measured at 5.5 K over an ordered 4×8 area (blue) and over an area including point defects and the domain boundaries of the 4×8 superstructure (black).

Fig. 2. (a-c) Differential conductance (dI/dV ) maps at 78 K for three sample-bias voltages (Vb) shown in the figures. The band bottom of the metallic surface state is located at 150 meV below EF, and the map indicates LDOS of the metallic state at the corresponding energy. The conductance peaks are on the zig-zag parts of the top of the chain structures. (d) Simultaneously-obtained topography image (6×6 nm2, Vb = 0.5V).

Figure 1 shows the results of STM/STS for the Au-adsorbed Ge(001) surface consisting of the well-known atomic chain structure. The atomic images largely depend on the sample bias voltage. The symmetry of the surface structure has been considered to be c(8×2) while a short range order of a 4×8 superstructure can be seen on the surface. The tunneling spectra always show a dip at EF as in Fig. 1d. However, by fitting the data to the (E-EF)α dependence, α is always larger than 0.7 at the ordered 4×8 area while α is closed to 0.5 at the areas including point defects and domain boundaries of the 4×8 superstructure. Thus, the square-root behavior is ascribed to the surface disorder.

We have measured differential conductivity (dI/dV) maps in the bias-voltage (Vb) region corresponding to the metallic surface state to study the spatial distribution of the surface LDOS. The results shown in Fig. 2 exhibit no 1D channel of high LDOS, which is expected for a 1D surface state. The observed LDOS is consistent with the ARPES observation of the 2D metallic state.


References
  • 1. C. Blumenstein et al., Nat. Phys. 7, 776 (2011).
  • 2. K. Nakatsuji et al., Phys. Rev. B 80, 081406 (2009).
  • 3. J. Park et al., Phys. Rev. B 90, 165410 (2014).
Authors
  • F. Komori, J. Parka, K. Nakatsuji, T.-H.Kimb S. K. Songa,b, and H. W. Yeoma,b
  • aInstitute for Basic Science
  • bPohang University of Science and Technology