The structure of silicene fabricated on a Ag(111) surface was determined using reflection high-energy positron diffraction with a linac-based brightness-enhanced intense positron beam. From the rocking curve analysis, the silicene was verified to have a buckled structure with a spacing of 0.83Å between the top and the bottom Si layers. The distance between the bottom Si layer in the silicene and the first Ag layer was determined to be 2.14Å. These results agree with the theoretically predicted values from a previous study [Phys. Rev. Lett. 108, 155501 (2012)] within an error of ±0.05Å.
The structure of germanene on an Al(111) surface has been experimentally investigated using the total-reflection high-energy positron diffraction (TRHEPD) method. The observed spot intensities are asymmetric, revealing no mirror symmetry in the atomic coordinates of germenene with respect to the 〈110〉 direction. Quantitative TRHEPD rocking curve analysis, based on dynamical diffraction theory, has revealed that the germanene layer has a 3×3 structure with asymmetrical buckling due to the protrusion of one of the Ge atoms in the unit cell, which is unlike the structural model proposed in previous studies. The magnitude of the buckling was found to be 0.94 Å, and the spacing between the germanene and the Al(111) substrate to be 2.51 Å. The new structure proposed in the present investigations, though different from that reported in studies before, does not contradict the other characteristics which were found experimentally in the previous studies. RECEIVED
The atomic configuration and electronic band structure of Pt-induced nanowires on a Ge(001) surface are investigated using scanning tunneling microscopy, reflection high-energy positron diffraction, and angle-resolved photoemission spectroscopy. A previously proposed theoretical model, composed of Ge dimers on the top layer and buried Pt arrays in the second and fourth layers [Vanpoucke et al., Phys. Rev. B 77, 241308(R) (2008)], is found to be the fundamental structure of the observed nanowires. At low temperatures (T < 80 K), each Ge dimer is alternately tilted in the surface normal direction (asymmetric), causing a p(4 × 4) periodicity. At high temperatures (T > 110 K), each Ge dimer is flat with respect to the horizontal axis (symmetric), giving rise to p(4 × 2) periodicity. Upon the above phase transition, the electronic band dispersion related to the Ge dimers in the deeper energy region shifts to the Fermi level.
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