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[en] TaNiSe is the strongest candidate for the long conjectured excitonic insulator state. It is a direct zero gap semiconductor at high temperature, and undergoes at T = 326 K a semiconductor-insulator transition simultaneous with an orthorhombic-monoclinic q = 0 structure transition. Our low temperature STM evidences its layered structure with rippling atomic chains. The local spectroscopy reveals the opening of the excitonic insulator gap and the spectral weight shift with decreasing temperature. High resolution STM topography shows a local distortion associated with the structure transition. This distortion may play an important role in the formation of the excitonic state, as also supported by our band structure calculations.
[en] We report on the 5.5√3x5.5√3- R30"0 overlayers superstructure observed by the scanning tunneling microscopy on the Ge(111) surface. It shows pronounced effects of the local density of states leading to the strong dependence of STM images on the bias voltage and some dynamic changes of images at 300 K. This overlayer is tentatively interpreted as graphene formed in small submonolayer amounts due to the pyrolysis of hydrocarbon constituents of the residual atmosphere of the vacuum chamber during the annealing of a Ge(111) sample at 900 K. We suggest a model of the graphene/Ge(111)- 5.5 √3x5.5√3-R30"0. Heteroepitaxial interface, featuring the reconstructed Ge(111) substrate with no long-range order under the graphene layer, the latter being corrugated due to spatial variations of the interatomic geometry of the Ge(111) and graphene(0001) atomic lattices with extremely large mismatch
[en] We examine the double-layer B -type steps on As-terminated vicinal Ge(001) surfaces. The currently accepted structure is a chemically inert bulklike structure without any gap state, and with all the chemical bonds of the Ge and As atoms being satisfied. However, we show that the need for optimizing the p3 pyramidal angles of the threefold coordinated As atoms drives unusual atomic rearrangement. This leads to a more stable reconstruction involving odd-membered (5-7-5) rings at the step edge. Comparison between theoretical and experimental scanning tunneling microscopy images yields excellent agreement