Our scanning tunneling microscopy and electron diffraction experiments revealed that a new twodimensional allotrope of Bi forms on the Si111-7 7 surface. This pseudocubic f012g-oriented allotrope is stable up to four atomic layers at room temperature. Above this critical thickness, the entire volume of the film starts to transform into a bulk single-crystal (001) When the size and shape of the materials are downsized to a nanometer scale, they often reveal anomalous atomic structures as well as exotic functional properties that do not exist in bulk. A variety of novel structures discovered in nanoparticles and nanowires have been attracting broad interest [1][2][3][4]. Group V elements are known to show rich allotropic transformation because their semimetallic bonding character can be easily shifted to either the metallic or covalent side, for example, by changing the applied pressure [5][6][7][8]. In this context, a question arises whether a new allotrope can be realized by tuning the dimensionality or size, such as thickness of the film, instead of tuning the pressure or temperature.In this Letter, we report on our finding that ultrathin film of Bi really shows such an allotropic transformation as a function of thickness on the scale of several atomic layers. Our electron diffraction and scanning tunneling microscopy (STM) experiments revealed that, over the wetting layer formed initially on the Si111-7 7 surface, Bi grows with a new f012g-oriented phase whose structure is significantly different from bulk Bi and that it transforms into the bulk-like single-crystal Bi(001) phase above the critical thickness which increases with the substrate temperature. Our ab initio theory revealed that the f012g phase with even-number layers is stabilized by forming a puckered-layer structure. The resulting film is very flat, compared to the growth of any known metal films, reflecting the inherent two-dimensional (2D) structure of this f012g phase. To the best of our knowledge, puckered-layer structure was never observed before except for the case of black phosphorus, which is a famous teratoid phase in group V elements.Bismuth ultrathin films were grown in ultrahigh vacuum (UHV) and characterized in situ using STM and reflection high-energy electron diffraction (RHEED). The spot-profile-analyzing low-energy electron diffraction (SPA-LEED) was performed with momentum reso-

The plate geometry in northeast Asia has been a long‐standing question, with a major issue being whether the Sea of Okhotsk and northern Japanese islands are better regarded as part of the North American plate or as a separate Okhotsk plate. This question has been difficult to resolve, because earthquake slip vectors along the Kuril and Japan trenches are consistent with either Pacific‐North America or Pacific‐Okhotsk plate motion. To circumvent this difficulty, we also use slip vectors of earthquakes along Sakhalin Island and the eastern margin of the Japan Sea and compare them to the predicted Eurasia‐Okhotsk and Eurasia‐North America motions. For a model with a separate Okhotsk plate, we invert 10 Eurasia‐Okhotsk and 255 Pacific‐Okhotsk slip vectors with Pacific‐North America and Eurasia‐North America NUVEL‐1 data. Alternatively, for a model without an Okhotsk plate, those Eurasia‐Okhotsk and Pacific‐Okhotsk data are regarded as Eurasia‐North America and Pacific‐North America data, respectively. The model with an Okhotsk plate fits the data better than one in which this region is treated as part of the North American plate. Because the improved fit exceeds that expected purely from the additional plate, the data indicate that the Okhotsk plate can be resolved from the North American plate. The motions on the Okhotsk plate's boundaries predicted by the best fitting Euler vectors are generally consistent with the recent tectonics. The Eurasia‐Okhotsk pole is located at northernmost Sakhalin Island and predicts right‐lateral strike slip motion on the NNE striking fault plane of the May 27, 1995, Neftegorsk earthquake, consistent with the centroid moment tensor focal mechanism and the surface faulting. Along the northern boundary of the Okhotsk plate, the North America‐Okhotsk Euler vector predicts left‐lateral strike slip, consistent with the observed focal mechanisms. On the NW boundary of the Okhotsk plate, the Eurasia‐Okhotsk Euler vector predicts E‐W extension, discordant with the limited focal mechanisms and geological data. This misfit may imply that another plate is necessary west of the Magadan region in southeast Siberia, but this possibility is hard to confirm without further data, such as might be obtained from space‐based geodesy.

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We discuss the results of the scanning tunneling microscopy (STM) investigations and ab initio calculations of the structure and stability of the quasicubic Bi{012} film formed in the initial stage of the bismuth deposition on the Si(111)-7×7 surface at room temperature. Results of our STM experiments show that paired-layer Bi{012} film grows on top of the initially formed wetting layer, with the Si 7×7 lattice preserved underneath. The pairing of the layers in the {012} film leads to the substantial stabilization of the film when it consists of an even number of layers and only even-number layered Bi{012} islands are observed to be stable. The buckling of the atoms in the topmost paired layer induced by the relaxation of the film is evidenced by the high-resolution STM images.

The three-dimensional (3D) island structure was prepared by molecular beam epitaxy for the lattice mismatched InAs/GaAs(001) system and its images showing atomic structure on faceted planes were taken in situ by ultrahigh vacuum scanning tunneling microscopy (STM). The (113), (114), and (215) faceted planes are observed for the 3D islands. Based on the STM images, atomic structural models are proposed for the faceted surfaces. The surface structure of the (113) faceted planes we propose is different from those observed on the flat GaAs(113) surface.