A new structural model for the Si(331)-(12×1) reconstruction is proposed. Based on scanning tunneling microscopy images of unprecedented resolution, low-energy electron diffraction data, and firstprinciples total-energy calculations, we demonstrate that the reconstructed Si(331) surface shares the same elementary building blocks as the Si (110)-(16×2) surface, establishing the pentamer as a universal building block for complex silicon surface reconstructions.The study of semiconductor surface reconstructions has been an area of active research for many years and has gained tremendous importance with the advent of low-dimensional heteroepitaxial semiconductor nanostructures such as quantum dots and quantum wires [1]. The creation of a surface results in broken bonds, called dangling bonds. Dangling bonds are energetically unfavorable causing surface atoms to rearrange or reconstruct. This often results in highly complex atomic structures, whose determination remains a formidable challenge and requires the complementary role of different experimental and theoretical methods. In order to lower the surface energy, silicon surfaces adopt a variety of strategies allowing to reduce the number of dangling bonds. Despite the large number of known surface reconstructions, one frequently encounters common elementary structural building blocks [2,3]. Identifying these building blocks is important not only for a better understanding of these surfaces, but also serves as a guide for the elaboration of new structural models. Two of the most important strategies, encountered for instance on Si(100) [4] and Si(111) [5], are respectively the formation of dimers, where two surface atoms pair up to eliminate their dangling bonds, and the appearance of adatoms, which bond to three surface atoms thus saturating three dangling bonds. An important step towards the understanding of high-index group IV surfaces with a surface normal in between the (111) and (100) direction was the introduction of an additional reconstruction element by Dabrowski et al. [6]. They proposed a six-fold coordinated surface self-interstitial which is captured by a conglomerate of surface atoms [7,8]. This concept was subsequently adapted by An [9] and theoretically analyzed by Stekolnikov [10,11] to explain the pairs of pentagons observed in scanning tunneling microscopy (STM) images of the reconstructed Si(110) surface.In this letter we focus on the atomic structure of the Si(331)-(12×1) reconstruction. We present highresolution STM images resolving for the first time rows of * Electronic address: corsin.battaglia@unine.ch; URL: http://www.unine.ch/phys/spectro 1a)), is an important surface, since it is the only confirmed planar silicon surface with a stable reconstruction located between (111) and (110). Since the discovery of the Si(331)-(12×1) reconstruction more than 17 years ago [12] several structural models containing dimers and adatoms have been proposed [13,14]. However, none of these models is able to explain the pentagons observed in our STM image...

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We present an ab-initio study of the phase transition cd→β-tin in Si and Ge under hydrostatic and non-hydrostatic pressure. For this purpose we have developed a new method to calculate the influence of non-hydrostatic pressure components not only on the transition pressure but also on the enthalpy barriers between the phases. We find good agreement with available experimental and other theoretical data. The calculations have been performed using the plane-wave pseudopotential approach to the density-functional theory within the local-density and the generalized-gradient approximation implemented in VASP.