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We searched for the structure of calcium in phases IV and V by a metadynamics simulation based on ab initio calculations, and found two structures. One is a tetragonal lattice which consists of two helical chains along the c axis. The other is an orthorhombic lattice of four zigzag chains. We have calculated the x-ray diffraction patterns and enthalpies of the two structures discovered by our simulation. From comparisons of the patterns with the experimental x-ray patterns of the phases IV and V and from the pressure dependence of the enthalpies, we conclude that the structure with a helical pattern corresponds to phase IV and the structure with a zigzag pattern to phase V. Studies have revealed that many elements have complex structures with low symmetry at high pressures; McMahon and Nelmes have reviewed complex lattice structures of elements under high pressure. 1 Some elements have interesting structures beyond the simple cubic ͑sc͒ lattice at high pressure. Phosphorus has the sc structure ͑P-III͒ from 10 to 107 GPa, 2 which transforms, at 107 GPa, to a complex structure 3 ͑P-IV͒ whose structure has long been unidentified. But it has recently been theoretically predicted 4 and experimentally confirmed 5 to be an incommensurately modulated structure.Another example is calcium, which shows interesting successive structural phase transitions under pressure, where the transitions are from a closest-packed to a less close-packed structure. The fcc structure at ambient pressure transforms to the bcc at 20 GPa and then to the sc structure ͑Ca-III͒ at 32 GPa. 6 In 2005 Yabuuchi et al. pressurized calcium further and observed new structural phase transitions. 7 The sc transforms to phase IV ͑Ca-IV͒ at 113 GPa and then to phase V ͑Ca-V͒ at 139 GPa. 7 Very recently an increase of the superconducting transition temperature T c in the high-pressure phases has also been reported, i.e., 25 K at 161 GPa in Ca-V, which is the highest recorded for an element. 8 This result has attracted much interest in relation to pressure-induced electronic s-d transfer and the increase of the superconducting T c with pressure, which has been discussed also in Sr and Ba under pressure. [9][10][11] Information about the structures of Ca-IV and Ca-V is crucial for the clarification of the high T c in calcium.In this work, we have explored the crystal structures of Ca-IV and Ca-V using an ab initio metadynamics simulation 12 in which we employed density functional theory in the generalized gradient approximation ͑GGA͒. Metadynamics is a method used to find neighboring local minima by filling the potential wells with an artificial Gaussian potential ͑GP͒. To avoid rigid rotation of the system, we treated only the symmetric part of the cell matrix defined by h = ͑a ជ , b ជ , c ជ͒, where a ជ, b ជ , and c ជ are the lattice vectors. We update the simulation cell by the steepest-descent method with a stepping parameter ␦h: h ij t+1 = h ij t + ␦hF ij t / ͉F ij t ͉, where t is the number of updates of h and F is the driving force for the update. If we...

Image based modeling methods has been well studied for generating a 3D model from an image sequence. Most of them require redundant and huge spatio-temporal images for estimating a scene depth. It is not good characteristic for taking a higher resolution of texture. A route panorama is a continuous panoramic image along a path. It is suitable for modeling large environments such as a city or town. The panorama captured by a line scan sensor also has advantage for capturing higher resolution easily. In this paper, we propose a method for depth estimation from the panorama. The route panorama has color drifts that correspond to the distances of captured objects. We use these color drifts to estimate the depth of an image. The proposed method detects the color drift by window matching using Belief Propagation. It also uses a Gaussian Pyramid to stabilize the estimation and decrease its computation cost. We confirmed that the proposed method estimated depth maps from a single high-resolution panorama in experiments.

Abstract. We searched for the structure of calcium in phase V by a metadynamics based on the first-principles calculation, and found a new structure, where the positions of the calcium atoms in the unit cell have a zigzag pattern. Calculating the x-ray diffraction pattern of the new structure and comparing it with the experimental patterns of Ca-IV and Ca-V, we conclude that the new zigzag structure is a candidate of the structure of Ca-V.