We present two new techniques for the inversion of first-arrival times to estimate velocity structure. These travel-time inversion techniques are unique in that they do not require the calculation of ray paths. First-arrival times are calculated using a finite-difference scheme that iteratively solves the eikonal equations for the position of the wavefront. The first inversion technique is a di...

This article is concerned with a version of photoacoustic computed tomography, that uses line detectors (instead of point-like ones) for the recording of induced acoustical data. The three dimensional imaging problem, consisting in reconstructing the unknown density function from recorded data, is reduced to a series of two dimensional problems: First, the initial data of the two dimensional wa...

The whole mol-ecule of the title compound, [NiCl2(C3H4N2)2(NH3)2], is generated by inversion symmetry. The Ni(II) ion, which is located on an inversion center, has a distorted octa-hedral coordination environment and is surrounded by two ammine N atoms and two Cl atoms in the equatorial plane, with two N atoms of two imidazole groups occupying the axial positions. The imidazole ring makes a dih...

Piezoelectricity allows precise and robust conversion between electricity and mechanical force, and arises from the broken inversion symmetry in the atomic structure. Reducing the dimensionality of bulk materials has been suggested to enhance piezoelectricity. However, when the thickness of a material approaches a single molecular layer, the large surface energy can cause piezoelectric structur...

The Kirkwood-Buff integrals for two-component mixtures in one-dimensional systems are calculated directly. The results are applied to square-well particles and found to agree with those obtained by the inversion of the Kirkwood-Buff theory of solutions.

Dirac semimetals associated with bulk fermions are well-known in topological electronic systems. In sharp contrast, three-dimensional (3D) phonons crystalline solids still unavailable. Here we perform symmetry arguments and first-principles calculations to systematically investigate 3D all space groups inversion symmetry. The results show that there two categories of depending on their protecti...