We present x-ray powder diffraction (XRPD) and neutron diffraction measurements on the slightly underdoped iron pnictide superconductor Ba(1-x)K(x)Fe2As2, Tc = 32K. Below the magnetic transition temperature Tm = 70K, both techniques show an additiona
l broadening of the nuclear Bragg peaks, suggesting a weak structural phase transition. However, macroscopically the system does not break its tetragonal symmetry down to 15 K. Instead, XRPD patterns at low temperature reveal an increase of the anisotropic microstrain proportionally in all directions. We associate this effect with the electronic phase separation, previously observed in the same material, and with the effect of lattice softening below the magnetic phase transition. We employ density functional theory to evaluate the distribution of atomic positions in the presence of dopant atoms both in the normal and magnetic states, and to quantify the lattice softening, showing that it can account for a major part of the observed increase of the microstrain.
Variation of the geometrical and electronic properties of the gold materials in different dimensions has been investigated by $ab$ $initio$ method, taking into account the spin-orbit (SO) interaction. It is found that SO effects in different dimens
ional Au materials depend greatly on fundamental symmetry and dimensionality. For single walled gold nanotubes (SWGNTs), SO interaction decreases significantly the conducting channel number of achiral SWGNT (4, 0), and leads to spin splitting at Fermi level of chiral SWGNT, indicating that quasi-1D SWGNT can be a good candidate for the spin-electron devices. Furthermore, our results suggest that cage cluster might be synthesizable experimentally by taking gold tube structure as parent material.
Using the first principles calculations we have studied the vibrational modes and Raman spectra of a (10, 10) single-walled carbon nanotube (SWNT) bundle under hydrostatic pressure. Detailed analysis shows that the original radial breathing mode (RBM
) of the SWNT bundle disappears after the structural phase transition (SPT). And significantly a RBM-like mode appears at about 509 cm^{-1}, which could be considered as a fingerprint of the SPT happened in the SWNT bundle, and further used to determine the microscopic structure of the bundle after the SPT.
Using the first-principles spin density functional approach, we have studied magnetism of a new type of all-carbon nanomaterials, i.e., the carbon nanowires inserted into the single-walled carbon nanotubes. It is found that if the 1D carbon nanowire
density is not too higher, the ferromagnetic ground state will be more stable than the antiferromagnetic one, which is caused by weak coupling between the 1D carbon nanowire and the single-walled carbon nanotube. Also, both dimerization of the carbon nanowire and carbon vacancy on the tube-wall are found to enhance the magnetic moment of the composite.
The structural, electronic, optical and vibrational properties of the collapsed (10,10) single-walled carbon nanotube bundle under hydrostatic pressure have been studied by the first-principles calculations. Some features are observed in the present
study: First, a collapsed structure is found, which is distinct from both of the herringbone and parallel structures obtained previously. Secondly, a pseudo-gap induced by the collapse appears along the symmetry axis textit{$Gamma $X}. Thirdly, the relative orientation between the collapsed tubes has an important effect on their electronic, optical and vibrational properties, which provides an efficient experimental method to distinguish unambiguously three different collapsed structures.
The linear polarizability absorption spectra of the double-walled carbon nanotubes (DWNTs) have been calculated by using the tight-binding (TB) model and sum-over-state (SOS) method, supplemented by the first principles CASTEP calculations. It is fou
nd that the chiral symmetries of both outer and inner tubes in the DWNTs can always be identified distinctly by the characteristic peaks in the absorption spectra of the DWNTs, no matter what kind of the outer tube is, offering a powerful experimental tool to measure precisely the chiral angle of the inner tube of a DWNT.
