arXiv subjects
Publications and source records attributed to Xiaoping Yang.
In Carnot groups of step 3, all subriemannian geodesics are proved to be normal. The proof is based on a reduction argument and the Goh condition for minimality of singular curves. The Goh condition is deduced from a reformulation and a calculus of the end-point mapping which boils down to the graded structures of Carnot groups.
Reduced dimensionality and strong electronic correlations, which are among the most important ingredients for cupratelike high-Tc superconductivity, characterize also the physics of nickelate-based heterostructures. Starting from the local-density approximation we arrive at a simple two-band model for quasi-two-dimensional 2D LaNiO3 /LaAlO3 heterostructures and extend it by introducing an appropriate hopping in the z direction to describe the dimensional crossover to three dimensions 3D. Using dynamical mean-field theory, we study the effects of electronic correlations with increasing interaction strength along the crossover from 2D to 3D. Qualitatively, the effects of electronic correlations are surprisingly similar, albeit quantitatively larger interaction strengths are required in three dimensions for getting a Mott-Hubbard insulating state. The exchange parameters of an effective Kugel-Khomskii-type spin-orbital model are also derived and reveal strong antiferromagnetic tendencies.
We investigate the electronic and magnetic properties of hydrogenated carbon nanotubes using ab initio spin-polarized calculations within both the local density approximation (LDA) and the generalized gradient approximation (GGA). We find that the combination of charge transfer and carbon network distortion makes the spin-polarized flat-band appear in the tube's energy gap. Various spin-dependent ground state properties are predicted with the changes of the radii, the chiralities of the tubes and the concentration of hydrogen (H). It is found that strain or external electric field can effectively modulate the flat-band spin-splitting, and even induce an insulator-metal transition.
The origin of x-ray magnetic circular dichroism (XMCD) at the Cu L2,3 edge in YBa2Cu3O7/La(1-x)Ca(x)MnO3 superlattices is revealed by performing first-principle electronic structure calculation using fully-relativistic spin-polarized linear muffin-tin orbital and projected augmented plane wave methods. We show that the XMCD spectra at the Cu L2,3 edges are proportional to the difference of the densities of majority- and minority-spin Cu 3z2-1 states. Although the Cu 3z2-1 states lie well below the Fermi level, a small number of majority-spin 3z2-1 holes is created by the Cu 3z2-1 - O p_z - Mn 3z2-1 hybridization across the interface. Even this tiny number of holes is sufficient to produce appreciable Cu L2,3 XMCD. The robustness of this conclusion is verified by studying the influence of doping, atomic relaxation, correlation effects, and antiferromagnetic order in a CuO2 plane on the XMCD spectra.
Using the local density approximation and its combination with dynamical mean-field theory, we show that electronic correlations induce a single-sheet, cuprate-like Fermi surface for hole-doped 1/1 LaNiO3 /LaAlO3 heterostructures, even though both eg orbitals contribute to it. The Ni 3d 3z^2-1 orbital plays the role of the axial Cu 4s-like orbital in the cuprates. These two results indicate that "orbital engineering" by means of heterostructuring should be possible. As we also find strong antiferromagnetic correlations, the low-energy electronic and spin excitations in nickelate heterostructures resemble those of high-temperature cuprate superconductors.
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 additional 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.
Here we present a combined study of the slightly underdoped novel pnictide superconductor Ba(1-x)K(x)Fe(2)As(2) by means of X-ray powder diffraction, neutron scattering, muon spin rotation (muSR), and magnetic force microscopy (MFM). Commensurate static magnetic order sets in below Tm ~ 70 K as inferred from the emergence of the magnetic (1 0 -3) reflection in the neutron scattering data and from the observation of damped oscillations in the zero-field-muSR asymmetry. Transverse-field muSR below Tc shows a coexistence of magnetically ordered and non-magnetic states, which is also confirmed by MFM imaging. We explain such coexistence by electronic phase separation into antiferromagnetic and superconducting/normal state regions on a lateral scale of several tens of nanometers. Our findings indicate that such mesoscopic phase separation can be considered an intrinsic property of some iron pnictide superconductors.
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 dimensional 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.
Combining a classical force field, a tight-binding model, and first-principles calculations, we have studied structural, electronic, and optical properties of double-walled carbon nanotube (DWNT) bundles under hydrostatic pressure. We find that the outer tube acts as a protection shield for the inner tube and the inner tube increases the structure stability and the ability to resist the pressure of the outer tube. Moreover, the collapsed structures of the double-walled carbon nanotube bundle called ``parallel'' and ``in-between'' are more stable than the one called ``herringbone''. The structural phase transition induces a pseudogap along symmetry line \textit{$Γ$X}. Furthermore, the optical properties change greatly after the collapse and a strong anisotropy appears in the collapsed structure. This provides an efficient experimental way to detect structural phase transitions in DWNT bundles.
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{$Γ$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 electronic and transport property of a radially deformed double-walled carbon nanotube (DWNT) intramolecular junction (IMJ) has been studied by the tight-binding (TB) model combined with the first-principle calculations. The geometrical structures of the DWNT IMJ have been first optimized in energy by the universal force field (UFF) method. It is found that when heavily squashed, the DWNT will become an insulator-coated metallic wire, and the conductance near the Fermi level has been significantly changed by the radial squash. Specially, several resonance conductance peaks appear at some energies in the conduction band of the squashed DWNT IMJ. Finally, we have also investigated the conductance variation due to change of the length of the central semiconductor in the squashed DWNT IMJ. Furthermore, a promising pure carbon nanoscale electronic device is proposed based on the DWNT IMJ.
Three kinds of the response mechanisms to the external pressure have been found for double-walled carbon nanotube (DWNT) bundle, depending strongly on their average radius and symmetry. The small-diameter DWNT bundle undergoes a small discontinuous volume change, and then deform continuously. The intermediate-diameter DWNT bundle collapses completely after a structure phase transition (SPT). Significantly, two SPTs exist for the larger-diameter DWNT bundle if the outer tube has no $C_{6}$ or $C_{3}$ symmetry. It would be interesting to search for signatures of these different structural transformations by experimentally investigating mechanical, optical and thermal response functions of DWNT bundle.
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 found 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.
The geometrical and electronic structures of the 4 Å diameter perfect and deformed (5, 3) single-walled gold nanotube (SWGT) have been studied based upon the density-functional theory in the local-density approximation (LDA). The calculated relaxed geometries show clearly significant deviations from those of the ideally rolled triangular gold sheet. It is found that the different strains have different effects on the electronic structures and density of states of the SWGTs. And the small shear strain can reduce the binding energy per gold atom of the deformed SWGT, which is consistent with the experimentally observed result. Finally, we found the finite SWGT can show the metal-semiconductor transition.
The Co-doped anatase TiO$_{2}$, a recently discovered room-temperature ferromagnetic insulator, has been studied by the first-principles calculations in the pseudo-potential plane-wave formalism within the local-spin-density approximation (LSDA), supplemented by the full-potential linear augmented plane wave (FP-LAPW) method. Emphasis is placed on the dependence of its electronic structures and linear optical properties on the Co-doping concentration and oxygen vacancy in the system in order to pursue the origin of its ferromagnetism. In the case of substitutional doping of Co for Ti, our calculated results are well consistent with the experimental data, showing that Co is in its low spin state. Also, it is shown that the oxygen vacancy enhances the ferromagnetism and has larger effect on both the electronic structure and optical properties than the Co-doping concentration only.