Q. Q. Zheng

Symmetry and magnetic properties of transition metal clusters

Abstract The magnetic properties of 55-atom Fe, Co and Ni clusters are studied using the local spin-density formalism. The dependence of magnetic moment on cluster symmetry is found to be also cluster size dependent. The symmetry of cluster plays an important role in determining the charge distribution. The surface magnetism enhancement are found to be decreased from Fe to Ni.

The giant magnetic moment and electronic correlation effect in ferromagnetic nitride Fe16N2

The on-site Coulomb interaction correction (LDA+U) and the generalized gradient approximation (GGA) seek to improve upon the accuracy of the local-spin-density approximation (LSDA) in the calculation of the electronic structure of the novel nitride Fe16N2. The non-locality of the exchange-correlation interaction included by the GGA approach does not apparently affect the calculated magnetic moment. However, the LDA+U approach gave a good result which matches the experimentally determined giant magnetic moment more closely than any previous calculations. This fact draws attention to an unusual correlation effect in the nitride Fe16N2 for the first time.

Electronic structure and magnetic properties of Ni clusters

A systematic study on the ground state electronic structure and magnetic properties of Nin (n=2–39 and 55) clusters are performed using the density functional calculation with the local spin-density approximation. The binding energy per atom increases monotonically. The occupation numbers do not change significantly for n>8, which helps us to understand the experimental observation of the photoelectron spectroscopy. The obtained average magnetic moment per atom is larger than the bulk value, and the evolvement of these moments towards the bulk is in good agreement with the experimental trend. However, the average moments of “core” atoms are comparable to or even larger than that of surface atoms, which indicates that the usual conclusion of surface enhancement on magnetism in transition metal clusters may be improper for these Ninu200a(n=14–39) clusters.

Electronic structures of magnesium clusters

Abstract Electronic structures of magnesium clusters (up to 57 atoms) are studied by the spin-unrestricted density functional theory with a local density approximation. As the cluster size increases, more 3p electrons will be involved in the sp hybridization. The obtained energy gap, binding energy and electronic structure suggest that the convergence of Mg clusters toward the bulk bonding is slower than that of Be clusters.

Electronic structure of PrBa2Cu3O7 and YBa2Cu3O7: A local spin density approximation with the on-site Coulomb interaction study

Using the self-consistent linear muffin-tin orbitals and the atomic sphere approximation method, the band structure calculations for PrBa2Cu3O7 and YBa2Cu3O7 are performed in the regime of the local spin density approximation with the on-site Coulomb interaction (LSDA+U). The strong electron-electron correlation effects are considered for both the Pru200a4f and Cuu200a3d states. Comparing with the LSDA calculations, the LSDA+U calculations show that, while the essential feature of energy band of YBa2Cu3O7 is preserved, the electronic structure of PrBa2Cu3O7 is quite different. For the bands arisen from the CuO2 planes, a gap with a value of 0.5 eV is opened, indicating the insulating or semiconducting character of the CuO2 planes. The pdσ bands associated with the CuO chains are across the Fermi level, revealing the metallic character of the CuO chains.

Ab-initio molecular dynamics studies of a Ga13 cluster

Abstract We report the results on the structural and electronic properties of a Ga13 cluster carried out by the ab-initio molecular dynamics method. We found an icosahedron with some Jahn-Teller distortion to be the lowest energy structure for the Ga13 cluster. The charge density in the cluster is much lower than that in the bulk α-Ga, which implies that the bonding is still metallic.

First-principles study of the magnetic properties of 4d impurities in Cun clusters

The magnetic properties of small CunTM clusters (n = 3−7, 12, TM = Ru, Rh, Pd) have been studied using a linear combination of the atomic orbital approach with the density functional formalism, and the Kohn-Sham equation is solved self-consistently using the discrete variational method (DVM). It is found that the magnetic moments of Rh in the D3h structure and Ru in the Td structure are quenched to zero (we call this phenomenon the symmetry-quenching effect). It is also found that Ru and Rh are magnetic (except for symmetry-quenching), and Pd is nearly nonmagnetic. The effects of symmetry, coordination number and atomic distance on the magnetic moments of 4d impurities are discussed.

Ground state properties of a high-spin Mn12O12 molecule in an organic compound

The electronic structure and the magnetism of a Mn12O12 molecule at ground state have been studied by density functional theory with a local spin density approximation. We have found that the magnetic moments of Mn ions in the tetrahedron and those of Mn ions in the crown of the Mn12O12 molecule align antiferromagnetically. The average moment per Mn ion is about 3.07μB in the tetrahedron and 4.07μB in the crown. The total spin amounts to 20.0μB which is in agreement with recent experimental results. The significant difference of magnetic moments between Mn ions at two sites is attributed to the different exchange splitting of 3d orbitals. However, the charge difference between the two kinds of Mn ions is as small as 0.22 electrons. The charge density and the spin density exhibit strong directional dependence, which indicates the strong anisotropy in this molecule.

Magnetism and Jahn-Teller effect in LaMnO3

Combining charge transfer with the Jahn-Teller electron-phonon coupling, the magnetic properties of LaMnO3 and the influence of the Jahn-Teller effect are studied. The interaction parameters of super exchange coupling between manganese spins are obtained. In the ground state, manganese spins are antiferromagnetically ordered but canted by a small angle. The spectrum of the low-energy spin excitation is very close to that observed experimentally. Behaviors of magnetization and spin correlation functions of LaMnO3 exhibit certain two-dimensional ferromagnetic characters, even though it is an A-type antiferromagnet with anisotropic super exchange coupling. These results agree with several experiments. Furthermore, it is expected that the super exchange interactions depend on the electron-phonon coupling strength exponentially, and the Neel temperature will exhibit isotope shift.

The magnetic properties of 4d impurities in Agn clusters

Abstract The magnetic properties of small AgnTM clusters (n = 3–7, 12; TM = Ru, Rh, Pd) have been studied using a linear combination of atomic orbitals approach with the density functional formalism, and the Kohn-Sham equation is solved self-consistently with the discrete variational method (DVM). The symmetry-quenching effect is found that the magnetic moments of Rh in D3h structure and Ru in Td structure are quenched to zero. Meanwhile, it is found that Ru and Rh are magnetic (except for symmetry-quenching), and Pd is nearly nonmagnetic. The effects of symmetry, coordination number and atomic distance on the magnetic moment of 4d impurities are discussed.