Dehong Yu

Collisional fragmentation of endohedral fullerenes

Abstract The fragmentation patterns of the endohedral fullerenes ions, (Gd@C 82 ) + and (La@C 82 ) + , produced by energetic collisions with various atomic and molecular targets have been measured over an entire mass range extending from the parent endohedral to the bare metal ion. The fragmentation spectra confirm the endohedral nature of these fullerenes by the production of singly and doubly charged endohedral fragments M@C 2 n (30 ⩽ n ⩽ 40). The fragmentation pattern includes a substantial component of empty fullerenes that extend from C 80 to less than C 60 dependent on the collision partner. The empty fullerenes are accompanied by small metal-containing fragment ions, MC 2 n + (0 ⩽ n ⩽ 5), produced in hard central collisions with the target atom. The distribution of MC 2 n + fragments differs for the two endohedrals and is dependent on the collision partner.

Enhancement of Co substitution induced by Eu codoping in ZnO-based diluted magnetic semiconducting thin films

To avoid the occurrence of doped magnetic ion clustering is a challenge in fabrication of diluted magnetic semiconductors (DMSs). In this work, we report the intrinsic ferromagnetic behavior in Co-doped ZnO DMSs induced by Eu codoping. Both structural parameters and magnetic properties demonstrate the existence of an interaction between Co and Eu ions. The observation of multiplet structures for the localized Co 3d states in x-ray absorption and x-ray magnetic circular dichroism characterization evidences that the codoped Eu plays an important role in facilitating the Co substitution of Zn, leading to intrinsic ferromagnetism.

Magnetic Excitations in Polyoxotungstate-Supported Lanthanoid Single-Molecule Magnets: An Inelastic Neutron Scattering and ab Initio Study

Inelastic neutron scattering (INS) has been used to investigate the crystal field (CF) magnetic excitations of the analogs of the most representative lanthanoid-polyoxometalate single-molecule magnet family: Na9[Ln(W5O18)2] (Ln = Nd, Tb, Ho, Er). Ab initio complete active space self-consistent field/restricted active space state interaction calculations, extended also to the Dy analog, show good agreement with the experimentally determined low-lying CF levels, with accuracy better in most cases than that reported for approaches based only on simultaneous fitting to CF models of magnetic or spectroscopic data for isostructural Ln families. In this work we demonstrate the power of a combined spectroscopic and computational approach. Inelastic neutron scattering has provided direct access to CF levels, which together with the magnetometry data, were employed to benchmark the ab initio results. The ab initio determined wave functions corresponding to the CF levels were in turn employed to assign the INS transitions allowed by selection rules and interpret the observed relative intensities of the INS peaks. Ultimately, we have been able to establish the relationship between the wave function composition of the CF split LnIII ground multiplets and the experimentally measured magnetic and spectroscopic properties for the various analogs of the Na9[Ln(W5O18)2] family.

Cooling field tuned magnetic phase transition and exchange bias-like effect in Y0.9Pr0.1CrO3

Cooling magnetic field dependence of magnetic phase transition has been observed in Y0.9Pr0.1CrO3. GzFx order (spin structure of PrCrO3) is dominant after zero field cooling (ZFC), whereas GxFz order (spin structure of YCrO3) is dominant after cooling under a field higher than 100 Oe. Positive/negative exchange bias-like effect, with large vertical shift and small horizontal shift, has been observed after FC/ZFC process. The vertical shift can be attributed to the frozen ordered Pr3+ and Cr3+ spins in magnetic domains, because of the strong coupling between Pr3+ and Cr3+ sublattices; while the horizontal shift is a result of the pinning of spins at the interfaces. The frozen structure is generated by the field used for the measurement of the initial magnetization curve of M(H) for the ZFC cooled sample, while it is generated by the cooling field for the sample cooled under a cooling field higher than 100 Oe.

Carbonate-Bridged Lanthanoid Triangles: Single-Molecule Magnet Behavior, Inelastic Neutron Scattering, and Ab Initio Studies

Optimization of literature synthetic procedures has afforded, in moderate yield, homogeneous and crystalline samples of the five analogues Na11[{RE(OH2)}3CO3(PW9O34)2] (1-RE; RE = Y, Tb, Dy, Ho, and Er). Phase-transfer methods have allowed isolation of the mixed salts (Et4N)9Na2[{RE(OH2)}3CO3(PW9O34)2] (2-RE; RE = Y and Er). The isostructural polyanions in these compounds are comprised of a triangular arrangement of trivalent rare-earth ions bridged by a μ3-carbonate ligand and sandwiched between two trilacunary Keggin {PW9O34} polyoxometalate ligands. Alternating-current (ac) magnetic susceptibility studies of 1-Dy, 1-Er, and 2-Er reveal the onset of frequency dependence for the out-of-phase susceptibility in the presence of an applied magnetic field at the lowest measured temperatures. Inelastic neutron scattering (INS) spectra of 1-Ho and 1-Er exhibit transitions between the lowest-lying crystal-field (CF) split states of the respective J = 8 and (15)/2 ground-state spin-orbit multiplets of the Ho(III) and Er(III) ions. Complementary ab initio calculations performed for these two analogues allow excellent reproduction of the experimental magnetic susceptibility and low-temperature magnetization data and are in reasonable agreement with the experimental INS data. The ab initio calculations reveal that the slight difference in coordination environments of the three Ln(III) ions in each complex gives rise to differences in the CF splitting that are not insignificant. This theoretical result is consistent with the observation of multiple relaxation processes by ac magnetic susceptibility and the broadness of the measured INS peaks. The ab initio calculations also indicate substantial mixing of the MJ contributions to the CF split energy levels of each Ln(III) ion. Calculations indicate that the CF ground states of the Ho(III) centers in 1-Ho are predominantly comprised of contributions from small MJ, while those of the Er(III) centers in 1-Er are predominantly comprised of contributions from large MJ, giving rise to slow magnetic relaxation. Although no direct evidence for intramolecular RE···RE magnetic coupling is observed in either magnetic or INS studies, on the basis of the ab initio calculations, we find noncollinear magnetic axes in 1-Er that are coplanar with the erbium triangle and radially arranged with respect to the triangles centroid; thus, we argue that the absence of magnetic coupling in this system arises from dipolar and antiferromagnetic superexchange interactions that cancel each other out.

Electric-field-induced AFE-FE transitions and associated strain/preferred orientation in antiferroelectric PLZST.

Electric-field-induced, antiferroelectric-ferroelectric (AFE-FE) phase transitions are common for AFE materials. To date, the strain and preferred orientation evolution as well as the role of the intermediate FE state during the successive AFE-FE-AFE phase transitions has not been clear. To this end, we have herein studied a typical AFE Pb0.97La0.02(Zr0.56Sn0.33Ti0.11)O3 (PLZST) material using in-situ neutron diffraction. It is striking that the AFE-FE phase transition is not fully reversible: in the electric-field-induced FE state, the induced strain exhibits an elliptical distribution, which in turn leads to significant preferred orientation in the final AFE state after withdrawal of the applied electric-field. The ω-dependent neutron diffraction patterns show clear evidence of the induced strain distribution and associated preferred orientation arising from the AFE-FE phase transition. The current work also provides an explanation for several temperature and electric-field dependent dielectric anomalies as well as unrecovered strain change which appear in AFE materials after exposure to sufficiently high electric fields.

Lifetime measurements with fullerene ions in a heavy-ion storage ring

Positive and negative fullerene ions have been stored in a heavy-ion storage ring. We found confinement times (storage lifetimes) of the order of 2 to 10 s, the limit being set by collisional destruction in the background gas. The data also show a short decay time of the order of some milliseconds. This component was considerably longer for the negative C 60 and C 70 ions than the neighboring negative C 58 and C 68 ions. We discuss the possible processes that may account for the observation of the ms-decay times. The present technique may provide an excellent way to produce and study cold clusters

Excitation of He 3(3)P and Ne 3p states by polarized electrons

Integrated Stokes parameters for the transitions in helium (388.9 nm) and neon (640.2 nm) and (626.7 nm) have been measured over the electron energy range from threshold to 300 eV after excitation by polarized electrons. The measured zero values of for both the He 388.9 and Ne 640.2 nm photons are consistent with the LS coupling of the upper states for each transition. Similarly, the measured non-zero values of for the 626.7 nm photons provide clear evidence of the breakdown of LS coupling for the Ne state due to the spin - orbit interaction of the atomic electrons. Non-zero values of the Stokes parameter indicate strong electron exchange effects, particularly near threshold. Negative ion resonances show considerable effects on the polarizations of the subsequent radiations. The effect of different core configurations on the alignment of excited states is discussed.

Charge transfer in collisions involving multiply charged C60 molecules

Electron capture in collisions of C602+ and C603+ molecular ions with atomic and molecular gases has been studied at impact energies around 100 keV. The cross-section dependence on the target-ionization potentials is discussed, and a simple over-barrier model is evoked to explain the energy dependences. The cross sections for endothermic processes are discussed in the light of a simple two-state model, and a general understanding of their behaviour is obtained.

Formation and fragmentation of Cn+ clusters produced from a C60/C70 mixture in an electron-impact ion source

Various fullerene ions are generated in a standard plasma ion source from a vaporized mixture of C60/C70. Except C60+ and C70+, the fullerene ions are formed by fragmentation of C60 or C70 excited by electron impact. Information on the structure and stability of the fullerene ions is obtained by studying unimolecular dissociation and collision-induced fragmentation of C60+, C58+ and C56+ in H2 and Ar target gases.