H. H. Hsieh

Orbital-Assisted Metal-Insulator Transition in VO2

We found direct experimental evidence for an orbital switching in the V 3d states across the metal-insulator transition in VO2. We have used soft-x-ray absorption spectroscopy at the V L2,3 edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.

Valence, spin, and orbital state of Co ions in one-dimensional Ca3Co2O6 : An x-ray absorption and magnetic circular dichroism study

We have investigated the valence, spin, and orbital state of the Co ions in the one-dimensional cobaltate Ca3Co2O6 using x-ray absorption and x-ray magnetic circular dichroism at the Co-L2,3 edges. The Co ions at both the octahedral Cooct and trigonal Cotrig sites are found to be in a 3+ state. From the analysis of the dichroism we established a low-spin state for the Cooct and a high-spin state with an anomalously large orbital moment of 1.7B at the Co trig ions. This large orbital moment along the c-axis chain and the unusually large magnetocrystalline anisotropy can be traced back to the double occupancy of the d2 orbital in trigonal crystal field.

Magnetic versus crystal-field linear dichroism in NiO thin films

We have detected strong dichroism in the Ni L-2,L-3 x-ray absorption spectra of a monolayer NiO film. The dichroic signal appears to be very similar to the magnetic linear dichroism observed for thicker antiferromagnetic NiO films. Detailed analysis reveals, however, that the dichroism is caused by crystal-field effects in the monolayer film, which is a nontrivial effect because the high spin Ni 3d(8) ground state is not split by low-symmetry crystal fields. We present a practical method for identifying the independent magnetic and crystal-field contributions to the linear dichroic signal in spectra of NiO films with arbitrary thicknesses and lattice strains. Our findings are also relevant for 3d(5) and 3d(3) systems such as LaFeO3, Fe2O3, VO, LaCrO3, Cr2O3, and Mn4+ thin films.

Soft x-ray magnetic circular dichroism study on Gd-doped EuO thin films

We report on the growth and magnetic properties of Gd-doped

Epitaxial and layer-by-layer growth of EuO thin films on yttria-stabilized cubic zirconia (001) using MBE distillation

\mathrm{EuO}

Direct observation of electron doping in La0.7Ce0.3MnO3 using x-ray absorption spectroscopy

thin films. We prepared samples with Gd concentrations up to

Determining the crystal-field ground state in rare earth heavy fermion materials using soft-x-ray absorption spectroscopy.

11%

Controlling orbital moment and spin orientation in CoO layers by strain

by means of molecular beam epitaxy under distillation conditions. Using soft x-ray magnetic circular dichroism at the Eu and Gd

Different look at the spin state of Co3+ ions in a CoO5 pyramidal coordination

{M}_{4,5}

Orbitally Driven Spin-Singlet Dimerization in S=1 La4Ru2O10

edges, we found that