Showing papers by "Maurits W. Haverkort published in 2012"

Multiplet ligand-field theory using Wannier orbitals

Abstract: We demonstrate how ab initio cluster calculations including the full Coulomb vertex can be done in the basis of the localized Wannier orbitals which describe the low-energy density functional (local-density approximation) band structure of an infinite crystal, e.g., the transition-metal $3d$ and oxygen $2p$ orbitals. The spatial extent of our $3d$ Wannier orbitals (orthonormalized $N$th-order muffin-tin orbitals) is close to that found for atomic Hartree-Fock orbitals. We define ligand orbitals as those linear combinations of the O $2p$ Wannier orbitals which couple to the $3d$ orbitals for the chosen cluster. The use of ligand orbitals allows for a minimal Hilbert space in multiplet ligand-field theory calculations, thus reducing the computational costs substantially. The result is a fast and simple ab initio theory, which can provide useful information about local properties of correlated insulators. We compare results for NiO, MnO, and SrTiO${}_{3}$ with x-ray absorption, inelastic x-ray scattering, and photoemission experiments. The multiplet ligand-field theory parameters found by our ab initio method agree within $\ensuremath{\sim}10%$ with known experimental values.

Spin-orbital separation in the quasi-one-dimensional Mott insulator Sr2CuO3.

Abstract: When viewed as an elementary particle, the electron has spin and charge. When binding to the atomic nucleus, it also acquires an angular momentum quantum number corresponding to the quantized atomic orbital it occupies. Even if electrons in solids form bands and delocalize from the nuclei, in Mott insulators they retain their three fundamental quantum numbers: spin, charge and orbital1. The hallmark of one-dimensional physics is a breaking up of the elementary electron into its separate degrees of freedom2. The separation of the electron into independent quasi-particles that carry either spin (spinons) or charge (holons) was first observed fifteen years ago3. Here we report observation of the separation of the orbital degree of freedom (orbiton) using resonant inelastic X-ray scattering on the one-dimensional Mott insulator Sr2CuO3. We resolve an orbiton separating itself from spinons and propagating through the lattice as a distinct quasi-particle with a substantial dispersion in energy over momentum, of about 0.2 electronvolts, over nearly one Brillouin zone.

Determining the In-Plane Orientation of the Ground-State Orbital of CeCu2Si2

Abstract: We have successfully determined the hitherto unknown sign of the ${B}_{4}^{4}$ Stevens crystal-field parameter of the tetragonal heavy-fermion compound ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ using vector $\stackrel{^}{\mathbf{q}}$-dependent nonresonant inelastic x-ray scattering experiments at the cerium ${N}_{4,5}$ edge. The observed difference between the two different directions, $\stackrel{^}{\mathbf{q}}\ensuremath{\parallel}[100]$ and $\stackrel{^}{\mathbf{q}}\ensuremath{\parallel}[110]$, is due to the anisotropy of the crystal-field ground state in the (001) plane and is observable only because of the utilization of higher than dipole transitions possible in nonresonant inelastic x-ray scattering. This approach allows us to go beyond the specific limitations of dc magnetic susceptibility, inelastic neutron scattering, and soft x-ray spectroscopy, and provides us with a reliable information about the orbital state of the $4f$ electrons relevant for the quantitative modeling of the quasiparticles and their interactions in heavy-fermion systems.

Crystal-field ground state of the orthorhombic Kondo insulator CeRu2Al10

Abstract: We have succeeded in establishing the crystal-field ground state of CeRu${}_{2}$Al${}_{10}$, an orthorhombic intermetallic compound recently identified as a Kondo insulator. Using polarization-dependent soft x-ray absorption spectroscopy at the Ce ${M}_{4,5}$ edges, together with input from inelastic neutron and magnetic susceptibility experiments, we were able to determine unambiguously the orbital occupation of the $4f$ shell and to explain quantitatively both the measured magnetic moment along the easy $a$ axis and the small ordered moment along the $c$ axis. The results provide not only a platform for a realistic modeling of the spin and charge gap of CeRu${}_{2}$Al${}_{10}$, but demonstrate also the potential for soft x-ray absorption spectroscopy to obtain information not easily accessible by neutron techniques for the study of Kondo insulators in general.

Atomic and itinerant effects at the transition-metal x-ray absorption K pre-edge exemplified in the case of V 2 O 3

Abstract: X-ray absorption spectroscopy is a well-established tool for obtaining information about orbital and spin degrees of freedom in transition-metal and rare-earth compounds. For this purpose usually the dipole transitions of the $L$ ($2p$ to $3d$) and $M$ ($3d$ to $4f$) edges are employed, whereas higher order transitions such as quadrupolar $1s$ to $3d$ in the $K$ edge are rarely studied in that respect. This is due to the fact that usually such quadrupolar transitions are overshadowed by dipole-allowed $1s$ to $4p$ transitions and, hence, are visible only as minor features in the pre-edge region. Nonetheless, these features carry a lot of valuable information, similar to the dipole $L$-edge transition, which is not accessible in experiments under pressure due to the absorption of the diamond anvil pressure cell. We recently performed a theoretical and experimental analysis of such a situation for the metal-insulator transition of (V${}_{(1\ensuremath{-}x)}$Cr${}_{x}$)${}_{2}$O${}_{3}$. Since the importance of the orbital degrees of freedom in this transition is widely accepted, a thorough understanding of quadrupole transitions of the vanadium $K$ pre-edge provides crucial information about the underlying physics. Moreover, the lack of inversion symmetry at the vanadium site leads to on-site mixing of vanadium $3d$ and $4p$ states and related quantum mechanical interferences between dipole and quadrupole transitions. Here we present a theoretical analysis of experimental high-resolution x-ray absorption spectroscopy at the V $K$ pre-edge measured in partial fluorescence yield mode for single crystals. We carried out density functional as well as configuration interaction calculations in order to capture effects coming from both itinerant and atomic limits.

Mott versus Slater-type metal-insulator transition in Mn-substituted Sr3Ru2O7

Abstract: We present a temperature-dependent x-ray-absorption (XAS) and resonant elastic x-ray scattering (REXS) study of the metal-insulator transition (MIT) in Sr${}_{3}$(Ru${}_{1\ensuremath{-}x}$Mn${}_{x}$)${}_{2}$O${}_{7}$. The XAS results reveal that the MIT drives the onset of local antiferromagnetic correlations around the Mn impurities, a precursor of the long-range antiferromagnetism detected by REXS at ${T}_{\mathrm{order}}l{T}_{\mathrm{MIT}}$. This establishes that the MIT is of the Mott type (electronic correlations) as opposed to Slater type (magnetic order). While this behavior is induced by Mn impurities, the $(\frac{1}{4},\frac{1}{4},0)$ order exists for a wide range of Mn concentrations, and points to an inherent instability of Sr${}_{3}$Ru${}_{2}$O${}_{7}$.

Unoccupied electronic structure of TiOCl studied using x-ray absorption near-edge spectroscopy.

Abstract: We study the unoccupied electronic structure of the spin- quantum magnet TiOCl using x-ray absorption near-edge spectroscopy (XANES) at the Ti L and O K edges. We acquire data both in total electron and fluorescence yield modes (TEY and FY, respectively). While only the latter allows us to access the unconventional low-temperature spin–Peierls (SP) phase of TiOCl, the signal is found to suffer from significant self-absorption in this case. Nevertheless, we conclude from FY data that effects of the SP distortion on the electronic structure are absent in the incommensurate intermediate phase within experimental accuracy. The similarity of room-temperature FY and TEY data, the latter not being obscured by self-absorption, allows us to use TEY spectra for comparison with simulations. These are performed by means of cluster calculations in D4h and D2h symmetries using two different codes. We extract values of the crystal-field splitting and parameterize our results using the commonly seen notation of Slater, Racah and Butler. In all cases, good agreement with published values from other studies is found.