31 research outputs found
Digital light processing stereolithography of hydroxyapatite scaffolds with bone-like architecture, permeability, and mechanical properties
This work deals with the additive manufacturing and characterization of hydroxyapatite scaffolds mimicking the trabecular architecture of cancellous bone. A novel approach was proposed relying on stereolithographic technology, which builds foam-like ceramic scaffolds by using three-dimensional (3D) micro-tomographic reconstructions of polymeric sponges as virtual templates for the manufacturing process. The layer-by-layer fabrication process involves the selective polymerization of a photocurable resin in which hydroxyapatite particles are homogeneously dispersed. Irradiation is performed by a dynamic mask that projects blue light onto the slurry. After sintering, highly-porous hydroxyapatite scaffolds (total porosity ~0.80, pore size 100-800 µm) replicating the 3D open-cell architecture of the polymeric template as well as spongy bone were obtained. Intrinsic permeability of scaffolds was determined by measuring laminar airflow alternating pressure wave drops and was found to be within 0.75-1.74 × 10−9m2, which is comparable to the range of human cancellous bone. Compressive tests were also carried out in order to determine the strength (~1.60 MPa), elastic modulus (~513 MPa) and Weibull modulus (m = 2.2) of the scaffolds. Overall, the fabrication strategy used to print hydroxyapatite scaffolds (tomographic imaging combined with digital mirror device [DMD]-based stereolithography) shows great promise for the development of porous bioceramics with bone-like architecture and mass transport properties
Quasimolecular =3/2 moments in the cluster Mott insulator GaTaSe
Quasimolecular orbitals in cluster Mott insulators provide a route to tailor
exchange interactions, which may yield novel quantum phases of matter. We
demonstrate the cluster Mott character of the lacunar spinel GaTaSe
using resonant inelastic x-ray scattering (RIXS) at the Ta edge.
Electrons are fully delocalized over Ta tetrahedra, forming quasimolecular
=3/2 moments. The modulation of the RIXS intensity as function of
the transferred momentum q allows us to determine the cluster wavefunction,
which depends on competing intracluster hopping terms that mix states with
different character. This mixed wavefunction is decisive for the macroscopic
properties since it affects intercluster hopping and exchange interactions and
furthermore renormalizes the effective spin-orbit coupling constant. The
versatile wavefunction, tunable via intracluster hopping, opens a new
perspective on the large family of lacunar spinels and cluster Mott insulators
in general.Comment: 7 pages, 4 figures, plus supplementary informatio
Spin-orbit coupling in a half-filled shell: the case of KReCl
The half-filled shell of the configuration usually, in LS
coupling, hosts a S = 3/2 ground state with quenched orbital moment. This state
is not Jahn-Teller active. Sufficiently large spin-orbit coupling has
been predicted to change this picture by mixing in orbital moment, giving rise
to a sizable Jahn-Teller distortion. In KReCl we study the
electronic excitations using resonant inelastic x-ray scattering (RIXS) and
optical spectroscopy. We observe on-site intra- excitations below 2 eV
and corresponding overtones with two intra- excitations on adjacent
sites, the Mott gap at 2.7 eV, -to- excitations above 3 eV, and
charge-transfer excitations at still higher energy. The intra-
excitation energies are a sensitive measure of and Hund's coupling
. The sizable value of 0.29 eV places KReCl into
the intermediate coupling regime, but is not
sufficiently large to drive a pronounced Jahn-Teller effect. We discuss the
ground state wavefunction in a Kanamori picture and find that the S = 3/2
multiplet still carries about 97 % of the weight. However, the finite admixture
of orbital moment allows for subtle effects. We discuss small
temperature-induced changes of the optical data and find evidence for a
lowering of the ground state by about 3 meV below the structural phase
transitions.Comment: 16 pages, 14 figure
RIXS interferometry and the role of disorder in the quantum magnet BaTiIrO
Motivated by several claims of spin-orbit driven spin-liquid physics in
hexagonal BaTiIrO hosting Ir2O9 dimers, we report on
resonant inelastic x-ray scattering (RIXS) at the Ir L3 edge for different x.
We demonstrate that magnetism in BaTiIrO is governed by an
unconventional realization of strong disorder, where cation disorder affects
the character of the local moments. RIXS interferometry, studying the RIXS
intensity over a broad range of transferred momentum q, is ideally suited to
assign different excitations to different Ir sites. We find pronounced Ir-Ti
site mixing. Both ions are distributed over two crystallographically
inequivalent sites, giving rise to a coexistence of quasimolecular singlet
states on Ir2O9 dimers and spin-orbit entangled j=1/2 moments of 5d
Ir ions. RIXS reveals different kinds of strong magnetic couplings for
different bonding geometries, highlighting the role of cation disorder for the
suppression of long-range magnetic order in this family of compounds.Comment: 12 pages, 9 figure
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RIXS interferometry and the role of disorder in the quantum magnet Ba3 Ti3-x Irx O9
Motivated by several claims of spin-orbit-driven spin-liquid physics in hexagonal Ba3Ti3-xIrxO9 hosting Ir2O9 dimers, we report on resonant inelastic x-ray scattering (RIXS) at the Ir L3 edge for different x. We demonstrate that magnetism in Ba3Ti3-xIrxO9 is governed by an unconventional realization of strong disorder, where cation disorder affects the character of the local moments. RIXS interferometry, studying the RIXS intensity over a broad range of transferred momentum q, is ideally suited to assign different excitations to different Ir sites. We find pronounced Ir-Ti site mixing. Both ions are distributed over two crystallographically inequivalent sites, giving rise to a coexistence of quasimolecular singlet states on Ir2O9 dimers and spin-orbit-entangled j=1/2 moments of 5d5Ir4+ ions. RIXS reveals different kinds of strong magnetic couplings for different bonding geometries, highlighting the role of cation disorder for the suppression of long-range magnetic order in this family of compounds
Electronic excitations in J=0 Os halides studied by RIXS and optical spectroscopy
We demonstrate that the cubic antifluorite-type halides KOsCl,
KOsBr, and RbOsBr are excellent realizations of non-magnetic
J=0 compounds. The magnetic susceptibility shows the corresponding Van-Vleck
type behavior and no sign of defects. We investigate the electronic excitations
with two complementary techniques, resonant inelastic x-ray scattering (RIXS)
and optical spectroscopy. This powerful combination allows us to thoroughly
study, e.g., on-site intra- excitations and -to-
excitations as well as inter-site excitations across the Mott gap and an
exciton below the gap. In this way, we determine the electronic parameters with
high accuracy, altogether yielding a comprehensive picture. In KOsCl,
we find the spin-orbit coupling constant =0.34 eV, Hund's coupling
=0.43 eV, the onset of excitations across the Mott gap at =2.2 eV,
the cubic crystal-field splitting 10Dq=3.3 eV, and the charge-transfer energy
=4.6 eV. With =1.3, KOsCl is in the
intermediate-coupling regime. In a -only Kanamori picture, the above
values correspond to =0.41 eV and =0.28 eV, which is
very close to results reported for related iridates. In the tetragonal
phase at 5 K, the non-cubic crystal field causes a peak splitting of the J=1
state as small as 4 meV. Compared to KOsCl, the bromides KOsBr
and RbOsBr show about 12-14 % smaller values of 10Dq and ,
while the spin-orbit-entangled intra- excitations below 2 eV and hence
and are reduced by less than 4 %. Furthermore, the Mott gap in
KOsBr is reduced to about 1.8 eV.Comment: 14 pages, 14 figure
RIXS observation of bond-directional nearest-neighbor excitations in the Kitaev material NaIrO
Spin-orbit coupling locks spin direction and spatial orientation and
generates, in semi-classical magnets, a local spin easy-axis and associated
ordering. Quantum spin-1/2's defy this fate: rather than spins becoming locally
anisotropic, the spin-spin interactions do. Consequently interactions become
dependent on the spatial orientation of bonds between spins, prime theoretical
examples of which are Kitaev magnets. Bond-directional interactions imply the
existence of bond-directional magnetic modes, predicted spin excitations that
render crystallographically equivalent bonds magnetically inequivalent, which
yet have remained elusive experimentally. Here we show that resonant inelastic
x-ray scattering allows us to explicitly probe the bond-directional character
of magnetic excitations. To do so, we use a scattering plane spanned by one
bond and the corresponding spin component and scan a range of momentum transfer
that encompasses multiple Brillouin zones. Applying this approach to
NaIrO we establish the different bond-directional characters of
magnetic excitations at 10 meV and 45 meV. Combined with the observation of
spin-spin correlations that are confined to a single bond, this experimentally
validates the Kitaev character of exchange interactions long proposed for this
material.Comment: 6 pages, 5 figures, plus 4 pages Supplementary Information (incl. 5
figures
Quasimolecular electronic structure of the spin-liquid candidate Ba3InIr2O9
The mixed-valent iridate Ba3InIr2O9 has been discussed as a promising candidate for quantum spin-liquid behavior. The compound exhibits Ir4.5+ ions in face-sharing IrO6 octahedra forming Ir2O9 dimers with three t2g holes per dimer. Our results establish Ba3InIr2O9 as a cluster Mott insulator. Strong intradimer hopping delocalizes the three t2g holes in quasimolecular dimer states while interdimer charge fluctuations are suppressed by Coulomb repulsion. The magnetism of Ba3InIr2O9 emerges from spin-orbit entangled quasimolecular moments with yet unexplored interactions, opening up a new route to unconventional magnetic properties of 5d compounds. Using single-crystal x-ray diffraction we find the monoclinic space group C2/c already at room temperature. Dielectric spectroscopy shows insulating behavior. Resonant inelastic x-ray scattering reveals a rich excitation spectrum below 1.5 eV with a sinusoidal dynamical structure factor that unambiguously demonstrates the quasimolecular character of the electronic states. Below 0.3 eV, we observe a series of excitations. According to exact diagonalization calculations, such low-energy excitations reflect the proximity of Ba3InIr2O9 to a hopping-induced phase transition based on the condensation of a quasimolecular spin-orbit exciton. The dimer ground state roughly hosts two holes in a bonding j=12 orbital and the third hole in a bonding j=32 orbital
Spin-orbit coupling and crystal-field splitting in Ti-doped Ca2RuO4 studied by ellipsometry
In Ca2RuO4, the competition of spin-orbit coupling and tetragonal
crystal field splitting has been discussed controversially for
many years. The orbital occupation depends on , which allows
us to address this ratio via the optical spectral weights of the lowest
intersite Mott-Hubbard excitations. We study the optical conductivity of
CaRuTiO in the range of 0.75 - 5 eV by ellipsometry,
using the large single crystals that can be grown for small Ti concentrations.
Based on a local multiplet calculation, our analysis results in at 15 K. The dominant crystal field yields a ground
state close to xy orbital order but spin-orbit coupling is essential for a
quantitative description of the properties. Furthermore, we observe a
pronounced decrease of with increasing temperature, as expected
based on the reduction of octahedral distortions.Comment: 12 pages, 10 figure