178 research outputs found
Electronic structure of single-crystalline MgAlB probed by x-ray diffraction multipole refinements and polarization-dependent x-ray absorption spectroscopy
X-ray diffraction multipole refinements of single-crystalline
MgAlB and polarization-dependent near-edge x-ray absorption
fine structure at the B 1 edge reveal a strongly anisotropic electronic
structure. Comparing the data for superconducting compounds (, 1.0)
with those for the non-superconductor () gives direct evidence for a
rearrangement of the hybridizations of the boron bonds and underline the
importance of holes in the -bonded covalent states for the
superconducting properties of the diborides. The data indicate that Mg is
approximately divalent in MgB and suggest predominantly ionic bonds between
the Mg ions and the two-dimensional B rings. For AlB (), on the other
hand, about 1.5 electrons per Al atom are transferred to the B sheets while the
residual 1.5 electrons remain at the Al site which suggests significant
covalent bonding between the Al ions and the B sheets. This finding together
with the static electron deformation density points to almost equivalent
electron counts on B sheets of MgB and AlB\@, yet with a completely
different electron/hole distribution between the and bonds
Suppression of spin-state transition in epitaxially strained LaCoO_{3}
Epitaxial thin films of LaCoO_{3} (E-LCO) exhibit ferromagnetic order with a
transition temperature T_c = 85 K, while polycrystalline thin LaCoO_{3} films
(P-LCO) remain paramagnetic. The temperature-dependent spin-state structure for
both E-LCO and P-LCO was studied by x-ray absorption spectroscopy at the Co
L_{2,3} and O K edges. Considerable spectral redistributions over temperature
are observed for P-LCO. The spectra for E-LCO, on the other hand, do not show
any significant changes for temperatures between 30 K and 450 K at both edges,
indicating that the spin state remains constant and that the epitaxial strain
inhibits any population of the low-spin (S = 0) state with decreasing
temperature. This observation identifies an important prerequisite for
ferromagnetism in E-LCO thin films.Comment: 5 pages, 5 figures, submitted to Physical Review
YBaCuO/LaCaMnO bilayers: Interface coupling and electric transport properties
Heteroepitaxially grown bilayers of ferromagnetic LaCaMnO
(LCMO) on top of superconducting YBaCuO (YBCO) thin films were
investigated by focusing on electric transport properties as well as on
magnetism and orbital occupation at the interface. Transport measurements on
YBCO single layers and on YBCO/LCMO bilayers, with different YBCO thickness
and constant LCMO thickness \,nm, show a significant reduction of
the superconducting transition temperature only for \,nm,with
only a slightly stronger suppression in the bilayers, as compared to the
single layers. X-ray magnetic circular dichroism (XMCD) measurements confirm
recently published data of an induced magnetic moment on the interfacial Cu by
the ferromagnetically ordered Mn ions, with antiparallel alignment between Cu
and Mn moments. However, we observe a significantely larger Cu moment than
previously reported, indicating stronger coupling between Cu and Mn at the
interface. This in turn could result in an interface with lower transparency,
and hence smaller spin diffusion length, that would explain our electric
transport data, i.e.smaller suppression. Moreover, linear dichroism
measurements did not show any evidence for orbital reconstruction at the
interface, indicating that a large change in orbital occupancies through
hybridization is not necessary to induce a measurable ferromagnetic moment on
the Cu atoms.Comment: 8 Figure
Extremely Small Energy Gap in the Quasi-One-Dimensional Conducting Chain Compound SrNbO
Resistivity, optical, and angle-resolved photoemission experiments reveal
unusual one-dimensional electronic properties of highly anisotropic
SrNbO. Along the conducting chain direction we find an extremely small
energy gap of only a few meV at the Fermi level. A discussion in terms of
typical 1D instabilities (Peierls, Mott-Hubbard) shows that neither seems to
provide a satisfactory explanation for the unique properties of SrNbO.Comment: 4 pages, 3 figure
Hole distribution for (Sr,Ca,Y,La)_14 Cu_24 O_41 ladder compounds studied by x-ray absorption spectroscopy
The unoccupied electronic structure for the Sr_14Cu_24O_41 family of two-leg
ladder compounds was investigated for different partial substitutions of Sr^2+
by Ca^2+, leaving the nominal hole count constant, and by Y^3+ or La^3+,
reducing the nominal hole count from its full value of 6 per formula unit.
Using polarization-dependent x-ray absorption spectroscopy on single crystals,
hole states on both the chain and ladder sites could be studied. While for
intermediate hole counts all holes reside on O sites of the chains, a partial
hole occupation on the ladder sites in orbitals oriented along the legs is
observed for the fully doped compound Sr_14Cu_24O_41. On substitution of Ca for
Sr orbitals within the ladder planes but perpendicular to the legs receive some
hole occupation as well.Comment: 10 pages RevTeX style with 7 embedded figures + 1 table; accepted by
Phys. Rev.
- …