376 research outputs found
Synthesis and electronic properties of Ruddlesden-Popper strontium iridate epitaxial thin films stabilized by control of growth kinetics
We report on the selective fabrication of high-quality SrIrO and
SrIrO epitaxial thin films from a single polycrystalline SrIrO
target by pulsed laser deposition. Using a combination of X-ray diffraction and
photoemission spectroscopy characterizations, we discover that within a
relatively narrow range of substrate temperature, the oxygen partial pressure
plays a critical role in the cation stoichiometric ratio of the films, and
triggers the stabilization of different Ruddlesden-Popper (RP) phases. Resonant
X-ray absorption spectroscopy measurements taken at the Ir -edge and the O
-edge demonstrate the presence of strong spin-orbit coupling, and reveal the
electronic and orbital structures of both compounds. These results suggest that
in addition to the conventional thermodynamics consideration, higher members of
the SrIrO series can possibly be achieved by kinetic
control away from the thermodynamic limit. These findings offer a new approach
to the synthesis of ultra-thin films of the RP series of iridates and can be
extended to other complex oxides with layered structure.Comment: 7 pages, 6 figure
Effect of Cr spacer on structural and magnetic properties of Fe/Gd multilayers
In this work we analyse the role of a thin Cr spacer between Fe and Gd layers
on structure and magnetic properties of a [Fe(35A)/Cr(tCr)/Gd(50A)/Cr(tCr)]x12
superlattice. Samples without the Cr spacer (tCr=0) and with a thin tCr=4A are
investigated using X-ray diffraction, polarized neutron and resonance X-ray
magnetic reflectometry, SQUID magnetometery, magneto-optical Kerr effect and
ferromagnetic resonance techniques. Magnetic properties are studied
experimentally in a wide temperature range 4-300K and analysed theoretically
using numerical simulation on the basis of the mean-field model. We show that a
reasonable agreement with the experimental data can be obtained considering
temperature dependence of the effective field parameter in gadolinium layers.
The analysis of the experimental data shows that besides a strong reduction of
the antiferromagnetic coupling between Fe and Gd, the introduction of Cr
spacers into Fe/Gd superlattice leads to modification of both structural and
magnetic characteristics of the ferromagnetic layers
EXAFS study of lead-free relaxor ferroelectric BaTi(1-x)Zr(x)O3 at the Zr K-edge
Extended X-ray absorption fine structure (EXAFS) experiments at the Zr K-edge
were carried out on perovskite relaxor ferroelectrics BaTi(1-x)Zr(x)O3 (BTZ) (x
= 0.25, 0.30, 0.35), and on BaZrO3 for comparison. Structural information up to
4.5 A around the Zr atoms is obtained, revealing that the local structure
differs notably from the average Pm-3m cubic structure deduced from X-ray
diffraction. In particular, our results show that the distance between Zr atoms
and their first oxygen neighbors is independent of the Zr substitution rate x
and equal to that measured in BaZrO3, while the X-ray cubic cell parameter
increases linearly with x. Furthermore, we show that the Zr atoms tend to
segregate in Zr-rich regions. We propose that the relaxor behavior in BTZ is
linked to random elastic fields generated by this particular chemical
arrangement, rather than to random electric fields as is the case in most
relaxors.Comment: 13 pages, 12 figures, 4 tables. Submitted to Phys. Rev.
A 4-unit-cell superstructure in optimally doped YBa2Cu3O6.92 superconductor
Using high-energy diffraction we show that a 4-unit-cell superstructure,
q0=(1/4,0,0), along the shorter Cu-Cu bonds coexists with superconductivity in
optimally doped YBCO. A complex set of anisotropic atomic displacements on
neighboring CuO chain planes, BaO planes, and CuO2 planes, respectively,
correlated over ~3-6 unit cells gives rise to diffuse superlattice peaks. Our
observations are consistent with the presence of Ortho-IV nanodomains
containing these displacements.Comment: Corrected typo in abstrac
Theory of Room Temperature Ferromagnet V(TCNE)_x (1.5 < x < 2): Role of Hidden Flat Bands
Theoretical studies on the possible origin of room temperature ferromagnetism
(ferromagnetic once crystallized) in the molecular transition metal complex,
V(TCNE)_x (1.5<x<2) have been carried out. For this family, there have been no
definite understanding of crystal structure so far because of sample quality,
though the effective valence of V is known to be close to +2. Proposing a new
crystal structure for the stoichiometric case of x=2, where the valence of each
TCNE molecule is -1 and resistivity shows insulating behavior, exchange
interaction among d-electrons on adjacent V atoms has been estimated based on
the cluster with 3 vanadium atoms and one TCNE molecule. It turns out that
Hund's coupling among d orbitals within the same V atoms and antiferromagnetic
coupling between d oribitals and LUMO of TCNE (bridging V atoms) due to
hybridization result in overall ferromagnetism (to be precise, ferrimagnetism).
This view based on localized electrons is supplemented by the band picture,
which indicates the existence of a flat band expected to lead to ferromagnetism
as well consistent with the localized view. The off-stoichiometric cases (x<2),
which still show ferromagnetism but semiconducting transport properties, have
been analyzed as due to Anderson localization.Comment: Accepted for publication in J. Phys. Soc. Jpn. Vol.79 (2010), No. 3
(March issue), in press; 6 pages, 8 figure
Charge-Stripe Ordering From Local Octahedral Tilts: Underdoped and Superconducting La2-xSrxCuO4 (0 < x < 0.30)
The local structure of La2-xSrxCuO4, for 0 < x < 0.30, has been investigated
using the atomic pair distribution function (PDF) analysis of neutron powder
diffraction data. The local octahedral tilts are studied to look for evidence
of [110] symmetry (i.e., LTT-symmetry) tilts locally, even though the average
tilts have [010] symmetry (i.e., LTO-symmetry) in these compounds. We argue
that this observation would suggest the presence of local charge-stripe order.
We show that the tilts are locally LTO in the undoped phase, in agreement with
the average crystal structure. At non-zero doping the PDF data are consistent
with the presence of local tilt disorder in the form of a mixture of LTO and
LTT local tilt directions and a distribution of local tilt magnitudes. We
present topological tilt models which qualitatively explain the origin of tilt
disorder in the presence of charge stripes and show that the PDF data are well
explained by such a mixture of locally small and large amplitude tilts.Comment: 11 two-column pages, 11 figure
Incommensurate lattice distortion in the high temperature tetragonal phase of La(Sr,Ba)CuO
We report incommensurate diffuse (ICD) scattering appearing in the
high-temperature-tetragonal (HTT) phase of La(Sr,Ba)CuO
with observed by the neutron diffraction technique. For
all compositions, a sharp superlattice peak of the low-temperature-orthorhombic
(LTO) structure is replaced by a pair of ICD peaks with the modulation vector
parallel to the CuO octahedral tilting direction, that is, the diagonal
Cu-Cu direction of the CuO plane, above the LTO-HTT transition temperature
. The temperature dependences of the incommensurability for all
samples scale approximately as , while those of the integrated intensity
of the ICD peaks scale as . These observations together with
absence of ICD peaks in the non-superconducting sample evince a
universal incommensurate lattice instability of hole-doped 214 cuprates in the
superconducting regime.Comment: 6 pages, 6 figure
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