171 research outputs found
Experimental probing of the anisotropy of the empty p states near the Fermi level in MgB2
We have studied the Boron K-edge in the superconductor MgB2 by electron
energy loss spectroscopy (EELS) and experimentally resolved the empty p states
at the Fermi level that have previously been observed within an energy window
of 0.8eV by soft x-ray absorption spectroscopy. Using angular resolved EELS, we
find that these states at the immediate edge onset have pxy character in
agreement with predictions from first-principle electronic structure
calculations.Comment: 15 pages, 5 figure
Local boron doping quantification in homoepitaxial diamond structures
The capability of transmission electronmicroscopy (TEM) using the high angle annular dark fieldmode (HAADF,also labelled Z-contrast) to quantify boron concentration, in the high doping range between 1019cm−3 and 1021cm−3, is demonstrated. Thanks to the large relative variation of atomic number Z between carbon and boron, doping concentration maps and profiles are obtained with a nanometer-scale resolution. A novel numerical simulation procedure allows the boron concentration quantification and demonstrates the high sensitivity and
spatial resolution of the technique.4 page
Direct observation of nm-scale Mg- and B-oxide phases at grain boundaries in MgB2
Here we describe the results of an atomic resolution study of the structure
and composition of both the interior of the grains, and the grain boundaries in
polycrystalline MgB2. We find that there is no oxygen within the bulk of the
grains but significant oxygen enrichment at the grain boundaries. The majority
of grain boundaries contain BOx phases smaller than the coherence length, while
others contain larger areas of MgO sandwiched between BOx layers. Such results
naturally explain the differences in connectivity between the grains observed
by other techniques
Direct measurement of the low-temperature spin-state transition in LaCoO3
LaCoO(3) exhibits an anomaly in its magnetic susceptibility around 80 K associated with a thermally excited transition of the Co(3+)-ion spin. We show that electron energy-loss spectroscopy is sensitive to this Co(3+)-ion spin-state transition, and that the O K edge prepeak provides a direct measure of the Co(3+) spin state in LaCoO(3) as a function of temperature. Our experimental results are confirmed by first-principles calculations, and we conclude that the thermally excited spin-state transition occurs from a low to an intermediate spin state, which can be distinguished from the high-spin state
Inversion of two-band superconductivity at the critical electron doping of (Mg,Al)B-2
Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)AlxB2. A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximate to 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T-c< 7 K up to about x approximate to 55. Only the pi band had detectable density of states for 0.33 less than or similar to x less than or similar to 0.55, implying an inversion of the two-band hierarchy of MgB2 in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T-c are discussed
Observation of coherent oxide precipitates in polycrystalline MgB2
Here we describe the results of an atomic resolution study of oxygen
incorporation into bulk MgB2. We find that ~20-100 nm sized precipitates are
formed by ordered substitution of oxygen atoms onto boron lattice sites, while
the basic bulk MgB2 crystal structure and orientation is preserved. The
periodicity of the oxygen ordering is dictated by the oxygen concentration in
the precipitates and primarily occurs in the (010) plane. The presence of these
precipitates correlates well with an improved critical current density and
superconducting transition behavior, implying that they act as pinning centers.Comment: Submitted to Applied Physics Letters, 6 pages, 3 figure
Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb
High purity niobium (Nb), subjected to the processing methods used in the
fabrication of superconducting RF cavities, displays micron-sized surface
patches containing excess carbon. High-resolution transmission electron
microscopy and electron energy-loss spectroscopy measurements are presented
which reveal the presence of nanoscale NbC coherent precipitates in such
regions. Raman backscatter spectroscopy on similar surface regions exhibit
spectra consistent with the literature results on bulk NbC but with
significantly enhanced two-phonon scattering. The unprecedented strength and
sharpness of the two-phonon signal has prompted a theoretical analysis, using
density functional theory (DFT), of phonon modes in NbC for two different
interface models of the coherent precipitate. One model leads to overall
compressive strain and a comparison to ab-initio calculations of phonon
dispersion curves under uniform compression of the NbC shows that the measured
two-phonon peaks are linked directly to phonon anomalies arising from strong
electron-phonon interaction. Another model of the extended interface between Nb
and NbC, studied by DFT, gives insight into the frequency shifts of the
acoustic and optical mode density of states measured by first order Raman. The
exact origin of the stronger two-phonon response is not known at present but it
suggests the possibility of enhanced electron-phonon coupling in transition
metal carbides under strain found either in the bulk NbC inclusions or at their
interfaces with Nb metal. Preliminary tunneling studies using a point contact
method show some energy gaps larger than expected for bulk NbC.Comment: Phys. Rev. B, accepte
Aluminum Oxide Layers as Possible Components for Layered Tunnel Barriers
We have studied transport properties of Nb/Al/AlOx/Nb tunnel junctions with
ultrathin aluminum oxide layers formed by (i) thermal oxidation and (ii) plasma
oxidation, before and after rapid thermal post-annealing of the completed
structures at temperatures up to 550 deg C. Post-annealing at temperatures
above 300 deg C results in a significant decrease of the tunneling conductance
of thermally-grown barriers, while plasma-grown barriers start to change only
at annealing temperatures above 450 deg C. Fitting the experimental I-V curves
of the junctions using the results of the microscopic theory of direct
tunneling shows that the annealing of thermally-grown oxides at temperatures
above 300 deg C results in a substantial increase of their average tunnel
barriers height, from ~1.8 eV to ~2.45 eV, versus the practically unchanged
height of ~2.0 eV for plasma-grown layers. This difference, together with high
endurance of annealed barriers under electric stress (breakdown field above 10
MV/cm) may enable all-AlOx and SiO2/AlOx layered "crested" barriers for
advanced floating-gate memory applications.Comment: 7 pages, 6 figure
Polymorphism in Ruddlesden-Popper : Discovery of a Hidden Phase with Distinctive Layer Stacking
We report the discovery of a novel form of Ruddlesden-Popper (RP) oxide,
which stands as the first example of long-range, coherent polymorphism in this
class of inorganic solids. Rather than the well-known, uniform stacking of
perovskite blocks ubiquitously found in RP phases, this newly discovered
polymorph of the bilayer RP phase adopts a novel stacking
sequence in which single and trilayer blocks of octahedra alternate
in a 1313 sequence. Crystals of this new polymorph are described in space group
Cmmm, although we note evidence for a competing Imcm variant. Transport
measurements at ambient pressure reveal metallic character with evidence of a
charge density wave transition with onset at T = 134 K, which lies intermediate
between that of the standard 2222 polymorph of (space group
Amam) and the trilayer RP phase, . The discovery of such
polymorphism could reverberate to the expansive range of science and
applications that rely on RP materials, particularly the recently reported
signatures of superconductivity with as high as 80 K above 14 GPa in
bilayer .Comment: 37 pages, 10 figure
- …