125 research outputs found
Seebeck effect in the conducting LaAlO_{3}/SrTiO_{3} interface
The observation of metallic behavior at the interface between insulating
oxides has triggered worldwide efforts to shed light on the physics of these
systems and clarify some still open issues, among which the dimensional
character of the conducting system. In order to address this issue, we measure
electrical transport (Seebeck effect, Hall effect and conductivity) in
LaAlO_{3}/SrTiO_{3} interfaces and, for comparison, in a doped SrTiO_{3} bulk
single crystal. In these experiments, the carrier concentration is tuned, using
the field effect in a back gate geometry. The combined analysis of all
experimental data at 77 K indicates that the thickness of the conducting layer
is ~7 nm and that the Seebeck effect data are well described by a
two-dimensional (2D) density of states. We find that the back gate voltage is
effective in varying not only the charge density, but also the thickness of the
conducting layer, which is found to change by a factor of ~2, using an electric
field between -4 and +4MV/m at 77K. No enhancement of the Seebeck effect due to
the electronic confinement and no evidence for two-dimensional quantization
steps are observed at the interfaces.Comment: 15 pages, 5 figure
The Crossover beteween Aslamazov-Larkin and Short Wavelength Fluctuations Regimes in HTS Conductivity Experiments
We present paraconductivity (AL) measurements in three different high
temperature superconductors: a melt textured sample, a
epitaxial thin film and a highly textured
tape. The crossovers between different temperature
regimes in excess conductivity have been analysed. The Lawrence-Doniach (LD)
crossover, which separates the 2D and 3D regimes, shifts from lower to higher
temperatures as the compound anisotropy decreases. Once the LD crossover is
overcome, the fluctuation conductivity of the three compounds shows the same
universal behaviour: for all the curves bend down
according to the law. This asymptotic behaviour was
theoretically predicted previously for the high temperature region where the
short wavelength fluctuations (SWF) become important.Comment: 4 pages, Revtex, 1 PostScript figure available upon request
([email protected]); submitted to Physical Review B rapid communication
Influence of Mandrel s Surface on the Mechanical Properties of Joints Produced by Electromagnetic Compression
Electromagnetic compression of tubular profiles with high electrical conductivity is an innovative joining process for the manufacturing of lightweight structures. Taking conventional interference fits into account, the contact area s influence on the joint s quality seems to be of significance, as e.g. the contact area and the friction coefficient between the joining partners determine an allowed axial load or torsional momentum proportionally. Therefore, different contact area surfaces were prepared by shot peening and different machining operations and strategies. The mandrel s surfaces were prepared by shot peening with glass beads and Al2O3 particles. Alternatively, preparation was done using simultaneous five axis milling, because potential joining partners in lightweight frame structures within the Transregional Collaborative Research Centre SFB/TR10 would be manufactured similarly. After that, the manufactured surfaces were characterized by measuring the surface roughness and using confocal whitelight microscopy. After joining by electromagnetic compression, the influence of different mandrel s surface conditions on the joint s mechanical properties were analyzed by tensile tests. Finally, conclusions and design rules for the manufacturing of joints by electromagnetic compression are given
The Randomized Shortened Dental Arch Study: Tooth Loss
The evidence concerning the management of shortened dental arch (SDA) cases is sparse. This multi-center study was aimed at generating data on outcomes and survival rates for two common treatments, removable dental prostheses (RDP) for molar replacement or no replacement (SDA). The hypothesis was that the treatments lead to different incidences of tooth loss. We included 215 patients with complete molar loss in one jaw. Molars were either replaced by RDP or not replaced, according to the SDA concept. First tooth loss after treatment was the primary outcome measure. This event occurred in 13 patients in the RDP group and nine patients in the SDA group. The respective Kaplan-Meier survival rates at 38 months were 0.83 (95% CI: 0.74-0.91) in the RDP group and 0.86 (95% CI: 0.78-0.95) in the SDA group, the difference being non-significant
Development of design principles for form-fit joints in lightweight frame structures
Based on fundamental technological investigations, alternative joining strategies using electromagnetic forming (EMF) for the flexible production of lightweight frame structures are developed in the collaborative research project SFB/TR10. The results of these investigations will also be used to create general design principles for the joining process itself as well as for the joining zone. The focus of this article will be on dominating form-fit joints of aluminum frame structures and the parameters which have a significant influence on the strength of those joints. For the development of design principles regarding the joining zone, the groove geometry of the connection elements was varied in terms of size and shape, and the influence of those variations was analyzed. In terms of the joining process itself the effect on the joint strength of different forming pressures for a given groove geometry was also investigated. In the first step these experiments were performed on solid mandrels. In order to reduce the weight of the structure, experiments were then performed with hollow connection elements and similar groove geometries to analyze how the reduced stiffness of those elements affected the strength of the joints
In-situ Magnesium Diboride Superconducting Thin Films grown by Pulsed Laser Deposition
Superconducting thin films of MgB2 were deposited by Pulsed Laser Deposition
on magnesium oxide and sapphire substrates. Samples grown at 450C in an argon
buffer pressure of about 10-2 mbar by using a magnesium enriched target
resulted to be superconducting with a transition temperature of about 25 K.
Film deposited from a MgB2 sintered pellet target in ultra high vacuum
conditions showed poor metallic or weak semiconducting behavior and they became
superconducting only after an ex-situ annealing in Mg vapor atmosphere. Up to
now, no difference in the superconducting properties of the films obtained by
these two procedures has been evidenced.Comment: 10 pages, 4 figure
Epitaxial MgB2 thin films on ZrB2 buffer layers: structural characterization by synchrotron radiation
Structural and superconducting properties of magnesium diboride thin films
grown by pulsed laser deposition on zirconium diboride buffer layers were
studied. We demonstrate that the ZrB2 layer is compatible with the MgB2 two
step deposition process. Synchrotron radiation measurements, in particular
anomalous diffraction measurements, allowed to separate MgB2 peaks from ZrB2
ones and revealed that both layers have a single in plane orientation with a
sharp interface between them. Moreover, the buffer layer avoids oxygen
contamination from the sapphire substrate. The critical temperature of this
film is near 37.6 K and the upper critical field measured at Grenoble High
Magnetic Field Laboratory up to 20.3 T is comparable with the highest ones
reported in literature.Comment: 14 pages, 8 figures, submitted to Supercond. Sci. and Techno
Anisotropy in c-oriented MgB2 thin films grown by Pulsed Laser Deposition
The electronic anisotropy in MgB2, is still a not completely clear topic;
high quality c-oriented films are suitable systems to investigate this
property. In this work we present our results on MgB2 superconducting thin
films grown on MgO and sapphire substrates. The films are deposited in high
vacuum, at room temperature, by Laser Ablation, starting from two different
targets: pure Boron and stoichiometric MgB2. In both cases, to obtain and
crystallize the superconducting phase, an ex-situ annealing in magnesium vapor
is needed. The films were characterized by Synchrotron radiation diffraction
measurements; the films turned out to be strongly c-oriented, with the c-axis
perpendicular to the film surface and an influence of the substrate on
crystallographic parameters is observed. Resisivity measurements with the
magnetic field perpendicular and parallel directions to the film surface
evidenced an anisotropic upper critical field behavior. The Hc2 ratios (h)
resulted in the range 1.2-1.8; this difference will be discussed also in
comparison with the literature data.Comment: presented at ISS 2001,Kobe,Japan. submitted to Physica
Magnetoresistivity in MgB2 as a probe of disorder in p- and s-bands
In this paper we present normal state magnetoresistivity data of magnesium
diboride epitaxial thin films with different levels of disorder, measured at
42K in magnetic fields up to 45 Tesla. Disorder was introduced in a controlled
way either by means of neutron irradiation or by carbon doping. From a
quantitative analysis of the magnetoresistivity curves with the magnetic field
either parallel or perpendicular to the plane of the film, we extract the ratio
of the scattering times in p- and s-bands. We demonstrate that the undoped
unirradiated thin film has p scattering times smaller than s ones; upon
irradiation, both bands become increasingly more disordered; eventually the
highly irradiated sample (neutron fluence 7.7X1017 cm-2) and the C-doped sample
have comparable scattering times in the two types of bands. This description of
the effect of disorder in the two kinds of bands on transport is consistent
with the residual resistivity values and with the temperature dependence of the
resistivity.Comment: 19 pages, 3 tables, 2 figure
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