267 research outputs found
Current distribution in wide YBCO tapes
The need of a better mechanical behaviour and the stabilization of coated conductors for applications, as Magnets, cables or Fault Current Limiters, has motivated the lamination of tapes with stainless steel or copper alloys, increasing so the elastic modulus of the conductors and their mechanical performance. Some of the stainless steels used are magnetic, thus introducing some perturbations of the current flow when energizing the conductor. In order to detect these possible perturbations, the magnetic self field in the surface of the tape has been explored by Hall mapping technique at several current loads in a monotonically driven cyclic sequence. By increasing current steps when loading up, crossing the critical field threshold, and decreasing down to remanent state. Deviation from the expected magnetic map has been observed. In this work, we will report on the resulting measurements, and the current flow is calculated by solving the inverse problem for a 12 mm wide stainless steel reinforced Coated Conductor tape. We discuss on the likely origin of the observed perturbations.We would like to acknowledge the support of Nanoselect project of the CONSOLIDER program, and EU-FP7-ECCOFLOW project, and Xermae
Magnetic transitions in Pr2NiO4 single crystal
The magnetic properties of a stoichiometric Pr2NiO4 single crystal have been examined by means of the temperature dependence of the complex ac susceptibility and the isothermal magnetization in fields up to 200 kOe at T=4.2 K. Three separate phases have been identified and their anisotropic character has been analyzed. A collinear antiferromagnetic phase appears first between TN = 325 K and Tc1 = 115 K, where the Pr ions are polarized by an internal magnetic field. At Tc1 a first modification of the magnetic structure occurs in parallel with a structural phase transition (Bmab to P42/ncm). This magnetic transition has a firstâorder character and involves both the outâofâplane and the inâplane spin components (magnetic modes gx and gxcyfz, respectively). A second magnetic transition having also a firstâorder character is also clearly identified at Tc2 = 90 K which corresponds to a spin reorientation process (gxcyfz to cxgyaz magnetic modes). It should be noted as well that the outâofâphase component of Ïac shows a peak around 30 K which reflects the coexistence of both magnetic configurations in a wide temperature interval. Finally, two fieldâinduced transitions have been observed at 4.2 K when the field is directed along the c axis. We propose that the highâfield anomaly arises from a metamagnetic transition of the weak ferromagnetic component, similarly to La2CuO4
Ultra-high critical current densities of superconducting YBaCuO thin films in the overdoped state
Doping is one of the most relevant paths to tune the functionality of
cuprates, it determines carrier density and the overall physical properties of
these impressive superconducting materials. We present an oxygen doping study
of YBaCuO (YBCO) thin films from underdoped to overdoped
state, correlating the measured charge carrier density, , the
hole doping, , and the critical current density, . Our results
show a continuous increase of with charge carrier density,
reaching 90 MA/cm at 5 K for -doping at the Quantum Critical Point
(QCP), linked to an increase of the superconducting condensation energy. The
ultra-high achived corresponds to a third of the depairing
current, i.e. a value 60 % higher than ever reported in YBCO films. The
overdoped regime is characterized by a sudden increase of ,
associated to the reconstruction of the Fermi-surface at the QCP. Overdoping
YBCO opens a promising route to extend the current carrying capabilities of
REBCO coated conductors for applications
Effect of strain and magnetic field on the critical current and electric resistance of the joints between HTS coated conductors
Engineering of devices and systems such as magnets, fault current limiters or cables, based on
High Temperature Superconducting wires requires a deep characterization of the possible
degradation of their properties by handling at room temperature as well as during the service
life thus establishing the limits for building up functional devices and systems.
In the present work we report our study regarding the mechanical behavior of spliced joints
between commercial HTS coated conductors based on YBCO at room temperature and
service temperature, 77 K.
Tensile tests under axial stress and the evolution of the critical current and the electric
resistance of the joints have been measured. The complete strain contour for the tape and the
joint has been obtained by using Digital Image Correlation. Also, tensile tests under external
magnetic field have been performed and the effect of the applied field on the critical current
and the electric resistance of the joints has been studied.
Finally, a preliminary numerical study by means of Finite Element Method (FEM) of the
mechanical behavior of the joints between commercial HTS is presented
Effect of strain and magnetic field on the critical current and electric resistance of the joints between HTS coated conductors
Engineering of devices and systems such as magnets, fault current limiters or cables, based
on High Temperature Superconducting wires requires a deep characterization of the possible
degradation of their properties by handling at room temperature as well as during the service
life thus establishing the limits for building up functional devices and systems.
In the present work we report our study regarding the mechanical behavior of spliced joints
between commercial HTS coated conductors based on YBCO at room temperature and service
temperature, 77 K.
Tensile tests under axial stress and the evolution of the critical current and the electric resistance
of the joints have been measured. The complete strain contour for the tape and the joints
has been obtained by using Digital Image Correlation. Also, tensile tests under external magnetic
field have been performed and the effect of the applied field on the critical current and
the electric resistance of the joints has been studied. Additionally, fatigue tests under constant
cyclic stress and loading-unloading ramps have been carried out in order to evaluate the electromechanical
behavior of the joints and the effect of maximum applied stress on the critical current.
Finally, a preliminary numerical study by means of the Finite Element Method (FEM) of the
electromechanical behavior of the joints between commercial HTS is presented
Vortex liquid entanglement in twinned YBa_2Cu_3O_7 /Y_2BaCuO_5 composite superconductors
The angular dependence of the in-plane resistivity Ï(T,H, Ξ) of melt textured YBa_2Cu_3O_7/Y_2BaCuO_5 composites has been measured in a large range of magnetic fields and temperatures and from them, the intrinsic anisotropy of the superconducting state has been verified following the anisotropic Ginzburg-Landau approach. The influence of correlated defects like twin boundaries and quenched disorder generated by Y_2BaCuO_5 precipitates on the pinning behavior of these composites in the liquid vortex state is analyzed, and the corresponding phase diagram is determined and compared to that of twinned single crystals. We show that the irreversibility line displays an upwards shift due to twin boundary pinning enabling to define a ''quenched'' Bose glass transition. A new region in the vortex liquid state is identified where twin boundary pinning defines a partially entangled liquid vortex state characterized by a short-range c-axis vortex coherence. The transition to the entangled liquid phase is experimentally determined. The relevance of this depinning line and its unique position with respect to twinned single crystals is discussed. [S0163- 1829(99)01741-5]
Observation of insulator-metal transition in EuNiO under high pressure
The charge transfer antiferromagnetic (T =220 K) insulator EuNiO
undergoes, at ambient pressure, a temperature-induced metal insulator MI
transition at T=463 K. We have investigated the effect of pressure (up
to p~20 GPa) on the electronic, magnetic and structural properties of
EuNiO using electrical resistance measurements, {151}^Eu nuclear
resonance scattering of synchrotron radiation and x-ray diffraction,
respectively. With increasing pressure we find at p =5.8 GPa a transition
from the insulating state to a metallic state, while the orthorhombic structure
remains unchanged up to 20 GPa. The results are explained in terms of a gradual
increase of the electronic bandwidth with increasing pressure, which results in
a closing of the charge transfer gap. It is further shown that the
pressure-induced metallic state exhibits magnetic order with a lowervalue of
T (T ~120 K at 9.4 GPa) which disappears between 9.4 and 14.4 GPa.Comment: 10 pages, 3 figure
A hidden Goldstone mechanism in the Kagom\'e lattice antiferromagnet
In this paper, we study the phases of the Heisenberg model on the \kagome
lattice with antiferromagnetic nearest neighbour coupling and
ferromagnetic next neighbour coupling . Analysing the long wavelength, low
energy effective action that describes this model, we arrive at the phase
diagram as a function of . The interesting part of
this phase diagram is that for small , which includes , there is
a phase with no long range spin order and with gapless and spin zero low lying
excitations. We discuss our results in the context of earlier, numerical and
experimental work.Comment: 21 pages, latex file with 5 figure
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