997 research outputs found
Influence of the sample geometry on the vortex matter in superconducting microstructures
The dependence of the vortex penetration and expulsion on the geometry of
mesoscopic superconductors is reported. Hall magnetometry measurements were
performed on a superconducting Al square and triangle. The stability of the
vortex patterns imposed by the sample geometry is discussed. The
field-temperature diagram has been reconstructed showing the transitions
between states with different vorticity. We have found that the vortex
penetration is only weakly affected by the vortex configuration inside the
sample while the expulsion is strongly controlled by the stability of the
vortex patterns. A qualitative explanation for this observation is given.Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev.
Linear magnetic flux amplifier
By measuring the critical current versus the applied magnetic field
of an Al superconducting loop enclosing a soft Permalloy magnetic
dot, we demonstrate that it is feasible to design a linear magnetic flux
amplifier for applications in superconducting quantum interference devices. The
selected dimensions of a single-domain Permalloy dot provide that the
preferential orientation of the magnetization is rotated from the perpendicular
direction. By increasing an applied magnetic field, the magnetization of the
dot coherently rotates towards the out-of-plane direction, thus providing a
flux gain and an enhancement of the sensitivity. As a result of a pronounced
shape anisotropy, the flux gain generated by the dot can be tuned by adjusting
the dimensions of the dot.Comment: to appear in Applied Physics Letter
Flux Confinement in Mesoscopic Superconductors
We report on flux confinement effects in superconducting submicron line, loop
and dot structures. The main idea of our study was to vary the boundary
conditions for confinement of the superconducting condensate by taking samples
of different topology and, through that, modifying the lowest Landau level
E_{LLL}(H). Since the critical temperature versus applied magnetic field
T_{c}(H) is, in fact, E_{LLL}(H) measured in temperature units, it is varied as
well when the sample topology is changed. We demonstrate that in all studied
submicron structures the shape of the T_{c}(H) phase boundary is determined by
the confinement topology in a unique way.Comment: 10 pages, 5 EPS figures, uses LaTeX's sup.sty, contribution to a
special issue of "Superlattices and Microstructures
Tunable Field Induced Superconductivity
We investigate the transport properties of a thin superconducting Al layer
covering a square array of magnetic dots with out-of-plane magnetization. A
thorough characterization of the magnetic properties of the dots allowed us to
fine-tune their magnetic state at will, hereby changing the influence of the
dots on the superconductor in a continuous way. We show that even though the
number of vortex-antivortex pairs discretely increases with increasing the
magnetization of the dots, no corresponding discontinuity is observed in the
resistance of the sample. The evolution of the superconducting phase boundary
as the magnetic state of the dots is swept permits one to devise a fully
controllable and erasable field induced superconductor
Vortex melting and decoupling transitions in YBaCuO single crystals
The vortex correlation along the c-axis in high quality single crystals of
YBaCuO has been investigated as a function of temperature T
in different magnetic fields, using the quasi-flux transformer configuration. A
simultaneous sharp drop associated with the vortex lattice melting is observed
in both the primary and secondary voltages(V and V). Just above
the melting temperature, the vortices form three-dimensional line liquid with
the correlation length along the c direction t, the sample
thickness. The temperature where a resistive peak in R develops
corresponds to the decoupling temperature T at which the vortices loose
their correlation along the c-direction and they dissolve into the two
dimensional pancake vortices. The H-T phase diagram for the
YBaCuO single crystal is obtained.Comment: 1 Text file, 3 eps figure
Magnetic-field dependence of the spin states of the negatively charged exciton in GaAs quantum wells
We present high-field (<50 T) photoluminescence measurements of the binding energy of the singlet and triplet states of the negatively charged exciton in a 200-Angstrom quantum well. Comparing our data with those of other groups and with theoretical predictions we clearly show how the singlet, "bright" and "dark" triplet states may be identified according to the high-field dependence of their binding energies. We demonstrate that a very consistent behavior of the binding energy in a magnetic field has been observed in quantum wells of different widths by different groups and conclude that the triplet state found in this, as well as nearly all other experiments, is undoubtedly the bright triplet. By combining our data with that in the literature we are able to present the generic form of the binding energy of the spin states of the charged exciton in a magnetic field, which reveals the predicted singlet to dark triplet ground state transition at about 20 T
Resistive switching in ultra-thin La0.7Sr0.3MnO3 / SrRuO3 superlattices
Superlattices may play an important role in next generation electronic and
spintronic devices if the key-challenge of the reading and writing data can be
solved. This challenge emerges from the coupling of low dimensional individual
layers with macroscopic world. Here we report the study of the resistive
switching characteristics of a of hybrid structure made out of a superlattice
with ultrathin layers of two ferromagnetic metallic oxides, La0.7Sr0.3MnO3
(LSMO) and SrRuO3 (SRO). Bipolar resistive switching memory effects are
measured on these LSMO/SRO superlattices, and the observed switching is
explainable by ohmic and space charge-limited conduction laws. It is evident
from the endurance characteristics that the on/off memory window of the cell is
greater than 14, which indicates that this cell can reliably distinguish the
stored information between high and low resistance states. The findings may
pave a way to the construction of devices based on nonvolatile resistive memory
effects
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