70 research outputs found
Negative Refraction in Ferromagnet/Superconductor Superlattices
Negative refraction, which reverses many fundamental aspects of classical
optics, can be obtained in systems with negative magnetic permeability and
negative dielectric permittivity. This Letter documents an experimental
realization of negative refraction at millimeter waves, finite magnetic fields
and cryogenic temperatures utilizing a multilayer stack of ferromagnetic and
superconducting thin films. In the present case the superconducting
YBa_2Cu_3O_7 layers provide negative permittivity while negative permeability
is achieved via ferromagnetic (La:Sr)MnO_3 layers for frequencies and magnetic
fields close to the ferromagnetic resonance. In these superlattices the
refractive index can be switched between positive and negative regions using
external magnetic field as tuning parameter.Comment: 4 Pages, 3 Figures, Phys. Rev. Lett., accepte
Microwave absorption in YBa[2]Cu[3]O[7-delta]-manganite superlattices
3 pages Erreur sur l'affiliation : CEA/DSM/IRAMIS et non CEA/DSM/DRECAMInternational audienceWe report on results of direct microwave absorption measurements of Re1−x Bx MnO3 /YBa2 Cu3 O7−δ half metal/d-wave superconductor superlattices (where Re - La and Pr) and (B - Sr and Ca) for microwave frequencies in the range from 9 to 20 GHz. The measurements of the entire heterostructure were performed mostly at temperatures below the superconducting transition. The obtained results strongly depend on the microwave frequency and can be qualitatively described within the theoretical model of high-frequency properties of Josephson junctions with a ferromagnetic barrier proposed by S. Takahashi, S. Hikino, M. Mori, J. Martinek, and S. Maekawa, Phys. Rev. Lett. 99, 057003 (2007)
Andreev bound states in ferromagnet-superconductor nanostructures
We discuss the properties of a ferromagnet - superconductor heterostructure
on the basis of a Hubbard model featuring exchange splitting in the ferromagnet
and electron - electron attraction in the superconductor. We have solved the
spin - polarized Hartree - Fock - Gorkov equations together with the Maxwell's
equation (Ampere's law) fully self-consistently. We have found that a Proximity
Effect - Fulde - Ferrell - Larkin - Ovchinnikov state is realized in such a
heterostructure. It manifests itself in an oscillatory behavior of the pairing
amplitude in the ferromagnet and spontaneously generated spin polarized current
in the ground state. We argue that it is built up from the Andreev bound
states, whose energy can be tuned by the exchange splitting and hence can
coincide with the Fermi energy giving rise to a current carrying -state.
We also suggest experiments to verify these predictions.Comment: 12 pages, 5 figures included; to be published in Physica
Proximity induced metal/insulator transition in superlattices
The far-infrared dielectric response of superlattices (SL) composed of
superconducting YBaCuO (YBCO) and ferromagnetic La%
CaMnO (LCMO) has been investigated by ellipsometry. A drastic
decrease of the free carrier response is observed which involves an unusually
large length scale of d20 nm in YBCO and d10
nm in LCMO. A corresponding suppression of metallicity is not observed in SLs
where LCMO is replaced by the paramagnetic metal LaNiO. Our data suggest
that either a long range charge transfer from the YBCO to the LCMO layers or
alternatively a strong coupling of the charge carriers to the different and
competitive kind of magnetic correlations in the LCMO and YBCO layers are at
the heart of the observed metal/insulator transition. The low free carrier
response observed in the far-infrared dielectric response of the magnetic
superconductor RuSrGdCuO is possibly related to this effect
Spin-polarized quasiparticle injection effects in YBCO thin films
We report detailed transport studies on ferromagnet-superconductor
heterostructures. Epitaxial heterostructures of half-metal colossal
magnetoresistive La2/3Ca1/3MnO3 (HM-CMR) and high Tc superconducting YBa2Cu3O7
(YBCO) are grown on SrTiO3 (100) single crystal substrates by pulsed laser
deposition. Using the HM--CMR layer as source for spin-polarized
quasiparticles, we show the effect of injection of spin-polarized
quasiparticles into the ab-plane and along the c-axis of YBCO. The results show
a drop in the ab-plane resistance Rab (T) in the case of injection along the
c-axis that is discussed to be related to the opening of a pseudogap.Comment: 6 pages, 2 figure
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