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Register a new userRevealing the ortho-II Band Folding in YBa2Cu3O7-delta Films
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We present an angle-resolved photoelectron spectroscopy study of YBa2Cu3O7-delta films in situ grown by pulsed laser deposition. We have successfully produced underdoped surfaces with ordered oxygen vacancies within the CuO chains resulting in a clear ortho-II band folding of the Fermi surface. This indicates that order within the CuO chains affects the electronic properties of the CuO2 planes. Our results highlight the importance of having not only the correct surface carrier concentration, but also a very well ordered and clean surface in order that photoemission data on this compound be representative of the bulk.
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Temperature dependences of the magnetic moment have been measured in YBa_2Cu_3O_{7-delta} thin films over a wide magnetic field range (5 <= H <= 10^4 Oe). In these films a paramagnetic signal known as the paramagnetic Meissner effect has been observed. The experimental data in the films, which have strong pinning and high critical current densities (J_c ~ 2 times 10^6 A/cm^2 at 77 K), are quantitatively shown to be highly consistent with the theoretical model proposed by Koshelev and Larkin [Phys. Rev. B 52, 13559 (1995)]. This finding indicates that the origin of the paramagnetic effect is ultimately associated with nucleation and inhomogeneous spatial redistribution of magnetic vortices in a sample which is cooled down in a magnetic field. It is also shown that the distribution of vortices is extremely sensitive to the interplay of film properties and the real experimental conditions of the measurements.
We have developed a masked ion irradiation technique to engineer the energy landscape for vortices in oxide superconductors. This approach associates the possibility to design the landscape geometry at the nanoscale with the unique capability to adjust depth of the energy wells for vortices. This enabled us to unveil the key role of vortex channeling in modulating the amplitude of the field matching effects with the artificial energy landscape, and to make the latter govern flux dynamics over an usually wide range of temperatures and applied fields.
In this letter, we present the superconducting property characterization of a phase pure reasonably good quality YBa2Cu3O7-{delta} sample. Studied compound is crystallized in orthorhombic Pmmm space group with lattice parameters a, b, and c are 3.829(2) {AA}, 3.887(1) {AA} and 11.666(3) {AA} respectively. Bulk superconductivity is observed below 90K as evidenced by resistivity and dc/ac magnetization measurements. The resistivity under magnetic field ({rho}TH) measurements showed clearly both the intra-grain and inter-grain transitions, which are supplemented by detailed (varying frequency and amplitude) ac susceptibility studies as well. The upper critical field at 0K i.e., Hc2(0) being determined from {rho}TH measurements with 50% criteria of resistivity drope is ~ 70 Tesla. Studied polycrystalline YBa2Cu3O7-{delta} is subjected to detailed heat capacity (CP) studies. Cp exhibited well defined anomaly at below 90 K, which decreases with applied field. Though the Cp anomaly/peak at Tc reduces with applied field, the same is not completely suppressed in high applied fields of up to 12 Tesla. The Sommerfeld constant ({gamma}) and Debye temperature ({Theta}D) as determined from low temperature fitting of CP(T) data to Sommerfeld-Debye model, are 10.65 mJ/mole-K2 and 312.3 K respectively. The results are compared with existing literature on bulk polycrystalline superconducting YBa2Cu3O7-{delta} sample
Using spin polarized neutron reflectivity experiments, we demonstrate an unusual proximity behaviour when the superconductor (SC) and the ferromagnet (FM) are coupled through an insulator (I) in YBa2Cu3O7-{delta} (SC)/SrTiO3 (I)/La0.67Sr0.33MnO3 (FM) heterostructures. We have observed an unexpected magnetic modulation at the interface region of the FM below the superconducting transition temperature. The magnetization of the FM layer at the I/FM interface was drastically reduced as compared to the magnetization in the rest of the FM layer. This result indicates that the Cooper pairs tunnel across the insulator and interact with the local magnetization at the interface region (extending ~ 30 {AA}) of the FM causing modification of the magnetization at the interface. This unexpected magnetic behavior cannot be explained on the basis of the existing theoretical models. However, the length scale associated here clearly suggests the long range proximity effect as a result of tunneling of Cooper pairs.
We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa$_2$Cu$_3$O$_{7-x}$/LaMnO$_{3+delta}$/YBa$_2$Cu$_3$O$_{7-x}$) tunnel junctions. At a large bias voltage $Vsim 1$ V we observe a step-like onset of excess current that occurs below the superconducting transition temperature $T<T_c$ and is easily suppressed by a magnetic field. The phenomenon is attributed to a novel type of the superconducting proximity effect of non-equilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunneling process. The occurrence of a strongly non-equilibrium population is confirmed by the detection of photon emission at large bias voltage. Since the conduction band in our ferromagnetic insulator is strongly spin polarized, the long-range (20 nm) of the observed proximity effect provides evidence for an unconventional spin-triplet superconducting state.
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