Superconductivity depression in ultrathin YBa2Cu3O7−δ layers in La0.7Ca0.3MnO3/YBa2Cu3O7−δ superlattices

Abstract

We report on the depression of the superconducting critical temperature of ultra superlattices. The thickness of the manganite layer is kept at 15 unit cells and the YBCO thickness is varied between N=12 and N=1 unit cells. The structural analysis using x-ray diffraction and electron microscopy shows sharp interfaces with little structural disorder. While a critical temperature, Tc=85 K is found for 12 YBCO unit cells, superconductivity is completely suppressed for YBCO layer thickness below 3 unit cells. The possible interaction between superconductivity and magnetism is investigated.2 Ferromagnetic (F) / superconductor (S) heterostructures have recently attracted much interest for applications in spin injection (three terminal) devices [1]. High Tc superconductors (HTS) and colossal magnetoresistance (CMR) oxides are interesting candidate materials because the low carrier density of the HTS and the almost full spin polarization of the CMR oxides can be combined to yield high sensitivity (gain) fast devices. A reduction of the critical current consistent with suppression of superconductivity by spin polarized quasiparticle injection has been reported by several groups in recent years [2,3], opening the door to practical devices based on complex oxides. The samples reported so far involve quite thick (50-100 nm ) YBCO layers, thus shadowing interface effects. However, interface properties are expected to play a dominant role in the physics of CMR/HTS F/S heterostructures, and extrinsic (interface alloying or roughness) or intrinsic factors (proximity effect) may deeply influence the performance of the devices. The use of superlattices (instead of bilayers) allows an in depth characterization of the interfaces with conventional structure probes like x-ray diffraction (XRD) or transmission electron microscopy (TEM). The presence of magnetism and superconductivity in this kind of samples has been reported before [4,5].In this letter we report on the growth of LCMO/YBCO superlattices with ultrathin (1 to 12 unit cells) YBCO layers and fixed LCMO thickness (15 unit cells), to investigate how robust is the superconductivity of the YBCO when its thickness is reduced in presence of magnetic layers. We have found that superconductivity is depressed in presence of the adjacent LCMO layers. A structural analysis with XRD and TEM is used to explore the influence of interface disorder on the depression of the superconductivity.Samples were grown in a high pressure (3.4 mbar) pure oxygen sputtering system at high temperatures (900 ºC). Individual YBCO films on STO (100) given the small thickness of the layer is consistent with the absence of interdiffusion.At this point it seems very unlikely that the systematic depression of the critical temperature when the YBCO thickness is reduced might result of extrinsic factors like deoxygenation or roughness. Another source of Tc depression in ultrathin layers is the reduced dimensionality [9]. In this context, we compare the depression of the critical temperature when the Y...

Competition between Covalent Bonding and Charge Transfer at Complex-Oxide Interfaces

“…1(c), show that the interfaces are chemically abrupt within the precision of the technique, limited by the unavoidable formation of amorphous layers during specimen preparation (Supplemental Material [33]). The abrupt interface is consistent with previous x-ray work [23].…”

“…The presence of a significant number of oxygen vacancies is unlikely because the samples are grown in a high-oxygen pressure environment [23]. However, oxygen vacancies (difficult to detect) dope the system with electrons.…”

“…A high magnification Z-contrast image of a La 0.7 Ca 0.3 MnO 3 =YBCO=La 0.7 Ca 0.3 MnO 3 trilayer [23] is shown in Fig. 1(a).…”

Spin diffusion versus proximity effect at ferromagnet/superconductorLa0.7Ca0.3MnO3∕YBa2Cu3OPeña1, Visani2, García‐Barriocanal3 et al. 2006Phys. Rev. BSelf Cite541251View full textAdd to dashboardCite

“…Further details about growth and structure can be found elsewhere. 19,[27][28][29] In particular transmission electron microscopy shows flat interfaces over nanometer length scales. 29 X-ray refinement techniques using the SUPREX 9.0 software were used to obtain quantitative information about the interface roughness at longer length scales.…”

“…[6][7][8][9][10][11][12][13] In many cases the F-S competition is obscured by interface disorder like roughness, interdiffusion, or interface alloying. With the ͑re͒dis-covery of colossal magnetoresistance ͑CMR͒ materials there has been renewed activity in the field with heterostructures involving high-T c superconductors ͑HTSs͒ and CMR materials, [14][15][16][17][18][19] which incorporate a number of interesting new ingredients. ͑1͒ F and S oxides can be chosen with the same crystalline structure, well-matching lattice parameters, and good chemical compatibility, which allows the growth of highly perfect interfaces, despite the larger complexity of these materials as compared to single-element or alloy transition metals.…”