Suppression of superconductivity in YBCO/LCMO superlattices

Abstract

The competition of superconductivity and magnetism in superlattices composed of alternating YBa2Cu3O 7−d and La0.67Ca0.33MnO3 thin films is investigated using low-energy optical spectroscopy. The thickness of the superconducting YBCO layers is varied from 30 nm to 20 nm while the thickness of the magnetic LCMO layers is kept constant at 20 nm. We clearly observe that the superconducting condensate density in the superconducting state of superlattice is drastically reduced by the magnetic subsystem which may be connected with proximity effects that distort the gap symmetry and thus suppress superconductivity. PACS numbers: 72.25.Mk; 74.78.Bz; 75.70.Cn; Since the early 1960s the interplay of superconductivity (SC) and ferromagnetism (FM) has continuously drawn attention [1]. At first glance SC and FM exclude each other because the magnetic exchange field breaks the Cooper pairs. Nevertheless the possible coexistence of both phenomena due to a spatially modulated order parameter was suggested by Fulde, Ferrell, Larkin and Ovchinnikov [2]; the correspondent state was never observed in conventional SC. A good way to study the effect is to bring the nano-sized layers of the two materials into contact and to view the SC-FM interaction on the level of the proximity effects. The exploration of multilayer systems composed of alternating conventional SC and metallic FM layers was subject of recent reviews [3,4].In the last years artificially grown superlattices (SLs) consisting of high-T c superconductors and manganese oxides which exhibit the phenomenon of colossal magnetoresistance have attracted increasing interests for various reasons. On one hand, SLs were developed as an important tool to explore the interplay between the two antagonistic SC and FM ground states, and on the other hand the injection of spin-polarized carriers can lead to new SC switching devices [5]. Due to the structural similarity of the two classes of perovskite compounds, it is possible to construct a unique combination of SC cuprate and strongly FM manganese layers. It has been demonstrated by different groups [6,7,8,9] that although SC and FM are preserved in each subsystem, the superconducting transition temperature T c and the Curie temperature of the magnetic ordering T mag are considerably suppressed. Transport and magnetization measurements clearly indicate that spin-polarized quasiparticle injection in YBa 2 Cu 3 O 7−d /La 0.67 Ca 0.33 MnO 3 (YBCO/LCMO) SL can strongly decrease T c and the critical current J c [10,11]. Particularly interesting is the almost complete spin polarization in LCMO and the low carrier density in YBCO which seems to be suppressed even further in the case of SL, as was shown by optical spectroscopy [12]. The advantage of optical techniques is that one can directly monitor the temperature dependence of the "strength" of the conducting and supercon- ducting condensates expressed by the spectral weight, i.e. the area under the dynamical conductivity spectrum. Up to now, however, optical measurements failed to even de...

Magnetism and superconductivity in oxide ferromagnet/superconductor heterostructures

“…Recent elipsometric measurements [19] of far-infrared properties of LaCaMnO/YBCO superlattices demonstrate a hole charge transfer from LCMO to YBCO layers. This observation is also confirmed by low energy optical spectroscopy [20] indicating a reduced density of superconducting condensate at the interface between the layers. Similar effect takes place in LSMO/YBCO superlattices.…”

Ferromagnetic/superconducting bilayer structure: A model system for spin diffusion length estimation

“…A review on this topic is given by Izyumov et al 9 . In the past few years, much attention has been paid to junctions consisting of La 2/3 Ca 1/3 MnO 3 (LCMO), a material that shows a colossal magnetoresistance (CMR) effect, and of YBa 2 Cu 3 O 7−δ (YBCO), a high-T c superconductor [10][11][12][13][14][15][16][17] . Experiments with these junctions allow to obtain information about the spin-dependent properties of high-T c superconductors that can lead to the design of new superconducting devices such as "spintronic devices", like transistors with high gain current and high speed.…”

Microwave absorption in YBa2Cu3O7−δ-manganite superlattices

“…These exhibit unique properties, such as the negative refraction index discovered recently at high magnetic fields for millimeter waves [4]. While some experimental low-frequency data have been obtained on simple structures like S/F bilayers and F/S/F trilayers [5], the high-frequency properties of [S/F] n superlattices have been investigated by ellipsometry only in the far-infrared and sub-mm spectral ranges [6,7], where a substantial suppression of both superconducting and magnetic properties has been found in the case of…”