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Register a new userInterfacial roughness and proximity effects in superconductor/ferromagnet CuNi/Nb heterostructures
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We report an investigation of the structural and electronic properties of hybrid superconductor/ferromagnet (S/F) bilayers of composition Nb/Cu$_{60}$Ni$_{40}$ prepared by magnetron sputtering. X-ray and neutron reflectometry show that both the overall interfacial roughness and vertical correlations of the roughness of different interfaces are lower for heterostructures deposited on Al$_2$O$_3$(1$bar{1}$02) substrates than for those deposited on Si(111). Mutual inductance experiments were then used to study the influence of the interfacial roughness on the superconducting transition temperature, $T_C$. These measurements revealed a $sim$ 4% higher $T_C$ in heterostructures deposited on Al$_2$O$_3$, compared to those on Si. We attribute this effect to a higher mean-free path of electrons in the S layer, caused by a suppression of diffusive scattering at the interfaces. However, the dependence of the $T_C$ on the thickness of the ferromagnetic layer is not significantly different in the two systems, indicating a weak influence of the interfacial roughness on the transparency for Cooper pairs.
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We review the present status of the experimental and theoretical research on the proximity effect in heterostructures composed of superconducting (S) and ferromagnetic (F) thin films. First, we discuss traditional effects originating from the oscillatory behavior of the superconducting pair wave function in the F-layer. Then, we concentrate on recent theoretical predictions for S/F layer systems. These are a) generation of odd triplet superconductivity in the F-layer and b) ferromagnetism induced in the S-layer below the superconducting transition temperature $T_{c}$ (inverse proximity effect). The second part of the review is devoted to discussion of experiments relevant to the theoretical predictions of the first part. In particular, we present results of measurements of the critical temperature $T_{c}$ as a function of the thickness of F-layers and we review experiments indicating existence of odd triplet superconductivity, cryptoferromagnetism and inverse proximity effect.
Heterostructures made of itinerant ferromagnets and superconductors are studied. In contrast to most previous models, ferromagnetism is not enforced as an external Zeeman field but induced in a correlated single-band model (CSBM) that displays itinerant ferromagnetism as a mean-field ground state. This allows us to investigate the influence of an adjacent superconducting layer on the properties of the ferromagnet in a self-consistent Bogoliubov-de Gennes approach. The CSBM displays a variety features not present in the Zeeman exchange model that influence the behavior of order parameters close to the interface, as e.g. phase separation and the competition between magnetism and superconducting orders.
We report observation of strong magnetic proximity coupling in a heterostructured superconductor Sr$_2$VO$_3$FeAs, determined by the upper critical fields $H_{c2}(T)$ measurements up to 65 T. Using the resistivity and the radio-frequency measurements for both $H parallel ab$ and $H parallel c$, we found a strong upward curvature of $H_{c2}^c(T)$, together with a steep increase of $H_{c2}^{ab}(T)$ near $T_c$, yielding the anisotropic factor $gamma_H=H_{c2}^{ab}/H_{c2}^c$ up to $sim$ 20, the largest value among iron-based superconductors. These are attributed to the Jaccarino-Peter effect, rather than to the multiband effect, due to strong exchange interaction between itinerant Fe spins of the FeAs layers and localized V spins of Mott-insulating SrVO$_3$ layers. These findings provide evidence for strong antiferromagnetic proximity coupling, comparable with the intralayer superexchange interaction of SrVO$_3$ layer and sufficient to induce magnetic frustration in Sr$_2$VO$_3$FeAs.
We report the low-energy electrodynamics of a moderately clean A15 superconductor (SC) following ultrafast excitation to understand and manipulate terahertz (THz) quasi--particle (QP) transport by tuning pump photoexcitation of from competing orders. Using 35-fs optical pulses, we observe a non-thermal enhancement in the low frequency conductivity, opposite to that observed for THz pump, which persists up to an additional critical temperature, above the SC one, from an electronic order in the Martensitic normal state. In the SC state, the fluence dependence of pair breaking kinetics together with an analytic model provides evidence for a `one photon-to-one pair non-resonant energy transfer during the laser pulse. Such initial transfer of photon energy $hbaromega$ to QPs at the {em quantum} limit, set by $2Delta_{SC}/hbaromega$=0.33$%$, is more than one order of magnitude smaller than in previously studied BCS SCs, which we attribute to strong electron--phonon coupling and possible influence of phonon condensation in A15 SCs.
We have investigated the structural, magnetic and superconducting properties of [Nb(1.5nm)/Fe(x)]$_{10}$ superlattices deposited on a thick Nb(50nm) layer. Our investigation showed that the Nb(50nm) layer grows epitaxially at 800$^circ$C on Al$_2$O$_3$(1$bar{1}$02) substrate. Samples grown at this condition posses a high residual resistivity ratio of 15-20. By using neutron reflectometry we show that Fe/Nb superlattices with $x<$ 4 nm form a depth-modulated FeNb alloy with concentration of iron varying between 60% and 90%. This alloy has properties of a weak ferromagnet. Proximity of this weak ferromagnetic layer to a thick superconductor leads to an intermediate phase that is characterized by co-existing superconducting and normal-state domains. By increasing the thickness of the Fe layer to $x$ = 4 nm the intermediate phase disappears. We attribute the intermediate state to proximity induced non-homogeneous superconductivity in the periodic Fe/Nb structure.
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