Proximity effect in multilayer structures with alternating ferromagnetic and normal layers
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Solid-state physics
London penetration depth
The penetration depth /spl lambda/(T) dependence on temperatures for high T/sub c/ superconducting YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films stored in various environments was measured by a well-designed microwave dielectric resonator. A d-wave T/sup 2/ dependence was observed at low temperatures, while an exponential dependence of the penetration depth /spl lambda/(T) relevant to the s wave was detected as temperature increases due to thermal fluctuation. An abnormal upturn of the penetration depth at temperatures below 10 K attributed to the surface current carried by the defect surface-induced Andreev bound states can be apparently observed without applying heavy-ion bombardment from this relatively higher frequency measurement. Readers who endeavor to start this kind of measurement can use the well-modified dielectric cavity in conjunction with the detailed measuring procedure.
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The equation giving the current density as a functional of the vector potential for an impurity superconductor derived by Mattis and Bardeen is used to compute the temperature dependence of the penetration depth of impure superconductors. Results of calculations for different values of the ratio of the coherence distance to the mean free path and also for different values of the ratio of the coherence distance to the London penetration depth are given. The results are applied to tin as an example, and appreciable deviations from the ${[1\ensuremath{-}{(\frac{T}{{T}_{c}})}^{4}]}^{\ensuremath{-}\frac{1}{2}}$ temperature dependence of the penetration depth are found for all values of the mean free path.
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We have investigated the doping dependence of the penetration depth vs. temperature in electron doped Pr$_{2-x}$Ce$_x$CuO$_{4-δ}$ using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a $d_{x^2-y^2}$ pairing gap in the underlying spectrum, we find nodeless behavior of the low-$T$ penetration depth in underdoped case, in accord with experimental results. As doping increases, a linear-in-$T$ behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.
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The magnetic penetration depth formula in MgB 2 is more complex than that in BCS superconductors due to the existence of exotic two energy gap in MgB 2 . A new simplified relationship between the penetration depth and temperature is presented, which is derived from the two-fluid model by means of numerical fitting method, and the physical meaning is relatively clear.
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We have investigated the behaviour of the depairing current J_{dp} in ferromagnet/superconductor/ferromagnet (F/S/F) trilayers as function of the thickness d_s of the superconducting layers. Theoretically, J_{dp} depends on the superconducting order parameter or the pair density function, which is not homogeneous across the film due to the proximity effect. We use a proximity effect model with two parameters (proximity strength and interface transparency), which can also describe the dependence of the superconducting transition temperature T_c on d_s. We compare the computations with the experimentally determined zero-field critical current J_{c0} of small strips (typically 5~ \mu m wide) of Fe/Nb/Fe trilayers with varying thickness d_{Nb} of the Nb layer. Near T_c the temperature dependence J_{c0}(T) is in good agreement with the expected behaviour, which allows extrapolation to T = 0. Both the absolute values of J_{c0}(0) and the dependence on d_{Nb} agree with the expectations for the depairing current. We conclude that J_{dp} is correctly determined, notwithstanding the fact that the strip width is larger than both the superconducting penetration depth and the superconducting coherence length, and that J_{dp}(d_s) is correctly described by the model.
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Magnetic penetration depth \ensuremath{\lambda} in Nb/Al and Nb/Cu superconducting bilayers, its absolute values and temperature dependencies under the variations of material and sample parameters are systematically investigated. Measurements of \ensuremath{\lambda} are performed by using the two-coil mutual inductance technique and the results are compared with a recent microscopic theory [Phys. Rev. B 59, 14 630 (1999)] based upon Eilenberger-Usadel's quasiclassical equations of superconductivity. Our results show that the $\ensuremath{\lambda}(T)$ characteristics at low temperatures usually follow a power law, or a linear temperature dependence, if the sublayer thicknesses are less than several times of their coherence length and the interface resistance is small. A two-gap, steplike feature in the $\ensuremath{\lambda}(T)$ curve of the Nb/Al bilayer system will develop around the sublayer transition temperature of Al when the interface resistance increases or when the Al layer thickness becomes much larger than its coherence length. These results can be well described within the framework of the microscopic theory.
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At present, there is no complete theory of superconductivity, which entails the need to solve a number of important research problems. In particular, is no expression capable of describing the critical temperature of all superconducting materials. Superconductors in the transition from their superconducting to normal state are considered. To this end, an analogy between the plasmons in metal and oscillations of superconducting electrons is taken, based of which expressions for the critical temperature and superconducting particle velocity as functions of coherence length and London penetration depth are obtained. The obtained dependences show that the critical temperature increases with increasing the coherence length and decreases with increasing the London penetration depth. The possible coherence length and London penetration depth values for possible room-temperature superconductors are given. The theoretical values of the superconductor critical temperature are in good agreement with the experimentally measured values for organic and cuprate superconductors. Based on the obtained expression for the critical temperature, a generalization of the expression for some alloys, metal compounds, Chevrel phases, and iron pnictides is proposed.
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London penetration depth AL was calculated for superconductors having the lifetime broadened Bardeen-Cooper-Schrieffer (BCS) density of states. With the increase of gap states, .1L increases and the temperature dependence deviates from the ordinary BCS form. The model gives a T2 dependence of AL in the case of weak-coupling BCS. The results are discussed referring some experimental results of magnetic field penetration depth of oxide superconductors.
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To address the issue of the pairing state in high-temperature superconductors, four different experimental investigations have been carried out on a single high-quality, untwinned single crystal of ${\mathrm{LuBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. These include measurements of the specific heat in magnetic fields, both near the transition temperature and at low temperatures, investigations of the angular dependence of the transverse magnetic moment in the nonlinear Meissner regime, and the measurement of microwave properties leading to the temperature dependences of the penetration depth and the surface resistance at low temperatures. Some of the results raise questions relating to the existence of a pairing state with line nodes in the energy gap, whereas others appear to be consistent with this picture. The results taken together suggest that studies of the pairing state be examined critically for possible alternative explanations. \textcopyright{} 1996 The American Physical Society.
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