Anomalous transport in normal-superconducting and ferromagnetic-superconducting nanostructures

Abstract

We have calculated the temperature dependence of the conductance variation (δS(T )) of mesoscopic superconductor normal metal(S/N) structures, in the diffusive regime, analysing both weak and strong proximity effects. We show that in the case of a weak proximity effect there are two peaks in the dependence of δS(T ) on temperature. One of them (known from previous studies) corresponds to a temperature T1 of order of the Thouless energy (ǫ T h ), and another, newly predicted maximum, occurs at a temperature T2 where the energy gap in the superconductor ∆(T2) is of order ǫ T h . In the limit L φ < L the temperature T1 is determined by Dh/L 2 φ (L φ is the phase breaking length), and not ǫ T h . We have also calculated the voltage dependence δS(V ) for a S/F structure (F is a ferromagnet) and predict non-monotonic behaviour at voltages of order the Zeeman splitting.Pacs numbers: 74.25.fy, 72.10Bg, 73.40Gk, 74.50.+r Since the late 1970's it has been known that the conductance (G) of S/N mesoscopic structures depends on temperature (T ) (and voltage (V )) in a non-monotonic way (see reviews [1,2]). This behaviour was first predicted in Ref.[3] where a simple point S/N contact was analysed. The authors of Ref.[3], using a microscopic theory and assuming that the energy gap in the superconductor (∆) is much less than the Thouless energy ǫ T h ≡hD/L 2 (D is the diffusion constant), showed that the zero-bias conductance G coincides at zero temperature with its normal state value (G n ). With increasing T , G exhibits a non-monotonic behaviour, increasing to a maximum of G max ≈ 1.25G n at T m ≈ ∆(T m ) and then decreasing to G n for T > T m .Recently mesoscopic S/N structures have been fabricated in which the limit ∆ >> ǫ T h is realised. In this case Nazarov and Stoof [4] (also see [5][6][7]) argued that the temperature dependence of the conductance G has a similar non-monotonic behaviour with a maximum at a temperature comparable with the Thouless energy, while simultaniously Volkov, Allsopp and Lambert [8] predicted that the voltage dependence of the conductance in an S/N mesoscopic structure (Andreev interferometer) has a similar form with a maximum at eV m ≈ ǫ T h . This non-monotonic behaviour has been observed both in very short S/N contacts [9] and in longer mesoscopic S/N structures [10][11][12]21]. In ref [6] it was noted that the conductance δG = G − G n consists of two contributions. The first, δG DOS , is negative due to a proximity effect induced decrease in the density of states (DOS) of the normal wire which makes contact with a superconducting strip [13]. The other contribution δG MT (positive) is analogous to the Maki-Thompson (MT) contribution to the paraconductivity of S/N/S and N/S/N mesoscopic structures and was calculated in [14]. At T = 0 and V = 0 both contributions to the conductance are equal, as T or V increase the contribution δG MT dominates until a maximum is reached, then both these contributions decay.During the past decade a great deal of interest in the transport properties of N/S n...

Non-equilibrium supercurrent through mesoscopic ferromagnetic weak links

“…It has been predicted that devices with tunnel junctions [2] and systems with good metallic contacts [3] can enter a peculiar mesoscopic non-equilibrium state at low temperatures, which even allows reversing the supercurrent, turning the system into a π-junction. This phenomenon has been verified experimentally [1].Another phenomenon of high interest in superconducting mesoscopics is the combination of ferromagnetic (F) elements with superconductors (S) [4][5][6][7][8]. A strong exchange interaction h in the ferromagnet is expected to suppress the superconducting proximity effect, and hence also the supercurrent.…”

Ferromagnetic Josephson Junction with Precessing Magnetization

“…Such non analytic temperature dependence was already met in the field of transport in N/S junctions [13].…”

“…2). Let us now compare both main contributions (13) and (15) to I c . They have quite different dependences on the device parameters.…”

Resonant peak in the density of states in normal-metal/diffusive-ferromagnet/superconductor junctions

“…The order parameter oscillations also lead to nonmonotonous dependence of T c in SF bilayers on the F-layer thickness 9,10,11,12,13 . Effects of resonant transmission in conductivity of SF structures were discussed in Ref 14,15,16 .Another interesting consequence of the oscillations of the pair amplitude is the spatially damped oscillating behavior of the density of states (DOS) in a ferromagnet predicted theoretically 17,18,19,20 in various regimes. The energy dependent DOS calculated in the clean 18 and the dirty 21 limits exhibits rich structures.…”

“…The order parameter oscillations also lead to nonmonotonous dependence of T c in SF bilayers on the F-layer thickness 9,10,11,12,13 . Effects of resonant transmission in conductivity of SF structures were discussed in Ref 14,15,16 .…”