In this talk, Matthias Eschrig discussed the modification of the pairing

state of a superconductor (SC) due to proximity with a magnetic material.

He began by reviewing the case of a ferromagnet (F) to SC junction, studied

by Buzdin in 1982. Due to the Fermi energy mismatch in the F, characterized

by the parameter J, one expects a split Fermi surface. This furthermore implies

pairing at a nonzero wavevector q, i.e., oscillations in the pairing phi(z)

as a function of the position z in the F region:

phi(z) ~ Exp[-z/xi1]*Exp[i z/xi2]

Where xi1 and xi2 are parameters that can be determined theoretically. xi1

decreases with increasing T, while xi2 increases with increasing T.

Matthias discussed two experiments verifying this picture, each with

a SC-F-SC function. The first (by Kontos et al) showed a transtion between 0 and Pi phases

of the junction (signalled by a vanishing of the critical Josephson current Ic)

with increasing width df of the F region and the second (by Ryazonov

et al) showed a Pi-to-0 transition with increasing T

At this point Dirk Morr pointed out that one can have a zero of Ic without

a transition between Pi and 0 states. But the location of the transition

agreed with theory.

Turning to the case of an interface between a SC and a half metal Ferromagnet,

the main topic, naively one expects no proximity effect. However, recent experiments

by Keizer et al, Nature 2006, on Josephson junctions with NbTiN SC

linked by Cr02 half metal, showed a large-distance Josephson effect.

An initial clue was that the Josephson effect was observed to be

very sensitive to surface properties. The experimentalists

observed hysteresis of the Fraunhofer diffraction pattern. After

subtracting the hysteresis, the pattern was shifted by Pi from

the usual case.

Matthias's work on this problem was published in 2003 in PRL and in 2006 on

cond-mat, and is based on the notion that the important physics occurs

at the interface.

The first effect to consider is spin mixing. Thus, one expects different

phase shifts of spin-up and spin-down fermions scattering at such an

interface, characterized by an angle theta. By itself, this leads to

singlet (S) - triplet (T) mixing, the magnitude of which is proportional

to Sin theta.

To understand the experiments, however, additional scattering properties

must be included. The additional properties included were surface scattering

at the interfaces that were assumed to have a local interface magnetization

m. The two relevant interface magnetizations, m1 and m2, can be labelled by

their angles alpha_i with respect to the magnetization M of the FM regime

and also by the angle between them. The resulting critical Josephson current

is sensitive to the angles alpha, and theta, while the shift in the

Fraunhofer diffraction pattern depends on the difference between the

local interface magnetizations m1 and m2. Future work will focus

on determining the precise physical mechanism behind the interface

magnetizations m1 and m2.

## Friday, August 31, 2007

### M. Eschrig: "The pairing state near superconductor/half metal interfaces"

Posted by DSheehy at 9:39 AM

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## 4 comments:

one smal story, Interesting in monochrome. Reminds me how I once spotted a butterfly but when I got my camera I discovered it was dead. I was too sad to take pictures.

Hugs and blessings,

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