Ted Forgan

Birmingham Condensed Matter Website Ted Forgan

Ted Forgan

School of Physics and Astronomy, University of Birmingham, U.K.

Birmingham Condensed Matter Physics Group

Pauli-limiting effects in the flux lattice of CeCoIn5


Superconductivity at high B –

what determines Bc2?

2

02 2

cB and

)0(0

Fv

=>

F

cc

m

eB

2

02 )0(

*

)0(

2

22

2

*)()0(

F

cc k

mTB =

>

‘Heavy fermion’ materials have low Tc (~ 1 K)

but high Bc2 (~ 10 T)


Superconductivity and magnetic fields – effect of

electron spin

What energy balance decides if paramagnetism

is important?

FF

PauliBB

222 )0(

Pauli paramagnetism favours normal state

- because paired antiparallel electrons cannot line up with B

=> )0(PauliBB

cPauli TB =>

2

22

2

*)()0(

F

cc k

mTB Compare

=>


Pairing controlled by spin energy

k

~ 0 exp (iqz)

Flux lines modulated along z

Carries a supercurrent // B !

Maybe ~ cos (qz) – Larkin & Ovchinnikov (same time)

F

k

2B

with q = ( k + k )

BzF

B

v

Bq

2

i.e.

This was predicted by Fulde & Ferrell in 1964

but

~BB

“LOFF / FFLO”

k

paired

1//~~ B

Fvq


Speed of sound Phase Diagram

Maybe FFLO in CeCoIn5

proposed shape of flux lines

i.e. flux lines turn

into “strings of sausages”


NMR on CeCoIn5 versus B and T

Kumagai et al. cond-mat/0605394


Flux Lattice in CeCoIn5, B // c

d-wave superconductivity +

paramagnetic effects??

Hexagonal =>

Rhombic =>

Square =>

Rhombic =>

Hexagonal


d-wave heavy-fermion CeCoIn5

B – T Phase Diagram

Note that we could go

very close to Hc2 – just 50 mT below !


Results in niobium near Bc2

linear

scale

logscale

Note: intensity

falls rapidly to zero near

Bc2 in a conventional material

Diffracted Intensity vs. T at B = 0.2 T


d-wave heavy-fermion CeCoIn5

flux lines show stronger field contrast at high

field – even up to Bc2 !

Published in ‘Science’(Bianchi et al) Jan ‘08

|F|2

B

Electron spins lining up parallel in the de-paired

flux line cores...

Is there a sign of FFLO –for B // c ??


Ted Forgan

Documents