Ted Forgan
description
Transcript of 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
Birmingham Condensed Matter Website Ted Forgan
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)
Birmingham Condensed Matter Website Ted Forgan
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
=>
Birmingham Condensed Matter Website Ted Forgan
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
Birmingham Condensed Matter Website Ted Forgan
Speed of sound Phase Diagram
Maybe FFLO in CeCoIn5
proposed shape of flux lines
i.e. flux lines turn
into “strings of sausages”
Birmingham Condensed Matter Website Ted Forgan
NMR on CeCoIn5 versus B and T
Kumagai et al. cond-mat/0605394
Birmingham Condensed Matter Website Ted Forgan
Flux Lattice in CeCoIn5, B // c
d-wave superconductivity +
paramagnetic effects??
Hexagonal =>
Rhombic =>
Square =>
Rhombic =>
Hexagonal
Birmingham Condensed Matter Website Ted Forgan
d-wave heavy-fermion CeCoIn5
B – T Phase Diagram
Note that we could go
very close to Hc2 – just 50 mT below !
Birmingham Condensed Matter Website Ted Forgan
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
Birmingham Condensed Matter Website Ted Forgan
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 ??
Birmingham Condensed Matter Website Ted Forgan