Ted Forgan
description
Transcript of Ted Forgan
![Page 1: Ted Forgan](https://reader035.fdocuments.in/reader035/viewer/2022081520/568153b0550346895dc1b1f8/html5/thumbnails/1.jpg)
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
![Page 2: Ted Forgan](https://reader035.fdocuments.in/reader035/viewer/2022081520/568153b0550346895dc1b1f8/html5/thumbnails/2.jpg)
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)
![Page 3: Ted Forgan](https://reader035.fdocuments.in/reader035/viewer/2022081520/568153b0550346895dc1b1f8/html5/thumbnails/3.jpg)
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
=>
![Page 4: Ted Forgan](https://reader035.fdocuments.in/reader035/viewer/2022081520/568153b0550346895dc1b1f8/html5/thumbnails/4.jpg)
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
![Page 5: Ted Forgan](https://reader035.fdocuments.in/reader035/viewer/2022081520/568153b0550346895dc1b1f8/html5/thumbnails/5.jpg)
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”
![Page 6: Ted Forgan](https://reader035.fdocuments.in/reader035/viewer/2022081520/568153b0550346895dc1b1f8/html5/thumbnails/6.jpg)
Birmingham Condensed Matter Website Ted Forgan
NMR on CeCoIn5 versus B and T
Kumagai et al. cond-mat/0605394
![Page 7: Ted Forgan](https://reader035.fdocuments.in/reader035/viewer/2022081520/568153b0550346895dc1b1f8/html5/thumbnails/7.jpg)
Birmingham Condensed Matter Website Ted Forgan
Flux Lattice in CeCoIn5, B // c
d-wave superconductivity +
paramagnetic effects??
Hexagonal =>
Rhombic =>
Square =>
Rhombic =>
Hexagonal
![Page 8: Ted Forgan](https://reader035.fdocuments.in/reader035/viewer/2022081520/568153b0550346895dc1b1f8/html5/thumbnails/8.jpg)
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 !
![Page 9: Ted Forgan](https://reader035.fdocuments.in/reader035/viewer/2022081520/568153b0550346895dc1b1f8/html5/thumbnails/9.jpg)
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
![Page 10: Ted Forgan](https://reader035.fdocuments.in/reader035/viewer/2022081520/568153b0550346895dc1b1f8/html5/thumbnails/10.jpg)
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 ??
![Page 11: Ted Forgan](https://reader035.fdocuments.in/reader035/viewer/2022081520/568153b0550346895dc1b1f8/html5/thumbnails/11.jpg)
Birmingham Condensed Matter Website Ted Forgan