Puzzles in the Iron-Based Superconductors (IBS) From
Specific Heat Measurements
Bulk transition width in IBS vs annealing
• Usually, when annealing a superconductor and Tc
increases, then the bulk transition width (as measured
by the specific heat) goes down.
• Example – A-15 Nb3Al
Irradiated Nb3Al
o
o
o
o oo
o
o
o
o
Fluence Tc(K) Tc(K)
0
18.8
0.6
1x1018 17.9
1.0
n/cm2
Annealing of IBS
Gofryk et al. were
the first to anneal
(2 weeks, 800 oC)
Co-doped BaFe2As2
They got Tc~1.3 K
Tc=25 K, C/Tc=34
annealed
unannealed
Warning – when estimating transition width (see previous
slides), do not be confused by plots like the one below
Expanded View of Our Annealed Data
26.6
49
Annealing of IBS
Tc~1.0 K
Question: What about Transition Width in un-doped IBS,
unannealed LiFeAs
Tc
=1.2 K
Why is the bulk superconducting transition in
IBS so broad, even after annealing?
• Not reasonable to assume that IBS are in the strong pair breaking
regime. If that were so, then much higher Tc’s would be potentially
possible.
Puzzle 2: Specific Heat C vs Tc in Superconductors
• Background/Progress Report
Bud’ko, Ni, Canfield noticed that C/Tc  Tc
in doped BaFe2As2 (PRB 2009)
~2
Kim et al., 2011: This C/Tc  Tc2 is a.) true for most IBS and b.)
not true for BCS superconductors, heavy Fermions, (cuprates?)
BCS: C/Tc =1.43
Note:
BNC is not unusual
based on large
C/Tc at high Tc but
rather the quick fall
off in C/Tc at lower
Tc
Instead of plotting C/Tc vs Tc for the elements, try C/Tc vs 
BNC for IBS compared to Cuprates (avoiding pseudogap)
Result of careful measurement of C/Tc in
annealed Ba(Fe1-xCox)2As2
X
X for optimally doped
xopt
Result: no apparent
difference in rate of
falloff of C/Tc for
under- vs over-doped, i.
e. C/Tc seems
disconnected from the
SDW magnetism
Result of careful measurement of C/Tc in
annealed Ba(Fe1-xCox)2As2
• It’s not just annealing to try to optimize C, it’s also a question of
normalizing C based on finite  at low temperature in the
superconducting state
Finite 
annealed
unannealed
New Subject: This C/Tc correlation with Tc is
useful for identifying materials that do not belong
to the class of IBS
C/Tc = 41 mJ/molK2
Tc = 3.7 K, RRR=650
KFe2As2
e. g. KFe2As2,
RbFe2As2, and
CsFe2As2 – all with
Tc’s < 4 K
Recent work of Bud’ko et al., PRB 89, 014510
(2014) (note that plot is of C, not C/Tc
C

C

C

C
C/Tc is an important key to estimating sample
quality, e. g. in Ca0.33Na0.67Fe2As2
C/Tc = 39
mJ/molK2
S. Johnston
et al., 2014
C/Tc = 105 mJ/molK2
Kim et al.,
2014
“the specific heat jump at Tc obtained for this material
scales relatively well with its Tc”
105
66
39
Question: As samples become better (larger C values), how
will the exponent  in C/Tc ~ Tc vary?