Anisotropic superconducting gap revealed by angle resolved...

Anisotropic superconducting gap revealed by angle resolved specific heat and STM

measurements in iron pnictide superconductors

Hai-Hu Wen

National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China.

Institute of Physics, CAS, Beijing 100190, China

KITP 2011, Santa Barbara

http://www.google.com/imgres?imgurl=http://www.logoun.com/Upfiles/BeyondPic/2008-08/2008814590900321.jpg&imgrefurl=http://www.logoun.com/News.aspx%3FPage%3D58&usg=__VGWD05I88VNLID8vfvlZZ5vBns8=&h=355&w=300&sz=24&hl=zh-CN&start=1&zoom=1&um=1&itbs=1&tbnid=vMSd8b1atZco-M:&tbnh=121&tbnw=102&prev=/images%3Fq%3D%25E5%258D%2597%25E4%25BA%25AC%25E5%25A4%25A7%25E5%25AD%25A6logo%26um%3D1%26hl%3Dzh-CN%26newwindow%3D1%26sa%3DN%26ndsp%3D20%26tbs%3Disch:1&ei=QkA1TdfXHYLLgQed1JCgCw

Nanjing University Established in 1902

BeijingNanjing

Shanghai

Acknowledgements:

• IOP, CAS: Bing Zeng, Cong Ren, Lei Shan, HuanYang, Bing Shen

• IOP, CAS: Tao Xiang (Simulation)

• Naval Research Lab, USA: Igor I Mazin, (Theory and LDA calclation)

• Houston university: Shuheng Pan (STM)

• Nanjing University: Qianghua Wang (theory)

• ……

Outline of the talk

• Brief introduction to the pairing symmetry of iron pnictide superconductors

• Bulk evidence for anisotropic gaps in FeSe0.45Te0.55revealed by angle resolved specific heat

• Doping induced evolution from nodeless to nodal gaps in Ba(Fe1-xCox)2As2 and Ba(Fe1-xNix)2As2 revealed by specific heat, point contact tunneling measurements.

• Specific heat and STM measurements in Ba1-xKxFe2As2• Concluding remarks

Outline of the talk

• Brief introduction to the pairing symmetry of iron pnictide superconductors

••• Bulk evidence for anisotropic gaps in Bulk evidence for anisotropic gaps in Bulk evidence for anisotropic gaps in FeSeFeSeFeSe0.450.450.45TeTeTe0.550.550.55revealed by angle resolved specific heat revealed by angle resolved specific heat revealed by angle resolved specific heat

••• Doping induced evolution from Doping induced evolution from Doping induced evolution from nodelessnodelessnodeless to nodal gaps to nodal gaps to nodal gaps in in in Ba(FeBa(FeBa(Fe111---xxxCoCoCoxxx)))222AsAsAs222 and and and Ba(FeBa(FeBa(Fe111---xxxNiNiNixxx)))222AsAsAs222 revealed by revealed by revealed by specific heat, point contact tunneling measurements. specific heat, point contact tunneling measurements. specific heat, point contact tunneling measurements.

••• Specific heat and STM measurements in Specific heat and STM measurements in Specific heat and STM measurements in BaBaBa111---xxxKKKxxxFeFeFe222AsAsAs222••• Concluding remarksConcluding remarksConcluding remarks

Debates on the gap structure in the iron pnictide superconductors

It seems that nodal gap exists in LaFePO, BaFe2As2-xPx, KFe2As2, in other systems it remains unclear yet. The FS may be fully gapped but with strong gap modulation.

ARPES: Most of the time Isotropic full gap at the FSs.

• H. Ding et al., EPL2008; • L. Zhao et al., CPL2008

Superfluid density: power law T-dependence: nodal gap or S± pairing underimpurity scattering? In LaFePO, one finds clear evidence of nodal gap. Overdopinginduces the appearance of c-axis nodes.

• R. Prozorov PRL2009, PRB2009 • J. D. Fletcher et al., PRL2009, • C. W. Hicks et al. PRL2009 • C. Martin et al., PRB2010• J.-Ph.Reid et al. PRB2010

NMR: Low-T, 1/T1T shows power law of T: nodal gap or the S± pairing under impurity scattering? Normal state 1/T1T: Coincidence of the vanishing SC and AF spin fluctuation

• K. Matano et al., EPL2008 • F. L. Ning PRL 2010

Thermodynamics and transport : s-wave with a very small gap; c-axis nodes.

X. G. Luo PRB2009 G. Mu PRB2009

Red: xzGreen:yzBlue: xy

These were calculated using a five-orbitaldxz ,dyz ,dxy ,dx2−y2 ,d3z2−r2 tight-binding fit to the density functionaltheory band-structure calculations

Orbital weights on different FSs. • Maier, T. A., Graser, S., Scalapino, D. J. & Hirschfeld, P. J. Rev. B 79, 224510 (2009).

• Kuroki, K. et al. Phys. Rev. B 79, 224511 (2009).

• Wang, F., Zhai, H. & Lee, D. H. EPL 85, 37005 (2009).

Orbital dependent interaction via AF spin fluctuation

The nodes can appear, but not enforced by symmetry and ataccidental positions!

Horizontal but segment like nodal lines on hole pockets

Electron-FS: highly anisotropic s-wave, accidental nodes are expectable!

Implications for ab/cpenetration depth and thermal conductivity

S. Graser, A. Kemper, T. A. Maier, H.-P. Cheng, P. J. Hirschfeld, and D. J. Scalapino. Phys. Rev. B 81, 214503 (2010)

axis

Other Similar studies: • K. Kuroki et al., PRB79, 224511 (2009).• F. Wang, H. Zhai, D. H. Lee, EPL 85, 37005 (2009).

Expected gap in 122Courtesy of S. Graser

Outline of the talk

••• Brief introduction to the pairing symmetry of iron Brief introduction to the pairing symmetry of iron Brief introduction to the pairing symmetry of iron pnictidepnictidepnictide superconductors superconductors superconductors

• Bulk evidence for anisotropic gaps in FeSe0.45Te0.55revealed by angle resolved specific heat

••• Doping induced evolution from Doping induced evolution from Doping induced evolution from nodelessnodelessnodeless to nodal gaps to nodal gaps to nodal gaps in in in Ba(FeBa(FeBa(Fe111---xxxCoCoCoxxx)))222AsAsAs222 and and and Ba(FeBa(FeBa(Fe111---xxxNiNiNixxx)))222AsAsAs222 revealed by revealed by revealed by specific heat, point contact tunneling measurements. specific heat, point contact tunneling measurements. specific heat, point contact tunneling measurements.


0 2 4 6 8 10 12 14 16 18 20 22 24

-300

-250

-200

-150

-100

-50

0

0 100 200 3000.0

0.2

0.4

0.6

0.8

1.0

1.2

ρ(mΩ

.cm

)

(a)

H=0 T11 12 13 14 15 16

0.0

0.4

0.8

(b)

T(K)

H = 0, 0.2, 0.5, 1, 2, 3, 4, 5,6, 7, 8, 9 T

11 12 13 14 15 16

0.0

0.4

0.8

(c)

ρ(

mΩ

.cm

)

T(K)

FC H=10 Oe

M

(em

u/m

ol)

T (K)

ZFC H=10 Oe

FeSe0.4Te0.6 single crystals

J. K. Dong et al., PRB80, 024518(2009).

In-plane thermal transport measurements finds a negligible κ0/T at T=0 K, this excludes a vertical line nodes, like a d-wave gap. While the strong field dependence suggests a very small gap, or a highly anisotropic gap.

cmKWvkc

kT

FFB 2

0

20 /140

6μκ =

Δ= Universal thermal conductivity

coefficient at T=0 for line nodes!

T. Hanagury et al., Science 428, 474 (2010)

Magnetic field–induced change in QPI intensitiesindicates the s±-wave symmetry.

Δ = 1.4 me V!

Quasiparticle DOS in Mixed State with Vortices

Two components for quasiparticles: Localized core state ( s-wave ) and de-localized extended states in d-wave case.

0 2 4 6 8 10 12

0.0

0.2

0.4

0.6

0.8

1.0Extracted data at T=0 K

0.063 0.069 0.075 0.09 0.11 0.15 0.22 H0.5

Δγ / Δγ

(12T

)

μ0H(T)

• S-wave, Ce/T∝ H• D-wave, Ce/T∝ H1/2

Doppler shift effect

• Phys. Rev. B70, 214505(2004).

• Phys. Rev. B 72, 134507(2005).

Angle Resolved Specific Heat: a powerful tool to detect the nodal gap symmetry

βααρ

δ sin)sincos( )( ,)(,

ixF

iyF

HFs vv

EvvmE vvrr −=•=

2/1

0

)( )2(~Φ

=π

HvaE Fi

H h( )

FS

ixF

iyF

k

ii vv

EN H

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡−⎟

⎟⎠

⎞⎜⎜⎝

⎛

Δ=

2)(,

)(,

2)()(

0 sincos,1min ααvv

H

Δ

N(E)

H=0

H≠0

E

Y. Matsuda, J. Phys.: Cond. Matter 18, R705 (2006).

Angle Resolved Specific Heat:

a powerful tool to detect the nodal gap symmetry

⎥⎦

⎤⎢⎣

⎡−+= ∫Γ )(sin2cos,1min2

22

2

0

00 αϕϕπ

ϕπ AdNNN M

yxk kk sinsin01. d-wave: Δ=Δ

2. Extended s-wave: yxk kk coscos0Δ=Δ

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎥⎦

⎤⎢⎣

⎡⎥⎦

⎤⎢⎣

⎡+−

+

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎥⎦

⎤⎢⎣

⎡⎥⎦

⎤⎢⎣

⎡+−

+=

∫

∫Γπ

π

ϕααϕ

ϕπϕ

ϕααϕ

ϕπϕα

2

022

2

20

21

2

022

2

20

21

0

tan)sincos(tan

2cos,1min

2

1tan)sincostan(

2cos,1min

2)(

hhAdN

hhAdNNN

M

M

200

222 /2 ΔΦ= πHvaA Fh

S. Graser et al., Phys. Rev. B 77, 180514(R) (2008)

Δ = a+b(coskx+cosky)


• dxy-wave gap?

• Extended S-wave gap

With ellipticity ratio h=a/b of the M-FS smaller than 1.2

4-fold oscillations of SH with in-plane field

Bing Zeng et al. arXiv:1007.3597. Nature Communications 2010

4-fold oscillations of SH with 9T in-plane field

1 2 3 4 5

-0.2

0.0

0.2

0.4

ΔC/T

max

-min (m

J/m

ol-K

2 )

T (K)

Crossover at 2.92 K C/T(max-min)

• Vorontsov, A. B. and Vekhter, I. Phys. Rev. Lett. 105, 187004 (2010)

• Chubukov, A. V. and Eremin, I. PRB82, 060504(R)(2010) .

• Vorontsov, A. B. and Vekhter, I. Phys. Rev. B 75, 224501 (2007). Phys. Rev. B 79, 064525 (2009).

• An, K. et al., Phys. Rev. Lett. 104, 037002 (2010).

For d-wave (2D case)

For Iron-pnictides

Fe-F

e

Fe-S

e-Fe

B. Zeng ……HHW, Nature Comm. 1, 157 (2010)

The free energy corresponding to two different kind of vortex patterns are quite close to each other, which makes this crossover!

-100 -80 -60 -40 -20 0 20 40 60 80 1003.8

3.9

4.0

4.1

4.2

4.3

4.4

raw

C (μ

J/K)

θ(degree)

FeSe 0T T=2.6K FeSe 0T T=2.65K FeSe 0T T=2.7K FeSe 9T T=2.6K FeSe 9T T=2.65K FeSe 9T T=2.7K Nb 0.4T T=2.6K Nb 0.4T T=2.65K

Comparison of the data of FeSe0.4Te0.6 at H=9T, 0T and the Nb


2

20

4

2

0

2

/3)(

/66.1/924.0

)(/12.0/

KmolmJ

KmolmJKmolmJ

T

KmolmJTC

ee

he

ph

ee

heph

−≈+

−=

−=

=

+++=

−=Δ

γγ

γ

β

βγ

γγγγγ

If at 9 T

%85.1/12.0/

/5.1 2

==Δ

−==

ee

ee

he KmolmJ

γγ

γγ

Theory prediction: 5-10%

• Vertical d-wave nodes on the hole pockets: unlikely

• Extended s-wave: accidental minimum of gaps on M-FS are possible

B. Zeng ……HHW. Nature Comm. 1, 157 (2010)

Qualitatively consistent with the theory


• Chubukov, A. V. and Eremin, I. PRB82, 060504(R)(2010)

0 50 100 150 200 250 300 350-2

0

2

4 r=0 r=0.5 r=0.7 r=1 r=2

Δ e(m

eV)

ϕ(degree)

Extended s-wave gap:

Δ = a+b(coskx+cosky)

On the M-FS: ]2sin1[0 ϕre −Δ=Δ• |r|1: accidental nodes around ϕ=45

Our present case: r=0.5-1

More overdoping Stronger intra-pocket scatteringA pair of accidental nodes expected??


• Chubukov, A. V. and Eremin, I. PRB82, 060504(R)(2010)

B. Zeng…HHW, Nature Comm. 1, 157 (2010)

Outline of the talk


••• Bulk evidence for anisotropic gaps in Bulk evidence for anisotropic gaps in Bulk evidence for anisotropic gaps in FeSeFeSeFeSe0.450.450.45TeTeTe0.550.550.55revealed by angle resolved specific heatrevealed by angle resolved specific heatrevealed by angle resolved specific heat

• Doping induced evolution from nodeless to nodal gaps in Ba(Fe1-xCox)2As2 and Ba(Fe1-xNix)2As2 revealed by specific heat, point contact tunneling measurements.


Doping induced evolution of the pairing interaction: from inter-pocket to intra-pocket dominated scattering?

]2sin1[0 ϕre −Δ=Δ

G.E. Blonder, M. Tinkham, and T.M. Klapwijk, Phys. Rev. B 25, 4515 (1982).

Probability:A: Andreev reflection, two electron tunnelingB: Single particle tunneling

BTK theory: Theory / ModelAndreev reflection

N S

e

h

BTK fitting:

Four parameters: T-temperature;Δ-SC gap;Z-barrier height;Γ-broadening parameter.

SC gap

+

+

__

T=2K, Δ0=5meV, α=45o, Γ=1meV.

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 300.60.81.01.21.41.61.82.02.22.42.62.83.0

dI/d

V (n

orm

. u.)

Bias voltage (mV)

Z=5

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 300.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

dI/d

V (n

orm

. u.)

Bias voltage (mV)

Z=2

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 300.8

1.0

1.2

1.4

1.6

1.8

2.0

dI/d

V (n

orm

. u.)

Bias voltage (mV)

Z=1

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 300.8

1.0

1.2

1.4

1.6

1.8

dI/d

V (n

orm

. u.)

Bias voltage (mV)

Z=0.5

Barrier strength:

Underdoped

Optimal doped

Overdoped

Cong Ren et al., to be published

[ ]φ2cos1 rreM

h

−−Δ=Δ

Δ=ΔΓ isotropic gap

anisotropic gap

MGwwGG )1( −+= Γ

r = 0.5Crossover pointof nodal gap

This unfortunately cannot distinguish the vertical nodes or the horizontal nodes.

• Measure the SH for an optimally doped and an very overdoped non-SC samples.

•Determine the phonon part from the overdoped sample

•Remove the phonon part and get the electronic part of the superconducting sample.

0 200 400 600 800 1000 12000

100

200

300

400

500

600

0

100

200

300

400

500

600

0 50 100 150 2000

50

100

150

0 50 100 150 2000

50

100

150

BaFe1.9Ni0.1As2 BaFe1.7Ni0.3As2

H=0T

BaFe1.84Co0.16As2 BaFe1.4Co0.6As2

C/T

(mJ/

mol

K2 )

T2 (K2)

(a)

(b)

H=0T

Specific heat in Ba(Fe1-xCox)As2 and Ba(Fe1-xNix)2As2:possible segments of nodal lines or point nodes

• The low temperature part has a quadratic T-dependence

• The global fitting is more close to a single band d-wave! (but it is not!)

• Models with two components:d+s or s1+s2

20 Te αγγ +=

0 10 2002468

0 10 20 300

2

4

6

8

0 5 10 15 20 25 300

10

20

30

40

50

60

BaFe1.84Co0.16As2 s-wave d-wave s+d s1+s2

γ e-γ

0 (m

J/m

ol K

2 )

(a)

0 5 10 15 20 250

10

20

30

40

50

(b)

BaFe1.9

Ni0.1

As2

s-wave d-wave s+d s1+s2

T (K)

γ e

-γ0 (

mJ/

mol

K2 )

T2 (K2)

γ e (m

J/m

ol K

2 )

γ e (m

J/m

ol K

2 )

T2 (K2)

Optimally doped Ba(Fe1-xCox)As2 and Ba(Fe1-xNix)2As2

Specific heat in Ba(Fe1-xCox)As2 and Ba(Fe1-xNix)2As2:possible segments of nodal lines or point nodes

0 10 20 30 400

10

20

30

(d)

T2 (K2)

0T 0.5T 1T 2T 3T 5T 7T 9T

BaFe1.9

Ni0.1

As2 H//ab

0 10 20 30 400

10

20

(b)

0T 0.5T 1T 2T 3T 5T 7T 9T

BaFe1.84Co0.16As2 H//ab

0 10 20 30 400

10

20

C/T

(mJ/

mol

K2 ) 0T

0.25T 0.5T 0.75T 1T 2T 3T 5T 7T 9T

BaFe1.84

Co0.16

As2 H//c

(a)

0 10 20 30 400

10

20

30

(c)

C/T

(mJ/

mol

K2 )

T2 (K2)

0T 0.5T 1T 2T 3T 5T 7T 9T

BaFe1.9Ni0.1As2 H//c

HEHNEEN

HvaE FH

∝=Δ∝

Φ=

)0,()(

~2

2

0

π

Both vertical or horizontal line nodes are impossible! But small segments of line nodes or point nodes are possible!

How to reconcile γe = γ0 + αT2 and γe-γ0∝ H

1. Strong pair breaking within S± pairingLike the penetration depth Δλ∝T2

Difficulty: the cleaner sample, the smaller γ0, but “2” is robust!

2. Impurity scattering with line nodes:Difficulty: linear field dependence γe-γ0∝ H, it would be

nonlinear, close to H1/2.

3. Point nodes or small segments of line nodes on some FS: fit both γe = γ0 + αT2 and γe-γ0∝ H

Outline of the talk


••• Bulk evidence for anisotropic gaps in Bulk evidence for anisotropic gaps in Bulk evidence for anisotropic gaps in FeSeFeSeFeSe0.450.450.45TeTeTe0.550.550.55revealed by angle resolved specific heat revealed by angle resolved specific heat revealed by angle resolved specific heat

••• Doping induced evolution from Doping induced evolution from Doping induced evolution from nodelessnodelessnodeless to nodal gaps to nodal gaps to nodal gaps in in in Ba(FeBa(FeBa(Fe111---xxxCoCoCoxxx)))222AsAsAs222 and and and Ba(FeBa(FeBa(Fe111---xxxNiNiNixxx)))222AsAsAs222 revealed by revealed by revealed by specific heat, point contact tunneling measurements.specific heat, point contact tunneling measurements.specific heat, point contact tunneling measurements.

• Specific heat and STM measurements in Ba1-xKxFe2As2••• Concluding remarksConcluding remarksConcluding remarks

Specific heat in Ba1-xKxFe2As2

2/10298|/ molKmJTCcT

−=Δ

2/50 molKmJn ≥γ

Gang Mu et al., Phys. Rev. B79, 174501 (2009)

2

43.1

≈Δ

=Δ

cn

cn

TCTC

γ

γ

Strong coupling

Weak coupling

Extremely strong mass enhancement!Consistent with ARPES data

H. Ding et al., arXiv:0812.0534

Strong renormalization of the electron mass

http://arxiv.org/abs/0812.0534

There are at least two components, one is s-wave with the gap of about 6 meV.

Concrete support

Air spring

Sound insulation room•• Temperature : 1.8Temperature : 1.8--50 K 50 K •• Magnetic field : 0Magnetic field : 0--9 T9 T•• Spatial resolution (z direction): 2 pm Spatial resolution (z direction): 2 pm •• Spatial resolution (Spatial resolution (xyxy direction): 1 direction): 1 ÅÅ•• Energy resolution : 40Energy resolution : 40μμVV•• Scan range at 4.2 K : 250Scan range at 4.2 K : 250××250 nm250 nm22

-150 -100 -50 0 50 100 1500.0

0.5

1.0

1.5

2.0

dI/d

V

Bias Voltage (mV)

Typical tunneling spectroscopy of Bi2212Typical tunneling spectroscopy of Bi2212

STM image of BiSTM image of Bi--based HTS based HTS cupratescuprates（（700700××700 700 ÅÅ22））

With the great help of Prof. Shuheng Pan

Low temperature cleavage leads to a stable surface state

Multigap evidence from STM measurements in Ba0.6K0.4Fe2As2

L. Shan et al,, PRB (R) 2011, in press.

Spatially resolved spectra in zero magnetic field at a temperature of 2K

Comparison of two dI/dV spectra measured in bright and dark areas, respectively.

On this kind of stable surface, the spectrum is dominated by the 4 meV gap. The bottom is rounded, sometime touches zero!

Nodes or anisotropic gap?

-15 -10 -5 0 5 10 150.0

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1.4

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2.0

dI/d

V (n

orm

aliz

ed)

Bias voltage (mV)-15 -10 -5 0 5 10 15

0.0

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dI/d

V (n

orm

aliz

ed)

Bias voltage (mV)

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V (n

orm

aliz

ed)

Bias voltage (mV)

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1.4

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1.8

2.0

dI/d

V (n

orm

aliz

ed)

Bias voltage (mV)

T=2K (site16-pt8)

Δ =3.5 (meV) Δ(θ)=4·abs[cos(2θ)] (meV)

-15 -10 -5 0 5 10 150.0

0.2

0.4

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2.0

dI/d

V (n

orm

aliz

ed)

Bias voltage (mV)-15 -10 -5 0 5 10 15

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2.0

dI/d

V (n

orm

aliz

ed)

Bias voltage (mV)

Δ(θ)=4·(abs[cos(2θ)])0.5 (meV)

Δ(θ)=3.2·(abs[cos(2θ)]+0.3) Δ(θ)=3.5·((abs[cos(2θ)])0.5+0.1)Δ1(θ)=3.2·((abs[cos(2θ)])0.5+0.1)--75%Δ2(θ)=6.5·(abs[cos(2θ)]+0.1)------25%

Two gap

2.17×10-15 Wb 1.98×10-15 Wb Wb15

0 1007.2−×=Φ

L. Shan et al.Nature Physics,In press.

Andreev bound state within vortex cores is observed!

The Andreev bound states peaked at a negative bias, -0.5 meV.

Comparison between the vortex bound states inBa0.6K0.4Fe2As2, Ba(Fe1-xCox)2As2, cuprates and 2H-NbSe2

Ba0.6K0.4Fe2As2Ba(Fe1-xCox)2As2

Origin of the difference:

• Fundamental difference of hole-and electron-doped samples

• Different scattering strength, Co-doping to the FeAs-planes

F. H. Hess et al., PRL64, 2711(1990).

2H-NbSe2

S. H. Pan et al., PRL85, 1536(2000).

Bi-2212

Different ground state:• Correlation• competing order• itinerancy

Concluding Remarks(1) A fourfold oscillation of the low-T SH was observed in FeSe0.45Te0.55 with the in-plane

magnetic field. This can be explained as highly anisotropic gaps with deep gap minima (along the Fe-Fe bond direction) on the electron-FS. This is consistent with the extended s-wave model with strong gap modulation on the electron pockets, as expected by the 5-orbital tight binding calculations.

(2) Optimally Co-doped and K-doped Ba-122 do not show the fourfold oscillation, while overdoped Ba(Fe1-xCox)2As2 shows some kind of oscillation. This is consistent with the doping dependence of the point contact tunneling data and the field dependence of the SH coefficient. Our data support the picture that the electron doping induces stronger gap anisotropy in the electron pocket.

(3) STM measurements on Ba0.6K0.4Fe2As2• Well ordered vortex lattice with a distorted triangular structure

• Rounded or flat bottom of spectrum may suggest the nodeless gap but with anisotropy in the optimally doped BaK-122. The tunneling spectrum generally exhibit the two gap feature. The Andreev bound states (peaked at a negative bias -0.5 mV) within vortex cores were observed, in sharp contrast to Co-doped FeAs122.

Thank you for your attention

and happy new year!

1/T1T measures the q-intergral of the imaginary part of the dynamical spin susceptibility, reflecting the summation of all different q-modes of spin fluctuations.

ffqqATTq

hf /),('')(/12

1rr χ∑∝

F. L. Ning et al., PRL104, 037001(2010)

1. Underdoped region: AF and SC compete for DOS.2. The strong temperature dependence in high-T is induced by multiband

effect and together with novel scattering. Gradually one band dominates in the overdoped region, then the SC vanishes!

( )2hehhee

HR σσμσμσ

++

=

iiii men /2τσ =

iiii me /τμ =

Superconductivity relies crucially on the multiband effect!

Lei Fang et al., Phys.Rev.B80, 140508 (R) (2009)

Anisotropic superconducting gap revealed by angle resolved specific heat and STM measurements in iron pnictide superconductorsNanjing UniversityOutline of the talkOutline of the talkDebates on the gap structure in the iron pnictide superconductorsOutline of the talkAngle Resolved Specific Heat: �a powerful tool to detect the nodal gap symmetry Outline of the talkOutline of the talkMultigap evidence from STM measurements in Ba0.6K0.4Fe2As2Comparison of two dI/dV spectra measured in bright and dark areas, respectively. Andreev bound state within vortex cores is observed!Concluding Remarks

Anisotropic superconducting gap revealed by angle resolved...

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