2012年6月29日，中科院物理所

极低温热导率探测超导体的超导能隙对称性

李世燕

复旦大学物理系

主要内容：

1. 极低温下的热输运测量

2. 常规超导体

3. 铜氧化物高温超导体

4. 重费米子超导体

5. 铁基超导体

3He-4He dilution fridgeT→7 mK; H→17 T κ = α

QΔT

1、极低温下的热输运测量

第一步 做一个好的样品杆

混合室主温度计不受磁场影响

样品在磁场中心 用标定温度计检验样品处温度

第二步 做一个好的热导率测量单元

Heat leak< 1%

第三步 测量热导率(in situ calibration)

Heater off

Heater on

κ = α QΔT

κ = κelectrons + κphonons + κmagnons + κspinons + …

κ = 1/3 C v l

Tκ

2T

electrons ~ T

phonons ~ T3

0

比热 速度 平均自由程

FERMIONS (Electrons)κ ∝ Ce ∝ T

BOSONS (Phonons)κ ∝ Cph ∝ T 3

κ/T = A + BT2

Τ → 0 分离各种准粒子的贡献

V3Si， Tc = 17 K常规超导体s波能隙

Δ = Δ0

由于s波能隙的存在，T→0时费米子态密度为0：

κ0/T = 0

Tl2Ba2CuO6+δ， Tc = 15 K高温超导体d波能隙

Δ = Δ0cos(2φ)

由于d波能隙在φ=45o处为零，杂质将会在能隙节点处产生有限的费米子态密度，T→0时：

κ0/T = c

Τ → 0 热导率探测超导能隙结构

κ0/Τ=0 验证绝缘体

κ/T = a + bTα-1 其中声子项bTα-1, 2 ≤ α ≤ 3声子的specular reflections导致α < 3

Specular Reflection of Phonons

Specularreflection lph = f(T)

κph/T~ Tα, α <2R. O. Pohl* and B. Stritzker, PRB 25, 3608 (1982).

Sapphire

V3Si s-wave SC(thermal insulator, κel =0)

Fit data to κ/T = κo/T + BTα κo/T = 0

α = 1.7

L/L0 验证电子体系满足Wiedemann-Franz law

LuNi2B2C at 7 TAg wire

κ / T = L0 / ρ0

2. 常规超导体

Nb single s-wave gap

MgB2 two s-wave gaps

NbSe2 multiple s-wave gaps

LuNi2B2C multiple anisotropic s-wave gaps

CuxTiSe2 s-wave gap

Nb: single s-wave gap

Tc = 9.1 KHc2 = 4.5 kG

T = 1.98 K

J. Lowell and J. B. Sousa, JLTP 3, 65 (1970)

MgB2: a two-band s-wave superconductor

X. X. Xi , RPP 71, 116501 (2008)

Δσ ~ 6.8 meVΔπ ~ 1.8 meV

Tc = 39 K

H // cH // ab

T = 0.60 K

A. V. Sologubenko et al., PRB 66, 014504 (2002)

Hc2 ∝ Δ2 / vF2

NbSe2: a multi-band s-wave superconductor

T. Yokoya et al., Science 294, 2518 (2001)

Tc = 7.2 K

5.3 K: Δ1 ~ 0 meV, Δ2 = 1.0 meV, Δ3 = 0.9 meV0 K: Δ1 < 0.2 meV, Δ2 = 1.22 meV, Δ3 = 1.13 meV

NbSe2

V3Si

E. Boaknin et al., PRL 90, 117003 (2003)

LuNi2B2C: multiple anisotropic s-wave gaps

Tc ~ 16 K, Hc2 ~ 7 T

E. Boaknin et al., PRL 87, 237001 (2001)

ARPES YNi2B2C

Δmax = 3.2 meV, Δmin = 1.5 meV

Δmax / Δmin ~ 2.1

热导率实验难以区分以下两种情况：1、多个各向同性的能隙，大小不同2、单个各向异性能隙

T. Baba et al., PRB 87, 180509(R) (2010)

超导体CuxTiSe2超导能隙的对称性

CuxTiSe2的相图

CDW量子临界点出现超导态：非常规超导体？

Morosan et al., Nat. Phys. 2, 544 (2006)

重费米子材料CePd2Si2的相图

Mathur et al., Nature 394, 39 (1998)

ρ, κ Cu0.06TiSe2的电阻率和热导率

Cu0.06TiSe2中的s波超导电性

确定Cu0.06TiSe2具有s波超导能隙：

1）κ0/T = 0

2）κ0(H)/T与s波超导体行为一致

S. Y. Li et al., PRL 99, 107001 (2007)

3. 铜氧化物高温超导体

Evidences for d-wave gap

Coexisting/competing orders in La2-xSrxCuO4

Wiedemann-Franz law in the normal state of Pr2-xCexCuO4

⎟⎟⎠

⎞⎜⎜⎝

⎛+=

F

2

2

F2B0

vv

vv

dn

3kκhT

A. Durst and P. A. Lee, PRB 62, 1270 (2000).M. J. Graf et al., PRB 53,15147 (1996).

• universal• no Fermi-liquid or vertex corrections

Thermal transport (T << γ , γ << Δ)

Fermi-liquid theory of nodal quasiparticles

simple dx2-y2 case:

node2

1vφ∂Δ∂

⋅=Fkh

22

22

21

2 vv kkE F ++= h

Fkh0

22v Δ

=

near the nodes

Quasiparticle dispersion

•universal κ0/T verifiedTaillefer et. al., PRL 79, 483 (1997).

•quantitative agreement with ARPES

Experiments (at optimal doping)

Chiao et. al., PRB 62, 3354 (2000).

Evidences for d-wave gap in cuprates

Louis Taillefer et al., PRL 79, 483 (1997)

YBCO y=6.9: κ0/T = 0.19 mW/K2cm nodes, supporting nodal gap

Chiao et al., PRB 62, 3554 (2000)

Bi-2212

Takeya et al., PRL 88, 077001 (2002)

LSCO

Ando et al., PRL 92, 247004 (2004)

BSLCO

Proust et al., PRL 89, 147003 (2002)

Tl-2201

Impurity effect Universal heat conduction

Louis Taillefer et al., PRL 79, 483 (1997)

Η > 0 d-wave superconductor in field

YBCO: Chiao et al., PRL 82, 2943 (1999)

d-wave：κ0(H)/T ～ H1/2 Tl-2201

Volovik effect: Doppler shift due to the superfluid flow around the vortices, which yields a H1/2 growth in quasiparticle DOS

Coexisting/competing orders in La2-xSrxCuO4

LSCO x = 0.101) SDW order in H = 02) Field enhances SDW

Lake et al., Nature 415, 299 (2002)

In underdoped LSCO:field suppresses κ0/T

Hawthorn et al., PRL 90, 197004 (2003)Sun et al., PRL 90, 117004 (2003)

x = 0.144 thermal conductivity and neutron

S. Y. Li et al., preprint

The increase of SDW phasecauses the decease of κ0/T！

thermal metal-insulator crossover Demler et al., PRL 87, 067202 (2001)

Khaykovich et al., PRB 71, 220508 (2005)

x = 0.144

H = 60 T metal-insulator crossover in LSCO

MI-crossover near optimal doping ρ ~ log(1/T)

Ando et al., PRL 75, 4662 (1995)Boebinger et al., PRL 77, 5417 (1996)

How to understand this MI-crossover and log(1/T) dependence？

neutron, κ, ρ phase diagram of LSCO

S. Y. Li et al., preprint

Understanding transport in LSCO:

1) The decrease of κ0/T is caused bythe increase of SDW phase

2) MI-crossover near optimal dopingin the normal state is:

Metal → SDW + Metal

3) 0.08 ≤ p ≤ 0.13, SDW + Metal ⇒ρ ~ log(1/T), like granular metal

Wiedemann-Franz law in the normal state of Pr2-xCexCuO4

ρ

T

ρ0

T 2

κ00//T

κT

e

22

00

0

3π

σκ

⎟⎠⎞

⎜⎝⎛==

ekL

TB

overdoped Tl-2201, Tc = 15 KWiedemann-Franz law satisfied

Proust et al., PRL 89, 147003 (2002)

L0

ρ0(13T)

zerofield3 T

6 T

10 T

14 T

Pr1.85Ce0.15CuO4 optimal doping

μΩcm 8.260 =ρ

T < 0.3 K, downturn problem:due to electron-phonon decoupling

0.3 ~ 0.6 K, κe/σT ≈ 1.8L0Wiedemann-Franz violatedHill et al., Nature 414, 711 (2001)

4. 重费米子超导体

Ce2PdIn8

PrOs4Sb12

H. Hegger et al., PRL 84, 4986 (2000) Times cited: 551C. Petrovic et al., EPL 53, 354 (2001) Times cited: 324C. Petrovic et al., JPCM 13, L337 (2001) Times cited: 381

N. D. Mathur et al., Nature 394, 39 (1998) Times cited: 881

CeRhIn5, TN = 3.8 K; CeIrIn5, Tc = 0.4 K; CeCoIn5, Tc = 2.3 K.

CeIn3, TN = 10.1 K,Tc = 0.2 K@26 kbar.

Field-induced QCP and nodal superconductivity in Ce2PdIn8

J. D. Thompson, Z. Fisk, JPSJ 81, 011002 (2012)

QCP at Hc2 ρ(T) of CeCoIn5

In CeCoIn5, a field-induced AF QCP is located at Hc2 = 5.1 T.

J. Paglione, Louis Taillefer et al., PRL 91, 246405 (2003)

In CeCoIn5, a field-induced AF QCP is located at Hc2 = 5 T.

A. Bianchi et al., PRL 91, 257001 (2003)

QCP at Hc2 C(T) of CeCoIn5

FFLO Possible in CeCoIn5

2nd order

1st order

Specific heat Specific heat, magnetization,

A. Bianchi et al., PRL 91, 187004 (2003) H. A. Raddovan et al., Nature 425, 51 (2003)

FFLO Spatial symmetry spontaneously broken

A novel superconducting phase with spatially inhomogeneous order parameter.Predicted by Fulde, Ferrell, Larkin and Ovchinnikov in 1964.

CeCoIn5 Phase diagram

NMR

K. Kumagai et al., PRL 97, 227002 (2006)

Q phase neutron, NMR, μSR

M. Kenzelmann et al., Science 321, 1652 (2008)

Coupled SDW and SC in the Q-phase for both H//ab and H//cOrignin of Q phase? Really FFLO?

1 Nature, 2 Science, over 30 PRL CeCoIn5 papers on this issueMore compounds show this kind of phase diagram?

A. Aperis et al., PRL 104, 216403 (2010)

CeMIn5 M = Rh, Ir

CeRhIn5, field-induced magnetic orderNot coupled to superconductivity

H. Shakeripour et al., NJP 11, 055065 (2009)

CeIrIn5, Tc ~ 0.4 K

T. Park et al., Nature 440, 65 (2006)

Ce2MIn8 M = Co, Rh, Ir

Ce2CoIn8, Tc ~ 0.4 K

Genfu Chen et al., JPSJ 71, 2836 (2002)

Ce2RhIn8, Tc ~ 2 K@2.3GPa; Ce2IrIn8, NSC

Ce2PdIn8 new 218 compound

D. Kaczorowski et al., PRL 103, 027003 (2009)

Ce2PdIn8, Tc ~ 0.68 KCeIn3

ρ(Τ) Ce2PdIn8

Tc = 0.68 K, ΔTc = 20 mK, bulk Hc2 = 2.32 T, Hc2(onset) = 2.4 T

ρ ~ T at Hc2(onset)= 2.4 T.

Very thin crystals: t = 40 μm; no CeIn3 phase

ρ(Τ) Ce2PdIn8

ρ ~ AT2 at H > Hc2 A ∝ A0 (H - Hc2)α

Phase diagram Ce2PdIn8

In Ce2PdIn8, a field-induced AF QCP is located at Hc2 = 2.32 T.

κ(Τ) Ce2PdIn8

Gap with nodes, likely d-waveA first-order phase transiton near Hc2 = 2.32 T.

J. K. Dong et al., PRX 1, 011001 (2011)

κ(Η,θ) CeCoIn5 in rotating field

Four-fold sysmmetry: supporting d-wave gap

K. Izawa et al., PRL 87, 057002 (2001)

dilution fridge: 7 mK;vector magnet: 9T–3T

κ(Η,θ) Our second system

C/Τ Ce2PdIn8

H || ab H || c

Field-induced QCP near Hc2 in both H || c and H || ab

Y. Tokiwa, P. Gegenwart et al., PRB 84, 140507(R) (2011)

C/Τ Ce2PdIn8

Y. Tokiwa, P. Gegenwart et al., PRB 84, 140507(R) (2011)

CeCoIn5

No second phase was detected in Ce2PdIn8 by C/T.Hope with probes sensitive to magnetic order (NMR, μSR, ...),maybe in next generation of cleaner and larger single crystals.

Summary on Ce2PdIn8

Based on ρ and κ of Ce2PdIn8, we find:

A field-induced AF QCP near Hc2 = 2.32 T;

Nodal superconductivity, likely d-wave;

First-order transition near Hc2 = 2.32 T.

Ce2PdIn8 is another promising compound to study the “Q-phase, FFLO state”, as in CeCoIn5.The comparison between them may provide the key to understand the origin of Q phase, and the possible FFLO state.

J. K. Dong et al., PRX 1, 011001 (2011)

Multiple gap and multiple symmetries in PrOs4Sb12

PrOs4Sb12的晶体结构Filled skutterudite

PrOs4Sb12 Tc = 1.85 KFirst Pr-based heavy fermion SC

Bauer et al., PRB 65, 100506(R) (2002)

κ PrOs4Sb12和PrRu4Sb12的超导能隙

H = 0PrOs4Sb12: κ0/T = 0.46 mW/cmK2

(1/17 κN0/T) Δ存在节点PrRu4Sb12: κ0/T = 0.058 mW/cmK2

(1/140 κN0/T) Δ不存在节点

Hc1 < H < Hc2PrOs4Sb12: a small d-wave gap

+ a large s-wave gapPrRu4Sb12: a small s-wave gap

+ a large s-wave gap

R. Hill, S. Y. Li et al., PRL 101, 237005 (2008)

5. 铁基超导体

The 122 compounds:a) optimally electron-doped BaFe1.9Ni0.1As2b) heavily electron-doped BaFe1.73Co0.27As2c) extremely hole-doped KFe2As2d) Isovalently doped Ba(Fe1-xRux)2As2 and

BaFe2(As1-xPx)2

The 111 compound: NaFe1-xCoxAs

The 11 compound: FeSex

Open issues

Discovery of LaO1-xFxFeAs with Tc = 26 K

Y. Kamihara et al., JACS 130, 3296 (2008)Times Cited: 2740

23 February, 2008

SmO1-xFxFeAs with Tc = 43 K

X. H. Chen et al., Nature 453, 761 (2008)Times Cited: 869

25 March, 2008

Zoo of iron-based high-Tc supercondcutors

1、“1111” system: (La,Sm,Ce,Nd,Pr,Gd)OFeAs1) F substitution of O2) O deficiency3) Th4+ substitution of Gd3+

4) Co substitution of Fe

2、“122” system: (Ba,Sr,Ca,Eu)Fe2As21) K substitution of Ba 2) Co,Ni substitution of Fe3) Isovalent P or Ru substitutions

3、 “111” system LiFeAs, NaFe1-xCoxAs

4、 “11” system FeSex, FeTe1-xSex

5、 AxFe2-ySe2 (A = K, Rb, Cs)

The 122 compounds:

a) optimally electron-doped BaFe1.9Ni0.1As2

b) heavily electron-doped BaFe1.73Co0.27As2

c) extremely hole-doped KFe2As2

d) Isovalently doped Ba(Fe1-xRux)2As2 and BaFe2(As1-xPx)2

Phase diagram of 122 system

Phase diagram

Electron-doped Hole-doped

M. Neupane et al., PRB 83, 094522 (2011)

theory band structure and pairing state for LaFeAsO1-xFx

D. J. Singh et al., PRL 100, 237003 (2008) I. I. Mazin et al., PRL 101, 057003 (2008)

Multi-band s±-wave: interband hoping via QAF = (π, π)

Multiple nodeless gaps

ARPES SC gap structure in Ba0.6K0.4Fe2As2

H. Ding et al., EPL 83, 47001 (2008)

s±: Fermi surface nesting

P. Richard et al., PRL 102, 047003 (2009)

Nodeless gap

κ0/T hole-doped Ba1-xKxFe2As2 (Tc ~ 30 K)

k-dependent gap

X. G. Luo, Louis Taillefer, et al., PRB 80, 140503(R) (2009)

First heat transport measurement on iron-based superconductors

Nodeless superconducting gap in BaFe1.9Ni0.1As2

Tc = 20.3 K, optimal doping κ0 / T = 0, nodeless gap

κ0(Η)/T BaFe1.9Ni0.1As2 single crystal

L. Ding, S. Y. Li et al., New J. Phys. 11, 093018 (2009)

Hc2 ~ 50 T

Why different from Ba1-xKxFe2As2?For NbSe2, ΔL/ΔS ~ 3

ARPES Ba0.6K0.4Fe2As2 vs BaFe1.85Co0.15As2

H. Ding et al., EPL 83, 47001 (2008)

Δ0(α) ~ 12.5 meVΔ0(β) ~ 5.5 meV

Δ0(γ and δ) ~ 12.5 meV

ΔL/ΔS ~ 2.2

K. Terashima et al., PNAS 106, 7330 (2009)

α hole pocket disappearsΔ0(β) ~ 6.6 meV

Δ0(γ and δ) ~ 5.0 meV

ΔL/ΔS ~ 1.3

κ0 / T = 0, nodeless gap

Multiple nodeless gap in BaFe1.73Co0.27As2

Hc2 ≈ 17.0 T

κ0(Η)/T SC gaps of BaFe1.73Co0.27As2

Multiple nodeless gaps, ΔL/ΔS > 3

J. K. Dong, S. Y. Li et al., PRB 80, 024518 (2009)

ARPES BaFe1.85Co0.15As2 vs BaFe1.7Co0.3As2

For Co0.3-doped sample, the hole pocket disapprears, and Tc = 0 K.

To explain the κ0/T(H) of our Co0.27-doped sample, there should be a very small β-hole pocket and Δ(β) must be much larger than Δ(γ and δ).

It chould be an evidence for interband superconductivity:Δ1/Δ2 = (N2/N1)1/2

It will be very interesting to check this with low-temperature ARPES.

Y. Sekiba et al., New J. Phys. 11, 025020 (2009)

κ0(Η)/Τ Ba(Fe1-xCox)2As2 data of Taillefer’s group

M. A. Tanatar, Louis Taillefer et al., PRL 104, 067002 (2010)

Their results of BaFe1.772Co0.228As2 (Tc = 10.1 K) are consistent with ours.

They explained it with highly anisotropic gap, i.e., deep minima.

κ0(Η)/T theoretical curves for two ±s-wave gaps

Yunkyu Bang, PRL 104, 217001 (2010)

Nodal superconductivity in KFe2As2

Phase diagram

Electron-doped Hole-doped

M. Neupane et al., PRB 83, 094522 (2011)

Tc ~ 3 K RRR ~ 100

χ, ρ(Τ) susceptibility and resistivity

κ/Τ SC gap of KFe2As2

J. K. Dong, S. Y. Li et al., PRL 104, 087005 (2010)

a large κ0/T at H = 0 T κ0(H)/T similar to that in d-wave Tl2201Superconducting gap with nodes, likely d-wave.

Why different from other dopings?

In KFe2As2, the interband hoping is suppressed, due to the absence of electron pocket, therefore one does not expect s±-wave.

The nodal superconductivity results from the intraband pairing through AF spin fluctuations, likely d-wave.

ARPES Ba0.6K0.4Fe2As2 vs KFe2As2

T. Sato et al., PRL 103, 047002 (2009)

K(Fe1-xCox)2As2 anomalous impurity effect

Universal heat

conduction

X. H. Chen, S. Y. Li groups, arXiv:1206.2030

KFe2As2 d-wave or accidental nodal s-wave?

Dirty KFe2As2, ρ0 = 2.32 μΩcm Clean KFe2As2: ρ0 = 0.21 μΩcm

(1) Universal heat conduction (2) Field dependence for the clean KFe2As2 agrees with

calculation for a clean d-wave superconductor

Louis Taillefer group, arXiv:1201.3376, PRL in press

s-wave nodal?s-wave

strong SFvanishing SF

Supconducting gap for electron and hole doping

Nodal gap in Ba(Fe1-xRux)2As2 and BaFe2(As1-xPx)2

Isovalent substitutionP for As in Fe2As2 block Zhi Ren, Zhu’an Xu et al., PRL 102, 137002 (2009)

EuFe2(As0.7P0.3)2

Isovalent doping Phase diagram of BaFe2(As1-xPx)2

S. Kasahara et al., PRB 81, 184519 (2010)

BaFe2(As0.67P0.33)2 Nodal superconducting gap

K. Hashimoto et al., PRB 81, 220501 (2010)

Thermal conductivityPenetration depth

BaFe2(As0.7P0.3)2 Nodal gap in α hole pocket at kz = π

Y. Zhang, D. L. Feng et al., Nature Physics 8, 371 (2012)

ARPES

LaFePO(Tc = 6 K) Nodal superconducting gap

J. D. Fletcher et al., PRL 102, 147001 (2009) C. W. Hicks et al., PRL 103, 127003 (2009)

LiFeP(Tc = 4.5 K) Nodal superconducting gap

K. Hashimoto et al., PRL 108, 047003 (2012)

Isovalent doping Ru for Fe, instead of P for As

Nodal gap in LaFeOP,BaFe2(As1-xPx)2, LiFeP.

Why the P substitution is so special for inducing nodal superconductivity?

Ba(Fe1-xRux)2As2 Phase diagram

F. Rullier-Albenque et al., PRB 81, 224503 (2010)

Nodal gap?

Ba(Fe0.64Ru0.36)2As2 resistivity and susceptibility

Tc = 20.2 K, optimal dopingρ ~ T1.31 for 30 < T < 90 K

S. Kasahara et al., PRB 81, 184519 (2010)

BaFe2(As1-xPx)2

Ba(Fe0.64Ru0.36)2As2 thermal conductivity

A significant κ0/T at H = 0 T,clear evidence for gap nodes

Ba(Fe0.64Ru0.36)2As2 field dependence of κ0/T

κ0/T ~ H1/2, similar to Tl2201 and BaFe2(As0.67P0.33)2,further evidence for gap nodes

X. Qiu, S. Y. Li et al., PRX 2, 011010 (2012)

hPn decreases upon P and Ru substitution

BaFe2(As1-xPx)2 Ba(Fe1-xRux)2As2

S. Kasahara et al., PRB 81, 184519 (2010) F. Rullier-Albenque et al., PRB 81, 224503 (2010)

hPn crucial for nodal gap?

Threshold value of hPn: 1.33 ÅFor our Ba(Fe0.64Ru0.36)2As2: hPn = 1.317 Å,

hPn = 1.33 Å corresponds to Ba(Fe0.75Ru0.25)2As2

K. Hashimoto et al., PRL 108, 047003(2012)

For Ba(Fe0.77Ru0.23)2As2: hPn = 1.333 Å.For BaFe2(As0.82P0.18)2: hPn = 1.340 Å.

Undoped Ba(Fe0.77Ru0.23)2As2 and BaFe2(As0.82P0.18)2

Ba(Fe0.77Ru0.23)2As2 and BaFe2(As0.82P0.18)2 themal conductivity

Nodal gap in both underdopedcompounds with hPn slightly higher than 1.33 Å.X. Qiu, S. Y. Li et al., PRX 2, 011010 (2012)

Multiple nodeless gaps in NaFe1-xCoxAs

Phase diagram

A. F. Wang et al., arXiv:1204.3522

Band structure: α hole pocket and γ(δ) electron pockets

ARPES NaFe0.95Co0.05As

Z.-H. Liu et al., PRB 84, 064519 (2011)

Isotropic gap in α hole pocket and γ(δ) electron pockets

ARPES NaFe0.95Co0.05As

Z.-H. Liu et al., PRB 84, 064519 (2011)

Penetration depth NaFe1-xCoxAs

K. Cho et al., arXiv:1201.2966

Penetration depth NaFe1-xCoxAs

K. Cho et al., arXiv:1201.2966

Claim nodal gap from underdopded to overdoped regime.

More experiments are highly desiredto clarify this issue.

NaFe1-xCoxAs resistivity and susceptibility

S. Y. Zhou, S. Y. Li et al., arXiv:1204.3440

NaFe1-xCoxAs thermal conductivity

S. Y. Zhou, S. Y. Li et al., arXiv:1204.3440

Clear evidence for nodeless gap.

Supporting ARPES, and inconsistent with penetration depth experiments.

Structure of prototype FeSe Tc ~ 8 K (0 GPa); Tc ~ 37 K (8.9 GPa)

F. Hsu et al., PNAS 105, 14262 (2008) S. Medvedev et al., Nat. Mater. 8, 630 (2009)

Multiple nodeless gaps in FeSex

Our FeSex single crystals (lmax ~ 1 mm)

U. Patel et al., APL 94, 082508 (2009)

Hc2 ≈ 20.1 T

J. K. Dong et al., PRB 80, 024518 (2009)

κ0/Τ(Η) SC gaps of FeSex

J. K. Dong, S. Y. Li et al., PRB 80, 024518 (2009)

Multiple nodeless gaps, ΔL/ΔS ~ 3

Other evidences for nodeless gapsin FeTe1-xSex

STM Nodal gap in FeSe crystalline film

Can-li Song et al., Science 332, 1410 (2011)

Open issues

1) Nodeless superconducting gap in optimally doped BaFe1.9Ni0.1As2

Open issue: are there conclusive evidences for s±-wave state?

2) Multiple nodeless gaps in heavily overdoped BaFe1.73Co0.27As2

Open issue: highly anisotropic gap, or isotropic gaps with significantdifferent magnitudes? Low-temperature ARPES needed.

3) Nodal superconducting gap in KFe2As2

Open issue: d-wave, or accidental nodes? (Taillefer group’s new worksupport d-wave, but ARPES at 2 K disagrees.)

4) Nodal superconducting gap in isovalently doped Ba(Fe1-xRux)2As2

Open issue: where is the gap and what’s the origin of this kind of nodal gap induced by P and Ru doping?

5) Nodeless superconducting gap in NaFe1-xCoxAs

Open issue: how to explain the nodal behavior of the penetrationdepth? (we are cross-checking samples with Ames lab)

6) Multiple nodeless gaps in FeSex

Open issue: how to explain the STM data of FeSe thin film which shows nodal behavior?

Plenary Speakers: 闻海虎 中国 14位Keynote Speakers: 陈仙辉Invited Speakers: 鲍威

丁洪方明虎封东来靳常青李世燕孙力玲王楠林王强华向涛薛其坤周兴江戴鹏程 美国胡江平 美国郑国庆 日本

过去十年，我们不但在超导材料研究方面继续取得好的成绩，而且逐渐拥有了：ARPES, STM, neutron, NMR, μSR, optical, 极低温, 强磁场, 高压，……

未来十年，值得期待！