NMR investigation on iron-based superconductors Interplay ...online.itp.ucsb.edu/online/ferrosup-m11/nakai/pdf/... · susceptibility Korringa law :T 1 T = const. (Characteristics

Mini workshop on iron-based superconductors (Minipgm)

Kavli Insitute for Theoretical Physics, Santa Barbara,10 Jan. 2011

NMR investigation on iron-based superconductors

Interplay between magnetism and superconductivity

Yusuke Nakai

Department of Physics, Kyoto UniversityJST – TRIP

Collaborators

NMR

K. Ishida S. Kitagawa T. Iye

Sample: BaFe2(As1-xPx)2

Kyoto Univ.

T. ShibauchiS. Kasahara Y. Matsuda

Sample: LaFeAs(O1-xFx)

Tokyo Inst. of Technology

Y. Kamihara H. Hosono

Kyoto Univ.

Contents

1. Introduction to iron-based superconductors & NMR

4.Summary

3.Comparison with other iron-based families

●P-concentration dependence of magnetic fluctuations

●Comparison with K- & Co-doped BaFe2As2

●Suppression of internal field at low P concentrations

2. NMR results in 122 superconductorBaFe2(As1-xPx)2

Contents


4.Summary






Hyperfine Interaction between Nuclear and Electronic Spins

Nuclear spins ( I ) are coupled with electronic spins ( S ) polarized by magnetic field (H0).

H0

Nuclear spin

mn = gnħI

Electron spin me = geħS

Knight Shift (proportional to spin susceptibility c(q = 0))

)0(AA

2

neA0n0

Loc0

q

Nc

ggg H

S

H

HHK

Hamiltonian of the nuclear spins

HZ = -gnħ I H0 + A I S = -gnħ I HLoc

A: Hyperfine coupling tensor

HLoc = H0 -gnħA

S = H0 -gnħ

A{S + dS }

Av. static dynamics

Local field at the nuclear site

Hyperfine interaction

Dynamical component of hyperfine interaction

SISISISI zzn dddgdg2

1

Nuclear spin-lattice relaxation rate: 1 /T1

Dynamical susceptibility

Korringa law :T1T = const. (Characteristics of Metal)

nMTT

T

T

1

1Magnetic fluctuations

2D FM n=3/2, AFM n=1

3D FM n= 1, AFM n=1/2

Moriya et al.

q-sum information on spin fluctuations with very low energy (~mK order)

⇔Neutron experiments

1/T1 = const. for a paramagnetic state of local moment systems

Superconductivity in BaFe2As2 (122) system

H.Chen et al. EPL85, 17006 (2009)X.F.Wang et al. arXiv. 0811.2920

BaFe2-xCoxAs2Ba1-xKxFe2As2

ele.-doping

hole-doping

BaFe2As2:Magnetic and Structural ordering at ~135 KBad metal

Isovalentdoping

Isovalent P substitution induces superconductivity.

S. Jiang et al. J. Phys: Cond.Matt. 21 382203 (2009)

⇔Cuprates (Mott insulator)

Isovalent P doped superconductor BaFe2(As1-xPx)2

Superconductivity with Tc =30 K is realized by isovalent P doping.High-quality single crystal

dHvA signal observed above x ~0.4.

resistivityKasahara et al., Phys. Rev. B 81 (2010) 184519Jiang et al. J. Phys: Cond. Mat. 21 382203 (09)

Quite sharp SC transition

Form Factor at As (P) site

AF

BaFe2As2

D D

D Applres HH

T < TSDW: Internal field (D) at the As site

Happl || c-axis:

Happl || a or b-axis:22

D Applres HH

NMR spectra indicate that internal field D is parallel to the c-axis.

K. Kitagawa et al., JPSJ 78 (2009) 063706.

Hyperfine Fields at As (P) site

c

ab

ab

ASS

SAC

SCA

21

2

1~A

HAs : HF at the 75As siteSFe : Spin at the Fe siteÃ : HF coupling tensor

S1[2] : off-diagonal terms related with(p, 0)[(0, p

C : off-diagonal terms related with (p, p) SF

Ai : diagonal terms

Hyperfine field determined by neighboring Fe spins

Fe3

Fe2Fe4

Fe1

Hyperfine fields due to the stripe spin correlations

|dSa| |dSb| |dSc|As

K. Kitagawa et al., JPSJ 78 (2009) 063706.

Fe spin in the ab-plane creates hyperfine fields along c-axis at As & P sites.

Stripe AF spin fluctuations in BaFe2(As1-xPx)2 (x ~0.33)

dSxFe dHc @As

dSzFe dHa @As

Stripe (p,0) AFM

(T1T)-1H//ab ∝|dHab|2 + |dHc|

2∝1/2|dSz|2+|dSx|

2

(T1T) -1H//c ∝ |dHa|2 + |dHb|2 = 2|dHab|2 ∝|dSz|

2

Isotropic spin component |dSx|=|dSy| =|dSz| : (T1T) -1H//ab / (T1T) -1

H//c = 1.5

x

z

S

S

S

d

d

d 04AsH

Tc

BaFe2(As1-xPx)2,

x ~ 0.33

Anisotropy in (T1T)-1 can provide information on q-vector of spin fluctuations.

(T1T)

-1ab

/ (T

1T)

-1c

1.5

Contents


4.Summary






Overview of NMR results in BaFe2(As1-xPx)2 (0.2 <= x <= 0.64)

Knight shift is temperature independent. Knight shift is nearly unchanged between x=0.20 and 0.56.Significant AFM fluctuations at the optimal P-concentration (x=0.33, Tc=30K).The AFM fluctuations are suppressed by P-substitution, and (T1T)-1=const.

is observed at x=0.64 (Tc~7 K). Fermi liquid

31P Knight shift 31P (T1T )-1

Kspin

Kspin∝cspin = mB2N(EF)

)0( qc q

q )0~,( c

Evolution of AFM fluctuations by P substitution for As

31P (T1T )-1

AFM

P dependence of a and b

P dependence of q

a(s

K )

-1

b(s

)-1a

b

q

T

ba

TT1

1

Moriya, Takahashi, Ueda, JPSJ 59 (1990) 2905.

2D AFM spin fluctuations

A possible QCP near an optimum concentration (x ~ 0.33)

A QCP inferred from 1/T1T coincides with a Tc maximum in the SC dome of BaFe2(As1-xPx)2.

P dependence of q and Tc31P (T1T )-1

q

T

ba

TT1

1

QCP

Summary of Experimental Results : r , m* and (T1T )-1

Resistivity

~T

~T 2

Ta

(T1T)-1

Effective mass

YN et al., PRL 105(2010) 107003.

Comparison: Ba(Fe1-xCox)2As2

Ba(Fe,Co)2As2

(ele. doping)

75As (T1T )-175As Knight shift

Korb

cspin = mB2N(EF)

N(EF) decreases, and AF fluctuations are suppressed by Co(electron) doping.

Ning et al., PRL 104 (2010) 037001.

Comparison: (Ba,K)Fe2As2

Fukazawa et al., JPSJ 78 (2009) 033704.; ibid 78 (2009) 083712.Yashima et al., ibid 78 (2009) 103702.

(Ba,K)Fe2As2

(hole doping)

75As Knight shift 75As (T1T )-1

Korb

cspin = mB2N(EF)

N(EF) increases with K(hole) doping

Strong Energy Dependence of DOS: Possible Origin of c (q=0,T)

Singh and Du, PRL, 100, 237003 (2008).DOS of LaFeAsO

Knight shift & (T1T)-1 dataElectron doping :

DOS decreaseHole doping :

DOS increaseIsovalent substitution:

DOS slightly decrease

Theoretical work considering the band structure effect appears consistent with the data.

H. Ikeda, JPSJ 77(2008)123707.

Contents


4.Summary






Positive link between AF fluctuations & SC in stoichiometric FeSe

T. Imai et al., PRL 102 (2009) 177005.

At 0 GPa, AF fluctuations grows prior to superconducting transition.Tc & AF fluctuations increase under pressure while c(q=0) unchanged.Resistivity rab ~ T is observed below 100 K. Non Fermi liquid behaviorSimilar to 122 superconductors.

Knight shift ~

(T1T)-1 ~

)0( qc

q

q )0~,( c

LaFeAs(O1-xFx) : Resistivity

Powdered samples and resistive data are provided by Hosono’s group.

LaFeAs(O1-xFx) (x=0.0, 0.04, 0.07, 0.11 and 0.14)

r –T plot

x=0.0(non-SC)

x=0.04 (Tc=16.3 K)

x=0.07(22.5 K)

x=0.11(22.5K)

x=0.14(12.3K)

LaFeAs(O1-xFx) : Resistivity

LaFeAs(O1-xFx) (x=0.0, 0.04, 0.07, 0.11 and 0.14)

FL behaviors characterized by

r~T 2 was observed up to 150K

r –T 2 plotx=0.0(non-SC)

x=0.04 (Tc=16.3 K)

x=0.07(22.5 K)

x=0.11(22.5K)

x=0.14(12.3K)

Evolution of magnetic fluctuations by F-doping in LaFeAs(O1-xFx)

x=0.0(non-SC)

x=0.04(Tc=16.3 K)

Tc=

16.3 K (x=0.04)

x=0.07(22.5 K)

22.5 K (x=0.07)

x=0.11(22.5K)

22.5 K (x=0.11)

x=0.14(12.3K)

12.3 K (x=0.14)

⇔ Tc changes only by a factor of 2.1/T1T depends strongly on F-doping (2 orders difference!!)

H~9.9T

As-NMR

YN et al. JPSJ 77, 073701 (2008).YN et al. New J. Phys. 11, 045004 (2009).

(T1T)-1 ~q

q )0~,( c

Different Phase diagrams: LaFeAs(O1-xFx) & Ba-122

Ba(Fe1-xCox)2As2

TN, TS are gradually suppressed.(2nd order like)Coexistence is suggested.

LaFeAs(O1-xFx)

TN, TS are sharply suppressed(1st order like).Phase separation is suggested.

Relation between antiferromagnetism and superconductivity appears different.

Luetkens et al., Nature Mater. 8, 305309 (2009). Laplace et al., Phys. Rev. B 80, 140501 (2009).

Clean single crystalline samples of 1111 are needed for clarifyng this issue.

111 Superconductor LiFeAs: Tcmax= 18 K

Ma et al., PRB 82 (2010) 180501(R).(T1T)-1

Lower Tc

~10KMedium Tc

~16K

Knight shift

Li et al., JPSJ 79 (2010) 083702.

(T1T)-1

Higher Tc

~17KStrength of spin fluctuations appears to depend on synthesis process.Lower Tc samples seem to have stronger AF spin fluctuations.

Systematic investigation on sample characterization is needed.

New high-Tc superconductor AyFe2-xSe2 Tc up to 40 K (122 structure)

M. Fang et al., arXiv:1012.5236.

Similar to the phase diagram of high-Tc cuprates.

Strong electron correlation plays a role??

By tuning Fe vacancies, AFM insulators turn into high-Tc superconductors.

J. Guo et al., PRB 82 (2010) 180520(R)Resistance

Alkali-metal intercalation in between FeSe layers induces a Tc of 30 K in K0.8Fe2Se2.

77Se NMR study on K0.8Fe2-xSe2 (Tc = 30 K)

K decreases on cooling.Likely to be spin-singlet.

Knight shift

(T1T)-1 No enhancement of (T1T)-1 at low T.Korringa law holds (T1TK2 = const.)

Fermi liquid

Issue to be clarifiedAbsence of spin fluctuations despite its high Tc?Fe NMR would be very important.

W. Yu et al., arXiv:1101.1071.

1/T1 in SC state

No Hebel-Slichter coherence peak?

Summary

Behavior of spin susceptibility is likely to originate from the DOS at EF, as inferred from the Knight shift of P-, K-, &Co-doped BaFe2As2.

AFM fluctuations with itinerant 2D nature are largely changed by Co & P substitution, and a QCP is suggested at optimal concentration for P-& Co-doped BaFe2As2.

Similar positive correlation between AFM fluctuations & SC is also observed for stoichiometric FeSe.

AFM fluctuations likely play a crucial role for superconductivity in the 122 & 11 systems.

However...Correlation between spin fluctuations & SC is weak in LaFeAs(O1-xFx).This may be also the case for newly discovered K1-yFe2-xSe2.

NMR investigation on iron-based superconductors Interplay ...online.itp.ucsb.edu/online/ferrosup-m11/nakai/pdf/... · susceptibility Korringa law :T 1 T = const. (Characteristics

Documents