Characterization and Uncertainties in the New Superconductor A x Fe 2-y Se 2 (A= K, Rb)

Transcription

1 Characterization and Uncertainties in the New Superconductor A x Fe 2-y Se 2 (A= K, Rb) Hai-Hu Wen National Lab of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 2193, China National Lab for Superconductivity,Institute of Physics, Chinese Academy of Sciences, Beijing 119, China Novel Superconductivity, Minneapolis, April 22-24, 211

2 Acknowledgements to: Fei Han, Bin Zeng, Chunhong Li, Bing Shen, Lei Shan, Cong Ren, Institute of Physics, CAS, China Igor Mazin, Naval Research Lab, USA (Theory) Xigang Luo and L. Tailleffer, Sherbrook Univ. Canada: (Thermal conductivity) Fanlong Ning and Takashi Imai, McMaster Univ. Canada (NMR) Ruslan Prozorov, Ames National Lab, USA: (penetration depth) Gengfu Chen et al., Renmin University (K 1-x Fe 2-y Se 2 )

3 Outline of the talk Brief introduction to the Fe-based superconducting systems and the basic understanding about the pairing symmetry The new superconducting family K x Fe 2-y Se 2 and the possible challenges Nodeless gap revealed by low-t specific heat Ordered Fe-vacancies, insulating ground state and superconductivity Intrinsic percolative superconductivity in K x Fe 2-y Se 2 Concluding remarks

4 Outline of the talk Brief introduction to the Fe-based superconducting systems and the basic understanding about the pairing symmetry The new superconducting family K x Fe possible challenges Fe 2 --yse Nodeless gap revealed by low-t T specific heat Se 2 and the Ordered Fe-vacancies, insulating ground state and superconductivity Intrinsic percolative superconductivity in K x Fe Concluding remarks Se 2 Fe 2 --yse

5 Known FeAs- and NiAs-based superconductors K 25 K K 39 K 3K? 4 K 47 K FeSe M. K. Wu Y. Takano LiFeAs NaFeAs Paul Chu C. Q. Jin LaFeAsO H. Hosono X. H. Chen N. L. Wang Z. X. Zhao Z. A. Xu CaFeAsF H. Hosono D. Johrendt H. H. Wen BaFe 2 As 2 BaNi 2 As 2 KFe 2 Se 2 FeAs: D. Johrendt FeSe X. L. Chen NiAs: J. D. Thompson F. Ronning (Sr 3 Sc 2 O 5 )Fe 2 As 2 H. H. Wen D. Johrendt (Sr 4 Sc 2 O 6 )Fe 2 P 2 (Sr 4 V 2 O 6 )Fe 2 As 2 FeP: Shimoyama V-FeAs: H. H. Wen D. Johrendt Ca 4 (Mg,Ti) 3 O 8 Fe 2 As Shimoyama

6 Band structure calculations: LaOFeP Fe 2+ 3d 6 : five orbitals There are five bands crossing the Fermi energy. Hole pockets around Γ; Electron pockets around M S. Lebegue, PRB 75, 3511 (27): Without considering the AF order D. Singh et al. PRL28 G. Xu, et al., EPL28

7 k AF Pairing: interpocket scattering of of electrons via via exchanging the the spin spin fluctuations S ± or extended s-wave Δ~ cos k x + cos k y Δ~ cos k x cos k y Eliashberg equation Δ( k') 1 Δ( k) = V ( k, k') tanh βe( k' ) 2E( k') 2 Γ M I. I. Mazin, et al. PRL11, 573 (28) K. Kuroki et al., PRL11, 874 (28) Z.J. Yao et al, NJP. 11, 259 (29) F. Wang et al., PRL12, 475 (29) Multiband + AF spin fluctuation

8 Superconductivity relies crucially on the multiband effect! R H σeμe + σhμh = ( σ + σ ) 2 e h σ = n e / m i i 2 τ i i μ = i e τ / m i i i Lei Fang et al., Phys. Rev. B8, 1458 (R) (29) (with I. Mazin) 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!

9 H c1 : Ba.6 K.4 Fe 2 As 2 H c1 (Oe) H c1 (Δ a ) Sample No. 1 H c1 (Δ a ) Sample No. 2 H c1 (Oe) H c1 (Oe) T (K) T (K) H c1 (Δ b ) T (K) (a) H c1 (Δ b ) (b) ρ = xρ + ( 1 x) ρ Δ Δ x Δ Δ x s a b = a b = a s = 1.6 ± = 9.1±.72 = 2.2 ± = 8.8 ±.7 C. Ren et al.,.3mev.3mev.3mev b s.2mev Phys. Rev. Lett.11, 2576 (28).

10 Multigap evidence from STM measurements in Ba.6 K.4 Fe 2 As 2 L. Shan et al., PRB 83, 651 (R) (211).

11 1/ T T 1 q r 2 r Ahf ( q) χ''( q, f ) / 1/T 1 T measures the q-intergral of the imaginary part of the dynamical spin susceptibility, reflecting the summation of all different q-modes of spin fluctuations. F. L. Ning et al., PRL14, 371(21) f

12 Red: xz Green:yz Blue: xy Orbital weights on different FSs. Maier, T. A., Graser, S., Scalapino, D. J. & Hirschfeld, P. J. Rev. B 79, These were calculated using a five-orbital (29). d xz,d yz,d xy,d x2 y2,d 3z2 r2 tight-binding fit to Kuroki, K. et al. Phys. Rev. B 79, the density functional (29). theory band-structure calculations Wang, F., Zhai, H. & Lee, D. H. EPL 85, 375 (29).

13 Orbital dependent interaction via exchanging AF spin fluctuation Gap anisotropy should be a normal thing to see! The nodes can appear, but not enforced by symmetry and at accidental positions!

14 Clear gap anisotropy on the electron FSs B. Zeng HHW. Nature Comm. 1, 157 (21) Chubukov, A. V. and Eremin, I. PRB82, 654(R)(21) Vorontsov, A. B. and Vekhter, I. Phys. Rev. Lett. 15, 1874 (21)

15 Outline of the talk Brief introduction to the Fe-based superconducting systems and the basic understanding about the pairing symmetry The new superconducting family K x Fe 2-y Se 2 and the possible challenges Nodeless gap revealed by low-t T specific heat Ordered Fe-vacancies, insulating ground state and superconductivity Intrinsic percolative superconductivity in K x Fe Concluding remarks Se 2 Fe 2 --yse

16 J. G. Guo et al. Phys. Rev. B 82, 1852 (R) (21) C. H. Li et al., arxiv: , PRB. D. M. Wang, et al., Phys. Rev. B 83, (211). R. Hu et al., Supercond. Sci. Technol. 24, 656 (211).

17 M. H. Fang et al., Arxiv Insulating due to Band gap or Strong correlation?

18 arxiv

19 T. Qian, et al, arxiv:

20 X. P. Wang et al., EPL93, 571(211)

21 Theoretical proposals for the gap symmetry arxiv arxiv arxiv arxiv T. A. Maier, S. Graser, P. J. Hirschfeld and D. J. Scalapino Five band tight binding fit + RPA(SF) Nodeless d-wave Fa Wang, Dunhai Lee Two stage FRG Nodeless d-wave or nodal extended s-wave I. I. Mazin Argument based on the stryctural symmetry T. Saito, S. Onari and H. Kontani ten-orbital Hubbard- Holstein model Conventional S or other kind of S± (nodeless d leads to the nodes at FS at k z =π/2) S++ or d-wave Depending on the SF and Orbital fluctuation

22 Outline of the talk Brief introduction to the Fe-based superconducting systems and the basic understanding about the pairing symmetry The new superconducting family K x Fe possible challenges Fe 2 --yse Nodeless gap revealed by low-t specific heat Se 2 and the Ordered Fe-vacancies, insulating ground state and superconductivity Intrinsic percolative superconductivity in K x Fe Concluding remarks Se 2 Fe 2 --yse

23 Single band estimate: H H c c2 ab c2 () 42T () 145T

24 Anisotropic GL equation ρ = ( GL H / ) f H c 2 GL ab H c 2 ( θ ) = H c2 / sin ( θ ) + γ cos ( θ ) C. H. Li et al., arxiv: PRB in press. More 2D than other FeAs-122 systems Supported by the ARPES data

25 Nodeless gap revealed by Low-T specific heat 5-5 M (emu/mol) KFe 2 Se 2 mass=8.93mg H= 2 oe T (K) 8 T c C/T (mj/mol K 2 ) T H//c 9T Sample from the same Batch Prepared by G. F. Chen et al. using flux method T (K)

26 Nodeless gap revealed by Low-T specific heat C/T (mj/mol K 2 ) H//c T H//c 9T T c C/T (mj/mol K 2 ) T 2 (K 2 ) T 2 (K 2 ) H//c T H//c 9T fit of T fit of 9T C( T, H ) / T = γ ( H ) + βt γ ().394mJ γ (9T ) 1.4mJ η.3mj Θ D = = 212 K / molk / molk / molk ηt ( 4 12π k N Z / 5β ) B A 1/ 3 4

27 Nodeless gap revealed by Low-T specific heat ΔC T c = mJ / molk T ± Not considering the phase separation 2 ΔC T c = mJ / molk T ± 2 Ba.6 K.4 Fe 2 As 2 ΔC T Tc 1mJ / molk G. Mu et al., PRB 29 2 ΔC T c = mJ / molk T ± 2 Ba(Fe 1-x Co x ) 2 As 2 ΔC T T c 3mJ / molk 2 B. Zeng et al., Phys. Rev. B 83, (211) SmFeAsO.85 F.15 ΔC T 15mJ / molk c T 2

28 Nodeless gap revealed by Low-T specific heat C/T (mj/mol K 2 ) (a) T 2 (K 2 ) T 1T 3T 5T 7T 9T B. Zeng et al., Phys. Rev. B 83, (211) ΔC/T (mj/mol K 2 ) Δγ(H) (mj/mol K 2 ) (b) (c) T 2 (K 2 ) d-wave H (T)

29 Nodeless gap revealed by Low-T specific heat C cal-s nodeless gap scaling 1T 3T 5T 7T 9T.12*x -2 (s-wave theory) S wave C C T core core 3 γ T n c2 H H () c2 γ n = H () T H 2 d wave C C T vol vol 2 T g H T f H T H Simon-Lee scaling PRL1996 C cal-d T 3T 5T 7T 9T T/H.5 (K/T.5 ) d-wave scaling Taking H c2 () = 48 T we get γ n = 5.8mJ / molk ΔC 1.93 γ T n c 2 (1.43 BCS weak coupling) C cal-s C cal-d s-wave d-wave 1 1 T / H.5 (a) (b) Z. Y. Liu et al. about s-wave in Sr.1 La.9 CuO 2, EPL69, 263 (25).

30 Outline of the talk Brief introduction to the Fe-based superconducting systems and the basic understanding about the pairing symmetry The new superconducting family K x Fe possible challenges Fe 2 --yse Nodeless gap revealed by low-t T specific heat Se 2 and the Ordered Fe-vacancies, insulating ground state and superconductivity Intrinsic percolative superconductivity in K x Fe Concluding remarks Se 2 Fe 2 --yse

31 Co-existence of AF order T N = 559 K) and superconductivity! With ordered moment of μ B /Fe! arxiv

32 Random distributed Fe-vacancies vs. Superconductivity We used the flux method to grow the KFe 2 As 2 (nominal composition) single crystals. The output are very nice, dark and shiny crystals, but insulating. The EDX analysis indicates that it is: EDX: K.67 Fe 1.45 Se 2 : close to the ordered state K.8 Fe 1.6 Se 2, or K 1 Fe 1.5 Se 2. 3.x x1 8 2.x1 8 Intensity ρ (Ω mm) 1.5x1 8 1.x1 8 5.x T (K) θ (degree) F. Han et al., arxiv:

33 Ordered State of Fe-vacancies vs. Superconductivity (a) ρ (mω cm) (c) ρ (mω cm) 2.5x1 7 2.x x1 7 1.x1 7 5.x lnρ (mω cm) not annealed 1 mev /T (K -1 ) T (K) annealed at 3 o C for 1 h T (K) (b) ρ (mω cm) (d) ρ (mω cm) annealed at 2 o C for 1 h T (K) annealed at 4 o C for 1 h T (K) M (emu/mol) FC ZFC not annealed annealed at 2 o C for 1 h annealed at 3 o C for 1 h annealed at 4 o C for 1 h diamagenetic signal (emu/mol) T (K) H = 5 Oe annealing temperature ( o C) F. Han et al., arxiv: Post annealing and fast quenching of insulating K.6 Fe 1.5 Se 2 induces superconductivity, even with quite large superconducting volume.

34 Ordered State of Fe-vacancies vs. Superconductivity not annealed annealed at 2 o C for 1h annealed at 3 o C for 1h annealed at 4 o C for 1h Intensity (arb. units) θ (degree) Post annealing and fast quenching does not alter the structure very much. Even the lattice constant does not change!

35 Ordered State of Fe-vacancies vs. Superconductivity (a) (c) ρ (mω cm) M (emu/mol) x x1 6 2.x x1 6 1.x1 6 5.x1 5. FC ZFC H = 5 Oe Post annealed and fast qucenched T (K) (b) M (emu/mol) FC ZFC 2 days later days later T (K) H = 5 Oe T (K) The randomly distributed Fe-vacancies lead to superconductivity. The disordered Fe vacancies relaxed back to the ordered state?

36 Post-annealing leads to the formation of some small grains with rich K, but the average composition and the structure does not change too much. The small K-rich grains seem having the iso-structure. Ordered State of Fe-vacancies vs. Superconductivity (a) (b) relative content K Fe Se K:.65 Fe:1.47 Se: plot (c) (d) 1 K.6642 Fe Se 2 2 K.6837 Fe Se 2 3 K.6732 Fe Se 2 4 K.6788 Fe Se 2 average content: K.675 Fe Se 2

37 Post-annealed K 1-x Fe 2-y Se 2 ρ (Ω cm) R H (1-8 m 3 /C) T (K) Multiband still plays the important role

38 Outline of the talk Brief introduction to the Fe-based superconducting systems and the basic understanding about the pairing symmetry The new superconducting family K x Fe possible challenges Fe 2 --yse Nodeless gap revealed by low-t T specific heat Se 2 and the Ordered Fe-vacancies, insulating ground state and superconductivity Intrinsic percolative superconductivity in K x Fe 2-y Se 2 Concluding remarks

39 Intrinsic percolative superconductivity in K 1-x Fe 2-y Se 2 M (emu/cm 3 ) M(emu/cm 3 ) ZFC FC H = 2 Oe T(K) K x Fe 2-y Se 2 MHL ZFC μ H (T) (a) T = 2 K (c) M (emu/cm 3 ) M (emu/cm 3 ) T = 2 K K x Fe 2-y Se 2 (b) T = 2 K μ H (T) Ba.6 K.4 Fe 2 As 2 BaFe 1.84 Co.16 As 2 (d) M(emu/cm 3 ) K 5 K 8 K 11 K 14 K 17 K 2 K K 25 K 26 K 27 K 28 K 29 K 3 K H>H p H=H p H c1 <H<H p (a) (b) B. Shen et al. arxiv: H(T)

40 Intrinsic percolative superconductivity in K 1-x Fe 2-y Se 2 M(emu/cm 3 ) K 3 K 4 K 5 K 6 K 7 K 8 K 9 K 1 K 11 K 12 K 13 K 14 K 15 K 16 K 17 K 18 K 19 K 2 K (a) M (emu/cm 3 ) K 1-x Fe 2-y Se 2 at T = 2 K M(emu/cm 3 ) the first H p1 the first H p1 the second H p H(Oe) the second H p2 (b) M (emu/cm 3 ) -2-4 B. Shen et al. arxiv: Meissner Screening at T = 2 K H (Oe) Ba.6 K.4 Fe 2 As 2

41 Concluding Remarks (1) Enormous data now have proofed that, the key ingredients for superconductivity are: multi-band or multigap, and AF spin fluctuation. (2) Hall coefficient in K 1-x Fe 2-y Se 2 was shown to be negative, temperature dependent, indicating the dominance of the electron-like charge carriers, but the multi-band effect are certainly present. (3) The anisotropy in K 1-x Fe 2-y Se 2 is clearly larger than the counterparts Co- and K- doped BaFe 2 As 2, indicating weaker warping effect. (4) The low-t specific heat was measured, showing a sharp SH anomaly. The field dependence at low temperatures together with the scaling give the evidence of nodeless gaps. (5) The superconductivity can be recovered through fast quenching of the Fe-vacancy ordered state (even around Fe 1.5 ), suggesting that the superconducting state may not co-exist with the Fe-vacancy ordered state. (6) Evidence of intrinsic percolative superconductivity from the magnetic penetrating measurements. Cautious are needed to draw any conclusions about the coexistence of SC and AF order (from the Fe vacancy ordered state). How to interpret the ARPES data? The bands near Γ are certainly involved, how?