Superconducting properties of FeSe 0.5 Te 0.5

Transcription

1 Superconducting properties of FeSe 0.5 Te 0.5 C.V. Tomy G. Balakrishnan and M.R. Lees Dept. of Physics, University of Warwick, UK Pradip Das and A.K. Grover Department of CMP&MS, TIFR Ravi P. Singh, Anil K Yadav, and S. Marik Department of Physics, IIT Bombay

2 Outline: Introduction Resistance Heat capacity Pressure Dependence Magnetization Conclusions

3 Introduction 1111 structure As Fe 122 structure Parent : LaOFeAs/LaOFeP Similar Tc ~4 building K blocks; no spacing LaO layers 1-x F x FeAs : hole doping : Tc ~ 28 K : first report (0 x 0.1) Ba La can be replaced by other rare earths. Se La O Sm 1111 has the highest Tc ~ 55 K A(Fe 1-x Co x )AsF another 1111 structure : electron-doped; A = Ca, Sr : Tc ~ 22 K Superconducting electrons flow in the planes that contain Fe Pnictogen,replaced by Chalcogen preserves the layer s charge balance,

4 122 - introduction 122 structure As Fe Similar building Parent : AFe 2 As 2 (A = Ba,Sr,Ca) blocks; no spacing layers A 1-x B x Fe 2 As 2 : hole doping : Tc ~ 22 K : Ba B = K, Cs, Na. Se Sm 1111 has the highest Tc ~ 55 K A(Fe 1-x Co x ) 2 As 2 : electron-doped; Superconducting electrons flow in the planes that contain Fe Pnictogen,replaced by Chalcogen preserves the layer s charge balance,

5 11 - Structure - comparison 1111 structure As 122 structure Similar building blocks; no spacing layers Fe Ba Se La O Pnictogen,replaced by Chalcogen preserves the layer s charge balance,

6 How it started! : 1 st report in 2008 T c ~ 8 K only with Se deficiency 1-

7 Fe-Se : Phase diagram -FeSe hexagonal NiAs type -Fe 1-x Se Tetragonal PbO type Fe 7 Se 8 FeSe 2 H Okamoto, J. Phase Equi. 1991

8 Fe-Se : Phase diagram Cava Group McQueen etal, PRB (2009)

9 Structure Tetragonal : PbO type hexagonal : NiAs type Edge sharing FeSe 4 tetrahedra stacked layer by layer

10 Structure - comparison 1111 structure 122 structure Similar building blocks; no spacing layers As Fe Ba Se La O Pnictogen,replaced by Chalcogen preserves the layer s charge balance,

11 Structure : Fe As(Se) layers Top (c-axis) view side (b-axis) view

12 Te substitution (FeSe 1-x Te x ) Fang et al, arxiv:

13 Sample Preparation FeSe 0.5 Te 0.5 Step 1: Stoichiometric Fe, Se and Te in evacuated quartz tube at 650C for 24 hrs Step 2: Seal in another evacuated quartz tube (while cooling, quartz tube breaks) Step 3: In Bridgeman Furnace: heated to 970C, pulled out at 2-5 mm/hour Step 4: As grown crystals can be sliced off to smaller crystals Step 5 : Characterised by X-ray diffraction

14 X-ray powder diffraction

15 Resistance Measurements WHH VALUES T c decreased by only 5.5% in 9 T dtc/db = K/tesla T c decreased by only 11% in 9 T dt c /db = K/tesla

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17 Heat capacity Measurements C(T c ) = 210 mjmol 1 K 1 Raw data T c C tot C ph BCS fit Energy gap with nodal points/lines?? Does not fit nicely! C ph 2 Debye + 1 Einstein + C el = n T Single crytal 0 T 3 T 6 T 9 T

18 Isofield Magnetization No ve Meissner component!!! McQueen etal, PRB (2009)

19 Isofield Magnetization

20 Isofield Magnetization H c H // c

21 Pressure dependence dt c /dp = +0.7 K / kbar

22 Isothermal Magnetization Ba(Fe 0.93 Co 0.07 ) 2 As 2

23 Isothermal Magnetization

24 Low fields (mt) Yang et al., PRB (1993)

25 Isothermal Magnetization

26 Isothermal Magnetization

27 j c (A cm -2 ) j c (nomalized) Isothermal Magnetization Jc 40x H // c FeSe 0.5 Te K 7 K 9 K 11 K FeSe 0.5 Te 0.5 H // c 5 K 7 K 9 K 11 K H(Oe) H (Oe)

28 M (emu) M(normalized) Isothermal Magnetization - relaxation T FeSe 0.5 Te 0.5 H // c 9 K (a) 4 T time (sec) H(kOe) 60 90

29 M (emu) Isothermal Magnetization - relaxation Prozorov, Canfield, PRB 78 (2008) 0.14 collective plastic FeSe 0.5 Te 0.5 H // c 9 K 0.06 Ba(Fe 0.93 Co 0.07 ) 2 As H(kOe) 60 90

30 Nm) Nm) Torque Measurements Nm) FeSe 0.5 Te 0.5 T = 6 K T = 10 K T = 20 K 0-10 deg H = 500 Oe (deg) 150 T = 6 k T = 10 k T = 20 k FCC 40 FeSe 0.5 Te 0.5 T = 6 k T =10 k T = 20 k -40 ZFC FCW (deg) T (K) 16 20

31 M (10-3 emu) M (10 3 emu) Fe 1 Se 0.5 Te 0.5 : off- stoichiometry Fe 1.03 Se 0.5 Te H = 100 Oe T (K) ZFC FCC FCW Fe 1.03 Se 0.5 Te 0.5 H c T = 2 K H (koe)

32 M (emu) M (10-6 emu) Fe 1 Se 0.5 Te 0.5 : off- stoichiometry Fe 1.01 Se 0.5 Te H c T (K) 10 H = 10 Oe 12 ZFC FC Fe 1.01 Se 0.5 Te 0.5 H c T= 2 K H (Oe) 50

33 M (emu) M (10-3 emu) Fe 1 Se 0.5 Te 0.5 : off- stoichiometry -2-4 Fe 0.99 Se 0.5 Te H c H = 10 Oe ZFC FC T (K) Fe 0.99 Se 0.5 Te 0.5 H c 5K 7K H (koe)

34 Comparison : Fe based superconductors Layered materials, small coherence length Parent compounds : poor conductors (semi-metals) (insulators for cuprates) AFM and SDW in parent compounds, weakening of AFM key factor in the appearance of superconductivity (parent compounds are AFM in cuprates) Unconventional pairing : s wave variant : s ; two different sets of Fermi surfaces; each by itself is conventional but with a phase difference of. Sign of order parameters is opposite (d waves in cuprates) Tc highest in F-doped Sm-1111 compound Electron and hole doped

35 Comparison : Fe based superconductors Superconducting electrons flow in the planes that contain Fe. Superconductivity is largely 2D. Inter-layer coupling gives a 3 rd dimensionality. Pnictogen (Chalcogen) atoms protrude above and below the plane. Pn(Ch) atoms are much bigger than Fe atoms, form edgesharing tetrahedra (corner-sharing octahedra in cuprates; size difference between Cu and O atoms are much smaller) Due to Fe atoms close-packing, all five 3d orbitals contribute charge carriers. (only one Cu 3d orbital in cuprates) p-orbitals of Pn(Ch) atoms hybridize with the five 3d orbitals leading to Multicomponent Fermi surface; electron and hole bands like in other semimetals like graphite, boron nitride, etc.

36 Conclusion Interesting compound. Similar structural building blocks as that of other Pnictides Much simpler unit cell and easy to prepare Large increase in T c with pressure other substitutions to increase T c? Interesting magnetic properties Can be obtained in single crystals More studies are expected.

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38 Ba 0.6 K 0.4 Fe 2 As 2 3-D plot of superconducting gap size ( ) at 15 K

39 Ag/LaFeAsO1 xfx point-contact junction

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41 Alaska Subedi,...D.J. Singh, PRB (2008)

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