1. The Low-Temperature Specific Heat of Chalcogen- based FeSe J.-Y. Lin, 1 Y. S. Hsieh, 1 D. Chareev, 2 A. N. Vasiliev, 3 Y. Parsons, 4 and H. D. Yang 4 1 Institute of Physics/National Chiao Tung University, Hsinchu 30010, Taiwan 2 Institute of Experimental Mineralogy, Cherngolovka, Moscow Region 142432, Russia 3 Department of Low temperature Physics, Moscow State University, Moscow 119991, Russia 4 Department of Physics, University of California, Santa Babara, CA 93106, USA 4 Department of Physics, National Sun Yat-sen University, Kaohsiung 804, Taiwan

2. Contents Introduction to Fe-based superconductors Specific heat as a probe of the superconducting order parameter Experiments and results Conclusions

3. A brief introduction to iron-based superconductors

4. Structure

5. 70 Nb 3 Ge MgB 2 Metallic alloys LSCO YBCO TI - cuprate Hg - cuprate Cuprates e-doped LaOFeP e-doped LaOFeAs e-doped SmOFeAs Fe-based superconductors The Race to Beat Cuprates? The crusade of Room Temperature superconductors? ？

6. The order parameter in Fe-based superconductors remains elusive. To get insight into the pairing mechanism, it is crucial to determine the gap structure in the superconductors like FeSe or pnictides. Though with lower T c, FeSe has the simplest structure, and this very simplicity could provide the most appropriate venue of understanding both the order parameter and the superconducting mechanism of Fe-base superconductors. Motivation

7. Johnston, 2010

9. ( Subedi et al., 2008 )

10. Specific heat as the probe Revealing the superconducting order parameter from the specific heat Information from k-space integration. Non phase-sensitive. Surprisingly selective if well excuted

11. FeSe single crystals

12. FeSe single crystal

16.  n =5.73 mJ/mol K 2  =210 K nearly identical to the results of polycrystals from T. M. McQueen et al. (2009)

17.  C/  n T c =1.65 Weak limit BCS isotropic s-wave:  C/  n T c =1.43

18. C. P. Sun et al. (2004)

19.  =  0 cos2  =  e (1+  cos2  )

20. Nicholson et al. (2011)

22. H c2 =13.1 T?  /  n =0~0.69 Quasi-linear  (H) in high H was also observed in 122. (J. S. Kim et al. 2010)

25.  n (mJ/mo l K 2 ) Ө D (K) C/nTcC/nTc H c2,H//c (T) H c2,H  c (T) 5.732101.651.5513.127.9

26. Bang, 2010

28. Anisotropic H c2

29. STM on FeSe C. L. Song et al., 2011

30. Comparison between FeSe and Fe(Se,Te) FeSe Song et al., 2011 Fe(Se,Te) Hanaguri et al., 2010

31. The fitting parameters

33. Conclusions for FeSe Existence of low-energy excitations more than in an isotropic s-wave. Gap anisotropy. S + exntended s. Probably No accidental nodes. Existence of an isotropic s-wave. H c2,H//c  13.1 T and H c2,H  c  27.9 T. The anisotropy in H c2 is about 2.1.

36. Fig. 4 The specific heat of MgB 2. The dashed lines are determined by the conservation of entropy around the anomaly and used to estimate ΔC/T c. Inset: Entropy difference ΔS by integration of ΔC/T.