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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

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

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A brief introduction to iron-based superconductors

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Structure

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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? ？

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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

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Johnston, 2010

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( Subedi et al., 2008 )

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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

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FeSe single crystals

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FeSe single crystal

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

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

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C. P. Sun et al. (2004)

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= 0 cos2 = e (1+ cos2 )

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Nicholson et al. (2011)

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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)

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n (mJ/mo l K 2 ) Ө D (K) C/nTcC/nTc H c2,H//c (T) H c2,H c (T) 5.732101.651.5513.127.9

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Bang, 2010

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Anisotropic H c2

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

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

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The fitting parameters

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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.

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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.

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