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Study of superconductivity on βpyrochlore KOs 2 O 6

Study of superconductivity on βpyrochlore KOs 2 O 6. Kitaoka Lab. Hirofumi Naruwa. Ref.) Z. Hiroi et al ., Phys. Rev. B 76 (2007) 014523 M.Yoshida et al., Phys. Rev. Lett. 98 (2007) 197002. Contents. Contents. ・ Introduction Cage compounds

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Study of superconductivity on βpyrochlore KOs 2 O 6

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  1. Study of superconductivity on βpyrochloreKOs2O6 Kitaoka Lab. Hirofumi Naruwa Ref.) Z. Hiroi et al., Phys. Rev. B 76 (2007) 014523 M.Yoshida et al., Phys. Rev. Lett. 98 (2007) 197002

  2. Contents Contents ・Introduction Cage compounds Influence to Tc from the guest in the cage BCS superconductivity Crystal structure of KOs2O6 ・Experimental results & discussion Rattling of KOs2O6 1/T1T of KOs2O6 and RbOs2O6 Specific heatof KOs2O6 Superconductivity of KOs2O6 ・Summary

  3. Cage compounds NO3 Ag Si Ag6O8cage Introduction Ag cage compound Ag6O8AgNO3 Silicon clathrate compounds ・Cage compounds have Polyhedral cages and guest molecules inside the cages. ・Guest molecules influence the physical properties very much. 多面体の

  4. Influence to Tc from the guest in the cage O Ｎ O O BF4 F F B F F Tc=0.3K Introduction Ag6O8AgHF2 Ag6O8AgBF4 Ag6O8AgNO3 Ag6O8AgX Ag6O8cage Ag6O8cage Ag6O8cage Ag6O8cage HF2 NO3 H X F F Tc=1.04K Tc=1.38K the superconducting property is affected by the presence of molecules inside the cages K. Kawashima et. al., JPSJ

  5. BCS superconductivity Introduction Cooper-pair electron-phonon coupling constant + λ λ Superconductivity is caused by the formation of Cooper-pairs. ⊿C/γTc C/ γT Cs Cn T / Tc Tc tends to be higher in the materials, which has a stronger Cooper pairing interaction the jump of specific heat is a good measure for strength of the Cooper pairing interaction

  6. Crystal structure of KOs2O6 βpyrochloreKOs2O6 K from Hiroi Lab. HP Introduction KOs2O6 O alkali metal RbOs2O6 CsOs2O6 K rattling OsO6 KOs2O6Tc=9.6K RbOs2O6 Tc=6.3K CsOs2O6 Tc=3.3K compare thesethree β pyrochlorematerials to study why Tc of KOs2O6 is the highest

  7. Rattling of KOs2O6 rion K rattling K Experimental results & discussion βpyrochloreKOs2O6 Einstein frequency lattice constant Cs Rb ωE（Ｋ） K harmonic oscillation lattice constant / rion free space rattling phonon ⇒　Rattling would be prominent in K-compound.

  8. 1/T1T of KOs2O6 and RbOs2O6 Experimental results & discussion T1(Nuclear spin-lattice relaxation time) pulse magnetic field t external magnetic field YBa2Cu3O7 T1 Al (high-temperature cuprate superconductor) (BCS superconductor ) Tc Coherent peak 1/T1T=const 1/ T1 (s-1) 1/ T1 (s-1) ∝ T3 ∝ exp(-⊿/kBT) T(K) T(K)

  9. RbOs2O6,KOs2O6 Coherent peak ⇒BCS 1/T1T=const KOs2O6 RbOs2O6 87Rb-NMR 1/ T1∝ exp(-⊿/kBT) 1/ T1∝ exp(-⊿/kBT) ⇒BCS exp(-⊿/kBT )

  10. specific heatof KOs2O6 Experimental results & discussion Cp/ T (mJ K-2mol-1) Tc(Cs) Tc(Rb) Tc(K) T(K) Cs＜Rb＜Kλ increase Cs＜Rb＜K Tc increase

  11. superconductivityof KOs2O6 K K K K K CsOs2O6 RbOs2O6 KOs2O6 effective mass large + Experimental results & discussion βpyrochlore BCS cage lattice constant / rion rattling free space phonon Rattling would be prominent in K-compound. ⇒Cs＜Rb＜Kλ increase anharmonicitic potential harmonicitic potential λ λ coefficient of electronic specific heat This electron pair links more strongly than BCS one.

  12. Summary Summary ・Free space for the guest atoms seem to influence superconductivity of βpyrochlore. ・ The results from specific heat and 1/T1T were different from those of BCS superconductors. It may be due to the rattling in free space.

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