Kitaoka Laboratory Yoichi Mugino

1. Enhancing the Superconductor Transition Temperature of the Heavy Fermion Compound CeIrIn5in the Absence of Spin Correlations Kitaoka Laboratory Yoichi Mugino Ref. S. Kawasaki et al., PRL 94, 037007 (2005)

2. Contents • Introductions Heavy-fermion system • About CeRh1-xIrxIn5 • Motivation • Experimental data Nuclear Quadrupole Resonance (NQR) method Spin-lattice relaxation rate • Summary

3. Introduction conduction electrons Polarization conduction electrons Jcf Jcf 4ｆ electrons The indirect f-f interaction is mediated by conduction electrons. Magnetic moment is screened by hybridization between conduction electrons and f electrons Heavy-fermion system Ce3+ : [Xe] 4f1 RKKY interaction Kondo effect Localization(局在) Itinerancy(遍歴) Jcf : exchange interaction 4ｆ electrons

4. Introduction TK TRKKY : RKKY interaction : Kondo effect Temperature (K) AFM QCP HF 0 Pressure Jcf Phase diagram for heavy fermion systems The superconductivity in some HF compounds takes place frequently near QCP. QCP (quantum critical point)：量子臨界点

5. Introduction • Narrow superconducting phase • CeRh2Si2 • CePd2Si2 • CeIn3 (R. Movshovich et al., 1996) (F. M. Grosche et al., 1996) (N. D. Mathur et al., 1998) (b) Wide superconducting phase CeCu2Ge2 CeCu2Si2 CeRhIn5 (D. Jaccard et al., 1992) (F. Thomas et al., 1993) (H. Hegger et al., 2000) Pressure-induced superconductors

6. Introduction SC2 SC1 Recent topics New mechanism for superconductivity was suggested ! SC2 appears where antiferromagnetism disappears Another mechanism for SC is suggested. The origin of this SC phase is still unknown QCP H.Q.Yuan et al., Science 302 (2003) 2104 New Journal 6 (2004) 132

7. About CeRh1-xIrxIn5 Ir(Rh) CeRhIn5: Antiferromagnet (TN=3.8K) CeIrIn5: Superconductor(TC=0.4K) CeRhIn5 TN = 3.8 K CeIrIn5 Tc = 0.4 K Rh→Ir = applying pressure

8. Motivation Clarify a pressure induced superconductivity and spin correlations in CeIrIn5 via the 115In nuclear-spin-lattice-relaxation rate measurements

9. Iz=±9/2 115In-NQR 4νQ Iz=±7/2 3νQ Iz=±5/2 2νQ Iz=±3/2 1νQ Iz=±1/2 G. –q. Zheng et al., PRB 70, 014511 (2004) 115In-NQR In the case of η= 0,

10. 1/T1T of CeRh1-xIrxIn5 CeRh1-xIrxIn5 x decreases →the peak of 1/T1T enhances Magnetic fluctuations enhance, and TC goes up

11. Tc Below TC 1/T1∝T3 　→ line-node d-wave SC Assume the line-node gap function and residual density of state (RDOS) Superconducting energy-gap of CeRh1-xIrxIn5

12. Superconducting energy-gap of CeRh1-xIrxIn5 The increase of TC correspond to the increase of Δ0

13. 1/T1T of CeIrIn5 under pressure CeIrIn5 By applying pressure, →1/T1T～const. (fermi liquid) Nonmagnetic, but TC goes up

14. Superconducting energy-gap of CeIrIn5 Assume the line-node gap function → 1/T1～ T3 Superconducting gap is constant (Δ0=2.5kBTC)

15. Compared energy-gap x=1.0→0.6 SC gap changes with TC x=1.0 under pressure SC gap is constant (2Δ0=5.0kBTC) The origin of SC2 is different from SC1 !

16. TC The value of γ(= C/T) is scaled to (1/T1T)1/2 Superconducting property R. Borth et al., Physica B 312-313 (2002) 136-137

17. Summary • In CeRh1-xIrxIn5, the superconductivity is mediated by antiferromagnetic fluctuations. • In CeIrIn5, the external pressure drastically suppresses antiferromagnetic fluctuations. • Above 1.0 GPa, the superconductivity is realized in the heavy-fermion state without antiferromagnetic spin fluctuations. • The increase of TC in CeIrIn5 under pressure may be relevant with the increase of bandwidth.