Unique Spin Dynamics and Unconventional Superconductivity in the Layered Heavy Fermion CompoundCeIrIn5: NQR Evidence

Abstract

We report measurements of the 115 In nuclear spin-lattice relaxation rate (1/T1) between T =0.09 K and 100 K in the new heavy fermion (HF) compound CeIrIn5. At 0.4 K ≤ T ≤ 100 K, 1/T1 is strongly T -dependent, which indicates that CeIrIn5 is much more itinerant than known Ce-based HFs. We find that 1/T1T , subtracting that for LaIrIn5, follows a ( 1 T +θ ) 3 4 variation with θ=8 K. We argue that this novel feature points to anisotropic, due to a layered crystal structure, spin fluctuations near a magnetic ordering. The bulk superconductivity sets in at 0.40 K below which the coherence peak is absent and 1/T1 follows a T 3 variation, which suggests unconventional superconductivity with line-node gap. PACS: 74.25.Ha, 74.70Tx, 76.60.Gv The emergence of superconductivity near a magnetic instability in cerium (Ce)-based heavy fermion (HF) compounds is one of the most intriguing phenomena in strongly correlated electron systems. Except for CeCu 2 Si 2 which is superconducting at ambient pressure with T c =0. . In spite of efforts and progress, however, knowledge about this class of superconductors is still limited because of difficult experimental conditions. The recently discovered new family of Ce-based heavy electron systems, CeMIn 5 (M=Rh, Ir) with M=Ir being a superconductor already at ambient pressure [6,7], provides new opportunities for studying the nature of the superconductivity in the vicinity of a magnetic instability, the interplay between magnetic excitations and superconductivity, etc. In particular, CeIrIn 5 is suitable for studies using microscopic experimental probes that can be applied more easily at ambient pressure.CeMIn 5 (M=Rh, Ir) consists of alternating layers of CeIn 3 and MIn 2 . CeRhIn 5 is an antiferromagnet with T N =3.8 K but becomes superconducting below T c =2.1 K under pressures larger than 1.6 GPa [6]. In CeIrIn 5 , the resistivity is already zero at ambient pressure below 1.2 K, but the Meissner effect and the jump in the specific heat are found only at 0.4 K [7]. The electronic specific heat coefficient γ is found to be 750 mJ/mol K 2 [7], which suggests a large mass enhancement. Recent de Haas-van Alphen Oscillation in CeIrIn 5 also reveals a cyclotron mass that is ∼20 times larger than the band mass, consistent with the specific heat result [8].In this Letter, we report a measurement using local probe, the 115 In nuclear quadrupolar resonance (NQR) study in CeIrIn 5 down to 90 mK, at zero magnetic field.From the temperature (T ) dependence of the nuclear spin lattice relaxation rate (1/T 1 ), we find that CeIrIn 5 is much more itinerant than known Ce-compounds such as CeCu 2 Si 2 [9], and show that this compound is located near a magnetic ordering with anisotropic spin fluctuations due to the layered crystal structure. No anomaly was found at 1.2 K in the NQR quantities, but 1/T 1 shows an abrupt decrease at 0.40 K below which the NQR intensity also decreases as does the ac susceptibility, confirming a bulk superconductivity below T c =0.40 K. The lack of coherence peak in 1/T...

Enhancing the Superconducting Transition Temperature of the Heavy Fermion CompoundCeIrIn5in the Absence of Spin Correlations

“…Antiferromagnet CeRhIn 5 becomes superconducting at relatively lower critical pressure P c ∼ 1.6 GPa and yet exhibits a higher T c ∼ 2 K [1]. Measurements of nuclear-quadrupole-resonance (NQR) [4,5,6,7], thermal transport and heat capacity [8] on CeMIn 5 found that the superconductivity is unconventional, with line-nodes in the superconducting gap function. NQR [4,5,7,9] and inelastic neutron diffraction [10] measurements also found strong antiferromagnetic spin fluctuations in the normal state.…”

“…Measurements of nuclear-quadrupole-resonance (NQR) [4,5,6,7], thermal transport and heat capacity [8] on CeMIn 5 found that the superconductivity is unconventional, with line-nodes in the superconducting gap function. NQR [4,5,7,9] and inelastic neutron diffraction [10] measurements also found strong antiferromagnetic spin fluctuations in the normal state. In addition, in CeIrIn 5 , the antiferromagnetic spin fluctuations are found to be anisotropic [4], namely, a magnetic correlation length ξ plane within the tetragonal plane grows up more dominantly than ξ c along the c-axis associated with their two-dimensional crystal structure.…”

Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

“…Monthoux et al [134], however, emphasize that this kind of magnetically mediated superconductivity is sensitive to small changes in the band structure, possibly yielding an explanation why CeRhIn 5 with a c/a ratio intermediate between CeIrIn 5 and CeCoIn 5 only exhibits superconductivity below T c = 110 mK. For the entire series of CeT In 5 compounds, both the specific heat [135] and the spin-lattice relaxation rate divided by temperature 1/T 1 T , measured by means of NQR [136][137][138], provide compelling evidence for the existence of line nodes in the superconducting energy gap and therefore unconventional superconductivity.…”

Zero-field NMR and NQR measurements of the antiferromagnet URhIn5

“…[5][6][7] CeRhIn 5 , an antiferromagnet with Néel temperature T N = 3.8 K at ambient pressure, becomes superconducting near 2 K, with suppression of the antiferromagnetism, at applied pressure of P * ∼ 2 GPa. 8 Systematic nuclear magnetic resonance (NMR) investigations of the Ce115 systems have established that these superconductors have d-wave superconducting gaps, [9][10][11][12] and that antiferromagnetic (AFM) spin interactions play an active role in the superconducting pairing. [13][14][15][16] In the Pu115 systems, NMR measurements show d-wave-like superconducting gap behavior, 17,18 with T c 's an order of magnitude higher than those for the Ce115.…”