NMR STUDY OF THE COEXISTENCE OF SUPERCONDUCTIVITY AND ANTIFEROMAGNETISM IN HEAVY FERMION SYSTEM Ce(Ir, Rh)In_5

CeMIn_5 （M=Ir, Co, Rh）是一类新的重费米子化合物。CelrIn_5和CeCoIn_5是超导体，T_c分别为400mK和2.3K。CeRhIn5为反铁磁体，T_N=3.7K，但在高压下成为超导体。此外，Ce(Rh_<1-x>Ir_x)In_5在较宽的Ir浓度范围内表现出反铁磁性（AF）和超导性（SC）的共存。本研究的目的是通过核四极共振（NQR）来研究Ce(Rh_<1-x>Ir_x)In_5中反铁磁性和超导性是如何共存的，并探讨在压力下CeRhIn_5中AF和SC的可能共存。我们发现在CeRhln5中AF和SC在微观上共存，在1.6 Gpa<P<1.9 Gpa的压力范围内。在此压力范围内，磁矩显著减小。在Ce(Rh_<1-x>Ir_x)In_5中，我们在0.35<x<0.55范围内发现了AF和SC共存的有力证据。具体来说，当x=0.5时，在3K时观察到T_N，随后在T_c=0.9K时发生SC。对测量光谱的详细分析表明，相同的电子参与了SC和AF。1/T_1在T_N处有一个宽峰，在T_c以下有一个T^3的变化，后一个性质表明像CeIrIn_5那样的非常规SC。我们进一步发现，在共存区，1/T_1的T^3依赖性被小于0.4K的T线性变化所取代，这可能是由于与共存磁相关的低洼磁激励。

CeMIn_5 (M=Ir, Co, Rh) is a new class of heavy fermion compounds. CelrIn_5 and CeCoIn_5 are superconductors with T_c=400mK and 2.3K, respectively. CeRhIn5 is an antiferromagnet with T_N=3.7K but becomes superconducting under high pressures. Also, upon alloying, Ce(Rh_<1-x>Ir_x)In_5 was suggested to exhibit a coexistence of antiferromagnetism (AF) and superconductivity (SC) over a wide range of Ir concentration, x.The purpose of this study is to investigate, through nuclear quadrupole resonance (NQR), how the coexistence of antiferrormagnetism and superconductivity takes place in Ce(Rh_<1-x>Ir_x)In_5 and to explore a possible coexistence of AF and SC in CeRhIn_5 under pressure.We have found that in AF and SC coexist microscopically in CeRhln5, in the pressure range of 1.6 Gpa<P<1.9 Gpa. In this pressure range, the magnetic moment was reduced substantially.In Ce(Rh_<1-x>Ir_x)In_5, we have found strong evidence for the microscopic coexistence of AF and SC in the range of 0.35<x<0.55. Specifically, for x=0.5,T_N is observed at 3K with a subsequent onset of SC at T_c=0.9K. Detailed analysis of the measured spectra indicate that the same electrons participate in both SC and AF. The nuclear spin-lattice relaxation rate.1/T_1 shows a broad peak at T_N and follows a T^3 variation below T_c, the latter property indicating unconventional SC as in CeIrIn_5. We further find that, in the coexistence region, the T^3 dependence of 1/T_1 is replaced by a T-linear variation below 0.4K, likely due to low-lying magnetic excitations associated with the coexisting magnetism.