Coexistence of antiferromagnetism and superconductivity in heavy-fermion systems

Abstract

We report the novel pressure(P )−temperature(T ) phase diagrams of antiferromagnetism (AF) and superconductivity (SC) in CeRhIn5, CeIn3 and CeCu2Si2 revealed by the NQR measurement. In the itinerant helical magnet CeRhIn5, we found that the Néel temperature TN is reduced at P ≥ 1.23 GPa with an emergent pseudogap behavior. The coexistence of AF and SC is found in a narrow P range of 1.63 − 1.75 GPa, followed by the onset of SC with line-node gap over a wide P window 2.1 − 5 GPa. In CeIn3, the localized magnetic character is robust against the application of pressure up to P ∼ 1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations. In CeCu2Si2, the SC and AF coexist on a microscopic level once its lattice parameter is expanded. We remark that the underlying marginal antiferromagnetic state is due to collective magnetic excitations in the superconducting state in CeCu2Si2. An interplay between AF and SC is discussed on the SO(5) scenario that unifies AF and SC. We suggest that the SC and AF in CeCu2Si2 have a common mechanism.

Theory of superconductivity in strongly correlated electron systems

“…These compounds are quasi-2D materials with the layered structure, which has been confirmed by the de Haas-van Alphen (dHvA) measurements [ 481,482,483,484,485]. The P − T phase diagram in CeRhIn 5 [ 477] and the phase diagram in the alloy system CeRh 1−x Ir x In 5 [ 486] indicate that the superconductivity appears in the neighborhood of the AF phase. The power-law behavior of the resistivity in the normal phase with T 1.3 in CeIrIn 5 (T c = 0.4K) [ 465] and T in CeCoIn 5 (T c = 2.3K) [ 466] implies the strong quasi-2D AF spin fluctuation.…”

“…The ground state of CeCu 2 (Si 1−x Ge x ) 2 , with increasing x, changes from the SC phase to the AF phase through the coexistent phase, owing to negative chemical pressures by Ge substitution for Si [ 476,477]. This property is similar to that of the organic superconductor, as is explained in Sec.…”

Interacting Antiferromagnetic Droplets in Quantum CriticalCeCoIn5

“…In the disordered state, T ÿ1 1 is independent of doping above 5 K and shows an unusual T 1=4 dependence. This behavior contrasts with the evolution of T ÿ1 1 with pressure in pure CeCoIn 5 , CeRhIn 5 , CeIn 3 , and CeCu 2 Si; Ge 2 , where the magnitude of T ÿ1 1 in the disordered state changes dramatically as the ground state evolves from antiferromagnetic to superconducting [15,16]. Pressure tuning modifies the Kondo interaction, J, and changes in the ground state are reflected in T ÿ1 1 through modifications of the spin fluctuation spectrum.…”

SO(5)theory of antiferromagnetism and superconductivity

“…13 predicts that the ordering wave vectors take discrete values that correspond to different Mott insulating PDW lobes. For long-range interactions, PDW phases are very densely packed, so experimentally we may observe an almost continuous dependence of incommensuration on doping, such as the one discussed by Yamada et al ͑1998͒ Kitaoka et al, 2002. and Wakimoto et al ͑2000͒. However, different states in the hierarchy are not equivalent.…”

“…Noting that D = D Si ͓1͑V Ge − V Si ͒x / V Ge ͔ for Ge doping and that D increases with pressure, one can draw a combined phase diagram as a function of lattice density D ͑Kitaoka et al, 2002͒. Kitaoka et al ͑2002͒ showed that the phase diagram of Fig. 46 could be understood in terms of an SO͑5͒ superspin picture.…”