Exotic magnetism and superconductivity in Ge-substitutedCeCu2Si2:A Cu NQR study

Abstract

We report Cu NQR results on Ge-doped heavy-fermion superconductors CeCu 2 (Si 1Ϫx Ge x ) 2 (0Ͻxр0.2) and compare with previous results on Ce 0.99 Cu 2.02 Si 2 (xϭ0). Once only 1% Ge is substituted for Si to expand the lattice, an antiferromagnetic ͑AFM͒ order sets in at T N ϳ0.7 K, followed by the onset of superconductivity at T c ϭ0.5 K. The sudden emergence of AFM order due to the slight Ge doping reinforces that an exotic magnetic phase at xϭ0 is in fact a marginal AFM state where slowly fluctuating AFM waves propagate over a long distance. The appearance of internal fields throughout the sample that is deduced from the NQR spectral shape below T N , excludes the presence of phase segregation between the superconducting ͑SC͒ and the AFM phases in the coexistent state below T c . The 1/T 1 result does not show significant reduction below T c , followed by a T 1 Tϭconst behavior. This indicates that the SC phase is in a gapless regime, dominated by magnetic excitations due to the coexistence of AFM and SC phase. As Ge content increases, T N is progressively increased, while T c is steeply decreased. As a result of the suppression of the slowly fluctuating AFM waves in the samples with more than xϭ0.06, their magnetic properties above T N progressively change to those in a localized regime as observed in CeCu 2 Ge 2 . The exotic interplay between magnetism and superconductivity in 0рxϽ0.06 is discussed in the context of a SO͑5͒ theory that unifies superconductivity and antiferromagnetism.

Theory of superconductivity in strongly correlated electron systems

“…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.…”

“…The remarkable AF spin fluctuation in these compounds is actually implied by some measurements. The power smaller than two of temperature (T 1∼1.5 ) is observed in the resistivity [ 460], and the NMR/NQR 1/T 1 T increases at low temperatures [ 476,479,480]. These properties are typical ones in the nearly AF Fermi-liquid in quasi-2D and 3D systems.…”

“…5.1.1). The effect of the dominant AF spin fluctuation has been observed as deviation from the conventional Fermi-liquid theory in the physical quantities, such as the resistivity, the specific heat [ 510], and the NMR/NQR 1/T 1 [ 472,476]. Then, the AF spin fluctuation may be responsible for the anisotropic even-parity superconductivity, which is observed in these compounds.…”

31P NMR investigations on the ferromagnetic quantum critical system YbNi4P2Sarkar1, Khuntia2, Krellner3 et al. 2012Phys. Rev. BSelf Cite132140View full textAdd to dashboardCite

“…For example in a 4f heavy-fermion, like CeCu 2 Si 2 (1/T 1 T ) levels into a constant value ("Kondo saturation") just below the Kondo temperature T K and above T K in most cases a n = -1 power law (constant 1/T 1 behaviour) is observed. 25,26 In contrast to that in the two Yb based correlated system YbRh 2 Si 2 and YbNi 4 P 2 , there is no Kondo saturation and a n < 1 power law is observed. The absence of the "Kondo saturation" might be related to the presence of ferromagnetic correlations whereas the high temperature behaviour might originate from the CEF splitting.…”

Nuclear spin-lattice relaxation rate in thed-wave superconducting state with coexisting antiferromagnetism

“…There are now several heavy fermion compounds showing clear experimental evidence of the microscopically homogeneous coexistence of AFM and SC correlations [3][4][5][6] . However, the direct consequences of the coexistence of AFM and SC has not yet been clearly resolved.…”