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Register a new userMagnetism and superconductivity in CeRh_{1-x}Ir_xIn_5 heavy fermion materials
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We report on zero-field muon spin relaxation studies of cerium based heavy-fermion materials CeRh_{1-x}Ir_xIn_5. In the superconducting x=0.75 and 1 compositions muon spin relaxation functions were found to be temperature independent across T_c; no evidence for the presence of electronic magnetic moments was observed. The x=0.5 material is antiferromagnetic below T_N=3.75 K and superconducting below T_c=0.8 K. Muon spin realxation spectra show the gradual onset of damped coherent oscillations characteristic of magnetic order below T_N. At 1.65 K the total oscillating amplitude accounts for at least 85% of the sample volume. No change in muon precession frequency or amplitude is detected on cooling below T_c, indicating the microscopic coexistence of magnetism and superconductivity in this material.
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We present a systematic ^{115}In NQR study on the heavy fermion compounds CeRh_{1-x}Ir_xIn_5 (x=0.25, 0.35, 0.45, 0.5, 0.55 and 0.75). The results provide strong evidence for the microscopic coexistence of antiferromagnetic (AF) order and superconductivity (SC) in the range of 0.35 leq x leq 0.55. Specifically, for x=0.5, T_N is observed at 3 K with a subsequent onset of superconductivity at T_c=0.9 K. T_c reaches a maximum (0.94 K) at x=0.45 where T_N is found to be the highest (4.0 K). Detailed analysis of the measured spectra indicate that the same electrons participate in both SC and AF order. 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 (T_c=0.4 K). We further find that, in the coexistence region, the T^3 dependence of 1/T_1 is replaced by a T-linear variation below Tsim 0.4 K, with the value frac{(T_1)_{T_c}}{(T_1)_{low-T}} increasing with decreasing x, likely due to low-lying magnetic excitations associated with the coexisting magnetism.
$rm CePt_3Si$ is a novel ternary compound exhibiting antiferromagnetic order at $T_N approx 2.2$ K and superconductivity (SC) at $T_c approx 0.75$ K. Large values of $H_{c2} approx -8.5$ T/K and $H_{c2}(0) approx 5$ T indicate Cooper pairs formed out of heavy quasiparticles. The mass enhancement originates from Kondo interaction with a characteristic temperature $T_K approx 8$ K. NMR and $mu$SR measurements evidence coexistence of SC and long range magnetic order on a microscopic scale. Moreover, $rm CePt_3Si$ is the first heavy fermion SC without an inversion symmetry. This gives rise to a novel type of the NMR relaxation rate $1/T_1$ which is very unique and never reported before for other heavy fermion superconductors. Studies of Si/Ge substitution allow us to establish a phase diagram.
Superconductivity in lanthanide- and actinide-based heavy-fermion metals can have different microscopic origins. Among others, Cooper pair formation based on fluctuations of the valence, of the quadrupole moment or of the spin of the localized 4f/5f shell have been proposed. Spin-fluctuation mediated superconductivity in CeCu2Si2 was demonstrated by inelastic neutron scattering to exist in the vicinity of a spin-density-wave quantum critical point. The isostructural HF compound YbRh2Si2 which is prototypical for a Kondo-breakdown quantum critical point has so far not shown any sign of superconductivity down to approximately 10mK. In contrast, results of de-Haas-van-Alphen experiments by Shishido et al. (J. Phys. Soc. Jpn. 74, 1103 (2005)) suggest superconductivity in CeRhIn5 close to an antiferromagnetic quantum critical point beyond the spin-density-wave type, at which the Kondo effect breaks down. For the related compound CeCoIn5 however, a field-induced quantum critical point of spin-density-wave type is extrapolated to exist inside the superconducting phase.
We report Eu-local-spin magnetism and Ni-doping-induced superconductivity (SC) in a 112-type ferroarsenide system Eu(Fe$_{1-x}$Ni$_{x}$)As$_2$. The non-doped EuFeAs$_2$ exhibits two primary magnetic transitions at $sim$100 and $sim$ 40 K, probably associated with a spin-density-wave (SDW) transition and an antiferromagnetic ordering in the Fe and Eu sublattices, respectively. Two additional successive transitions possibly related to Eu-spin modulations appear at 15.5 and 6.5 K. For the Ni-doped sample with $x$ = 0.04, the SDW transition disappears, and SC emerges at $T_mathrm{c}$ = 17.5 K. The Eu-spin ordering remains at around 40 K, followed by the possible reentrant magnetic modulations with enhanced spin canting. Consequently, SC coexists with a weak spontaneous magnetization below 6.2 K in Eu(Fe$_{0.96}$Ni$_{0.04}$)As$_2$, which provides a complementary playground for the study of the interplay between SC and magnetism.
Understanding the origin of superconductivity in strongly correlated electron systems continues to be at the forefront of unsolved problems in all of physics. Among the heavy f-electron systems, CeCoIn5 is one of the most fascinating, as it shares many of the characteristics of correlated d-electron high-Tc cuprate and pnictide superconductors, including the competition between antiferromagnetism and superconductivity. While there has been evidence for unconventional pairing in this compound, high-resolution spectroscopic measurements of the superconducting state have been lacking. Previously, we have used high-resolution scanning tunneling microscopy techniques to visualize the emergence of heavy-fermion excitations in CeCoIn5 and demonstrate the composite nature of these excitations well above Tc. Here we extend these techniques to much lower temperatures to investigate how superconductivity develops within a strongly correlated band of composite excitations. We find the spectrum of heavy excitations to be strongly modified just prior to the onset of superconductivity by a suppression of the spectral weight near the Fermi energy, reminiscent of the pseudogap state in the cuprates. By measuring the response of superconductivity to various perturbations, through both quasiparticle interference and local pair-breaking experiments, we demonstrate the nodal d-wave character of superconducting pairing in CeCoIn5.
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