Dynamical magnetic susceptibility in the lamellar cobaltate superconductor Na_xCoO_2⋅y\cdot yH_2O

We systematically analyze the influence of the superconducting gap symmetry and the electronic structure on the dynamical spin susceptibility in superconducting Na_xCoO_2$\cdot y$H_2O within a three different models: the single a_{1g}-band model with nearest-neighbor hoppings, the realistic three-band t_{2g}-model with, and without e'_g pockets present at the Fermi surface. We show that the magnetic response in the normal state is dominated by the incommensurate antiferromagnetic spin density wave fluctuations at large momenta in agreement with experimental temperature dependence of the spin-lattice relaxation rate. Also, we demonstrate that the presence or the absence of the e'_g-pockets at the Fermi surface does not affect significantly this conclusion. In the superconducting state our results for d_{x^2-y^2}- or d_{xy}-wave symmetries of the superconducting order parameter are consistent with experimental data and exclude nodeless $d_{x^2-y^2} + id_{xy}$-wave symmetry. We further point out that the spin-resonance peak proposed earlier is improbable for the realistic band structure of Na_xCoO_2$\cdot y$H_2O. Moreover, even if present the resonance peak is confined to the antiferromagnetic wave vector and disappears away from it.Comment: Published version, PACS: 74.70.-b; 75.40.Gb; 74.20.Rp; 74.25.J

Time-Reversal Symmetry-Breaking Superconductivity in Heavy Fermion PrOs4Sb12 detected by Muon Spin Relaxation

We report on muon spin relaxation measurements of the 4f^2-based heavy-fermion superconductor filled-skutterudite PrOs4Sb12. The results reveal the spontaneous appearance of static internal magnetic fields below the superconducting transition temperature, providing unambiguous evidence for the breaking of time-reversal symmetry in the superconducting state. A discussion is made on which of the spin or orbital component of Cooper pairs carries a nonzero momentum.Comment: 5 pages with 3 figure

Magnetism and superconductivity in CeRh_{1-x}Ir_xIn_5 heavy fermion materials

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.Comment: 6 pages with 3 figures. Revision with corrected axis label (mK) in Fig.

Staggered magnetism in LiV2_2O4_4 at low temperatures probed by the muon Knight shift

We report on the muon Knight shift measurement in single crystals of LiV2O4. Contrary to what is anticipated for the heavy-fermion state based on the Kondo mechanism, the presence of inhomogeneous local magnetic moments is demonstrated by the broad distribution of the Knight shift at temperatures well below the presumed "Kondo temperature" ($T^*\simeq 30$ K). Moreover, a significant fraction ($\simeq10$ %) of the specimen gives rise to a second component which is virtually non-magnetic. These observations strongly suggest that the anomalous properties of LiV2O4 originates from frustration of local magnetic moments.Comment: 11 pages, 5 figures, sbmitted to J. Phys.: Cond. Mat

Cu-spin dynamics in the overdoped regime of La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ probed by muon spin relaxation

Muon-spin-relaxation measurements have been performed for the partially Zn-substituted La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ with y=0-0.10 in the overdoped regime up to x=0.30. In the 3 % Zn-substituted samples up to x=0.27, exponential-like depolarization of muon spins has been observed at low temperatures, indicating Zn-induced slowing-down of the Cu-spin fluctuations. The depolarization rate decreases with increasing x and almost no fast depolarization of muon spins has been observed for x=0.30 where superconductivity disappears. The present results suggest that the dynamical stripe correlations exist in the whole superconducting regime of La_2-x_Sr_x_CuO_4_ and that there is no quantum critical point at x~0.19.Comment: 6 pages, 5 figure