Susceptibility Inhomogeneity and Non-Fermi-Liquid Behavior in Ce(Ru_{0.5}Rh_{0.5})_2Si_2

Magnetic susceptibility and muon spin rotation (\muSR) experiments have been carried out to study the effect of structural disorder on the non-Fermi-liquid (NFL) behavior of the heavy-fermion alloy Ce(Ru_{0.5}Rh_{0.5})_2Si_2. Analysis of the bulk susceptibility in the framework of disorder-driven Griffiths-phase and Kondo-disorder models for NFL behavior yields relatively narrow distributions of characteristic spin fluctuation energies, in agreement with \muSR linewidths that give the inhomogeneous spread in susceptibility. \muSR and NMR data both indicate that disorder explains the "nearly NFL" behavior observed above \sim2 K, but does not dominate the NFL physics found at low temperatures and low magnetic fields.Comment: 6 pages, 4 figures, REVTeX, submitted to Phys. Rev.

Griffiths phase of the Kondo insulator fixed point

Heavy fermion compounds have long been identified as systems which are extremely sensitive to the presence of impurities and other imperfections. In recent years, both experimental and theoretical work has demonstrated that such disorder can lead to unusual, non-Fermi liquid behavior for most physical quantities. In this paper, we show that this anomalous sensitivity to disorder, as well as the resulting Griffiths phase behavior, directly follow from the proximity of metallic heavy fermion systems to the Kondo insulator fixed point.Comment: 5 pages, 4 figures; Proceedings of the SCES, August 2000, to appear in the Journal of Magnetism and Magnetic Material

Actes du Congrès Collèges célébrations 92

Également disponible en version papierTitre de l'écran-titre (visionné le 19 août 2009

^{27}Al Impurity-Satellite NMR and Non-Fermi-Liquid Behavior in U_{1-x}Th_xPd_2Al_3

Non-Fermi-liquid (NFL) behavior in the f-sublattice-diluted alloy system U_{1-x}Th_xPd_2Al_3 has been studied using ^{27}Al nuclear magnetic resonance (NMR). Impurity satellites due to specific U near-neighbor configurations to ^{27}Al sites are clearly resolved in both random and field-aligned powder samples. The spatial mean Kbar and rms spread delta K of impurity satellite shifts, which are related to the mean chibar and rms spread delta chi of the inhomogeneous susceptibility, have been measured in field-aligned powders with the crystalline c axis both perpendicular and parallel to the external field. The relatively narrow lines observed at low temperatures suggest that disorder- induced inhomogeneity of the f-ion--conduction-electron hybridization is not the cause of NFL behavior in these alloys: at low temperatures the experimental values of delta chi(T)/chibar(T) are much smaller than required by disorder- driven models. This is in contrast to results in at least some alloys with disordered non-f-ion nearest neighbors to f ions ("ligand disorder"), where disorder-driven theories give good accounts of NFL behavior. Our results suggest that f-ion dilution does not produce as much inhomogeneity of the hybridization strength as substitution on ligand sites.Comment: 10 pages, 12 figures, REVTeX. Submitted to Phys. Rev.

High-field muSR studies of superconducting and magnetic correlations in cuprates above Tc

The advent of high transverse-field muon spin rotation (TF-muSR) has led to recent muSR investigations of the magnetic-field response of cuprates above the superconducting transition temperature T_c. Here the results of such experiments on hole-doped cuprates are reviewed. Although these investigations are currently ongoing, it is clear that the effects of high field on the internal magnetic field distribution of these materials is dependent upon a competition between superconductivity and magnetism. In La_{2-x}Sr_xCuO_4 the response to the external field above Tc is dominated by heterogeneous spin magnetism. However, the magnetism that dominates the observed inhomogeneous line broadening below x ~ 0.19 is overwhelmed by the emergence of a completely different kind of magnetism in the heavily overdoped regime. The origin of the magnetism above x ~ 0.19 is currently unknown, but its presence hints at a competition between superconductivity and magnetism that is reminiscent of the underdoped regime. In contrast, the width of the internal field distribution of underdoped YBa_2Cu_3O_y above Tc is observed to track Tc and the density of superconducting carriers. This observation suggests that the magnetic response above Tc is not dominated by electronic moments, but rather inhomogeneous fluctuating superconductivity.Comment: 28 pages, 11 figures, 104 reference

Ga NMR study of the local susceptibility in SrCr8Ga4O19: pseudogap and paramagnetic defects

We present the first Ga(4f) NMR study of the Cr susceptibility in the archetype of Kagome based frustrated antiferromagnets, SrCr$_{8}$Ga$_{4}$O$_{19}$. Our major finding is that the susceptibility of the frustrated lattice goes through a maximum around 50 K. Our data also supports the existence of paramagnetic ``clusters'' of spins, responsible for the Curie behavior observed in the macroscopic susceptibility at low T. These results set novel features for the constantly debated physics of geometrically frustrated magnets.Comment: 4 pages, 5 figures Submitted to Phys. Rev. Let

Spin dynamics and spin freezing in the triangular lattice antiferromagnets FeGa2S4 and NiGa2S4

Magnetic susceptibility and muon spin relaxation (muSR) experiments have been carried out on the quasi-2D triangular-lattice spin S = 2 antiferromagnet FeGa2S4. The muSR data indicate a sharp onset of a frozen or nearly-frozen spin state at T* = 31(2) K, twice the spin-glass-like freezing temperature T_f = 16(1) K. The susceptibility becomes field dependent below T*, but no sharp anomaly is observed in any bulk property. A similar transition is observed in muSR data from the spin-1 isomorph NiGa2S4. In both compounds the dynamic muon spin relaxation rate lambda_d(T) above T* agrees well with a calculation of spin-lattice relaxation by Chubukov, Sachdev, and Senthil in the renormalized classical regime of a 2D frustrated quantum antiferromagnet. There is no firm evidence for other mechanisms. At low temperatures lambda_d(T) becomes temperature independent in both compounds, indicating persistence of spin dynamics. Scaling of lambda_d(T) between the two compounds is observed from ~T_f to ~1.5T*. Although the muSR data by themselves cannot exclude a truly static spin component below T*, together with the susceptibility data they are consistent with a slowly-fluctuating "spin gel" regime between T_f and T*. Such a regime and the absence of a divergence in lambda_d(T) at T* are features of two unconventional mechanisms: (1) binding/unbinding of Z_2 vortex excitations, and (2) impurity spins in a nonmagnetic spin-nematic ground state. The absence of a sharp anomaly or history dependence at T* in the susceptibility of FeGa2S4, and the weakness of such phenomena in NiGa2S4, strongly suggest transitions to low-temperature phases with unconventional dynamics.Comment: 13 pages, 6 figures, accepted for publication in Physical Review

Muon spin rotation and relaxation in magnetic materials

A review of the muon spin rotation and relaxation ($\mu$SR) studies on magnetic materials published from July 1993 is presented. It covers the investigation of magnetic phase diagrams, of spin dynamics and the analysis of the magnetic properties of superconductors. We have chosen to focus on selected experimental works in these different topics. In addition, a list of published works is provided.Comment: Review article, 59 pages, LaTeX with IoP macro

The Nature of the Near-IR Core Source in 3C 433

We report the analysis of near-infrared imaging, polarimetric and spectroscopic observations of the powerful radio galaxy 3C433, obtained with the HST and UKIRT telescopes. The high spatial resolution of HST allows us to study the near-nuclear regions of the galaxy (<1 kpc). In line with previous observations, we find that 3C433 has an unresolved core source that is detected in all near-IR bands, but dominates over the host galaxy emission at 2.05 um. Our analysis reveals: (1) the presence of a dust lane aligned close to perpendicular (PA=70\pm5\degr) to the inner radio jet axis (PA=-12\pm2\degr); (2) a steep slope to the near-IR SED ($\alpha=5.8\pm0.1$; F$_{\nu}\propto\nu^{-\alpha}$); (3) an apparent lack of broad permitted emission lines at near-IR wavelengths, in particular the absence of a broad Pa$\alpha$ emission line; and (4) high intrinsic polarization for the unresolved core nuclear source ($8.6\pm1$ per cent), with an E-vector perpendicular (PA=83.0\pm 2.3\degr) to the inner radio jet. Using five independent techniques we determine an extinction to the compact core source in the range 3<A_V<67 mag. An analysis of the long wavelength SED rules out a synchrotron origin for the high near-IR polarization of the compact core source. Therefore, scattering and dichroic extinction are plausible polarizing mechanisms, although in both of these cases the broad permitted lines from the AGN are required to have a width >10^4 km/s (FWHM) to escape detection in our near-IR spectrum. Dichroic extinction is the most likely polarization mechanism because it is consistent with the various available extinction estimates. In this case, a highly ordered, coherent toroidal magnetic field must be present in the obscuring structure close to the nucleus.Comment: 12 pages, 5 figures, accepted for publication in MNRA

Singular Fermi Liquids

An introductory survey of the theoretical ideas and calculations and the experimental results which depart from Landau Fermi-liquids is presented. Common themes and possible routes to the singularities leading to the breakdown of Landau Fermi liquids are categorized following an elementary discussion of the theory. Soluble examples of Singular Fermi liquids (often called Non-Fermi liquids) include models of impurities in metals with special symmetries and one-dimensional interacting fermions. A review of these is followed by a discussion of Singular Fermi liquids in a wide variety of experimental situations and theoretical models. These include the effects of low-energy collective fluctuations, gauge fields due either to symmetries in the hamiltonian or possible dynamically generated symmetries, fluctuations around quantum critical points, the normal state of high temperature superconductors and the two-dimensional metallic state. For the last three systems, the principal experimental results are summarized and the outstanding theoretical issues highlighted.Comment: 170 pages; submitted to Physics Reports; a single pdf file with high quality figures is available from http://www.lorentz.leidenuniv.nl/~saarloo