Evidence for short-range antiferromagnetic fluctuations in Kondo-insulating YbB12

The spin dynamics of mixed-valence YbB12 has been studied by inelastic neutron scattering on a high-quality single crystal. In the Kondo-insulating regime realized at low temperature, the spectra exhibit a spin-gap structure with two sharp, dispersive, in-gap excitations at E = 14.5 and approximately 20 meV. The lower mode is shown to be associated with short-range correlations near the antiferromagnetic wave vector q0 = (1/2, 1/2, 1/2). Its properties are in overall agreement with those expected for a "spin exciton'' branch in an indirect hybridization gap semiconductor.Comment: 4 pages, 4 figures ; submitted to Physical Review Letter

The effect of uniaxial pressure on the magnetic anomalies of the heavy-fermion metamagnet CeRu2Si2

The effect of uniaxial pressure (P_u) on the magnetic susceptibility (X), magnetization (M), and magnetoresistance (MR) of the heavy-fermion metamagnet CeRu2Si2 has been investigated. For the magnetic field along the tetragonal c axis, it is found that characteristic physical quantities, i.e., the temperature of the susceptibility maximum (T_max), the pagamagnetic Weiss temperature (Q_p), 1/X at 2 K, and the magnetic field of the metamagnetic anomaly (H_M), scale approximately linearly with P_u, indicating that all the quantities are related to the same energy scale, probably of the Kondo temperature. The increase (decrease) of the quantities for P_u || c axis (P_u || a axis) can be attributed to a decrease (increase) in the nearest Ce-Ru distance. Consistently in MR and X, we observed a sign that the anisotropic nature of the hybridization, which is believed to play an important role in the metamagnetic anomaly, can be controlled by applying the uniaxial pressure. PACS numbers: 75.20.Hr, 71.27.+a, 74.62.FjComment: 7 pages, ReVTeX, 6 EPS figures : Will appear in Phys. Rev.

High energy spin excitations in YBa_2 Cu_3 O_{6.5}

Inelastic neutron scattering has been used to obtain a comprehensive description of the absolute dynamical spin susceptibility $\chi'' (q,\omega)$ of the underdoped superconducting cuprate YBa_2 Cu_3 O_{6.5} ($T_c = 52 K$) over a wide range of energies and temperatures ($2 meV \leq \hbar \omega \leq 120 meV$ and $5K \leq T \leq 200K$). Spin excitations of two different symmetries (even and odd under exchange of two adjacent CuO_2 layers) are observed which, surprisingly, are characterized by different temperature dependences. The excitations show dispersive behavior at high energies.Comment: 15 pages, 5 figure

Pressure Evolution of the Magnetic Field induced Ferromagnetic Fluctuation through the Pseudo-Metamagnetism of CeRu2Si2

Resistivity measurements performed under pressure in the paramagnetic ground state of CeRu2Si2 are reported. They demonstrate that the relative change of effective mass through the pseudo metamagnetic transition is invariant under pressure. The results are compared with the first order metamagnetic transition due to the antiferromagnetism of Ce0.9La0.1Ru2Si2 which corresponds to the "negative" pressure of CeRu2Si2 by volume expansion. Finally, we describe the link between the spin-depairing of quasiparticles on CeRu2Si2 and that of Cooper pairs on the unconventional heavy fermion superconductor CeCoIn5.Comment: 5 pages, 6 figures, accepted for publication in J. Phys. Soc. Jp

Spontaneous deformation of the Fermi surface due to strong correlation in the two-dimensional t-J model

Fermi surface of the two-dimensional t-J model is studied using the variational Monte Carlo method. We study the Gutzwiller projected d-wave superconducting state with an additional variational parameter t'_v corresponding to the next-nearest neighbor hopping term. It is found that the finite t'_v<0 gives the lowest variational energy in the wide range of hole-doping rates. The obtained momentum distribution function shows that the Fermi surface deforms spontaneously. It is also shown that the van Hove singularity is always located very close to the Fermi energy. Using the Gutzwiller approximation, we show that this spontaneous deformation is due to the Gutzwiller projection operator or the strong correlation.Comment: 4 pages, 3 eps figures, revte

Novel Coexistence of Superconductivity with Two Distinct Magnetic Orders

The heavy fermion Ce(Rh,Ir)In5 system exhibits properties that range from an incommensurate antiferromagnet on the Rh-rich end to an exotic superconductor on the Ir-rich end of the phase diagram. At intermediate composition where antiferromagnetism coexists with superconductivity, two types of magnetic order are observed: the incommensurate one of CeRhIn5 and a new, commensurate antiferromagnetism that orders separately. The coexistence of f-electron superconductivity with two distinct f-electron magnetic orders is unique among unconventional superconductors, adding a new variety to the usual coexistence found in magnetic superconductors.Comment: 3 figures, 4 page

Signal phylogéographique détécté via les marqueurs microsatellites nuclaires entre les structures génétiques spatiales des populations de Moabi situées de part et d'autre de l'équateur (Gabon-Cameroun)

Les patrons de structuration génétique spatiale et les forces abiotiques expliquant la formation de ces structures ont été très peu étudiés sur de larges échelles géographiques climatiquement contrastées chez les populations d'arbres. Notre étude présente dés éléments de réponse sur la dynamique génétique spatiale du Moabi (Baillonella toxisperma Pierre, Sapotaceae), une espèce endémique du bassin guinéo-congolais, représentée à très faibles densités (5-7 adultes/km2). Quinze marqueurs microsatellites nucléaires ont été utilisés pour génotyper 530 individus issus de différentes régions du Cameroun et du Gabon. A l'aide de modèles bayésiens nous avons détecté trois grandes structures génétiques spatiales : un groupe situé au Cameroun (Cluster 1) et deux groupes situés au Gabon, dont l'un continental (Cluster 2) et l'autre le long de la côte atlantique (Cluster 3). Les paramètres de diversité génétique variaient de façon modérée d'un groupe à l'autre (Henuc = 0,49-0,58 ; Rnuc = 6,9-8,9). Les indices de différentiation génétique étaient essentiellement élevés entre le groupe du Cameroun et ceux du Gabon (respectivement [Cluster1 vs Cluster 2 : Fstnuc = 0.207*** ; Rstnuc = 0.223*] et [Cluster 1 vs Cluster 3 : Fstnuc = 0.113*** ; Rstnuc = 0.278***]), mettant ainsi en évidence un signal phylogéographique fort. Les premières pistes d'explications de ce signal phylogéographique pourraient être : (i) l'effet de l'inversion des saisons due au fait que le Gabon et le Cameroun se trouvent de part et d'autre de l'équateur climatique - fait qui serait susceptible d'affecter la synchronisation de la floraison entre les arbres en fleurs - ou (ii) différents scénarios historiques de patrons de colonisation ou de recolonisation des forêts du bassin guinéo-congolais. (Texte intégral

Insulator-to-metal transition in Kondo insulators under strong magnetic field

Magnetization curve and changes of the single-particle excitation spectra by magnetic field are calculated for the periodic Anderson model at half-filling in infinite spatial dimension by using the exact diagonalization method. It is found that the field-induced insulator-to-metal transition occurs at a critical field $H_c$, which is of the order of the single ion Kondo temperature. The transition is of first order, but could be of second order in the infinite system size limit. These results are compared with the experiments on the Kondo insulator YbB$_{12}$.Comment: 11 pages, REVTEX, no figures; 7 figures available on request; To appear in Phys. Rev. B, Mar.15, 199