[en] We present specific heat data for the heavy fermion superconductor CeCoIn₅ close to the upper critical field H_c2 for magnetic fields applied along the [1 0 0] crystallographic axis. For fields above 10 T, the superconducting phase transition becomes first-order. In the same field range, we observe a second specific heat anomaly within the superconducting state, which we attribute to a Fulde-Ferrell-Larkin-Ovchinnikov state

[en] Measurements of the electrical conductivity σ(T) and magnetoconductivity Δσ(H) of icosahedral Al₇₀Re₁₀Pd₂₀ and Al₇₀Re_8.6Pd_21.4 quasicrystals at low temperatures and in high magnetic fields H are reported. Below 0.2 K, σ of Al₇₀Re₁₀Pd₂₀ varies as σ(T)=σ₀+aT^1/2 with σ₀=30Ω^-1cm^-1 and a<0. The conductivity decreases with increasing magnetic field. For Al₇₀Re_8.6Pd_21.4 σ(T) decreases with decreasing T and saturates at the level σ₀=1.7 Ω^-1cm^-1 below 0.1 K. In magnetic fields up to 40kOe, Δσ(H) is negative below 1K and σ(T,H) remains temperature-independent at the lowest temperatures. The optical conductivity σ(ω) is obtained from reflectivity data in the frequency range between 15 and 10⁵cm^-1. The main contributions to σ(ω) are a broad absorption signal centered at about 0.1 eV and a sizable peak with its maximum at 2.6 eV. copyright 1997 The American Physical Society

[en] We report the results of measurements of the thermoelectric power S of stoichiometric CaB₆ and vacancy-doped Ca_1-δB₆ between 5 and 300 K. The thermopower for both materials is surprisingly large at room temperature. Across the whole temperature range covered, S is negative and the temperature dependence is most probably dictated by band-structure effects. The phenomenological interpretation of our data involves a calculation of S(T), using the Boltzmann equation in the relaxation time approximation and assuming a band of defect states in proximity to the lower edge of the conduction band. Good agreement with our data is found by considering acoustic phonon and ionized impurity scattering for the electrons in the conduction band, which is well separated from the valence band. (author)

[en] From small-angle neutron scattering studies of the flux line lattice (FLL) in CeCoIn₅, with magnetic field applied parallel to the crystal c-axis, we obtain the field and temperature dependence of the FLL form factor (FF), which is a measure of the spatial variation of the field in the mixed state. We extend our earlier work (Bianchi et al 2008 Science 319 177) to temperatures up to 1250 mK. Over the entire temperature range, paramagnetism in the flux line cores results in an increase of the FF with field. Near H_c2 the FF decreases again, and our results indicate that this fall-off extends outside the proposed Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) region. Instead, we attribute the decrease to a paramagnetic suppression of Cooper pairing. At higher temperatures, a gradual crossover toward more conventional mixed state behavior is observed.

No abstract available

[en] We have measured the resistivity and magnetisation of several samples of the Eu_1-xCa_xB₆ series under pressure in order to probe the evolution of itinerant electron ferromagnetism in CaB₆ from a localised moment ordered state in EuB₆. Starting with pure EuB₆, the Curie temperature T_C is reduced upon increasing x. In particular, we have investigated the magnetic field and pressure dependence of T_C, which is enhanced by both of these externally tuneable control parameters. At T_C, there is a large peak in the resistivity, which is eliminated by moderately strong magnetic fields. (author)

[en] Zero field (ZF) and transverse field (TF) muon spin relaxtion and rotation (μSR) experiments have been carried out in the Cd-doped heavy fermion super conductor CeCoIn₅ to investigate its superconducting state.

[en] The CeMIn₅ (where M: Co,Ir) family of heavy fermion systems is currently in vogue due to the delicate interplay between unconventional superconductivity and magnetism observed in these systems. A putative quantum critical point - which lies in the vicinity of the superconducting regime in the temperature-magnetic field phase space of these systems - manifests itself in a host of novel properties. An additional ambient pressure phase space was opened up by Cd substitution on the In site, which acts as an efficient electronic tuning agent and shifts the ground state from a superconducting to an antiferromagnetic one. Here, we present sensitive measurements of the magnetoresistance in the CeCo(In_0.925Cd_0.075)₅ system in the temperature range 0.05 K≤T ≤4 K and with magnetic fields of up to 15 T. At fields larger than the superconducting upper critical field, features corresponding to a possible destabilization of the antiferromagnetic order are observed. Measurements performed with the magnetic field applied along different crystallographic directions indicate that this feature is strongly anisotropic. The implications of our results are discussed in the context of the crystallographic and magnetic anisotropy of these systems.

[en] The heavy-fermion (HF) metals are very susceptible to chemical substitution. In these compounds the Kondo coupling between a lattice of local moments and the conduction band creates quasiparticle excitations with large effective masses and the dopants disrupt the coherent Kondo coupling. We study the effect of Yb doping on the Pauli-limited, HF superconductor, CeCoIn₅ via de Haas-van Alphen (dHvA) measurements. Yb acts as a non-magnetic divalent substitution for Ce, equivalent to hole doping on the rare-earth site. Our main goal consists in the systematic investigation of the dHvA oscillations on Ce_1-xYb_xCoIn₅ in order to elucidate the evolution of the Fermi surface as a function of Yb. The dHvA data were obtained on high-quality single crystals with different concentrations of Yb atoms. The experiment was performed in a top-loading dilution refrigerator by use of a capacitive torque cantilever technique at temperatures down to 20 mK in magnetic fields up to 18 T.

[en] Starting from the heavy-fermion superconductor CeCoIn₅ with a superconducting T_c=2.3 K, doping with cadmium induces antiferromagnetic order in CeCo(In_1-xCd_x)₅ above a critical Cd concentration x_c ∼0.005 with a subtle interplay between magnetism and antiferromagnetism. We report on elastic neutron scattering experiments of the heavy-fermion alloy CeCo(In_1-xCd_x)₅ with x=0.0075 to study the magnetic structure and the influence of superconductivity on the antiferromagnetism. Below T_N=2.4 K and down to lowest temperatures T < 100 mK commensurate antiferromagnetic order with a propagation vector τ=(1/2 1/2 1/2) was detected. The transition into the superconducting state at T_c=1.7 K is accompanied by a kink in the magnetic intensity followed by a saturation towards lower temperatures at a reduced value. These results indicate a coexistence of antiferromagnetism and superconductivity, but reveal at the same time a strong influence of the superconducting state on the magnetic order. Our results are discussed in comparison to other heavy-fermion superconductors.