Results 1 - 10 of 89
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[en] In YbRh2Si2 pronounced non-Fermi-liquid (NFL) effects are observed in thermodynamic, magnetic and transport properties above a weak antiferromagnetic (AF) phase transition at TN = 70 mK. The AF order is suppressed to TN → 0 either by i) the application of small critical magnetic fields Bc0 or ii) a slight expansion of the crystal lattice by substituting 5% of the Si atoms by Ge in YbRh2(Si1-xGex)2. In both cases the NFL behavior extends to lowest temperatures. For B > Bc0 (with Bc0 = 0 for x=0.05) we observe a weakly polarized Landau FL at lowest temperatures which fulfills the Kadowaki-Woods relation A/γ2 = const between the coefficients A of the resistivity and γ of the specific heat. The 1/(B-Bc0) divergence of A(B) indicates singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass at the quantum critical point. (author)
[en] In the itinerant metamagnet Sr3Ru2O7, a first order metamagnetic transition is suppressed to a quantum critical end point (QCEP) at T=0 by applying a magnetic field μ0 Hc = 7.85 T along the c-axis. Quantum critical behaviour is observed in thermal expansion and can be explained in terms of 2d ferromagnetic fluctuations. Below 1 K, a new phase with a strongly enhanced residual resistivity forms in the vicinity of the QCEP. When H has a small in-plane component, the resistivity becomes anisotropic, which is discussed in terms of formation of an electronic nematic fluid organized into domains. We present in-plane thermal expansion and magnetostriction measurements obtained by high-resolution capacitive dilatometry. Our findings suggest that inside the nematic phase, the four-fold symmetry is broken by a uniform lattice distortion with ΔL/L of the order of 10-6. The results are discussed in the context of a recent model for domain formation via partial orbital ordering.
[en] The S=((1)/(2)) antiferromagnetic Heisenberg model with the Dzyaloshinsky-Moriya interaction is applied to a semimetallic compound Yb4As3. The field dependent specific heat and the magnetization of this model are calculated by the numerical quantum transfer-matrix method and compared with experiment. New specific heat experimental results for the polydomain sample in the field applied are presented and compared with the theoretical predictions obtained without adjustable microscopic parameters
[en] The upper critical field Hc2(T) of a high quality single crystal of UBe13 is studied with very low noise resistivity measurements. It shows a large but finite slope of -45 T/K, an unusual temperature dependence with an inflexion point at T/Tc ∼0.5 and a large saturated limit for T → 0 of Hc2(0) = 14 T. The complete temperature dependence of Hc2(T) can be described by a simple model of strong coupling superconductor, assuming a full Pauli limitation and the occurrence of a nonuniform superconducting state (FFLO state) at low temperature
[en] Low-temperature electrical-resistivity, ρ(T), measurements on single crystals of the structurally disordered ThAsSe reveal an anomalous scattering mechanism, which is apparently derived from two-level systems. For the ThAsSe specimen displaying a resistivity saturation at millikelvin temperatures, a crossover from a logarithmic to a non-Fermi-liquid behavior Δρ∝1-aT1/2 is observed upon cooling below ≅4 K. A comparison of experimental results with the theoretical ones yields a characteristic energy scale of the order of a few K for ThAsSe
[en] We present a systematic study on the influence of antiferromagnetic and ferromagnetic phases of Eu2+ moments on the superconducting phase upon doping the As site by isovalent P, which acts as chemical pressure on EuFe2As2. In this contribution, we shall report the magnetic susceptibility, magnetization, resistivity, specific heat and thermal conductivity measurements performed on P doped EuFe2As2 single crystals. Bulk superconductivity with transition temperatures of 22 K and 28 K is observed for x = 0.16 and 0.20 samples respectively. The Eu ions order antiferromagnetically for x = 0.13, while a crossover is observed for x = 0.22 whereupon the Eu ions order ferromagnetically. We discuss in detail the coexistence of superconductivity and magnetism in a tiny region of the phase diagram and comment on the competition of ferromagnetism and superconductivity in the title compound.
[en] YbRh2Si2 is a prototype heavy-fermion metal which displays a magnetic, field-induced antiferromagnetic (AF) quantum critical point (QCP). It has attracted much attention due to an additional low-energy scale T*(B) merging at the QCP, whose origin is still under discussion. Here, we present our recent thermodynamic, magnetic and electrical transport measurements on different single crystalline samples of charge-carrier doped Yb(Rh1-xTx)2Si2 (=Fe,Ni,Ru) at temperatures down to 15 mK and in magnetic fields up to 7 T. The partial substitution of Rh by either Fe or Ni introduces holes or electrons, respectively. The evolution of the single-ion Kondo scale is similar to isoelectronic Co substitution in accordance with the chemical pressure effect. However, while chemical pressure has little influence on T*(B) in isoelectronic doped samples, we observe a drastic reduction or increase of B*(T = 0) by Fe- or Ni-doping, respectively. As the AF order is completely suppressed by Fe-doping, a heavy Fermi liquid ground state (without the T*(B) anomaly) is observed. These results are compared to measurements on samples where Rh is partially substituted by Ru. Here the chemical pressure effect is minimized in order to investigate the pure charge doping effect.
[en] The authors address the relationship between heavy-fermion (HF) superconductivity (sc) and antiferromagnetic order (afm) in U- and Ce-based compounds. Usually, sc and small-moment afm coexist in U-systems (e.g. UPt3 and URu2Si2). For CeCu2Ge2, on the other hand, external pressure was found to suppress large-moment afm, before sc develops at p approx-gt 7 GPa. As is discussed in the present paper, in UPd2Al3 sc even coexists with large-moment afm, and in CeCu2Si2 sc competes with and expells open-quotes phase Aclose quotes (presumably a disordered phase of low-moment afm). The authors advocate that the difference in the phenomenological behavior of Ce- and U-compounds is related to differences in the electronic structures of their open 4f and 5f shells
[en] Non-Fermi liquid (NFL) behavior in the normal state of the heavy-fermion superconductor UBe13 is studied by means of low-temperature measurements of the specific heat, C, and electrical resistivity, ρ, on a high-quality single crystal in magnetic fields up to 15.5 T. At B=0, unconventional superconductivity forms at Tc=0.9 K out of an incoherent state, characterized by a large and strongly temperature dependent ρ(T). In the magnetic field interval 4 T≤B≤10 T, ρ(T) follows a T3/2 behavior for Tc(B)≤T≤1 K, while ρ is proportional to T at higher temperatures. Corresponding non-Fermi liquid behavior is observed in C/T as well and hints at a nearby antiferromagnetic (AF) quantum critical point (QCP) covered by the superconducting state. We speculate that the suppression of short-range AF correlations observed by thermal expansion and specific heat measurements below TL∼0.7 K (B=0) yields a field-induced QCP, TL→0, at B=4.5 T
[en] Geometrically frustrated magnetic materials host new exotic states such as quantum spin liquids (QSL). A recent example is the QSL candidate YbMgGaO, with the triangular lattice of pseudospin-1/2 Yb ions. A crucial part, to understand the physics of YbMgGaO, is to identify leading interaction terms that may extend beyond nearest neighbors. To this end, we studied KBaYb(BO), where the Yb ions are also arranged on a triangular lattice, whereas the distance between nearest neighbors is comparable to the next nearest neighbor distance in YbMgGaO. Our polycrystalline samples confirm the rhombohedral symmetry of KBaYb(BO) with the space group . Magnetic susceptibility data down to 1.8 K revealed a Curie-Weiss temperature of about 20 mK with antiferromagnetic couplings. Furthermore, we report heat capacity down to 60 mK, which also indicates a very small exchange coupling.