T. Cichorek

Divergence of the Grüneisen Ratio at Quantum Critical Points in Heavy Fermion Metals

We present low-temperature volume thermal expansion, beta, and specific heat, C, measurements on high-quality single crystals of CeNi2Ge2 and YbRh2(Si0.95Ge0.05)(2) which are located very near to quantum critical points. For both systems, beta shows a more singular temperature dependence than C, and thus the Grüneisen ratio Gamma proportional to beta/C diverges as T-->0. For CeNi2Ge2, our results are in accordance with the spin-density wave (SDW) scenario for three-dimensional critical spin fluctuations. By contrast, the observed singularity in YbRh2(Si0.95Ge0.05)(2) cannot be explained by the itinerant SDW theory but is qualitatively consistent with a locally quantum critical picture.

Pronounced Enhancement of the Lower Critical Field and Critical Current Deep in the Superconducting State of PrOs4Sb12

We have observed an unexpected enhancement of the lower critical field H(c1)(T) and the critical current I(c)(T) deep in the superconducting state below T approximately 0.6 K (T/T(c) approximately 0.3) in the filled skutterudite heavy fermion superconductor PrOs(4)Sb(12). From a comparison of the behavior of H(c1)(T) with that of the heavy fermion superconductors U(1-x)Th(x)Be(13) and UPt(3), we speculate that the enhancement of H(c1)(T) and I(c)(T) in PrOs(4)Sb(12) reflects a transition into another superconducting phase that occurs below T/T(c) approximately 0.3. An examination of the literature reveals unexplained anomalies in other physical properties of PrOs(4)Sb(12) near T/T(c) approximately 0.3 that correlate with the features we have observed in H(c1)(T) and I(c)(T).

Specific heat and disorder in the mixed state of non-magnetic borocarbides

The temperature and magnetic-field dependence of the specific heat cp(T, H) in the superconducting (sc) mixed state as well as the upper critical field Hc2(T ) have been measured for polycrystalline YxLu1−xNi2B2C and Y(Ni1−yPty)2B2C samples. The linear-in- T electronic specific-heat contribution γ(H) · T exhibits significant deviations from the usual linear-in-H law resulting in a disorder-dependent negative curvature of γ(H). The Hc2(T )d ata point to the quasi-clean limit for (Y, Lu)-substitutions and to a transition to the quasi-dirty limit for (Ni, Pt)-substitutions. The γ(H)-dependence is discussed in the unitary d-wave as well as in the quasi-clean s-wave limits. From a consideration of γ(H )d ata only,d-wave pairing cannot be ruled out.

Low-energy excitations of the semimetallic one-dimensional S=1/2 antiferromagnet Yb4As3 ☆

We investigate a new route to quasi-one-dimensional spin-chain systems which originates from the charge-ordering transition out of a homogeneous mixed-valence state. We present evidence that Yb4As3 and P, Sb doped mixed crystals are well described by this mechanism. Many thermodynamic and low-temperature transport heavy-fermion-like properties can be explained by the existence of low-lying quasi-one-dimensional spin excitations in the Yb3+-chains. The observation of soliton excitations in a transverse external field gives further support to the existence of spin chains in Yb4As3. We also present recent results on spin-glass behavior at very low temperature caused by the interchain coupling and disorder. In addition, the yet unexplained magnetotransport effects are discussed.

Crystallochemistry and Kondo-like behaviour of the thorium and uranium arsenoselenides

Abstract We have grown single crystals of uranium and thorium arsenoselenide and arsenosulphide by the chemical vapour transport method. The crystals show a Kondo-like effect and its contribution to the resistivity was estimated by the determination of the reciprocal residual resistivity ratio (RRRR) ρ (4,2)/ ρ (300). The ratio varied from 0.70 (for UAsS) to 2.10 (for ThAsSe). A high resolution transmission electron microscopy study of ThAsSe crystals reveals a perfect ordering of cations. The X-ray diffraction examination showed that the unit cells of the examined crystals belong to a tetragonal system with the PbFCl-type structure (space group P4/nmm (no. 129)). Anomalously large anisotropic displacement factors have been observed for all the atoms composing the compounds examined. An interdependence between the displacement factor and RRRR has allowed to ascribe both phenomena to the same source. It indicates that the two-level system Kondo model is most suitable for the theoretical description of the systems.

Thermo-chemical properties and electrical resistivity of Zr-based arsenide chalcogenides

abstract Ternary phases in the systems Zr–As–Se and Zr–As–Te were studied using single crystals of ZrAs1.40(1)Se0.50(1) and ZrAs1.60(2)Te0.40(1) (PbFCl-type of structure, space group P4/nmm) as well as ZrAs0.70(1)Se1.30(1) and ZrAs0.75(1)Te1.25(1) (NbPS-type of structure, space group Immm). The characterization covers chemical compositions, crystal structures, homogeneity ranges and electrical resistivities. At 1223 K, the Te-containing phases can be described with the general formula ZrAsxTe2–x, with 1.53(1)≤x≤1.65(1) (As-rich) and 0.58(1)≤x≤0.75(1) (Te-rich). Both phases are located directly on the tie-line between ZrAs2 and ZrTe2, with no indication for any deviation. Similar is true for the Se-rich phase ZrAsx–ySe2–x with 0.70(1)≤x≤0.75(1). However, the compositional range of the respective As-rich phase ZrAsx–ySe2–x (0.03(1)≤y≤0.10(1); 1.42(1)≤x≤1.70(1)) is not located on the tie-line ZrAs2–ZrSe2, and exhibits a triangular region of existence with intrinsic deviation of the composition towards lower non-metal contents. Except for ZrAs0.75Se1.25, from the homogeneity range of the Se-rich phase, all compounds under investigation show metallic characteristics of electrical resistivity at temperatures >20 K. Related uranium and thorium arsenide selenides display a typical magnetic field-independent rise of the resistivity towards lower temperatures, which has been explained by a non-magnetic Kondo effect. However, a similar observation has been made for ZrAs1.40Se0.50, which, among the Zr-based arsenide chalcogenides, is the only system with a large concentration of intrinsic defects in the anionic substructure.

Unusual magnetic properties of the low-dimensional quantum magnet Na2V3O7

We report the results of low-temperature measurements of the specific heat Cp(T), ac susceptibility chi(T) and 23Na nuclear magnetic resonance NMR of Na2V3O7. At liquid He temperatures Cp(T)/T exhibits broad field-dependent maxima, which shift to higher temperatures upon increasing the applied magnetic field H. Below 1.5 K the ac magnetic susceptibility chi(T) follows a Curie-Weiss law and exhibits a cusp at 0.086 mK which indicates a phase transition at very low temperatures. These results support the previous conjecture that Na2V3O7 is close to a quantum critical point (QCP) at mu_{0}H = 0 T. The entire data set, including results of measurements of the NMR spin-lattice relaxation 1/T1(T), reveals a complex magnetic behavior at low temperatures. We argue that it is due to a distribution of singlet-triplet energy gaps of dimerized V moments. The dimerization process evolves over a rather broad temperature range around and below 100 K. At the lowest temperatures the magnetic properties are dominated by the response of only a minor fraction of the V moments.

Crystallographic disorder and electron scattering on structural two-level systems in ZrAs1.4Se0.5

Single crystals of ZrAs1.4Se0.5 (PbFCl-type structure) were grown by chemical vapour transport. While their thermodynamic and transport properties are typical for ordinary metals, the electrical resistivity exhibits a shallow minimum at low temperatures. Application of strong magnetic fields does not influence this anomaly. The minimum of the resistivity in ZrAs1.4Se0.5 apparently originates from interaction between the conduction electrons and structural two-level systems. Significant disorder in the As–Se substructure is inferred from x-ray diffraction and electron microprobe studies.

Low-temperature specific heat for off- and near-stoichiometric UAsSe

Abstract Two UAsSe single crystals with slight variations of the As/Se content ratio have been investigated. The development of the low-T upturn in the resistivity is accompanied by smooth decrease of the Curie temperature. The sample with T C =116.9 K and almost invisible upturn could be regarded as a near-stoichiometric, slightly disordered one, whereas the specimen with T C =101.5 K appears to be an off-stoichiometric, more strongly disordered one. Clear differences between both electronic and lattice contributions to the specific heat for off- and near-stoichiometric UAsSe crystals have also been found. We ascribe the smaller low-T specific heat for the off-stoichiometric sample in comparison to the near-stoichiometric one to a weaker hybridization between 5f and conduction-electron states accompanied by the relative shift of 5f band with regard to EF.

Single-ion Kondo Scaling of the Coherent Fermi Liquid Regime in Ce1 xLaxNi2Ge2

Thermodynamic and transport properties of the La-diluted Kondo lattice CeNi(2)Ge(2) were studied in a wide temperature range. The Ce-rich alloys Ce(1-x)La(x)Ni(2)Ge(2) were found to exhibit distinct features of the coherent heavy Fermi liquid. At intermediate compositions (0.7≤x≤0.9), non-Fermi liquid properties have been observed, followed by the local Fermi liquid behavior in the dilute limit. The 4f-electron contribution to the specific heat was found to follow the predictions of the Kondo-impurity model in both the local as well as the coherent regimes, with the characteristic Kondo temperature decreasing rapidly from about 30 K for the parent compound CeNi(2)Ge(2) to about 1 K in the most dilute samples. The specific heat does not show any evidence for the emergence of a new characteristic energy scale related to the formation of the coherent Kondo lattice.