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The temperature dependencies of the lower critical field Hc1(T) of several filled-skutterudite superconductors were investigated by local magnetization measurements. While LaOs4As12 and PrRu4As12 exhibit the Hc1(T) dependencies consistent with the single-band BCS prediction, for LaRu4As12 (the superconducting temperature Tc = 10.4 K) with a similar three-dimensional Fermi surface, we observe a sudden increase in Hc1(T) deep in a superconducting state below about 0.32Tc. Remarkably, a rapid rise of Hc1(T) at approximately the same reduced temperature 0.27Tc is also found for the heavy-fermion compound PrOs4Sb12, (Tc similar or equal to 1.78 K), in fair accord with the earlier macroscopic study. We attribute the unusual Hc1(T) dependencies of LaRu4As12 and PrOs4Sb12 to a kink structure in their superfluid densities due to different contributions from two nearly decoupled bands. Whereas LaRu4As12, is established as a two-band isotropic s-wave superconductor, nonsaturating behavior of Hc1(T) is observed for PrOs4Sb12, indicative of an anisotropic structure of a smaller gap. For this superconductor with broken time-reversal symmetry, our findings suggest a superconducting state with multiple symmetries of the order parameters.
In the known topological semimetals, conventional charge carriers exist in addition to relativistic quasiparticles, and thus a disentangling of their conduction properties remains challenging. Here, we address an unsaturated extreme magnetoresistance (XMR) with a marked deviation from the semiclassical B2 behavior that is commonly credited to the presence of topologically protected electronic states. For the topologically trivial semimetal LuAs, we observe a nonsaturating XMR with a nonquadratic magnetic-field dependence gained up to nearly 60 T. Remarkably, this diamagnetic material exhibits a very large magnetostriction that provides solid evidence for a field-dependent variation of electron and hole concentrations. We show that an underlying strain-induced change in the charge-carrier densities can give rise to an unsaturated XMR even in a moderately imbalanced semimetal. Our finding is of importance as well for topological semimetals in which the number of conventional charge carriers can be continuously altered with increasing field, and hence some of their high-field properties may not necessarily reflect the presence of massless quasiparticles.
Of all stoichiometric filled-skutterudite superconductors, LaRu4As12 has the highest critical field and temperature. Here we report on a detailed Fermi-surface investigation of LaRu4As12 by means of de Haas-van Alphen measurements and density-functional-theory calculations. We find evidence for a nearly spherical and a multiply connected Fermi-surface sheet. The different effective masses and mass enhancements for the two sheets support two-band superconductivity, which was inferred from previous specific-heat measurements. Furthermore, quantum oscillations persist as well in the superconducting phase. We use two models to describe the additional damping, yielding energy gaps differing by a factor of 5.
We present the results of our comprehensive investigation on the antiferromagnetic heavy-fermion superconductor Ce3PtIn11 carried out by means of electrical transport, heat capacity and ac magnetic susceptibility measurements, performed on single-crystalline specimens down to 50 mK in external magnetic fields up to 9 T. Our experimental results elucidate a complex magnetic field -temperature phase diagram which contains both first- and second-order field-induced magnetic transitions and highlights the emergence of field stabilized phases. Remarkably, a prominent metamagnetic transition was found to occur at low temperatures and strong magnetic fields. In turn, the results obtained in the superconducting phase of Ce3PtIn11 corroborate an unconventional nature of Cooper pairs formed by heavy quasiparticles. The compound is an almost unique example of a heavy fermion system in which superconductivity may coexist microscopically with magnetically ordered state.
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We address the origin of the magnetic-field-independent -AT1/2 term observed in the low-temperature resistivity of several As-based metallic systems of the PbFCl structure type. For the layered compound ZrAs1.58Se0.39, we show that vacancies in the square nets of As give rise to the low-temperature transport anomaly over a wide temperature regime of almost two decades in temperature. This low-temperature behavior is in line with the nonmagnetic version of the two-channelKondo effect, whose origin we ascribe to a dynamic Jahn-Teller effect operating at the vacancy-carrying As layer with a C4 symmetry. The pair-breaking nature of the dynamical defects in the square nets of As explains the lowsuperconducting transition temperature Tc approximate to 0.14 K of ZrAs1.58Se0.39 compared to the free-of-vacancies homologue ZrP1.54S0.46 (Tc approximate to 3.7 K). Our findings should be relevant to a wide class of metals with disordered pnictogen layers.
The heat capacity, magnetic, and electrical transport properties of the binary intermetallic compound Y9Co7 are re-investigated through careful low-temperature measurements performed on one and the same high-quality polycrystalline sample. Our results indicate a local coexistence of itinerant ferromagnetism and bulk superconductivity in Y9Co7 below Tsc = 2.95 K, as opposite to a previous conviction that superconductivity occurs in the paramagnetic phase embedded in a basically normal magnetic environment. Since the clean-limit condition is satisfied for a pure sample of Y9Co7, the question whether magnetic fluctuations contribute in the formation of Cooper pairs in this sole d-band ferromagnetic superconductor remains open.
We present some physical properties of the filled skutterudite compound NdOs4As12, which exhibits a ferromagnetic transition at TC approximate to 1.1 K, evidenced by distinct anomalies in susceptibility, electrical resistivity, and specific-heat C(T) measurements. At B = 0, the heat capacity displays a Schottky-like term with maximum values at about 0.93 K. Additionally, the C(T) data show enhanced values in the low-temperature paramagnetic state. Upon small external fields B ≤ 0.25 T, a Schottky-like peak shifts above the ferromagnetic transition. Anomalous low-energy excitations of a quartet ground state of the Nd3+ multiplet are attributed to a lowering of the Th cubic point symmetry due to structural disorder rather than to the Zeeman splitting due to a molecular field. We expect that our findings will be also relevant for the study of exotic heavy-fermion behavior in filled skutterudites.
properties of the filled skutterudite compound LaRu4As12 were investigated both as a function of temperature andmagnetic field. At T ≪ Tc = 10.45 K, the electronic contribution to the specific heat was found to be at variance with the predictions for an s-wave superconductor with one energy gap. In constant applied fields up to Hc2(0) approximate to 10.2 T, multiple gaps have been further inferred from both a nonlinear magnetic-field dependence of the specific heat in the zero-temperature limit and a positive curvature of the upper critical field in the vicinity of Tc. Thus, LaRu4As12 appears to be a rare multiple-gap superconductor with cubic symmetry. Intriguing evidence for multiband effects is observed in the compound with enhanced superconducting properties as compared to other skutterudite superconductors.
Thermodynamic and transport properties of the La-diluted Kondo lattice CeNi2Ge2 were studied in a wide temperature range. The Ce-rich alloys Ce1−xLaxNi2Ge2 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 CeNi2Ge2 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.
The effect of La doping of CeRhSb has been studied with the help of electrical resistivity, magnetic susceptibility, and specific-heat measurements for the system of Ce1-xLaxRhSb with x < 0.2. Particular emphasis is put on the extreme low-doping regime, x similar to 0.02. Namely, we show that the resistivity increase induced by appearance of the Kondo gap for x = 0 is transformed into that caused by itinerant f holes and appears already for x = 0.02. The studies indicate also that a spin-glasslike behavior, intermixed with a ferromagnetic component in these materials, is induced by statistical distribution of La ions. Our study also shows that the Kondo gap in CeRhSb is very sensitive to the magnitude of hybridization V between the 4f-electron and the conduction-electron states. The results for the system Ce1-xLaxRhSb are interpreted in terms of (narrow) Kondo-hole band, located near at the Fermi level, that gives rise to an abrupt transition from the Kondo-insulator regime to a metallic state via an intermediate step (at very low doping) involving formation of the ferromagnetic clusters containing a Kondo hole (i.e., magnetic polarons). Finally, a comparison between Ce1-xLaxRhSb with Ce1-xLaxNiSn systems is carried out to contrast the concept of the polarons in Ce1-xLaxRhSb with their absence in the latter system due to the dominant antiferromagnetic interaction among the 4f(1) spins.
Magnetization, electrical-resistivity, specific-heat, and thermopower measurements were performed on single crystals of the filled skutterudite compound PrRu4As12. These measurements reveal a superconducting transition near 2.5 K that is quantified by: (1) magnetic susceptibility data which display an onset to a Meissner state at Tc approximate to 2.5 K and complete diamagnetic shielding as T → 2.0 K, (2) electrical-resistivity data which show a transition to a zero-resistance state at Tc approximate to 2.5 K, and (3) specific-heat and thermopower data which display "jumps" at Tc approximate to 2.5 K. Furthermore, the electronic contribution to the specific heat follows the BCS prediction for a superconductor. For temperatures above Tc, magnetization measurements indicate local moment behavior where μeff = 3.52 μ(B)/Pr3+ ion. Analysis of the data for T>Tc, using a crystalline electric field model, suggests that PrRu4As12 has a Γ1 ground state with a Γ4 first excited state at TΓ4 approximate to 95 K.
Single crystals of HfAs1.7Se0.2 are grown by chemical transport reaction and their chemical composition characterized in detail by various analytical methods. Chemical analyses and crystal structure investigations by single-crystal X-ray diffraction as well as powder diffraction with synchrotron radiation reveal a tetragonal PbFCI structure type with strong disorder caused by a significant arsenic deficiency (As0.9) on the 2a site and mixed occupancy of the 2c site (As0.8Se0.2). HfAs1.7Se0.2 is a diamagnetic metal which transforms into a superconducting state at Tc= 0.52 K. Similar to other PbFCI-type arsenide selenides, the title compound displays a magnetic-field-independent -AT1/2 term in the low-temperature electrical resistivity. This unusual term presumably originates from the electron scattering of structural two-level systems. According to the experimental results, HfAs1.7Se0.2 appears to be a rare example of a nonmagnetic Kondo material.