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Register a new userHeavy Fermion Superconductivity in the Quadrupole Ordered State of PrV2Al20
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PrV$_2$Al$_{20}$ is a rare example of a heavy fermion system based on strong hybridization between conduction electrons and nonmagnetic quadrupolar moments of the cubic $Gamma_3$ ground doublet. Here, we report that a high-quality single crystal of PrV$_2$Al$_{20}$ exhibits superconductivity at $T_{rm c}=$ 50 mK in the antiferroquadrupole-ordered state under ambient pressure. The heavy fermion character of the superconductivity is evident from the specific heat jump of $Delta C/T sim 0.3$ J/mol K$^2$ and the effective mass $m^*/m_0 sim 140$ estimated from the temperature dependence of the upper critical field. Furthermore, the high-quality single crystals exhibit double transitions at $T_{rm Q}$ = 0.75 K and $T^{*}$ = 0.65 K associated with quadrupole and octapole degrees of freedom of the $Gamma_3$ doublet. In the ordered state, the specific heat $C/T$ shows a $T^3$ dependence, indicating the gapless mode associated with the quadrupole and/or octapole order. The strong sensitivity to impurity of the superconductivity suggests unconventional character due to significant quadrupolar fluctuations.
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We grew single crystals of the recently discovered heavy fermion superconductor UTe2, and measured the resistivity, specific heat and magnetoresistance. Superconductivity (SC) was clearly detected at Tsc=1.65K as sharp drop of the resistivity in a high quality sample of RRR=35. The specific heat shows a large jump at Tsc indicating strong coupling. The large Sommerfeld coefficient, 117mJ K-2mol-1 extrapolated in the normal state and the temperature dependence of C/T below Tsc are the signature of unconventional SC. The discrepancy in the entropy balance at Tsc between SC and normal states points out that hidden features must occur. Surprisingly, a large residual value of the Sommerfeld coefficient seems quite robust (gamma_0/gamma ~ 0.5). The large upper critical field Hc2 along the three principal axes favors spin-triplet SC. For H // b-axis, our experiments do not reproduce the huge upturn of Hc2 reported previously. This discrepancy may reflect that Hc2 is very sensitive to the sample quality. A new perspective in UTe2 is the proximity of a Kondo semiconducting phase predicted by the LDA band structure calculations.
We report the observation of heavy-fermion superconducitivity in CeCoIn5 at Tc =2.3 K. When compared to the pressure-induced Tc of its cubic relative CeIn3 (Tc ~200 mK), the Tc of CeCoIn5 is remarkably high. We suggest that this difference may arise from magnetically mediated superconductivity in the layered crystal structure of CeCoIn5 .
The interplay of magnetism and unconventional superconductivity (d singlet wave or p triplet wave) in strongly correlated electronic system (SCES) is discussed with recent examples found in heavy fermion compounds. A short presentation is given on the formation of the heavy quasiparticle with the two sources of a local and intersite enhancement for the effective mass. Two cases of the coexistence or repulsion of antiferromagnetism and superconductivity are given with CeIn3 and CeCoIn5. A spectacular example is the emergence of superconductivity in relatively strong itinerant ferromagnets UGe2 and URhGe. The impact of heavy fermion matter among other SCES as organic conductor or high Tc oxide is briefly pointed out.
Specific heat and magnetization measurements have been performed on high-quality single crystals of filled-skutterudite PrFe_4P_{12} in order to study the high-field heavy-fermion state (HFS) and low-field ordered state (ODS). From a broad hump observed in C/T vs T in HFS for magnetic fields applied along the <100> direction, the Kondo temperature of ~ 9 K and the existence of ferromagnetic Pr-Pr interactions are deduced. The {141}-Pr nuclear Schottky contribution, which works as a highly-sensitive on-site probe for the Pr magnetic moment, sets an upper bound for the ordered moment as ~ 0.03 mu_B/Pr-ion. This fact strongly indicates that the primary order parameter in the ODS is nonmagnetic and most probably of quadrupolar origin, combined with other experimental facts. Significantly suppressed heavy-fermion behavior in the ODS suggests a possibility that the quadrupolar degrees of freedom is essential for the heavy quasiparticle band formation in the HFS. Possible crystalline-electric-field level schemes estimated from the anisotropy in the magnetization are consistent with this conjecture.
PuCoGa$_5$ has emerged as a prototypical heavy-fermion superconductor, with its transition temperature ($T_csimeq18.5$ K) being the highest amongst such materials. Nonetheless, a clear description as to what drives the superconducting pairing is still lacking, rendered complicated by the notoriously intricate nature of plutoniums 5$f$ valence electrons. Here, we present a detailed $^{69,71}$Ga nuclear quadrupole resonance (NQR) study of PuCoGa$_5$, concentrating on the systems normal state properties near to $T_c$ and aiming to detect distinct signatures of possible pairing mechanisms. In particular, the quadrupole frequency and spin-lattice relaxation rate were measured for the two crystallographically inequivalent Ga sites and for both Ga isotopes, in the temperature range 1.6 K - 300 K. No evidence of significant charge fluctuations is found from the NQR observables. On the contrary, the low-energy dynamics is dominated by anisotropic spin fluctuations with strong, nearly critical, in-plane character, which are effectively identical to the case of the sister compound PuCoIn$_5$. These findings are discussed within the context of different theoretical proposals for the unconventional pairing mechanism in heavy-fermion superconductors.
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