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Register a new userHybrid gap structure in the heavy-fermion superconductor CeIrIn$_5$
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Added by
Hamideh Shakeripour
Publication date
2006
fields
Physics
and research's language is
English
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The thermal conductivity $kappa$ of the heavy-fermion superconductor CeIrIn$_5$ was measured as a function of temperature down to $T_c$/8, for current directions perpendicular ($J parallel a$) and parallel ($J parallel c$) to the tetragonal c axis. For $J parallel a$, a sizable residual linear term $kappa_0 / T$ is observed, as previously, which confirms the presence of line nodes in the superconducting gap. For $J parallel c$, on the other hand, $kappa / T to 0$ as $T to 0$. The resulting precipitous decline in the anisotropy ratio $kappa_c / kappa_a$ at low temperature rules out a gap structure with line nodes running along the c-axis, such as the d-wave state favoured for CeCoIn$_5$, and instead points to a hybrid gap of $E_g$ symmetry. It therefore appears that two distinct superconducting states are realized in the Ce$M$In$_5$ family.
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We report a systematic study of temperature- and field-dependent charge ($boldsymbol{rho}$) and entropy ($mathbf{S}$) transport in the heavy-fermion superconductor CeIrIn$_5$. Its large positive thermopower $S_{xx}$ is typical of Ce-based Kondo lattice systems, and strong electronic correlations play an important role in enhancing the Nernst signal $S_{xy}$. By separating the off-diagonal Peltier coefficient $alpha_{xy}$ from $S_{xy}$, we find that $alpha_{xy}$ becomes positive and greatly enhanced at temperatures well above the bulk $T_c$. Compared with the non-magnetic analog LaIrIn$_5$, these results suggest vortexlike excitations in a precursor state to unconventional superconductivity in CeIrIn$_5$. This study sheds new light on the similarity of heavy-fermion and cuprate superconductors and on the possibility of states not characterized by the amplitude of an order parameter.
In quasi-two dimensional Ce(Ir,Rh)In$_5$ system, it has been suggested that the phase diagram contains two distinct domes with different heavy fermion superconducting states. We here report the systematic pressure dependence of the electron transport properties in the normal state of CeRh$_{0.2}$Ir$_{0.8}$In$_{5}$ and CeIrIn$_{5}$, which locates in first and second superconducting dome, respectively. We observed non-Fermi liquid behavior at low temperatures in both compounds, including non-quadratic $T-$dependence of the resistivity, large enhancement of the Hall coefficient, and the violation of the Kohlers rule in the magnetoresistance. We show that the cotangent of Hall angle $cot Theta_H$ varies as $T^2$, and the magnetoresistance is quite well scaled by the Hall angle as $Delta rho_{xx}/rho_{xx}propto tan^2Theta_H$. The observed transport anomalies are common features of Ce$M$In$_{5}$ ($M$=Co, Rh, and Ir) and high-$T_c$ cuprates, suggesting that the anomalous transport properties observed in CeIrIn$_{5}$ are mainly governed by the antiferromagnetic spin fluctuations, not by the Ce-valence fluctuations which has been proposed to be the possible origin for the second superconducting dome.
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.
We report $^{115}$In nuclear quadrupolar resonance (NQR) measurements on the heavy-fermion superconductor PuCoIn$_5$, in the temperature range $0.29{rm K}leq Tleq 75{rm K}$. The NQR parameters for the two crystallographically inequivalent In sites are determined, and their temperature dependence is investigated. A linear shift of the quadrupolar frequency with lowering temperature below the critical value $T_c$ is revealed, in agreement with the prediction for composite pairing. The nuclear spin-lattice relaxation rate $T_1^{-1}(T)$ clearly signals a superconducting (SC) phase transition at $T_csimeq 2.3$K, with strong spin fluctuations, mostly in-plane, dominating the relaxation process in the normal state near to $T_c$. Analysis of the $T_1^{-1}$ data in the SC state suggests that PuCoIn$_5$ is a strong-coupling $d$-wave superconductor.
The superconducting order parameter of the first heavy-fermion superconductor CeCu2Si2 is currently under debate. A key ingredient to understand its superconductivity and physical properties is the quasiparticle dispersion and Fermi surface, which remains elusive experimentally. Here we present measurements from angle-resolved photoemission spectroscopy. Our results emphasize the key role played by the Ce 4f electrons for the low-temperature Fermi surface, highlighting a band-dependent conduction-f electron hybridization. In particular, we find a very heavy quasi-two-dimensional electron band near the bulk X point and moderately heavy three-dimensional hole pockets near the Z point. Comparison with theoretical calculations reveals the strong local correlation in this compound, calling for further theoretical studies. Our results provide the electronic basis to understand the heavy fermion behavior and superconductivity; implications for the enigmatic superconductivity of this compound are also discussed.
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