Intermetallic compounds containing $f$-electron elements display a wealth of superconducting phases, which are prime candidates for unconventional pairing with complex order parameter symmetries. For instance, superconductivity has been found at the border of magnetic order as well as deep within ferromagnetically and antiferromagnetically ordered states, suggesting that magnetism may promote rather than destroy superconductivity. Superconducting phases near valence transitions or in the vicinity of magnetopolar order are candidates for new superconductive pairing interactions such as fluctuations of the conduction electron density or the crystal electric field, respectively. The experimental status of the study of the superconducting phases of $f$-electron compounds is reviewed.

Superconducting phases of f -electron compounds

Multipolar interactions in f -electron systems: The paradigm of actinide dioxides

We introduce the development of conductive organic molecular assemblies including organic metals, superconductors, single component conductors, conductive films, and conductive liquids, particularl...

Development of Conductive Organic Molecular Assemblies: Organic Metals, Superconductors, and Exotic Functional Materials

Recent advances in the design and synthesis of organic synthetic metals have yielded materials that have the highest superconducting transition temperatures ( T c ≈ 13 kelvin) reported for these systems These materials have crystal structures consisting of alternating layers of organic donor molecules and inorganic anions Organic superconductors have various electronic and magnetic properties and crystal structures that are similar to those of the inorganic copper oxide superconductors (which have high T c values); these similarities include highly anisotropic conductivities, critical fields, and short coherence lengths The largest number of organic superconductors, including those with the highest T c values, are charge-transfer salts derived from the electron donor molecule BEDT-TTF or ET [bis(ethylenedithio)-tetrathiafulvalene] The synthesis and crystal structures of these salts are discussed; their electrical, magnetic, and band electronic structure properties and their many similarities to the copper oxide superconductors are treated as well

Organic superconductors--new benchmarks.

In this paper we review experimental and theoretical results on higher electronic multipoles in solids with strong correlations. Recent experiments and their theoretical interpretation have confirmed the ordering of octupoles and even higher multipoles in rare-earth and actinide compounds with f electrons. The concept of multipoles is critically examined in point groups where spherical tensors of different ranks mix. Using a phenomenological approach, we demonstrate how linear and nonlinear couplings of different multipoles lead to rich phase diagrams and anomalies in physical observables. As actual representative systems, we first consider Ce x La 1- x B 6 , for which resonant X-ray scattering probed the octupole order for the first time, and NpO 2 , where quadrupoles induced by the octupole order have been observed. We then consider a class of compounds called skutterudites as the most convenient system for systematic study. Particular emphasis is placed on the ordering of scalar components from fourth-...

Multipole Orders and Fluctuations in Strongly Correlated Electron Systems

Specific heat and magnetization measurements have been performed on high-quality single crystals of filled skutterudite ${\mathrm{PrFe}}_{4}{\mathrm{P}}_{12}$ 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 $\ensuremath{\sim}9\mathrm{K}$ and the existence of ferromagnetic Pr-Pr interactions are deduced. The ${}^{141}\mathrm{Pr}$ 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 $\ensuremath{\sim}0.03{\ensuremath{\mu}}_{B}/\mathrm{Pr}$ 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.

https://arxiv.org/pdf/cond-mat/0201046.pdf

Anomalous heavy-fermion and ordered states in the filled skutterudite PrFe 4 P 12

We have measured both magnetoresistance and Hall effect in $\mathrm{Ce}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ to clarify the large resistivity state ascribed to the Kondo insulating one and the origin of the phase transition near $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ reported in the specific-heat measurement. We found unusual temperature $(T)$ dependence both in the electrical resistivity $\ensuremath{\rho}\ensuremath{\sim}{T}^{\ensuremath{-}1∕2}$ and the Hall coefficient ${R}_{H}\ensuremath{\sim}{T}^{\ensuremath{-}1}$ over the wide temperature range of about two orders of magnitude below $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which can be explained as a combined effect of the temperature dependences of carrier density and carrier scattering by spin fluctuation. An anomaly related with the phase transition has been clearly observed in the transport properties, from which the $H\text{\ensuremath{-}}T$ phase diagram is determined up to $14\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Taking into account the small entropy change, the phase transition is most probably the spin density wave one. Both the electrical resistivity and Hall resistivity at $0.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is largely suppressed about an order of magnitude by magnetic fields above $\ensuremath{\sim}3\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, suggesting a drastic change of electronic structure and a suppression of spin fluctuations under magnetic fields.

Transport properties in Ce Os 4 Sb 12 : Possibility of the ground state being semiconducting

We discovered a new organic superconductor, (DMET)2Au(CN)2, which is a radical salt of an asymmetrical donor. The room-temperature conductivity of this salt was very high (2500 S·cm−1) at ambient pressure. This salt showed a metal-insulator transition at about 25 K at ambient pressure, but it exhibited superconductivity below 0.80 K at 5 kbar.

New Organic Superconductor, (DMET)2Au(CN)2

We discovered superconductivity at ambient pressure in (DMET) 2 I 3 at 0.47 K and in (DMET) 2 IBr 2 at 0.59 K (midpoints), where DMET is (dimethyl(ethylenedithio)diselenadithiafulvalene), while in (DMET) 2 I 2 Br, superconductivity was not observed down to 0.5 K. In the DMET salts, those with smaller molar volume exhibit higher transition temperature, contrary to the BEDT-TTF salts. The effect of the disorder of the anion orientation on superconductivity is discussed.

On Ambient-Pressure Superconductivity in Organic Conductors: Electrical Properties of (DMET)2I3, (DMET)2I2Br and (DMET)2IBr2

We report transport measurements on high quality RERu4Sb12 single crystals under magnetic fields up to 14 T and temperatures down to 0.3 K. CeRu4Sb12 exhibits anomalous features in all the transport properties, compared to those for RE = La, Pr and Nd. The thermoelectric power (TEP) shows a huge positive peak over 100 μ V K−1 near 90 K. The Hall coefficient, ~2.0 × 10−9m3 C−1 at room temperature, exhibits a drastic increase below ~100 K to RH ≈ 5.2× 10−8m3 C−1 at 0.3 K. At low temperatures below ~3 K, the TEP exhibits an apparent anomaly correlated with the non-Fermi-liquid behaviour reported in the resistivity and specific heat. We have succeeded in observing Shubnikov–de Haas oscillations both in the magnetoresistance and Hall effect. This result along with the Hall effect suggests small semimetallic Fermi surfaces with highly correlated electrons at low temperatures in contrast with the large FS for LaRu4Sb12. NdRu4Sb12 exhibits anomalies suggesting a ferromagnetic ground state at around 1.4 K above which the resistivity exhibits a negative temperature coefficient up to 6 K.

https://iopscience.iop.org/article/10.1088/0953-8984/14/45/317/pdf

T. Namiki

Papers

Anomalous heavy-fermion and ordered states in the filled skutterudite PrFe 4 P 12

Transport properties in Ce Os 4 Sb 12 : Possibility of the ground state being semiconducting

New Organic Superconductor, (DMET)2Au(CN)2

On Ambient-Pressure Superconductivity in Organic Conductors: Electrical Properties of (DMET)2I3, (DMET)2I2Br and (DMET)2IBr2

Transport properties in the filled-skutterudite compounds RERu4Sb12 (RE—La, Ce, Pr and Nd); an exotic heavy fermion semimetal CeRu4Sb12