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

We implemented the charge self-consistent combination of density-functional theory and dynamical mean-field theory (DMFT) in two full-potential methods, the augmented plane-wave and the linear muffin-tin orbital methods. We categorize the commonly used projection methods in terms of the causality of the resulting DMFT equations and the amount of partial spectral weight retained. The detailed flow of the dynamical mean-field algorithm is described, including the computation of response functions such as transport coefficients. We discuss the implementation of the impurity solvers based on hybridization expansion and an analytic continuation method for self-energy. We also derive the formalism for the bold continuous time quantum Monte Carlo method. We test our method on a classic problem in strongly correlated physics, the isostructural transition in Ce metal. We apply our method to the class of heavy-fermion materials ${\text{CeIrIn}}_{5}$, ${\text{CeCoIn}}_{5}$, and ${\text{CeRhIn}}_{5}$ and show that the $\text{Ce}\text{ }4f$ electrons are more localized in ${\text{CeRhIn}}_{5}$ than in the other two, a result corroborated by experiment. We show that ${\text{CeIrIn}}_{5}$ is the most itinerant and has a very anisotropic hybridization, pointing mostly toward the out-of-plane In atoms. In ${\text{CeRhIn}}_{5}$ we stabilized the antiferromagnetic DMFT solution below 3 K, in close agreement with the experimental Neel temperature.

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Dynamical mean-field theory within the full-potential methods: Electronic structure of CeIrIn 5 , CeCoIn 5 , and CeRhIn 5

With only a few exceptions that are well understood, conventional superconductivity does not coexist with long-range magnetic order (for example, ref. 1). Unconventional superconductivity, on the other hand, develops near a phase boundary separating magnetically ordered and magnetically disordered phases. A maximum in the superconducting transition temperature T(c) develops where this boundary extrapolates to zero Kelvin, suggesting that fluctuations associated with this magnetic quantum-critical point are essential for unconventional superconductivity. Invariably, though, unconventional superconductivity masks the magnetic phase boundary when T 0 K magnetic field-pressure phase diagram of CeRhIn5 is well described with a theoretical model developed to explain field-induced magnetism in the high-T(c) copper oxides, but in which a clear delineation of quantum-phase boundaries has not been possible. These experiments establish a common relationship among hidden magnetism, quantum criticality and unconventional superconductivity in copper oxides and heavy-electron systems such as CeRhIn5.

Hidden magnetism and quantum criticality in the heavy fermion superconductor CeRhIn5.

We carried out the de Haas–van Alphen (dHvA) experiment for an antiferromagnet CeRhIn 5 at high pressures up to 3 GPa. The cross-sectional areas of the Fermi surfaces due to main dHvA branches α i and β 2 at ambient pressure, which are well explained by two kinds of nearly cylindrical Fermi surfaces of a non-4 f reference compound LaRhIn 5 , are unchanged up to about 2.3 GPa, while the corresponding cyclotron masses m c * increase steeply above a pressure P * = 1.6 GPa where pressure-induced superconductivity sets in : m * c = 5.5 m 0 at ambient pressure, 20 m 0 at 1.6 GPa and 60 m 0 at 2.2 GPa for branch β 2 , for example. Above 2.4 GPa, new dHvA branches appear, which are in good agreement with the corresponding dHvA branches of a 4 f -itinerant heavy fermion superconductor CeCoIn 5 , indicating that the 4 f electron of CeRhIn 5 becomes itinerant and significantly contributes to the volume of the Fermi surface. The 4 f -electron character is thus changed from localized to itinerant at a critical pressur...

A Drastic Change of the Fermi Surface at a Critical Pressure in CeRhIn5: dHvA Study under Pressure

Origin of hydrogen trapping site in vanadium carbide precipitation strengthening steel

We report magnetic neutron-diffraction and electrical resistivity studies on single crystals of the heavy-fermion antiferromagnet CeRhIn 5 at pressures up to 2.3 GPa. These experiments show that the staggered moment of Ceand the incommensurate magnetic structure change weakly with applied pressure up to 1.63 GPa, where resistivity, specific heat and nuclear quadrupole resonance measurements confirm the presence of bulk superconductivity. This work places important constraints on an interpretation of the relationship between antiferromagnetism and unconventional superconductivity in CeRhIn 5 .

Magnetic structure of CeRhIn5 as a function of pressure and temperature

Magnetocaloric and magnetic properties of SmFe 0.5 Mn 0.5 O 3 complex perovskite

Structural and magnetic properties of LaCrO3 half-doped with Al

The structural, magnetic and electrical transport properties of La2MnFeO6 bulk samples prepared by combustion synthesis have been investigated. The X-ray powder diffraction (XRD) patterns with Rietveld refinement at room temperature shows that samples prepared by combustion synthesis are formed in single phase with an orthorhombic structure space group Pnma (62). The temperature dependent magnetization shows a formation of ferromagnetic cluster at 150K with short range interactions and a long range ferromagnetic order below 75 K. The magnetic field dependence of the magnetization shows a typical paramagnetic behavior at room temperature and a ferromagnetic-like behavior at low temperatures. The low value of the magnetization at high magnetic field can indicate the formation of a weak ferromagnetism or a metamagnetic phase. Results of the temperature dependence of the resistivity show that the transport process in this material can be attributed to small polarons.

Ferromagnetic Cluster on La2FeMnO6

We have performed electron spin resonance (ESR) and dc magnetization measurements on single crystals of La{sub 2(1-x)}Sr{sub 1+2x}Mn{sub 2}O{sub 7} up to 800 K with special emphasis on the x=0.4 composition. The ESR linewidth shows behavior similar to that observed in the three-dimensional perovskites and above -500 K can be described by a universal expression {Delta}H{sub pp}(T)=[C/T{sub x}(T)]{Delta}H{sub pp}({sup {infinity}}). The linewidth and the resonance field become anisotropic below -500 K. The anisotropy in the resonance field is proportional to the magnetization M, and we concluded that it is intrinsic to the system. We show that demagnetization effects can explain only part of the anisotropy. The remainder arises from short-range uniaxial terms in the Hamiltonian that are associated with the crystal field and Dzialozhinsky-Moriya interactions. The anisotropy in the linewidth is attributed to the easy-plane ferromagnetic ordering, which also arises from the short-range anisotropy.

N. O. Moreno

Papers

Magnetic structure of CeRhIn5 as a function of pressure and temperature

Magnetocaloric and magnetic properties of SmFe 0.5 Mn 0.5 O 3 complex perovskite

Structural and magnetic properties of LaCrO3 half-doped with Al

Ferromagnetic Cluster on La2FeMnO6

Electron spin resonance above T{sub}c in layered manganites.