John L. Sarrao

Plutonium-based superconductivity with a transition temperature above 18 K

Plutonium is a metal of both technological relevance and fundamental scientific interest. Nevertheless, the electronic structure of plutonium, which directly influences its metallurgical properties, is poorly understood. For example, plutoniums 5f electrons are poised on the border between localized and itinerant, and their theoretical treatment pushes the limits of current electronic structure calculations. Here we extend the range of complexity exhibited by plutonium with the discovery of superconductivity in PuCoGa5. We argue that the observed superconductivity results directly from plutoniums anomalous electronic properties and as such serves as a bridge between two classes of spin-fluctuation-mediated superconductors: the known heavy-fermion superconductors and the high-Tc copper oxides. We suggest that the mechanism of superconductivity is unconventional; seen in that context, the fact that the transition temperature, Tc ≈ 18.5 K, is an order of magnitude greater than the maximum seen in the U- and Ce-based heavy-fermion systems may be natural. The large critical current displayed by PuCoGa5, which comes from radiation-induced self damage that creates pinning centres, would be of technological importance for applied superconductivity if the hazardous material plutonium were not a constituent.

High-temperature weak ferromagnetism in a low-density free-electron gas

The magnetic properties of the ground state of a low-density free-electron gas in three dimensions have been the subject of theoretical speculation and controversy for seven decades. Not only is this a difficult theoretical problem to solve, it is also a problem which has not hitherto been directly addressed experimentally. Here we report measurements on electron-doped calcium hexaboride (CaB6) which, we argue, show that—at a density of 7× 1019 electrons cm−3—the ground state is ferromagnetically polarized with a saturation moment of 0.07 µB per electron. Surprisingly, the magnetic ordering temperature of this itinerant ferromagnet is 600 K, of the order of the Fermi temperature of the electron gas.

Hidden magnetism and quantum criticality in the heavy fermion superconductor CeRhIn5

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 Tc 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 < Tc, preventing proof of a magnetic quantum-critical point. Here we report specific-heat measurements of the pressure-tuned unconventional superconductor CeRhIn5 in which we find a line of quantum–phase transitions induced inside the superconducting state by an applied magnetic field. This quantum-critical line separates a phase of coexisting antiferromagnetism and superconductivity from a purely unconventional superconducting phase, and terminates at a quantum tetracritical point where the magnetic field completely suppresses superconductivity. The 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-Tc 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.

Superconductivity and quantum criticality in CeCoIn5

Electrical resistivity measurements on a single crystal of the heavy-fermion superconductor CeCoIn5 at pressures to 4.2 GPa reveal a strong crossover in transport properties near P(*) approximately 1.6 GPa, where T(c) is a maximum. The temperature-pressure phase diagram constructed from these data provides a natural connection to cuprate physics, including the possible existence of a pseudogap.

Incommensurate magnetic structure of CeRhIn 5

The magnetic structure of the heavy fermion antiferromagnet

Superconductivity and magnetism in a new class of heavy-fermion materials

{\mathrm{CeRhIn}}_{5}

Fermi surface of the heavy-fermion superconductor CeCoIn5: The de Haas–van Alphen effect in the normal state

is determined using neutron diffraction. We find a magnetic wave vector

Superconductivity in Cerium- and Plutonium-Based `115' Materials

{\mathbf{q}}_{M}=(1/2,1/2,0.297),

Photoemission and the electronic structure of PuCoGa5.

which is temperature independent up to

Nanomagnetic droplets and implications to orbital ordering in La1-xSrxCoO3

{T}_{N}=3.8 \mathrm{K}.