Donavan Hall

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.

Magnetic enhancement of superconductivity from electron spin domains.

Since the discovery of superconductivity, there has been a drive to understand the mechanisms by which it occurs. The BCS (Bardeen–Cooper–Schrieffer) model successfully treats the electrons in conventional superconductors as pairs coupled by phonons (vibrational modes of oscillation) moving through the material, but there is as yet no accepted model for high-transition-temperature, organic or ‘heavy fermion’ superconductivity. Experiments that reveal unusual properties of those superconductors could therefore point the way to a deeper understanding of the underlying physics. In particular, the response of a material to a magnetic field can be revealing, because this usually reduces or quenches superconductivity. Here we report measurements of the heat capacity and magnetization that show that, for particular orientations of an external magnetic field, superconductivity in the heavy-fermion material CeCoIn5 is enhanced through the magnetic moments (spins) of individual electrons. This enhancement occurs by fundamentally altering how the superconducting state forms, resulting in regions of superconductivity alternating with walls of spin-polarized unpaired electrons; this configuration lowers the free energy and allows superconductivity to remain stable. The large magnetic susceptibility of this material leads to an unusually strong coupling of the field to the electron spins, which dominates over the coupling to the electron orbits.

Coexistence of magnetism and superconductivity in CeRh 1 − x Ir x In 5

We report a thermodynamic and transport study of the phase diagram of

Electronic structure of CeRhIn 5 : de Haas–van Alphen and energy band calculations

{\mathrm{CeRh}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{In}}_{5}.

Anomalous superconductivity and field-induced magnetism in CeCoIn 5

Bulk superconductivity is observed over a broad range of doping,

Fermi-surface measurements on the low-carrier density ferromagnet Ca 1 − x La x B 6 and SrB 6

0.3lxl~1,

Extension of the temperature-magnetic field phase diagram of CeB 6

including a substantial range

Localized f electrons in Ce x La 1 − x RhIn 5 : de Haas–van Alphen measurements

(0.3lxl0.6)

Magnetization measurements on single crystals of superconducting Ba0.6K0.4BiO3

over which it coexists with magnetic order (which is observed for

Fermi surface properties of low-concentration Ce x La 1 − x B 6 : de Haas–van Alphen

0l~xl0.6).