Spin and charge quantum phase transitions in narrow-band metals

F. M. Grosche

University of Cambridge, UK


    The threshold of magnetism in narrow-band metals is frequently associated with anomalous metallic states and unconventional superconductivity. Despite comprehensive progress in recent years, the border of ferromagnetic order remains comparatively little explored in heavy fermion materials. In the uniaxial local moment ferromagnet CeAgSb2, both hydrostatic pressure and transverse magnetic field can be used to suppress magnetic order. We discuss transport, magnetic and thermodynamic properties on approaching the quantum phase transition.

    An alternative scenario arises in the vicinity of a structural, or - more generally - charge density quantum phase transition. In a number of material families of current interest, structural and magnetic transitions appear in close proximity. Examples include certain Ce-based heavy fermion systems (Kondo volume collapse), as well as the lattice distortions near the onset of electronic order in Sr3Ru2O7 and in some iron pnictide systems. In the non-magnetic cubic superconductor Sr3Ir4Sn13, a low temperature superlattice distortion can be studied in isolation. Substitution of Sr by Ca and application of hydrostatic pressure suppress the superlattice transition and lead towards a structural quantum critical point. We present the resulting superconducting phase diagram and discuss the normal state transport properties at the critical pressure in terms of a low-lying, weakly dispersive phonon branch.

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