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The 5 K superconductor La [ABSTRACT FROM AUTHOR]

The discovery of iron pnictides and iron chalcogenides as a new class of unconventional superconductors in 2008 has generated an enormous amount of experimental and theoretical work that identifies these materials as correlated metals with multi-orbital physics, where magnetism, nematicity, and superconductivity are competing phases that appear as a function of pressure and doping. A microscopic understanding of the appearance of these phases is crucial in order to determine the nature of superconductivity in these systems. Here, we review our recent theoretical efforts to describe and understand from first principles the properties of iron pnictides and chalcogenides with special focus on (i) pressure dependence, (ii) effects of electronic correlation, and (iii) origin of magnetism and superconductivity. [ABSTRACT FROM AUTHOR]

Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high-temperature superconductivity. In transition metal materials, collapses are usually driven by so-called spin-state transitions, the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S = 5/2 mineral hauerite (MnS2) undergoes an unprecedented (ΔV ∼22 %) collapse driven by a conceptually different magnetic mechanism. Using synchrotron X-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic ground state using in situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S = 1/2 moments are quenched by dimerization. Our results show how the emergence of metal-metal bonding can stabilize giant spin-lattice coupling in Earth's minerals. [ABSTRACT FROM AUTHOR]

Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high-temperature superconductivity. In transition metal materials, collapses are usually driven by so-called spin-state transitions, the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S = 5/2 mineral hauerite (MnS2) undergoes an unprecedented (ΔV ∼22 %) collapse driven by a conceptually different magnetic mechanism. Using synchrotron X-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic ground state using in situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S = 1/2 moments are quenched by dimerization. Our results show how the emergence of metal-metal bonding can stabilize giant spin-lattice coupling in Earth's minerals. [ABSTRACT FROM AUTHOR]