Pressure-driven magnetic and structural transitions in the 122-pnictides

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Date/Time:Thursday, 14 Nov 2013 - Saturday, 16 Nov 2013
Location:PHYSICS Hall Room 3
Channel:Condensed Matter Physics
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Michael Widom

Carnegie Mellon University

Pnictides of the family AFe2As2, where A is an alkali earth element,
exhibit several phase transitions in their structure and magnetic order
as functions of applied pressure. We employ density functional theory
total energy calculations at T=0K to model these transitions for the
entire set of alkali earths (A=Ca, Sr, Ba, Ra) which form the 122
family. Three distinct types of transition occur: an enthalpic
transition in which the striped antiferromagnetic orthorhombic (OR-AFM)
phase swaps thermodynamic stability with a competing tetragonal phase; a
magnetic transition in which the OR-AFM phase loses its magnetism and
orthorhombicity; a lattice parameter anomaly in which the tetragonal
c-axis collapses. We identify this last transition as a "Lifshitz
transition" caused by a change in Fermi surface topology. Depending on
the element A, the tetragonal state exhibiting the Lifshitz transition
might be metastable (A=Ca) or stable (A=Sr, Ba and Ra).