Magnetoelastic Transitions in Iron-based Superconductors: Bulk and Thin Films
The surprising discovery of superconductivity in fluorine-doped arsenic-oxide LaO1-xFxFeAs  has established a new class of high-temperature superconductors . The normal state of these systems displays an intriguing interplay between structural and magnetic transitions that persists in the superconducting region of the phase diagram. In the seminar I will discuss how this interplay is largely dominated by symmetry-specific magnetoelastic couplings . I will show, in particular, that i) the existence of tricritical points in the generic phase diagram  and ii) the variations of transition temperatures induced by uniaxial pressure  are two direct consequences of these couplings. In addition, I will discuss specific results for thin films . Here, this magnetostructural interplay combined with epitaxial strain can radically modify the Kittel law for the size of the structural domains as a function of the film thickness. The new regime that appears in the ultrathin limit gives an access to the response functions of the nematic order parameter that complements bulk experiments .
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