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Molecular solids – model systems with variable many-body interactions

M. Lang - J.W. Goethe-University, Frankfurt (Main), SFB-TR49, Germany


Solids, made up of molecular units containing open shell transition metal ions or stable organic -radicals, provide flexible systems for exploring correlated electrons in reduced dimensions. In the course of an interdisciplinary research program at Frankfurt University, a variety of molecular materials has been synthesized and investigated in detail. One focus of these investigations lies on low-dimensional quantum magnets with moderate exchange coupling constants J, weak enough for laboratory magnets to tune the systems across field-induced quantum critical points.
Likewise, molecular metals, formed by the combination of stable -radicals with suitable inorganic linkers, have emerged as exemplary systems for studying the interplay of strong electron-electron and electron-phonon interactions in low dimensions. Of particular interest are the quasi-2D charge-transfer salts of the -(BEDT-TTF)2X family where transitions between various states such as Mott-insulating, antiferro¬magnetically ordered, anomalous metallic and superconducting can be induced by small variations of chemical (substitution) or physical (pressure, temperature or magnetic field) parameters.

 

In the talk we will discuss examples from both classes of molecule-based materials. Particular emphasis is placed on an oxalate-based Cu(II) (S = ½)-chain system. Here we study the anomalous magnetocaloric response close to the field-induced quantum critical point which may provide a most efficient cooling mechanism for reaching sub-Kelvin temperatures. In addition, we discuss the lattice effects observed at the Mott transition in an organic conductor indicative for the intricate role of the lattice degrees of freedom.

 

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