Research highlights

Frustated magnets, the interaction of disorder and frustration at low temperatures.  

Disorder meets quantum fluctuations in a complex lattice

Frustrated magnets are systems displaying competing magnetic interactions. The interactions between electronic spins in a solid will usually create magnetic order below a characteristic transition temperature. Disorder, frustration and dimensionality may conspire to decrease this temperature, paving the way for another ingredient to come into play: quantum fluctuations. In a magnetic solid, electronic spins fluctuate in a rate that is determined by thermal energy. However, since spins interact, magnetic order emerges below the transition temperature. Disorder and frustration may cause magnetic solids to display, instead, a spin glass state, where the spins are frozen in a random configuration. In a low dimensional spin system, like spins distributed in layers, this glassy state may not be fully developed down to very low temperatures. Then, quantum fluctuations will compete with this frozen state, possibly giving rise to some exotic ground state. Our work explores the magnetic properties of an intrinsically disordered, frustrated and low dimensional magnet, in which the spins remain in a dynamic magnetic state down to very low temperatures. Our sample displays experimental signatures seen both in quantum spin liquid candidates and spin glasses, offering a very exciting playground to study the competition between these novel states of frustrated magnets.   

Local properties of itinerant magnets

Chemistry shows up at a traditional physical system

In solids, local moment magnetism originates from spins that are pinned down to specific sites in the system, like in insulators, while itinerant magnetism originates from spins that walk free throughout the solid, like the spins  of conduction electrons in a metal.  Local moment magnetism is strongly dependent on the chemical (or local) properties of the material and our work suggests that in certain instances this is also the case for itinerant magnets. In solids, electronic spins are the main source of magnetism.  The complexity of the magnetic properties emerges because of the interaction between nearby spins. In metals, conduction electrons are spread in the system and their density dictates the magnetic properties of the solid. However, some metallic magnets display a structural building block called a coordination structure, like insulators. In our work, we investigated the local structural and electronic properties of a series of itinerant magnets and we discovered that the probed local properties are relevant to understanding their itinerant magnetic properties. 

Magnetism and superconductivity

Spin excitations in the Iron based superconductors: necessary, but not sufficient. 

In superconducting materials, electrons are found in pairs that carry electric current without dissipation. For pairing up electrons, you need a glue and our work advance the understanding that in the iron based superconductors (IBS) this glue is provided by the iron spins. A number of distinct mechanisms may lead to the formation of electron pairs. In a solid, the flipping of electronic spins give rise to spin excitations possessing a wave-like character, that can transfer energy and momentum to electrons. This is believed to be the mechanism behind superconductivity in the IBS. To probe the spin excitations, one can apply a technique called resonant inelastic x-ray scattering (RIXS). Previous RIXS experiments of IBS found spin excitations that are weakly dependent on the fact that the material is a superconductor or not. Puzzled by these results, we investigated by RIXS the spin excitations of some close relatives of the IBS which, however, never become superconductors. We found that the spin excitations are strongly scattered along certain directions, suggesting that spin excitations are a necessary ingredient to superconductivity in the IBS, albeit not a sufficient one.