April 10 Colloquium
Marc Janoscheck, Los Alamos National Laboratory
Investigating Strongly Correlated Electron Materials by Means of Neutron Scattering
Abstract: Materials governed by strong electronic correlations exhibit a broad set of interesting properties and ground states, such as heavy fermion behavior, complex magnetism, the Mott insulator, and unconventional superconductivity, and furthermore show potential for application spanning from thermoelectricity to spintroncis. Here it is generally believed that being able to manipulate and control these states that emerge through the cooperative behavior of in the order of 1023 electrons is critical to develop tomorrow’s devices and applications. Using two examples we will demonstrate that neutron scattering represents a powerful tool in shedding light on the properties of such complex materials.
First, we will report our work on the B20 helimagnet MnSi. The discovery of a new topological form of magnetic order in the family of cubic Dzyaloshinskii-Moriya helimagnets crystallizing in the B20 structure has recently attracted increasing scientific interest. This new magnetic state consists of a lattice of skyrmions, i.e., magnetic vortices characterized by a topological winding number , which is however only thermodynamically stable in a little pocket of the phase diagram at finite field close to the critical temperature Tc. In the skyrmion phase of MnSi spin transfer torques have been observed at record low current densities j = 106A m-2 making these materials promising for spintronic applications . Here it was argued in Ref.  that the skyrmion lattice phase only exists due to strong renormalizations attributed to thermal fluctuations, which in turn motivated our detailed small angle neutron scattering (SANS) study of the magnetic fluctuations in MnSi. Our study demonstrates that the properties of MnSi are indeed governed by strongly-interacting critical fluctuations, resulting in a fluctuation-induced first-order transition at Tc. Notably, we show that within a self-consistent Hartree-Fock-Brazovskii theory the critical magnetic fluctuations determined by our SANS measurements quantitatively reproduce the observed specific heat and magnetic susceptibility data . While our study contributes to our better understanding of the skyrmion-lattice compounds, we note that fluctuation-induced first-order transitions are of interest for a wide range of topics such liquid crystals, superconductors, cold atom systems or even phase transitions in the early universe (see references in Ref. ).
In the second part of the talk, we turn to our recent inelastic neutron scattering of δ-Pu. Pu is arguably the most complex elemental metal known because its 5f electrons are tenuously poised at the edge between localized and itinerant configurations . Despite its 60-year old history, our collective understanding of this complicated element is still vague. However, it is clear that Pu’s complex structure leads to emergent behavior—all a direct consequence of its 5f electrons—including six allotropic phases, large volumetric changes associated with these transitions of up to 25%, and mechanical properties ranging from brittle α-Pu to ductile δ-Pu . Pu also exhibits a Pauli-like magnetic susceptibility, electrical resistivity and a Sommerfeld coefficient of the specific heat that are an order of magnitude larger than in any other elemental metal . Finally, while experiments find no sign for static magnetism in Pu, most theories that use the correct volume predict a magnetically ordered state . This discrepancy might be reconciled by recent DMFT calculations that suggest that Pu fluctuates between localized and itinerant valence configurations . Our measurements demonstrate the existence of magnetic fluctuations centered around 84 meV, in agreement with these calculations putting the understanding of the “missing” magnetism and electronic structure of Pu within reach for the first time.
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Host: Dietrich Belitz