Investigating Strongly Correlated Electron Materials by Means of Neutron Scattering
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 future energy, memory and spintronic applications. 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, I will report on our recent neutron diffraction experiments on heavy fermion compound URu2Si2 that represents a prime example of a material with such emergent behavior: its hidden-order (HO) phase occurs below T0 = 17.5 K and coexists with superconductivity below Tc = 1.5 K. Early neutron scattering experiments have demonstrated the presence of a small antiferromagnetic moment of only ∼0.03 parallel to the tetragonal c-axis in the HO phase that is, however, too small to account for the entropy of ∼0.2Rln(2) associated with the observed specific-heat anomaly. This led to the terminology HO to allude to the unknown identity of the corresponding order parameter (OP) that eluded identification for almost three decades. Two recent models (rank-5 super spin / hastatic order) aiming to explain the ‘HO’ state , have proposed an additional small magnetic moment of 0.015 parallel to the tetragonal basal plane.
We have carried out a careful neutron scattering experiment that shows no evidence for an in- plane moment. Based on the statistics of our measurement, we establish that the upper experimental limit for the size of any possible in-plane component is S < 0.001 . This new limit is therefore an important constraint for present  and future theories that aim to model the OP of the notorious HO phase of URu2Si2. We will also discuss results on neutron diffraction measurements on a series of single crystals of URu2-xFexSi2, where Fe substitution serves as chemical pressure  to tune URu2Si2 from the HO in the neighboring large moment antiferromagnetic phase.
Further, I will present our extensive inelastic neutron scattering study on high-quality single-crystals of the heavy fermion compound CeRhIn5. CeRhIn5 exhibits antiferromagnetic order with a propagation vector k = [0.5,0.5,0.297] below TN = 3.8 K that can be suppressed to zero temperature by application of pressure. This results in an antiferromagnetic quantum critical point at Pc = 2.3 GPa around which a broad superconducting dome emerges with a maximal Tc = 2.3 K . This has led to the common belief that the unconventional superconductivity in CeRhIn5  is mediated by quantum critical magnetic fluctuations. In contrast the strength of magnetic exchange interactions was unknown before our measurements, likely because of the strong neutron absorption of this compound. Combining high-quality materials synthesis with novel neutron focusing techniques allowed us—for the first time—to determine the full spin wave spectrum in CeRhIn5 and determine the magnetic exchange energy J = 0.37 in very good agreement with TN . Our results are crucial to improve our understanding of unconventional superconductivity in heavy fermion materials.