MFN Scholar: Brian Light

Report on Malcolm F. Nicol Graduate Scholar Research Activities

Mixed valence 4-f electron systems have been of great interest recently.  Particular focus has been on rare-earth intermetallic compounds containing europium.  They exhibit a range of properties as well as intermediate valences associated with the transition from the Eu2+ configuration to the Eu3+ configuration.  A Eu ion can be either divalent (Eu2+: 4f7S = 7/2, L = 0, J = 7/2) or trivalent (Eu3+:4f6S = L = 3, J = 0). Divalent europium has a larger volume and magnetic moment (7 μB/Eu), while trivalent europium has a smaller volume and no magnetic moment1.

In some Europium compounds, the valence state becomes unstable and can change from an almost divalent state at high temperatures to a trivalent one at low temperatures with the probability decreasing with temperature.  This valence transition is similar to the Kondo effect, where the magnetic moment from localized f electrons is quenched.   For rare-earth and actinide compounds, the competition between the Kondo effect and Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction induces phenomena such as heavy fermion, unconventional superconductivity, and non-Fermi liquid behavior.  Many Ce- and Yb-based heavy fermion compounds have been extensively examined.  My current work focuses on examining potential Eu-based heavy fermion compounds.

Much of the time spent while a Malcolm F. Nicol Graduate Scholar was exploring various high-pressure techniques and researching possible projects that lead me to the study of Europium intermetallic systems.   High-pressure X-ray diffraction experiments of EuMn2Si2 and EuCo2Si2 have been performed at Argonne National Laboratory at the Advanced Photon Source at sector 16 of the High Pressure Collaborative Access Team (HPCAT).   A Princeton symmetric type diamond anvil cell (DAC) loaded with ruby for pressure measurement and neon as a pressure-transmitting medium were used for these experiments. Heat capacity and magnetization measurements were performed in a Quantum Design PPMS system at UNLV.  Future temperature dependent measurements of Europium valence under pressure are planned to be performed using Resonant Inelastic X-ray Scattering at the APS.