Electronic and Quantum Magnetism Unit (Yejun Feng)

Electrons in solids are typically not alone, but interact with each other. Just like people in a society, they interact in pairwise fashion and also collectively in an ensemble. Their behavior necessarily reflects the multi-facet complexity in a solid, such as the dichotomy between charge and spin degrees of freedom, lattice with regards to both phonons and electronic band structure, interaction between spin and orbital moments, in addition to competition and frustration among all. Furthermore, the collective behavior of electrons is governed by other global and local conditions such as dimensionality, site symmetry and anisotropy, and spatial inhomogeneity and defects.  The kaleidoscopic expression of electronic behavior spans over phenomena such as magnetism, charge ordering, and superconductivity, and leads to applications of many device and energy materials.  The topic of electron correlation has been a cornerstone of condensed matter physics and materials science for decades. 

Correlated electron states can often be tuned into different ground states by one or more athermal controlling parameters, through a process known as a quantum phase transition, which potentially involves critical fluctuations of a quantum nature. This permits new paradigms in condensed matter physics as well as new avenues to explore in the search for functional materials. However, experimental efforts to identify the driving mechanism of quantum phase transitions and fluctuation effects are challenging due to the large number of degrees of freedom in bulk systems. Many issues remain unresolved in our understanding of quantum criticality, such as universality classes and their associated critical exponents, non-Fermi liquid behavior in the quantum critical region, and the Fermi surface evolution across the athermal phase boundary. 

The Electronic and Quantum Magnetism Unit has its research program focusing on comprehensive experimental studies of correlated electrons, quantum magnets, and superconductors under pressure/magnetic field tuning. Our approach of employing direct probes of order parameters in the multi-dimensional Pressure-Magnetic Field- Temperature phase space seeks to illustrate model systems of various Hamiltonians, and better clarify above-mentioned issues with microscopic insight of evolving states and their competition. With precisely- controlled and well-characterized sample environments, in situ tunability including pressures up to 30 GPa, magnetic fields up to 14 T, and cryogenic temperatures down to 10 mK, we have the capability to actively control and discern some of the most complicated fundamental physics in correlated electrons.