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General Research Area:

Materials by Chemical Design

Research Summary:

Strongly correlated electron systems, heavy fermions, local moment and itinerant electron magnetism, superconductivity, charge density wave transitions.

QUANTUM CRITICALITY IN HEAVY FERMION COMPOUNDS: Phase transitions are driven by the tendency of systems to minimize their energy with respect to extrinsic parameters (pressure, magnetic field etc.). Very often phase transitions occur at finite temperatures, where the thermal fluctuations become smaller than a characteristic energy, in which case the energy is lowered via a symmetry change (broken symmetry) and the nature of the material changes, for example from liquid to solid as water freezes. We are interested in phase transitions that take place at T = 0, known as quantum critical transitions, given that quantum and not thermal fluctuations are now at play. These often occur in a special class of metals known as heavy fermions. We are focusing on the Yb-based heavy fermion compounds, and their properties in the vicinity of the quantum critical point (QCP). Spin fluctuations modify the electron states near a QCP, leading to the breakdown of the Fermi Liquid (FL) theory. We aim at understanding how different control parameters (chemical doping, magnetic field, pressure) affect the non-Fermi liquid (NFL) state.

SUPERCONDUCTIVITY: Despite the success of BCS theory in explaining conventional superconductivity, many questions remain both about conventional and high temperature superconductors. We are particularly interested in answering questions regarding the competition between density waves and superconductivity, both collective electronic states caused by Fermi surface instabilities. In addition, the recent discovery of Fe-based layered superconductors has revitalized the pursuit of high temperature superconductivity. The proximity to a magnetic instability due to the presence of the Fe ions appears key for the occurrence of superconductivity at surprisingly high temperatures, albeit smaller than in the cuprates. We are interested in the design and discovery of new superconducting materials related to these Fe-based compounds, and with improved properties for practical applications. Understanding superconductivity in the vicinity of magnetism and particularly the correlations between crystal structure and superconducting properties is also at the core of our focus on superconducting materials.

ITINERANT ELECTRON MAGNETISM: Magnetic order can occur in at least two classes of compounds: One consists of compounds based on rare earth ions, which have unfilled f-shell electrons acting as localized magnetic moments. Many compounds based on transition metals owe their magnetic properties to the more delocalized d-electrons, and are therefore known as itinerant magnets. Although Stoner theory provides the grounds for understanding the complex magnetism associated with itinerant moments, a unified theory for d-electron magnetism is lacking, particularly due to the limited number of known such compounds. It is our goal to design, discover and synthesize novel itinerant ferromagnetic compounds, and to answer questions regarding the role of spin fluctuations and the quantum criticality in these systems.

REDUCED DIMENSIONAL SYSTEMS AND DENSITY WAVES: Charge density waves (CDWs) are periodic modulations of the electron density in solids. They are collective states that arise due to intrinsic instabilities often present in low dimensional electronic systems. We are interested in studying the competition between CDW transitions and other cooperative electronic phenomena (for example superconductivity) as a function of pressure or chemical doping. These parameters modify the Fermi surface or the electron-phonon coupling, and thus tip the balance in favor of one or the other state as the choice of ground state.