University of Minnesota
School of Physics & Astronomy


Ionic Liquids - Quantum Phase Transitions

Allen Goldman
Allen Goldman
Alex Schumann

Allen Goldman is a condensed matter experimentalist working on the properties of materials at low temperatures. His research involves the study of quantum phase transitions. These are transitions that are found at absolute zero with an external parameter of the system such as magnetic field, disorder, chemical composition or charge density, controlling the transition.

The parameter, in effect controls the quantum mechanical ground state of the system. In conventional phase transitions, such as the liquid-gas transition or the transition to ferromagnetism or superconductivity temperature is the control parameter.

A standard way to bring about a quantum phase transition is to study a series of compounds with slightly different chemical. For example, by increasing the strontium concentration in Lanthanum Strontium Copper Oxide, samples with different Strontium concentrations evolve from insulating to metallic with high temperature superconductivity occurring in the latter. The problem with this approach is that changing the chemical composition also changes the degree of disorder. It also requires multiple samples, at least one for each composition. A way around this is to incorporate the compound of interest in a capacitor configuration and to induce charges electrostatically. Goldman and his students have used such an approach to induce superconductivity in amorphous insulating films.

They have recently been employing what he describes as a “wild technique” that involves the use of ionic liquids. While ordinary liquids such as water are predominantly made of electrically neutral molecules, ionic liquids are largely made of ions. Goldman and his students have configured capacitor structures that use ionic liquids as the dielectric. They are using these structures to study quantum phase transitions in high temperature superconductors, in ultrathin films of metals, and in insulators such as strontium titanate. The ions are mobile at temperatures down to about 220 degrees above absolute zero. At temperatures below that they and the liquid are frozen. Charges are first induced in the object of study at temperatures at which the ions are mobile, and then the structure is cooled to low temperatures to carry out the study. The great advantage of using an ionic liquid as a dielectric is that charge transfers that are up to two to three orders of magnitude greater than those attainable using more conventional dielectrics can be achieved. With ionic liquids physicists can monitor a single variable and tune the properties of a material continuously. Ionic liquids, when applied to high temperature superconductors, also offer the promise of exploring the entire range of properties from insulator to superconductor in the same sample. Goldman says ionic liquids have opened up new possibilities for condensed matter physics.

Ionic liquids have many applications, such as powerful solvents and electrically conducting fluids. They are are important for electrical battery applications.(link to article about Woods alternative energy). Ionic liquids are dielectrics, that is they are an insulator that can be polarized by an electrical field. A theory of the response of Ionic liquids to electric fields was developed in 1853 by Helmholtz. They have more recently been the subject of intense interest.

Although ionic liquids have already been used in promising technical applications, Goldman says that fundamental physics applications of the materials are only just beginning to be exploited. “The remarkably high charge transfers allow you to study physics you wouldn’t be able to study. We are interested in using it as a tool.” Goldman says that his group and collaborators in Spain and Israel are working on using ionic liquids to study quantum phase transitions in a variety of superconductors and magnetic materials.