Phase Transitions in Quantum Systems

Both experimental and theoretical studies of phase transitions in quantum systems are carried out in our department. The work of Professors Washburn and Kveshchenko is particularly concerned with the breakdown of Landau’s Fermi liquid approximation that allows one to treat the entire effect of the carrier interactions with the lattice and other carriers as an effective mass of an essentially free quasiparticle. This breakdown occurs when the interactions between electrons are particularly strong.

In spite of long-standing theoretical predictions for non-interacting carriers in disordered materials, there is mounting evidence that the carrier interactions create dramatic "amomalies" in the carriers' behavior. For example the carriers in a disordered two-dimensional system (such as garden-variety MOSFETs) exhibit an apparent phase transition from insulator to metal as the density of carriers changes.

Experiments carried out in Professor Washburn’s group to study the phase transitions and other correlation effects are performed at very low temperature in a dilution refrigerator (0.01 < T < 4K) in magnetic fields ranging from very low up to 15 T. Since the energy scale 0.01K is so small, the excitation voltages used in the experiments are usually less than 1 microvolt requiring a heavily shielded circuit and phase-sensitive detectors to amplify the signals.

Other systems in which such breakdowns occur are also of both great fundamental interest and potential practical importance: high temperature superconducting and carbon-based compounds, colossal magnetoresistance and heavy fermion materials, low-density electron gas in semiconductor heterostructures, and so forth. Professor Kveshchenko’s theoretical research has been focused on developing an adequate description of and gaining new insights into the nature of the experimentally observed “anomalous” (apparently non-Fermi-liquid-like) spectral and transport properties of the systems of electrons such systems, as well as linking the properties of these materials to their structure and assessing feasibility of their practical applications. Current topics include: superconducting, magnetic, and carbon-based materials; quasiparticle transport in disordered d-wave superconductors; and the phase diagram and transport properties of compressible Quantum Hall states.

Professor Hernandez’s theoretical research is focused on the liquid-vapor phase transition in systems composed of atoms whose valence electrons are “weakly” bound, such as the alkali fluids, in which the dilute vapor is insulating while the dense liquid is metallic. In a recent series of studies, conducted in collaboration with a group in Madrid (Spain), Professor Hernandez has tried to understand the experimental data, and predict the results of measurements when these are unavailable, on the structural, thermodynamic and electronic properties of the alkali-atom fluids, up to 2000-3000 K and for densities below that of the liquid at the triple point. To date there are results allowing an understanding of all features of the phase diagram and, also, of the electrical conductivity. Clustering effects have been predicted.