The quasiparticle dynamics about the Fermi surface in the normal state of a superconductor is arguably the most important characterization in terms of understanding the mechanism behind the superconducting pairing. One of the key research directions in the Analytis lab is to perform detailed, three-dimensional studies of the Fermi surface using magneto-transport, quantum oscillations and thermal transport techniques. Materials we study include Fe-based superconductors, thin-film cuprates, ruthenates and topological superconductors.
Quantum spin liquids have been an active area of research since their first proposal by P. W. Anderson in 1973. While many quantum spin liquids have been theorized, only a few materials have been found to be candidate spin liquids. We are working toward finding more material systems in which this phase of matter would be possible. In a quantum spin liquids, the frustrated interactions between spins causes them quantum mechanically fluctuate between degenerate ground states. While interactions would normally cause the spins to order, these fluctuations allow the spins to remain “liquid”, even close to absolute zero. Quantum spin liquids are an exciting phase of matter because nearly all theoretical models predict exotic excitations — particles with fractional quantum numbers or with statistics that are neither fermionic nor bosonic. A particular type of excitation possible in these systems may even be used for topological quantum computing.
Dimensionality plays an important role in condensed matter systems, particularly with regard to superconductivity. When systems are one dimensional (metallic in one direction but insulating in all others) bizarre things can occur, including the separation of spin and charge – the so-called Luttinger liquid. However, a material will have a given dimensionality in equilibrium and it is difficult to understand how these effects can be distinguished. One of our projects is to develop an experimental apparatus where the electronic overlap in different crystalline directions can by continuously tuned. Through this, we hope to observe how the properties of a quasi-two dimensional liquid evolve toward a one dimensional liquid.
The theory of phase transitions driven by thermal fluctuations is one of the centerpieces of condensed matter physics. Even at zero temperature however, quantum fluctuation can destroy order and drive a phase transition that is tuned by some nonthermal parameter (doping, magnetic field, pressure, strain …). Experiments cannot access absolute zero temperature, but the presence of the quantum phase transition creates a region of parameter space where quantum and thermal fluctuations compete. An appropriate theoretical description has not yet been established for this region, but various experiments have found strong deviations from the standard (and highly successful) theory of metals, Landau’s theory of the Fermi liquid.