Research - J. A. Sauls Theoretical Condensed Matter

Edge States and the Ground State of a Topological Superfluid

Sat, 29 Sep 2012 20:02:51 -0500

Broken symmetries in bulk condensed matter systems have implications for the spectrum of Fermionic excitations bound to surfaces and topological defects. The A-phase of superfluid 3He, described by the Anderson-Morel (AM) state, is the realization of a two-dimensional broken time-reversal topological superfluid. The 2D AM state belongs to the same topological class as the integer Quantum Hall Effect. The AM state is also representative of spin-triplet, chiral superconductors discussed as possible ground states for the superconductors Sr2RuO4 and UPt3.

Vortices in Unconventional Superconductors

Sat, 8 Oct 2011 13:26:13 -0500

Superconductors with a multi-component order parameter can possess topologically stable vortices and inhomogeneous phases that are not possible in single-component superconductors.This research project focusses on the electronic and magnetic structure of topological defects, quantized vortices, vortex lattices, composite mass/charge and spin vortices and domain walls that are possible ground states and metastable states in unconventional superconductors and superfluids, particularly the class of chiral spin-triplet superconductors and thin films of superfluid 3He-A. Among the novel vortex states are stable doubly quantized vortices and composite mass/charge and spin vortices with half a flux quantum. Observation of these structures would provide fingerprints of broken time-reversal symmetry in thin films of superfluid 3He-A, and possible realizations of odd-parity, spin-triplet pairing in exotic superconductors such as Sr2RuO4 and UPt3. The magnetic structure and relative stability of competing topological phases in these materials is an unsolved problem of current interest both theoretically and experimentally. This theoretical project also involves a collaboration with experimental groups at Northwestern and Notre Dame using NMR and small angle neutron scattering (SANS) to interpret and identify the superconducting phases in Sr2RuO4, UPt3, as well as novel vortex phases in other exotic superconductors.

Crystalline Phases of Superfluid 3He Films

Fri, 25 Mar 2011 12:13:15 -0500

The phases of superfluid 3He provide a beautiful example of spontaneously broken symmetry in condensed matter physics, exhibiting properties common to superconductors, nematic liquid crystals, and anti-ferromagnets. Many of the unique physical properties of superfluid 3He, including the spectrum of low-energy excitations, are connected to the breaking of orbital and spin rotation symmetries in combination with global gauge symmetry that is associated with superfluidity and superconductivity. However, translational symmetry has generally been assumed to be preserved even in reduced dimensions.
We predict that in thin films and cavities a new crystalline phase of superfluid 3He with ``stripe order’’ is the stable ground state over a wide range of film thickness (left figure). The mechanism responsible for the spontaneous breaking of translation symmetry in the plane of the film is competition between surface de-pairing and domain-wall formation (figures below) between degenerate ground states, and is generic to 3He confined in at least one spatial dimension. The stripe phase of superfluid 3He represents a new class of SuperSolid in which density wave order nucleates in the presence of a pre-existing superfluid phase.

Heat Transport by 3He through Aerogel

Tue, 8 Sep 2009 11:51:25 -0500

Transport of heat, sound and light through liquid Helium (3He) provides an ideal medium in which to study the structure of silica aerogel. We have obtained exact solutions to the Boltzmann equation for heat transport through liquid 3He that fills the open space of the aerogel structure.2 This result provides a method for quantitative analysis of transport experiments, and a means of testing theoretical models for the role of fractal correlations in the spatial arrangement of SiO2 clusters on the transport of mass, energy and magnetization in these remarkable solids.

Superconducting Spintronics

Mon, 20 Apr 2009 15:04:51 -0500

Conventional electronic devices are based on voltage control of electron charge currents in metals, semi-conductors and superconductors. By controlling the electron spin (and magnetic moment) in solid-state heterostructures, spintronics offers a new paradigm for making devices and circuits with wide ranging functionality. Supercondutors provide new and versatile elements for constructing spintronic devices. This is because of the proximity coupling between superconductors and ferromagnets, on scales ranging from nanometers to microns. Quantum mechanical coherence between superconducting leads provides a unique method for voltage control (via the a.c. Josephson effect) of spin polarized currents (see recent publications). The nonlinear I-V characteristic of the dc spin current due to MAR (left panel) is shown in the right panel for various models of the FM junction.

Condensed Matter Physics

Sun, 3 Feb 2008 20:44:19 -0600

Condensed matter physics is the field of study and inquiry into the fundamental properties of matter and radiation, and the physical phenomena that result from their interactions. It is also a field of physics which has led to many technological applications that have revolutionized modern society - from the transistor and silicon based electronics, from magnetic memory storage and liquid crystal displays in our laptops to ultra-sensitive superconducting magnetometers (called SQUIDS) for mapping brain activity to nuclear magnetic resonance imaging for medical screening and analysis, etc.
At a fundamental level condensed matter physics is a diverse field of research in large part because systems composed of very large numbers (N ≈ 1023) of atoms and molecules exhibit an exhibit a seemingly unlimited variety of macroscopic phases and correspondingly an enormous breadth of physical phenomena. It is the latter that is at the root of the many technological developments.
Theoretical research in condensed matter physics involves the discovery of new concepts related to the collective behavior of enormous numbers of atomic constituents, combined with the application of statistical mechanics and quantum theory to describe and predict the behavior of macroscopic matter. The concept of ``spontaneous symmetry breaking'' was developed as an organizing principle in condensed matter physics from the theory of phase transitions and emergent physical properties of the lower symmetry phase of matter. The ideas and mathematics underlying the connections between symmetry, symmetry breaking, phase transitions, collective behavior and emergent properties of matter are so powerful and general that the conceptual framework of `spontaneous symmetry breaking' is a cornerstone of nearly every sub-field of physics and physical sciences - from the forces governing the `families' of sub-atomic particles to the regular structures observed in crystals or the patterns that evolve in non-equilibrium fluid motion.