Charles E. Campbell, Associate Fellow

Simulations of Macromagnetic Structure and Simulations of Quantum Fluid Systems at Low Temperature

These researchers executed a program of simulations of condensed matter systems, particularly in the two very different areas of micromagnetic materials and of quantum fluids. The principal effort in the former was on the determination of magnetic microstructure of magnetic materials on the scale of 50 nanometers to 100 microns, which were also studied experimentally in the Physics Department’s Magnetic Microscopy Center (MMC). Of most interest were the mechanisms for spontaneous magnetic reversal, the structure of domain walls, and the effect of temperature, anisotropy, shape, and disorder on the writing to and stability of systems such as the microscopically thin permalloy films that were simulated by the group. At a similar level of interest was the appearance of unique structures in nanoscale nickel particles. Simulations also produced a new understanding of possible magnetic structures in such particles in the size range 50 nm to one micron, which should be very important technologically; these particles were grown and studied in the MMC.

One unanticipated outcome of this group’s results is that it appears that the Zeeman energy introduced by a pulsed magnetic field may decay into non-linear spin-waves, an effect suggested recently as one of the principal sources of energetic damping within grains. The smalldiameter systems that they have simulated thus far are ideal test beds for this possibility, which is one of the outstanding problems in present micromagnetics. The researchers are running spatially resolved continuous wave (CW) simulations to learn the source of the modes that are spun off from the main ferromagnetic response due to the presence of minority domains. The simulations seem to show spin waves carrying away some of the energy. The group’s next task is to characterize these modes quantitatively.

Concerning the other area of this computationally intensive research, the researchers’ quantum fluids simulation projects include potentially ground-breaking research in two areas: the impact of 3He (i.e., fermion isotopic impurities) on the (boson) superfluid phase transition in liquid 4He and the development of time-resolved quantum Monte Carlo simulations at finite temperatures. With regard to the former, the very low miscibility of 3He in liquid 4He requires very large simulation sizes, while the infamous sign problem in quantum Monte Carlo simulations of fermion systems requires the development of highly accurate importance sampling functions to successfully complete these simulations.

A related area of study for these researchers was identical particle scattering from a weakly coupled Bose condensed gas. Calculations were made of the scattering states and cross sections for a Bose condensed dilute gas trapped in a spherical square well of finite depth. The interactions were then treated in the scattering length approximation. The Gross-Pitaevskii and Bogoliubov equations for bound and scattering states were solved. The results indicated that there are transparency effects reminiscent of those conjectured to occur for strongly coupled systems. When incident particle wavelengths λ are comparable to the well size α, exchange-induced transparency enhancement is only dramatic for particular combinations of well depth, inter-action strength and particle number. For particles with large momenta (α/λ >> 1), however, exchange with the condensate results in enhanced transmission for all coupling strengths. Calculating the rate of decay of the scattering states to leading order in anharmonic corrections to the Bogoliubov approximation, the researchers found the corresponding inelastic cross sections to be extremely small.

Research Group

Jesse Berezovsky, Undergraduate Student Researcher
Paul Crowell, Faculty Collaborator
E. Dan Dahlberg, Faculty Collaborator
Eckhard Krotscheck, Institut für Theoretische Physik, Universität Linz, Linz, Austria
Andrew B. Kunz, National Institute of Standards and Technology
Ming Yan, Grauduate Student Researcher
Robert Zillich, Department of Chemistry, University of California, Berkeley