Kai Liu

Ph.D. - The John Hopkins University, 1998
Assistant Professor, Experimental Condensed Matter Physics
Professor Kai Liu joined the department in July 2001

The ability to manipulate matter at the atomic scale and to artificially tailor the material properties is at the heart of nanoscience and nanotechnology. Nanostructured materials provide the ideal arena for such explorations. They typically have structural or chemical restrictions on the nanometer scale along one or more of the dimensions, artificially designed and realized. Examples include 2-dimensional ultrathin films, 1-dimensional nanowires, 0-dimensional nanodots/nanoparticles, and more complex patterned structures that could have a combination of these characteristics. Due to their intricate nanostructures, extremely small length scales, low dimensionality, and interplay among constituents, nanostructured materials often exhibit new and enhanced properties over their bulk counterparts. These novel properties can also be tailored through extra degrees of freedom, such as structure and materials, etc. Our research focus on the study of magnetism and spin-dependent transport, particularly in nanostructured materials, to investigate the key issues of the interactions amongst spin, charge, and lattice of the material. We hope to understand how to preserve the coherence of electron spin over space and time, and try to devise mechanisms to encode and decode information using the electron spin, as well as its charge.

One area of research is magnetism in reduced dimensions, where surface and interfacial effects as well as single-domain behaviors are dominant. The magnetization reversal processes are complex and fascinating. The interplay between size confinement and proximity effect is particularly interesting. For example, in ultrafine nanomagnets, each particle is smaller than the typical magnetic domain size, so all its magnetic moments are aligned in one direction. A high-density array of such nanomagnets may be used to store information, as a magnetic storage media. How will the magnets behave individually and collectively? Can we use such arrays to push the present magnetic recording density up another decade? Will the magnetization be stable enough to overcome thermal fluctuations? How will the proximity to other materials, such as an antiferromagnet or a superconductor, influence the assembly?

Along the transport front, we are interested in novel materials and mechanisms that allow us to transfer information via the electron spins. Particularly desirable are materials with high spin polarization, or large imbalance between "spin-up" and "spin-down" electrons. Using such spin-polarized electrons, one can easily turn on/off the conduction by a magnetic field, just like manipulating the charge flow of electrons by an electric field. We are also interested in discriminating mechanisms for electron transport, such as spin-dependent scattering or tunneling effects. These materials and properties may be used to realize novel devices such as non-volatile magnetic random access memories (MRAM) that do not need constant power feed.

Typically we synthesize materials using magnetron sputtering, e-beam and thermal evaporation, electrodeposition, arc melting, and high temperature sintering. Additional processing is done by photo-, e-beam lithography and self-assembly nano-lithography. Further characterizations are done by X-ray diffraction, SEM, TEM, AFM, EDX, SQUID, VSM, AGM, MOKE and transport measurements. More information at: http://www.physics.ucdavis.edu/faculty/kliu.