John

Research Expertise and Interest

condensed matter physics and materials science

Research Description

My research group is concerned with the properties of novel magnetic, semiconducting, and superconducting materials especially in thin film form. We use specific heat, magnetic susceptibility, electrical resistivity, and other measurements as a function of temperature in order to test and develop models for materials which challenge our understanding of metallic behavior.



Current research includes: effects of spin on transport and tunneling, including studies of amorphous magnetic semiconductors and spin injection from ferromagnets into Si; finite size effects on magnetic and thermodynamic properties; formation of perpendicular magnetic anisotropy in magnetic thin films and effects of the vapor-deposition growth process on the structure of thin film amorphous and crystalline materials.



We also have an extensive effort in development of calorimetry for thin films, small bulk samples, and nano-scale biological systems. We use Si-microfabrication techniques to create membrane-based micro/nanocalorimeters that allow us to measure films weighing micrograms or less from 1-500K and in magnetic fields at present to 8T. We are working with researchers at the national high magnetic field lab to extend the magnetic field range to 45T in steady state and 100T in pulsed fields, and the temperature range to 0.3K.

Featured in the Media

Please note: The views and opinions expressed in these articles are those of the authors and do not necessarily reflect the official policy or positions of UC Berkeley.
April 6, 2020

The Search for the Perfect Mirror

The Atlantic
Natalie Wolchover and Quanta
Physics professor Frances Hellman is profiled here for her work with a 1,000-person team to find something close to an "ideal glass" for the Laser Interferometer Gravitational-Wave Observatory, or LIGO. The observatory uses pure glass mirrors to detect gravitational waves that flutter past Earth, altering space-time. "LIGO at this point is literally limited by" flaws in the glass they're using, Professor Hellman says. Despite the detectors' "astonishing vibration isolation, damping, all kinds of stuff that has led to the extraordinary sensitivity," she says, "the one thing they haven't been able to get rid of are these funny little atomic motions in the mirror coatings."
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July 19, 2019

This New Liquid Is Magnetic, and Mesmerizing

New York Times
Knvul Sheikh
A team of scientists, including researchers at Berkeley and the Lawrence Berkeley National Laboratory, has co-created a new magnetic material that is liquid and can change shape. It was accomplished using a special 3D printer to inject iron oxide nanoparticles into millimeter-scale droplets of toluene. The researchers believe the liquid magnets would be helpful in delivering drugs to specific locations in the body, and to create "soft robots." But that's just a start. As lead author Thomas Russell, a polymer scientist at the University of Massachusetts, Amherst, says: "We hope these findings will enable people to step back and think of new applications for liquid magnets. ... Because until now, people in material sciences haven't thought this was possible at all." Berkeley co-authors include physics professor Frances Hellman; Alejandro Ceballos, a doctoral alum from the Hellman lab, now a postdoc at the Lawrence Livermore National Laboratory; and graduate materials science and engineering student Yufeng Jiang.
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