Professor Hoffman is interested in how electrons behave within exotic materials. Her research team at Harvard has designed and constructed three low-temperature scanning probe microscopes to visualize and manipulate this behavior directly. Innovative techniques include quasiparticle interference imaging to extract the band structure of materials at the nanoscale, and force microscopy to trigger nanoscale electronic phase transitions. Materials of particular interest include high temperature superconductors, topological insulators, and strongly correlated vanadates, all of which present deep physics questions as well as potential for novel applications.
Superconductivity – the lossless conduction of electrical current – arises from the pairing of electrons on the Fermi surface. The discovery of high Tc superconductors launched hopes for widespread application, but significant challenges remain. For example, motion of vortices – quanta of magnetic flux – causes noise and dissipation in superconducting devices. Vortices can be pinned by the controlled introduction of defects into the superconductor. The Hoffman lab uses magnetic force microscopy (MFM) to manipulate individual superconducting vortices and directly quantify their interaction and pinning forces in picoNewtons. Furthermore, alternative order parameters may weaken superconductivity by competing for the Fermi surface electrons. The Hoffman lab uses scanning tunneling microscopy (STM) to visualize and understand this competition and its effect on the Fermi surface.
Topological insulators are 3D insulators with 2D metallic surface states. The robust spin-polarization of these surface states, and their protection against backscattering, suggests their utility for dissipationless spintronics devices. Furthermore, predicted topological behavior in proximity to superconducting or magnetic materials has led to numerous proposals for fault-tolerant quantum computing, as well as magnetoelectric effects for low-power-consumption electronics. The Hoffman lab uses STM to understand and quantify these behaviors, as well as to identify new topological materials. A newly constructed molecular beam epitaxy (MBE) system, coupled to the existing STM, will allow atomic precision growth of novel heterostructures to identify emergent behavior at the interfaces between topological and broken-symmetry materials.
Faculty Assistant: Helen Abraha
17 Oxford Street
Cambridge, MA 02138