Michael Crommie — Watching Electrons, One Atom at a Time

University of California, Berkeley · Department of Physics · Kavli Energy NanoScience Institute · Lawrence Berkeley National Laboratory (Materials Sciences Division)  ·  Field: condensed matter physics (scanning tunneling microscopy of low-dimensional materials)

A condensed summary of the research of Michael F. Crommie, the physicist who turned the scanning tunneling microscope into a camera for quantum mechanics — imaging the ripples of electron waves, building “artificial atoms” out of individual iron atoms, and, more recently, photographing an electron ice that had been predicted and unseen for ninety years.

The idea in one paragraph

Most of what we know about materials comes from averaged measurements — resistance, magnetization, light absorption over a whole sample. Crommie’s career is built on the opposite: putting an atomically sharp metal tip a fraction of a nanometre above a surface and reading the electrons out one location at a time. A scanning tunneling microscope (STM) does two things at once — it sees the local density of electronic states, and it can push single atoms around. Crommie uses both halves together: build a structure atom by atom, then watch what the electrons do inside it. Over three decades the samples changed — bare copper, then graphene, then stacked two-dimensional crystals — but the method is one long through-line: make quantum mechanics something you can look at.

1993: standing waves and the quantum corral

As a postdoc in Don Eigler’s group at IBM Almaden (after a Berkeley PhD with Alex Zettl), Crommie cooled a Cu(111) surface to a few kelvin and imaged faint circular ripples spreading from step edges and defects. They were standing waves in a two-dimensional electron gas — the surface-state electrons behaving exactly like water waves scattering off rocks, their wavelength giving a direct read-out of the electronic dispersion (Nature 363, 524, 1993).

Then came the image that ended up on textbook covers: Crommie, Lutz and Eigler dragged 48 iron atoms into a closed ellipse — a “quantum corral” — and watched the trapped electrons settle into the discrete standing-wave patterns of a particle in a box (Science 262, 218, 1993). It was a picture of a boundary-value problem. The work shared the AAAS Newcomb Cleveland Prize.

Kondo, and the mirage

Inside those corrals the group found something stranger. Place a single magnetic cobalt atom on the metal and the STM sees a sharp dip at the Fermi level — the Kondo resonance, the spectroscopic fingerprint of a lone spin being screened by a sea of conduction electrons (Science 280, 567, 1998). Put that atom at one focus of an elliptical corral and a phantom copy of the Kondo signal appears at the empty other focus: the “quantum mirage,” electronic information projected across empty space by wave interference. The same STM-as-scalpel approach let the group map how magnetic impurities locally poison superconductivity (Science 275, 1767, 1997).

The pivot to graphene (≈2007 onward)

When isolated graphene arrived, Crommie moved the technique onto a material where the electrons themselves are exotic — massless, relativistic Dirac fermions. Highlights from the Berkeley/LBNL years:

  • Charge puddles. STM maps showed graphene’s Dirac point isn’t flat: it breaks into electron and hole “puddles” from stray substrate charges — the thing that limits device quality (Nature Physics 5, 722, 2009).
  • Strain = magnetic field. Nanobubbles of graphene strain the lattice so severely that the electrons respond as if in a >300-tesla magnetic field — pseudo-Landau levels with no real magnet (Science 329, 544, 2010).
  • Artificial nuclei / atomic collapse. By assembling supercritically charged calcium-ion clusters on gated graphene, the group observed “atomic collapse” resonances — a relativistic instability predicted for hypothetical nuclei with Z ≳ 170, realised here at a millionth of that charge (Science 340, 734, 2013).
  • A camera for chemistry. Non-contact AFM with a CO-tipped probe resolved the actual bonds in single molecules before and after an on-surface reaction — reactant and product drawn like a textbook scheme (Science 340, 1434, 2013).

Designer matter: graphene nanoribbons

A long-running collaboration with Berkeley chemist Felix Fischer and theorist Steven Louie builds graphene nanoribbons from the bottom up — polymerising precisely designed molecules on a gold surface so the ribbon has an exact width and edge, then reading its electronic structure with the STM. By choosing the monomer they engineer the band gap (Nature Nanotechnology 10, 156, 2015), stitch ribbons of different widths into atomically sharp heterojunctions, and — following a Louie prediction — make ribbons whose topology forces a localised electronic state at every junction, so a ribbon superlattice becomes a tunable one-dimensional chain of interacting electrons: metal, semiconductor, or a row of spin qubits depending on spacing (Nature 560, 204, 2018).

What the lab does now

Crommie’s Berkeley group (with strong ties to LBNL’s Materials Sciences Division and the Kavli ENSI) runs cryogenic, gate-capable STM/STS — and increasingly CO-tip non-contact AFM — on:

  • Moiré and stacked 2D crystals: twisted graphene and semiconducting transition-metal-dichalcogenide bilayers hosting correlated insulators, the quantum anomalous Hall effect, and generalized Wigner crystals — now imaged directly and watched as they melt under an electric field.
  • On-surface synthesised nanocarbons: triangulene ribbons, nitrogen-doped and core-doped ribbons, open-shell nanographenes with magnetic edge states, and the contact engineering needed to turn them into transistors.
  • Van der Waals phonon and exciton engineering: using heterostructure interfaces to control terahertz phonons and electron–hole fluids.

The consistent bet: if you can build a quantum system atom by atom and put a probe right on top of it, you can see the physics before you have to explain it.

Media & further reading

Key references

  • Crommie, Lutz & Eigler, “Imaging standing waves in a two-dimensional electron gas,” Nature 363, 524–527 (1993). doi:10.1038/363524a0
  • Crommie, Lutz & Eigler, “Confinement of electrons to quantum corrals on a metal surface,” Science 262, 218–220 (1993). doi:10.1126/science.262.5131.218
  • Madhavan et al. (Crommie), “Tunneling into a single magnetic atom: spectroscopic evidence of the Kondo resonance,” Science 280, 567–569 (1998). doi:10.1126/science.280.5363.567
  • Levy et al. (Crommie), “Strain-induced pseudo–magnetic fields greater than 300 tesla in graphene nanobubbles,” Science 329, 544–547 (2010). doi:10.1126/science.1191700
  • Wang et al. (Crommie), “Observing atomic collapse resonances in artificial nuclei on graphene,” Science 340, 734–737 (2013). doi:10.1126/science.1234320
  • Rizzo et al. (Crommie/Fischer/Louie), “Topological band engineering of graphene nanoribbons,” Nature 560, 204–208 (2018). doi:10.1038/s41586-018-0376-8
  • Li et al. (Feng Wang/Crommie), “Imaging two-dimensional generalized Wigner crystals,” Nature 597, 650–654 (2021). doi:10.1038/s41586-021-03874-9
  • Google Scholar profile 157wzqoAAAAJ (citation total)