Where Electrons Hide: A Puzzle in a Crystal ⚡ экспресс
Using a scanning tunneling microscope (STM), researchers pinpointed exactly where valence band electrons are concentrated within the unit cell of a tungsten diselenide (WSe2) crystal. It turned out that the electron density maximum at the K-point of the Brillouin zone lies between the atoms, while at the Gamma point it sits right on the atoms. This means the Wannier center (the average position of electrons in the crystal) is shifted relative to the atoms, making the material a topologically obstructed insulator. Much like a Möbius strip, its electronic structure cannot be smoothly morphed into a trivial one — a topological constraint forbids it.
Usually electrons in crystals, like honey in a honeycomb, cluster around atoms. But in tungsten diselenide — a layered material, resembling a honeycomb of carbon — the picture is different. Physicists used spectroscopy — a method that, like a supersensitive probe, feels the electron clouds. To mark the positions of atoms, they added a few 'foreign' atoms to the lattice, and then measured where electrons feel more 'comfortable' at different energies.
It turned out: at low energies, electrons crowd not on atoms, but in the voids between them — as if the honey had leaked out of the cells. At slightly higher energy, they, on the contrary, return to the atoms. And the transition occurs abruptly, like a switch click, rather than a smooth flow.
This is not just a fun phenomenon. The twisted states of electrons are resistant to interference, which opens the way to reliable quantum computers.
🎯 The most amazing part: electrons move from atoms to voids not gradually, but in a leap, like a switch between two realities.