A key prediction of physicists Sin-Itiro Tomonaga and J.M Luttinger, known as TLL theory, is that exciting one electron in a 1D wire leads to a collective, organised response from every electron in the wire.
Due to this behaviour, TLL theory predicts that a moving electron in 1D will semmingly split in two and travel at different speeds, despite the fact that electrons are fundamental particles that have no constituent parts. This breakup, known as spin-charge separation, instead involves two inherent properties of the electron - negative charge and angular momentum, or ‘spin’.
In a study, ProfessorRandy Hulet of Rice University and physicist, Thierry Giamarchi of the University of Geneva, used another type of fermion - ultracold lithium atoms cooled to within 100 billionths of a degree of absolute zero. This was to verify the predicted speed that charge waves move in 1D and offer confirmation that 1D charge waves increase their speed in proportion to the strength of the interaction between them.
“In a 1D wire, electrons can move to the left or to the right, but they cannot go around other electrons," said Prof Hulet. "If you add energy to the system, they move, but because they're fermions and can't share space, that movement, or excitation, causes a kind of chain reaction.
"One electron moves, and it nudges the next one to move and the next, and so on, causing the energy you've added to move down the wire like a wave.”
In their experiments, Hulet's team used lithium atoms as stand-ins for electrons. The atoms are trapped and slowed with lasers that oppose their motion. The slower they go, the colder the lithium atoms become, and at temperatures colder than any in nature, the atoms behave like electrons. More lasers are used to form optical waveguides, 1D tubes wide enough for just one atom.
"We can use a magnetic field in our experiment to tune the strength of the repulsive interaction between the lithium atoms," Prof Hulet said. “Stronger or weaker electron interactions can produce wholly different effects, but it's extraordinarily difficult to study this with electrons because of the inability to directly control interactions. With ultracold atoms, we can essentially dial the interaction strength to any level we want and watch what happens."
"Charge excitations are predicted to move faster with increasing interaction strength, and we showed that," he said. "Giamarchi used TLL theory to predict how the charge wave would behave in our ultracold atoms, and his predictions were borne out in our experiments."
Having that ability to control interactions also sets the stage for testing the next TLL prediction: The speed of charge waves and spin waves diverge with increasing interaction strength, meaning that as electrons are made to repel one another with greater force, charge waves will travel faster and spin waves will travel slower.