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John O | September 2018

Researchers use ultra-cold lithium atoms to prove 50-year-old theory about 1-D electronics

By Josh Perry, Editor


Researchers from Rice University (Dallas, Texas) and the University of Geneva (Switzerland) used lithium atoms cooled to within 100 billionths of a degree of absolute zero to verify a theory first proposed in 1963 that exciting one electron in a 1-D wire leads to a collective response from every electron in the wire.


Using ultracold lithium atoms confined by intersecting laser beams, physicists from Rice University and the University of Geneva confirmed a 1963 prediction that the charge wave from an excited electron moves faster in a one-dimensional electron gas as interaction strength between the electrons increases. (Jeff Fitlow/Rice University)


According to a report from Rice, the increasing application of carbon nanotubes and nanowires in electronics and the ongoing quest for miniaturization of electronics has prompted a return to the Tomonaga-Luttinger liquid (TLL) theory about electron interaction.


“Stranger still, because of this collective behavior, TLL theory predicts that a moving electron in 1D will seemingly split in two and travel at different speeds, despite the fact that electrons are fundamental particles that have no constituent parts,” the article explained. “This strange breakup, known as spin-charge separation, instead involves two inherent properties of the electron — negative charge and angular momentum, or ‘spin.’”


Rice researchers used lithium atoms in place of electrons and verified the predicted speed that charge waves move in 1-D system and confirmed that 1-D charge waves increase speed in proportion to the strength of the interaction. As a researcher noted, the fermions (antisocial particles unwilling to share space) have nowhere to go and exciting one causes a chain reaction down the length of the wire.


“The atoms are trapped and slowed with lasers that oppose their motion,” the article added. “The slower they go, the colder the lithium atoms become, and at temperatures far colder than any in nature, the atoms behave like electrons. More lasers are used to form optical waveguides, one-dimensional tubes wide enough for just one atom.”


Magnetic fields were used to excite the atoms and tune the strength of the interactions between them. This is the first time that researchers have viewed the correlation between atom interactions and the speed of the collective charge waves. Researchers will study spin waves next.


The research was recently published in Physical Review Letters. The abstract stated:


“We present measurements of the dynamical structure factor S(q,ω) of an interacting one-dimensional Fermi gas for small excitation energies. We use the two lowest hyperfine levels of the 6Li atom to form a pseudospin-1/2 system whose s-wave interactions are tunable via a Feshbach resonance.


“The atoms are confined to one dimension by a two-dimensional optical lattice. Bragg spectroscopy is used to measure a response of the gas to density (“charge”) mode excitations at a momentum q and frequency ω, as a function of the interaction strength.


“The spectrum is obtained by varying ω, while the angle between two laser beams determines q, which is fixed to be less than the Fermi momentum kF. The measurements agree well with Tomonaga-Luttinger theory.”

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