A new discovery led by Princeton University could improve our understanding of the behavior of electrons under extreme conditions in quantum materials. The discovery provides experimental evidence that this familiar building block of matter behaves as if it were made up of two particles: one particle that gives the electron its negative charge and another that provides its magnet-like property, known as spin.
“We believe this is the first tangible evidence of spin-charge separation,” said Nai Phuan Ong, Eugene Higgins professor of physics at Princeton and lead author of the article published this week in the journal. Physics of nature.
The experimental results respond to a prediction made decades ago to explain one of the most mind-boggling states of matter, quantum spin liquid. In all materials, the spin of an electron can point up or down. In the familiar magnet, all rotations point uniformly in one direction throughout the sample when the temperature drops below a critical temperature.
However, in spinning liquid materials, the spins are unable to establish a uniform pattern even when cooled very close to absolute zero. Instead, the turns are constantly changing in a tightly coordinated and entangled choreography. The result is one of the most entangled quantum states ever, a state of great interest to researchers in the growing field of quantum computing.
To mathematically describe this behavior, the Princeton physicist, Nobel Prize winner Philip Anderson (1923-2020), who first predicted the existence of spin liquids in 1973, offered an explanation: in the quantum regime, an electron can be thought of as composed of two particles, one carrying the negative charge of the electron and the other containing its spin. Anderson called the particle containing a spin a spinon.
In this new study, the team looked for signs of the spinon in a spin liquid made up of ruthenium and chlorine atoms. At temperatures a fraction of Kelvin above absolute zero (or about -452 degrees Fahrenheit) and in the presence of a high magnetic field, ruthenium chloride crystals enter a spin liquid state.
Graduate student Peter Czajka and Tong Gao, Ph.D. 2020, connected three highly sensitive crystal thermometers sitting in a bath maintained at temperatures near zero absolute degrees Kelvin. They then applied the magnetic field and a small amount of heat to one edge of the crystal to measure its thermal conductivity, an amount that expresses how well it conducts thermal current. If spinons were present, they should appear as an oscillating pattern in a graph of thermal conductivity versus magnetic field.
The oscillating signal they were looking for was tiny – just a few hundredths of a degree change – so the measurements required extremely precise control of the temperature of the sample as well as careful calibrations of the thermometers in the strong magnetic field.
The team used the purest crystals available, those grown at the US Department of Energy’s Oak Ridge National Laboratory (ORNL) under the direction of David Mandrus, professor of materials science at the University of Tennessee-Knoxville, and Stephen Nagler, business researcher at ORNL. Neutron scattering division. The ORNL team has studied in depth the liquid quantum spin properties of ruthenium chloride.
In a series of experiments conducted over nearly three years, Czajka and Gao detected temperature oscillations compatible with spinons with increasing resolution, providing proof that the electron is composed of two particles coherent with the Anderson’s prediction.
“People have been looking for this signature for four decades,” said Ong. “If this discovery and the interpretation of spinons are validated, it would significantly advance the field of quantum spin liquids.”
Czajka and Gao spent last summer confirming experiences under COVID restrictions that required them to wear masks and maintain their social distancing.
“From a purely experimental standpoint,” Czajka said, “it was exciting to see results that actually break the rules you learn in elementary physics classes.”
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Material provided by Princeton University. Original written by Catherine Zandonella. Note: Content can be changed for style and length.