Researchers in the US may have found the first hard evidence that the electron is made up of two distinct particles. According to N Phuan Ong and colleagues at Princeton University, the observation of “spin-charge separation” in a material known as a quantum spin liquid suggests that the electron is not a structureless “point” particle as is commonly thought, but instead behaves like it contains two separate entities.
Quantum spin liquids (QSLs) are solid magnetic materials that cannot arrange their magnetic moments (or spins) into a regular and stable pattern. This so-called “frustrated” behaviour is very different from that of ordinary ferromagnets (in which all the spins point in the same direction, either “up” or “down”), or antiferromagnets (in which the spins point in alternating directions, “up-down” or “down-up”).
Quantum mechanics describes this frustration by suggesting that the orientation of the spins is not rigid. Instead, it constantly changes direction in a fluid-like way to produce an entangled ensemble of spin-ups and spin-downs. Thanks to this behaviour, a spin liquid will remain in a liquid state even at temperatures near absolute zero, where most materials usually freeze solid.
The holon and the spinon
To describe this behaviour in mathematical terms, the late Nobel laureate Philip W Anderson, who predicted the existence of spin liquids in 1973, proposed that in the quantum regime, an electron might in fact be composed of two distinct particles. The first, known as a “holon”, would bear the electron’s negative charge, while the second “spinon” particle would carry its spin. Anderson later suggested that this spin-charge separation might provide a microscopic mechanism to explain the high superconducting transition temperatures (Tc) that were observed in copper oxides, or cuprates, beginning in the late 1980s.
In the new study, Ong and graduate students Peter Czajka and Tong Gao set out to identify signs of the spinon in ruthenium (III) chloride, RuCl3. This antiferromagnetic material closely resembles the ideal Kitaev honeycomb model for a spin liquid, and undergoes a transition to a spin liquid at a temperature of 0.5 K in the presence of a strong magnetic field (between 7 and 11.5 Tesla). This is the field interval over which the spin liquid state is stable, Ong explains.
In their experiments, which they detail in Nature Physics, the researchers placed crystals of RuCl3 in an ultracold bath held at temperatures just above absolute zero. They then applied the magnetic field and a small amount of heat to one edge of a crystal and monitored its thermal conductivity. According to theory, spinons, if they are present, should appear as an oscillating pattern in the plot of thermal conductivity versus applied magnetic field.
Extremely small signal
The amount of heat applied is extremely small, equating to a temperature change of just a few hundredths of a degree. This meant that the researchers had to control the temperature of their sample very carefully, while using very sensitive thermometers to measure how it changed. They also made their measurements on the purest crystals available, as provided by David Mandrus’ group at the University of Tennessee-Knoxville and Stephen Nagler in the Neutron Scattering Division of Oak Ridge National Laboratory.
Promising quantum spin liquid candidate may fall short
In their study, which was performed over nearly three years, Ong, Czajka and Gao detected temperature oscillations that imply the presence of spinons. These freely moving spin excitations can be considered as (uncharged) analogues to electrons in a metal even though RuCl3 is an excellent insulator with a large electronic bandgap.
In their previous work, the researchers found that these oscillations slowly die away as the temperature increases from 0.5 to 5 K, replaced by an in-plane thermal Hall effect. “We have been investigating this effect in much detail and have many questions surrounding the twin phenomena,” Ong notes. “Is the thermal Hall effect quantized? Where does it come from? And what is its physical nature? We will be providing a detailed report on this study in a forthcoming paper,” he tells Physics World.