First Signs of Higgs Mechanism in a Magnet

Jülich, 7 August 2012 – British physicist Peter Higgs attracted widespread attention recently. Researchers at CERN announced the probable evidence of the Higgs boson, which he predicted back in the 1960s. The Higgs mechanism as proposed by him explains how elementary particles attain their mass – and also plays a role beyond elementary particle physics. Using neutron scattering experiments, an international team of researchers has found initial indications that this very mechanism could explain a phase transition emanating from exotic magnetic states in Yb₂Ti₂O₇ crystals that occurs at temperatures close to the absolute zero. During cooling of a state referred to as quantum spin ice, they have now for the first time observed indications for the spontaneous exchange with the Higgs field predicted by Higgs in a magnet. The findings have been published in the high-impact journal Nature Communications (DOI: 10.1038/ncomms1989).

Phase transitions describe how a material changes from one state into another. An everyday example is the melting of ice. In addition, there are phase transitions from electronic and magnetic states. For example, the magnetization of iron occurs below a particular critical temperature based only on the electromagnetic interactions between electrons and their magnetic moments, or spins. However, not all magnetic phase transitions can be explained in this manner. A team of German, Taiwanese, Japanese and British scientists have presented findings with initial experimental indications for a "Higgs transition" in Yb₂Ti₂O₇ crystals at temperatures close to absolute zero.

The existence of this phase transition has been known for a long time but not exactly what happens during this process. Experiments with polarized neutrons at Forschungszentrum Jülich’s outstation laboratory at the Heinz Maier-Leibnitz research neutron source (FRM II) in Garching near Munich shed light on the issue. Such experiments make it possible to measure the magnetic structure of materials with an atomic resolution. The high intensity of the neutron source at Garching also made it possible to detect the weak scattering signals from the sample. The experiments there could also be conducted at the needed low temperatures.

Scientists from Forschungszentrum Jülich and research institutions in Taiwan, Japan and the United Kingdom first turned their attention to the phase above 210 millikelvins and identified it as an exotic quantum spin ice with magnetic monopoles. Physicist Paul Dirac had already suggested that such magnetic monopoles existed back in 1931. However, for a long time, only magnetic dipoles could be verified experimentally. Like a bar magnet, they possess two opposing poles that cannot be separated. In 2009, magnetic monopoles were observed for the first time in “classical” spin ice. These behave like individual isolated north or south poles, similar to individual magnetic charges. The spins arrange themselves in the same pattern as water molecules in ice, whereby the quantum spin ice that was studied possesses a significantly smaller magnetic moment than normal spin ice.

"At temperatures exceeding 210 millikelvins, the magnetic monopoles of the quantum spin ice form a very complex pattern. Below 210 millikelvins, in contrast, the magnetic moments abruptly arrange themselves in parallel, or ferromagnetically, as is the case in iron," explains Dr. Yixi Su from the Jülich Centre for Neutron Science (JCNS). In quantum physics, this type of transition at extremely low temperatures is known as Bose-Einstein condensation. However, it assumes that the involved particles have a mass. The magnetic monopoles affected, however, are normally massless, and this also applies to the observed quantum spin ice. These are "quasi-particles", which first occur through the interaction of several electrons and migrate through the crystal like a sort of wave. For this reason, the researchers assume that they have observed the typical characteristics of a phase transition based on the Higgs mechanism. "As far as we know, this would then be the first demonstration of a Higgs transition in a magnet," Su reports.

As an integral component of the Standard Model of physics, the Higgs mechanism explains why particles – as well as electrons and quarks, from which atomic nuclei are composed – have mass in the first place. Responsible for this is the Higgs field, which is present throughout the entire universe. The field itself defies direct observation. However, as a result, elementary particles – as well as quasi-particles such as the magnetic monopoles in this case – can interact and, in so doing, obtain their mass.

Understanding such quantum-mechanical electromagnetic phenomena in detail is essential for understanding modern physics. Today’s information technology, to name but one example, is built upon it. The researchers now want to use Yb₂Ti₂O₇ as a model system to investigate fascinating properties of quantum spin liquids. Once again, they will use neutron scattering experiments. "No other method is sensitive enough at the moment," says Su.

Original Publication:

L.-J. Chang, S. Onoda, Y. Su, Y.-J. Kao, K.-D. Tsuei, Y. Yasui, K. Kakurai & M. R. Lees Higgs transition from a magnetic Coulomb liquid to a ferromagnet in Yb₂Ti₂O₇ Nature Communications 3:992 (2012), DOI: 10.1038/ncomms1989.