In his book A Different Universe: Reinventing Physics from the Bottom Down, published in 2005, physicist and Nobel laureate Robert B. Laughlin predicts new discoveries of many such organizing principles and laws of nature in complex systems. According to Laughlin, we are still unaware of the great majority of organizing principles. So the physicists in Roderich Moessner’s group still have much work to do.
Theoretical research into complex systems fascinates him for two reasons. “For one thing, we work in small groups, which gives us a high degree of flexibility and variety,” says Moessner. The second reason is probably more important: “Particle and astrophysicists have one large universe. We have many small quasiuniverses,” he says jokingly. “For us, every material is a potentially new type of complex system.” The variety within solid-state physics, says Moessner, derives from the countless possible combinations of the elements in the periodic table. So there are many new phenomena yet to be discovered.
The results of the scientists’ research into miniature quasiuniverses should not remain purely theoretical. Experiments on the monopole hypothesis are now on the agenda. Preparations for these are underway at several locations. “Whatever the results of the experiments, I am confident that they will teach us something important,” says the physicist. He does not yet envisage direct technical applications of the magnetic monopoles. However, the good news, says Moessner, is that it will hopefully be possible to create magnetic monopoles should they be needed for a specific application. “This demonstrates the level of control that we are now able to exercise in materials physics and the physics of complex systems.”
Spin ice forms in a crystal lattice of tetrahedra that are joined at their corners. It is composed of the elements holmium or dysprosium, titanium and oxygen. Spin ice owes its name to a similarity with water ice. The latter consists of oxygen and hydrogen atoms. Each oxygen atom has four neighboring hydrogen atoms. Two of these – the two with which the oxygen atom forms a water molecule – are closer to the oxygen than the other two. Experts call this the ice rule. Spin ice has a similar local arrangement of the magnetic moments of the spins located at each corner of the tetrahedron. Magnetic moments resemble minute bar magnets, each with a north and a south pole. Like the needle of a compass, the magnetic moments are pivoted at each of the four corners of the tetrahedra that make up spin ice. In the lowest-energy and thus preferred state of spin ice, two north poles in each tetrahedron point to its center, and two away from it – a magnetic version of the ice rule.