Most metals expand as their temperature rises. The Eiffel Tower, for example, stands about 10 to 15 centimeters taller in summer than in winter due to thermal expansion. However, this effect is highly undesirable for many technical applications. As a result, researchers have long sought materials that maintain a constant length regardless of temperature. One such material is Invar, an iron-nickel alloy known for its extremely low thermal expansion. The physical explanation for this property, however, remained unclear until recently.

Now, a collaboration between theoretical researchers at the Vienna University of Technology (TU Wien) and experimentalists at the University of Science and Technology Beijing has led to a significant breakthrough. Using complex computer simulations, they have unraveled the invar effect in detail and developed a so-called pyrochlore magnet—an alloy with even better thermal expansion properties than Invar. Over an exceptionally wide temperature range of more than 400 Kelvins, its length changes by only about one ten-thousandth of one percent per Kelvin.