Most metals expand when their temperature rises. The Eiffel Tower, for example, is about 10 to 15 centimeters taller in summer than in winter due to its thermal expansion. However, this effect is extremely undesirable for many technical applications.

For this reason, scientists have long been searching for materials that always have the same length regardless of the temperature. Invar, for example, an alloy of iron and nickel, is known for its extremely low thermal expansion. How this property can be explained physically, however, was not entirely clear until now.

Now, a collaboration between theoretical researchers at TU Wien (Vienna) and experimentalists at University of Science and Technology Beijing has led to a decisive breakthrough.