The existence of materials with a negative refractive index was first predicted by the Russian physicist Victor Veselago in 1967. He speculated that materials in which the electric permittivity and the magnetic permeability are both less than zero would refract light in the opposite direction compared to conventional materials. The electric permittivity is the ability of a material to store electrical charge and the magnetic permeability is the ability of a material to acquire magnetisation in magnetic fields.

In 2000, John Pendry, a theorist at Imperial College in London, showed that such materials could also behave as perfect lenses and so overcome the "diffraction limit" of the resolution of optical devices. Although negative-refractive-index materials do not occur naturally, several groups have successfully made them recently from metamaterials and photonic crystals.

The new material, made by Andrei Pimenov and colleagues of the University of Augsburg and co-workers at the Polish Academy of Sciences and Northern Illinois University consists of a multilayer stack of ferromagnetic manganese oxide and superconducting copper oxide thin films. With no external applied magnetic field the sample is superconducting and has negative electric permittivity. Then, when an external magnetic field of 3 Tesla is applied, the magnetic permeability becomes negative close to the so-called resonance field. In this way, both conditions for negative refraction are satisfied.

Pimenov and colleagues were able to directly determine the refractive index of the material as a function of applied magnetic field by measuring the field dependence of the transmittance and phase shift of the sample. Moreover, they found that the index of refraction can be tuned between negative and positive values by varying the strength of the applied magnetic field.

The physicists now plan to try and reduce the value of the magnetic field required to achieve negative magnetic permeability. They will also look at replacing the ferromagnetic layers with antiferromagnetic materials, which have "internal magnetism". This means that negative refraction could be observed in the absence of an external magnetic field, says Pimenov.