Science 345, 1487–1490 (2014)

Condensed-matter physicists often perform experiments at low temperatures to minimize the undesired effects of thermal excitation. Yet, if the phenomenon being studied has potential practical applications, it can also be important to know what happens at high temperatures. Hans Malissa and colleagues at the University of Utah, the University of Queensland and the University of Regensburg have now studied the effect of nuclear spin excitations in organic light-emitting diodes (OLEDs) at room temperature; these experiments are particularly challenging because at room temperature the nuclear spin excitation is one million times smaller than the thermal energy.

The effects of external magnetic fields on the conductivity of organic semiconductors have previously been attributed to the hyperfine interaction between the nuclear and the electron spins. Malissa and colleagues confirmed this prediction with two types of experiment. In the first, they used a technique known as electron spin-echo envelope modulation to study OLEDs made of the polymer MEH-PPV, and monitored the way in which the current evolves after the electron spin has been excited by a microwave pulse. In particular, they measured a signal modulated by a frequency typical of the hydrogen nuclear spin in the polymer. By replacing hydrogen with deuterium in the polymer the modulation frequency changed accordingly, illustrating that the interaction of the electron spin and the nuclear spin affects the current in these materials.

A more direct confirmation of the effect of the hyperfine interaction came from the second experiment. Here, the researchers used a technique known as electron–nuclear double resonance, in which the current is measured after excitation of the hydrogen nuclear spin with a radiofrequency pulse. Again, a clear modulation of the current was observed.