A sharp burst of pure sound can cause the resistance of manganite to fall to nearly one hundred thousandth of its original value, according to a team of physicists in the UK, US and Japan. This effect, dubbed "colossal phonoresistance" is believed to be caused by interactions between electrons and quantized lattice vibrations called phonons. The researchers believe that the effect could be used to make terahertz radiation detectors and ultimately shed light on high temperature superconductivity (Nature 449 72).
Physicists know that the electrical resistance of certain manganese oxides called manganites can drop by as much as ten orders of magnitude when the materials are exposed to a magnetic field. While a full explanation of why this colossal magnetoresistance (CMR) occurs has evaded researchers, physicists have suspected for some time that it is related to interactions between electrons and phonons.
Now, an international team led by Andrea Cavalleri at Oxford University has performed an experiment that provides further insight into the role of phonons in CMR. The team fired a short terahertz (THz) laser pulse at a manganite sample while monitoring its electrical resistance by measuring the current flowing through it. When the energy of the laser is tuned to a specific phonon frequency, the resistance of sample drops dramatically for about 5 ns before returning to its original value.
According to Cavalleri, the pulse – which is about 300 fs in duration — is long enough to create phonons at a specific frequency (about 17 THz). However, it is short enough to avoid exciting electrons and other phonons at other frequencies. This allowed the team to conclude that the drop in resistance was caused exclusively by interactions between 17 THz phonons and electrons in their equilibrium state. The pulse was also short enough to ensure that the electrons did not heat up, which means that CMR does not necessarily require the electrons to be “hot”.
By exciting only 17 THz phonons and not heating the sample the team has managed to avoid the “chicken and egg” problem, which normally makes it very difficult to study materials such as manganite. In such materials the electrons interact with each other via phonons and if an experiment excites both the electrons and phonons it can be impossible to determine, for example, what is a cause of CMR and what is an effect of CMR.
The “chicken and egg” problem also affects those studying cuprate high-temperature superconductors and Cavalleri and colleagues now plan to use the technique to gain a better understanding of the role of electron-phonon interactions in these materials.
Cavalleri told physicsworld.com that there could be practical applications for colossal phonoresistance – particularly because it works at room temperature. It could be used, for example, to make THz radiation detectors and other THz optoelectronic devices. He also believes that the technique could be used to change the magnetic properties of certain materials using a THz laser pulse.
