For a recent colloquium at the University of Bath, Professor Andrea Cavalleri was invited to talk about his work on ‘controlling solid phases with light’. A team led by Andrea at the Max Planck Institute for the Structure and Dynamics of Matter have been studying the response of materials using intense laser pulses of low frequency. I asked Andrea about the motivation for the work in their most recent paper, and the future of controlling matter with light.
Mostly, scientists have probed matter non-resonantly using lasers operating at frequencies near the visible part of the spectrum. More recently however, radiation in the mid infra-red to THz region of the spectrum has provided direct access to the low energy excitations in solids, such as phonons, excitons, and many others. Due to advances in high power sources, this type of radiation can be used not only to probe matter, but to drive it into a new transient state.
Andrea’s team have been ultimately seeking to control matter with light by driving solids into a non-equilibrium state, and this includes the optical enhancement of superconductivity.
“Our goal in particular is to use light to generate phases that do not occur spontaneously at equilibrium, like for example superconductivity at higher temperatures. Traditionally, quantum-phase discovery has been achieved by materials synthesis or by using pressure or strong magnetic fields, but the ability to generate sculpted, coherent light fields opens new possibilities which are only starting to be explored.”
Schematic LTT crystal structure for LESCOx. ©2016 IOP Publishing Ltd. All rights reserved.
In 2011, the team were able to measure superconducting-like optical properties in the cuprate, LESCO at temperatures far greater than the equilibrium transition temperature using these methods. Later, the same was found for YBCO and x-ray diffraction experiments hinted at intense light pulses forcing a structural change to the crystal as the driving mechanism for enhanced superconductivity.
Now, the team have focused on potassium doped buckyballs to see if the same effects can be seen. Buckyballs (Buckminsterfullerene) are large carbon cages made up of 60 atoms. When doped with potassium to form K3C60, the material becomes superconducting with an equilibrium transition temperature of 20 K.
In their experiments, a mid-infrared laser pulse (the ‘pump’) was firstly used to excite K3C60 powder in the range of 80 – 200 meV photon energy. Shortly after (about 1 ps), the excited state was measured using a second pulse (the ‘probe’) of THz frequency, which is used to detect changes in the reflectivity. It was found that the mid-infrared pulses had induced a large increase in carrier mobility and the opening of a gap in the optical conductivity which then disappears around a picosecond later.
These superconducting-like optical properties mimic the same signatures seen when cooling K3C60 below its equilibrium transition temperature. And so for a fleeting second (…or picosecond), light had driven the material into a superconducting state! What’s even more interesting is that the state seemingly survives up to around 100 K, which is around 5 times greater than the equilibrium transition temperature.
Similar to the team’s work on YBCO, they explain their results by light pulses distorting the crystal lattice which may increase the electron-phonon coupling, favouring stronger superconductivity. However, the results for K3C60 are even more convincing than their previous work on the cuprates since the interpretation of these experiments was difficult as their highly unconventional superconduting state is still not fully understood. In addition, the cuprates are two dimensional materials with peculiar transport properties.
“The novelty of our result on K3C60 resides in the fact that it demonstrates that light-induced superconductivity is a far more general phenomenon than previously envisaged. Unlike cuprates, K3C60 is a fully three-dimensional molecular solid, where the equilibrium superconducting state is quite conventional and fairly well understood.”
The optical signatures seen in the team’s experiments are extremely short lived and the superconducting-like state only exists on the order of picoseconds. Currently, this is too short for the traditional superconducting tests of zero electrical resistance and expulsion of magnetic fields (Meissner effect) to be performed. However, the optical signatures are clear and the work of Cavalleri et al. paves the way for controlling phenomena in solids with light.
“The possibilities offered by intense, sculpted light fields at THz frequencies for controlling and switching complex matter is enormous.”
An application using current methods of short light pulses could be to create a highly efficient switchable superconductor device. Conversely, if the light-induced state can be maintained for any significant time, the idea of creating optically pumped room temperature superconductors could become a reality.
“Great efforts are being aimed at developing new THz sources, which can produce picosecond-to-nanosecond narrow band, high field pulses, tunable in the broadest possible spectral range. These may be used to drive transient superconductors in a quasi-persistent mode, thus extending their lifetime.”
It is clear to see that we have only scratched the surface of this relatively new phenomenon, but the future is truly exciting.
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