Possible light-induced superconductivity in K3C60 at high temperatures

Superconductors have long been confined to a limited number of applications, with their highest transition temperature being 200 K (under extremely high pressure conditions), and are at present mainly used in magnets for nuclear magnetic resonance tomographs, fusion devices and particle accelerators. One fundamental problem, perhaps the most important of contemporary condensed matter physics, is finding a way to raise the critical temperature up to ambient conditions and develop cheap, scalable superconductor-based solutions for lossless energy distribution.
In 2013, researchers from Cavalleri’s group at the Max Planck Institute for the Structure and Dynamics of Matter (Hamburg, Germany) investigated ways to dynamically control superconductivity in high-temperature cuprate superconductors. In these experiments the samples were excited with strong femtosecond midinfrared light pulses (pump) tuned to be resonant with some specific lattice vibrations. The light-induced changes in the low-energy electrodynamics of the material were detected with a delayed pulse in the terahertz range (probe). This resonant excitation led to the observation of a transient superconducting-like phase at unprecedented temperatures, far beyond equilibrium Tc. However, the understanding of the microscopic mechanism underlying these experimental observations is not entirely clear, due to the intrinsic complexity of the physics of these oxides.
Max Planck researchers of the same group, in collaboration with an Elettra research team, decided to investigate with a similar technique a completely different material: the fulleride K3C60. This solid is composed of so-called Buckminster fullerene molecules (C60) arranged in a face centered cubic lattice. Intercalated potassium ions (K+) act as spacers between the negatively charged C603- molecules. K3C60 is a superconductor below 20 K (see Fig. 1). The steady state optical response of K3C60 was thoroughly characterized in a broad frequency range and as a function of temperature by exploiting the high brightness of the synchrotron radiation extracted from the infrared beamline SISSI at Elettra.

Figure 1. Equilibrium optical response of K3C60 measured at Elettra: (a) Reflectivity, (b), real and (c) imaginary part of the optical conductivity, showing the opening of a gap upon cooling below T(blue curves), as expected for a textbook superconductor. Red curves are measured at 25 K, blue curves at 10 K.

After these measurements, K3C60 was irradiated with strong femtosecond mid-infrared light pulses. The pump pulse was tuned to match the characteristic C60 intramolecular vibrational frequencies, while a terahertz probe pulse was used to observe the response of the superconducting gap.
Strikingly, for a wide range of temperatures up to at least 100 K (5 times Tc), the light-induced phase exhibited a strong increase of reflectivity,a gap in the real part of the optical conductivity and a low-frequency divergence of the imaginary part, strongly resembling the spectral hallmarks of the equilibrium superconducting transition (see Fig. 2). These observations are possibly indicative of a novel non-equilibrium superconducting phase induced by the laser field.
This discovery, achieved on a material far simpler than cuprates, may lead to a more comprehensive understanding of the phenomenon of light-induced superconductivity, and potentially give hints in the direction of synthesizing novel superconductors with higher critical temperatures.

Figure 2. Same quantities as in Figure 1, measured at T > Tc (100 K) at equilibrium and after laser excitation. Strikingly, the response observed in the transient state is very similar to that of the equilibrium superconductor of Figure 1. 


This research was conducted by the following research team:

M. Mitrano1, A. Cantaluppi1,2, D. Nicoletti1,2, S. Kaiser1, A. Perucchi3, S. Lupi4, P. Di Pietro3, D. Pontiroli5, M. Riccò5, S. R. Clark1,6,7, D. Jaksch7,8 and A. Cavalleri1,2,7

Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany.
The Hamburg Centre for Ultrafast Imaging, Hamburg, Germany.
INSTM UdR Trieste-ST and Elettra–Sincrotrone Trieste S.C.p.A.,  Basovizza, Trieste, Italy.
CNR-IOM and Dipartimento di Fisica, Università di Roma “Sapienza”, Roma, Italy.
Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Parma, Parma, Italy.
Department of Physics, University of Bath, UK.
Department of Physics, Oxford University, Clarendon Laboratory, Oxford, UK.
Centre for Quantum Technologies, National University of Singapore, Singapore.

Contact person:

Andrea Cavalleri, email: andrea.cavalleri@mpsd.mpg.de
Andrea Perucchi, email: andrea.perucchi@elettra.eu



M. Mitrano, A. Cantaluppi, D. Nicoletti, S. Kaiser, A. Perucchi, S. Lupi, P. Di Pietro, D. Pontiroli, M. Riccò, S. R. Clark, D. Jaksch & A. Cavalleri “Possible light-induced superconductivity in K3C60 at high temperature” Nature 530, 461 (2016), DOI:10.1038/nature16522

Last Updated on Monday, 21 March 2016 13:39