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Pressure tuning of light-induced superconductivity in K3C60

Unlike ordinary metals, superconductors have the unique capability of transporting electrical currents without any loss. Nowadays, their technological application is hindered by their low operating temperature, which in the best case can reach -70 degrees Celsius. Researchers of the group of Prof. A. Cavalleri at the Max Planck Institute of the Structure and Dynamics of Matter (MPSD) in Hamburg have routinely used intense laser pulses to stimulate different classes of superconducting materials. Under specific conditions, they have detected evidences of superconductivity at unprecedented high temperatures, although this state persisted very shortly, just for a small fraction of a second.
An important example is that of K3C60, an organic molecular solidformed by weakly-interacting C60 “buckyball” molecules (60 carbon atoms bond in the shape of a football),which is superconducting at equilibrium below a critical temperature of -250 degrees Celsius. In 2016, Mitrano and coworkers at the MPSD discovered that tailored laserpulses, tuned to induce vibrations of the C60 molecules,can induce a short-lived, highly conducting state with properties identical to those of a superconductor, up to a temperature of at least -170 degrees Celsius, far higher than the equilibrium critical temperature (Mitrano et al., Nature, 530, 461–464 (2016)). 

In their most recent investigation, A. Cantaluppi, M. Buzzi and colleagues at MPSD in Hamburg went a decisive step further by monitoring the evolution of the light-induced state in K3C60 once external pressure was applied by a diamond anvil cell (Figure 1). At equilibrium, when pressure is applied, the C60 molecules in the potassium-doped fulleride are held closer to each other. This weakens the equilibrium superconducting state and significantly reduces the critical temperature. The steady state optical response of K3C60 at different pressures and temperatures was determined via Fourier-transform infrared spectroscopy, by exploiting the high brightness of the synchrotron radiation available at the infrared beamline SISSI at Elettra.  

 

Figure 1.  Light-induced superconductivity in K3C60 was investigated at high pressure in a Diamond Anvil Cell. © Jörg Harms / MPSD
 

Understanding whether the light-induced highly-conducting state found in K3C60 is also suppressed by external pressure is key in ascertaining its superconducting nature and can provide new hints to unveil the physical mechanism behind light-induced high-temperature superconductivity.
K3C60 was systematically investigated, in presence of photo-excitation, for pressures varying from ambient conditions up to 2.5 GPa, which corresponds to 25,000 times the atmospheric pressure. The authors measured a strong reduction in photo-conductivity with increasing pressure (Figure 2a). Such behaviour is very different from that found in conventional metals, while it is fully compatible with the phenomenology of a superconductor, thus standing for a first unambiguous interpretation of the light-induced state in K3C60 as a transient superconducting phase. Importantly for stronger optical excitations, an incipient, transient superconductor was obtained at temperatures far above the -170 degrees Celsius hypothesized previously, and rather all the way to room temperature (Figure 2b).
 

Figure 2Pressure dependence of the low-frequency conductivity of K3C60. Blue diamonds are extrapolated zero-frequency conductivities extracted from Drude-Lorentz fits of the transient optical spectra, as a function of pressure and for three different temperatures: 100 K (a), 200 K (b), and 300 K (c). Red squares are the corresponding zero-frequency conductivities determined at equilibrium. Reproduced with permission from Cantaluppi et al., Macmillan Publishers Limited.


A universal picture able to describe the physical mechanism behind the phenomenon of light-induced high-temperature superconductivity in K3C60 is still missing and the ultimate goal of obtaining a stable room-temperature superconductor is not around the corner yet. Nonetheless, the novel approach introduced by the MPSD team, which combines optical excitation with the application of other external stimuli, as external pressure, shall pave the way in this direction, allowing for generation, control, and understanding of new phenomena in complex materials.
This work was supported by the ERC Synergy Grant “Frontiers in Quantum Materials’ Control” (Q-MAC), the Hamburg Centre for Ultrafast Imaging (CUI), and the priority program SFB925 of the Deutsche Forschungsgemeinschaft. The experiments were performed in the laboratories of the Center for Free-Electron Laser Science (CFEL), a joint enterprise of DESY, the Max Planck Society, and the University of Hamburg. The research was carried out in close collaboration with scientists of University of Parma and of the ELETTRA Synchrotron Facility, Trieste, Italy.


 

This research was conducted by the following research team:

Alice Cantaluppi1,2, Michele Buzzi1, Gregor Jotzu1, Daniele Nicoletti1,2, Matteo Mitrano1, Daniele Pontiroli3, Mauro Riccò3, Andrea Perucchi4, Paola Di Pietro4, Andrea Cavalleri1,2,5


Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany
The Hamburg Centre for Ultrafast Imaging, Hamburg, Germany
Dipartimento di Scienze Matematiche, Fisiche e Informatiche, Università degli Studi di Parma, Parma, Italy
INSTM UdR Trieste-ST and Elettra–Sincrotrone Trieste, Trieste, Italy.
Department of Physics, Oxford University, Clarendon Laboratory, Oxford UK.


Contact persons:

Andrea Cavalleri, email: andrea.cavalleri@mpsd.mpg.de


Reference

A. Cantaluppi, M. Buzzi, G. Jotzu, D. Nicoletti, M. Mitrano, D. Pontiroli, M. Riccò, A. Perucchi, P. Di Pietro and A. Cavalleri, “Pressure tuning of light-induced superconductivity in K3C60”,  Nature Physics, advanced online (2018) (DOI: 10.1038/s41567-018-0134-8)

 
Last Updated on Tuesday, 24 July 2018 07:23