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Liquid carbon can be disclosed if one is ultrafast enough

Understanding the properties and polymorphism of carbon at high pressures and temperatures is one of the main problems in the physics of materials under extreme conditions. The motivations to tackle such a challenging research are both technological and scientific. Carbon and its compounds are being increasingly employed as structural materials for high temperature devices with applications in nuclear technology, space launch programs as well as for scientific instrumentation. On the other hand carbon, one of the most abundant elements in the universe, can be naturally found under extreme conditions in the astrophysical context, as in planet cores. Remarkably, research on extrasolar planets, recently rewarded with the Nobel prize in physics, has predicted and demonstrated the existence of carbon-based exoplanets using models developed for carbon in extreme environments.
Among the diverse condensed forms of carbon that are stable under high pressure and temperature, the liquid phase (l-C) is probably the most difficult one to study. Access tol-C in the laboratory under stationary conditions is practically unfeasible due to the very high melting point (about 4800 K, see Fig. 1) and the carbon tendency to sublimate at low pressures and high temperatures. As a result, current knowledge of the structural and electronic properties of l-C is still today rather limited.
At the FERMI FEL, beamline EIS-TIMEX, a novel approach combining FEL and fs-laser radiation has been developed for generating liquid carbon under controlled conditions and monitoring its properties at the atomic scale. The method has been put to the test depositing a huge amount (5 eV/atom, 40 MJ/kg) of optical energy delivered by an ultrashort laser pulse (less than 100 fs, 10-13 s) into a self-standing amorphous carbon foil (a-C, thickness about 80 nm) and subsequently probing the excited sample volume with the FEL pulse varying both the FEL photon energy across the C K-edge (~ 283 eV) and delay between FEL and laser. We obtained a time-resolved x-ray absorption spectroscopy (tr-XAS, Fig. 2a) of l-C with a record time resolution of less than 100 fs. 
This method allowed us to monitor the formation of the liquid carbon phase occurring in about 300 fs after absorption of the laser pump pulse as an effect of the induced constant volume (isochoric) heating of the carbon sample. The time evolution of the measured x-ray absorption spectra is shown in Fig. 2a. Theoretical calculations, in agreement with the experimental tr-XAS data, reveal that the obtained l-C sample has a temperature of about 14200 K at a pressure of about 50 GPa (0.5 Mbar) (Fig. 1). 

Figure 1.    Sketch of the presumed phase diagram of carbon. Ultrafast isochoric heating, represented by the red curve, gives immediate access to the carbon liquid phase that persists for a few hundreds of femtoseconds prior to hydrodynamic expansion. 

Theoretical p-projected electron density of states (p-DOS, Fig 2b) of l-C is in very good agreement with XAS data and confirm that l-C under the obtained thermodynamic conditions is a metallic liquid. The atomic structure is dominated by carbon atom stripes as an effect of the pronounced sp1 hybridization. The process leading to the l-C phase has nonthermal nature as the melting mechanism is not driven by the phonon system, but by sudden change of the electronic structure of C.
The excited carbon sample volume is expected to rapidly start expanding just after 0.5-1 ps, therefore the generated liquid carbon phase can be assumed to be stable only for a few hundreds of fs. This is the lifetime of laser driven l-C. If you want to see it, you have to be ultrafast!


Figure 2.    (a) Ultrafast nonthermal melting dynamics in carbon revealed by tr-XAS at the C K-edge; blue curve: a-C, red curve: l-C. (b1) Electron distribution function (Fermi-Dirac) in l-C. (b2) p-DOS of l-C.  (b3) unoccupied p-DOS in l-C compared with the rescaled XAS spectrum of l-C.


This research was conducted by the following research team:

E. Principi1, S. Krylow2, M. E. Garcia2, A. Simoncig1, L. Foglia1, R. Mincigrucci1, G. Kurdi1, A. Gessini1, F. Bencivenga1, A. Giglia3, S. Nannarone3, C. Masciovecchio1


Elettra-Sincrotrone Trieste S.C.p.A., Basovizza (TS), Italy
Theoretical Physics and Center for Interdisciplinary Nanostructure Science and Technology (CINSAT) Universität Kassel, Kassel, Germany
IOM-CNR, Trieste, Italy

Contact persons:

Emiliano Principi, email: emiliano.principi@elettra.eu




E. Principi, S. Krylow, M. E. Garcia, A. Simoncig, L. Foglia, R. Mincigrucci, G. Kurdi, A. Gessini, F. Bencivenga, A. Giglia, S. Nannarone, and C. Masciovecchio, “Atomic and electronic structure of solid-density liquid carbon”,

Phys. Rev. Lett. 125, 155703 (2020), https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.155703

Last Updated on Wednesday, 21 October 2020 14:41