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Extreme pressure, temperature yields solitary superconductor

Posted: 07 Jul 2014     Print Version  Bookmark and Share

Keywords:Nihon University  superconductor  organic molecule  metal-dithiolate 

Researchers have discovered, about 30 years ago, that certain organic molecules become superconducting at low temperatures. This discovery served as an impetus to a number of investigations into the properties of these lightweight, low-cost and easy-to-modify materials. Despite much recent progress, chemists remain puzzled by one aspect of these compounds: all known molecular superconductors need the cooperative action of two or more different molecular species to move electrons without resistance.

HengBo Cui and Reizo Kato from the RIKEN Condensed Molecular Materials Laboratory in collaboration with Hayao Kobayashi and Akiko Kobayashi from Nihon University have realised a crucial goal in the search for metal-like organic molecules by uncovering the first molecular superconductor containing only one component.

Single-component nickel-organic molecule

The diamond anvil cell used to induce superconductivity in a single-component nickel-organic molecule. (Reproduced, with permission, from Ref. 1 2014 American Chemical Society)

Superconducting organic crystals are designed around the principle of charge-transfer complexes, where strong interactions between distinct 'donor' and 'acceptor' components move electrons through normally insulating carbon bonds. By squeezing the charge-transfer structures together using diamond anvil cells, tools that allow crystals to be compressed at pressures of up to millions of atmospheres, resistance-free electrical transport can occur at temperatures near absolute zero.

The electron donors and acceptors in molecular superconductors are normally individual ionic compounds. However, Kobayashi's team has recently spearheaded investigations into metal-dithiolate complexes that contain a complete charge-transfer system in a single molecule. These crystals, in which a central gold or nickel acceptor atom is flanked on two sides by extended aromatic donor rings infused with sulphur atoms, have a high intrinsic conductivity and exhibit metallic behaviour at low temperatures.

The researchers partnered with Masaaki Sasa from Fujitsu to explore numerous metal-dithiolate synthetic derivatives. They eventually found a promising compound, nickel bis(trifluoromethyl)tetrathiafulvalenedithiolate (Ni(hfdt)2). This molecule has bulky fluorinated end-groups on its dithiolate rings that trigger 2D layer stacking in the crystal state, a highly favourable arrangement for metal-like conductivity.

After carefully manipulating the tiny, submillimetre-sized Ni(hfdt)2 crystals into their diamond anvil cell device, the team measured how its electrical behaviour changed with pressure and temperature. At a pressure of about 8.1 gigapascals, they found that the resistivity suddenly plunged to zero at a temperature of 5.5 kelvin, clear evidence that they had discovered a single-component molecular superconductor. High-level theoretical calculations confirmed these experimental findings by revealing the critical point at which pressure converts Ni(hfdt)2 from an insulator to a superconductor.

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