Superconducting Nanocrystals

Superconductivity in the films of colloidal nanocrystals

Superconductivity is a fascinating physical phenomenon that allows one to experience quantum mechanics with a measuring tool as simple as an ohmmeter. At sufficiently low temperatures it is present in a variety of materials: metals, organic compounds, and copper oxides. The search for superconducting materials operating at ever higher temperature and magnetic fields is motivated by the tremendous impact they would have for energy and technology. Some of the best existing materials such as the Copper Oxides superconductors have naturally a strongly modulated composition of the material at the nanoscale, and the nanoscale of the material is expected to play a significant role. 

It is this question that researchers at the University of Chicago MRSEC are trying to address experimentally by making “nanoengineered” materials.  In the past year, they developed a new method for the preparation of monodisperse Pb nanocrystals, Fig 1a. As prepared, the particles are covered with an oxide shell and organic ligands that prevent coupling between nanocrystals, making them a perfect system for studying properties of the “isolated” particles. Measurements of magnetic susceptibly proved that on the individual level, the particles are superconducting1.   They also showed that, when the diameter varied from 20 nm to 4.4 nm, superconductivity was destroyed below the size of 5 nm, which agrees well with theoretical prediction.

The next step was to turn on the coupling between particles and observe how the macroscopic superconductivity emerges.   The researchers found that the insulating lead oxide shell, could be easily converted to the more conductive lead sulfide, Fig 1b, and that this transformation reduced the resistance of the films by more than 10 orders of magnitude.  Depending on the extent of the conversion, films show either insulating or metallic behavior at normal temperatures. Below the superconducting transition temperature, the insulating samples become “superinsulators” and show an exponential increase in resistance2, Fig. 2a, while the metallic films undergo a transition to the macroscopic superconducting state with nonzero resistance3 at low temperature, Fig. 2b. 

 
Fig. 2a Increase in resistance after the superconducting transition at 7 K due to the opening of the superconducting gap, which is destroyed by the application of magnetic field (pink curve).   Fig. 2b Superconducting transition below 7K for a film that showed metallic behavior in the normal state (Inset).

 

These experiments were made possible after a recent MRSEC purchase of Physical Property Measurement System (PPMS) that allows convenient and rapid low temperature experiments in high magnetic field. Current and future efforts are targeted towards achieving macroscopic superconducting state with zero resistance. It is expected that these films will have high critical current, which is important for practical applications. 

References:

1.  Zolotavin, P.; Guyot-Sionnest, P., Meissner Effect in Colloidal Pb Nanoparticles. ACS Nano 2010, 4, 5599-5608.

2.  Adkins, C. J.; Thomas, J. M. D.; Young, M. W., Increased Resistance Below the Superconducting Transition in Granular Metals. Journal of Physics C-Solid State Physics 1980, 13, 3427-3438.

3.   Halperin, B. I.; Refael, G.; Demler, E., Resistance in Superconductors. Int. J. Mod. Phys. B 2010, 24, 4039-4080.