Metallic Hydrogen Superconductor

Superconductors are materials that permit electricity to travel freely, without resistance, so they could dramatically improve the efficiency of power transmission technologies. Metallic hydrogen should be just such a superconductor.

Liquid metallic hydrogen is thought to exist in the high-gravity interiors of Jupiter and Saturn.
Scientists have predicted that electricity would flow, uninhibited, through a material made by compressing hydrogen into a metal. But so far, on Earth, researchers have been unable to use such compression techniques to squeeze hydrogen under high enough pressures to convert it into a metal. University at Buffalo chemists have now proposed an alternative solution for metallizing hydrogen by adding sodium to hydrogen which they think might make it possible to convert the compound into a superconducting metal under significantly lower pressures.

NaH₉, which does not occur naturally on Earth but is expected to be a stable compound, is predicted to become metallic at an experimentally achievable pressure of about 250 gigapascals, about 2.5 million times Earth's standard atmospheric pressure, but less than the pressure at Earth's core which is about 3.5 million atmospheres.

Reference
University at Buffalo (2011, June 13). Under pressure, sodium, hydrogen could undergo a metamorphosis, emerging as superconductor. ScienceDaily. Retrieved June 15, 2011, from http://www.sciencedaily.com­ /releases/2011/06/110613162240.htm

Further Reading
Metals and Non-metals
Kinetic Theory of Gases

Study Questions

1. Draw up a table listing the properties of metals and non-metals.
2. In what ways is elemental hydrogen like a non-metal?
3. In what ways is elemental hydrogen like a metal?
4. Use the Kinetic Theory of Gases to explain what you expect to happen as elemental hydrogen at atmospheric pressure is subjected to increasing pressure.
5. Using the Kinetic Theory of Gases, describe two ways that scientists could, in theory, make solid hydrogen.
6. Using the Kinetic Theory of Gases, explain why hydrogen might exist as a liquid in the interior of the planet Jupiter.
   
7. If 250 gigapascals is about 2.5 million times Earth's standard atmospheric pressure, what does the prefix "giga" stand for?
   
8. Why do you think that chemists suggest adding sodium to hydrogen to create a solid material capable of conducting electricity?
   

Heavy Fermions

Scientists are interested in studying heavy fermion behaviour because it could lead to the design of new materials for high temperature super-conductors.

Cornell University Scientists imaging the electronic properties of a material composed of uranium, ruthenium and silicon, have found that the effects of heavy fermions begin to appear as the material is cooled below 55K, and, an even more unusual electronic phase transition occurs below 17.5K.

This phase transition was studied using spectroscopic imaging scanning tunneling microscopy (SI-STM) which measures the wavelength of electrons on the surface of the material in relation to their energy. From the wavelength and energy measurements scientists calculated the effective electron mass and found that these electrons were either very heavy, or, that they were acting like very heavy electrons because they were being slowed down. This suggests that these electrons are interacting with the uranium atoms, that is, acting as particles rather than acting as a wave.

Reference:
A. R. Schmidt, M. H. Hamidian, P. Wahl, F. Meier, A. V. Balatsky, J. D. Garrett, T. J. Williams, G. M. Luke & J. C. Davis. Imaging the Fano lattice to 'hidden order' transition in URu₂Si₂. Nature, 2010; DOI: 10.1038/nature09073

Study Questions

1/ What is a fermion?

2/ What is a super-conductor?

3/ What could high temperature superconductors be used for?

4/ What is the atomic symbol for:

• uranium
• ruthenium
• silicon

5/ To which group of the Periodic Table do each of the following elements belong?

• uranium
• ruthenium
• silicon

6/ Convert the following temperatures in Kelvin to ^oC.

• 55K
• 17.5K

7/ What is the relationship between mass, energy and wavelength that would allow Scientists to calculate the effective mass of an electron?

8/ Why would electrons appear to be heavier if they are slowed down?

Iron and Superconductors

About 100 years ago, scientists discovered materials that could conduct electrons without losing energy to resistance, but, these "superconductors" had to be very cold. The electron-electron repulsion in these low-temperature superconductors was so weak that electrons could overcome it, pair up and move freely.

In 1986, scientists discovered new materials that became superconductors at temperatures above 100K. These high-temperature superconductors were made of layers of copper alloys sandwiched between layers of nonconducting material that were doped with trace amounts of material that could contribute a few extra electrons to the mix. If these materials were not doped with insulating material they did not conduct electricity as the electrons locked themselves at a distance from their neighbours. This locked pattern was named the "Mott localization".

In 2008 a second class of high-temperature superconductors was discovered. These pnictides are iron-based superconductors which are also layered and need to be doped. However, undoped pnictides are not Mott insulators.

Early in 2010, scientists replaced arsenic atoms in one of the intervening layers of a pnictide with slightly smaller phosphorous atoms. This brought the iron atoms a little closer together and further away from the Mott tipping point.

Rice University researchers are now using iron oxychalcogenides which are layered materials like pnictides, but with greater distance between the iron atoms, and this greater distance is enough to push the system into a Mott insulating state.

A better understanding of the behaviour of high-temperature superconductors is essential to future improvements in electric generators, MRI scanners, high-speed trains and other devices.

Reference:
Jian-Xin Zhu, Rong Yu, Hangdong Wang, Liang L. Zhao, M. D. Jones, Jianhui Dai, Elihu Abrahams, E. Morosan, Minghu Fang, and Qimiao Si. Band Narrowing and Mott Localization in Iron Oxychalcogenides La₂O₂Fe₂O(Se,S)₂. Physical Review Letters, 2010; 104 (21): 216405 DOI: 10.1103/PhysRevLett.104.216405