# Presentation: 611844

## In principle, high-temperature superconduc-tors can carry enormous electric currents I for use in electric power grids or to generate huge magnetic fields. In practice, I is severely limited by the presence of grain boundaries; their full oxygenation is essential for high I. We have discovered a novel way to determine the degree of oxygenation through a high pressure experiment. In addition, we have studied the properties of single grain boundaries in YBCO bicrystals and find that I increases sharply (+15%/GPa) if the grain boundary is compressed. Both results may help lead to higher I’s in future device applications.

- In principle, high-temperature superconduc-tors can carry enormous electric currents *I* for use in electric power grids or to generate huge magnetic fields. In practice, *I *is severely limited by the presence of grain boundaries; their full oxygenation is essential for high *I*. We have discovered a novel way to determine the degree of oxygenation through a high pressure experiment. In addition, we have studied the properties of single grain boundaries in YBCO bicrystals and find that *I* increases sharply (+15%/GPa) if the grain boundary is compressed. Both results may help lead to higher *I*’s in future device applications.

- Magnetic susceptibility versus temperature for a bicrystalline YBCO ring. Magnetic flux applied at low temperatures begins to enter the ring above 67 K at ambient pressure. Under pressure the ring can carry significantly more current *I*.

**Notes:**
The high-temperature superconducting oxides hold great promise for important technological applications which can affect our everyday lives. The realization of this potential has been hindered by two main factors:  (a) superconducting transition temperatures remain at subambient levels, the highest value at ambient pressure being at -140 C, and (b) the inability of grain boundaries in polycrystalline materials to tolerate large electric currents. Very high currents are required for applications in electric power grids or to generate enormous magnetic fields. We have carried out an extensive series of high-pressure studies of single grain boundaries in bicrystalline rings of superconducting YBCO. As seen in the figure, applying a 10 Oe magnetic field to such a ring at low temperatures leads to a ring current through the grain boundary which keeps all flux out of the ring. At ambient pressure this current is maintained until temperatures above 67 K are reached, whereupon the ring current can no longer be maintained and flux begins to enter the ring. Under 0.6 GPa pressure (6,000 atmospheres!) the ring current can be maintained to a higher temperature near 70 K, indicating that the current carrying capacity  of the ring has been significantly enhanced.

In a separate experiment the pressure was released at low temperatures and the current carrying capacity was found to change with annealing time at ambient temperatures. This relaxation behavior arises from the diffusion of oxygen ions in the grain boundary region. This is only possible if vacant oxygen sites are available. If these empty sites are filled with oxygen ions, the grain boundary will be able to carry higher electric currents. The present technique thus affords a novel method for testing whether the grain boundaries in a given material have been fully oxygenated.

## Enhancing the Current Carrying Capacity of 

**Education:**
- One undergraduate student (Forrest Rogers-Marcovitz) and two graduate students (Takahiro Tomita and Mathew Debessai) contributed to this project.  Forrest graduated in 2006 in the Engineering Department at Washington University with a major in physics. The Ph.D. thesis research of Takahiro is primarily derived from this project; Takahiro received his doctorate degree in December 2005 semester and returned to a postdoctoral position in Kobe, Japan. Mathew has been working in our group for just over one year.
- Our group website:
- _[http://wuphys.wustl.edu/~jss/](http://wuphys.wustl.edu/~jss/)_
- contains full information on the past and current research activities of our group, including the latest preprints, as well as a description of the major research facilities available.

**Societal Impact:**
- Superconducting technologies utilizing the cuprate oxides can potentially have a major impact on everyday life, but many applications cannot yet be realized due that grain boundaries in the cuprates sharply limit their electric current carrying capacity. The present project has developed two novel procedures which may help lead to significant improvements in this regard.

**Enhancing the Current Carrying Capacity of **
**YBCO Superconductors through High Pressure****James S. Schilling, Washington University,**  DMR-0404505

**Notes:**
Basic research on the superconducting and magnetic properties of matter under high pressure conditions affords students an excellent preparation for successful careers in both industry and academia. Former students Andy Cornelius and Craig Looney have tenured professorships at the U of Nevada and Merrimack U, respectively; Stefan Klotz has a permanent position at the U of Paris and is also the Editor-in-Chief of the journal “High Pressure Research”. Jost Diederichs has been one of the most successful development scientists ever hired by the innovative US company Quantum Design. Sascha Sadewasser has a permanent position at the Hahn-Meitner Research Institute in Berlin. Shanti Deemyad and Takahiro Tomita are postdoctoral research associates at the Harvard U and the U of Kobe (Japan), respectively.

Even though it is still necessary to cool the best superconducting materials down to temperatures of -140 C or lower, superconducting technologies utilizing the high-temperature superconductors are beginning to make an impact on some major areas, for example in magnetically levitated trains in Japan or magnet leads for superconducting magnets, now being offered by Quantum Design. Although it is very possible that a room temperature superconductor will someday be discovered, this may not happen for quite some time, perhaps only when a further class of superconducting materials is discovered.