# Synopsis: At last, single crystals

#### Anisotropic thermodynamic and transport properties of single-crystalline Ba1−xKxFe2As2 (x=0 and 0.45)

N. Ni, S. L. Bud’ko, A. Kreyssig, S. Nandi, G. E. Rustan, A. I. Goldman, S. Gupta, J. D. Corbett, A. Kracher, and P. C. Canfield

Published July 10, 2008

The discovery of superconductivity in iron arsenide compounds has stimulated intense research with new and important results appearing almost daily. Until very recently, all of the reports have been on polycrystalline samples. With single crystals, however, it is possible to avoid disorder and make measurements along definite crystallographic directions, which is essential in these materials because they are highly anisotropic.

In the July 10th issue of Physical Review B, Ni Ni, Paul Canfield and colleagues at Ames Laboratory and Iowa Sate University report the growth of the first single crystals of the parent (non-superconducting) compound $\text{Ba}{\text{Fe}}_{2}{\text{As}}_{2}$, as well as of the potassium-doped superconductor ${\text{Ba}}_{1-x}{\text{K}}_{x}{\text{Fe}}_{2}{\text{As}}_{2}$ (so far studied only in polycrystalline form [1,2]). The group presents the details of the growth process and the thermodynamic and transport properties of the single crystals. They verify that $\text{Ba}{\text{Fe}}_{2}{\text{As}}_{2}$ has similar structural and magnetic properties as $\text{LaFeAsO}$, the first iron arsenide material found to be superconducting upon doping with fluorine. However, because $\text{Ba}{\text{Fe}}_{2}{\text{As}}_{2}$ is an intermetallic compound (rather than an oxide), it is much easier to synthesize in single-crystal form.

It was expected that in compounds such as $\text{LaFeAsO}$—just as in the cuprates—oxygen would play an important role for high-temperature superconductivity. However, non-oxide compounds such as $\text{Ba}{\text{Fe}}_{2}{\text{As}}_{2}$ prove that oxygen is not an essential ingredient—a realization that could have far-reaching consequences. - Alexios Klironomos

[1] M. Rotter, M. Tegel, D. Johrendt, I. Schellenberg, W. Hermes, and R. Poettgen, Phys. Rev. B. 78, 020503 (2008).

[2] M. Rotter, M. Tegel, and D. Johrendt, arXiv:0805.4630 (unpublished).