Superconductors: Riding the wave

04/26/2013

The coexistence of superconductivity and a wave-like, charge-ordered phase in iron-doped 1T-TaS2 offers a deeper understanding of high-temperature superconductivity

Tantalum (Ta) atoms organize into a ‘Star of David’ pattern in the charge-density wave (CDW) phase of 1T-TaS2.
Tantalum (Ta) atoms organize into a ‘Star of David’ pattern in the charge-density wave (CDW) phase of 1T-TaS2.

© 2013 Ran Ang

Understanding the origin of superconductivity in high-temperature superconductors remains a major question in physics and is crucial for the development of superconductors that can operate at higher temperatures. However, unraveling the phenomenon is complicated by the fact that electronic properties — such as magnetism — which usually prevent superconductivity, appear to play an important role in the superconducting state at such temperatures. An international team led by Takashi Takahashi and colleagues from the AIMR at Tohoku University has now shown that superconductivity may coexist with these competing states1.

Superconductivity originates from the existence of pairs of electrons that can roam a crystal without losing energy. In conventional superconductors the formation of such electron pairs is facilitated by oscillations of the atoms in the crystal. However, in high-temperature superconductors other factors including magnetism seem to play a role, despite conventional theory determining that magnetism and superconductivity cannot coexist.

The researchers studied 1T-TaS2, a material consisting of layers of the elements tantalum (Ta) and sulfur (S), as well as its iron-doped derivatives. At lower temperatures the tantalum atoms in 1T-TaS2 arrange into a so-called ‘Star of David’ pattern (see image) and exhibit an insulating behavior in which periodic variations in the density of electrons — known as charge-density waves (CDWs) — arise across the material. Typically, superconductivity and the CDW state are considered to be mutually exclusive. Yet the researchers found that when some of the tantalum atoms were replaced with iron, superconductivity could occur.

Takahashi and colleagues used high-resolution angle-resolved photoemission spectroscopy (ARPES) — a technique in which light pushes electrons out of a material — to investigate 1T-TaS2. Measuring the energy and momentum of the ejected electrons provides a direct window into a material’s electronic state. The researchers identified a shallow electron pocket, believed to be characteristic of superconductivity, within 1T-TaS2 in the nearly commensurate CDW (NCCDW) state — a CDW phase that forms at higher temperatures and particular concentrations of iron. “The unique feature of this method is to reveal the electronic states responsible for the CDW state as well as the superconducting state. This would not be possible with other experimental approaches,” explains team member Ran Ang.

The team was able to show that superconductivity and the CDW state are not separated within 1T-TaS2 crystals but instead coexist, representing an intrinsic property and offering valuable insight into the origin of high-temperature superconductivity. In addition, the ability to control the CDW phase — by switching between the material’s normal metallic and insulating states — has potential for application in electronic devices.

References

  1. Ang, R., Tanaka, Y., Ieki, E., Nakayama, K., Sato, T., Li, L. J., Lu, W. J., Sun, Y. P. & Takahashi, T. Real-space coexistence of the melted Mott state and superconductivity in Fe-substituted 1T-TaS2. Physical Review Letters 109, 176403 (2012). | article

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.