Anomalous compression in CrAs - Dr. Lin Wang

NOVEMBER 17, 2015

Using synchrotron X-ray diffraction combined with theoretical modeling, a team led by scientists from HPSTAR,  Dr. Zhenhai Yu, Dr. Qingyang Hu, Dr. ChunyuLi, and Dr. Lin Wang performed detailed structure study on CrAs, an important bulk superconductivity material, at high pressures. They observed anomalous anisotropy with a pressure-induced isostructural transition. This crystal change agrees well with the conditions at which bulk CrAs becoming superconducting. The results shed light on the structural and related electronic responses to high pressure, which play a key role toward understanding the superconductivity of CrAs. Their findings are published in recent PNAS (“Anomalous anisotropic compression behavior ofsuperconducting CrAs under high pressure,” doi:10.1073/pnas.1520570112).

Superconductivity has been observed in a majority of 3d transition-metal compounds, except for the Cr- and Mn-based compounds. However, a recent discovery shows superconductivity in CrAs under external pressures (Wei Wu, etal. Nat. Commun. 5, 5508, 2014), which remains metallic state even down to 350 mK. This result again demonstrates that external pressure is an effective and unique approach to tune crystal as well electronic structure of materials.

At ambient conditions, CrAs possess aparamagnetic metallic and the electrons are in itinerant status. It has been shown to exhibit a transition from the antiferromagnetic state to a (low Tc) superconducting state under high pressures and the antiferromagnetic state coexisted with superconducting state between 0.3 ~ 0.8 GPa. The superconductivity appears when the itinerant state occurred around ~ 2 K. The applied pressure shortens the atomic distance and consequently resulted in the expansion of energy band and Fermi surface crossing, thus caused the enhancement of itinerancy.

Diamond-anvil-cell is utilized to achieve the high-pressure conditions in this study. Through a detailed analysis of pressure-dependent (up to 1.8 GPa) synchrotron x-ray diffraction data measured of CrAs, the researchers found (111) peak shows an abrupt change during~0.18 - 0.35 GPa. Additionally, b axis expands first and then shrinks with pressure increasing. This structure evolution was attributed to an isostructural phase transition.This agrees with the pressures, at which CrAs turned to be superconductor. Furthermore, they used theoretical simulations to find possible reason for the connection between the anomalous compression behavior and superconducting in CrAs at the same pressure conditions.

Caption: Anomalous varing of  unit cell parameters a, b,c, and volume of CrAs at high pressures.