Engineering topics

Zhang, Yunwei

Publications and source records attributed to Zhang, Yunwei.

2 records

DOE OSTI2023

Exchange field enhanced upper critical field of the superconductivity in compressed antiferromagnetic EuTe2

Abstract Understanding the interplay between superconductivity and magnetism has been a longstanding challenge in condensed matter physics. Here we report high pressure studies on the C -type antiferromagnetic semiconductor EuTe 2 up to 36.0 GPa. A structural transition from the I4/mcm to the C2/m space group is identified at ~16 GPa. Superconductivity is observed above ~5 GPa in both structures. In the low-pressure phase, magnetoresistance measurements reveal strong couplings between the local moments of Eu 2+ and the conduction electrons of Te 5 p orbits. The upper critical field of superconductivity is well above the Pauli limit. While EuTe 2 becomes nonmagnetic in the high-pressure phase and the upper critical field drops below the Pauli limit. Our results demonstrate that the high upper critical field of EuTe 2 in the low-pressure phase is due to the exchange field compensation effect of Eu 2+ and the superconductivity in both structures may arise in the framework of the Bardeen-Cooper-Schrieffer theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Candidate structure for the H 2 -PRE phase of solid hydrogen

Experimental progress finally reached the metallic solid hydrogen phase, which was predicted by Wigner and Huntington over 80 years ago. However, the different structures in the phase diagram are still being debated due to the difficulty of diffraction experiments for high-pressured hydrogen. The determination of crystal structures under extreme condition is both of the basic condensed matter physics, and in planetary science: the behavior of giant gaseous planets (e.g., Jupiter and Saturn) strongly depends on the properties of inner high-pressured hydrogen. This work describes possible structures appearing under high pressures of 400~600 GPa. We applied a structural search using particle swarm optimization with density functional theory (DFT) to propose several candidate structures. For these structures, we performed fixed-node diffusion Monte Carlo simulations combined with DFT zero-point energy corrections to confirm their relative stability. We found P2 1 /c-8 as a promising candidate structure for the H 2 -PRE phase. P2 1 /c-8 is predicted the most stable at 400 and 500 GPa. P2 1 /c-8 reproduces qualitatively the IR spectrum peaks observed in the H 2 -PRE phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗