Lim Jinhyuk, Kim J S, Hire Ajinkya C, Quan Yundi, Hennig R G, Hirschfeld P J, Hamlin J J, Stewart G R, Olinger Bart
Department of Physics, University of Florida, Gainesville, FL 32611, United States of America.
Los Alamos National Laboratory, Los Alamos, NM 87545, United States of America.
J Phys Condens Matter. 2021 Jun 3;33(28). doi: 10.1088/1361-648X/abeace.
A15 NbSi is, until now, the only 'high' temperature superconductor produced at high pressure (∼110 GPa) that has been successfully brought back to room pressure conditions in a metastable condition. Based on the current great interest in trying to create metastable-at-room-pressure high temperature superconductors produced at high pressure, we have restudied explosively compressed A15 NbSi and its production from tetragonal NbSi. First, diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 NbSi material to traceas a function of pressure.is suppressed to ∼5.2 K at 88 GPa. Then, using these() data for A15 NbSi, pressures up to 92 GPa were applied at room temperature (which increased to 120 GPa at 5 K) on tetragonal NbSi. Measurements of the resistivity gave no indication of any A15 structure production, i.e. no indications of the superconductivity characteristic of A15 NbSi. This is in contrast to the explosive compression (up to∼ 110 GPa) of tetragonal NbSi, which produced 50%-70% A15 material,= 18 K at ambient pressure, in a 1981 Los Alamos National Laboratory experiment. This implies that the accompanying high temperature (1000 °C) caused by explosive compression is necessary to successfully drive the reaction kinetics of the tetragonal → A15 NbSi structural transformation. Our theoretical calculations show that A15 NbSi has an enthalpy vs the tetragonal structure that is 70 meV atomat 100 GPa, while at ambient pressure the tetragonal phase enthalpy is lower than that of the A15 phase by 90 meV atom. The fact that 'annealing' the A15 explosively compressed material at room temperature for 39 years has no effect shows that slow kinetics can stabilize high pressure metastable phases at ambient conditions over long times even for large driving forces of 90 meV atom.
到目前为止，A15 NbSi是唯一一种在高压（约110吉帕）下制备的“高温”超导体，并且已成功在亚稳态下恢复到室温压力条件。基于目前人们对尝试制备在室温压力下呈亚稳态的高压高温超导体的浓厚兴趣，我们重新研究了爆炸压缩的A15 NbSi及其由四方相NbSi的制备过程。首先，对爆炸压缩的A15 NbSi材料进行了高达88吉帕的金刚石对顶砧压力测量，以追踪超导转变温度随压力的变化。在88吉帕时，超导转变温度被抑制到约5.2开尔文。然后，利用这些A15 NbSi的数据，在室温下对四方相NbSi施加高达92吉帕的压力（在5开尔文时增加到120吉帕）。电阻率测量结果未显示有任何A15结构生成，即没有显示出A15 NbSi超导特性的迹象。这与1981年洛斯阿拉莫斯国家实验室的实验中四方相NbSi的爆炸压缩（高达约110吉帕）情况形成对比，那次实验产生了50%-70%的A15材料，在环境压力下超导转变温度为18开尔文。这意味着爆炸压缩所伴随的高温（1000℃）对于成功驱动四方相→A15 NbSi结构转变的反应动力学是必要的。我们的理论计算表明，在100吉帕时，A15 NbSi相对于四方相结构的原子焓为70毫电子伏特，而在环境压力下，四方相的原子焓比A15相低90毫电子伏特。在室温下对爆炸压缩的A15材料进行39年的“退火”处理却没有效果，这一事实表明，即使对于90毫电子伏特/原子的大驱动力，缓慢的动力学过程也能在环境条件下长时间稳定高压亚稳相。
