Structure and Metallicity of Phase V of Hydrogen - Drs. Philip Dalladay-Simpson and Ross T. Howie

JUNE 18, 2018

A new study from a team of scientists, which comprised of HPSTAR’s Philip Dalladay-Simpson and Ross T. Howie, proposed a promising candidate crystal structure for their recently discovered phase of hydrogen, phase V. Most poignantly the work,  which was recently published Physical Review Letters,  provides further evidence that phase V is indeed a stepping stone towards hydrogen’s elusive metallic form.

Metallic hydrogen is a longstanding topic in the field of condensed matter of physics since its prediction over 80 years ago. Since, this state of matter has been suggested to exhibit exotic  dissapationless states of matter at high temperatures, such as superconductivity and superfluidity.  However, in these regimes, complete characterisation of hydrogen is difficult as experimentalists become diagnostically poor, previously only finding Raman spectroscopy as a suitable probe. Fortunately, hydrogen has exceedingly rich vibrational properties and in this study, through the interplay of new theoretical techniques and experimental observation it is possible to ascertain the first structural knowledge of phase V.

In this study, the team used a newly developed saddle-point ab initio random structure searching methods (sp-AIRSS), to find candidate model crystal structures for phase V, reporting three new   structures which suitably describe the characteristics for phase V. Interestingly, the study proposed that the structure with the longer bond lengths and the smallest band gap, as the most promising model structure for hydrogen phase V. Noteworthy, as both these properties reflect the continuous metallisation and onset of dissociation in the hydrogen system, corroborating the previous experimental results on phase V.

“It is exciting that our newly discovered phase V, a phase exhibiting marked changes, has finally found a working model”, said Dr. Dalladay Simpson, one of the co-authors and a staff scientist at HPSTAR.  “This study is integral to answering the longstanding standing questions regarding the emergence of metallic hydrogen in a condensed form.”

2016年北京高压科学研究中心研究员Ross Howie 所在的研究团队率先在金属氢的研究中取得突破性发现，在380 万大气压的条件下，观测到了氢的又一新相——这一新相可能是人们长期寻找的金属相的前身。由于在此极端条件下，样品尺寸太小，无法通过实验的手段进行定量测量，比如第五相的晶体，电子结构。因此Howie研究团队借助理论计算—saddle-point 从头算的方法来寻找适合第五相的结构。通过晶体结构，电子结构，及金属化的计算，他们发现小的禁带宽度与长的键长结构相匹配，从而最终给出了第五相最理想的晶体结构。