Observation of superconductivity in Fe honeycomb lattice - Drs. Wenge Yang and Yonggang Wang

MAY 16, 2018

After continuous insistence on the pressure-driven spin-crossover in transition metal chalcogenides, a group of scientists co-led by Drs. Wenge Yang and Yonggang Wang of HPSTAR, firstly observed the emergence of superconductivity in Fe-based honeycomb lattice under compression. Before this, all the iron-based high-Tc superconductors structurally adopt FeSe-type square Fe lattice. The discovery provides a state-of-art demonstration for the pursuit of transition metal based superconductors under extreme conditions. This work is published by Nature Communications.

The discovery of Fe-based superconductors (FeSCs), with the highest transition temperature (Tc) up to 55 K, has attracted worldwide research efforts since the discovery of 26 K SC in LaO1-xFxFeAs in 2008. Similar to the high-Tc cuprate family adopting square CuO2 layers, all of the FeSCs structurally possess infinite antifluorite-like Fe2X2 layers (X denotes a pnictide or a chalcogenide element). Therefore, it’s widely believed that the common Fe2X2 layer is the essential building unit for the rational design of FeSCs, and SC can be generated by either chemical doping or external pressure.

In the pursuit of abrupt pressure-driven spin-crossover (SCO) in transition metal chalcogenides, we became aware of the essential role of metal-center geometry (dimensionality of the transition metal sublattice) in the synergetic behavior of SCO under high pressure. In MnX (X = S, Se) with a three-dimensional structure, we found that the pressure-driven SCO occurred gradually within a wide pressure range (~20 GPa), although the accompanying pressure-driven large volume collapse and Mott transition were believed to assist the occurrence of SCO (Angew. Chem. Int. Ed., 2016, 55, 10350). The structural phase transition of MnX under compression is from NaCl-type to MnP-type. When move to the two-dimensional MnPX3 (X = S, Se) system with a honeycomb Mn lattice, we realized a “cooperative” pressure-driven SCO phenomenon as expected (J. Am. Chem. Soc., 2016, 138, 15751). The close communication between Mn2+ centers within the honeycomb layer results in an in-plane lattice collapse, a semiconductor-to-metal transition and also a sharp pressure-driven SCO simultaneously. The phase transition of MnPX3 under compression is quasi-iso-structural.

From MnPX3 to FePX3, is there any difference under high pressure? “Of cause, they will.” said Dr. Wenge Yang, “The low spin Mn2+ (d5) is of S = 1/2, while, the low spin state of Fe2+ (d6) is of S = 0. By pressure-driven SCO, we are able to make such d6 transition metal chalcogenides nonmagnetic. And they are possible to be superconducting!” Just as what they expected, along with the cooperative pressure-driven SCO, superconductivity was observed at ~5 K in the high-pressure, nonmagnetic phase of FePSe3. The authors used in situ powder/single-crystal XRD, X-ray emission spectra and X-ray absorption spectra to probe the details of the structural and electronic changes during the phenomenon as possible as they can. “The SC originated from a parent antiferromagnetic material has strong similarities with the unconventional superconductors such as high-Tc cuprate SCs, FeSCs and heavy Fermion SCs” said Dr. Yonggang Wang, the leading author of this work. “Maybe one can explore a lot more new nonmagnetic, superconducting phases via pressure-driven SCO along this trend. Now I am working on other pressure-driven zero spin systems.”

Caption: Tetragonal Fe lattice in FeSe-type superconductors and honeycomb lattice in FePSe3.

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搜狐网：http://www.sohu.com/a/231754423_199523

知社学术圈：http://www.yidianzixun.com/article/0J3nxIbo

含磁性过渡金属离子的超导和超导材料一直是物理、化学和材料领域研究的热点。以近十年来发现的高温铁基超导体为例，几乎都具有FeSe型的四方或类四方层状结构基元，人们也普遍认为这样的铁离子的格子构型是产生超导的前提。在由北京高压科学研究中心的王永刚和杨文革研究员负责的一项研究中，研究者们首次在高压相FePSe3的蜂窝状类六方铁格子中发现超导。伴随着等结构相变和绝缘体金属相变，FePX3 (X = S, Se)在高压作用下发生Fe2+离子自旋量子数从S = 2到S = 0的高低自旋转变，继而在无磁性的高压相FePSe3中观察到转变温度约为5 K的超导现象。这是人们首次在具有蜂窝状类六方铁格子的硫族化合物中发现超导。压力诱导的自旋交叉为新型含磁性离子超导化合物的探索提供了一种有效的途径。该研究结果发表在《自然-通讯》。