High-pressure effects on the iron-free layered pnictide, BaPd2As2 – and pulsed field experiment in Ba2Ti2Fe2As2O4 up to 60T done in Wuhan - Dr. Mahmoud Abdel-Hafiez

APRIL19, 2018

The compound BaPd2As2 represents the end member of the Pd-doped series and exists in the form of two types of crystal structures: ThCr2Si2-structure type (I4/mmm) and CeMg2Si2-structure type (P4/mmm). The former structure has bulk superconductivity, while in the latter structure only filamentary superconductivity was observed below 2 K. This shows that the crystal structure has a predominant effect on the superconducting properties of these systems. A new study led by Dr. Mahmoud Abdel-Hafiez together with Drs. Yongsheng Zhao and Wenge Yang from HPSTAR, reports for the first time on the high pressure effects on the iron-free layered pnictide superconductor. A combination of X-ray diffraction, specific heat, transport, and magnetization were used to determine the structure and explain the suppression of superconductivity of this compound with increasing pressure. This study is published in Physical Review B 97, 134508 (2018).

On the other hand, a new iron-based oxypnictide superconductor, Ba2Ti2Fe2As4O (Ba22241), was discovered and subjected to an effective self-doping property. This offers an alternative and efficient route for inducing superconductivity, instead of achieving it mechanically or chemically via element substitution. The self-doping stems from an interlayer electronic interaction since this compound contains not only the same Fe2As2 layers as in other Fe-based superconductors but also another conducting Ti2O sheet, which makes it very distinctive. A new study led by Dr. Mahmoud Abdel-Hafiez represnts for the first time pulsed field experiment done in Wuhan up to 60T. This study is published in Physical Review B 97, 115152 (2018).

While the layered 122 iron arsenide superconductors are highly anisotropic, unconventional, and exhibit several forms of electronic orders that coexist or compete with superconductivity in different regions of their phase diagrams, we find in the absence of iron in the structure that the superconducting characteristics of the end member BaPd2As2 are surprisingly conventional. Such behavior differs significantly from all layered superconductors previously known, especially from the unconventional iron-based superconductors with various competing or coexisting electronic orders.

We have determined the Hc2(T ) phase diagram for Ba22241 from electrical-transport measurements in magnetic fields up to 60 T aligned both within the ab plane and along the c axis. This study led by Mahmoud Abdel-Hafiez of HPSTAR and was in collaboration with universtities of Moscow, Liege and Frannkfurt.

Through extreme conditions such as high pressure and high magnetic filed experiments, many fundamental problems in condensed matter physics will be solved. These are the centeral aims of our research group, said Dr. Abdel-Hafiez.

Caption: Up: Left panel: Temperature dependence ofmagnetic moment at different values of applied pressure. The inset shows the P-T phase diagram which illustrates the decrease of Tc with the applied pressure. Right panels: The 3D Fermi surfaces in theBrillouin zone under pressures of 0 and 5GPa. BaPd₂As₂ shows a multiband Fermi surface. Below: Phase diagram of Hc2 vs temperature of Ba22241 up to 60 T for the field applied parallel and perpendicular to c. Tc has been estimated through the onset values as explained in the text. Solid symbols (from the PPMS) and open symbols (from the pulsed magnet) represent Hc2 for H // ab in blue and Hc2 for H // c in red.

122型铁基超导体通常表现出很高的超导各向异性，非传统的超导电性以及一些奇异的电磁耦合性质。北京高压科学研究中心的Mahmoud Abdel-Hafiez 研究员发现对于不含铁的122型超导体BaPd₂As₂却表现出传统超导体所具有的各向同性的超导性质。他们进一步的高压晶体结构测量表明BaPd₂As₂在高压下的超导转变温度变现出线性压力抑制现象，这与传统超导体在高压下的电，磁性能的变相一致。另外该研究团队对新型超导材料Ba₂Ti₂Fe₂As₄O (Ba22241)进行了高磁场下的电输运测量确定了其上临界场相图。