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High-Pressure Effects on Isotropic Superconductivity in the Iron-Free Layered Pnictide Superconductor Bapd2as2

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High-Pressure Effects on Isotropic Superconductivity in the Iron-Free Layered Pnictide Superconductor Bapd2as2 CORE Metadata, citation and similar papers at core.ac.uk Provided by Kyutacar : Kyushu Institute of Technology Academic Repository High-pressure effects on isotropic superconductivity in the iron-free layered pnictide superconductor BaPd2As2 著者 Abdel-Hafiez M., Zhao Y., Huang Z., Cho C.-w., Wong C. H., Hassen A., Ohkuma M., Fang Y.-W., Pan B.-J., Ren Z.-A., Sadakov A., Usoltsev A., Pudalov V., Mito M., Lortz R., Krellner C., Yang W. journal or Physical Review B publication title volume 97 number 13 page range 134508-1-134508-9 year 2018-04-12 URL http://hdl.handle.net/10228/00007071 doi: info:doi/10.1103/PhysRevB.97.134508 PHYSICAL REVIEW B 97, 134508 (2018) High-pressure effects on isotropic superconductivity in the iron-free layered pnictide superconductor BaPd2As2 M. Abdel-Hafiez,1,2,3,* Y. Zhao,4 Z. Huang,5 C.-w. Cho,5 C. H. Wong,6 A. Hassen,7 M. Ohkuma,8 Y.-W. Fang,9 B.-J. Pan,10 Z.-A. Ren,10 A. Sadakov,11 A. Usoltsev,11 V. Pudalov,11 M. Mito,8 R. Lortz,5 C. Krellner,2 and W. Yang4,12 1Center for High Pressure Science and Technology Advanced Research, Beijing, 100094, China 2Institute of Physics, Goethe University Frankfurt, 60438 Frankfurt/M, Germany 3National University of Science and Technology “MISiS,” Moscow 119049, Russia 4Center for High Pressure Science and Technology Advanced Research, Shanghai, 201203, China 5Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 6Institute of Physics and Technology, Ural Federal University, Yekaterinburg, Russia 7Faculty of Science, Physics Department, Fayoum University, 63514-Fayoum, Egypt 8Graduate School of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan 9Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241, China 10Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China 11P.N. Lebedev Physical Institute, Russian Academy of Sciences, 119991 Moscow, Russia 12High Pressure Synergetic Consortium (HPSynC), Carnegie Institution of Washington, Argonne, Illinois, 60439, USA (Received 17 November 2017; revised manuscript received 26 March 2018; published 12 April 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. Here we report on complementary measurements of specific heat, mag- netic susceptibility, resistivity measurements, Andreev spectroscopy, and synchrotron high pressure x-ray diffrac- tion measurements supplemented with theoretical calculations for BaPd2As2. Its superconducting properties are completely isotropic as demonstrated by the critical fields, which do not depend on the direction of the applied field. Under the application of high pressure, Tc is linearly suppressed, which is the typical behavior of classical phonon- mediated superconductors with some additional effect of a pressure-induced decrease in the electronic density of states and the electron-phonon coupling parameters. Structural changes in the layered BaPd2As2 have been studied by means of angle-dispersive diffraction in a diamond-anvil cell. At 12 GPa and 24.2 GPa we observed pressure induced lattice distortions manifesting as the discontinuity and, hence discontinuity in the Birch-Murnaghan equation of state. The bulk modulus is B0 = 40(6) GPa below 12 GPa and B0 = 142(3) GPa below 27.2 GPa. DOI: 10.1103/PhysRevB.97.134508 I. INTRODUCTION of parallel spins are adjacent to chains with opposite spin direction [9]. In addition to this SDW order a spontaneous In conventional superconductors, the electron-phonon in- breaking of rotational symmetry (nematic order) from a teraction gives rise to the attraction between electrons near high-temperature tetragonal (C4)symmetrytoalowtemper- the Fermi surface with opposite momenta and opposite spins, ature orthorhombic structure occurs [9]. The nematic order which eventually causes superconductivity and conservation of is most likely due to an electronic instability that causes the time-reversal symmetry [1]. The discovery of superconduc- pronounced in-pane anisotropy of the Fe-As layers [10]. Upon tivity up to 55 K in iron-based pnictides has been at the forefront doping, both forms of static orders coexist with supercon- of interest in the scientific community over the last few years ductivity in a wide range of their phase diagram, and it is [2–5]. These materials have multiple Fermi pockets with elec- still unclear whether they are beneficial or competing with tronlike and holelike dispersion of carriers. For the BaFe2As2 superconductivity. The compound BaPd2As2 in the focus of (BFA) parent compound, the electron doping is induced by this paper represents the end member of the Pd-doped series substitution of Ni, Co, Rh, and Pd at the Fe sites, which have and exists in the form of two types of crystal structures: more d electrons than Fe [6–8]. The parent compound BFA ThCr2Si2-structure type (I4/mmm) and CeMg2Si2-structure itself is not superconducting but features two forms of highly type (P 4/mmm)[11,12]. The former structure has bulk su- correlated electronic orders. One is the antiferromagnetic spin perconductivity with Tc = 3.85 K, while in the latter struc- density wave (SDW) of stripe-type order in which chains ture 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 *mahmoudhafi[email protected] [13–15]. Given that Fe likely has a leading effect on the high 2469-9950/2018/97(13)/134508(9) 134508-1 ©2018 American Physical Society M. ABDEL-HAFIEZ et al. PHYSICAL REVIEW B 97, 134508 (2018) temperature superconductivity in doped BFA, the effect of used for 4-probe measurements. The DC magnetization was partial substitution of Fe on superconducting properties in measured in the Quantum Design SQUID VSM magnetometer general and on their anisotropy evolution remains unaddressed. with magnetic fields applied parallel and perpendicular to The heavily hole-doped compounds differ from their optimally the ab plane. For the parallel field configuration the sample doped counterparts by the presence of particularly strong was attached with vacuum grease to a quartz sample holder, electronic correlations [16], which is seen in their substantially which allowed us to rotate the sample at room temperature in larger reported mass enhancements compared to those of the the grease so that different in-plane field orientations could optimally doped compounds. For instance, it is well established be realized. AC magnetic susceptibility measurements were that CaFe2As2 is less correlated than KFe2As2 at ambient pres- performed with an AC field at a frequency of 10 Hz and sure [17,18]. Therefore, it is necessary to deeper investigate amplitude of 3.86 Oe over the pressure range of 0–5 GPa in a the evolution of physical properties of the BFA system under commercial SQUID magnetometer. Pressure was attained by various doping, including end members of the 122 family with a miniature diamond anvil cell (DAC) which was designed fully substituted Ba, Fe, and As. to be inserted into the SQUID magnetometer [20]. In the In particular, due to the lack of systematic experiments, a sample chamber, the crystals were immersed into a pressure comprehensive and complete picture on the iron-free layered transmitting medium, Apiezon J oil, together with a piece of pnictide superconductor BaPd2As2 is still lacking. Here, we ruby as the manometer. Pressure calibration was performed performed thermodynamic, transport, magnetotransport, local using the ruby fluorescence method at room temperature [19]. spectroscopy measurements, and first principle calculations in Multiple Andreev reflection effect (MARE) spectroscopy of order to unravel the nature of the superconductivity in the superconductor-normal metal-superconductor (SnS) junctions BaPd2As2 system. Specifically, we examined the electronic was performed using the break-junction technique [21,22]. properties of BaPd2As2 single crystal using a combination MARE occurs in the ballistic regime of SnS junctions and of specific heat (down to 400 mK), magnetic susceptibility, causes excess current at low bias voltages (so-called foot) and resistivity measurements (down to 400 mK and under pres- a subharmonic gap structure. This structure consists of a series sure up to 11 GPa), Andreev reflection spectroscopy (down of dips in dynamic conductance, each dip corresponding to = to 1.4 K), and first-principles calculations. We found that voltage values Vn 2/en, where is the superconducting e n = BaPd2As2 as the end member of the substituted series of gap, is the elementary charge, and 1, 2,...is the sub- harmonic order. First-principles calculations are performed BaFe2−x Pdx As2 appears to behave surprisingly different from with plane-wave density functional theory (DFT) implemented the parent compound BaFe2As2. While highly unconventional superconductivity can be induced by doping or application of in Quantum Espresso [23]. The in situ high-pressure x-ray = ˚ pressure in BaFe2As2, the end member BaPd2As2 in which diffraction(λ 0.4066 A) measurement was performed with all iron
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