Parallel session A: High pressure synthesis & characterization of functional materials

Resume : The layered transition-metal dichalcogenides (TMDs) 1T-TaS2 and 1T-TaSe2 exhibits several interesting phases including various charge density wave (CDW), Mott insulating state, and superconductivity (SC) when subjected to the external pressure. Until now, these pressure phase diagrams are not universal because they rely on various factors such as electronic correlations. To understand the nature of various phase diagrams, the electrical transport and ac susceptibility of 1T-TaSe2-xSx were investigated in a cubic anvil pressure apparatus up to 15 GPa. The universal superconducting phase diagram was revealed on the border of CDWs. The application of high hydrostatic pressure suppresses CDWs more sensitively and thoroughly with the critical pressures Pc(x) ~ 4.70-6.55 GPa. Pressure-induced superconducting state coexists with various CDWs, and then bulk SC emerges along with the complete collapse of various CDWs. Meanwhile, the superconducting transition temperature increases monotonously up to ~ 7.3 K at 15 GPa without a dome-like shape. The results clarify that the superconducting cooper-pairing can be associated with CDWs instability near Pc(x)[1, 2]. References [1] B. S. Wang, et al., Phys. Rev. B. (rapid) 95, 220501(R) (2017). [2] B. S. Wang, et al., Phys. Rev. B. (rapid) (2018) (in press).

Resume : One of the common features of unconventional superconducting systems such as the heavy-fermions, high transition-temperature cuprates and iron pnictides is that the superconductivity emerges in the vicinity of long-range antiferromagnetically ordered state. In this talk I will present the discovery of ferromagnetism on the verge of the superconducting dome in NdFeAsOF through the application of external high pressure. The ambient pressure superconductivity at $T_{c} \sim$ 45.4\,K is fully suppressed at $P_{c} \sim$ 21 GPa. Upon further increase of the pressure, the ferromagnetism associated with the order of rare-earth subsystem is induced at the border of superconductivity. We also show that the temperature evolution of the electrical resistivity as a function of pressure is consistent with a crossover from a Fermi-liquid to non-Fermi-liquid to Fermi-liquid. These results give access to the high-pressure side of the superconducting phase diagram in 1111 type of materials.

Resume : Recently, a novel Jeff=3/2 state has been theoretically suggested in a series of ‘lacunar spinel’ compounds, GaT4X8 (T=Nb and Ta, X=S, Se, and Te) [1]. And later it has been experimentally confirmed for the case of GaTa4Se8 by using resonant inelastic x-ray scattering (RIXS) [2]. Now the key remaining question is about the fate of this magnetic state under pressure since this material exhibits the insulator-to-metal transition followed by superconducting transition as a function of pressure. Here we report and discuss our recent first-principles calculation result along with RIXS data. We found that the Jeff=3/2 moment is well maintained in the metallic region and presumably also in the superconducting phase. Our result suggests an interesting new playground to study the intriguing material phase in which Jeff=3/2 moment is residing in a metallic and superconducting phase. Reference [1] H. –S. Kim et al., Nature Comm. (2014) [2] M. Y. Jeong et al., Nature Comm. (2017)

Resume : Despite numerous studies on three-dimensional topological insulators (3D TIs), the controlled growth of high quality (bulk-insulating and high mobility) TIs remains a challenging subject. This study investigates the role of growth methods on the synthesis of single crystal stoichiometric BiSbTeSe2 (BSTS). Three types of BSTS samples are prepared using three different methods, namely melting growth (MG), Bridgman growth (BG) and two-step melting-Bridgman growth (MBG). Our results show that the crystal quality of the BSTS depend strongly on the growth method. Crystal structure and composition analyses suggest a better homogeneity and highly-ordered crystal structure in BSTS grown by MBG method. This correlates well to sample electrical transport properties, where a substantial improvement in surface mobility is observed in MBG BSTS devices. The enhancement in crystal quality and mobility allow the observation of well-developed quantum Hall effect at low magnetic field.

Resume : It has been recognized that high-energy X-ray or neutron diffraction is a powerful tool for studying crystalline, disordered and nano materials 1-7. Much wide reciprocal space can be probed by using high-energy X-ray or neutron beam. As a result, the atomic pair distribution function (PDF) can be directly obtained by Fourier transformation. Although the pair distribution function, G(r), is simply another representation of the diffraction data, real space exploration of the data has advantages especially in the case of materials with significant structural disorder 2,4. The total scattering, including Bragg peaks as well as diffuse scattering, contributes to the PDF, and is particularly useful for characterizing aperiodic distortions in crystals 4. It is promising for high-pressure sciences to use the PDF method to characterize the structural variation in short, intermediate and long-range orders under extreme conditions of high pressure and temperature 7-9. However, existing high-pressure PDF studies have been limited to low pressures (< 10GPa) 7,8, because it is very difficult to obtain decent X-ray total scattering data under high pressure by using a large unfocused beam, making PDF analysis at pressures higher than a pressure of 10 GPa not yet possible. Recently, we have succeeded in focusing high-energy X-rays down to a size of 10 micron 10,11. In this presentation, we present our recent progress of pair distribution function (PDF) determination under extreme conditions ususing diamond anvil cell and the high energy X-ray focusing abeam. Some examples of the compressibility properties of nanoparticles, such as n-Au and n-Pt, in the diamond anvil cell under quasi-hydrostatic conditions at high pressure will be presented. References : 1. T. Egami and S. J. L. Billinge, Pergamon, Oxford (1994). 2. J. L. Billinge Simon, in Zeitschrift für Kristallographie/International journal for structural, physical, and chemical aspects of crystalline materials (2004), Vol. 219, pp. 117. 3. T. Proffen, R. G. DiFrances co, S. J. L. Billinge, E. L. Bros ha and G. H. Kwei, Phys ical Review B 60 (14), 9973-9977 (1999). 4. S. J. L. Billinge and M. G. Kanatzidis , Chemical Communications (7), 749-760 (2004). 5. S. J. L. Billinge and I. Levin, Science 316 (5824), 561-565 (2007). 6. B. Gilbert, F. Huang, H. Zhang, G. A. Waychunas and J. F. Banfield, Science 305 (5684), 651-654 (2004). 7. C. D. Martin, S. M. Antao, P. J. Chupas , P. L. Lee, S. D. Shas tri and J. B. Paris e, Applied Phys ics Letters 86 (6), - (2005). 8. L. Ehm, L. A. Borkows ki, J. B. Paris e, S. Ghos e and Z. Chen, Applied Phys ics Letters 98 (2), - (2011). 9. X. Hong, L. Ehm and T. S. Duffy, Applied Phys ics Letters 105 (8), 081904 (2014). 10. X. Hong, L. Ehm, Z. Zhong, S. Ghos e, T. S. Duffy and D. J. Weidner, Scientific Report s 6, 21434 (2016). 11. H. Xinguo, S. D. Thomas , E. Lars and J. W. Donald, Journal of Phys ics : Condens ed Matter 27 (48), 485303 (2015).

Resume : Krypton is the only known noble gas apart from xenon to form compounds that are isolable in macroscopic amounts.[1,2] However, in all of them krypton adopts the 2 oxidation state, in contrast to xenon which forms compounds with an oxidation state as high as 8. Motivated by the prospects of obtaining novel compounds through reaction conducted at high pressure,[3–5] we present here theoretical investigations into the reactivity of Kr and F2 up to 200 GPa. In particular we focus on krypton tetrafluoride, KrF4, which in its most stable form is a molecular crystal in which krypton forms short covalent bonds with neighboring fluorine atoms thus adopting the 4 oxidation state. We find that this hitherto unknown compound can be stabilized at pressures below 50 GPa.[6] Our results indicate also that above 50 GPa a multitude of other krypton fluorides should be stable, among them KrF which exhibits covalent Kr–Kr bonds reminiscent of Xe-Xe bonds found in XeF.[7] Our results set the stage for future high-pressure synthesis of novel krypton compounds. [1] J. F. Lehmann, H. P. A. Mercier and G. J. Schrobilgen, Coord. Chem. Rev., 2002, 233–234, 1. [2] M. Lozinšek and G. J. Schrobilgen, Nat. Chem., 2016, 8, 732. [3] E. Zurek and W. Grochala, Phys. Chem. Chem. Phys., 2015, 17, 2917. [4] X. Li, A. Hermann, F. Peng, J. Lv, Y. Wang, H. Wang and Y. Ma, Sci. Rep., 2015, 5, 16675. [5] J. Botana, X. Wang, C. Hou, D. Yan, H. Lin, Y. Ma and M. Miao, Angew. Chemie Int. Ed., 2015, 54, 9280. [6] D. Kurzydłowski, M. Sołtysiak, A. Dżoleva and P. Zaleski-Ejgierd, Crystals, 2017, 7, 329. [7] F. Peng, J. Botana, Y. Wang, Y. Ma and M. Miao, J. Phys. Chem. Lett., 2016, 4562.

Resume : Hydrogen is recognized as a potential and extremely interesting energy carrier [1], which can facilitate efficient utilization of unevenly distributed renewable energy. Furthermore, hydrogen has also an extremely interesting chemistry and form compounds with most elements in the periodic table and with a variety of different types of bonds [2]. Here we report selected recent investigations using high hydrogen pressures generated using a metal hydride hydrogen compressor. This equipment was implemented at several synchrotron facilities for in situ powder X-ray diffraction studies of hydrogen release and uptake reactions [3]. We present hydrogen uptake properties of composites hydrides of lithium or sodium closo-boranes-hydride composites [4]. Metal borohydrides investigated under pressure can have extraordinary properties. Praseodymium borohydride appear as the first example of a compound with step wise negative thermal expansion [5]. [1] Ley, et al, Mater. Today, 2014, 17(3), 122. [2] M. Paskevicius, Chem. Soc. Rev. 2017, 46, 1565 [3] B. R. S. Hansen, et al, J. Appl. Cryst., 2015, 48, 1234 [4] S. R. H. Jensen, et al, 2018 submitted. [5] S.H. P. G.Doust, et al, Dalton Trans, 2018 Accepted.

Resume : Boron based materials have been proposed as good hydrogen storage materials. Boron nitride nanotubes (BNNTs) are not an exception due to their superior mechanical, optical and electronic properties with the potential of hydrogen absorption at room temperature. Despite the structural similarity, the physical properties of BNNTs are different from the ones of CNTs in many aspects. In fact, tunable wide band gaps (~5 eV), high oxidation resistance up to 1,100 oC, fast piezoelectricity properties, and large excitonic effect proposed BNNTs better candidates for the optical, mechanical electrical devices. The most significant feature of BNNTs is the strong adsorption properties of hydrogen due to the ionic character of B-N bond which may induce an extra dipole moment. In spite of the many interesting properties and structural similarities with CNTs, however, fundamental information about BNNTs including transport and energy storage properties is very little known. Samples of BN multi-walled nanotubes (BNMWNT) were synthesized chemical reactions of boron and nitrogen [7], and transport properties were compared to the ones of hexagonal boron-nitride (h-BN). To evaluate the structural stabilities and the amount of hydrogen storage, samples were pressurized using a Mao-type diamond anvil cell under neon and hydrogen environments, respectively. High-pressure X-ray diffraction (XRD) data were measured at HP-CAT beamline of Advanced Photon Source and PAL-5A beamline of Pohang Synchrotron Light Source. Our combined data of high-pressure XRD and Raman measurements revealed that BNNTs maintained the tubular structure to the maximum of 9.8 GPa but completed destroyed at higher pressure and at releasing to the ambient condition. XRD data measured from h-BN demonstrated that the main peaks corresponding to the structure disappear at 19.7 GPa, but recovered upon releasing the pressure. This demonstrates that the h-BN is more sustainable than the tubular shape of BN. Results of structural stabilities, electric conductivities and hydrogen storage properties under high pressure of hydrogen will be presented.

Resume : Distinctive superconducting behaviors between bulk and monolayer FeSe make it challenging to obtain a unified picture of all FeSe-based superconductors. Here, we investigate the ultrafast quasiparticle dynamics of an intercalated superconductor (Li1-xFex)OHFe1-ySe, which is a bulk crystal but shares a similar electronic structure with single-layer FeSe on SrTiO3. We obtain the electron-phonon coupling (EPC) constant λ (0.24 ± 0.03), which well bridges that of bulk FeSe crystal and single-layer FeSe/SrTiO3 [1]. Moreover, we find that such a positive correlation between λ and superconducting Tc holds among all known FeSe-based superconductors, even in line with reported FeAs-based superconductors. Our observation indicates possible universal role of EPC in the superconductivity of all known categories of iron-based superconductors, which is a critical step towards achieving a unified superconducting mechanism for all iron-based superconductors. References: [1] Y. C. Tian, W. H. Zhang, F. S. Li, Y. L. Wu, Q. Wu, F. Sun, G. Y. Zhou, L. L. Wang, X. C. Ma, Q. K. Xue, Jimin Zhao, Ultrafast dynamics evidence of high temperature superconductivity in single unit cell FeSe on SrTiO3. Phys. Rev. Lett. 116, 107001 (2016).