High Pressure as a Tool to Design & Synthesize new High Tc Superconductor, Hard Materials & Multifunctional Oxides

Resume : Orthovanadates have recently emerged as promising optical materials for birefringent solid-state laser applications [1], cathodoluminescent materials, thermophosphors, or scintillators [2]. At ambient conditions YVO4 crystallizes in zircon-type structure and transforms under pressure into scheelite-type phase. The change of energy band gap with pressure has been investigated theoretically. The fully relativistic calculations have been carried out with exchange-correlation potential in GGA U form (generalized gradient approximation including on-site Coulomb energies). Full-Potential Local-Orbital Minimum-Basis Scheme (FPLO-14) has been employed [3]. The failure of the band gap predictions within the pure GGA comes from not sufficient treatment of the strong correlations. On-site Coulomb potential (GGA U) applied to oxygen O(2p) orbitals provides a crucial insight into the width of the band gap calculated for the zircon-type structure. Also the influence of the potentials U on the remaining V(3d) and Y(4d) valence bands has been studied. The lattice parameters measured under pressure have been employed [4] to calculate evolution of XPS photoemission spectra. [1] W. Ryba-Romanowski, Cryst. Res. Technol. 38, (2003) 225. [2] D. F. Mullica et al., Inorg. Chim. Acta 248 (1996) 85. [3] K. Koepernik, H. Eschrig, Phys. Rev. B 59 (1999) 1743. [4] X. Wang et al., Phys. Rev. B 70 (2004) 064109. Work supported by the National Science Centre (Poland) grant: DEC-2011/01/B/ST3/02212 .

Resume : In strongly correlated electron systems, unusual superconductivity has been studied. Recently, a number of pressure induced superconductors have been found in the vicinity of a ordered phase of a quantum critical point (QCP). In the 4f magnetic ordered systems, the magnetic quantum criticality arises by tuning the competition between the RKKY interaction and the Kondo effect. While the coupling between spin and orbital degrees of freedom is unavoidable in transition metal systems, f-electron systems may provide an ideal nonmagnetic state where orbitals are the only active degree of freedom. One such nonmagnetic state can be found in the cubic 4f2 systems such as Pr based compounds when the crystal electric field (CEF) stabilizes the so-called G3 doublet state with an electric quadrupole moment. The orbital degree of freedom takes the form of the electric quadrupole moment, which may form a variety of electric ground states such as the quadrupolar order through indirect RKKY-type interaction. Its hybridization with a conduction electron may also induce a nonmagnetic form of the Kondo effect that quenches the quadrupolar moment and leads to an anomalous metallic state. Without the disadvantage of introducing disorder, pressure is one of the cleanest tuning parameters. The PrTr2Al20 (Tr=Ti and V), which crystallizes in a cubic structure [1], has demonstrated that the interplay of the quadrupole order and Kondo effect [2]. Here we present the discovery of the pressure-induced heavy fermion superconductivity in PrTi2Al20. The PrTi2Al20 single crystals were grown by the Al-flux method as described in the literature. We have used a cubic anvil and modified Bridgman anvil cell with in-situ pressure tuning in a 3He and dilution refrigerator to the study of the electrical phenomena. Electrical resistivity, AC susceptibility and AC calorimetric were measured on high quality single crystals of PrTi2Al20. A magnetic field was applied to determine the upper critical field HC2, as well as the effect of magnetic field on the under pressure. At ambient pressures we find a superconducting phase below 0.2 K which is suppressed by an extremely small field [2]. We confirm the increase of TQ at low pressure, and the increases of superconductivity over about 6 GPa, with an initially rather small HC2, which increases significantly as TQ starts to decrease, as found in the previous study. We find that at higher pressures the decrease of TQ continues, and is accompanied by a further enhancement of HC2, reaching over 5 Tesla, more than double the theoretical paramagnetic limit. We found that the transition temperature, Tc, and the effective mass, m*, associated with the superconductivity are dramatically enhanced as the system approaches the putative quantum critical point of the orbital order. Our experiment indicates that the strong orbital fluctuations may provide a nonmagnetic glue for Cooper pairing [3]. [1] S. Niemann, W. Jeitschko, J. Sol. State Chem. 114, 337–341 (1995). [2] A. Sakai et al., J. Phys. Soc. Jpn. 81, 083702 (2012). [3] K. Matsubayashi et al., Phys. Rev. Lett 109, 187004 (2012)

Resume : Among transition metal dichalcogenides, IrTe2 is a good example in which superconductivity appears due to suppression of first-order structural phase transition. Here, we have studied nanoscale structure of Ir1-xPtxTe2 to understand structural phase transition and appearance of superconductivity in this system. X-ray absorption measurements reveal Ir-Ir dimerization and appearance of longer Ir-Te bondlengths below the structural phase transition temperature in IrTe2. The local structure also reveals substantial changes as a function of pressure across the structural phase transition showing distinct atomic correlations. The results suggest that the phase transition in in IrTe2 should be an order-disorder like transition of Ir-Ir dimers assisted by Ir-Te bond correlations. X-ray absorption also reveal clear changes in the unoccupied 5d-electronic states and the local geometry with Pt substitution. There is an anomalous spectral weight transfer across the structural phase transition from trigonal to monoclinic, characterizing the reduced atomic structure symmetry. In addition, a gradual increase of the spectral weight transfer is observed in IrTe2, indicating that the low temperature phase is likely to have lower symmetry than the monoclinic. The results suggest that the interplay between inter-layer and intra-layer atomic correlations should have a significant role in the properties of Ir1-xPtxTe2 system.

Resume : Revealing the elementary excitation in the electron strongly correlated systems is the most fundamental means to understand the underlying quantum physics of their novel emergent phenomena, i.e. High T superconductivity. Currently, excitation measurements are typically carried out by angle-resolved photon electron emission spectroscopy (ARPES), scanning tunneling electron microscopy (STEM), or inelastic neutron scattering. However, none of these methods are really compatible with high-pressure conditions, which basically leaves the electron dynamics of the correlated materials at high pressure become a new field in the condensed matter physics to be explored. Hard x-ray resonant inelastic scattering (RIXS) is a new synchrotron method to study the elementary excitations in the electron-correlated-systems, which, mots importantly, is compromised with high-pressure samples environments. Here, we report the first high-pressure RIXS measurements on a high-pressure synthesized singe-crystal Sr3Ir2O7, a newly discovered spin-orbit coupling (SOC) assisted Mott insulator up to 40 GPa through a successful integration of a diamond anvil cell and a resonant inelastic X-ray scattering (RIXS) spectrometer. Our results revealed an interesting evolution of elementary excitation within Ir-5d valence band, such as spin-orbiton excitation, crystal field excitation, magnon excitation, with high-pressure. Moreover, we discovered a new type of excitation that only occurs above 20 GPa in this material. Our results not only demonstrated a first example to directly probe elementary excitation in the d valence band using x-ray at high-pressure, which also promises to a new way to study the electronic structures of electron correlated systems, including cuprates, at high pressure

Resume : Metallic (half-metallic) magnetic clusters embedded in non-magnet semiconductoring matrix provide a potential route toward spin-based devices. The basic advantage of such “hybrid” systems lies in significant remanent magnetization under ambient conditions that is not reached for the classical diluted ferromagnetic semiconductors. However, the control of magnetic characteristics in materials where native phase separations inevitable is still challenging. Here I will review a case of Mn-doped II-IV-V2 semiconductors with general chalcopyrite structure. The experimental and computational results obtained on two central compounds CdGeP2:Mn and ZnGeAs2:Mn will be reported. In contrast to traditional approaches for probing of magnetic semiconductors, I and my co-workers used a high-pressure as tool allowing us to reveal unconventional phase transitions, which caused by phase separated MnP clusters in CdGeP2:Mn. The second part of presentation will be focused on anomalous nature of isothermal magnetization hysteresis and novel pressure-structure-driven magnetoresistance in ZnGeAs2:Mn. Finally, I will show that structural transition in Mn-doped chalcopyrite lattice connected with pressure-induced metallization give rise to qualitative changes in band structure. The accumulated results support the fact that the pressure provides a convenient way to configure the response of cluster systems. This work supported by the RAS Presidium Program P34 “Matter at high pressures”

Resume : We study structural and electronic transitions in layered semiconducting materials MoX2 (X = S, Se, Te) upon increasing pressure, employing ab initio DFT based metadynamics and evolutionary search. In MoS2 we found at 20 GPa a layer sliding structural transition converting the 2Hc stacking into the 2Ha one.[1] This explains earlier X-ray and Raman data and has been recently experimentally confirmed.[2,3] In the same pressure region MoS2 undergoes a semiconductor-semimetal transition [1] as also verified by recent experiments.[3,4] The 2Ha structure remains stable until 130-140 GPa where we predict two competing transformation scenarios - chemical decomposition into MoS + S or transformation into a new metastable metallic structure (P4/mmm) with superconducting Tc=16 K.[5] We discuss the relation of our predictions to the recent experiments.[3,4] In MoSe2 and MoTe2 we instead predict metallization at 28 GPa and 13 GPa but no layer sliding transition.[6] The prediction for MoSe2 was also experimentally verified recently.[7] References: [1] L. Hromadová, R. Martoňák, and E. Tosatti, Phys. Rev. B 87, 144105 (2013) [2] N. Bandaru et al., J. Phys. Chem. C 118, 3230 (2014) [3] Z.-H. Chi et al., Phys. Rev. Lett. 113, 036802 (2014) [4] Z. Chi et al., arXiv:1503.05331 [5] O. Kohulák, R. Martoňák, and E. Tosatti, Phys. Rev. B 91, 144113 (2015) [6] M. Rifliková, R. Martoňák, and E. Tosatti, Phys. Rev. B 90, 035108 (2014) [7] Z. Zhao et al., arXiv:1504.08077, Nat. Comm. in press

Resume : Noble gases (He–Rn) were long considered as unable to form chemical bonds in neutral molecules, a prejudice refuted by the synthesis of ‘XePtF6’ by Neil Bartlett in 1962 [1]. After nearly half a century of superb (and difficult) experimental work, the chemistry of the heavier ‘noble gases’ (Ar – Rn) is well developed [2]. Yet the high-pressure (p > 1 GPa) chemical and physical properties of noble gas compounds are very poorly understood. To our knowledge there is only one experimental study on the high-pressure polymorphism of a genuine (i.e. not van der Waals complex) noble gas compound, XeF2 [3]. It has been claimed that this difluoride undergoes polymerization at 50 GPa, although our theoretical investigations indicate that XeF2 remains molecular up to 100 GPa [4], but undergoes auto-ionization to [XeF]+F– at a higher compression of 200 GPa. Our recent investigation indicate also that other novel noble gas compounds can be synthesized at relatively small pressures (p < 80 GPa). References: 1. Bartlett, N. Proc. Chem. Soc. 1962, 218. 2. W. Grochala, Chem. Soc. Rev. 2007, 36, 1696. 3. M. Kim, M. Debessai, and C.-S. Yoo, Nat. Chem., 2010, 2, 784. 4. D. Kurzydłowski, P. Zaleski-Ejgierd, W. Grochala, and R. Hoffmann, Inorg. Chem., 2011, 50, 3832.

Resume : In this talk I will present some recent results related to surface morphology of carbonates, as well as their behaviors under water and pressure effects. The hydrous phases are of considerable interest for their role as precursors to stable carbonate minerals. I will present a detailed recent results concerning structural and energetic stability of dry and hydrous surfaces of calcium carbonate polymorphs using two recently developed forcefields. In particular, I will show the mechanical characteristics of dry forms of calcium carbonate and hydrous phases. In addition I will talk about the stability of (001) and surfaces of monohydro-calcite, and the (001) surface of ikaite. At the end I will show and discuss the computed morphology pictures obtained from MD simulation and compared to observed SEM images. In addition further recent studies will be shown regarding the interface between clay and carbonate surfaces under anisotropic constrains as well as shear stress. Various mechanical properties will be presented with and without the presence of the water at the interface.

Resume : Cation ordering in transition-metal oxides often drastically modifies their physical and chemical properties. We here focus on some ordered perovskite-structure oxides, in which transition-metal ions are included at the A and B sites in perovskite ABO3 in ordered manners like AA’3B4O12 and AA’3B2B’2O12. In such compounds A-A, A-B, and B-B interactions compete and/or cooperate with each other and play important roles in giving rise to wide variety of functional properties. Note that many of this structure type compounds are stabilized under high pressure conditions. Recently, we succeeded in obtaining lots of new AA’3B4O12 and AA’3B2B’2O12 oxides by high-presure synthesis. Some of the compounds show intriguing properties. CaCu3Fe4O12 and LaCu3Fe4O12 contain unusually high valence states of iron, and instabilities of the high oxidation states at low temperatures are relieved by charge disproportionation in CaCu3Fe4O12 and by intersite charge transfer in LaCu3Fe4O12. The transitions give significant changes in the properties, and the intersite charge transfer in LaCu3Fe4O12 accomanies a large negative thermal expansion like behavior. The newly discovered A-and-B-site-ordered quadruple perovskite CaCu3Fe2Re2O12 shows ferrimagnetism with an unusually high magnetic transition temperature and large magnetization. More importantly, the electronic structure of CaCu3Fe2Re2O12 is half metallic, and the compound shows large magnetoresistance by conduction of spin-polarized electrons.