Spin density wave rather than tetragonal structure is prerequisite for superconductivity in La3Ni2O7-δ.

The majority of interactions in solids strongly depend on the interatomic distances. The application of pressure changes the lattice parameters and modifies the electronic and the phononic energy spectra of a material avoiding some of the undesirable effects induced by chemical doping, like lattice disorder, impurity phases and phase separations. Layered materials are particularly affected by the application of pressure because their anisotropic structure eases the contraction of the lattice along specific crystallographic directions. In this thesis I study the effect of pressure on the transport properties of two new layered materials: L4Fe2As2Te(1-x)O4 (L = Pr, Sm, Gd) superconductors and beta-Bi4I4 topological insulator. The discovery of Fe-based superconductors (Fe-SCs) provided a new material base for studying the mechanism of high temperature superconductivity. These materials present unexpectedly high critical temperatures (Tc ) despite the presence of magnetic atoms. The parent compounds of Fe-SCs are semimetals with an antiferromagnetic ground state in which the itinerant electrons form a periodic modulation of spin density called spin density wave (SDW). By changing the structural properties or by adding/removing carriers the SDW order can be suppressed and superconductivity emerges. Our group recently succeeded in synthesizing a new family of Fe-SCs that presents Tc up to 45 K upon optimum chemical doping and substitution. I performed a systematic study of the upper critical field and the pressure-dependent electrical resistivity of single crystals of Pr4Fe2As2Te(1-x)O4 and Sm4Fe2As2Te(1-x)O(4-y)Fy . Hall coefficient and magnetoresistance of Pr4Fe2As2Te(1-x)O4 revealed electrons as the dominant type of charge carriers. The results can be successfully fitted by a two-band model that allows the estimation of the temperature dependent electron and hole densities and mobilities. In view of the exceptionally high structural anisotropy of this new class of Fe-SCs I investigated the electronic anisotropy of Sm4Fe2As2Te(1-x)O(4-y)Fy by means of microfabrication techniques. The results show a ratio between the out-of-plane and in-plane electrical resistivities that is the highest reported so far in Fe-SCs. Topological insulators (TIs) form another class of materials that is currently attracting a lot of attention both for fundamental physics and potential applications. In TIs metallic surface states coexist with an insulating bulk. Most of the TIs known so far are either three-dimensional strongly bonded bulk materials or layered van der Waals crystals. Beta-Bi4I4 is a material composed of quasi-one-dimensional molecular chains that was theoretically predicted, and only recently experimentally verified, to be a novel strong topological insulator. Due to its particular structure, beta-Bi4I4 offers the possibility to study the interplay between different ground states like topological insulating phase, charge-density-wave and superconductivity. Here I present, for the first time, a study of the thermoelectric properties of beta-Bi4I4 single crystals. Electrical resistivity, Seebeck coefficient, thermal conductivity and Hall coefficient measurements reveal a possible charge density-wave-order (CDW) at low temperature. According to resistivity and thermoelectric power measurements, pressure suppresses the CDW order. Resistivity measurements performed in a diamond anvil cell up to 20 GPa demonstrate that superconductivity is induced in beta-Bi4I4 for pressure above 10 GPa. Preliminary X-ray diffraction measurements by synchrotron radiation, performed on the sample recovered at ambient pressure after the experiment, reveal that a new Bi-I phase is formed at high pressure. The possibility that the topological insulator state could coexist with superconductivity in beta-Bi4I4 is certainly a fascinating option but it remains for now an open question.

Flow accelerated corrosion of X65 steel gradual contraction pipe in high CO2 partial pressure environments

Global warming due to the increased atmospheric carbon dioxide concentration is the driving force for developing strategies that exploit CO2 as raw material to produce interesting compounds for industry. According to this approach, Acetobacterium woodii was modified to convert CO2 and H2 into acetone. Gas fermentation was performed at high pressure to debottleneck the issue of the low availability of gaseous substrates in the liquid medium. This work aimed to investigate the catalytic performance of a modified A. woodii strain for acetone synthesis at 10 bar providing an H2-CO2 blend. First, tests were performed to assess the ability of the biocatalyst to survive heterotrophically at high pressure. Moreover, a reference test was set up in autotrophy at atmospheric pressure to confirm that it produced both acetate and acetone. Feeding the strain at 10 bar with the H2-CO2 mix resulted in growth inhibition and formic acid production. This outcome suggested a metabolism impairment due to bicarbonate build-up in the reactor at high CO2 partial pressure. Thus, bacteria were grown at atmospheric pressure in a medium with an augmented exogenous salt concentration. Results confirmed that formic acid production and growth inhibition could be due to HCO3–. Furthermore, the modified A. woodii grown at atmospheric pressure in a sterile medium pressurized before inoculation showed the same outcomes. Finally, tests at 10 bar lowering the CO2 partial pressure indicated that this gas was responsible for formic acid production but was not the only inhibitory factor for autotrophic cell growth at high pressure.

PurposeThe purpose of this work is to design the wire beam electrode (WBE) of P110 steel and study its corrosion behavior and mechanism under high temperature and pressure.Design/methodology/approachPackaging materials of the new type P110 steel WBE and high pressure stable WBE structure were designed. A metallurgical microscope (XJP-3C) and scanning electron microscopy (EV0 MA15 Zeiss) with an energy dispersive spectrometer were used to analyze the microstructure and composition of the P110 steel. The electrochemical workstation (CS310, CorrTest Instrument Co., Ltd) with a WBE potential and current scanner was used to analyze the corrosion mechanism of P110 steel.FindingsAccording to the analysis of Nyquist plots at different temperatures, the corrosion resistance of P110 steel decreases with the increase of temperature under atmospheric pressure. In addition, Rp of P110 steel under high pressure is maintained in the range of 200 ∼ 375 Ωcm2, while that under atmospheric pressure is maintained in the range of 20 ∼ 160 Ωcm2, indicating that the corrosion products on P110 steel under high pressure is denser, which improves the corrosion resistance of P110 steel to a certain extent.Originality/valueThe WBE applied in high temperature and pressure environment is in blank. This work designed and prepared a WBE of P110 steel for high temperature and pressure environment, and the corrosion mechanism of P110 steel was revealed by using the designed WBE.

The discovery of high-temperature superconductivity (SC) with Tc ≈ 80 K in the pressurized La3Ni2O7 has aroused great interests. Currently, due to technical difficulties, most experiments on La3Ni2O7 can only be performed at ambient pressure (AP). Particularly, various experiments have revealed the presence of spin density wave (SDW) in the unidirectional diagonal double-stripe pattern with wave vector near (π/2, π/2) in La3Ni2O7 at AP. In this work, we employ first-principle calculations followed by the random phase approximation (RPA)-based study to clarify the origin of this special SDW pattern and the potential SC in La3Ni2O7 at AP. Starting from our density-functional-theory band structure, we construct an eight-band bilayer tight-binding model using the Ni-3dz2 and 3dx2−y2 orbitals, which is equipped with the standard multi-orbital Hubbard interaction. Our RPA calculation reveals an SDW order driven by Fermi-surface nesting with wave vector Q ≈ (0, 0.84π) in the folded Brillouin zone (BZ). From the view of the unfolded BZ, the wave vector turns to Q0 ≈ (0.42π, 0.42π), which is near the one detected by various experiments. Further more, this SDW exhibits an interlayer antiferromagnetic order with a unidirectional diagonal double-stripe pattern, consistent with recent soft X-ray scattering experiment. This result suggests that the origin of the SDW order in La3Ni2O7 at AP can be well understood in the itinerant picture as driven by Fermi surfaces nesting. In the aspect of SC, our RPA study yields an approximate s±-wave spin-singlet pairing with Tc much lower than that under high pressure. Further more, the Tc can be strongly enhanced through hole doping, leading to possible high-temperature SC at AP.

All in an engineer’s life

The upper critical fields Hc2 of polycrystalline samples of LnOFBiS2 (Ln = La, Nd) at ambient pressure (tetragonal structure) and high pressure (HP) (monoclinic structure) have been investigated via electrical resistivity measurements at various magnetic fields up to 8.5 T. The Hc2(T) curves for all the samples show an uncharacteristic concave upward curvature at temperatures below Tc, which cannot be described by the conventional one-band Werthamer–Helfand–Hohenberg theory. For the LaOFBiS2 sample under HP, as temperature is decreased, the upper critical field Honset, estimated from the onset of the superconducting transitions, increases slowly between 4.9 and 5.8 T compared with the slope of Honset(T) below 4.9 T and above 5.8 T. This anomalous behavior reveals a remarkable similarity in superconductivity between LaOFBiS2 samples measured under HP and synthesized under HP, although the crystal structures of the two samples were reported to be different. A reasonable explanation is that local atomic environment, which can be tuned by applying external pressure, is essential to the enhancement of Tc for BiS2-based superconductors. On the other hand, such anomalous behavior is very subtle in the case of NdOFBiS2 under HP, suggesting that the anisotropy of the upper critical field in the ab-plane and the possible lattice deformation induced by external pressure is weak. This explains why the pressure-induced enhancement of Tc for NdOFBiS2 is not as large as that for LaOFBiS2.

New Cage-Like Cerium Trihydride Stabilized at Ambient Conditions

Crystal structure and phase transitions at high pressures in the superconductor FeSe0.89S0.11

High-pressure experiments of argon hydrate and methane hydrate were performed using a diamond anvil cell in a pressure range of 0.2 to 10.0GPa at room temperature. In-situ X-ray diffractometry and optical microscopy revealed that two high pressure structures of argon hydrate, a primitive tetragonal structure and a body-centered orthorhombic structure, existed under pressures of up to 6.5GPa. The structural analysis showed that the tetragonal structure was composed solely of 14-hedra accommodating two argon atoms, and that the body-centered orthorhombic structure belonged to a“filled-ice”structure, i.e., a new type of structure in a water-guest system. As for methane hydrate, three high-pressure structures, a hexagonal structure, a primitive orthorhombic structure, and a body-centered orthorhombic structure, were found. The structural analysis indicated that the hexagonal structure was a modified structure of a hexagonal one reported at ambient pressure, and that the latter orthorhombic structure was the“filled-ice”structure.

High-pressure phase transition in brucite, Mg(OH)₂

Density wave (DW) order is believed to be correlated with superconductivity in the recently discovered high-temperature superconductor La3Ni2O7. However, experimental investigations of its evolution under high pressure are still lacking. Here, we explore the quasiparticle dynamics in bilayer nickelate La3Ni2O7 single crystals using ultrafast optical pump-probe spectroscopy under high pressures up to 34.2 GPa. At ambient pressure, the temperature-dependent relaxation dynamics demonstrate a phonon bottleneck effect due to the opening of an energy gap around 151 K. The energy scale of the DW-like gap is determined to be 66 meV by the Rothwarf-Taylor model. Combined with recent experiential results, we propose that this DW-like transition at ambient pressure and low temperature is spin density wave (SDW). With increasing pressure, this SDW order is significantly suppressed up to 13.3 GPa before it completely disappears around 26 GPa. Remarkably, at pressures above 29.4 GPa, we observe the emergence of another DW-like order with a transition temperature of approximately 135 K, which is probably related to the predicted charge density wave (CDW) order. Our study provides the experimental evidences of the evolution of the DW-like gap under high pressure, offering critical insights into the correlation between DW order and superconductivity in La3Ni2O7.

It is challenging to design electro-optic (EO) crystals with both a large bandgap and a high EO coefficient. We propose that some semiconductors synthesized under a high pressure environment can present both a large bandgap and a high linear EO coefficient because of the enhanced strength of the valence bond. The electronic band structures and linear EO coefficients of the $II\mathrm{Si}{\mathrm{N}}_{2}$ ($II=\mathrm{Mg}$, Sr, Ba) compounds are studied using first-principles calculation. Our calculated results predict that both $\mathrm{SrSi}{\mathrm{N}}_{2}$ and $\mathrm{BaSi}{\mathrm{N}}_{2}$, under a high-pressure condition, will crystalize into an orthorhombic structure with the same space group of $\mathrm{MgSi}{\mathrm{N}}_{2}$ under ambient pressure. The calculated bandgaps are 5.48, 4.41, and 3.57 eV, respectively, for $\mathrm{MgSi}{\mathrm{N}}_{2}, \mathrm{SrSi}{\mathrm{N}}_{2}$, and $\mathrm{BaSi}{\mathrm{N}}_{2}$, while the calculated linear EO coefficients are 1.55, 12.09, and 21.60 pm/V at a fiber communication wavelength of 1550 nm. The increasing linear EO coefficients can be interpreted by the enhanced bond strength and a reduced bandgap. This work indicates that it is feasible to enhance the linear EO coefficient with high pressure.

High pressure Raman spectroscopic study of BaFCl

This thesis presents the numerical simulation of fluid dynamics, as well as heat and mass transfer for drop impingement on a hot solid surface for low and high ambient pressures. The technical application ranges from effective thermal management strategies using spray cooling, safety aspects in high pressure nuclear reactors to process technology in chemical or food industry. It is reported in literature that wetting characteristics depend on the ambient pressure. Drop splash is suppressed at low ambient pressure. High ambient pressure encourages compressibility effects. The compressibility of both the liquid and vapour phase increases with increasing pressure. Thereby, the effects of compressibility on drop impingement is of interest. Up to now, no attempt has been made to investigate a full pressure range for the evaporative drop impingement process. In order to provide insights into evaporative drop impingement processes under various ambient pressures, numerical simulations are performed. CFD simulations are conducted using a finite volume discretisation method solving the Navier-Stokes equations. The volume of fluid method is utilised to resolve two-phase flow. The solver accounts for compressible fluid flow, heat and mass transfer due to evaporation across the free liquid-vapour interface, evaporation in the vicinity of the three-phase contact line, as well as for heat conduction within the solid substrate. The dynamic contact angle is implemented using a subscale model. Effects of low and high ambient pressure on the three-phase contact line are investigated in the well established so-called micro region model. The focus is the non-splashing drop-wall collision in a non-boiling, single-component evaporation regime. Ambient pressure ratios ranging between p/pcr = [8*10^{-3} ... 0.5], Reynolds and Weber numbers ranging between Re = [600 ... 1300] and We = [10 ... 50] are investigated. The wall temperature is above saturation but below Leidenfrost temperature. The wall superheat is in the order of 10 K. Different parameter studies are dedicated to investigate the influence of low and high ambient pressures on the evaporative drop impact processes. Within one parameter study, dimensional drop impact parameters are kept constant, such as drop diameter, impact velocity and wall superheat. Caused by the variation in ambient pressure, material properties of the fluid change. Consequently, non-dimensional groups are changing, indicating a shift in dominant forces. Another parameter study keeps non-dimensional groups constant. Further parameter studies focus on the influence of the vapour phase on the drop impact outcome, especially for high ambient pressure. Within this work, results are presented for different length scales. The modelling of the vicinity of an evaporating three-phase contact line indicates a strong influence of the ambient pressure on the apparent contact angle and the heat being transferred in the micro region. For increasing pressure, the contact angle increases whereas the transferred heat has a local maximum within the investigated pressure range. For the macro-scale drop impingement process, strong influence on the fluid dynamics and heat transfer is identified. In summary, numerical simulations of the evaporative drop impact and the modelling of micro-scale thermodynamic effects for low and high ambient pressure are investigated in the present thesis. The results increase the understanding of the influence of pressure on the fluid dynamics, as well as the heat and mass transfer. Correlations for the maximum spreading ratio, spreading duration, as well as transferred energy and mass are reported. The findings are expected to improve design concepts for technical applications within the investigated parameter range.