Function Of External Pressure Research Articles

Understanding External Pressure Effects and Interlayer Orbital Exchange Pathways in the Two-Dimensional Magnet─Chromium Triiodide

Recent experiments have shown that exfoliated few-layer CrI3, a prototypical van der Waals magnet, undergoes a phase transition from the high-temperature monoclinic structure to the low-temperature rhombohedral structure under pressure. To understand how the magnetism of CrI3 responds to these structural changes, we perform ab initio density functional theory simulations on bilayer CrI3. We simulate the interlayer lateral shift-dependent potential energy surface of bilayer CrI3 to examine the stability and magnetism as a function of external pressure. Using the hybrid PBE0 functional, we are able to give qualitatively correct exchange coupling energies, without using an on-site Coulomb interaction correction. Thus, we avoid using tunable parameters. The results show that under external pressure, the monoclinic crystal structure is destabilized in comparison with the rhombohedral structure, in agreement with the observed phase transition in few-layer CrI3 devices under pressure. We also look into the microscopic origins of the interlayer exchange coupling. We identify the competing orbital pathways that favor ferromagnetic and antiferromagnetic kinetic exchange, respectively, which are consistent with previous reports. This study opens a new direction of using hybrid functionals with Gaussian orbitals and a cluster-based approach for obtaining Heisenberg J values to accurately simulate the magnetic properties of 2D materials.

Fluorides of Silver Under Large Compression*.

The silver-fluorine phase diagram has been scrutinized as a function of external pressure using theoretical methods. Our results indicate that two novel stoichiometries containing Ag+ and Ag2+ cations (Ag3 F4 and Ag2 F3 ) are thermodynamically stable at ambient and low pressure. Both are computed to be magnetic semiconductors under ambient pressure conditions. For Ag2 F5 , containing both Ag2+ and Ag3+ , we find that strong 1D antiferromagnetic coupling is retained throughout the pressure-induced phase transition sequence up to 65 GPa. Our calculations show that throughout the entire pressure range of their stability the mixed-valence fluorides preserve a finite band gap at the Fermi level. We also confirm the possibility of synthesizing AgF4 as a paramagnetic compound at high pressure. Our results indicate that this compound is metallic in its thermodynamic stability region. Finally, we present general considerations on the thermodynamic stability of mixed-valence compounds of silver at high pressure.

Improvement in Sensing Characteristics of Silicon Microstructure based MEMS Capacitive Sensor for Automotive Applications

Capacitive pressure sensors have become a reasonable choice due to their low power consumption, energy efficiency, and robustness. In this paper, a thorough investigation of MEMS based capacitive pressure sensor with square diaphragm has been carried out. A comparative study of the diaphragm displacement and capacitance as a function of external pressure and temperature has been done by using various diaphragm materials such as Si, Poly Si, Si3N4 and 3C-SiC. The performance analysis of the device was envisaged with and without packaging stress by measuring various parameters. The simulation results are emphasized on the change in capacitance at different die bonding temperatures. Temperature dependence of capacitance at varying ambient pressure has also been realized. The present work will facilitate researchers for choosing a selective material for automotive applications.

THE EFFECTS OF PRESSURE AND TEMPERATURE ON THE MAGNETIZATION OF A PARABOLIC QUANTUM DOT IN A MAGNETIC FIELD

In this work , we present a theoretical study of the magnetization of two-electron GaAs parabolic quantum dot (QD) under the combined effects of external pressure, temperature and magnetic field. First, we obtain the eigenenergies by solving the two electron quantum dot Hamiltonian using the exact diagonalization method. The obtained results show that the energy levels of the quantum dot depend strongly on the pressure and temperature. Next, we investigate the dependence of magnetization of a quantum dot as a function of external pressure, temperature, confining frequency and magnetic field. The singlet-triplet transitions in the ground state of the quantum dot spectra and the corresponding jumps in the magnetization curves have been shown .The comparison shows that our results are in very good agreement with the reported works. PACS: 73.21.La, 65.80.-g Keywords: Pressure; temperature; magnetization; quantum dot; magnetic field; exact diagonalization.

The Effects of Pressure and Temperature on the Magnetic Susceptibility of Semiconductor Quantum Dot in A Magnetic Field

In this work, we present a theoretical study of the magnetic susceptibility (x of two-electron GaAs parabolic quantum dot (QD) under the combined effects of external pressure, temperature and magnetic field. We used the exact diagonalization method to obtain the eigenenergies by solving the two electron quantum dot Hamiltonian taking into account the dependence of the effective mass and dielectric constant on the hydrostatic pressure and temperature. The pressure and temperature show significant effects on the calculated QD spectra. Next, we investigate the behavior of the magnetization of a quantum dot as a function of external pressure, temperature, confining frequency and magnetic field. The singlet-triplet transitions in the ground state of the quantum dot spectra and the corresponding jumps in the magnetic susceptibility spectra have been shown. The comparison shows that our results are in very good agreement with the reported works.

Influence of Tow Architecture on Compaction and Nesting in Textile Preforms

Transverse compression response of tows during processes such as vacuum infusion or autoclave curing has significant influence on resin permeability in fabrics as well as the laminate thickness, fibre volume fraction and tow orientations in the finished composite. This paper reports macro –scale deformations in dry fibre assemblies due to transverse compaction. In this study, influence of weave geometry and the presence of interlacements or stitches on the ply-level compaction as well as nesting have been investigated. 2D woven fabrics with a variety of interlacement patterns - plain, twill and sateen- as well as stitched Non-crimp (NCF) fabrics have been investigated for macro-level deformations. Compression response of single layer and multilayer stacks has been studied as a function of external pressure in order to establish nesting behaviour. It appears that the degree of individual ply compaction and degree of nesting between the plies are influenced by tow architectures. Inter-tow spacing and stitching thread thickness appears to influence the degree of nesting in non-crimp fabrics.

Comment on “Pressure-induced structural and valence transition in AgO” by C. Hou, J. Botana, X. Zhang, X. Wang and M. Miao, Phys. Chem. Chem. Phys., 2016, 18, 15322

The recent article by Hou et al. has focused on a theoretical study of mixed valence compound AgO in order to elucidate the nature of the electronic structure of this system as a function of external pressure. The authors claim that '…the effects of pressure on the Ag valence state are not investigated yet.' This statement is incorrect in view of the theoretical study published in 2015 by Włodarska et al., which answers most of the questions posed by these authors, including the key one: '…does the Ag cation exhibit similar behaviour to the Au cation in M2Au2X6halides, experiencing mixed-valence to single-valence transition under pressure? If so, what is the mechanism?'.

Pressure-dependent calibration of the OH and HO<sub>2</sub> channels of a FAGE HO<sub>x</sub> instrument using the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC)

Abstract. The calibration of field instruments used to measure concentrations of OH and HO2 worldwide has traditionally relied on a single method utilising the photolysis of water vapour in air in a flow tube at atmospheric pressure. Here the calibration of two FAGE (fluorescence assay by gaseous expansion) apparatuses designed for HOx (OH and HO2) measurements have been investigated as a function of external pressure using two different laser systems. The conventional method of generating known concentrations of HOx from H2O vapour photolysis in a turbulent flow tube impinging just outside the FAGE sample inlet has been used to study instrument sensitivity as a function of internal fluorescence cell pressure (1.8–3.8 mbar). An increase in the calibration constants CHO and CHO2 with pressure was observed, and an empirical linear regression of the data was used to describe the trends, with ΔCHO = (17 ± 11) % and ΔCHO2 = (31.6 ± 4.4)% increase per millibar air (uncertainties quoted to 2σ). Presented here are the first direct measurements of the FAGE calibration constants as a function of external pressure (440–1000 mbar) in a controlled environment using the University of Leeds HIRAC chamber (Highly Instrumented Reactor for Atmospheric Chemistry). Two methods were used: the temporal decay of hydrocarbons for calibration of OH, and the kinetics of the second-order recombination of HO2 for HO2 calibrations. Over comparable conditions for the FAGE cell, the two alternative methods are in good agreement with the conventional method, with the average ratio of calibration factors (conventional : alternative) across the entire pressure range, COH(conv)/COH(alt) = 1.19 ± 0.26 and CHO2(conv)/CHO2(alt) = 0.96 ± 0.18 (2σ). These alternative calibration methods currently have comparable systematic uncertainties to the conventional method: ~ 28% and ~ 41% for the alternative OH and HO2 calibration methods respectively compared to 35% for the H2O vapour photolysis method; ways in which these can be reduced in the future are discussed. The good agreement between the very different methods of calibration leads to increased confidence in HOx field measurements and particularly in aircraft-based HOx measurements, where there are substantial variations in external pressure, and assumptions are made regarding loss rates on inlets as a function of pressure.

Pressure and temperature dependence of the zero-field splitting in the ground state of NV centers in diamond: A first-principles study

Nitrogen-vacancy centers in diamond (NV) attract great attention because they serve as a tool in many important applications. The NV center has a polarizable spin $S=1$ ground state and its spin state can be addressed by optically detected magnetic resonance (ODMR) techniques. The ${m}_{S}=0$ and ${m}_{S}=\ifmmode\pm\else\textpm\fi{}1$ spin levels of the ground state are separated by about 2.88 GHz in the absence of an external magnetic field or any other perturbations. This zero-field splitting (ZFS) can be probed by ODMR. As this splitting changes as a function of pressure and temperature, the NV center might be employed as a sensor operating at the nanoscale. Therefore, it is of high importance to understand the intricate details of the pressure and temperature dependence of this splitting. Here we present an ab initio theory of the ZFS of the NV center as a function of external pressure and temperature including detailed analysis on the contributions of macroscopic and microscopic effects. We found that the pressure dependence is governed by the change in the distance between spins as a consequence of the global compression and the additional local structural relaxation. The local structural relaxation contributes to the change of ZFS with the same magnitude as the global compression. In the case of temperature dependence of ZFS, we investigated the effect of macroscopic thermal expansion as well as the consequent change of the microscopic equilibrium positions. We could conclude that theses effects are responsible for about 15% of the observed decrease of ZFS.

Stability, electronic, magnetic and pressure effect of half-Heusler alloys CNaCa and SiNaCa: A first-principles study

We investigate the electronic and magnetic properties of CNaCa and SiNaCa by using full-potential local-orbital minimum-basis method. The calculations reveal the both alloys are half-metallic ferromagnets (HMF) with the same magnetic moment of 1.00μB per formula unit, the magnetic moment mainly originates from the strong spin-polarization of p electrons of C atom and partial involvement of d electrons of Ca atom. Both alloys have a large half-metallic gaps, even up to 0.609eV for CNaCa, meaning they are stable HMF at room temperature. The robustness of half-metallicity against the lattice constant is also calculated. Finally, lattice constant, magnetic moments, and electronic structure are calculated as a function of external pressure, which implies that the exchange splitting in crystal field gradually disappear and a transition occur from ferromagnetic to nonmagnetic states under a external pressure.

Pressure-Dependent Relaxation in the Photoexcited Mott InsulatorET–F2TCNQ: Influence of Hopping and Correlations on Quasiparticle Recombination Rates

We measure the ultrafast recombination of photoexcited quasiparticles (holon-doublon pairs) in the one dimensional Mott insulator ET-F(2)TCNQ as a function of external pressure, which is used to tune the electronic structure. At each pressure value, we first fit the static optical properties and extract the electronic bandwidth t and the intersite correlation energy V. We then measure the recombination times as a function of pressure, and we correlate them with the corresponding microscopic parameters. We find that the recombination times scale differently than for metals and semiconductors. A fit to our data based on the time-dependent extended Hubbard Hamiltonian suggests that the competition between local recombination and delocalization of the Mott-Hubbard exciton dictates the efficiency of the recombination.

Excited atoms in cavities of liquid He I: long-range interatomic repulsion and broadening of atomic lines

A theoretical analysis of the line broadening of localized atomic transitions in liquid helium is presented. It is shown that accurate information can be derived on the long-range part of the He*-He interaction as well as on the local structure near the He* emitters. The analysis confirms that in corona discharges in liquid helium the emitting He* atoms reside in cavities and that for known He*-He interaction the size of the cavities can be deduced from the line profile. The He*-He interaction was calculated using the full configuration interaction (CI) method as implemented in the Molpro package. The widths of atomic lines due to fluorescent transitions between different excited states of helium atoms were calculated as a function of external pressure in the range from 0.1 and 3.5 MPa using the static approximation method, and the input from the results of the CI calculation and cavity diameters calculated using the bubble model. The calculated widths showed excellent agreement with experimental data of liquid helium excited by corona discharges. A second, analytical analysis using a power function to represent the He*-He interaction showed qualitative agreement with the experimental data.

Shape of atomic lines emitted by cryoplasma in Helium

Experimental data on spectral shape of the line 706nm emitted by corona discharge in liquid and gaseous cryogenic helium are presented. The data have their explanation in a frame work of the bubble model. The long-range interaction between excited He* atoms and He atoms in the ground state was used for a description of the fluorescence measurements. The 'hump' like repulsive part of the inter-atomic potential at intermediate internuclear separations around 5 Å was calculated by using the full configuration interaction method as implemented in the MOLPRO code. The repulsion gives rise to the establishment of cavities around excited atoms in liquid helium and radius and profiles of the cavity boundaries were calculated for this potential and different pressures. The potential was used to simulate the atomic line fluorescence profile as a function of external pressure between 0.1 and 3.5 MPa. The fluorescence shows characteristic line shifts and widths as a function of pressure. By comparison with the theoretical predictions, we found evidence that for corona discharge-excited liquid helium the He* atoms reside in a cavity and hence emit from within the liquid phase. The measurements carried out at higher temperature 5.1K give spectra are characterised for the bubble states similar to the states were obtained in superfluid He II at 1.8 K

Noncollinear magnetic ordering in compressed FePd3ordered alloy: A first principles study

By means of ab initio calculations based on the density functional theory we investigated magnetic phase diagram of ordered FePd$_3$ alloy as a function of external pressure. Considering several magnetic configurations we concluded that the system under pressure has a tendency to non-collinear spin alignment. Analysis of the Heisenberg exchange parameters $J_{ij}$ revealed strong dependence of iron-iron magnetic couplings on polarization of Pd atoms. To take into account the latter effect we built an extended Heisenberg model with higher order (biquadratic) terms. Minimizing the energy of this Hamiltonian, fully parameterized using the results of ab initio calculations, we found a candidate for a ground state of compressed FePd$_3$, which can be seen as two interpenetrating "triple-Q" phases.

Experimental and theoretical characterization of the long-range interaction between He*(3s) and He(1s)

The long-range interaction between ${}^{{1,3}}S$ He${}^{*}(3s)$ and ${}^{1}\phantom{\rule{-0.16em}{0ex}}S$ He$(1s)$ was studied in bulk liquid helium by fluorescence measurements and by combined theoretical electronic structure and bosonic density functional theory calculations. The excited He${}^{*}$ atoms were produced in the liquid by corona discharge with subsequent impact excitation instigated by hot electrons from the discharge. The long-range contribution to the repulsive ``hump'' near 5 \AA{} in the ${}^{{1,3}}S$ He--${}^{1}\phantom{\rule{-0.16em}{0ex}}S$ He potentials was interrogated by monitoring He${}^{*}$ ${}^{{1,3}}S(3s)\ensuremath{\rightarrow}{}^{{1,3}}P(2p$) fluorescence profile characteristics as a function of external pressure between 0.1 and 3.5 MPa. Fluorescence line shifts and widths as a function of pressure were extracted from the experimental data and compared to the theoretical predictions, establishing that the nascent He${}^{*}$ atoms reside in a bubble state within the liquid. It was observed that the experimental data could only be consistently reproduced if the excited He${}^{*}$ atoms emit in a less-dense environment as compared with the rest of the bulk liquid.

First-principles study on the structure, elastic and electronic properties of hexagonal HfPtAl under pressure

We have investigated the structure, elastic and electronic properties in hexagonal HfPtAl by a first-principles ultrasoft pseudopotential of the plane wave within the density functional theory (DFT) plus the generalized gradient approximation (GGA) in the scheme of Perdew–Burke–Ernzerhof (PBE). All properties are calculated as a function of external pressure. The results of structural parameters have a good agreement with reported experiments and can predict the properties under pressure well. The hexagonal HfPtAl is mechanically stable and behaves in a ductile manner. The TDOS is occupied by Pt-d, Hf-d and Al-p.

High pressure studies of metal organic framework materials

This paper gives a brief overview of our recent work in the field of high pressure structural studies of metal organic framework materials. These are nanoporous materials which have found widespread application in the fields of gas separation and storage. Pressure provides a convenient tool to modify pore size and content without the need to alter the material chemically. Our techniques (high pressure single crystal diffraction and density functional theory) have allowed us to determine how the molecular frameworks change as a function of external pressure. The results have been surprising.

Insight into elastic behavior of calcium silicate hydrated oxide (C–S–H) under pressure and composition effect

The present work relates to the study of structural and elastic properties of Tobermorite 11Å as a function of external pressure and composition in terms of calcium to silicon ratio. Basing on the lattice dynamics method, the main aim of this work is precisely to shed light, for the first time, on the high pressure structural phase transition in Tobermorite 11Å and the possible correlation with some elastic quantities. In order to check the transferability of the potentials used we have, additionally, performed a single calculation based on the density functional theory (DFT) for a pressure of 15GPa in the case of Ca/Si=1. The variation of the unit cell parameters with pressure indicates that Tobermorite 11Å undergoes a structural instability around 15GPa along b-axis and around 20GPa along a-axis which is confirmed from our calculations of X-Rays diffraction patterns at various pressure values. We have also observed the anisotropic character of the Tobermorite structure for both cases (Ca/Si=1 and Ca/Si=0.83). Our results show that around 20GPa an important change appears in the elastic behaviour of Tobermorite. As pressure increases the calculated elastic quantities for Ca/Si=1 became closer to those evaluated for Ca/Si=0.83, which may stimulate further experimental and theoretical research on the matter.

In-plane and tunneling pressure sensors based on graphene/hexagonal boron nitride heterostructures

An in-plane pressure sensor (IPPS) consisting of graphene sandwiched by hexagonal boron nitride (h-BN) and a tunneling pressure sensor (TPS) consisting of h-BN sandwiched by graphene are demonstrated. The responses as function of external pressure are modeled. The current varies by 3 orders of magnitude as pressure increases from 0 to 5 nN/nm2. The IPPS current is negatively correlated to pressure, whereas TPS current exhibits positive correlation to pressure. The IPPS design is insensitive to the number of wrapping h-BN layers, indicating precise process control is unnecessary. The result paves a viable avenue towards realizing of atomic scale pressure sensors.

First-principles studies on magnetic properties of rocksalt structure MC (M=Ca, Sr, and Ba) under pressure

Magnetic properties of sp-electron half-metallic ferromagnets (HMFs) MC (M=Ca, Sr, and Ba) in rocksalt structure at ambient and elevated pressure have been investigated using first-principles within the generalized gradient approximation in the scheme of Perdew et al. [Phys. Rev. Lett. 77, 3865 (1996)]. Magnetic moments, lattice constants, and total energies are calculated as a function of external pressure. The calculations predict the occurrence of pressure-induced magnetic phase transitions and find that the main reason for the magnetic transitions of sp-electron HMFs is the band widening of anion p states.