Linear Pressure Coefficient Research Articles

Stress-Tuned Optical Transitions in Layered 1T-MX2 (M=Hf, Zr, Sn; X=S, Se) Crystals.

Optical measurements under externally applied stresses allow us to study the materials’ electronic structure by comparing the pressure evolution of optical peaks obtained from experiments and theoretical calculations. We examine the stress-induced changes in electronic structure for the thermodynamically stable 1T polytype of selected MX compounds (M=Hf, Zr, Sn; X=S, Se), using the density functional theory. We demonstrate that considered 1T-MX materials are semiconducting with indirect character of the band gap, irrespective to the employed pressure as predicted using modified Becke–Johnson potential. We determine energies of direct interband transitions between bands extrema and in band-nesting regions close to Fermi level. Generally, the studied transitions are optically active, exhibiting in-plane polarization of light. Finally, we quantify their energy trends under external hydrostatic, uniaxial, and biaxial stresses by determining the linear pressure coefficients. Generally, negative pressure coefficients are obtained implying the narrowing of the band gap. The semiconducting-to-metal transition are predicted under hydrostatic pressure. We discuss these trends in terms of orbital composition of involved electronic bands. In addition, we demonstrate that the measured pressure coefficients of HfS and HfSe absorption edges are in perfect agreement with our predictions. Comprehensive and easy-to-interpret tables containing the optical features are provided to form the basis for assignation of optical peaks in future measurements.

High pressure effects on the crystal and magnetic structure of ScMnO3

The crystal and magnetic structures of hexagonal manganite ScMnO3 have been studied by means of neutron diffraction at high pressures up to 2 GPa in the temperature range 10–300 К. At ambient pressure below TN = 130 K the triangular antiferromagnetic (AFM) state of mixed Г1 + Г2 irreducible representation is formed. Upon application of high pressure, the symmetry of magnetic structure persists up to 2 GPa. A significant suppression of the ordered magnetic moment and gradual spin reorientation were revealed under pressure. The baric dependence of Neel temperatures TN was obtained with the average linear pressure coefficient dTN/dP= 6.9 K/GPa, which is significantly larger with respect to related compounds like YMnO3. For the data obtained, one may expect modification of the magnetic ground state symmetry to the pure Г1 representation at pressures about 6 GPa.

Band structures and pressure dependence of the band gaps of GaN

The linear muffin-tin orbital method is used with the atomic-sphere approximation to study the ground-state properties, band structures and pressure dependence of the band gaps of zincblende GaN. The equilibrium lattice constant calculated is smaller than the experimental value by about 2%. It is found that the fundamental band gap is direct and that the value (3.90 eV) is in good agreement with the experimental value. The linear pressure coefficient for this band gap is much smaller than those for Ga.As and GaP. The total density of states and local density of. states are discussed. Moreover, the band structure (wurtzite) GaN is also studied. It is found that the band gap of wurtzite GaN is smaller than that of zincblende GaN, which agrees well with the experimental results.

Engineering Near-Infrared Light Emission in Mechanically Exfoliated InSe Platelets through Hydrostatic Pressure for Multicolor Microlasing.

γ-indium selenide (InSe) is a van der Waals semiconductor and holds great potentials for low-energy-consumption electronic and optoelectronic devices. Herein, we investigated the hydrostatic pressure engineered near-infrared (NIR) light emission of mechanically exfoliated γ-InSe crystals using the diamond anvil cell (DAC) technique. A record-wide spectral tuning range of 185 nm and a large linear pressure coefficient of 40 nm GPa-1 were achieved for spontaneous emissions, leading to ultrabroadband microlasing spectrally ranging from 1022 to 911 nm. This high emission tunability can be attributed to the compression of the soft intralayer In-Se bonds under high pressure, which suppressed the band gap shrinkage by increasing the interlayer interaction. Furthermore, two band gap crossovers of valence (direct-to-indirect) and conduction bands were resolved at approximately 4.0 and 7.0 GPa, respectively, resulting in pressure-sensitive emission lifetime and intensity. These findings pave the pathways for pressure-sensitive InSe-based NIR light sources, sensors and so on.

A novel approach to investigate temperature field evolution of water lubricated stern bearings (WLSBs) under hydrodynamic lubrication

Water lubricated stern bearings (WLSBs) are the critical component of ship propulsion system and have important effect on navigation safety. Operating temperature plays a main role on the performance of WLSBs. This paper aims to investigate the effect of operating conditions on bearing temperature characteristics under hydrodynamic lubrication. A novel CFD simulation method developed to improve calculation accuracy. Finite difference method was used to decrease the error of geometric modeling while the experiment and experimental correction formula were exploited to obtain improved boundary conditions. Based on the new method, the effects of operating conditions on temperature characteristics for two typical WLSBs were studied, and mechanisms of bearing temperature field evolution were discussed. Results show that the max friction coefficient appears when bearings are in low velocity and low load condition. Total heat flux density is a function of linear velocity, pressure and friction coefficient. Max temperature of bearing at 0.4 MPa decrease along with increased velocity, while decrease first and then increase at 0.2 MPa. Moreover, peak temperature appears at eccentric side and beyond minimum water film thickness position about 4–40°. High temperature area mainly located at the position of 80–140° in circumferential direction and 0.2–0.13 m in axial direction. With the increase of inlet water velocity, the max temperature of bearing changes slightly. It is appropriate to set the inlet velocity at 2 m/s to obtain better cooling performance. This work can provide theoretical basis for the operation monitoring of WLSBs and the development of new materials.

Behavior of Raman modes in InPBi alloys under hydrostatic pressure

Raman spectra of InPBi alloys with bismuth amount 0.3%-2.0% were measured under hydrostatic pressure in diamond anvil cell up to ∼4 GPa at room temperature. Two bismuth related Raman modes were identified and their evolutions under pressure were studied. The linear pressure coefficients of these two modes are determined to be 1.292 and 2.169 cm-1/GPa, respectively. The different behaviors of these two modes under pressure suggest that they may have distinct origins. InP related Raman modes were also investigated including two InP related modes caused by Bi doping.

High-pressure Raman scattering in bulk HfS2: comparison of density functional theory methods in layered MS2 compounds (M\u2009=\u2009Hf, Mo) under compression

We report high-pressure Raman-scattering measurements on the transition-metal dichalcogenide (TMDC) compound HfS2. The aim of this work is twofold: (i) to investigate the high-pressure behavior of the zone-center optical phonon modes of HfS2 and experimentally determine the linear pressure coefficients and mode Grüneisen parameters of this material; (ii) to test the validity of different density functional theory (DFT) approaches in order to predict the lattice-dynamical properties of HfS2 under pressure. For this purpose, the experimental results are compared with the results of DFT calculations performed with different functionals, with and without Van der Waals (vdW) interaction. We find that DFT calculations within the generalized gradient approximation (GGA) properly describe the high-pressure lattice dynamics of HfS2 when vdW interactions are taken into account. In contrast, we show that DFT within the local density approximation (LDA), which is widely used to predict structural and vibrational properties at ambient conditions in 2D compounds, fails to reproduce the behavior of HfS2 under compression. Similar conclusions are reached in the case of MoS2. This suggests that large errors may be introduced if the compressibility and Grüneisen parameters of bulk TMDCs are calculated with bare DFT-LDA. Therefore, the validity of different approaches to calculate the structural and vibrational properties of bulk and few-layered vdW materials under compression should be carefully assessed.

Structural phase stability, elastic parameters and thermal properties of YN from first-principles calculation

The structural phase stability, elastic parameters and thermodynamic properties of YN at normal and under high pressure are reported. The calculations are mainly performed using the full-potential linearized augmented plane wave method within the density functional theory. Both local density approximation (LDA) and generalized gradient approximation (GGA) are used to model the correlation-exchange potential. The calculated equilibrium lattice parameter and the bulk modulus show good accordance with the experimental and previous theoretical reports. The phase transition from the NaCl (B1) structure to the CsCl (B2) structure is found to occur at 131 GPa within GGA and 115 GPa within LDA. The linear pressure coefficients of the different elastic moduli being addressed here are also determined along with the mechanical and dynamical stability criteria which are shown to be satisfied for YN with B1 phase under normal conditions. Besides, the heat capacity and other thermodynamic parameters are examined and discussed versus temperature.

High pressure luminescence of Nd3+ in YAlO3 perovskite nanocrystals: A crystal-field analysis.

Pressure-induced energy blue- and red-shifts of the 4F3/2 → 4I9/2,11/2 near-infrared emission lines of Nd3+ ions in YAlO3 perovskite nano-particles have been measured from ambient conditions up to 29 GPa. Different positive and negative linear pressure coefficients have been calibrated for the emission lines and related to pressure-induced changes in the interactions between those Nd3+ ions and their twelve oxygen ligands at the yttrium site. Potentiality of the simple overlap model, combined with ab initio structural calculations, in the description of the effects of these interactions on the energy levels and luminescence properties of the optically active Nd3+ ion is emphasized. Simulations show how the energies of the 4f3 ground configuration and the barycenters of the multiplets increase with pressure, whereas the Coulomb interaction between f-electrons decreases and the crystal-field strength increases. All these effects combined explain the wavelength blue-shifts of some near-infrared emission lines of Nd3+ ions. Large pressure rates of various emission lines suggest that a YAlO3 perovskite nano-crystal can be a potential candidate for near-infrared optical pressure sensors.

High pressure sensitivity of anti-Stokes fluorescence in Nd3+ doped yttrium orthoaluminate nano-perovskites

Pressure-induced energy shifts of anti-Stokes emissions of Nd3+:YAlO3 nano-perovskite has been studied up to 10GPa. Emission lines have been identified as thermally-induced transitions between 4F7/2, 4S3/2, and 4F9/2 multiplets and the 4I9/2 ground state, which emerge as a consequence of the heat produced in the sample by using a confocal set up, which condenses the energy of the laser beam in a micro-sized spot. Blue and red-shifts have been observed with linear pressure coefficients that range from +3.15cm−1/GPa to −7.21cm−1/GPa. These results suggest the use of Nd3+:YAlO3 nano-perovskite in high pressure experiments as an optical pressure sensor in the wavelength interval from 660 to 760nm.

High-pressure behavior of superconducting boron-doped diamond

This work investigates the high-pressure structure of freestanding superconducting ($T_{c}$ = 4.3\,K) boron doped diamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0-30\,GPa range. High-pressure Raman scattering experiments revealed an abrupt change in the linear pressure coefficients and the grain boundary components undergo an irreversible phase change at 14\,GPa. We show that the blue shift in the pressure-dependent vibrational modes correlates with the negative pressure coefficient of $T_{c}$ in BDD. The analysis of x-ray diffraction data determines the equation of state of the BDD film, revealing a high bulk modulus of $B_{0}$=510$\pm$28\,GPa. The comparative analysis of high-pressure data clarified that the sp$^{2}$ carbons in the grain boundaries transform into hexagonal diamond.

Pressure dependence of refractive index, dielectric constants and optical phonon frequencies of indium arsenide

A pseudopotential approach has been employed so as to investigate the pressure dependence of refractive index, dielectric constants and optical phonon frequencies for InAs. At zero pressure our findings are compared to those reported in the literature and found to be generally in reasonably good agreement. The pressure dependence of all features of interest showed a monotonic behavior. The linear pressure coefficient of the refractive index and dielectric constants are calculated and reported. The transverse and longitudinal optical phonon frequencies are found to shift towards high frequencies when pressure is applied.

Pressure-Dependent Photoluminescence Study of Wurtzite InP Nanowires.

The elastic properties of InP nanowires are investigated by photoluminescence measurements under hydrostatic pressure at room temperature and experimentally deduced values of the linear pressure coefficients are obtained. The pressure-induced energy shift of the A and B transitions yields a linear pressure coefficient of αA = 88.2 ± 0.5 meV/GPa and αB = 89.3 ± 0.5 meV/GPa with a small sublinear term of βA = βB = -2.7 ± 0.2 meV/GPa(2). Effective hydrostatic deformation potentials of -6.12 ± 0.04 and -6.2 ± 0.04 eV are derived from the results for the A and B transitions, respectively. A decrease of the integrated intensity is observed above 0.5 GPa and is interpreted as a carrier transfer from the first to the second conduction band of the wurtzite InP.

Pressure effect on the structural and electronic properties of CuInS2

The pressure dependence of the bond length and energy gap in chalcopyrite CuInS2 between 0 and 40 GPa has been investigated using pseudopotentials plane-wave method within the generalized gradient approximation for the exchange-correlation potential. We found that the bond length decreases as the pressure increases. Also, the energy gap of CuInS2 expands as the pressure increases with a rate of 10.693 meV/GPa. The linear pressure coefficient calculated is approximately half the reported experimental value of 23 meV/GPA. Our calculated bulk modulus of 68.7 GPa is in good agreement with the available experimental and theoretical values. The present calculations show that the d-electrons of Cu ions are one of the important factors that dominate the contributions to the I-VI bonds and the energy gap in CuInS2.

Structural and electronic properties of boron- bismuth compound under pressure

<p>In the present work, we report first principles calculations of the pressure effect on the structural and electronic properties of Boron- Bismuth (BBi) compound in its zincblende phase. The pseudopotential plane wave (PPW) method in the framework of the density func-tional theory (DFT) within the local density approximation for the exchange-correlation functional, and the Hartwigzen-Goedecker-Hutter (HGH) scheme for the pseudopotential were used in the calculation. The unit cell volume, the molecular and crystal densities, the equation of state and also the linear and quadratic pressure coefficients of the energy band-gaps are investigated.</p>

First principle study of the physical properties of semiconducting binary antimonide compounds under hydrostatic pressures

First-principle calculations have been performed to investigate the structural phase transition, electronic, elastic, thermodynamical and optical properties of III-Sb compounds under hydrostatic pressure up to their first order transitions pressure (Zinc Blende to Rock Salt). Four different exchange–correlation functionals comprising Perdew–Burke–Ernzerhof generalized parameterization of gradient approximation, Wu-Cohen, local density approximation as well as modified Becke and Johnson were used. The structural properties such as phase transitions, equilibrium lattice parameters, bulk modulus and its first pressure derivative were obtained using an optimization method. Moreover, elastic constants, Young׳s modulus, shear modulus, Poisson׳s ratio, sound velocities for longitudinal and shear waves, Debye average velocity, Debye temperature and Grüneisen parameters were calculated up to the first order phase transition pressure. The obtained structural and elastic parameters are consistent with the available experimental data. The static calculations predict that Zinc Blende to Rock Salt phase transitions occur at 48.5, 9.5, 5.87 and 3.15GPa for BSb, AlSb, GaSb and InSb respectively. The optical properties of these compounds, such as dielectric function, refractive index and the optical band gap were also calculated for the radiation up to 14eV. In addition, the influence of the hydrostatic pressure on the elastic parameters, energy band structures and the refractive index of these compounds were investigated. The linear and quadratic pressure coefficients of the compounds were also calculated. Results have been discussed and compared with available experimental and theoretical data, which show an overall good agreement with the other studies.

Suppression of the charge-density-wave state inSr10Ca4Cu24o41by external pressure

The influence of external pressure on the charge-density-wave (CDW) ground state of the quasi-one-dimensional two-leg ladder compound ${\mathrm{Sr}}_{10}$${\mathrm{Ca}}_{4}$${\mathrm{Cu}}_{24}$${o}_{41}$ has been studied by optical reflectivity measurements as a function of temperature (10--300 K) and pressure $P$ (0.3--4.3 GPa) over the spectral range 580--6000 cm${}^{1}$. With increasing pressure the CDW transition temperature ${T}_{\text{CDW}}$ decreases with the linear pressure coefficient $\ensuremath{\approx}\ensuremath{-}$70 K/GPa, and above $\ensuremath{\approx}$3 GPa the CDW phase is suppressed at all temperatures. This behavior is similar to that in compounds ${\mathrm{Sr}}_{14\ensuremath{-}x}$${\mathrm{Ca}}_{x}$${\mathrm{Cu}}_{24}$${o}_{41}$ with increasing Ca content $x$ at ambient pressure, with the simple scaling $x\ensuremath{\approx}3\ifmmode\times\else\texttimes\fi{}P(\text{GPa})$. The size of the CDW gap decreases with increasing pressure, whereas the dimensionality of the high-temperature insulating phase in ${\mathrm{Sr}}_{10}$${\mathrm{Ca}}_{4}$${\mathrm{Cu}}_{24}$${o}_{41}$ within the ladder plane is hardly affected by external pressure.

Raman scattering from superhard rhenium diboride under high pressure

Lattice vibrational properties of superhard rhenium diboride (ReB2) were examined up to 8 GPa in a diamond anvil cell using Raman spectroscopy techniques. Linear pressure coefficients and mode Grüneisen parameters are obtained. Good agreement is found between the experimental and theoretical calculated Grüneisen parameters. Examination of the calculated mode Grüneisen parameters reveals that both B-B and Re-B covalent bonds play a dominant role in supporting the applied load under pressure. A comparison of vibrations parallel and perpendicular to the c-axis indicates that bonds along the c-axis tend to take greater loads. Our results agree with observations of elastic lattice anisotropy obtained from both in situ X-ray diffraction measurements and ultrasonic resonance spectra.

Pressure dependence of Morin transition in α-Fe2O3 hematite

We report high-pressure neutron diffraction data on the Morin-transition TM in α-Fe2O3 hematite, up to 8 GPa and under hydrostatic conditions. We find a strictly linear pressure coefficient of with no evidence for intermediate magnetic states as indicated by all previous non-hydrostatic measurements. The behaviour of TM is highly regular and can well be explained within the framework of Artman et al.'s single-ion model if a change of the single-ion anisotropy energy by +1%/GPa is assumed. Such a value seems not to be unusual for iron oxides.

Polaron physics and crossover transition in magnetite probed by pressure-dependent infrared spectroscopy

The optical properties of magnetite at room temperature were studied by infrared reflectivity measurements as a function of pressure up to 8 GPa. The optical conductivity spectrum consists of a Drude term, two sharp phonon modes, a far-infrared band at around 600 cm−1 and a pronounced mid-infrared absorption band. With increasing pressure both absorption bands shift to lower frequencies and the phonon modes harden in a linear fashion. Based on the shape of the MIR band, the temperature dependence of the dc transport data, and the occurrence of the far-infrared band in the optical conductivity spectrum, the polaronic coupling strength in magnetite at room temperature should be classified as intermediate. For the lower energy phonon mode an abrupt increase of the linear pressure coefficient occurs at around 6 GPa, which could be attributed to minor alterations of the charge distribution among the different Fe sites.