Hydrostatic Pressure Coefficients Research Articles

Uniaxial-stress dependence of Hall effect in an AlGaAs/GaAs modulation-doped heterojunction

Uniaxial compressive stress was applied to an AlGaAs/GaAs heterojunction. The uniaxial stress coefficients of sheet resistivity, sheet electron concentration, and mobility were obtained. The hydrostatic pressure coefficient of sheet resistivity was also obtained and was used to explain the different magnitude of the uniaxial stress coefficients of sheet electron concentration in the [110] and [11̄0] directions. We obtain a value for the piezoelectric constant e14 of AlAs to be −0.26 C/m2, compared to the value −0.225 C/m2 calculated by K. Hübner [Phys. Status Solidi B 57, 627 (1973)].

Photoluminescence of (111)InxGa1−xAs/GaAsstrained-layer quantum wells under hydrostatic pressure

The unexpectedly low values of the hydrostatic pressure coefficients of the band gaps of compressively strained (001) quantum wells are due to nonlinear elasticity effects in the interaction of external pressure and internal coherency strain. In (111)-oriented quantum wells compressive strain lowers the pressure coefficient, in contradiction to our previous report, but in agreement with recent work and in accordance with nonlinear elasticity theory. We compare recent experimental results with the predictions of the theory of the nonlinear elastic effects in strained layers.

Pressure and temperature dependence of the absorption edge of a thick Ga0.92In0.08As0.985N0.015 layer

We have studied the pressure and temperature dependence of the absorption edge of a 4-μm-thick layer of the alloy Ga0.92In0.08As0.985N0.015. We have measured the hydrostatic pressure coefficient of the energy gap of this alloy to be 51 meV/GPa, which is more than a factor two lower than that of GaAs (116 meV/GPa). This surprisingly large lowering of the pressure coefficient is attributed to the addition of only ∼1.5% nitrogen. In addition, the temperature-induced shift of the edge is reduced by the presence of nitrogen. We can explain this reduction by the substantial decrease of the dilatation term in the temperature dependence of the energy gap.

Prostacyclin counteracts the increase in capillary permeability induced by tumour necrosis factor-alpha.

To analyse how prostacyclin interferes with the short-term local circulatory effects of tumour necrosis factor-alpha (TNF alpha) in a skeletal muscle. An autoperfused sympathectomised cat gastrocnemius muscle enclosed in a plethysmograph. Arterial blood flow, total and segmental vascular resistances (large-bore arterial vessels, arterioles and veins), hydrostatic capillary pressure, tissue volume and capillary filtration coefficient were followed during local intra-arterial infusion of TNF alpha at various rates (2.5, 5.0 and 7.5 micrograms/kg per min) and during intra-arterial infusion of prostacyclin simultaneously with the highest dose of TNF alpha. The capillary filtration coefficient reflects the capillary surface for fluid exchange. Arterial infusion of TNFx had no influence on vascular resistance up to 5.0 micrograms/kg per min but induced vasodilation at 7.5 micrograms/kg per min. No effects on the recorded hydrostatic capillary pressure were observed. The capillary filtration coefficient and the capillary filtration increased with the infusion rate of TNF alpha, the former by 55%. Simultaneous arterial infusion of prostacyclin (350 ng/kg per min) caused further vasodilation and an increase in hydrostatic capillary pressure and completely restored the capillary filtration coefficient to control. The TNF alpha-induced filtration was partly restored. The local circulatory effect of TNF alpha is small apart from a graded increase in the capillary filtration coefficient, most likely reflecting an increase in the capillary permeability. The prostacyclin-induced decrease in capillary filtration coefficient most likely reflects a restoration of capillary permeability. The TNF alpha-induced transcapillary filtration is not fully reduced by prostacyclin due to a simultaneous increase in hydrostatic capillary pressure.

Thermodynamic properties of the spin-Peierls transition in CuGeO3

We present an investigation of the spin-Peierls transition atTSP=14.5 K in polycrystalline CuGeO3 through specific-heat and thermal-expansion measurements. Clear second-order phase-transition anomalies are found in both properties atTSP, although only a small entropy of S≈0.1 Rln2 is released at the transition. Most of the entropy is released atTSP<T<150 K, where the temperature dependence of the magnetic contribution to the specific heat as well as the thermal expansion exhibit extrema atT*≈40 K. These are caused by one-dimensional antiferromagnetic fluctuations along the Cu chains, possibly accompanied by structural fluctuations. Using Ehrenfest's relation, a hydrostatic pressure coefficient (∂TSP/∂p)p→0 ≈ (0.45±0.06) K/kbar is derived.

Influence of pressure on (001)GaAs surfaces.

This paper presents results of band-structure calculations for uniaxially stressed (001) GaAs surfaces. We discuss results for both ideal surfaces and 2\ifmmode\times\else\texttimes\fi{}1 reconstructed surfaces. Corresponding bulk-energy bands are projected onto the first surface Brillouin zone. The influence of uniaxial force on both surface and bulk energy bands is then discussed. The one-dimensional pressure coefficient of the fundamental band gap is calculated and compared with the corresponding hydrostatic pressure coefficient. Ideal surface-state energy bands were found to be relatively insensitive to the applied uniaxial force compared with the bulk bands. However, for 2\ifmmode\times\else\texttimes\fi{}1 reconstructed surfaces, the surface energy bands are sensitive to the applied uniaxial force (compared with the bulk bands): this originates primarily from the stronger dependence of the surface conduction band on pressure compared with the surface valence band. The influence of uniaxial force on the total density of states for ideal and 2\ifmmode\times\else\texttimes\fi{}1 reconstructed surfaces is discussed.

Dependence of ordering temperature on pressure, carrier density and U-U distance in uranium pnictides; pressure and hall effects examinations of UP 2 and USb 2

The basal plane Hall effect has been examined for UP 2 and USb 2 from 230 K to 465 K. It was found that the normal Hall coefficients are negative and in a single band approach correspond to a conduction electron densities equal to 1.2 and 0.53 fu -1 (fu-formula unit) for UP 2 and USb 2, respectively. The spontaneous Hall coefficients are positive and about three orders of magnitude higher than the normal ones. The hydrostatic pressure coefficients of antiferromagnetic ordering temperature were found equal to −20±2 KGPa −1 for UP 2 and +3.6±0.7 KGPa −1 for USb 2. The above quantities determined for the UX 2 pnictides are compared to that known for the corresponding U 3X 4 pnictides.

The Pressure Dependence of Transition Metal-Related Levels in GaAs

The hydrostatic pressure coefficients of V3+/2+ acceptor level in bulk GaAs and of the 0.48 eV trap (related to Νi 2 +/ 1 + double acceptor level) in VPE GaAs were measured by means of the DLTS technique. The obtained values are 94 meV/GPa and 196 meV/GPa relative to the bottom of the conduction band. For Ni 2 +/ 1 + level the strong pressure dependence of the capture cross-section activation energy (60 meV/GPa) was also observed.

TEMPERATURE AND HYDROSTATIC PRESSURE DEPENDENCE OF SUBBAND STRUCTURE IN GaAs/AlxGa1−xAs MULTIPLE QUANTUM WELL

The temperature and hydrostatic pressure dependence of GaAs/AlxGa1−xAs multiple quantum well subband structures has been examined theoretically. Such effects on the subband energy measured from the corresponding band optimum are negligible, and therefore, the temperature and the hydrostatic pressure coefficients of these multiple quantum wells are dominated by those coefficients of the bulk GaAs bandgap.

Optical investigation of Ni impurity in ZnSe under hydrostatic pressure

We have determined the hydrostatic pressure coefficients of all the intra-Ni 2+ ion optical transitions as well as of the valence-band Ni 2+/Ni 1+ level photoionization edge in ZnSe. Following the idea that transition-metal impurity levels can serve as reference points for measuring absolute pressure shifts of the band edges, we have evaluated the valence-band deformation potential of ZnSe and discussed the validity of such a procedure.

Measurement of the effect of hydrostatic pressure on the optical spectrum of CaClF:Sm 2+ using spectral holeburning

We have measured the spectral line shifts of some 4f-4f transitions of the Sm 2+ ions in CaClF for hydrostatic pressures up to one atmosphere. Measurements in this low pressure regime were possible using narrow spectral holes burnt in the inhomogeneous line-profile at low temperatures. Hydrostatic pressure coefficients of (-100±4) MHz/bar for the 5D 0A 1- 7F 0A 1 transition and (-85±7) MHz/bar for the 5D 1E- 7F 0A 1 transition were obtained. Thermal shifts of the spectral holes were also measured up to 22 K and from the temperature dependence we identified acoustic phonon Raman scattering as the dominant mechanism in this temperature range.

Hydrostatic and uniaxial pressure coefficients of CdTe.

The pressure dependence of direct band-gap CdTe has been measured by observing the direct exciton photoluminescence and donor-acceptor pair photoluminescence under hydrostatic pressures up to 35 kbar. A value of 6.5\ifmmode\pm\else\textpm\fi{}0.2 meV ${\mathrm{kbar}}^{\ensuremath{-}1}$ was obtained corresponding to a hydrostatic deformation potential $\ensuremath{\Xi}=2.74\ifmmode\pm\else\textpm\fi{}0.09$ eV. Using this value, the uniaxial stress data of Thomas J. Appl. Phys. 32, 2298 (1961) is reassessed to determine a best value of $\ensuremath{\Delta}=8.7\ifmmode\pm\else\textpm\fi{}0.3$ meV ${\mathrm{kbar}}^{\ensuremath{-}1}$, corresponding to ${D}_{u}=1.11$ eV and ${D}_{{u}^{\ensuremath{'}}}=2.57$ eV.

Properties of strained layer InxAl1−xAs/InP heterostructures

X-ray diffraction and photoluminescence measurements were made on InxAl1−xAs layers grown by molecular-beam epitaxy whose lattice constants are matched, in tension, or in compression, relative to their (100)-oriented InP substrates. Using a linear interpolation between the InAs and AlAs elastic stiffness coefficients, hydrostatic pressure coefficients, and shear deformation potentials, the strain-dependent fundamental band gaps were calculated and shown to be in good agreement with the experimentally measured data within the pseudomorphic limit. The calculated composition-dependent critical thickness for the onset of plastic deformation of these layers is shown to be in better agreement with the energy balance model of People and Bean than the mechanical equilibrium model of Matthews and Blakeslee.

On the elastic strain coefficients of the resistance of metals

The relation between the elastic strain sensitivity coefficient of resistance and the hydrostatic pressure coefficient of resistance is not simple, owing to the tensor nature of the resistivity of solids. We discuss this relation and derive a rigorous expression which demonstrates that both coefficients have to be measured in order to characterize a given material. Simple relations which often appear in the literature should be dealt with cautiously because they usually treat the resistivity of metals as a scalar, thus eliminating one of the piezoresistance coefficients.

Au acceptor levels in Si under pressure

The hydrostatic pressure coefficient of Au acceptor levels ET in Si was measured by transient capacitance method. Under the pressure range of 0–8 kbar, the pressure coefficient ∂(Ec−ET)/∂P=−1.9 meV/kbar. The electron capture cross section of Au acceptor centers is independent of pressure within experimental accuracy. For defect levels with defect potential of Td symmetry, the uniaxial stress coefficient ∂(Ēc−ĒT)/∂F is isotropic and equal to one-third of corresponding hydrostatic pressure coefficient. By comparing the present result of hydrostatic pressure coefficient with the uniaxial stress coefficient reported by X. C. Yau, G. G. Qin, S. R. Zeng, and M. H. Yuan [Acta Phys. Sin. 33, 377 (1984)], one concludes that the defect potential is far from Td symmetry. Therefore, the Au acceptor levels are unlikely to have been originated by simple gold substitutional or interstitial configuration in Si.

HYDROSTATIC PRESSURE DEPENDENCE OF GOLD ACCEPTOR LEVELS IN Si

The hydrostatic pressure coefficient of gold acceptor levels Er in Silicon was measured by transient capacitance method. Under the pressure range of 0-8 Kbar, thepressure coefficient (?(Ec-ET))/(?P) =-1.9 meV/kbar. The electron capture cross section of gold acceptor centers does not depend on pressure within the experimental accuracy. By comparing the present result of hydrostatic pressure coefficient with the uniaxial pressure coefficient reported in [13], we conclude that the defect potential is lack of Td symmetry. Therefore, the gold acceptor levels are not originated by simple gold substitutional or interstitial configuration in Si crystals.

STRESS DEPENDENCE OF DEFECT LEVELS IN SEMICONDUCTORS

New criterion for symmetric properties of defect potential in semiconductors is presented. For a degenerate energy level Es, we introduce the average Es of energy splitling under stress. Es is invariant under symmetry operation of Hamiltonian. Therefore, it is straight foward to reduce the stress coefficient tensor of Es by group theoretical method. For instance, for cubic semiconductors, if the defect level ET is withdefect potential of T group symmetry, the uniaxial stress coefficient of (?(Ec-ET))/(?F) or (?(ET-Ev))/(?F) is isotropic and equal to one third of corresponding hydrostatic pressurecoefficient. The case of defect potential with C3v symmetry is also discussed.The electron (or hole) emission and capture processes of defect levels under energy splitting are analysed. The weighted average of emission rate en and capture rate cn satisfy the following relation: en=gTgc-1cnN′ce(-(Bc-BT)/kT) By measuring the stress dependence of en., various schemes for determining the symmetry property of defect potential are discussed. The transient processes under defect level splitting are also discussed. When transitions are al lowed between the splitting states, the transient is exporential with single decay time constant en-1. When transitions between different states are forbidden, the transient is decay with multi-exponential time constants. en is determined by the initial transient slope.

Deep traps in GaAs under hydrostatic pressure

The influence of hydrostatic pressure on the energy levels of eight different deep traps in GaAs is determined by means of a transient capacitance technique. Hydrostatic pressure coefficients are shown to be decisive parameters in order to distinguish isoenergetic traps of different origin. For the EL6-, the M3- and the E3-traps (E c−0.31eV), strongly different pressure coefficients are found. The HK1- and HK2- hole traps exhibit only small pressure coefficients relative to the valence band edge.

Piezoelectric and pyroelectric properties of poly(vinylidene fluoride) films at high hydrostatic pressure

Abstract The pressure dependence of the piezoelectric hydrostatic pressure coefficient, d p, and the pyroelectric coefficient, p y, was determined from atmospheric pressure to 7 Kbar over a temperature range from -80°C to +48°C for poled, uniaxially oriented film. The effect of the glass transition on d p and p y as a function of pressure is compared to the pressure dependence of the glass transition temperature obtained from dielectric studies. The ratio of d p/py as a function of pressure and temperature is presented.

Pressure dependence of elastic constants and optical phonon frequencies in Se and Te. A study of the homological relationship between the vibrational modes in these materials

AbstractThe hydrostatic pressure coefficients for the elastic constants of Se and Te are shown to be large and positive in contrast to the negative mode Grüneisen parameters reported earlier for the optical vibrations. These results can be understood on the basis of the idea that the hydrostatic pressure promotes transfer of electrons from intrachain bonding orbitals to interchain bonding states. This strengthens the interchain “elastic” force constants while simultaneously weakening the intrachain “optic” force constants. The great similarity in the Se and Te lattices suggests that their vibrational spectra can be related by homology. Of the three lattice parameters describing each crystal, the intrachain bond angle is nearly identical and relatively pressure insensitive in the two lattices. Therefore, by a two‐step transformation in which the c/a‐ratio is first made equal by applying hydrostatic pressure, and secondly by properly scaling the mass and the unit cell, the two lattices can be made structurally identical. With this transformation and the available pressure data all zone centre phonons studied in the two materials could be related by a universal scaling factor for the “mode” force constants. Rough estimates for the higher‐order pressure coefficients can also be made.