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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Unexpected structural softening of interstitial boron solid solution WB{sub 3+x}

Using first-principles calculations, we reveal an unexpected structural softening in a recently proposed WB{sub 3+x} structural model that tries to explain the X-ray diffraction, high resolution TEM, pressure dependence of the normalized lattice c/a ratio, and hardness experimental results of the synthesized tungsten boride compounds with a nominal composition WB{sub 4}. We show that the interstitial boron in WB{sub 3+x}, which was proposed to strengthen the covalent bonding network, unexpectedly weakens the atomic bonding, resulting in a large reduction of its indentation strength to well below that of WB{sub 3}. This is in direct contradiction to the experimental results showing that synthesized WB{sub 4} is harder than WB{sub 3}. The unusual structural softening is attributed to the unique three-center covalent bonding formed by the interstitial boron atoms that can easily deform under indentation. Our results show that the proposed interstitial boron solid solution WB{sub 3+x} structure is incompatible with experimental results, which calls for further investigations to determine the crystal structure of the synthesized WB{sub 4}.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Theoretical prediction of the fundamental properties for the ternary Li{sub 2}PtH{sub 6} and Na{sub 2}PtH{sub 6}

Li{sub 2}PtH{sub 6} and Na{sub 2}PtH{sub 6} are good candidate for hydrogen storage. The structural, elastic, electronic and optical properties of Li{sub 2}PtH{sub 6} and Na{sub 2}PtH{sub 6} compounds have been investigated using pseudo-potential plane-wave method based on the density functional theory. Computed lattice constant and H atom positional parameter at equilibrium agree well with the available experimental data. A quadratic pressure dependence of the elastic stiffness is found. A set of isotropic elastic parameters and related properties, namely bulk and shear moduli, Young's modulus, Poisson's ratio, average sound velocity and Debye temperature are numerically estimated in the framework of the Voigt-Reuss-Hill approximation for Li{sub 2}PtH{sub 6} and Na{sub 2}PtH{sub 6} polycrystalline aggregate. The analyses of the band structure indicates that Li{sub 2}PtH{sub 6} and Na{sub 2}PtH{sub 6} are indirect gap semiconductors. The static dielectric constant and static refractive index are inversely proportional to the fundamental gap. Highlights: Black-Right-Pointing-Pointer We predict elastic moduli, energy gaps and optical parameters. Black-Right-Pointing-Pointer Electron effective mass is anisotropic. Black-Right-Pointing-Pointer Li{sub 2}PtH{sub 6} and Na{sub 2}PtH{sub 6} are indirect gap semiconductors. Black-Right-Pointing-Pointer The contribution to the optical spectra from main transitions are predicted.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Full potential study of the elastic, electronic, and optical properties of spinels MgIn{sub 2}S{sub 4} and CdIn{sub 2}S{sub 4} under pressure effect

The structural, elastic, electronic, and optical properties of cubic spinel MgIn{sub 2}S{sub 4} and CdIn{sub 2}S{sub 4} compounds have been calculated using a full relativistic version of the full-potential linearized-augmented plane wave with the mixed basis FP/APW+lo method. The exchange and correlation potential is treated by the generalized-gradient approximation (GGA). Moreover, the Engel-Vosko GGA formalism is also applied to optimize the corresponding potential for band structure calculations. The ground state properties, including the lattice constants, the internal parameter, the bulk modulus, and the pressure derivative of the bulk modulus are in reasonable agreement with the available data. Using the total energy-strain technique, we have determined the full set of first-order elastic constants C{sub ij} and their pressure dependence, which have not been calculated or measured yet. The shear modulus, Young's modulus, and Poisson's ratio are calculated for polycrystalline XIn{sub 2}S{sub 4} aggregates. The Debye temperature is estimated from the average sound velocity. Electronic band structures show a direct band gap ({Gamma}-{Gamma}) for MgIn{sub 2}S{sub 4} and an indirect band gap (K-{Gamma}) for CdIn{sub 2}S{sub 4}. The calculated band gaps with EVGGA show a significant improvement over the GGA. The optical constants, including the dielectric function {epsilon}({omega}), the refractive index n({omega}), the reflectivity R({omega}), and the energy loss function L({omega}) were calculated for radiation up to 30 eV. -- Graphical abstract: Calculated total and partial densities of states for MgIn{sub 2}S{sub 4} and CdIn{sub 2}S{sub 4}

36 MATERIALS SCIENCE↗
DOE OSTI2017

Nanocell with a pressure-controlled Rb atomic vapor column thickness: Critical influence of the thickness on optical processes

A new device is designed: it consists of a nanocell (NC) filled with Rb atom vapors and placed in a vacuum chamber. When the pressure in the chamber changes in the range 0–1 atm, the NC thickness is smoothly varied in the range L = 140–1700 nm, which is caused by the pressure-induced deformation of thin garnet windows in the chamber. The pressure dependence has excellent reproducibility even after many hundreds of cycles of letting in of air and its complete pumping out from the chamber. The accuracy of setting required thickness L is much better than in the wedge-gap NCs to be moved mechanically that were used earlier. The processes of Faraday rotation (FR) of a polarization plane, resonance absorption, and fluorescence are studied using the D{sub 1}-line narrow-band continuous laser radiation when the thickness changes from L = λ/2 (398 nm) to L = 2λ (1590 nm) at a step λ/2. The FR signal is shown to be maximal at L = λ/2 and 3λ/2 and to have the minimum spectral width (≈60 MHz). At L = λ and 2λ, the FR signal is minimal and has the maximum spectral width (≈200 MHz). The resonance absorption demonstrates the same oscillating behavior; however, the effect in the case of FR is much more pronounced. The oscillating effect is absent for resonance fluorescence: its spectral width and amplitude increase monotonically with L. The detected effects are explained and possible applications are noted.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Quantitative Collision Cross-Sections from FTICR Linewidth Measurements: Improvements in Theory and Experiment

Two corrections to the equation used in the cross-sectional areas by Fourier transform ion cyclotron resonance (“CRAFTI”) technique are identified. In CRAFTI, ion collision cross-sections are obtained from the pressure-dependent ion linewidths in Fourier transform mass spectra. The effects of these corrections on the accuracy of the cross-sections obtained using the CRAFTI technique are evaluated experimentally using the 20 biogenic amino acids and several crown ether complexes with protonated alkyl monoamines. Good absolute agreement is obtained between the CRAFTI cross-sections and the corresponding cross-sections obtained using both static drift ion mobility spectrometry and computational simulations. These results indicate that the CRAFTI cross-sections obtained using the updated equation presented here are quantitatively descriptive of the size and shape of the gas-phase ions. Cross-sections that differ by less than 3% are measured for the isobaric isomers n-butylamine and tert-butylamine complexed with the crown ethers. This level of precision is similar to what has been achieved previously using traveling wave ion mobility devices. These results indicate that CRAFTI can be used to probe subtle structural differences between ions with approximately the same precision as that achieved in traveling wave ion mobility devices. .

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

XO-2b: A HOT JUPITER WITH A VARIABLE HOST STAR THAT POTENTIALLY AFFECTS ITS MEASURED TRANSIT DEPTH

The transiting hot Jupiter XO-2b is an ideal target for multi-object photometry and spectroscopy as it has a relatively bright (V-mag = 11.25) K0V host star (XO-2N) and a large planet-to-star contrast ratio (R{sub p}/R{sub s} ≈ 0.015). It also has a nearby (31.″21) binary stellar companion (XO-2S) of nearly the same brightness (V-mag = 11.20) and spectral type (G9V), allowing for the characterization and removal of shared systematic errors (e.g., airmass brightness variations). We have therefore conducted a multiyear (2012–2015) study of XO-2b with the University of Arizona’s 61″ (1.55 m) Kuiper Telescope and Mont4k CCD in the Bessel U and Harris B photometric passbands to measure its Rayleigh scattering slope to place upper limits on the pressure-dependent radius at, e.g., 10 bar. Such measurements are needed to constrain its derived molecular abundances from primary transit observations. We have also been monitoring XO-2N since the 2013–2014 winter season with Tennessee State University’s Celestron-14 (0.36 m) automated imaging telescope to investigate stellar variability, which could affect XO-2b’s transit depth. Our observations indicate that XO-2N is variable, potentially due to cool star spots, with a peak-to-peak amplitude of 0.0049 ± 0.0007 R-mag and a period of 29.89 ± 0.16 days for the 2013–2014 observing season and a peak-to-peak amplitude of 0.0035 ± 0.0007 R-mag and 27.34 ± 0.21 day period for the 2014–2015 observing season. Because of the likely influence of XO-2N’s variability on the derivation of XO-2b’s transit depth, we cannot bin multiple nights of data to decrease our uncertainties, preventing us from constraining its gas abundances. This study demonstrates that long-term monitoring programs of exoplanet host stars are crucial for understanding host star variability.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Dependency of active pressure and equation of state on stiffness of wall

Autonomous motion and motility are hallmarks of active matter. Active agents, such as biological cells and synthetic colloidal particles, consume internal energy or extract energy from the environment to generate self-propulsion and locomotion. These systems are persistently out of equilibrium due to continuous energy consumption. It is known that pressure is not always a state function for generic active matter. Torque interaction between active constituents and confinement renders the pressure of the system a boundary-dependent property. The mechanical pressure of anisotropic active particles depends on their microscopic interactions with a solid wall. Using self-propelled dumbbells confined by solid walls as a model system, we perform numerical simulations to explore how variations in the wall stiffness influence the mechanical pressure of dry active matter. In contrast to previous findings, we find that mechanical pressure can be independent of the interaction of anisotropic active particles with walls, even in the presence of intrinsic torque interaction. Particularly, the dependency of pressure on the wall stiffness vanishes when the stiffness is above a critical level. In such a limit, the dynamics of dumbbells near the walls are randomized due to the large torque experienced by the dumbbells, leading to the recovery of pressure as a state variable of density.

42 ENGINEERING↗
DOE OSTI2011

Process Simulation of Cold Pressing and Sintering of Armstrong CP-Ti Powders

A computational methodology is presented for the process simulation of cold pressing and sintering of Armstrong CP-Ti powders. Since the powder consolidation is governed by specific pressure-dependent constitutive equations, solution algorithms were developed for the ABAQUS user material subroutine, UMAT, for computing the plastic strain increments based on an implicit integration of the nonlinear yield function, flow rule, and hardening equations. Sintering was simulated using a model based on diffusional creep using the user subroutine CREEP. The initial mesh, stress, and density for the simulation of sintering were obtained from the results of the cold pressing simulation, minimizing the errors from decoupling the cold pressing and sintering simulations. Numerical simulation results are presented for the cold compaction followed by a sintering step of the Ti powders. The numerical simulation results for the relative density were compared to those measured from experiments before and after sintering, showing that the relative density can be accurately predicted. Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. ACKNOWLEDGEMENTS This research was sponsored by the U.S. DOE, and carried out at ORNL, under Contract DE-AC05-00OR22725 with UT-Battelle, LLC. This research was sponsored by the U.S. DOE, EERE Industrial Technology Program Office under CPS Agreement # 17881.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Growth of polyaromatic molecules via ion-molecule reactions: An experimental and theoretical mechanistic study

The reactivity of naphthyl cations with benzene is investigated in a joint experimental and theoretical approach. Experiments are performed by using guided ion beam tandem mass spectrometers equipped with electron impact or atmospheric pressure chemical ion sources to generate C{sub 10}H{sub 7}{sup +} with different amounts of internal excitation. Under single collision conditions, C-C coupling reactions leading to hydrocarbon growth are observed. The most abundant ionic products are C{sub 16}H{sub 13}{sup +}, C{sub 16}H{sub n}{sup +} (with n=10-12), and C{sub 15}H{sub 10}{sup +}. From pressure-dependent measurements, absolute cross sections of 1.0{+-}0.3 and 2{+-}0.6 A{sup 2} (at a collision energy of about 0.2 eV in the center of mass frame) are derived for channels leading to the formation of C{sub 16}H{sub 12}{sup +} and C{sub 15}H{sub 10}{sup +} ions, respectively. From cross section values a phenomenological total rate constant k=(5.8{+-}1.9)x10{sup -11} cm{sup 3} s{sup -1} at an average collision energy of about 0.27 eV can be estimated for the process C{sub 10}H{sub 7}{sup +}+C{sub 6}H{sub 6}{yields} all products. The energy behavior of the reactive cross sections, as well as further experiments performed using partial isotopic labeling of reagents, support the idea that the reaction proceeds via a long lived association product, presumably the covalently bound protonated phenylnaphthalene, from which lighter species are generated by elimination of neutral fragments (H, H{sub 2}, CH{sub 3}). A major signal relevant to the fragmentation of the initial adduct C{sub 16}H{sub 13}{sup +} belongs to C{sub 15}H{sub 10}{sup +}. Since it is not obvious how CH{sub 3} loss from C{sub 16}H{sub 13}{sup +} can take place to form the C{sub 15}H{sub 10}{sup +} radical cation, a theoretical investigation focuses on possible unimolecular transformations apt to produce it. Naphthylium can act as an electrophile and add to the {pi} system of benzene, leading to a barrierless formation of the ionic adduct with an exothermicity of about 53 kcal mol{sup -1}. From this structure, an intramolecular electrophilic addition followed by H shifts and ring opening steps leads to an overall exothermic loss (-7.1 kcal mol{sup -1} with respect to reagents) of the methyl radical from that part of the system which comes from benzene. Methyl loss can take place also from the ''naphthyl'' part, though via an endoergic route. Experimental and theoretical results show that an ionic route is viable for the growth of polycyclic aromatic species by association of smaller building blocks (naphthyl and phenyl rings) and this may be of particular relevance for understanding the formation of large molecules in ionized gases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Compressional behavior of omphacite to 47 GPa

Omphacite is an important mineral component of eclogite. Single crystal synchrotron X-ray diffraction data on natural (Ca,Na)(Mg,Fe,Al)Si 2 O 6 omphacite have been collected at the Advanced Photon Source beamlines 13-BM-C and 13-ID-D up to 47 GPa at ambient temperature. Unit cell parameter and crystal structure refinements were carried out to constrain the isothermal equation of state and compression mechanism. The 3rd order Birch-Murnaghan equation of state (BM3) fit of all data gives V o = 423.9(3) Å3, K To = 116(2) GPa and K To ’ = 4.3(2). These elastic parameters are consistent with the general trend of the diopside-jadeite join. The eight-coordinated polyhedra (M2 and M21) are the most compressible, and contribute to majority of the unit cell compression, while the SiO 4 tetrahedra (Si1 and Si2) behave as rigid structural units and are the most incompressible. Axial compressibilities are determined by fitting linearized BM 3 equation of state to pressure dependences of unit cell parameters. Throughout the investigated pressure range, the b-axis is more compressible than the c-axis. Here, the axial compressibility of the α-axis is the largest among the three axes at 0 GPa, yet it quickly drops to the smallest at pressures above 5 GPa, which is explained by the rotation of the stiffest compression axis toward the a-axis with the increase of pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2020

High Pressure Infiltration/Expulsion of Aqueous NaCl in Planar Hydrophobic Nanopores

Pressure-driven permeation of water in a poorly wettable material results in a conversion of mechanical work into surface free energy representing a new form of energy storage, or absorption. When water is replaced by a concentrated electrolyte solution, the storage capacity of a nanoporous medium becomes comparable to high-end supercapacitors. The addition of salt can also reduce the hysteresis of the infiltration/expulsion cycle. Our molecular simulations provide a theoretical perspective into the mechanisms involved in the process, and underlying structures and interactions in compressed nanoconfined solutions. Specifically, we consider aqueous NaCl in planar confinements of widths of 1.0 nm and 1.64 nm and pressures of up to 3 kbar. Open ensemble Monte Carlo simulations utilizing fractional exchanges of molecules for efficient additions/removal of ions have been utilized in conjunction with pressure-dependent chemical potentials to model bulk phases under pressure. Confinements open to these pressurized bulk, aqueous electrolyte phases show reversibility at narrow pore sizes and irreversibility in wider ones, consistent with experiment, as well as strong hysteresis at both pore size. The addition of salt results in significant increases in the solid/liquid interfacial tension in narrower pores and associated infiltration and expulsion pressures. Furthermore, these changes are consistent with strong desalination effects at the lower pore size observed irrespective of external pressure and initial concentration.

25 ENERGY STORAGE↗
NASA NTRS1991

The high-pressure melting curve of iron - A technical discussion

The melting curve of iron is reliably determined to 105 GPa using the laser-heated diamond cell, in close agreement with independent measurements using piston-cylinder and large-volume presses or shock-wave experiments. In order to obtain reliable melting data from the internally heated diamond cell, whether by laser or Joule heating, temperature gradients across the sample must be quantitatively measured; otherwise, such 'wire heating' experiments can lead to significant underestimates of the melting temperature and its pressure dependence. The best estimate of the high-pressure melting curve of iron, as derived from the laser-heated diamond cell and Hugoniot temperature measurements, yields melting temperatures of 4800 + or - 200 K and 6700 + or - 400 K at 133 GPa and 243 GPa, respectively.

Williams, Quentin↗
DOE OSTI2015

Metal-insulator transition in Nd{sub 1−x}Eu{sub x}NiO{sub 3}: Entropy change and electronic delocalization

The metal-insulator (MI) phase transition in Nd{sub 1–x}Eu{sub x}NiO{sub 3}, 0 ≤ x ≤ 0.35, has been investigated through the pressure dependence of the electrical resistivity ρ(P, T) and measurements of specific heat C{sub P}(T). The MI transition temperature (T{sub MI}) increases with increasing Eu substitution and decreases with increasing pressure. Two distinct regions for the Eu dependence of dT{sub MI}/dP were found: (i) for x ≤ 0.15, dT{sub MI}/dP is nearly constant and ∼−4.3 K/kbar; (ii) for x ≥ 0.15, dT{sub MI}/dP increases with x and it seems to reach a saturation value ∼−6.2 K/kbar for the x = 0.35 sample. This change is accompanied with a strong decrease in the thermal hysteresis in ρ(P, T) between the cooling and warming cycles, observed in the vicinity of T{sub MI}. The entropy change (ΔS) at T{sub MI} for the sample x = 0, estimated by using the dT{sub MI}/dP data and the Clausius-Clapeyron equation, resulted in ΔS ∼ 1.2 J/mol K, a value in line with specific heat measurements. When the Eu concentration is increased, the antiferromagnetic (AF) and the MI transitions are separated in temperature, permitting that an estimate of the entropy change due to the AF/Paramagnetic transition be carried out, yielding ΔS{sub M} ∼ 200 mJ/mol K. This value is much smaller than that expected for a s = 1/2 spin system. The analysis of ρ(P, T) and C{sub P}(T) data indicates that the entropy change at T{sub MI} is mainly due to the electronic delocalization and not related to the AF transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1983

Calculation of symmetric vortex separation affecting subsonic bodies at high incidence

A method for calculating the longitudinal aerodynamic coefficients and the pressure distributions on a body at reasonably high angles of attack is presented. The body is represented by a combination of source elements and vortex-lattice elements, including separation of the vortices at increasing angles of attack. The method is self-consistent in that the body and the separated vortex wake are treated as an integrated interacting system. The location of the separation line can be included as an arbitrary input from experimental data or can be evaluated approximately by a pressure-dependent criterion. Calculated values of the aerodynamic coefficients and pressure distributions on cone-cylinder and ogive-cylinder bodies compare well, qualitatively and quantitatively, with experimental data, including simulation of the dependence on Reynolds number.

Almosnino, D.↗
DOE OSTI2022

Pressure-induced changes of valence fluctuation in β - YbAlB 4 probed by x-ray absorption spectroscopy

Here we report detailed valence fluctuation phenomena in the unique quantum-critical compound β-YbAlB 4 under pressures and at low temperatures. We directly observed the drastic change in the pressure dependence of the mean Yb valence at about 3 GPa with no pressure-induced structural transition. Below 3 GPa, the anomaly in the temperature dependences of the mean Yb valence was observed at about 55 K under ambient pressure, corresponding to a previously observed reduced Kondo coherence temperature, and this temperature rapidly decreases to 22 K at 2.2 GPa, in contrast to the monotonous decrease of the high Kondo temperature (~250K at ambient pressure). The present results reveal that this reduced Kondo coherence temperature is responsible for the unconventional quantum critical behaviors in β-YbAlB 4 . Above 3 GPa, it was found that the temperature dependence of the mean Yb valence exhibits a discontinuous change at low temperature (10.5 and 9 K under 4.8 and 5.5 GPa, respectively).

36 MATERIALS SCIENCE↗
DOE OSTI2024

Electronic structure and lattice dynamics of 1⁢𝑇−VSe 2 : Origin of the three-dimensional charge density wave

To characterize in detail the charge density wave (CDW) transition of 1⁢𝑇−VSe 2 , its electronic structure and lattice dynamics are comprehensively studied by means of x-ray diffraction, muon spectroscopy, angle resolved photoemission (ARPES), diffuse and inelastic x-ray scattering, and state-of-the-art first-principles density functional theory calculations. Resonant elastic x-ray scattering does not show any resonant enhancement at either V or Se, indicating that the CDW peak at the 𝐾 edges describes a purely structural modulation of the electronic ordering. ARPES experiments identify (i) a pseudogap at 𝑇 > 𝑇⁢ CDW , which leads to a depletion of the density of states in the ML-M'L' plane at 𝑇 < 𝑇⁢ CDW , and (ii) anomalies in the electronic dispersion reflecting a sizable impact of phonons on it. A diffuse scattering precursor, characteristic of soft phonons, is observed at room temperature (RT) and leads to the full collapse of the low-energy phonon (𝜔 1 ) with propagation vector (0.25 0 −0.3) r.l.u. Here, we show that the frequency and linewidth of this mode are anisotropic in momentum space, reflecting the momentum dependence of the electron-phonon interaction (EPI), hence demonstrating that the origin of the CDW is, to a much larger extent, due to the momentum dependent EPI with a small contribution from nesting. The pressure dependence of the 𝜔 1 soft mode remains nearly constant up to 13 GPa at RT, with only a modest softening before the transition to the high-pressure monoclinic C2/m phase. The wide set of experimental data is well captured by our state-of-the art first-principles anharmonic calculations with the inclusion of van der Waals corrections in the exchange-correlation functional. The comprehensive description of the electronic and dynamical properties of VSe 2 reported here adds important pieces of information to the understanding of the electronic modulations in the family of transition-metal dichalcogenides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2001

Band Anticrossing in Highly Mismatched Compound Semiconductor Alloys

Compound semiconductor alloys in which metallic anions are partially replaced with more electronegative isoelectronic atoms have recently attracted significant attention. Group IIIN(x)V(1-x), alloys with a small amount of the electronegative N substituting more metallic column V elements has been the most extensively studied class of such Highly Mismatched Alloys (HMAs). We have shown that many of the unusual properties of the IIIN(x),V(1-x) alloys can be well explained by the Band Anticrossing (BAC) model that describes the electronic structure in terms of an interaction between highly localized levels of substitutional N and the extended states of the host semiconductor matrix. Most recently the BAC model has been also used to explain similar modifications of the electronic band structure observed in Te-rich ZnS(x)Te(l-x) and ZnSe(Y)Te(1-y) alloys. To date studies of HMAs have been limited to materials with relatively small concentrations of highly electronegative atoms. Here we report investigations of the electronic structure of ZnSe(y)Te(1-y) alloys in the entire composition range, 0 less than or equal to y less than or equal to 1. The samples used in this study are bulk ZnSe(y)Te(1-y) crystals grown by either a modified Bridgman method or by physical vapor transport. Photomodulated reflection (PR) spectroscopy was used to measure the composition dependence of optical transitions from the valence band edge and from the spin-orbit split off band to the conduction band. The pressure dependence of the band gap was measured using optical absorption in a diamond anvil cell. We find that the energy of the spin-orbit split off valence band edge does not depend on composition and is located at about 3 eV below the conduction band edge of ZnSe. On the Te-rich side the pressure and the composition dependence of the optical transitions are well explained by the BAC model which describes the downward shift of the conduction band edge in terms of the interaction between localized Se states and the conduction band. On the other hand we show that the large band gap reduction observed on the Se-rich side of the alloy system is a result of an interaction between the localized Te level and the valence bands. This interaction leads to the formation of a Te-like valence band edge that strongly interacts with the light hole valence band. Calculations based on a modified k p model account for the reduction of the band gap and the large increase of the spin-orbit splitting observed in Se-rich ZnSe(y)Te(l-y) alloys. We will also discuss the importance of these new results for understanding of the electronic structure and band offsets in other highly mismatched alloy systems.

Yu, Kin Man↗