High dynamic pressure is used to synthesize metastable superconducting materials and to process superconductors into practical form. Specimens are subjected to shock pressures in the range 0.1-1 Mbar, temperatures up to several 1000 K and quenches at rates up to 1012 bar/s and 109 K/s. Shock waves are generated by impact of projectiles accelerated up to 3.5 km/s by the LLNL two-stage light-gas gun. Recovered specimens are characterized for superconductivity and structural properties. The gas gun, specimen recovery fixture, and phenomenology of the technique are described. Results are reviewed and include a quenched solid-solid phase transition (A15 Nb3Si); amorphous, fine-grained, and multiphase materials produced by shock-induced melting of powders and subsequent rapid solidification; and the shock-compaction in a metal matrix of thin powder layers of high-Tc oxides mixed with metal. The latter is a potential method for fabricating superconducting wire, which could be scaled to long lengths using explosives.
For several reasons, the nitrogen oxides are of interest as possible sources of new metastable polymeric materials which form only at high pressures and may possess superior mechanical properties and thermal/electrical conductivities. In this study, molecular electronic structure calculations, both semi-empirical and ab initio, have been conducted for oligomers of NO, N2O and NO2. A number of linear, cyclic, and stacked forms have been identified as local minima on the potential energy hypersurface. Satisfactory agreement with experiment was found for the known (SN)x. Calculations of the kinetics of depolymerization of the nitrogen oxide oligomers, i.e. of the energy barriers surrounding the metastable species, are under way.
Theoretical calculations by R. Bardo predict that under high pressure, p-Nitroaniline (PNA) forms semiconducting condensation products which possibly may be retained at ambient conditions. A combination phase-reaction P,T diagram has been determined in order to evaluate the correctness of the prediction and to indicate possible synthesis routes. The data were obtained by a combination of optical microscopy, x-ray diffraction, and Fourier transform infrared (FTIR) spectroscopy coupled with a diamond anvil high pressure cell. The results show, (1) a reversible crystallographic transition occurs at about 4.0 GPa; (2) a reversible transition at about 1.0 GPa and 140°C which effects the optical properties of the material also occurs; (3) the melting point increases to 245°C at 1.0 GPa; and (4) irreversible chemical reactions begin to be significant above 280°C and 1.0 GPa. The reaction products were investigated by mass spectrometry. A series of dimers with N or N-N linkages are formed under pressure at moderate temperatures. If temperatures at pressure are excessive, i. e., above 360°C, then the PNA reacts to form an amorphous material stable when heated to over 350°C.
The application of pressure is known to enormously enhance electrical conductivities of existing insulating and semiconducting organic polymers and organic salts. However, none of these compounds are metastable, so that their superior high-pressure characteristics are not retained when the pressure is removed. In order to meet SDI requirements for improved, highly-sensitive optical and infrared detectors, for example, any breakthrough metastable organic conductor must combine superior mechanical and electrical properties which are maintained over large ranges of pressure and temperature. Semiempirical and ab initio LCAO-SCF and CI "tight-binding" calculations of the ground and excited electronic states of p-nitroaniline (PNA) indicate that certain polymorphs may have the potential for becoming strong quasi-one-dimensional metastable semiconductors when processed with uniaxial shocks. These calculations were done for the head-to-tail orientation and at various intermolecular separations of the PNA monomers in (PNA)n, n=2 and 3. The results show that the excited states associated with the intense charge transfer band and a weaker band are strongly red-shifted from 4.4 ev at ambient pressure to optical gaps of 3.76 ev and 1.33 ev, respectively, at a pressure of roughly 400 kbar. The effect of these shifts is to greatly enhance intra-and inter-molecular charge transfer during excitation. The accompanying increase of the monomer dipole moment by 4.5D at 400 kbar may result in strong crystals due to the increased dipole-dipole interaction. The state at 1.33 ev may facilitate an insulator-to-semi-conductor transition. For this state, electronic charge hopping can occur between the NO2 and NH2 groups of nearest-neighbor monomers. The minimum activation energy for hopping is found to be 0.4 ev at 0°K.
Angular-resolved synchrotron radiation photoemission spectroscopy and reflection high energy electron diffraction (RHEED) are used to study the MBE growth and the thermal stability of α-Sn/CdTe(110) and CdTe/α-Sn(110) interfaces. Both interfaces are found to be stable, nonreactive, and abrupt for growth temperatures up to 100 C. At the α-Sn/CdTe interface we measured a valence band offset of ΔEv = 1.1eV. Shifts in the Sn4d core level and valence band spectra suggest a bandgap in a 50 monolayer (ML) thick α-Sn film of at least ΔEg ≥ 200meV. Stable superlattices of α-Sn/CdTe(110) have been grown at 100 C. The surface quality of the superstructure degrades after the growth of several α-Sn/CdTe periods. After the growth of 10 periods each 50 ML thick, the RHEED pattern shows mainly three dimensional bulk diffraction, indicating increased surface and interface roughness.
We report here the results of Hgl-xZnxTe-CdTe strained-layer superlattices grown by MBE. This superlattice system has been chosen in order to investigate the effect of the strain in Type III and Type I superlattices. Hgl-xZnxTe-CdTe superlattices with different strain have been grown on CdTe(111)B/GaAs(100) and CdTe(100)/GaAs(100) substrates and characterized in situ by electron diffraction and ex situ by X-ray diffraction, infrared transmission and Hall measurements. The high quality of Hgl-xZnxTe-CdTe super-lattices is attested by the presence of satellite peaks in the X-ray spectra. The values of hole mobilities between 5x103 up to 2x104cm2v-ls-1 at T-23K along (111)B growth orientation and up to 4.9x104cm2v-1s-1 at 5K along (100) growth orientation are obtained for Type III superlattices whereas in Type I superlattices hole mobility is between 200-300cm2v-ls-1. Hg1-xZnxTe-CdTe superlattices with n-type character have also been obtained. A p- to n-type transition is observed when increasing HgZnTe layer thickness and/or decreasing CdTe layer thickness. Electron mobilities as high as 2.5x10-5cm2v-ls-1 have been reached.
Magneto-transport and photo-transport measurements on MBE-grown HgTe/CdTe and HgZnTe/CdTe superlattices have been made. A detailed analysis of the magnetic-field-dependent Hall data shows that mixed conduction by more than one carrier species is significant at all temperatures. The T-dependence of the intrinsic carrier density has been used to characterize the bandgap and the product of the electron and hole density-of-states effective masses. Hole mobilities greater than 105 cm2/Vs are reported. Photo-Hall measurements employing CO2 laser-irradiation to generate excess electrons and holes are shown to yield minority electron mobilities and free-carrier lifetimes. Theoretical band-structure calculations by the tight-binding method are consistent with the experimental results in that they predict not only zero bandgap but also electron and hole effective masses which are both considerably less than O.Olmo. Calculated superlattice band structures are found to display a number of features which suggest the potential for large optical nonlinearities. Experimentally third-order nonlinear susceptibilities have been determined by the non-degenerate four-wave mixing technique, using a pair of grating tuned, Q-switched CO2 lasers. Even though the samples were grown without regard to optimizing the nonlinear optical properties, a x(3) of nearly 10-4 esu was observed in the non-degenerate four-wave mixing configuration.
HgTe and zero-gap HgCdTe have record, picosecond speed, optical nonlinearities at 10.6μ. They are caused by carrier temperature modulation, which produces large carrier density variations in zero-gap materials. The thermal process is only beginning to saturate at 1 MW/cm2; at that intensity , the dielectric constant of HgTe is modulated by about 10%. Further enhancement of these nonlinearities may be achieved with suitable doping or alloying.
A class of organic molecules which will photoisomerize when irradiated into resonances in the 2.7-3.4 micron region is described. These are single photon reactions which can have a high quantum efficiency and which can lead to optical nonlinearities. The optical properties of host-guest materials containing such photoisomerizable compounds are discussed.
This paper is concerned with the development and theoretical modeling of a new class of organic nonlinear optical media, namely the polyacene quinones. The research is a collaborative effort involving research groups with expertise in the following areas: synthesis (L. L. Miller and coworkers), laser spectroscopy (P. F. Barbara and coworkers), computational chemistry (J. Almlof and coworkers) and solid state chemistry (M. C. Etter and coworkers). Polyacene quinones are large, delocalized aromatic molecules that offer extraordinarly large electronic π delocalization lengths, which is an important attribute for the the realization of large nonlinear optical effects in organic media. Several large polyacene quinones have been synthesized and the methodology has been demonstrated to be general. Considerable progress has been made in studying the electronic structure and optical transitions of small polyacene quinones, which is a prerequisite to developing a model for the nonlinear optical properties of the polyacene quinones. The first degenerate-four-wave-mixing measurements have been made on a polyacene quinone.
The processing and properties of Types I and II fused quartz optics and Types III and IV synthetic fused silica optics are compared with new organometallic sol-gel derived (Types V and VI) gel-silica optics. Type V gel-silica has excellent transmission from 165 nm to 4200 nm with no OH absorption peaks. Other physical properties and structural characteristics of Type V gel-silica are similar to high grade fused silica but offer the advantages of near net shape cast4ng, including internal cavities, and a lower coefficient of thermal expansion of 0.2 x 10-6 cm/cm compared with 0.55 x 10-6 cm/cm. Optically transparent porous gel silica (Type VI) has a UV cut-off ranging from 250-300 nm. Type VI gel-silica optics has a density as low as 60% of Types I-V silica and can be impregnated with up to 30 to 40% by volume of a second phase optically active organic or inorganic compound.
We report the first observation of modification of the surface-refractive index of sol-gel silica glasses for waveguide fabrication. Techniques for fabrication and measurements of attenuation and index change are reported. Preliminary results are encouraging showing the viabilitysof the sol-gel technology in integrated optical applications.
A new concept in polymeric materials has been developed by Hoechst Celanese Research Division in Summit, New Jersey. By combining several functional side-groups with a polymeric backbone, novel materials which will combine mechanical integrity and ease of fabrication with other properties, such as non-linear optical activity and piezo- or pyro-electricity, are possible. This work is being sponsored by SDIO/IST and managed by AFOSR.
x(3) values have been determined by degenerate four wave mixing (DFNN) measurements for several new polymers prepared by condensation of tetraaminobenzene with a variety of bisalkylaminovinyldichloroquinones. By control of reaction c9Vitions, both open chain and fully fused ring polymers have been prepared and characterized. Comparable x(3) values (typically in the range 7 X 10 to 4 X 10 esu at 532 nm) are observed for the two forms of the polymer suggesting comparable electron delocalization lengths in the two forms. This observation is, in turn, suggestive of the importance of protonation effects in these nitrogen containing polymers. Derivatization with the vinylamine substituents not only affects polymer solubility and optical properties but also influences condensation polymerization kinetics. Open chain polymers of moderately high molecular weight can be prepared by reaction of the monomers in dimethylformamide am at ambient temperatures. The results for aminovinyl polymers are compared briefly with the results obtained for other ladder polymers prepared in our laboratory.
Monolayer and multilayer Langmuir-Blodgett films of two systems: (i) polyalkylthiophene and (ii) tetrakis cumylphenoxy phthalocyanines in pure form and mixed phase with octadecanol were prepared. We report here the first observation of a degenerate four wave mixing signal from a monolayer Langmuir-Blodgett film that of phthalocyanine, due to a large resonant third order optical nonlinearity, x(3) . The effective x(3) behaviors of both phthalocyanine and polythiophene Langmuir-Blodgett films were investigated by time-resolved femtosecond degenerate four wave mixing. The polythiophene Langmuir-Blodgett film when doped with iodine was found to be highly conductive. We have also observed a dramatic reduction of the nonlinear reflectivity as a consequence of iodine doping of polythiophene Langmuir-Blodgett films.
The nonlinear optical response of organic and polymeric materials arises from the polarization response of delocalized π-electrons. The susceptibility x(2) or x(3) is the macroscopic observable tensor sum of the molecular hyperpolarizabilities β or γ including the effects of molecular response and orientation within the sample. A two-dimensional thin film provides a different local environment for the π electrons responsible for the fast (femtosecond) purely electronic optical nonlinearity of organic materials from three-dimensional analogues. Langmuir-Blodgett thin film technology is well suited to production of two-dimensional films and layered structures incorporating molecules, even with different nonlinear optical properties in each layer. This paper describes a program for fabrication and evaluation of organic heterostructure thin films by the Langmuir-Blodgett technique for applications to nonlinear optics.
Novel functional polymers and guest-host polymer blends designed for optical and electronic applications are being synthesized and studied to obtain a fundamental understanding of the molecular factors that affect the technological potential of these systems. As a prototype for a polymer blend system in which one component is electronically active, blends of poly (g-phenylene vinylene)(PPV) with several water-soluble, inert, polymers have been studied. Though specific molecular interactions are generally absent in these systems, the presence of the second component has profound effects on many of the properties of PPV in the blend.
The sol-gel method for the preparation of oxide ceramics has been successfully applied for many materials in the past few years. Most recently, under SDIO/AFOSR sponsorship, this method has been applied to two types of materials. These are the high temperature superconducting ceramic oxides and the infrared transmitting sulfides. Bulk solids, fibers and thin films of YBa2Cu3O7 have been successfully prepared.
The crystal structure of the organic conductor derived from tetrathiofulvalene (TTF) and tetracyanoquinodimethane (TCNQ) is composed of segregated stacks of donor (TTF) and acceptor (TCNQ) molecules and ions. Charge transfer from donor to acceptor stacks gives a stable crystal where ions of the same charge pack with parallel molecular planes at short intermolecular distances. An ab initio quantum mechanical calculation using a 6-31G** basis set and the DMOL method has been carried out to examine the intermolecular bonding and Coulombic interactions in this crystal. Molecular relationships at the calculated energy minima are close to the observed crystal structure. Formation of extended intermolecular orbitals within each type of stack correlates with the intermolecular bonding which appears to be present in this material as shown by the short interplanar distances. A complete evaluation of the crystal stability could not be carried out owing to the complexity of the problem but the results show that as a stack of TCNQ anions is extended the energy of the highest occupied intermolecular orbital is reduced. Conversely, as a stack of TTF cations is extended, the energy of the highest occupied orbital is raised. The calculation is in agreement with the observed properties of the TTF-TCNQ crystal and suggests that interactions between molecular ions can be treated only by quantum mechanical methods.