Spontaneous Long-range Order Research Articles

Optical spectroscopic observation of spontaneous long range ordering in AlGaInP

We report the unambiguous optical spectroscopic observation of spontaneous long-range crystal ordering in AlGaInP epitaxial layers grown by metalorganic vapor phase epitaxy. Polarized spectroscopy demonstrates the existence of an in-plane anisotropy for samples grown on substrates oriented close to (100). This anisotropy results from the order-induced splitting of the normally degenerate zone-center valence-band states. Our measurements allow the size of the valence-band splitting to be measured and also allow a determination of the order-induced band-gap reduction. Both of these quantities are found to decrease with increasing misorientation of the substrate away from (100), indicating that the degree of ordering also decreases with increasing misorientation.

Raman scattering in single-variant spontaneously ordered GaInP2.

Raman scattering is used to study the phenomenon of spontaneous long-range ordering in ${\mathrm{GaInP}}_{2}$ epilayers. The measurements are performed in a series of single-variant ordered samples with different order parameter values. The samples have been grown by metalorganic vapor phase epitaxy on GaAs (001) substrates misoriented 6\ifmmode^\circ\else\textdegree\fi{} off towards the [111]B direction, and the ordering degree has been varied by changing the growth temperature. The Raman selection rules of the ordered phase are derived, using group theory. Polarized Raman scattering measurements allow the identification of modes associated with the trigonal atomic arrangement. \textcopyright{} 1996 The American Physical Society.

Optimization of Collagen Liquid Crystalline Assemblies: Influence of Sonic Fragmentation

We have previously demonstrated that highly concentrated solutions of type I calf skin collagen form liquid crystalline assemblies whose three-dimensional arrangements follow geometries described in extracellular matrices. Studying these self-assemblies in vitro intimates how structures of similar appearance might self-assemble in vivo. The spontaneous long-range order of collagen triple helices is easier to obtain once the protein is fragmented by ultrasonic waves. In the present work we show by X-ray diffraction and differential scanning calorimetry that the structural stability of collagen fragments is preserved after sonication times reaching 40 min. The sonicated polypeptide chains analyzed by SDS gel electrophoresis show after 30 min of sonication a wide polydispersity in molecular weights with a population ranging from 100 kDa and higher size aggregates to small fragments of 30 kDa. The sonicated molecular population, observed in electron microscopy after unidirectional platinum shadowing, shows after 20 min of sonication a preponderant population of fragments below 90 nm in length. The present results indicate that the best time for the emergence of liquid crystalline phases corresponds, under our experimental conditions, to sonications of 20 min. The cholesteric order, monitored in optical microscopy between crossed polars, is then significantly optimized both in time and in space.

Polarization-dependent XAFS Measurements of Spontaneous Ordering in MOCVD-Grown Ga0.5In0.5P on GaAs Substrates

Certain thin-film alloys deposited using epitaxial growth techniques have shown spontaneous long-range ordering and band gap reduction under particular growth conditions. One such thin film is Ga0.5In0.5P grown by MOCVD on single-crystal GaAs aligned 2° from the (100) axis to achieve lattice match. Quantitative results on the gallium nearest neighbor and second nearest neighbor distances, coordination number and disorder parameters will be presented and short range structural ordering will be discussed for several ordered and disordered samples. The data reported are obtained at room and liquid nitrogen temperatures using the total electron yield detection.

Compositional modulation and long-range ordering in GaP/InP short-period superlattices grown by gas source molecular beam epitaxy

Long-range ordering in a (GaP)2/(InP)2 short-period superlattice and a Ga0.525In0.475P buffer layer grown on a (001)GaAs substrate by gas source molecular beam epitaxy were studied. Transmission electron microscopy and low-temperature cathodoluminesence techniques were used to examine the microstructure of the short-period superlattice and to determine its band-gap energy. The superlattice layer was found to have a [001] long-range ordered structure with a band gap narrowing of about 130 meV, while the Ga0.525In0.475P layer had a 37 meV band-gap narrowing induced by spontaneous long-range ordering in the [111] direction. The ordered superlattice layer was found to have a growth-induced lateral periodic modulation of the composition along the [1̄10] direction. Within the modulating bands, which had a 200 Å periodicity, the In composition was found to vary from 42 to 56% while the Ga correspondingly varied between 58 and 44%.

Band gaps and spin-orbit splitting of ordered and disordered AlxGa1-xAs and GaAsxSb1-x alloys.

Spontaneous long-range ordering of the otherwise disordered isovalent semiconductor alloys A/sub x/B/sub 1-x/C has been recently observed in numerous III-V alloy systems exhibiting the CuAu-I, CuPt, and chalcopyrite structures. We present a theory for the ordering-induced changes in the Brillouin-zone-center electronic properties, with application to the Al/sub x/Ga/sub 1-//sub x/As and GaAs/sub x/Sb/sub 1-//sub x/ alloys. The dominant effect for these systems is shown to be level repulsion between different-symmetry states of the binary constituents which fold into equal-symmetry states in the ordered ternary structures. Strong variations in the band gaps, spin-orbit splittings, and charge densities among the three basic ordered structures reflect the different magnitudes of the symmetry-enforced coupling between the folded states. An extension of the model to the disordered alloys yields good agreement with the observed optical bowing parameters for the fundamental gaps; however, the positive (downward concave) bowing of the spin-orbit splitting observed in some common-cation semiconductor alloy remains an unexplained puzzle.

Ordering of isovalent intersemiconductor alloys.

Recent observations of spontaneous long-range ordering in alloys of isovalent semiconductors, despite independent evidence for repulsive interactions in these systems, are interpreted in terms of first-principles total-energy calculations for ordered compounds and cluster-variation-method calculations for the disordered phase.

Ordering-induced changes in the optical spectra of semiconductor alloys

It is shown how the recently predicted and subsequently observed spontaneous long-range ordering of pseudobinary A0.5B0.5C isovalent semiconductor alloys into the (AC)1(BC)1 superlattice structure (a CuAuI-type crystal) gives rise to characteristic changes in the optical and photoemission spectra. We predict new direct transitions and substantial splittings of transitions absent in the disordered alloy.

Upper bound to the spin correlation function of the n-vector model with random anisotropic interactions

We obtain an upper bound to the spin correlation function in the thermodynamic limit of the zero external field limit for the n-vector model with random anisotropic interactions. We find a sufficient condition for disappearance of the spontaneous long-range order.

ESR line shift in NiSnCl 6·6H 2O at very low temperatures

Electron spin resonance in a single-ground-state magnet, NiSnCl 6·6H 2O, has been studied at temperatures between 83 mK and 4.2 K. Below 1 K, the resonance lines show marked shifts, which can be described quantitatively in terms of exchange shift due to polarization effects treated by McMillan and Opechowski. The shifts give 2 J = −(0.55 ± 0.15) × 10 −2 cm −1 for the antiferromagnetic exchange interaction among the Ni 2+ ions in this crystal. The value of 2 J is sub-critical in the sense that no spontaneous long-range order can be expected in this singlet-ground-state magnet.