Electronic structure, magnetic ordering, and formation pathway of the transition metal oxide hydride LaSrCoO(3)H(0.7)

The role of the hydride anion in controlling the electronic properties of the transition metal oxide hydride LaSrCoO(3)H(0.7) is investigated theoretically by full potential DFT band structure calculation and experimentally by determination of the Neel temperature for three-dimensional magnetic ordering. The mechanism by which hydrogen is introduced into the solid is addressed by in situ X-ray diffraction studies of the formation of the oxide hydride, which reveal both a relationship between the microscopic growth of the observed oxide hydride order and the anisotropic broadening of the diffraction profile, and the existence of a range of intermediate compositions.     

Isolating the effect of torsional defects on mobility and band gap in conjugated polymers

Conjugated polymers represent a promising class of organic semiconductors with potential applications in a variety of molecular devices. Poly(3-alkylthiophene)s, in particular, are garnering interest due to their large charge carrier mobility and band gap in the visible region of the spectrum. Defects play a pivotal role in determining the performance of polymer electronics, and yet the function of specific types of defects is still largely unknown. Density functional theory calculations of alkyl-substituted oligothiophenes are used to isolate the effect of static inter-ring torsion defects on key parameters such as electronic coupling between rings and band gap. Results have potential implications both for the fundamental understanding of intramolecular charge transport and for improving processing in organic devices.     

Chiral II-VI semiconductor nanostructure superlattices based on an amino acid ligand

Reaction of L-cysteine with M(NO3)2 x xH2O (M = Cd, Zn) generates M(L-cysteinate), which feature one-dimensional substructures that can be viewed as fragments of bulk structures of CdS (rock salt high pressure phase) and ZnS (wurtzite) because of the bridging modes accessible to the sulfur atom of L-cysteine. The MS substructures are arranged in a regular and periodic fashion within the crystal via the carboxylate function of L-cysteine. Considering the structural similarities with bulk materials, the optical properties of M(L-cysteinate) were studied and indicate blue shifts of the band gap of 2.59 eV (M = Cd, compared to CdS rock salt) and 1.37 eV (M = Zn, compared to ZnS wurtzite) with respect to the bulk MS structures, due to the low dimensionality of the metal-sulfur arrangement. The chelating nature of the cysteine ligand imposes an unusual mer arrangement of three binding S moieties at Cd with a correspondingly high Cd coordination number in a chalcogenide-based material. Density of states calculations show strong electronic structure similarities with the bulk phases and rationalize the band gap changes.     

Wetting of mixed OHH(2)O layers on Pt(111)

We describe the effect of growth temperature and OHH(2)O composition on the wetting behavior of Pt(111). Changes to the desorption rate of ice films were measured and correlated to the film morphology using low energy electron diffraction and thermal desorption of chloroform to measure the area of multilayer ice and monolayer OHH(2)O exposed. Thin ice films roughen, forming bare (radical39 x radical39)R16 degrees water monolayer and ice clusters. The size of the clusters depends on growth temperature and determines their kinetic stability, with the desorption rate decreasing when larger clusters are formed by growth at high temperature. Continuous films of more than approximately 50 layers thick stabilize an ordered incommensurate ice film that does not dewet. OH coadsorption pins the first layer into registry with Pt, forming an ordered hexagonal (OH+H(2)O) structure with all the H atoms involved in hydrogen bonding. Although this layer has a similar honeycomb OH(x) skeleton to ice Ih, it is unable to reconstruct to match the bulk ice lattice parameter and does not form a stable wetting layer. Water aggregates to expose bare monolayer (OH+H(2)O), forming bulk ice crystallites whose size depend on preparation temperature. Increasing the proportion of water in the first layer provides free OH groups which stabilize the multilayer. The factors influencing multilayer wetting are discussed using density functional theory calculations to compare water adsorption on top of (OH+H(2)O) and on simple models for commensurate water structures. We show that both the (OH+H(2)O) structure and "H-down" water layers are poor proton acceptors, bonding to the first layer being enhanced by the presence of free OH groups. Formation of an ordered ice multilayer requires a water-metal interaction sufficient to wet the surface, but not so strong as to prevent the first layer relaxing to stabilize the interface between the metal and bulk ice.     

Chemical Bonding and Atomic Structure in Y(2) O(3) :ZrO(2) -SrTiO(3) Layered Heterostructures

Repeating boundaries: The buried interfaces in artificial heterostructures produced by sequential deposition of nanosized units are critical to their properties. With density functional theory it was shown that in Y(2) O(3) :ZrO(2) (YSZ) and SrTiO(3) (STO) heterostructures reconstruction of the interfaces between the component units is required to access the most favorable structure.     

Electric field effects in RUS measurements

Much of the power of the Resonant Ultrasound Spectroscopy (RUS) technique is the ability to make mechanical resonance measurements while the environment of the sample is changed. Temperature and magnetic field are important examples. Due to the common use of piezoelectric transducers near the sample, applied electric fields introduce complications, but many materials have technologically interesting responses to applied static and RF electric fields. Non-contact optical, buffered, or shielded transducers permit the application of charge and externally applied electric fields while making RUS measurements. For conducting samples, in vacuum, charging produces a small negative pressure in the volume of the material - a state rarely explored. At very high charges we influence the electron density near the surface so the propagation of surface waves and their resonances may give us a handle on the relationship of electron density to bond strength and elasticity. Our preliminary results indicate a charge sign dependent effect, but we are studying a number of possible other effects induced by charging. In dielectric materials, external electric fields influence the strain response, particularly in ferroelectrics. Experiments to study this connection at phase transformations are planned. The fact that many geological samples contain single crystal quartz suggests a possible use of the piezoelectric response to drive vibrations using applied RF fields. In polycrystals, averaging of strains in randomly oriented crystals implies using the “statistical residual” strain as the drive. The ability to excite vibrations in quartzite polycrystals and arenites is explored. We present results of experimental and theoretical approaches to electric field effects using RUS methods.     

Artificial construction of the layered Ruddlesden-Popper manganite La2Sr2Mn3O10 by reflection high energy electron diffraction monitored pulsed laser deposition

Pulsed laser deposition has been used to artificially construct the n = 3 Ruddlesden-Popper structure La(2)Sr(2)Mn(3)O(10) in epitaxial thin film form by sequentially layering La(1-x)Sr(x)MnO(3) and SrO unit cells aided by in situ reflection high energy electron diffraction monitoring. The interval deposition technique was used to promote two-dimensional SrO growth. X-ray diffraction and cross-sectional transmission electron microscopy indicated that the trilayer structure had been formed. A site ordering was found to differ from that expected thermodynamically, with the smaller Sr(2+) predominantly on the R site due to kinetic trapping of the deposited cation sequence. A dependence of the out-of-plane lattice parameter on growth pressure was interpreted as changing the oxygen content of the films. Magnetic and transport measurements on fully oxygenated films indicated a frustrated magnetic ground state characterized as a spin glass-like magnetic phase with the glass temperature T(g) ≈ 34 K. The magnetic frustration has a clear in-plane (ab) magnetic anisotropy, which is maintained up to temperatures of 150 K. Density functional theory calculations suggest competing antiferromagnetic and ferromagnetic long-range orders, which are proposed as the origin of the low-temperature glassy state.     

Excess low frequency conduction noise in a granular composite

Granular composites consisting of 25% nickel as 8 nm diameter particles dispersed in an aluminium oxide matrix display excess conduction noise. Co-deposited films with resistance per square about 10⁵ ohms and negative temperature coefficient show a noise power spectral density $\text{S}{}_{\mathrm{v}}\text{(?) =}\text{S}{}_{\mathrm{v}}\text{(1)}\text{?}{}^{\mathrm{\alpha }}$ where α ? 1.10 ± 0.03 over the accessible spectral range of 0.1 Hz ? ? ? 5000 Hz. The amplitude 3 × 10^?15 ? S_v(1) ? 5 × 10^?12 V²Hz^?1, appears to increase approximately quadratically as the applied voltage V_s up to V_s ? 2.5 V and as the first power of V_s for 2.5 ? V_s < 35 V.