Hydrogen-bonded ferroelectrics: Evidence for Slater configurations from E.P.R. studies of X-irradiated KH2AsO4 and NH4H2AsO4

The temperature dependence of the powder E.P.R. spectrum of the AsO₄ ^4- centre in x-irradiated samples of KH₂AsO₄ and NH₄H₂AsO₄ yields results which can be understood as arising from exchange processes taking place between the six configurations in the Slater model for this type of hydrogen-bonded ferroelectric and antiferroelectric. Knowing that the ⁷⁵As hyperfine parameters have rhombic symmetry in the compounds studied, it is possible to interpret certain specific features in the spectra as being associated with the Slater configurations of higher energy.     

Coherent manipulation of electron spins in the {Cu3} spin triangle complex impregnated in nanoporous silicon

We report on coherent manipulation of electron spins in an antiferromagnetically coupled spin triangle {Cu3-X} (X=As, Sb) impregnated in freestanding nanoporous silicon (NS) by using 240 GHz microwave pulses. Rabi oscillations are observed and the spin coherence time is found to be T(2)=1066 ns at 1.5 K. This demonstrates that the {Cu3-X}:NS hybrid material provides a promising scheme for implementing spin-based quantum gates. By measuring the spin relaxation times of samples with different symmetries and environments we give evidence that a spin chirality is the main decoherence source of spin triangle molecules.     

Growth and properties of microcrystalline germanium–carbide alloys grown using electron cyclotron resonance plasma processing

We report on the growth and properties of a new material, microcrystalline (Ge, C), which has potentially important optical, electrical and structural properties. The material was grown using a remote, low pressure electron cyclotron resosnance (ECR) plasma process on glass, stainless steel and c-Si substrates. The growth was done with hydrogen dilution and ion bombardment at temperatures of 350–400°C. We discovered that the optical absorption curve parallels that of c-Ge, with increased bandgaps as C is incorporated. We obtained up to 2% C incorporation, which increased the gap to 1.1 eV. At comparable bandgaps, the absorption coefficient of the (Ge, C) material is much larger than that of c-Si. Raman and X-ray measurements detected microcrystalline structure, and a dependence of grain size on the substrate used. The lattice constant contracted with C incorporation, approximately obeying Vegard’s law. Both undoped and n-doped materials were grown.     

Charge Carrier Transport 1n 1:1 Complexes of Tetracyanobenzene with Naphthalene Anthracene and Durene

Abstract

It is well established that crystals of charge transfer (CT) complexes formed between aromatic donors and acceptors are built up of linear stacks of closely packed molecules. There are two main types of CT crystal structure, (i) a parallel arrangement of linear stacks made up of the individual component molecules, AAAA…DDDD… and (ii) a parallel arrangement of linear stacks made up of alternating molecules, ADADA… In each case it is to be expected that the intermolecular forces between the face to face packed molecules in each stack are far greater than those forces between stacks. A considerable anisotropy in physical properties which are a strong function of intermolecular interactions is a natural result of these crystal structures. Such anisotropy has been reported in the few crystals of the ADAD structure which have been examined, for example, in triplet exciton motion in biphenxl² and anthracene tetracyanobenzene (TCNB) complexes³,⁴ and in charge carrier mobilities μ in anthracene⁵ and phenthrene-pyromellitic-acid-dianhydride⁶ as well as anthracene-trinitrobenzene⁷. Carrier mobilities which were reported were all low, < 0.1 cm²/V sec., and though the major interesting questions were posed, such as what is the effect of different donors on μ?; or are there separate conduction and valence bands for acceptors and donors?; the dearth of mobility data on a range of materials leaves these questions unanswered. We have recently measured the mobilities of holes in several TCNB complexes as a first step towards understanding carrier transport in such materials. These are a particularly interesting series of materials in that triplet motion is said to be primarily two dimensional in anthracene TCNB³, and unidimensional in biphenyl TCNB². Since the transport of charge, like triplet exciton motion, is dominated by exchange interactions, and since CT triplet     

Evidence of a ZnCr2Se4 spinel inclusion at the core of a Cr-doped ZnSe quantum dot

Herein we report doping of ZnSe by Cr ions leads to formation of small ZnCr(2)Se(4) spinel inclusions within the cubic sphalerite lattice of a 2.8 nm CrZnSe quantum dot (QD). The Cr ion incorporates as a pair of Cr(III) ions occupying edge-sharing tetragonal distorted octahedral sites generated by formation of three Zn ion vacancies in the sphalerite lattice in order to charge compensate the QD. The site is analogous to the formation of a subunit of the ZnCr(2)Se(4) spinel phase known to form as inclusions during peritectoid crystal growth in the ternary CrZnSe solid-state compound. The oxidation state and site symmetry of the Cr ion is confirmed by X-ray absorption near edge spectroscopy (XANES), crystal field absorption spectroscopy, and electron paramagnetic resonance (EPR). Incorporation as the Cr(III) oxidation state is consistent with the thermodynamic preference for Cr to occupy an octahedral site within a II-VI semiconductor lattice with a half-filled t(2g) d-level. The measured crystal field splitting energy for the CrZnSe QD is 2.08 eV (2.07 eV form XANES), consistent with a spinel inclusion. Further evidence of a spinel inclusion is provided by analysis of the magnetic data, where antiferromagnetic (AFM) exchange, a Curie-Weiss (C-W) temperature of θ = -125 K, and a nearest-neighbor exchange coupling constant of J(NN) = -12.5 K are observed. The formation of stable spinel inclusions in a QD has not been previously reported.     

Crystal growth and magnetic behavior of R6T13−xAlxMy phases (R=La, Nd; T=Mn, Fe; M=main group) grown from lanthanide-rich eutectic fluxes

Binary eutectic mixtures of early lanthanide metals and late transition metals have been explored as media for crystal growth of new intermetallic phases. A large family of R₆T_13−xAl_xM_y phases (R=La or Nd; T=Fe or Mn; M=main group elements) with the La₆Co₁₁Ga₃ structure type can be crystallized from La/Ni and Nd/Fe eutectics. The tetragonal structure of these compounds features slabs of transition metal atoms capped with mixed T/Al sites and separated by layers of lanthanide ions. The growth of large crystals of the lanthanum analogs allows for the study of the anisotropic magnetic properties of the transition metal slabs. For La₆Fe_13−xAl_xM_y analogs, these order antiferromagnetically with T_N strongly dependent on the Fe/Al ratio on the mixed sites. Growth of Mn analogs is reported for the first time; the transition metal slabs in La₆(Mn/Ni)₁₀Al₃ phases order ferromagnetically with a T_C of 200 K.