Vitrification, relaxation and free volume in glycerol-water binary liquid mixture:Spin probe ESR studies

Glass transition and relaxation of the glycerol-water (G-W) binary mixture system have been studied over the glycerol concentration range of 5-85 mol% by using the highly sensitive technique of electron spin resonance (ESR). For the water rich mixture the glass transition,sensed by the dissolved spin probe, arises from the vitrified mesoscopic portion of the binary system. The concentration dependence of the glass transition temperature manifests a closely related molecular level cooperativity in the system. A drastic change in the mesoscopic structure of the system at the critical concentration of 40 mol is confirmed by an estimation of the spin probe effective volume in a temperature range where the tracer reorientation is strongly coupled to the system dynamics.

Tetragonal bismuth bilayer: A stable and robust quantum spin hall insulator

Topological insulators (TIs) exhibit novel physics with great promise for new devices, but considerable challenges remain to identify TIs with high structural stability and large nontrivial band gap suitable for practical applications. Here we predict by first-principles calculations a two-dimensional (2D) TI, also known as a quantum spin Hall (QSH) insulator, in a tetragonal bismuth bilayer (TB-Bi) structure that is dynamically and thermally stable based on phonon calculations and finite-temperature molecular dynamics simulations. Density functional theory and tight-binding calculations reveal a band inversion among the Bi-p orbits driven by the strong intrinsic spin-orbit coupling, producing a large nontrivial band gap, which can be effectively tuned by moderate strains. The helical gapless edge states exhibit a linear dispersion with a high Fermi velocity comparable to that of graphene, and the QSHphase remains robust on a NaCl substrate. These remarkable properties place TB-Bi among the most promising 2D TIs for high-speed spintronic devices, and the present results provide insights into the intriguing QSH phenomenon in this new Bi structure and offer guidance for its implementation in potential applications.

Uncoupled surface spin induced exchange bias in α-MnO 2 nanowires

We have studied the microstructure, surface states, valence fluctuations, magnetic properties, and exchange bias effect in MnO2 nanowires. High purity α-MnO 2 rectangular nanowires were synthesized by a facile hydrothermal method with microwave-assisted procedures. The microstructure analysis indicates that the nanowires grow in the [0 0 1] direction with the (2 1 0) plane as the surface. Mn3+ and Mn2+ ions are not found in the system by X-ray photoelectron spectroscopy. The effective magnetic moment of the manganese ions fits in with the theoretical and experimental values of Mn4+ very well. The uncoupled spins in 3d3 orbitals of the Mn 4+ ions in MnO 6 octahedra on the rough surface are responsible for the net magnetic moment. Spin glass behavior is observed through magnetic measurements. Furthermore, the exchange bias effect is observed for the first time in pure α-MnO2 phase due to the coupling of the surface spin glass with the antiferromagnetic α-MnO2 matrix. These α-MnO2 nanowires, with a spin-glass-like behavior and with an exchange bias effect excited by the uncoupled surface spins, should therefore inspire further study concerning the origin, theory, and applicability of surface structure induced magnetism in nanostructures.

Studies of electronic structure by x-ray Raman and emission spectroscopy: applications to MgB2 superconductors and high pressure physics

X-ray Raman scattering and x-ray emission spectroscopies were used to study the electronic properties and phase transitions in several condensed matter systems. The experimental work, carried out at the European Synchrotron Radiation Facility, was complemented by theoretical calculations of the x-ray spectra and of the electronic structure. The electronic structure of MgB2 at the Fermi level is dominated by the boron σ and π bands. The high density of states provided by these bands is the key feature of the electronic structure contributing to the high critical temperature of superconductivity in MgB2. The electronic structure of MgB2 can be modified by atomic substitutions, which introduce extra electrons or holes into the bands. X ray Raman scattering was used to probe the interesting σ and π band hole states in pure and aluminum substituted MgB2. A method for determining the final state density of electron states from experimental x-ray Raman scattering spectra was examined and applied to the experimental data on both pure MgB2 and on Mg(0.83)Al(0.17)B2. The extracted final state density of electron states for the pure and aluminum substituted samples revealed clear substitution induced changes in the σ and π bands. The experimental work was supported by theoretical calculations of the electronic structure and x-ray Raman spectra. X-ray emission at the metal Kβ line was applied to the studies of pressure and temperature induced spin state transitions in transition metal oxides. The experimental studies were complemented by cluster multiplet calculations of the electronic structure and emission spectra. In LaCoO3 evidence for the appearance of an intermediate spin state was found and the presence of a pressure induced spin transition was confirmed. Pressure induced changes in the electronic structure of transition metal monoxides were studied experimentally and were analyzed using the cluster multiplet approach. The effects of hybridization, bandwidth and crystal field splitting in stabilizing the high pressure spin state were discussed. Emission spectroscopy at the Kβ line was also applied to FeCO3 and a pressure induced iron spin state transition was discovered.

A numerical study of the role of the vertical structure of vorticity during tropical cyclone genesis

An eight-level axisymmetric model with simple parameterizations for clouds and the atmospheric boundary layer was developed to examine the evolution of vortices that are precursors to tropical cyclones. The effect of vertical distributions of vorticity, especially that arising from a merger of mid-level vortices, was studied by us to provide support for a new vortex-merger theory of tropical cyclone genesis. The basic model was validated with the analytical results available for the spin-down of axisymmetric vortices. With the inclusion of the cloud and boundary layer parameterizations, the evolution of deep vortices into hurricanes and the subsequent decay are simulated quite well. The effects of several parameters such as the initial vortex strength, radius of maximum winds, sea-surface temperature and latitude (Coriolis parameter) on the evolution were examined. A new finding is the manner in which mid-level vortices of the same strength decay and how, on simulated merger of these mid-level vortices, the resulting vortex amplifies to hurricane strength in a realistic time frame. The importance of sea-surface temperature on the evolution of full vortices was studied and explained. Also it was found that the strength of the surface vortex determines the time taken by the deep vortex to amplify to hurricane strength.

Influence of lattice distortion on the Curie temperature and spin-phonon coupling in LaMn0.5Co0.5O3

Two distinct ferromagnetic phases of LaMn0.5Co0.5O3 having monoclinic structure with distinct physical properties have been studied. The ferromagnetic ordering temperature T-c is found to be different for both the phases. The origin of such contrasting characteristics is assigned to the changes in the distance(s) and angle(s) between Mn-O-Co resulting from distortions observed from neutron diffraction studies. Investigations on the temperature dependent Raman spectroscopy provide evidence for such structural characteristics, which affects the exchange interaction. The difference in B-site ordering which is evident from the neutron diffraction is also responsible for the difference in T-c. Raman scattering suggests the presence of spin-phonon coupling for both the phases around the T-c. Electrical transport properties of both the phases have been investigated based on the lattice distortion.

Electronic structure of square planar CuO46− clusters

The results of spin-polarized MSXagr calculations show that the ground state of the CuO 4 6– cluster is essentially non-magnetic in spite of odd number of electrons in the system for short Cu–O distances (1.90 Å) as found in the highT c superconductors. This arises due to the fact that the unpaired electron resides in a molecular orbital with primarily oxygen 3s character. The stability of this molecular orbital is found to be sensitive to the cluster geometry and thus, increase in Cu–O distance (as well as other changes affecting oxygen-oxygen distance) tend to favour a magnetic state. From these calculations we have also estimated the Coulomb correlation strength within the Cu 3d to be about 5.3 eV.

First-principles analysis of electron correlation, spin ordering and phonons in the normal state of FeSe1-x

We present first-principles density-functional-theory-based calculations to determine the effects of the strength of on-site electron correlation, magnetic ordering, pressure and Se vacancies on phonon frequencies and electronic structure of FeSe1-x. The theoretical equilibrium structure (lattice parameters) of FeSe depends sensitively on the value of the Hubbard parameter U of on-site correlation and magnetic ordering. Our results suggest that there is a competition between different antiferromagnetic states due to comparable magnetic exchange couplings between first- and second-neighbor Fe sites. As a result, a short range order of stripe antiferromagnetic type is shown to be relevant to the normal state of FeSe at low temperature. We show that there is a strong spin-phonon coupling in FeSe (comparable to its superconducting transition temperature) as reflected in large changes in the frequencies of certain phonons with different magnetic ordering, which is used to explain the observed hardening of a Raman-active phonon at temperatures (similar to 100 K) where magnetic ordering sets in. The symmetry of the stripe antiferromagnetic phase permits an induced stress with orthorhombic symmetry, leading to orthorhombic strain as a secondary order parameter at the temperature of magnetic ordering. The presence of Se vacancies in FeSe gives rise to a large peak in the density of states near the Fermi energy, which could enhance the superconducting transition temperature within the BCS-like picture.

A formate bridged Ni(II) sheet and its 3D analogue in presence of a linear organic linker: Synthesis, crystal structure and magnetic properties

Reaction of formamide with Ni(NO3)(2)center dot 6H(2)O under hydrothermal condition in a mixture of MeOH/H2O forms a two-dimensional formate bridged sheet Ni(HCOO)(2)(MeOH)(2) (1). X-ray structure analysis reveals the conversion of formamide to formate which acts as a bridging ligand in complex 1 where the axial sites of Ni(II) are occupied by methanol used as a solvent. An analogous reaction in presence of 4,4'-bipyridyl (4,4'-bipy) yielded a three-dimensional structure Ni(HCOO)(2)(4,4'-bpy) (2). DC magnetic measurements as a function of temperature and field established the presence of spontaneous magnetization with T-c (Curie temperature) = 17 and 20.8 K in 1 and 2, respectively, which can be attributed due to spin-canting. DFT calculations were performed to corroborate the magnetic results of 1 and 2. (C) 2010 Elsevier Ltd. All rights reserved.

Structure of Relativistic States

QCD factorization in the Bjorken limit allows to separate the long-distance physics from the hard subprocess. At leading twist, only one parton in each hadron is coherent with the hard subprocess. Higher twist effects increase as one of the active partons carries most of the longitudinal momentum of the hadron, x -> 1. In the Drell-Yan process \pi N -> \mu^- mu^+ + X, the polarization of the virtual photon is observed to change to longitudinal when the photon carries x_F > 0.6 of the pion. I define and study the Berger-Brodsky limit of Q^2 -> \infty with Q^2(1-x) fixed. A new kind of factorization holds in the Drell-Yan process in this limit, in which both pion valence quarks are coherent with the hard subprocess, the virtual photon is longitudinal rather than transverse, and the cross section is proportional to a multiparton distribution. Generalized parton distributions contain information on the longitudinal momentum and transverse position densities of partons in a hadron. Transverse charge densities are Fourier transforms of the electromagnetic form factors. I discuss the application of these methods to the QED electron, studying the form factors, charge densities and spin distributions of the leading order |e\gamma> Fock state in impact parameter and longitudinal momentum space. I show how the transverse shape of any virtual photon induced process, \gamma^*(q)+i -> f, may be measured. Qualitative arguments concerning the size of such transitions have been previously made in the literature, but without a precise analysis. Properly defined, the amplitudes and the cross section in impact parameter space provide information on the transverse shape of the transition process.

An electron-spin-resonance study of MN2+ doped calcium hydrazine carboxylate monohydrate

Single crystals of calcium hydrazine carboxylate, monohydrate have been studied by ESR of Mn2+ doped in the calcium sites. X-band ESR indicated a large crystal field splitting necessitating experiments at Q band. The analysis shows two magnetically inequivalent (but chemically equivalent) sites with g(xx) = 2.0042+/-0.0038, g(yy) = 2.0076 +/-00029, g(zz) =2.0314+/-0.001, A(zz) = 0.0099+/-0.0002 cm(-1), A(xx) = 0.0099+/-0.0002 cm(-1), A(yy) = 0.0082+/-0.0002cm(-1), D = 3/2D(zz) = 0.0558+/-0.0006cm(-1), and E = 1/2(D-xx-D-yy) = 0.0127+/-0.0002 cm(-1).One of the principal components of the crystal field, (D-zz), is found to be along the Ca<->Ca direction in the structure and a second one, (D-xx), along the perpendicular to the plane of the triangle formed by three neighbouring calciums. The A tensor is found to have an orientation different from that of the g and D tensors reflecting the low symmetry of the Ca2+ sites.

An investigation of the first-order spin-state transition in Fe(Phen)(2)(NCS)(2) EXAFS and infrared spectroscopy

The structure of Fe(Phen)(2)(NCS)(2) has been examined across the first-order spin-state transition by EXAFS with full multiple scattering analysis. The EXAFS data at 298 K can be satisfactorily assigned to the high-spin state, but the analysis of the low-temperature data at 90 K is not entirely unequivocal, although consistent with the predominant presence of the low-spin state. That some proportion of the high-spin state remains at low temperatures, well below the first-order transition, is clearly evidenced in the infrared spectra, suggesting possible sublattice ordering.

meso-substituted octabromoporphyrins: Synthesis, spectroscopy, electrochemistry and electronic structure

The free-base, copper(II) and zinc(II) derivatives of 5,10,15,20-tetraarylporphyrin (aryl = phenyl, 4-methylphenyl or 4-chlorophenyl) and the corresponding brominated 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin derivatives have been synthesized and their spectral and redox properties compared by UV/VIS, H-1 NMR, ESR and cyclic voltammetric methods. Substitution with the electron-withdrawing bromine groups at the pyrrole carbons has a profound influence on the UV/VIS and H-1 NMR spectral features and also on the redox potentials of these systems. On the other hand, electron-withdrawing chloro or electron-donating methyl groups at the para positions of the four phenyl rings have only a marginal effect on the spectra and redox potentials of both the brominated and the non-brominated derivatives. The ESR data for the copper(II) derivatives of ail these systems reveal that substitution at either the beta-pyrrole carbons and/or the para positions of the meso-phenyl groups does not significantly affect the spin-Hamiltonian parameters that describe the metal centre in each case. Collectively, these observations suggest that the highest-occupied (HOMO) and lowest-unoccupied molecular orbitals (LUMO) of the octabromoporphyrins involve the porphyrin pi-ring system as is the case with the non-brominated derivatives.-Investigations have been carried out to probe the electronic structures of these systems by three different approaches involving spectral and redox potential data as well as AMI calculations. The results obtained suggest that the electron-withdrawing beta-bromine substituents stabilize the LUMOs and, to a lesser degree, the HOMOs and that the extent of these changes can be fine-tuned, in a subtle way, by substituting at the meso-aryl rings of a given porphyrin.

Role of free valences and spin densities on the migrating atom in location of saddle points in radical-exchange reactions

The principle of the conservation of bond orders during radical-exchange reactions is examined using Mayer's definition of bond orders. This simple intuitive approximation is not valid in a quantitative sense. Ab initio results reveal that free valences (or spin densities) develop on the migrating atom during reactions. For several examples of hydrogen-transfer reactions, the sum of the reaction coordinate bond orders in the transition state was found to be 0.92 +/- 0.04 instead of the theoretical 1.00 because free valences (or spin densities) develop on the migrating atom during reactions. It is shown that free valence is almost equal to the square of the spin density on the migrating hydrogen atom and the maxima in the free valence (or spin density) profiles coincide (or nearly coincide) with the saddle points in the corresponding energy profiles.

Density-matrix renormalization-group studies of the spin-1/2 Heisenberg system with dimerization and frustration

Using the density-matrix renormalization-group technique, we study the ground-state phase diagram and other low-energy properties of an isotropic antiferromagnetic spin-1/2 chain with both dimerization and frustration, i.e., an alternation delta of the nearest-neighbor exchanges and a next-nearest-neighbor exchange J(2). For delta = 0, the system is gapless for J(2) < J(2c) and has a gap for J(2) > J(2c) where J(2c) is about 0.241. For J(2) = J(2c) the gap above the ground state grows as delta to the power 0.667 +/- 0.001. In the J(2)-delta plane, there is a disorder line 2J(2) + delta = 1. To the left of this line, the peak in the static structure factor S(q) is at q(max) = pi (Neel phase), while to the right of the line, q(max) decreases from pi to pi/2 as J(2) is increased to large values (spiral phase). For delta = 1, the system is equivalent to two coupled chains as on a ladder and it is gapped for all values of the interchain coupling.