Critical Properties at the Field-Induced Bose-Einstein Condensation in NiCl(2)-4SC(NH(2))(2)

We report new magnetization measurements on the spin-gap compound NiCl(2)-4SC(NH(2))(2) at the low-field boundary of the magnetic field-induced ordering. The critical density of the magnetization is analyzed in terms of a Bose-Einstein condensation of bosonic quasiparticles. The analysis of the magnetization at the transition leads to the conclusion for the preservation of the U(1) symmetry, as required for Bose-Einstein condensation. The experimental data are well described by quantum Monte Carlo simulations.

Direct Measurement of the Bose-Einstein Condensation Universality Class in NiCl(2)-4SC(NH(2))(2) at Ultralow Temperatures

In this work, we demonstrate field-induced Bose-Einstein condensation (BEC) in the organic compound NiCl(2)-4SC(NH(2))(2) using ac susceptibility measurements down to 1 mK. The Ni S=1 spins exhibit 3D XY antiferromagnetism between a lower critical field H(c1)similar to 2 T and a upper critical field H(c2)similar to 12 T. The results show a power-law temperature dependence of the phase transition line H(c1)(T)-H(c1)(0)=aT(alpha) with alpha=1.47 +/- 0.10 and H(c1)(0)=2.053 T, consistent with the 3D BEC universality class. Near H(c2), a kink was found in the phase boundary at approximately 150 mK.

Approximation to density functional theory for the calculation of band gaps of semiconductors

The local-density approximation (LDA) together with the half occupation (transitionstate) is notoriously successful in the calculation of atomic ionization potentials. When it comes to extended systems, such as a semiconductor infinite system, it has been very difficult to find a way to half ionize because the hole tends to be infinitely extended (a Bloch wave). The answer to this problem lies in the LDA formalism itself. One proves that the half occupation is equivalent to introducing the hole self-energy (electrostatic and exchange correlation) into the Schrodinger equation. The argument then becomes simple: The eigenvalue minus the self-energy has to be minimized because the atom has a minimal energy. Then one simply proves that the hole is localized, not infinitely extended, because it must have maximal self-energy. Then one also arrives at an equation similar to the self- interaction correction equation, but corrected for the removal of just 1/2 electron. Applied to the calculation of band gaps and effective masses, we use the self- energy calculated in atoms and attain a precision similar to that of GW, but with the great advantage that it requires no more computational effort than standard LDA.

Bose-Einstein condensation in antiferromagnets close to the saturation field

At zero temperature and strong applied magnetic fields the ground state of an anisotropic antiferromagnet is a saturated paramagnet with fully aligned spins. We study the quantum phase transition as the field is reduced below an upper critical H(c2) and the system enters a XY-antiferromagnetic phase. Using a bond operator representation we consider a model spin-1 Heisenberg antiferromagnetic with single-ion anisotropy in hypercubic lattices under strong magnetic fields. We show that the transition at H(c2) can be interpreted as a Bose-Einstein condensation (BEC) of magnons. The theoretical results are used to analyze our magnetization versus field data in the organic compound NiCl(2)-4SC(NH(2))(2) (DTN) at very low temperatures. This is the ideal BEC system to study this transition since H(c2) is sufficiently low to be reached with static magnetic fields (as opposed to pulsed fields). The scaling of the magnetization as a function of field and temperature close to H(c2) shows excellent agreement with the theoretical predictions. It allows us to obtain the quantum critical exponents and confirm the BEC nature of the transition at H(c2).

Direct measurement of spin correlations using magnetostriction

We demonstrate that the short-range spin correlator < S(i)center dot S(j)>, a fundamental measure of the interaction between adjacent spins, can be directly measured in certain insulating magnets. We present magnetostriction data for the insulating organic compound NiCl(2)-4SC(NH(2))(2), and show that the magnetostriction as a function of field is proportional to the dominant short-range spin correlator. Furthermore, the constant of proportionality between the magnetostriction and the spin correlator gives information about the spin-lattice interaction. Combining these results with the measured Young's modulus, we are able to extract dJ/dz, the dependence of the superexchange constant J on the Ni interionic distance z.

Magnetoelastic and thermal effects in the BiMn(2)O(5) lattice: A high-resolution x-ray diffraction study

High-resolution synchrotron x-ray diffraction measurements were performed on single crystalline and powder samples of BiMn(2)O(5). A linear temperature dependence of the unit cell volume was found between T(N)=38 and 100 K, suggesting that a low-energy lattice excitation may be responsible for the lattice expansion in this temperature range. Between T(*)similar to 65 K and T(N), all lattice parameters showed incipient magnetoelastic effects, due to short-range spin correlations. An anisotropic strain along the a direction was also observed below T(*). Below T(N), a relatively large contraction of the a parameter following the square of the average sublattice magnetization of Mn was found, indicating that a second-order spin Hamiltonian accounts for the magnetic interactions along this direction. On the other hand, the more complex behaviors found for b and c suggest additional magnetic transitions below T(N) and perhaps higher-order terms in the spin Hamiltonian. Polycrystalline samples grown by distinct routes and with nearly homogeneous crystal structure above T(N) presented structural phase coexistence below T(N), indicating a close competition amongst distinct magnetostructural states in this compound.

Detection of catechol using mixed Langmuir-Blodgett films of a phospholipid and phthalocyanines as voltammetric sensors

The combination of metallic phthalocyanines (MPcs) and biomolecules has been explored in the literature either as mimetic systems to investigate molecular interactions or as supporting layers to immobilize biomolecules. Here, Langmuir-Blodgett (LB) films containing the phospholipid dimyristoyl phosphatidic acid (DMPA) mixed either with iron phthalocyanine (FePc) or with lutetium bisphthalocyanine (LuPc(2)) were applied as ITO modified-electrodes in the detection of catechol using cyclic voltammetry. The mixed Langmuir films of FePc + DMPA and LuPc(2) + DMPA displayed surface-pressure isotherms with no evidence of molecular-level interactions. The Fourier Transform Infrared (FTIR) spectra of the multilayer LB films confirmed the lack of interaction between the components. The DMPA and the FePc molecules were found to be oriented perpendicularly to the substrate, while LuPc(2) molecules were randomly organized. The phospholipid matrix induced a remarkable electrocatalytic effect on the phthalocyanines; as a result the mixed LB films deposited on ITO could be used to detect catechol with detection limits of 4.30 x 10(-7) and 3.34 x 10(-7) M for FePc + DMPA and LuPc(2) + DMPA, respectively. Results from kinetics experiments revealed that ion diffusion dominated the response of the modified electrodes. The sensitivity was comparable to that of other non-enzymatic sensors, which is sufficient to detect catechol in the food industry. The higher stability of the electrochemical response of the LB films and the ability to control the molecular architecture are promising for further studies with incorporation of biomolecules.

Theoretical study of one- and two-photon absorption spectra of azoaromatic compounds

In this study, the one- and two-photon absorption spectra of seven azoaromatic compounds (five pseudostilbenes-type and two aminoazobenzenes) were theoretically investigated using the density functional theory combined with the response functions formalism. The equilibrium molecular structure of each compound was obtained at three different levels of theory: Hartree-Fock, density functional theory (DFT), and Moller-Plesset 2. The effect of solvent on the equilibrium structure and the electronic transitions of the compounds were investigated using the polarizable continuum model. For the one-photon absorption, the allowed pi ->pi(*) transition energy showed to be dependent on the molecular structures and the effect of solvent, while the n ->pi(*) and pi ->pi(*)(n) transition energies exhibited only a slight dependence. An inversion between the bands corresponding to the pi ->pi(*) and n ->pi(*) states due to the effect of solvent was observed for the pseudostilbene-type compounds. To characterize the allowed two-photon absorption transitions for azoaromatic compounds, the response functions formalism combined with DFT using the hybrid B3LYP and PBE0 functionals and the long-range corrected CAM-B3LYP functional was employed. The theoretical results support the previous findings based on the three-state model. The model takes into account the ground and two electronic excited states and has already been used to describe and interpret the two-photon absorption spectrum of azoaromatic compounds. The highest energy two-photon allowed transition for the pseudostilbene-type compounds shows to be more effectively affected (similar to 20%) by the torsion of the molecular structure than the lowest allowed transition (similar to 10%). In order to elucidate the effect of the solvent on the two-photon absorption spectra, the lowest allowed two-photon transition (dipolar transition) for each compound was analyzed using a two-state approximation and the polarizable continuum model. The results obtained reveal that the effect of solvent increases drastically the two-photon cross-section of the dipolar transition of the pseudostilbene-type compounds. In general, the features of both one- and two-photon absorption spectra of the azoaromatic compounds are well reproduced by the theoretical calculations.

cis-Bis{1,1-dibenzyl-3-[(furan-2-yl)carbonyl]thioureato-kappa 2 O,S}nickel(II)

In the title compound, [Ni(C(20)H(17)N(2)O(2)S)(2)], the NiII atom is coordinated by the S and O atoms of two 1,1-dibenzyl-3-[(furan-2-yl)carbonyl]thioureate ligands in a distorted square-planar geometry. The two O and two S atoms are mutually cis to each other. The Ni-S and Ni-O bond lengths lie within the range of those found in related structures. The dihedral angle between the planes of the two chelating rings is 20.33 (6)degrees.

1-furfuryl-3-furoylthiourea

The title compound, C11H10N2O3S, was synthesized from furoyl isothiocyanate and furfurylamine in dry acetone. The thiourea group is in the thioamide form. The trans-cis geometry of the thiourea group is stabilized by intramolecular hydrogen bonding between the carbonyl and cis-thioamide and results in a pseudo-S(6) planar ring which makes dihedral angles of 2.5 (3) and 88.1 (2)degrees with the furoyl and furfuryl groups, respectively. There is also an intramolecular hydrogen bond between the furan O atom and the other thioamide H atom. In the crystal structure, molecules are linked by two intermolecular N-H center dot center dot center dot O hydrogen bonds, forming dimers. These dimers are stacked within the crystal structure along the [010] direction.

N-(3-chlorophenyl) maleimide

The title compound, C10H6ClNO2, has a dihedral angle of 46.46 (5)degrees between the benzene and maleimide rings. A short intermolecular halogen-oxygen contact is observed, with a Cl center dot center dot center dot O distance of 3.0966 (13) angstrom. Both CO groups are involved in two C-H center dot center dot center dot O interactions, which gives rise to sheets parallel to (100). In addition, these sheets exhibit a pi-pi stacking interaction between the benzene and maleimide rings [mean interplanar distance of 3.337 (3) angstrom].

1-Benzyl-3-(2-furoyl)thiourea

In the title compound, C13H12N2O2S, the dihedral angle between the two aromatic ring planes is 87.52 (12)degrees. The molecule shows an intramolecular N-H center dot center dot center dot O hydrogen bond. The crystal structure is stabilized by intermolecular N-H center dot center dot center dot S and C-H center dot center dot center dot O hydrogen bonding.

Tris(N-benzoyl-N ',N ''-diphenylthioureato-kappa O-2,S)cobalt(III)

In the title compound, [Co(C20H15N2OS)(3)], the Co-III atom is coordinated by the S and O atoms of three N-benzoyl-N',N'-diphenylthiourea ligands in a slightly distorted octahedral geometry. The O and S atoms are in cis positions, while the positions between the O and S atoms are trans.

1-Furoyl-3-methyl-3-phenylthiourea

The title compound, C13H12N2O2S, crystallizes with two independent molecules in the asymmetric unit. The two molecules differ in the conformation of the thiocarbonyl and carbonyl groups, and show the typical geometric parameters of substituted thiourea derivatives. The crystal structure is mainly stabilized by intermolecular N-H center dot center dot center dot O hydrogen bonding.

Low-temperature study of a new nevirapine pseudopolymorph

The title compound (systematic name: 11-cyclopropyl-4-methyl-5,11-dihydro-6H-dipyrido[3,2-b: 2',3'-e][1,4] diazepin-6-one butanol 0.3-solvate), C15H14N4O center dot 0.3C(4)H(9)OH, was crystallized in a new triclinic pseudopolymorphic form, a butanol solvate, and the crystal structure determined at 150 K. The molecular conformation of this new form differs from that reported previously, although the main intermolecular hydrogen-bond pattern remains the same. N-H center dot center dot center dot O hydrogen bonds [N center dot center dot center dot O = 2.957 (3) angstrom] form centrosymmetric dimers and the crystal packing of this new pseudopolymorph generates infinite channels along the b axis.