Structure determination of medium-sized sodium clusters

Sodium cluster anions Na(n)(-) with n = 39-350 have been studied by low temperature photoelectron spectroscopy and density functional theory (DFT). The highly structured experimental spectra are in excellent agreement with the electronic density of states (DOS) of the DFT lowest energy structures. Even for the largest sizes, a pronounced sensitivity of the DOS on fine geometric details could be observed, allowing for a reliable identification of a specific icosahedral growth motif. The intermediate sizes between the closed-shell Mackay clusters with 55, 147, and 309 atoms form by growth of overlayers, which often exhibit a twist deformation with respect to regular (Mackay-type) ones.     

Spin-orbit-induced spin-density wave in a quantum wire

We present analysis of the interacting quantum wire problem in the presence of magnetic field and spin-orbit interaction. We show that an interesting interplay of Zeeman and spin-orbit terms, facilitated by the electron-electron interaction, results in the spin-density wave state when the magnetic field and spin-orbit axes are orthogonal. This strongly affects charge transport through the wire: With the spin-density wave stabilized, single-particle backscattering off a nonmagnetic impurity becomes irrelevant. The sensitivity of the effect to the direction of the magnetic field can be used for experimental verification of this proposal.     

Enhancement of two-photon absorption cross-section in macrocyclic thiophenes with cavities in the nanometer regime

The linear and nonlinear optical properties of two thiophene-based cyclic molecules have been investigated. These molecules represent nanometer sized cavities which may be useful for novel photonic devices. By virtue of long-range interactions, these chromophores serve as novel architectures for enhanced two-photon absorption (TPA) properties. Measurements of the different size ring structures showed a 550% increase in the TPA cross-section for the larger macrocycle. Electronic structure calculations have suggested an increase in coupling of the excited states in these systems as the ring size is increased. Measurements of the ultrafast transient absorption and fluorescence were carried out with these systems in order to probe the interaction between the chromophores. The results of the transient decays as well as fluorescence anisotropy decay times gives stronger proof to the suggestion of delocalized states in the cyclic macrocycles. These results provide information regarding the optical properties of these novel systems useful for potential applications in photonics.     

Mechanisms for fluorescence depolarization in dendrimers

We have investigated the fluorescence properties of dendrimers (Gn is the dendrimer generation number) containing four different luminophores, namely terphenyl (T), dansyl (D), stilbenyl (S), and eosin (E). In the case of T, the dendrimers contain a single p-terphenyl fluorescent unit as a core with appended sulfonimide branches of different size and n-octyl chains. In the cases of D and S, multiple fluorescent units are appended in the periphery of poly(propylene amine) dendritic structures. In the case of E, the investigated luminophore is noncovalently linked to the dendritic scaffold, but is encapsulated in cavities of a low luminescent dendrimer. Depending on the photophysical properties of the fluorescent units and the structures of the dendrimers, different mechanisms of fluorescence depolarization have been observed: (i) global rotation for GnT dendrimers; (ii) global rotation and local motions of the dansyl units at the periphery of GnD dendrimers; (iii) energy migration among stylbenyl units in G2S; and (iv) restricted motion when E is encapsulated inside a dendrimer, coupled to energy migration if the dendrimer hosts more than one eosin molecule.     

Dopamine and iron mediated fragmentation of galactocerebroside and cardiolipin in micelles

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and thin-layer chromatography (TLC) have been used to study dopamine and iron mediated free-radical transformation of lipids in their hydrophilic parts. It has been shown that the action of the dopamine/Fe²⁺ system on galactocerebroside or cardiolipin, which are the components of mixed micelles, results in formation of ceramide or phosphatidic acid and phosphatidylhydroxyacetone, respectively. These data, when combined with results obtained using the ascorbate/Fe²⁺/H₂O₂ oxidizing system with the same substrates, demonstrate that the formation of these products proceeds via an OH-radical induced fragmentation taking place in polar moiety of the starting lipids.      

Ising phases of Heisenberg ladders in a magnetic field

We show that Dzyaloshinskii-Moriya (DM) interactions can substantially modify the phase diagram of spin-1/2 Heisenberg ladders in a magnetic field provided they compete with exchange. For nonfrustrated ladders, they induce a local magnetization along the DM vector that turns the gapless intermediate phase into an Ising phase with broken translational symmetry, while for frustrated ladders, they extend the Ising order of the half-integer plateau to the surrounding gapless phases of the purely Heisenberg case. Implications for experimental ladder and dimer systems are discussed.     

FMR fine structure – A tool to investigate the spatial magnetic phase separation phenomena in manganites

An original approach is proposed to study the magnetic phase separation phenomenon. It is based on the registration of the noise‐like FMR Fine Structure (FMR FS) caused by the magnetic interparticle dipole–dipole interaction between spatially separated ferromagnetic regions. Data obtained for a La_0.7Pb_0.3MnO₃ single crystal point to the existence of spatially separated ferromagnetic regions. It is shown that FMR FS of the La_0.7Pb_0.3MnO₃ single crystal is temperature reversible and disappears at the maximum of magnetoresistance.

     

Emergence of the acoustic Green's function from thermal noise

The fluctuation-dissipation theorem is used to show how acoustic Green's functions corresponding to sound propagation in opposite directions between any two given points can be extracted from time series of thermal noise recorded at these points. The result applies to arbitrarily inhomogeneous, moving or motionless fluids with time-independent parameters, and demonstrates that the two-point correlation function of thermal noise contains as much information about the environment as can be obtained acoustically by placing transceivers at the two points.     

Synthesis of Triborylalkenes from Terminal Alkynes by Iridium‐Catalyzed Tandem C?H Borylation and Diboration

A two‐step reaction to convert terminal alkynes into triborylalkenes is reported. In the first step, the terminal alkyne and pinacolborane (HBpin) are converted into an alkynylboronate, which is catalyzed by an iridium complex supported by a SiNN pincer ligand. In the second step, treatment of the reaction mixture with CO generates a new catalyst which mediates dehydrogenative diboration of alkynylboronate with pinacolborane. The mechanism of the diboration remains unclear but it does not proceed via intermediacy of hydroboration products or via B₂pin₂.