Interparticle potential and drag coefficient in nematic colloids

Magneto-optic tweezers were used for measurements of liquid-crystal-mediated forces between spherical beads with tangential anchoring in thin nematic samples. Repulsive force, which results from the quadrupolar symmetry of defects around the immersed beads, decreases proportionally to 1/x6, with x being the bead separation. The velocity with which the particles are pushed apart also follows the same separation dependence. We thus find the effective drag coefficient gamma(eff) independent of x for surface-to-surface distances as small as 10% of the bead diameter.     

Ferroelectricity of perovskites under pressure

Ab initio simulations and experimental techniques are combined to reveal that, unlike what was commonly accepted for more than 30 years, perovskites and related materials enhance their ferroelectricity as hydrostatic pressure increases above a critical value. This unexpected high-pressure ferroelectricity is different in nature from conventional ferroelectricity because it is driven by an original electronic effect rather by long-range interactions.     

Magnetocaloric effect and multifunctional properties of Ni–Mn-based Heusler alloys

The studies of magnetocaloric properties, phase transitions, and phenomena related to magnetic heterogeneity in the vicinity of the martensitic transition (MT) in Ni–Mn–In and Ni–Mn–Ga off-stoichiometric Heusler alloys are summarized. The crystal structure, magnetocaloric effect (MCE), and magnetotransport properties were studied for the following alloys: Ni₅₀Mn_50−xIn_x, Ni_50−xCo_xMn₃₅In₁₅, Ni₅₀Mn_35−xCo_xIn₁₅, Ni₅₀Mn₃₅In₁₄Z (Z=Al, Ge), Ni₅₀Mn₃₅In_15−xSi_x, Ni_50−xCo_xMn_25+yGa_25−y, and Ni_50–xCo_xMn_32−yFeyGa₁₈. It was found that the magnetic entropy change, ΔS, associated with the inverse MCE in the vicinity of the temperature of the magneto-structural transition, TM, persists in a range of (125-5) J/(kg K) for a magnetic field change ΔH=5 T. The corresponding temperature varies with composition from 143 to 400 K. The MT in Ni₅₀Mn_50−xIn_x (x=13.5) results in a transition between two paramagnetic states. Associated with the paramagnetic austenite-paramagnetic martensite transition ΔS=24 J/(kg K) was detected for ΔH=5 T at T=350 K. The variation in composition of Ni₂MnGa can drastically change the magnetic state of the martensitic phase below and in the vicinity of TM. The presence of the martensitic phase with magnetic moment much smaller than that in the austenitic phase above TM leads to the large inverse MCE in the Ni₄₂Co₈Mn_32−yFeyGa₁₈ system. The adiabatic change of temperature (ΔT_ad) in the vicinity of TC and TM of Ni₅₀Mn₃₅In₁₅ and Ni₅₀Mn₃₅In₁₄Z (Z=Al, Ge) was found to be ΔT_ad=−2 K and 2 K for ΔH=1.8 T, respectively. It was observed that |ΔT_ad|≈1 K for ΔH=1 T for both types of transitions. The results on resistivity, magnetoresistance, Hall resistivity in some In-based alloys are discussed.     

Cold-field switching in PVDF-TrFE ferroelectric polymer nanomesas

Polarization reversal in ferroelectric nanomesas of polyvinylidene fluoride with trifluoroethylene has been probed by ultrahigh vacuum piezoresponse force microscopy in a wide temperature range from 89 to 326 K. In dramatic contrast to the macroscopic data, the piezoresponse force microscopy local switching was nonthermally activated and, at the same time, occurring at electric fields significantly lower than the intrinsic switching threshold. A "cold-field" defect-mediated extrinsic switching is shown to be an adequate scenario describing this peculiar switching behavior. The extrinsic character of the observed polarization reversal suggests that there is no fundamental bar for lowering the coercive field in ferroelectric polymer nanostructures, which is of importance for their applications in functional electronics.     

Electron-electron interaction in the magnetoresistance of graphene

We investigate the magnetotransport in large area graphene Hall bars epitaxially grown on silicon carbide. In the intermediate field regime between weak localization and Landau quantization, the observed temperature-dependent parabolic magnetoresistivity is a manifestation of the electron-electron interaction. We can consistently describe the data with a model for diffusive (magneto)transport that also includes magnetic-field-dependent effects originating from ballistic time scales. We find an excellent agreement between the experimentally observed temperature dependence of magnetoresistivity and the theory of electron-electron interaction in the diffusive regime. We can further assign a temperature-driven crossover to the reduction of the multiplet modes contributing to electron-electron interaction from 7 to 3 due to intervalley scattering. In addition, we find a temperature-independent ballistic contribution to the magnetoresistivity in classically strong magnetic fields.     

Self‐assembly of p‐tert‐butyl thiacalix[4]arenes and metal cations into nanoscale three‐dimensional particles

The shape of supramolecular aggregates based on stereoisomers of p‐tert‐butyl thiacalix[4]arenes functionalized with secondary, tertiary amide and hydrazide groups at the lower rim in cone, partial cone and 1,3‐alternate conformations with several metal cations were investigated by atomic force microscopy. The examined p‐tert‐butyl thiacalix[4]arenes form host–guest complexes; dimers, spherics ellipsoids and elongated nanoscale particles depending on the conformation of macrocycles, the nature of the binding centers and the nature of the metal cation. Only associates formed by p‐tert‐butyl thiacalix[4]arenes with morpholide groups at the lower rim in cone conformation with silver cations exhibited a higher antimicrobial activity. Copyright © 2012 John Wiley & Sons, Ltd.     

Novel complex phenomena in ferroelectric nanocomposites

A first-principles-based effective Hamiltonian is used to investigate finite-temperature properties of ferroelectric nanocomposites made of periodic arrays of ferroelectric nanowires embedded in a matrix formed by another ferroelectric material. Novel transitions and features related to flux-closure configurations are found. Examples include (i)?a vortex core transition, that is characterized by the change of the vortex cores from being axisymmetric to exhibiting a 'broken symmetry'; (ii)?translational mode of the vortex cores; (iii)?striking zigzag dipolar chains along the vortex core axis; and (iv)?phase-locking of ferroelectric vortices accompanied by ferroelectric antivortices. These complex phenomena are all found to coexist with a spontaneous electrical polarization aligned along the normal of the plane containing the vortices.     

Parahydrogen-Induced Polarization in Heterogeneous Hydrogenations over Silica-Immobilized Rh Complexes

The use of heterogeneous catalysts for parahydrogen-induced polarization (PHIP) of nuclear spins opens new horizons for production of hyperpolarized substances. Immobilization of homogeneous hydrogenation catalysts is a promising approach for designing the efficient heterogeneous catalytic systems capable of PHIP generation. Herein, we study the formation of PHIP in the gas-phase and in the liquid-phase hydrogenations of propyne and propylene catalyzed by silica-immobilized Rh complexes synthesized by the ligand-exchange anchoring of the Wilkinson’s complex RhCl(PPh₃)₃, the binuclear complex Rh₂Cl₂(C₈H₁₂)₂ and the cationic complex [Rh(C₈H₁₂)₂]⁺[BF₄]^? to the phosphine-modified silica gel. We consider the stability and the mechanistic aspects of the hydrogenation over the immobilized Wilkinson’s catalyst in terms of PHIP observation. Using a PASADENA (parahydrogen and synthesis allow dramatically enhanced nuclear alignment) effect, it is found, in particular, that liquid-phase propyne hydrogenation over the immobilized Wilkinsons’s catalyst at 70°C proceeds in a stable regime with a stereoselective cis addition of a hydrogen molecule, while in the gas phase at the same temperature the hydrogenation stereoselectivity is observed only for a short time after the reaction is started, and then the catalyst rapidly loses its activity. The reasons of the catalyst deactivation are discussed based on the literature data, the results of infrared spectroscopy study, and the comparison to the behavior of the immobilized binuclear and cationic Rh complexes. In addition, it is shown that the immobilized Wilkinson’s catalyst is reduced as temperature increases in the range of 90–130°C, as confirmed by X-ray photoelectron spectroscopy.     

Mesoscale and clusters of synchrony in networks of bursting neurons

We study the role of network architecture in the formation of synchronous clusters in synaptically coupled networks of bursting neurons. We give a simple combinatorial algorithm that finds the largest synchronous clusters from the network topology. We demonstrate that networks with a certain degree of internal symmetries are likely to have cluster decompositions with relatively large clusters, leading potentially to cluster synchronization at the mesoscale network level. We also address the asymptotic stability of cluster synchronization in excitatory networks of Hindmarsh-Rose bursting neurons and derive explicit thresholds for the coupling strength that guarantees stable cluster synchronization.