Anharmonic magnetic deformation of self-assembled molecular nanocapsules

High magnetic fields were used to deform spherical nanocapsules, self-assembled from bola-amphiphilic sexithiophene molecules. At low fields the deformation -- measured through linear birefringence -- scales quadratically with the capsule radius and with the magnetic field strength. These data confirm a long standing theoretical prediction (W. Helfrich, Phys. Lett. {\bf 43A}, 409 (1973)), and permits the determination of the bending rigidity of the capsules as (2.6$\pm$0.8)$\times 10^{-21}$ J. At high fields, an enhanced rigidity is found which cannot be explained within the Helfrich model. We propose a complete form of the free energy functional that accounts for this behaviour, and allows discussion of the formation and stability of nanocapsules in solution.Comment: 4 pages, 3 figures, accepted in Phys. Rev. Let

Collisional and thermal ionization of sodium Rydberg atoms I. Experiment for nS and nD atoms with n=8-20

Collisional and thermal ionization of sodium nS and nD Rydberg atoms with n=8-20 has been studied. The experiments were performed using a two-step pulsed laser excitation in an effusive atomic beam at atom density of about 2 10^{10} cm^{-3}. Molecular and atomic ions from associative, Penning, and thermal ionization processes were detected. It has been found that the atomic ions were created mainly due to photoionization of Rydberg atoms by photons of blackbody radiation at the ambient temperature of 300K. Blackbody ionization rates and effective lifetimes of Rydberg states of interest were determined. The molecular ions were found to be from associative ionization in Na(nL)+Na(3S) collisions. Rate constants of associative ionization have been measured using an original method based on relative measurements of Na_{2}^{+} and Na^{+} ion signals.Comment: 23 pages, 10 figure

Effects of hole-doping on the magnetic ground state and excitations in the edge-sharing CuO2_2 chains of Ca2+x_{2+x}Y2−x_{2-x}Cu5_5O10_{10}

Neutron scattering experiments were performed on the undoped and hole-doped Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$, which consists of ferromagnetic edge-sharing CuO$_2$ chains. It was previously reported that in the undoped Ca$_2$Y$_2$Cu$_5$O$_{10}$ there is an anomalous broadening of spin-wave excitations along the chain, which is caused mainly by the antiferromagnetic interchain interactions [Matsuda $et$ $al.$, Phys. Rev. B 63, 180403(R) (2001)]. A systematic study of temperature and hole concentration dependencies of the magnetic excitations shows that the magnetic excitations are softened and broadened with increasing temperature or doping holes irrespective of $Q$ direction. The broadening is larger at higher $Q$. A characteristic feature is that hole-doping is much more effective to broaden the excitations along the chain. It is also suggested that the intrachain interaction does not change so much with increasing temperature or doping although the anisotropic interaction and the interchain interaction are reduced. In the spin-glass phase ($x$=1.5) and nearly disordered phase ($x$=1.67) the magnetic excitations are much broadened in energy and $Q$. It is suggested that the spin-glass phase originates from the antiferromagnetic clusters, which are caused by the hole disproportionation.Comment: 8 pages, submitted to Phys. Rev.

In situ root identification through blade penetrometer testing – Part 1:interpretative models and laboratory testing

Root architecture and reinforcement are important parameters to measure the safety of vegetated slopes and stream banks against slope instability and erosion or to assess the stability of plants against environmental loading (e.g. windthrow of trees). However, these are difficult to measure without time-consuming sampling or counting procedures. Previous studies proposed using a penetrometer with an adapted geometry, and showed that individual root breakages could be detected as sudden drops in penetrometer resistance. However, there are no existing models to derive root properties from the measured traces. Here, several interpretative models are developed and their performance at identifying and characterising buried acrylonitrile butadiene styrene root analogues of varying diameter and architecture in sand are assessed. It was found that models, assuming the analogues broke in bending rather than tension, provided good predictions for the force–displacement behaviour. The simple analytical bending model developed here was shown to perform almost as well as more sophisticated numerical models. For all models, the predictions of additional penetrometer force required to break the root analogue were more accurate than predictions for lateral root displacement required to reach failure. The root analogue diameter and to a lesser extent the soil resistance and root angle were shown to affect the penetrometer resistance strongly. Root branching, root length and the distance between the point of load application and a root boundary (root tip or parent root) had a much smaller effect. When the root failure mechanism, root strength, root stiffness and soil resistance are known, an accurate prediction of the root diameter can be made based on the root peak resistance value identified from a blade penetration test. Penetrometer testing, a test which is easy to perform in the field, coupled with an accurate interpretative model might therefore be an effective method to rapidly quantify the spatial distribution, depths and diameters of roots. </jats:p

Influence of solvent quality on polymer solutions: a Monte Carlo study of bulk and interfacial properties

The effect of solvent quality on dilute and semi-dilute regimes of polymers in solution is studied by means of Monte Carlo simulations. The equation of state, adsorptions near a hard wall, wall-polymer surface tension and effective depletion potentials are all calculated as a function of concentration and solvent quality. We find important differences between polymers in good and theta solvents. In the dilute regime, the physical properties for polymers in a theta solvent closely resemble those of ideal polymers. In the semi-dilute regime, however, significant differences are found.Comment: 10 pages, 13 figure

Generation of single colour centers by focussed nitrogen implantation

Single defect centers in diamond have been generated via nitrogen implantation. The defects have been investigated by single defect center fluorescence microscopy. Optical and EPR spectra unambiguously show that the produced defect is the nitrogen-vacancy colour center. An analysis of the nitrogen flux together with a determination of the number of nitrogen-vacancy centers yields that on average two 2 MeV nitrogen atoms need to be implanted per defect center.Comment: 6 pages, 3 figure

Single photon emitters based on Ni/Si related defects in single crystalline diamond

We present investigations on single Ni/Si related color centers produced via ion implantation into single crystalline type IIa CVD diamond. Testing different ion dose combinations we show that there is an upper limit for both the Ni and the Si dose 10^12/cm^2 and 10^10/cm^2 resp.) due to creation of excess fluorescent background. We demonstrate creation of Ni/Si related centers showing emission in the spectral range between 767nm and 775nm and narrow line-widths of 2nm FWHM at room temperature. Measurements of the intensity auto-correlation functions prove single-photon emission. The investigated color centers can be coarsely divided into two groups: Drawing from photon statistics and the degree of polarization in excitation and emission we find that some color centers behave as two-level, single-dipole systems whereas other centers exhibit three levels and contributions from two orthogonal dipoles. In addition, some color centers feature stable and bright emission with saturation count rates up to 78kcounts/s whereas others show fluctuating count rates and three-level blinking.Comment: 7 pages, submitted to Applied Physics B, revised versio

The mass distribution of quark matter

We analyze lattice QCD results on the equation of state in terms of infinitely many non-interacting massive ideal gas components. We find that the entire pressure-temperature curve can be described by a temperature independent mass distribution at vanishing chemical potential. We collect strong indications for a mass gap in this distribution, conjectured to be related confinement.Comment: 4 pages 3 figures Late

Study of the Negative Magneto-Resistance of Single Proton-Implanted Lithium-Doped ZnO Microwires

The magneto-transport properties of single proton-implanted ZnO and of Li(7\%)-doped ZnO microwires have been studied. The as-grown microwires were highly insulating and not magnetic. After proton implantation the Li(7\%) doped ZnO microwires showed a non monotonous behavior of the negative magneto-resistance (MR) at temperature above 150 K. This is in contrast to the monotonous NMR observed below 50 K for proton-implanted ZnO. The observed difference in the transport properties of the wires is related to the amount of stable Zn vacancies created at the near surface region by the proton implantation and Li doping. The magnetic field dependence of the resistance might be explained by the formation of a magnetic/non magnetic heterostructure in the wire after proton implantation.Comment: 6 pages with 5 figure

Electrical current distribution across a metal-insulator-metal structure during bistable switching

Combining scanning electron microscopy (SEM) and electron-beam-induced current (EBIC) imaging with transport measurements, it is shown that the current flowing across a two-terminal oxide-based capacitor-like structure is preferentially confined in areas localized at defects. As the thin-film device switches between two different resistance states, the distribution and intensity of the current paths, appearing as bright spots, change. This implies that switching and memory effects are mainly determined by the conducting properties along such paths. A model based on the storage and release of charge carriers within the insulator seems adequate to explain the observed memory effect.Comment: 8 pages, 7 figures, submitted to J. Appl. Phy