Path integral based calculations of symmetrized time correlation functions. I

In this paper, we examine how and when quantum evolution can be approximated in terms of (generalized) classical dynamics in calculations of correlation functions, with a focus on the symmetrized time correlation function introduced by Schofield. To that end, this function is expressed as a path integral in complex time and written in terms of sum and difference path variables. Taylor series expansion of the path integral's exponent to first and second order in the difference variables leads to two original developments. The first order expansion is used to obtain a simple, path integral based, derivation of the so-called Schofield's quantum correction factor. The second order result is employed to show how quantum mechanical delocalization manifests itself in the approximation of the correlation function and hinders, even in the semiclassical limit, the interpretation of the propagators in terms of sets of guiding classical trajectories dressed with appropriate weights.     

Mapping electric fields generated by microelectrodes using optically trapped charged microspheres

In this work, we show a new technique to measure the direction and amplitude of the electric field generated by microelectrodes in a liquid environment, as often used in microfluidic devices. The method is based on the use of optical tweezers as a force transducer. A trapped, charged particle behaves as a probe. With this technique, it is possible to obtain a detailed map of the electric field, even for very complex electrode structures with a resolution below a micrometre and with a sensitivity as low as a few hundreds of V m(-1).     

Ultrafast pump-probe study of excited-state charge-transfer dynamics in umecyanin from horseradish root

We have applied femtosecond pump-probe spectroscopy to investigate the excited-state dynamics of umecyanin from horseradish roots, by exciting its 600-nm ligand-to-metal charge-transfer band with a 15-fs pulse and probing over a broad range in the visible region. The decay of the pump-induced ground-state bleaching is modulated by clearly visible oscillations and occurs exponentially with a time constant depending on the observed spectral component of the transmission difference signal, ranging from 270 fs up to 700 fs. The slower decaying process characterizes the spectral component corresponding to the metal-to-ligand charge-transfer transition. The excited-state decay rate is significantly lower than in other blue copper proteins, probably because of the larger energy gap between ligand- and metal-based orbitals in umecyanin. Wavelength dependence of the recovery times could be due to either the excitation of several transitions or the occurrence of intramolecular vibrational relaxation within the excited state. We also find evidence of a hot ground-state absorption, at 700 nm, persisting for several picoseconds. The vibrational coherence induced by the ultrashort pump pulse allows vibrational activity to be observed, mainly in the ground state, as expected in a system with fast excited-state decay. However, we find evidence of a rapidly damped oscillation, which we assign to the excited state. Finally, the Fourier transform of the oscillatory component of the signal presents additional bands in the low-frequency region which are assigned to collective motions of the protein.     

New insights into urea action on proteins: a SANS study of the lysozyme case

We present a study on lysozyme dissolved in mixtures of water and urea, which is ubiquitously used as a protein denaturant. Despite the wide use of urea, the basic molecular mechanisms inducing protein unfolding are not still clarified. Small-angle neutron scattering (SANS) experiments have been performed using little amounts of denaturant in solutions in order to investigate the urea effect on lysozyme preceding the unfolding process. A global fit strategy, applied to analyze SANS experiments, provides an estimation of the average composition of the solvent in the close vicinity of the protein surface and the change of the protein-protein interactions due to the presence of urea. In particular, the thermodynamic equilibrium constant responsible for cosolvent balancing between the bulk and solvation layer has been determined. It turns out that urea is preferentially driven to the protein surface, confirming literature results at infinite dilute conditions. SANS data also reveal a possible variation of the protein net charge as a function of urea concentration, opening new perspectives and questions about the protein surface architecture at the first stages of unfolding processes.     

Water as a solute: competitive shell formation in (He,Ne) mixed microdroplets

Quantum Monte Carlo (QMC) stochastic calculations are carried out for a series of mixed rare gas clusters containing He and Ne which further include one H(2)O molecule as a single dopant. The ab initio potentials employed in the calculations, and the structural details provided by the QMC results, clearly reveal that the differences in the interaction forces which exist between the two solvent adatoms and the molecular solute are causing strongly competing environments that generate preferential shell structuring when surrounding the water molecule. The different behaviour of the two solvents, and the energetics of mixing, are analyzed in detail for small aggregates and for larger mixtures, revealing structural effects which originate from the different networking between solvent adatoms.     

Efficient generation of Bessel beam arrays by means of an SLM

We use a Spatial Light Modulator (SLM) to produce arrays of Bessel beams by using multiple axicon phase-masks on the SLM. This approach utilises the whole of the SLM, rather than just a thin annular region (which is the case if the SLM is in the far-field of the generated Bessel beams). Using the whole SLM rather than just an annular region means that the required intensity on the SLM is an order of magnitude lower for a given power in the Bessel beams. Spreading the power over the whole SLM is important for high-power applications such as laser micromachining. We allow the axicons to overlap and interfere in the hologram, so the axial length of the Bessel beam core is maintained as we add more beams to the array.     

Microwave properties of DyBCO monodomain in the mixed state and comparison with other RE-BCO systems

We report on microwave measurements on DyBa₂Cu₃O_7?δ monodomains grown by the top-seeded melt-textured technique. We measured the field increase of the surface resistance R_s(H) in the a–b plane at 48.3 GHz. Measurements were performed at fixed temperatures in the range 70 K–T_c with a static magnetic field μ₀H < 0.8 T parallel to the c-axis. Low field steep increase of the dissipation, typical signature of the presence of weak links, is absent, thus indicating the single-domain behavior of the sample under study. The magnetic field dependence of R_s(H) is ascribed to the dissipation caused by vortex motion. The analysis of X_s(H) points to a free-flow regime, thus allowing to obtain the vortex viscosity as a function of temperature. We compare the results with those obtained on RE-BCO systems. In particular, we consider strongly pinned films of YBa₂Cu₃O_7?δ with nanometric BaZrO₃ inclusions.     

Hydrodynamics of Bose and Fermi superfluids at zero temperature: the superfluid nonlinear Schrödinger equation

We discuss the zero-temperature hydrodynamics equations of bosonic and fermionic superfluids and their connection with generalized Gross-Pitaevskii and Ginzburg-Landau equations through a single superfluid nonlinear Schrödinger equation.     

Rheology of sheared monodisperse granular suspensions

Sheared granular suspensions can either flow or be jammed. They show as well a ‘melting’ transition: partially ordered flowing states are found which can be melted into fully disordered arrangements of grains by sufficient shear. While these are well documented phenomena, the underlying mechanisms and their control parameters are still far from clear. Via Molecular Dynamics simulations, we study the rheology of a model system of sheared frictional monodisperse granular materials [7, 8]. In particular, we aim to understand the nature of a critical line separating crystallised and melted states and the “jammed” region in the phase diagram. We outline as well connections and differences with thermal glass formers and colloidal suspensions.     

A study of core and valence levels in β-PbO2 by hard X-ray photoemission

The core and valence levels of β-PbO₂ have been studied using hard X-ray photoemission spectroscopy (hν = 6000 eV and 7700 eV). The Pb 4f core levels display an asymmetric lineshape which may be fitted with components associated with screened and unscreened final states. It is found that intrinsic final state screening is suppressed in the near-surface region. A shift in the O 1s binding energy due to recoil effects is observed under excitation at 7700 eV. It is shown that conduction band states have substantial 6s character and are selectively enhanced in hard X-ray photoemission spectra. However, the maximum amplitude in the Pb 6s partial density of states is found at the bottom of the valence band and the associated photoemission peak shows the most pronounced enhancement in intensity at high photon energy.