Description of a computer program to calculate reacting supersonic internal flow fields with shock waves using viscous characteristics: Program manual and sample calculations

A computer program for calculating internal supersonic flow fields with chemical reactions and shock waves typical of supersonic combustion chambers with either wall or mid-stream injectors is described. The usefulness and limitations of the program are indicated. The program manual and listing are presented along with a sample calculation

On a modified-Lorentz-transformation based gravity model confirming basic GRT experiments

Implementing Poincar\'e's `geometric conventionalism' a scalar Lorentz-covariant gravity model is obtained based on gravitationally modified Lorentz transformations (or GMLT). The modification essentially consists of an appropriate space-time and momentum-energy scaling ("normalization") relative to a nondynamical flat background geometry according to an isotropic, nonsingular gravitational `affecting' function Phi(r). Elimination of the gravitationally `unaffected' S_0 perspective by local composition of space-time GMLT recovers the local Minkowskian metric and thus preserves the invariance of the locally observed velocity of light. The associated energy-momentum GMLT provides a covariant Hamiltonian description for test particles and photons which, in a static gravitational field configuration, endorses the four `basic' experiments for testing General Relativity Theory: gravitational i) deflection of light, ii) precession of perihelia, iii) delay of radar echo, iv) shift of spectral lines. The model recovers the Lagrangian of the Lorentz-Poincar\'e gravity model by Torgny Sj\"odin and integrates elements of the precursor gravitational theories, with spatially Variable Speed of Light (VSL) by Einstein and Abraham, and gravitationally variable mass by Nordstr\"om.Comment: v1: 14 pages, extended version of conf. paper PIRT VIII, London, 2002. v2: section added on effective tensorial rank, references added, appendix added, WEP issue deleted, abstract and other parts rewritten, same results (to appear in Found. Phys.

Photoinduced Electron Pairing in a Driven Cavity

We demonstrate how virtual scattering of laser photons inside a cavity via two-photon processes can induce controllable long-range electron interactions in two-dimensional materials. We show that laser light that is red (blue) detuned from the cavity yields attractive (repulsive) interactions whose strength is proportional to the laser intensity. Furthermore, we find that the interactions are not screened effectively except at very low frequencies. For realistic cavity parameters, laser-induced heating of the electrons by inelastic photon scattering is suppressed and coherent electron interactions dominate. When the interactions are attractive, they cause an instability in the Cooper channel at a temperature proportional to the square root of the driving intensity. Our results provide a novel route for engineering electron interactions in a wide range of two-dimensional materials including AB-stacked bilayer graphene and the conducting interface between LaAlO3 and SrTiO3

Back and forth from cool core to non-cool core: clues from radio-halos

X-ray astronomers often divide galaxy clusters into two classes: "cool core" (CC) and "non-cool core" (NCC) objects. The origin of this dichotomy has been the subject of debate in recent years, between "evolutionary" models (where clusters can evolve from CC to NCC, mainly through mergers) and "primordial" models (where the state of the cluster is fixed "ab initio" by early mergers or pre-heating). We found that in a well-defined sample (clusters in the GMRT Radio halo survey with available Chandra or XMM-Newton data), none of the objects hosting a giant radio halo can be classified as a cool core. This result suggests that the main mechanisms which can start a large scale synchrotron emission (most likely mergers) are the same that can destroy CC and therefore strongly supports "evolutionary" models of the CC-NCC dichotomy. Moreover combining the number of objects in the CC and NCC state with the number of objects with and without a radio-halo, we estimated that the time scale over which a NCC cluster relaxes to the CC state, should be larger than the typical life-time of radio-halos and likely shorter than about 3 Gyr. This suggests that NCC transform into CC more rapidly than predicted from the cooling time, which is about 10 Gyr in NCC systems, allowing the possibility of a cyclical evolution between the CC and NCC states.Comment: Accepted for publication in A&

Coherent Modulation of the YBa2Cu3O6+x Atomic Structure by Displacive Stimulated Ionic Raman Scattering

We discuss the mechanism of coherent phonon generation by Stimulated Ionic Raman Scattering, a process different from conventional excitation with near visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling between a directly excited infrared-active phonon mode and other Raman modes. We experimentally study the response of YBa2Cu3O6+x to the resonant excitation of apical oxygen motions at 20 THz by mid-infrared pulses, which has been shown in the past to enhance the interlayer superconducting coupling. We find coherent oscillations of four totally symmetric (Ag) Raman modes and make a critical assessment of the role of these oscillatory motions in the enhancement of superconductivity.Comment: 12 pages, 4 figure

Pump frequency resonances for light-induced incipient superconductivity in YBa2_2Cu3_3O6.5_{6.5}

Optical excitation in the cuprates has been shown to induce transient superconducting correlations above the thermodynamic transition temperature, $T_C$, as evidenced by the terahertz frequency optical properties in the non-equilibrium state. In YBa$_2$Cu$_3$O$_{6+x}$ this phenomenon has so far been associated with the nonlinear excitation of certain lattice modes and the creation of new crystal structures. In other compounds, like La$_{2-x}$Ba$_x$CuO$_4$, similar effects were reported also for excitation at near infrared frequencies, and were interpreted as a signature of the melting of competing orders. However, to date it has not been possible to systematically tune the pump frequency widely in any one compound, to comprehensively compare the frequency dependent photo-susceptibility for this phenomenon. Here, we make use of a newly developed optical parametric amplifier, which generates widely tunable high intensity femtosecond pulses, to excite YBa$_2$Cu$_3$O$_{6.5}$ throughout the entire optical spectrum (3 - 750 THz). In the far-infrared region (3 - 25 THz), signatures of non-equilibrium superconductivity are induced only for excitation of the 16.4 THz and 19.2 THz vibrational modes that drive $c$-axis apical oxygen atomic positions. For higher driving frequencies (25 - 750 THz), a second resonance is observed around the charge transfer band edge at ~350 THz. These observations highlight the importance of coupling to the electronic structure of the CuO$_2$ planes, either mediated by a phonon or by charge transfer.Comment: 47 pages, 21 figures, 2 table

Hybrid CO₂-Ti:sapphire laser with tunable pulse duration for mid-infrared-pump terahertz-probe spectroscopy

We describe a mid-infrared pump – terahertz-probe setup based on a CO2 laser seeded with 10.6 μm wavelength pulses from an optical parametric amplifier, itself pumped by a Ti:Al2O3 laser. The output of the seeded CO2 laser produces high power pulses of nanosecond duration, which are synchronized to the femtosecond laser. These pulses can be tuned in pulse duration by slicing their front and back edges with semiconductor-plasma mirrors irradiated by replicas of the femtosecond seed laser pulses. Variable pulse lengths from 5 ps to 1.3 ns are achieved, and used in mid-infrared pump, terahertz-probe experiments with probe pulses generated and electro-optically sampled by the femtosecond laser

A river model of space

Within the theory of general relativity gravitational phenomena are usually attributed to the curvature of four-dimensional spacetime. In this context we are often confronted with the question of how the concept of ordinary physical three-dimensional space fits into this picture. In this work we present a simple and intuitive model of space for both the Schwarzschild spacetime and the de Sitter spacetime in which physical space is defined as a specified set of freely moving reference particles. Using a combination of orthonormal basis fields and the usual formalism in a coordinate basis we calculate the physical velocity field of these reference particles. Thus we obtain a vivid description of space in which space behaves like a river flowing radially toward the singularity in the Schwarzschild spacetime and radially toward infinity in the de Sitter spacetime. We also consider the effect of the river of space upon light rays and material particles and show that the river model of space provides an intuitive explanation for the behavior of light and particles at and beyond the event horizons associated with these spacetimes.Comment: 22 pages, 5 figure

Pressure tuning of light-induced superconductivity in K3C60

Optical excitation at terahertz frequencies has emerged as an effective means to manipulate complex solids dynamically. In the molecular solid K3C60, coherent excitation of intramolecular vibrations was shown to transform the high temperature metal into a non-equilibrium state with the optical conductivity of a superconductor. Here we tune this effect with hydrostatic pressure, and we find it to disappear around 0.3 GPa. Reduction with pressure underscores the similarity with the equilibrium superconducting phase of K3C60, in which a larger electronic bandwidth is detrimental for pairing. Crucially, our observation excludes alternative interpretations based on a high-mobility metallic phase. The pressure dependence also suggests that transient, incipient superconductivity occurs far above the 150 K hypothesised previously, and rather extends all the way to room temperature.Comment: 33 pages, 17 figures, 2 table