An Ultrasonic analog for a laser

We report measurements on ultrasonic systems analogous to random lasers. One system entails unstable ultrasonic feedback between distinct transducers, another involves a piezoelectric device that emits spontaneously and by stimulation. Both systems are found to exhibit behaviors similar to those of lasers. Over a wide range of parameters we observe narrow single emission lines, sensitivity to linear cavity properties, complex multi-mode emissions, and line narrowing

Entrainment and stimulated emission of auto-oscillators in an acoustic cavity

We report theory, measurements and numerical simulations on nonlinear piezoelectric ultrasonic devices with stable limit cycles. The devices are shown to exhibit behavior familiar from the theory of coupled auto-oscillators. Frequency of auto-oscillation is affected by the presence of an acoustic cavity as these spontaneously emitting devices adjust their frequency to the spectrum of the acoustic cavity. Also, the auto-oscillation is shown to be entrained by an applied field; the oscillator synchronizes to an incident wave at a frequency close to the natural frequency of the limit cycle. It is further shown that synchronization occurs here with a phase that can, depending on details, correspond to stimulated emission: the power emission from the oscillator is augmented by the incident field. These behaviors are essential to eventual design of an ultrasonic system that would consist of a number of such devices entrained to their mutual field, a system that would be an analog to a laser. A prototype laser is constructed

Coherent backscattering of ultrasound without a source

Coherent backscattering is due to constructive interferences of reciprocal paths and leads to an enhancement of the intensity of a multiply scattered field near its source. To observe this enhancement an array of receivers is conventionally placed close to the source. Our approach here is different. In a first experiment, we recover the coherent backscattering effect (CBE) within an array of sources and a distant receiver using time correlation of diffuse fields. The enhancement cone has an excellent spatial resolution. The dynamics of the enhancement factor is studied in a second experiment using correlation of thermal phonons at the same ultrasonic frequencies, without any active source

Growth and characterisation of titanium sulphide nanostructures by surface-assisted vapour transport methods; from trisulphide ribbons to disulphide nanosheets

Surface Assisted Chemical Vapour Transport (SACVT) methods have been employed to grow nanostructures of titanium disulphide (TiS2) and titanium trisulphide (TiS3). SACVT reactions occur between titanium and sulphur powders to form TiSx species transported in the vapour phase to grow nanometric flower-like structures on titanium-coated silica substrates. The evolution of structure and composition has been followed by powder X-ray diffraction, electron microscopy and Raman spectroscopy. At 1 : 2 Ti : S ratios, the size and shape of the hexagonal 1T-TiS2 titanium disulphide structures formed can be varied from flower-like growths with 'petals' formed from nanosheets 10 nm thick to platelets microns across. Increasing the proportion of sulphur (Ti : S 1 : 4) enables TiS3 flower-like structures composed of radiating nanoribbons to grow at elevated temperatures without decomposition to TiS2. TEM/SAED suggests that individual trisulphide ribbons grow along the [010] direction. Magnetic properties of the disulphide nanomaterials have been determined using SQUID magnetometry and Raman spectra for disulphides suggest that their crystal and electronic structures may be more complex than expected for bulk, stoichiometric, CdI2-structured TiS2

Joint Density-Functional Theory of the Electrode-Electrolyte Interface: Application to Fixed Electrode Potentials, Interfacial Capacitances, and Potentials of Zero Charge

This work explores the use of joint density-functional theory, a new form of density-functional theory for the ab initio description of electronic systems in thermodynamic equilibrium with a liquid environment, to describe electrochemical systems. After reviewing the physics of the underlying fundamental electrochemical concepts, we identify the mapping between commonly measured electrochemical observables and microscopically computable quantities within an, in principle, exact theoretical framework. We then introduce a simple, computationally efficient approximate functional which we find to be quite successful in capturing a priori basic electrochemical phenomena, including the capacitive Stern and diffusive Gouy-Chapman regions in the electrochemical double layer, quantitative values for interfacial capacitance, and electrochemical potentials of zero charge for a series of metals. We explore surface charging with applied potential and are able to place our ab initio results directly on the scale associated with the Standard Hydrogen Electrode (SHE). Finally, we provide explicit details for implementation within standard density-functional theory software packages at negligible computational cost over standard calculations carried out within vacuum environments.Comment: 18 pages, 5 figures. Initially presented at APS March Meeting 2010. Accepted for publication in Physical Review B on Jul. 27, 201

The reprocessing features in the X-ray spectrum of the NELG MCG-5-23-16

We present results from the spectral analysis of the Seyfert 1.9 galaxy MCG-5-23-16, based on ASCA, BeppoSAX, Chandra and XMM-Newton observations. The spectrum of this object shows a complex iron Kalpha emission line, which is best modeled by a superposition of a narrow and a broad (possibly relativistic) iron line, together with a Compton reflection component. Comparing results from all (six) available observations, we do not find any significant variation in the flux of both line components. The moderate flux continuum variability (about 25% difference between the brightest and faintest states), however, does not permit us to infer much about the location of the line-emitting material. The amount of Compton reflection is lower than expected from the total iron line EW, implying either an iron overabundance or that one of the two line components (most likely the narrow one) originates in Compton-thin matter.Comment: Accepted for publication in A&

Cavity-enhanced optical detection of carbon nanotube Brownian motion

Optical cavities with small mode volume are well-suited to detect the vibration of sub-wavelength sized objects. Here we employ a fiber-based, high-finesse optical microcavity to detect the Brownian motion of a freely suspended carbon nanotube at room temperature under vacuum. The optical detection resolves deflections of the oscillating tube down to 50pm/Hz^1/2. A full vibrational spectrum of the carbon nanotube is obtained and confirmed by characterization of the same device in a scanning electron microscope. Our work successfully extends the principles of high-sensitivity optomechanical detection to molecular scale nanomechanical systems.Comment: 14 pages, 11 figure

Comparison of CDMA and FDMA for the MobileStar(sm) system

Spread-spectrum code division multiple access (CDMA) and single channel per carrier frequency division multiple access (FDMA) systems are compared for spectrum efficiency. CDMA is shown to have greater maximum throughput than FDMA for the MobileStar(sm) system which uses digital voice activated carriers and directive circularly polarized satellite antennas

Tidal streams in a MOND potential: constraints from Sagittarius

We compare orbits in a thin axisymmetric disc potential in MOND to those in a thin disc plus near-spherical dark matter halo predicted by a $\Lambda$CDM cosmology. Remarkably, the amount of orbital precession in MOND is nearly identical to that which occurs in a mildly oblate CDM Galactic halo (potential flattening q=0.9), consistent with recent constraints from the Sagittarius stream. Since very flattened mass distributions in MOND produce rounder potentials than in standard Newtonian mechanics, we show that it will be very difficult to use the tidal debris from streams to distinguish between a MOND galaxy and a standard CDM galaxy with a mildly oblate halo. If a galaxy can be found with either a prolate halo, or one which is more oblate than $q \sim 0.9$ this would rule out MOND as a viable theory. Improved data from the leading arm of the Sagittarius dwarf - which samples the Galactic potential at large radii - could rule out MOND if the orbital pole precession can be determined to an accuracy of the order of $\pm 1^o$.Comment: 7 pages, 3 figures. Final version accepted for publication in MNRAS. The modelling of the Sagittarius stream has been improved, but otherwise the conclusions remain the sam