25,185 research outputs found
Dynamical Interactions with Electronic Instruments
This paper examines electronic instruments that incorporate dynamical systems, where the behaviour of the instrument depends not only upon the immediate input to the instrument, but also on the past input. Five instruments are presented as case studies: Michel Waisviszâ Crackle-box, Dylan Menziesâ Spiro, no-input mixing desk, the authorâs Feedback Joypad, and microphone-loudspeaker feedback. Links are suggested between the sonic affordances of each instrument and the dynamical mechanisms embedded in them. These affordances are contrasted with those of non-dynamical instruments such as the Theremin and sample-based instruments. This is discussed in the context of contemporary, material-oriented approaches to composition and particularly to free improvisation where elements such as unpredictability and instability are often of interest, and the process of exploration and discovery is an important part of the practice
Phonons in aluminum at high temperatures studied by inelastic neutron scattering
Inelastic neutron scattering measurements on aluminum metal were performed at temperatures of 10, 150, 300, 525, and 775 K using direct-geometry Fermi chopper spectrometers. The temperature dependent phonon density of states (DOS) was determined from the scattering, and was used to fit Bornâvon KĂĄrmĂĄn models of lattice dynamics. The shifts in the phonon frequencies with increasing temperature were largely explained by the softening of the longitudinal force constants out to third nearest neighbors. A significant broadening of the phonon spectra at high temperatures was also measured. The phonon DOS was used to determine the vibrational contributions to the entropy of aluminum as a function of temperature. All other contributions to the entropy of aluminum were calculated or assessed, and the total entropy was in excellent agreement with the NIST-JANAF compilation [M. W. Chase, J. Phys. Chem. Ref. Data Monogr. 9, 59 (1998)]. Anharmonic effects were attributed to phonon-phonon interactions. The quasiharmonic approximation was generally successful, but its weaknesses are discussed
Electron-ion coupling in semiconductors beyond Fermi's golden rule
In the present work, a theoretical study of electron-phonon (electron-ion)
coupling rates in semiconductors driven out of equilibrium is performed.
Transient change of optical coefficients reflects the band gap shrinkage in
covalently bonded materials, and thus, the heating of atomic lattice. Utilizing
this dependence, we test various models of electron-ion coupling. The
simulation technique is based on tight-binding molecular dynamics. Our
simulations with the dedicated hybrid approach (XTANT) indicate that the widely
used Fermi's golden rule can break down describing material excitation on
femtosecond time scales. In contrast, dynamical coupling proposed in this work
yields a reasonably good agreement of simulation results with available
experimental data
The HARPS search for southern extra-solar planets XIX. Characterization and dynamics of the GJ876 planetary system
Precise radial-velocity measurements for data acquired with the HARPS
spectrograph infer that three planets orbit the M4 dwarf star GJ876. In
particular, we confirm the existence of planet "d", which orbits every 1.93785
days. We find that its orbit may have significant eccentricity (e=0.14), and
deduce a more accurate estimate of its minimum mass of 6.3 Earth masses.
Dynamical modeling of the HARPS measurements combined with literature
velocities from the Keck Observatory strongly constrain the orbital
inclinations of the "b" and "c" planets. We find that i_b = 48.9 degrees and
i_c = 48.1 degrees, which infers the true planet masses of M_b = 2.64 Jupiter
masses and M_c = 0.83 Jupiter masses, respectively. Radial velocities alone, in
this favorable case, can therefore fully determine the orbital architecture of
a multi-planet system, without the input from astrometry or transits.
The orbits of the two giant planets are nearly coplanar, and their 2:1 mean
motion resonance ensures stability over at least 5 Gyr. The libration amplitude
is smaller than 2 degrees, suggesting that it was damped by some dissipative
process during planet formation. The system has space for a stable fourth
planet in a 4:1 mean motion resonance with planet "b", with a period around 15
days. The radial velocity measurements constrain the mass of this possible
additional planet to be at most that of the Earth.Comment: 10 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
Interaction of Phonons and Dirac Fermions on the Surface of Bi2Se3: A Strong Kohn Anomaly
We report the first measurements of phonon dispersion curves on the (001)
surface of the strong three-dimensional topological insulator Bi2Se3. The
surface phonon measurements were carried out with the aid of coherent helium
beam surface scattering techniques. The results reveal a prominent signature of
the exotic metallic Dirac fermion quasi-particles, including a strong Kohn
anomaly. The signature is manifest in a low energy isotropic convex dispersive
surface phonon branch with a frequency maximum of 1.8 THz, and having a
V-shaped minimum at approximately 2kF that defines the Kohn anomaly.
Theoretical analysis attributes this dispersive profile to the renormalization
of the surface phonon excitations by the surface Dirac fermions. The
contribution of the Dirac fermions to this renormalization is derived in terms
of a Coulomb-type perturbation model
Quantum properties of dichroic silicon vacancies in silicon carbide
The controlled generation and manipulation of atom-like defects in solids has
a wide range of applications in quantum technology. Although various defect
centres have displayed promise as either quantum sensors, single photon
emitters or light-matter interfaces, the search for an ideal defect with
multi-functional ability remains open. In this spirit, we investigate here the
optical and spin properties of the V1 defect centre, one of the silicon vacancy
defects in the 4H polytype of silicon carbide (SiC). The V1 centre in 4H-SiC
features two well-distinguishable sharp optical transitions and a unique S=3/2
electronic spin, which holds promise to implement a robust spin-photon
interface. Here, we investigate the V1 defect at low temperatures using optical
excitation and magnetic resonance techniques. The measurements, which are
performed on ensemble, as well as on single centres, prove that this centre
combines coherent optical emission, with up to 40% of the radiation emitted
into the zero-phonon line (ZPL), a strong optical spin signal and long spin
coherence time. These results single out the V1 defect in SiC as a promising
system for spin-based quantum technologies
- âŠ