A Microscopic Description of Displacive Coherent Phonons

We develop a Hamiltonian-based microscopic description of laser pump induced displacive coherent phonons. The theory captures the feedback of the phonon excitation upon the electronic fluid, which is missing in the state-of-the-art phenomenological formulation. We show that this feedback leads to chirping at short time scales, even if the phonon motion is harmonic. At long times this feedback appears as a finite phase in the oscillatory signal. We apply the theory to BaFe$_2$As$_2$, explain the origin of the phase in the oscillatory signal reported in recent experiments, and we predict that the system will exhibit red-shifted chirping at larger fluence. Our theory also opens the possibility to extract equilibrium information from coherent phonon dynamics.Comment: 9 pages, 3 figure

Turbulent transport mechanisms in oscillating bubble plumes

The detailed investigation of an unstable meandering bubble plume created in a 2-m-diameter vessel with a water depth of 1.5 m is reported for void fractions up to 4% and bubble size of the order of 2.5 mm. Simultaneous particle image velocity (PIV) measurements of bubble and liquid velocities and video recordings of the projection of the plume on two vertical perpendicular planes were produced in order to characterize the state of the plume by the location of its centreline and its equivalent diameter. The data were conditionally ensemble averaged using only PIV sets corresponding to plume states in a range as narrow as possible, separating the small-scale fluctuations of the flow from the large-scale motions, namely plume meandering and instantaneous cross-sectional area fluctuations. Meandering produces an apparent spreading of the average plume velocity and void fraction profiles that were shown to remain self-similar in the instantaneous plume cross-section. Differences between the true local time-average relative velocities and the difference of the averaged phase velocities were measured; the complex variation of the relative velocity was explained by the effects of passing vortices and by the fact that the bubbles do not reach an equilibrium velocity as they migrate radially, producing momentum exchanges between high- and low-velocity regions. Local entrainment effects decrease with larger plume diameters, contradicting the classical dependence of entrainment on the time-averaged plume diameter. Small plume diameters tend to trigger ‘entrainment eddies' that promote the inward-flow motion. The global turbulent kinetic energy was found to be dominated by the vertical stresses. Conditional averages according to the plume diameter showed that the large-scale motions did not affect the instantaneous turbulent kinetic energy distribution in the plume, suggesting that large scales and small scales are not correlated. With conditional averaging, meandering was a minor effect on the global kinetic energy and the Reynolds stresses. In contrast, plume diameter fluctuations produce a substantial effect on these quantitie

Detection of squeezed phonons in pump-probe spectroscopy

Robust engineering of phonon squeezed states in optically excited solids has emerged as a promising tool to control and manipulate their properties. However, in contrast to quantum optical systems, detection of phonon squeezing is subtle and elusive, and an important question is what constitutes an unambiguous signature of it. The state of the art involves observing oscillations at twice the phonon frequency in time resolved measurements of the out of equilibrium phonon fluctuation. Using Keldysh formalism we show that such a signal is a necessary but not a sufficient signature of a squeezed phonon, since we identify several mechanisms that do not involve squeezing and yet which produce similar oscillations. We show that a reliable detection requires a time and frequency resolved measurement of the phonon spectral function

The STAR Silicon Strip Detector (SSD)

The STAR Silicon Strip Detector (SSD) completes the three layers of the Silicon Vertex Tracker (SVT) to make an inner tracking system located inside the Time Projection Chamber (TPC). This additional fourth layer provides two dimensional hit position and energy loss measurements for charged particles, improving the extrapolation of TPC tracks through SVT hits. To match the high multiplicity of central Au+Au collisions at RHIC the double sided silicon strip technology was chosen which makes the SSD a half million channels detector. Dedicated electronics have been designed for both readout and control. Also a novel technique of bonding, the Tape Automated Bonding (TAB), was used to fullfill the large number of bounds to be done. All aspects of the SSD are shortly described here and test performances of produced detection modules as well as simulated results on hit reconstruction are given.Comment: 11 pages, 8 figures, 1 tabl

Validation of the Meaf Score and Comparaison with Early Allograft Dysfunction Critera on Predicting 3 Months Graft Survival

International audienceno abstrac

Disappearance of back-to-back high pTp_T hadron correlations in central Au+Au collisions at sNN\sqrt{s_{NN}} = 200 GeV

Azimuthal correlations for large transverse momentum charged hadrons have been measured over a wide pseudo-rapidity range and full azimuth in Au+Au and p+p collisions at $\sqrt{s_{NN}}$ = 200 GeV. The small-angle correlations observed in p+p collisions and at all centralities of Au+Au collisions are characteristic of hard-scattering processes already observed in elementary collisions. A strong back-to-back correlation exists for p+p and peripheral Au + Au. In contrast, the back-to-back correlations are reduced considerably in the most central Au+Au collisions, indicating substantial interaction as the hard-scattered partons or their fragmentation products traverse the medium.Comment: submitted to Phys. Rev. Let

Azimuthal anisotropy and correlations in the hard scattering regime at RHIC

Azimuthal anisotropy ($v_2$) and two-particle angular correlations of high $p_T$ charged hadrons have been measured in Au+Au collisions at $\sqrt{s_{NN}}$=130 GeV for transverse momenta up to 6 GeV/c, where hard processes are expected to contribute significantly. The two-particle angular correlations exhibit elliptic flow and a structure suggestive of fragmentation of high $p_T$ partons. The monotonic rise of $v_2(p_T)$ for $p_T<2$ GeV/c is consistent with collective hydrodynamical flow calculations. At \pT>3 GeV/c a saturation of $v_2$ is observed which persists up to $p_T=6$ GeV/c.Comment: As publishe