Calculation of the Raman G peak intensity in monolayer graphene: role of Ward identities

The absolute integrated intensity of the single-phonon Raman peak at 1580 cm^{-1} is calculated for a clean graphene monolayer. The resulting intensity is determined by the trigonal warping of the electronic bands and the anisotropy of the electron-phonon coupling, and is proportional to the second power of the excitation frequency. The main contribution to the process comes from the intermediate electron-hole states with typical energies of the order of the excitation frequency, contrary to what has been reported earlier. This occurs because of strong cancellations between different terms of the perturbation theory, analogous to Ward identities in quantum electrodynamics

Evidence for Two Time Scales in Long SNS Junctions

We use microwave excitation to elucidate the dynamics of long superconductor / normal metal / superconductor Josephson junctions. By varying the excitation frequency in the range 10 MHz - 40 GHz, we observe that the critical and retrapping currents, deduced from the dc voltage vs. dc current characteristics of the junction, are set by two different time scales. The critical current increases when the ac frequency is larger than the inverse diffusion time in the normal metal, whereas the retrapping current is strongly modified when the excitation frequency is above the electron-phonon rate in the normal metal. Therefore the critical and retrapping currents are associated with elastic and inelastic scattering, respectively

Experimental determination of the rattle of simple models

The effect of the excitation frequency on the rattle boundaries of simple models was investigated. The frequency range investigated was from 40 to 4,000 Hz. A 1-inch steel ball was studied to determine the rattle boundary for both vertical motion and for the ball suspended as a pendulum. Effects of surface contact and weight were also studied. Results indicate that the shape of the rattle boundary depends on the particular configuration being investigated as well as the range of frequency being investigated. Although there was condiderable scatter in the data, the general trend indicates that the level of acceleration required for the onset of rattle was independent of excitation frequency

Frequency effect on streaming phenomenon induced by Rayleigh surface acoustic wave in microdroplets

Acoustic streaming of ink particles inside a water microdroplet generated by a surface acoustic wave(SAW) has been studied numerically using a finite volume numerical method and these results have been verified using experimental measurements. Effects of SAW excitation frequency, droplet volume, and radio-frequency (RF) power are investigated, and it has been shown that SAW excitation frequency influences the SAWattenuation length, lSAW , and hence the acoustic energy absorbed by liquid. It has also been observed that an increase of excitation frequency generally enhances the SAW streaming behavior. However, when the frequency exceeds a critical value that depends on the RF power applied to the SAW device, weaker acoustic streaming is observed resulting in less effective acoustic mixing inside the droplet. This critical value is characterised by a dimensionless ratio of droplet radius to SAWattenuation length, i.e., Rd/lSAW . With a mean value of Rd/lSAW  ≈ 1, a fast and efficient mixing can be induced, even at the lowest RF power of 0.05 mW studied in this paper. On the other hand, for the Rd/lSAW ratios much larger than ∼1, significant decreases in streaming velocities were observed, resulting in a transition from regular (strong) to irregular (weak) mixing/flow. This is attributed to an increased absorption rate of acoustic wave energy that leaks into the liquid, resulting in a reduction of the acoustic energy radiated away from the SAW interaction region towards the droplet free surface. It has been demonstrated in this study that a fast and efficient mixing process with a smaller RF power could be achieved if the ratio of Rd/lSAW  ≤ 1 in the SAW-droplet based microfluidics

Spectroscopic investigation of local mechanical impedance of living cells

The mechanical properties of PC12 living cells have been studied at the nanoscale with a Force Feedback Microscope using two experimental approaches. Firstly, the local mechanical impedance of the cell membrane has been mapped simultaneously to the cell morphology at constant force. As the force of the interaction is gradually increased, we observed the appearance of the sub-membrane cytoskeleton. We shall compare the results obtained with this method with the measurement of other existing techniques. Secondly, a spectroscopic investigation has been performed varying the indentation of the tip in the cell membrane and consequently the force applied on it. In contrast with conventional dynamic atomic force microscopy techniques, here the small oscillation amplitude of the tip is not necessarily imposed at the cantilever first eigenmode. This allows the user to arbitrarily choose the excitation frequency in developing spectroscopic AFM techniques. The mechanical response of the PC12 cell membrane is found to be frequency dependent in the 1 kHz - 10 kHz range. The damping coefficient is reproducibly observed to decrease when the excitation frequency is increased.Comment: 8 pages, 8 figure

Loading Bose condensed atoms into the ground state of an optical lattice

We optimize the turning on of a one-dimensional optical potential, V_L(x,t) = S(t) V_0 cos^2(kx) to obtain the optimal turn-on function S(t) so as to load a Bose-Einstein condensate into the ground state of the optical lattice of depth V_0. Specifically, we minimize interband excitations at the end of the turn-on of the optical potential at the final ramp time t_r, where S(t_r) = 1, given that S(0) = 0. Detailed numerical calculations confirm that a simple unit cell model is an excellent approximation when the turn-on time t_r is long compared with the inverse of the band excitation frequency and short in comparison with nonlinear time \hbar/\mu where \mu is the chemical potential of the condensate. We demonstrate using the Gross-Pitaevskii equation with an optimal turn-on function S(t) that the ground state of the optical lattice can be loaded with very little excitation even for times t_r on the order of the inverse band excitation frequency

Analysis of a duffing oscillator that exhibits hysteresis with varying excitation frequency and amplitude

Hysteresis, or jump phenomenon, are a common and severe nonlinear behaviour associated with the Duffing oscillator and the multi-valued properties of the response solution. Jump phenomenon can be induced by either varying the amplitude or the frequency of excitation. In this paper a new time and frequency domain analysis is applied to this class of system based on the response curve and the response spectrum map