Friction as Contrast Mechanism in Heterodyne Force Microscopy

The nondestructive imaging of subsurface structures on the nanometer scale has been a long-standing desire in both science and industry. A few impressive images were published so far that demonstrate the general feasibility by combining ultrasound with an Atomic Force Microscope. From different excitation schemes, Heterodyne Force Microscopy seems to be the most promising candidate delivering the highest contrast and resolution. However, the physical contrast mechanism is unknown, thereby preventing any quantitative analysis of samples. Here we show that friction at material boundaries within the sample is responsible for the contrast formation. This result is obtained by performing a full quantitative analysis, in which we compare our experimentally observed contrasts with simulations and calculations. Surprisingly, we can rule out all other generally believed responsible mechanisms, like Rayleigh scattering, sample (visco)elasticity, damping of the ultrasonic tip motion, and ultrasound attenuation. Our analytical description paves the way for quantitative SubSurface-AFM imaging.

High speed collision and reconnection of Abelian Higgs strings in the deep type-II regime

We study high speed collision and reconnection of cosmic strings in the type-II regime (scalar-to-gauge mass ratios larger than one) of the Abelian Higgs model. New phenomena such as multiple reconnections and clustering of small scale structure have been observed and reported in a previous paper, as well as the fact that the previously observed loop that mediates the second intercommutation is only a loop for sufficiently large beta = m_scalar^2/m_gauge^2. Here we give a more detailed account of our study, involving 3D numerical simulations with beta in the range 1 to 64, the largest value simulated to date, as well as 2D simulations of vortex-antivortex (v-av) collisions to understand the possible relation to the new 3D phenomena. Our simulations give further support to the idea that Abelian Higgs strings never pass through each other, unless this is the result of a double reconnection; and that the critical velocity (v_c) for double reconnection goes down with increasing mass ratio, but energy conservation suggests a lower bound around 0.77c. We discuss the qualitative change in the intermediate state observed for large mass ratios. We relate it to a similar change in the outcome of 2D v-av collisions in the form of radiating bound states. In the deep type-II regime the angular dependence of v_c for double reconnection does not seem to conform to semi-analytic predictions based on the Nambu-Goto approximation. We model the high angle collisions reasonably well by incorporating the effect of core interactions, and the torque they produce on the approaching strings, into the Nambu-Goto description of the collision. An interesting, counterintuitive aspect is that the effective collision angle is smaller because of the torque. Our results suggest differences in network evolution and radiation output with respect to the predictions based on Nambu-Goto or beta = 1 Abelian Higgs dynamics.

Higher order intercommutations in Cosmic String Collisions

We report the first observation of multiple intercommutation (more than two successive reconnections) of cosmic strings at ultra-high collision speeds, and the formation of ``kink trains with up to four closely spaced left- or right-moving kinks. We performed a flat space numerical study of abelian Higgs cosmic string intercommutation in the type-II regime $beta > 1$ (where $beta = m^2_{scalar} / m^2_{gauge}$) up to $beta = 64$, the highest value investigated to date. Our results confirm earlier claims that the minimum critical speed for double reconnection goes down with increasing $beta$, from $sim 0.98 c$ at $beta = 1$ to $sim 0.86 c$ for $beta = 64$. Furthermore, we observe a qualitative change in the process leading to the second intercommutation: if $beta geq 16$ it is mediated by a loop expanding from the collision point whereas if $1 < beta leq 8 $ the previously reported ``loop is just an expanding blob of radiation which has no topological features and is absorbed by the strings. The multiple reconnections are observed in the loop-mediated, deep type-II regime $beta geq 16$. Triple reconnections appear to be quite generic for collision parameters on the boundary between single and double reconnection. For $beta = 16$ we observe quadruple events. They result in clustering of small scale structure in the form of ``kink trains. Our findings suggest that, due to the core interactions, the small scale structure and stochastic gravitational wave background of abelian Higgs strings in the strongly type-II regime may be quite different from what would be expected from studies of Nambu-Goto strings or of abelian Higgs strings in the $beta approx 1$ regime.