K. Heeck

Halving the Casimir force with Conductive Oxides

The possibility to modify the strength of the Casimir effect by tailoring the dielectric functions of the interacting surfaces is regarded as a unique opportunity in the development of micro- and nanoelectromechanical systems. In air, however, one expects that, unless noble metals are used, the electrostatic force arising from trapped charges overcomes the Casimir attraction, leaving no room for exploitation of Casimir force engineering at ambient conditions. Here we show that, in the presence of a conductive oxide, the Casimir force can be the dominant interaction even in air, and that the use of conductive oxides allows one to reduce the Casimir force up to a factor of 2 when compared to noble metals.

Pressure dependence of the T/sub c/ of YBa/sub 2/Cu/sub 3/O/sub 7/ up to 170 kbar

The superconducting onset temperature T/sub co/ of single-phased YBa/sub 2/Cu/sub 3/O/sub 9-//sub delta/ (with deltaapprox. =2) measured resistively in a diamond anvil cell is found to increase at a rate dT/sub co//dp = 0.043 K kbar/sup -1/ up to 170 kbar. This is much weaker than for La-Ba-Cu-O for which dT/sub co//dp = 0.64 K kbar/sup -1/. The pressure dependence of the high-T/sub c/ superconductors measured so far cannot be explained within a standard electron-phonon Bardeen-Cooper-Schrieffer theory. Predictions of resonating-valence bonds and bipolaronic theories are discussed.

Fast imaging polarimeter for magneto-optical investigations

~typically, 0.5‐2.0 mm! is in direct contact with the superconductor and rotates the polarization vector of the incident linear polarized light over an angle f proportional to the local magnetic field B according to f5VlB. Between the crossed polarizers of a polarization microscope the square of the local magnetic field is thus imaged as an intensity map. Unfortunately, this type of setup has three main disadvantages: ~i! the sign of the rotation angle of the polarization vector and, hence, of the magnetic field, cannot be determined; ~ii! the setup is very insensitive for small magnetic fields/angles; and ~iii! the measured intensity distribution is strongly influenced by the inhomogeneous illumination. In this article, we report on a new method which circumvents all these problems. The transmitted intensity in a polarization microscope with a sample placed between the polarizer and analyzer is I5L sin 2 ~a1f!.L~a1f! 2 , where f is the rotation angle due to the sample, ~90°2a! is the angle between the polarizer and analyzer, and L is the incident intensity. For a50, i.e., with perfectly crossed polarizer and analyzer, the intensity depends quadratically on f for small f. This results in poor sensitivity close to f50 while the method gives no information on the sign of f .T o increase the sensitivity close to f50, sometimes a is set to a small nonzero value. We expand this idea by modulating the incident polarization direction. Since the sinusoidal modulation used for nonimaging Kerr measurements has the disadvantage that a lot of processing is needed to determine f and also has a response time lower than the modulation frequency ~which due to Nyquist’s 4 sampling theorem must be much lower than the image acquisition rate!, we use a modulation with only three fixed values of a. This is justified since, even if we take into account that there may be an intensity offset K ~due to camera noise, readout offset, or stray light! resulting in a measured intensity I.K1L~a1f! 2 , ~1! only three measurements suffice to determine K, L, and f. We choose to measure at a52a 0 , 0, and 1a 0 , where a 0 is typically of the order of the maximum rotation due to the sample ~a few degrees!; the corresponding intensities are denoted by I 2 , I 0 , and I 1 . It is easily verified that L and f can be found from such a measurement using

Fast determination of 2D current patterns in flat conductors from measurement of their magnetic field

Abstract An extremely fast method is presented to calculate the local current–density vector in a flat conductor from the z -component of the magnetic field measured above its surface, e.g. by means of magneto-optical indicators, Hall-probe arrays or scanning SQUIDs. The method may be used for samples of arbitrary thickness provided that the current vector has only x - and y -components. The method combines the conjugate gradient (CG) method and fast Fourier transform to invert the relevant Toeplitz matrix equation. For a current map of n × n pixels, the number of operations needed is of order n 2.8 only, compared to n 4.5 or higher for earlier methods. The increase in speed for 512×512 pixels is found to be a factor 135 with respect to the fastest existing CG method.

Fiber-top atomic force microscope

We present the implementation of an atomic force microscope (AFM) based on fiber-top design. Our results demonstrate that the performances of fiber-top AFMs in contact mode are comparable to those of similar commercially available instruments. Our device thus represents an interesting alternative to existing AFMs, particularly for applications outside specialized research laboratories, where a compact, user-friendly, and versatile tool might often be preferred.

No anomalous scaling in electrostatic calibrations for Casimir force measurements

In a recent paper [Phys. Rev. A 78, 020101(R) (2008)], Kim et al. have reported a large anomaly in the scaling law of the electrostatic interaction between a sphere and a plate, which was observed during the calibration of their Casimir force setup. Here we experimentally demonstrate that this behavior is not universal. Electrostatic calibrations obtained with our setup follow the scaling law expected from elementary electrostatic arguments, even when the electrostatic voltage that one must apply to minimize the force (typically ascribed to contact potentials) depends on the separation between the surfaces.

Ferrule-top nanoindenter: An optomechanical fiber sensor for nanoindentation

Ferrule-top probes are self-aligned all-optical devices obtained by fabricating a cantilever on the top of a ferruled optical fiber. This approach has been proven to provide a new platform for the realization of small footprint atomic force microscopes (AFMs) that adapt well to utilization outside specialized laboratories [D. Chavan et al., Rev. Sci. Instrum. 81, 123702 (2010); ibid. 82, 046107 (2011)]. In this paper we now show that ferrule-top cantilevers can be also used to develop nanoindenters. Our instrument combines the sensitivity of commercial AFM-based indentation with the ease-of-use of more macroscopic instrumented indenters available today on the market. Furthermore, the all-optical design allows smooth operations also in liquids, where other devices are much more limited and often provide data that are difficult to interpret. This study may pave the way to the implementation of a new generation user-friendly nanoindenters for the measurement of the stiffness of samples in material sciences and medical research.

Fibre-top cantilevers: design, fabrication and applications

Fibre-top cantilevers are a new generation of miniaturized devices obtained by carving tiny mechanical beams directly on the cleaved edge of an optical fibre. The light coupled from the other side of the fibre allows measurements of the position of the cantilever with sub-nanometre accuracy. The monolithic structure of the device, the absence of electronic contacts on the sensing head, and the simplicity of the working principle offer unprecedented opportunities for the development of scientific instruments for both standard applications and utilization beyond research laboratories. In this paper we review the results that our group has obtained over the last year in the development of this technology. We describe the working principle and the fabrication procedure, and we present a series of proof-of-concept experiments that demonstrate that fibre-top cantilevers can be used both for atomic force microscopy and for the detection of chemical species.

Microtorquemeter for magnetization measurements on small superconducting samples

A torquemeter with a noise level of ∼10−12 N m was developed and used for magnetization measurements (due to the Meissner–Ochsenfeld effect) on superconducting samples with a mass typically smaller than 10μg. The torquemeter consists of a very soft, thin metal beam, symmetrically placed between two capacitor plates, which are used for position measurement. Due to the symmetric setup electrostatic forces on the beam are largely compensated. A cheap but accurate electronic device replaces the conventional capacitance bridge and lock‐in amplifier. Calibration of this kind of apparatus is discussed in some detail. Experiments are reported on a 6 μg lead disk and on small crystals of YBa2Cu3O7 and Nd2−xCexCuO4−δ.

Halving the Casimir force with conductive oxides: experimental details.

limited space available, were omitted in the previous article. We discuss the performance of our setup in terms of stability of the calibration procedure and reproducibility of the Casimir force measurement. We also introduce and demonstrate a technique to obtain the spring constant of our force sensor. Furthermore, we present a thorough description of the experimental method, a comprehensive explanation of data elaboration and error analysis, and a complete characterization of the dielectric function and of the surface roughness of the samples used in the actual experiment.