First critical field measurements of superconducting films by third harmonic analysis

The temperature behaviour of the first critical field ($B_{C1}$) of superconducting thin film samples can be determined with high accuracy using an inductive and contactless method. Driving a sinusoidal current in a single coil placed in front of the sample, a non zero third harmonic voltage $V_{3}$ is induced in it when Abrikosov vortices enter the sample. Conditions to be satisfied for the quantitative evaluation of $B_{C1}$ using this technique are detailed. As validation test, different type II superconductors (Nb, NbN, MgB$_{2}$ and Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-d}$ under the form of thin films) have been measured. The comparison between experimental results, data presented in literature and theoretical predictions is presented and discussed.Comment: to be published in Journal of Applied Physic

Intrinsic and structural isotope effects in Fe-based superconductors

The currently available results of the isotope effect on the superconducting transition temperature T_c in Fe-based high-temperature superconductors (HTS) are highly controversial. The values of the Fe isotope effect (Fe-IE) exponent \alpha_Fe for various families of Fe-based HTS were found to be as well positive, as negative, or even be exceedingly larger than the BCS value \alpha_BCS=0.5. Here we demonstrate that the Fe isotope substitution causes small structural modifications which, in turn, affect T_c. Upon correcting the isotope effect exponent for these structural effects, an almost unique value of \alpha~0.35-0.4 is observed for at least three different families of Fe-based HTS.Comment: 4 pages, 2 figure

Reducing Polarization Mode Dispersion With Controlled Polarization Rotations

One of the fundamental limitations to high bit rate, long distance, telecommunication in optical fibers is Polarization Mode Dispersion (PMD). Here we introduce a conceptually new method to reduce PMD in optical fibers by carrying out controlled rotations of polarization at predetermined locations along the fiber. The distance between these controlled polarization rotations must be less than both the beat length and the mode coupling length of the fiber. This method can also be combined with the method in which the fiber is spun while it drawn. The incidence of imperfections on the efficiency of the method is analysed.Comment: 4 page

Ratchet Cellular Automata

In this work we propose a ratchet effect which provides a general means of performing clocked logic operations on discrete particles, such as single electrons or vortices. The states are propagated through the device by the use of an applied AC drive. We numerically demonstrate that a complete logic architecture is realizable using this ratchet. We consider specific nanostructured superconducting geometries using superconducting materials under an applied magnetic field, with the positions of the individual vortices in samples acting as the logic states. These devices can be used as the building blocks for an alternative microelectronic architecture.Comment: 4 pages, 3 figure

Study of the local field distribution on a single-molecule magnet-by a single paramagnetic crystal; a DPPH crystal on the surface of an Mn12-acetate crystal

The local magnetic field distribution on the subsurface of a single-molecule magnet crystal, SMM, above blocking temperature (T >> Tb) detected for a very short time interval (~ 10-10 s), has been investigated. Electron Paramagnetic Resonance (EPR) spectroscopy using a local paramagnetic probe was employed as a simple alternative detection method. An SMM crystal of [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-acetate) and a crystal of 2,2- diphenyl-1-picrylhydrazyl (DPPH) as the paramagnetic probe were chosen for this study. The EPR spectra of DPPH deposited on Mn12-acetate show additional broadening and shifting in the magnetic field in comparison to the spectra of the DPPH in the absence of the SMM crystal. The additional broadening of the DPPH linewidth was considered in terms of the two dominant electron spin interactions (dipolar and exchange) and the local magnetic field distribution on the crystal surface. The temperature dependence of the linewidth of the Gaussian distribution of local fields at the SMM surface was extrapolated for the low temperature interval (70-5 K)

Electron spin resonance on a 2-dimensional electron gas in a single AlAs quantum well

Direct electron spin resonance (ESR) on a high mobility two dimensional electron gas in a single AlAs quantum well reveals an electronic $g$-factor of 1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimum linewidth of 7 Gauss. The ESR amplitude and its temperature dependence suggest that the signal originates from the effective magnetic field caused by the spin orbit-interaction and a modulation of the electron wavevector caused by the microwave electric field. This contrasts markedly to conventional ESR that detects through the microwave magnetic field.Comment: 4 pages, 4 figure

Coplanar stripline antenna design for optically detected magnetic resonance on semiconductor quantum dots

We report on the development and testing of a coplanar stripline antenna that is designed for integration in a magneto-photoluminescence experiment to allow coherent control of individual electron spins confined in single self-assembled semiconductor quantum dots. We discuss the design criteria for such a structure which is multi-functional in the sense that it serves not only as microwave delivery but also as electrical top gate and shadow mask for the single quantum dot spectroscopy. We present test measurements on hydrogenated amorphous silicon, demonstrating electrically detected magnetic resonance using the in-plane component of the oscillating magnetic field created by the coplanar stripline antenna necessary due to the particular geometry of the quantum dot spectroscopy. From reference measurements using a commercial electron spin resonance setup in combination with finite element calculations simulating the field distribution in the structure, we obtain an average magnetic field of ~0.2mT at the position where the quantum dots would be integrated into the device. The corresponding pi-pulse time of ~0.3us fully meets the requirements set by the high sensitivity optical spin read-out scheme developed for the quantum dot

Axially open nonradiative structures: an example of single-mode resonator based on the sample holder

The concept of nonradiative dielectric resonator is generalized in order to include axially open configurations having rotational invariance. The resulting additional nonradiative conditions are established for the different resonance modes on the basis of their azimuthal modal index. An approximate chart of the allowed dielectric and geometrical parameters for the TE011 mode is given. A practical realization of the proposed device based on commercial fused quartz tubes is demonstrated at millimeter wavelengths, together with simple excitation and tuning mechanisms. The observed resonances are characterized in their basic parameters, as well as in the field distribution by means of a finite element method. The predictions of the theoretical analysis are well confirmed, both in the general behaviour and in the expected quality factors. The resulting device, in which the sample holder acts itself as single-mode resonating element, combines an extreme ease of realization with state-of-the-art performances. The general benefits of the proposed open single-mode resonators are finally discussed.Comment: 18 pages, 10 figure

Magnetic susceptibility of ultra-small superconductor grains

For assemblies of superconductor nanograins, the magnetic response is analyzed as a function of both temperature and magnetic field. In order to describe the interaction energy of electron pairs for a huge number of many-particle states, involved in calculations, we develop a simple approximation, based on the Richardson solution for the reduced BCS Hamiltonian and applicable over a wide range of the grain sizes and interaction strengths at arbitrary distributions of single-electron energy levels in a grain. Our study is focused upon ultra-small grains, where both the mean value of the nearest-neighbor spacing of single-electron energy levels in a grain and variations of this spacing from grain to grain significantly exceed the superconducting gap in bulk samples of the same material. For these ultra-small superconductor grains, the overall profiles of the magnetic susceptibility as a function of magnetic field and temperature are demonstrated to be qualitatively different from those for normal grains. We show that the analyzed signatures of pairing correlations are sufficiently stable with respect to variations of the average value of the grain size and its dispersion over an assembly of nanograins. The presence of these signatures does not depend on a particular choice of statistics, obeyed by single-electron energy levels in grains.Comment: 40 pages, 12 figures, submitted to Phys. Rev. B, E-mail addresses: [email protected], [email protected], [email protected]