Direct probing of band-structure Berry phase in diluted magnetic semiconductors

We report on experimental evidence of the Berry phase accumulated by the charge carrier wave function in single-domain nanowires made from a (Ga,Mn)(As,P) diluted ferromagnetic semiconductor layer. Its signature on the mesoscopic transport measurements is revealed as unusual patterns in the magnetoconductance, that are clearly distinguished from the universal conductance fluctuations. We show that these patterns appear in a magnetic field region where the magnetization rotates coherently and are related to a change in the band-structure Berry phase as the magnetization direction changes. They should be thus considered as a band structure Berry phase fingerprint of the effective magnetic monopoles in the momentum space. We argue that this is an efficient method to vary the band structure in a controlled way and to probe it directly. Hence, (Ga,Mn)As appears to be a very interesting test bench for new concepts based on this geometrical phase.Comment: 7 pages, 6 figure

Noise dephasing in the edge states of the Integer Quantum Hall regime

An electronic Mach Zehnder interferometer is used in the integer quantum hall regime at filling factor 2, to study the dephasing of the interferences. This is found to be induced by the electrical noise existing in the edge states capacitively coupled to each others. Electrical shot noise created in one channel leads to phase randomization in the other, which destroys the interference pattern. These findings are extended to the dephasing induced by thermal noise instead of shot noise: it explains the underlying mechanism responsible for the finite temperature coherence time $\tau_\phi(T)$ of the edge states at filling factor 2, measured in a recent experiment. Finally, we present here a theory of the dephasing based on Gaussian noise, which is found in excellent agreement with our experimental results.Comment: ~4 pages, 4 figure

Improving HTc Josephson Junctions (HTc JJ) by annealing: the role of vacancy-interstitial annihilation

We have studied the annealing effect in transport properties of High temperature Josephson Junctions (HTc JJ) made by ion irradiation. Low temperature annealing (80 degrees Celsius) increases the JJ transition temperature (TJ) and the Ic.Rn product, where Ic is the critical current and Rn the normal resistance. We found that the spread in JJ characteristics can be lowered by sufficient long annealing times. Using random walk numerical simulations, we showed that the characteristic annealing time and the evolution of the spread in JJ characteristics can be explained by a vacancy-interstitial annihilation process rather than by an oxygen diffusion one.Comment: 7 pages and 3 figures submitted to Applied Physics Letter

Study and optimization of ion-irradiated High-Tc Josephson nanoJunctions by Monte Carlo simulations

High Tc Josephson nanoJunctions (HTc JnJ) made by ion irradiation have remarkable properties for technological applications. However, the spread in their electrical characteristics increases with the ion dose. We present a simple model to explain the JnJ inhomogeneities, which accounts quantitatively for experimental data. The spread in the slit's width of the irradiation mask is the limiting factor.Monte Carlo simulations have been performed using different irradiation conditions to study their influence on the spread of the JnJ charcateristics. A "universal" behavior has been evidenced, which allows to propose new strategies to optimize JnJ reproducibility.Comment: 14 pages, 6 Figures. accepted in Journal of Applied Physic

Switching the magnetic configuration of a spin valve by current induced domain wall motion

We present experimental results on the displacement of a domain wall by injection of a dc current through the wall. The samples are 1 micron wide long stripes of a CoO/Co/Cu/NiFe classical spin valve structure. The stripes have been patterned by electron beam lithography. A neck has been defined at 1/3 of the total length of the stripe and is a pinning center for the domain walls, as shown by the steps of the giant magnetoresistance curves at intermediate levels (1/3 or 2/3) between the resistances corresponding to the parallel and antiparallel configurations. We show by electric transport measurements that, once a wall is trapped, it can be moved by injecting a dc current higher than a threshold current of the order of magnitude of 10^7 A/cm^2. We discuss the different possible origins of this effect, i.e. local magnetic field created by the current and/or spin transfer from spin polarized current.Comment: 3 pages, 3 figure

High-Quality Planar high-Tc Josephson Junctions

Reproducible high-Tc Josephson junctions have been made in a rather simple two-step process using ion irradiation. A microbridge (1 to 5 ?m wide) is firstly designed by ion irradiating a c-axis-oriented YBa2Cu3O7-? film through a gold mask such as the non-protected part becomes insulating. A lower Tc part is then defined within the bridge by irradiating with a much lower fluence through a narrow slit (20 nm) opened in a standard electronic photoresist. These planar junctions, whose settings can be finely tuned, exhibit reproducible and nearly ideal Josephson characteristics. This process can be used to produce complex Josephson circuits.Comment: 4 pages, 5 figures, to be published in Applied Physics Letter