Beyond the fundamental noise limit in coherent optical fiber links

It is well known that temperature variations and acoustic noise affect ultrastable frequency dissemination along optical fiber. Active stabilization techniques are in general adopted to compensate for the fiber-induced phase noise. However, despite this compensation, the ultimate link performances remain limited by the so called delay-unsuppressed fiber noise that is related to the propagation delay of the light in the fiber. In this paper, we demonstrate a data post-processing approach which enables us to overcome this limit. We implement a subtraction algorithm between the optical signal delivered at the remote link end and the round-trip signal. In this way, a 6 dB improvement beyond the fundamental limit imposed by delay-unsuppressed noise is obtained. This result enhances the resolution of possible comparisons between remote optical clocks by a factor of 2. We confirm the theoretical prediction with experimental data obtained on a 47 km metropolitan fiber link, and propose how to extend this method for frequency dissemination purposes as well

Spin-polarized tunneling spectroscopy in tunnel junctions with half-metallic electrodes

We have studied the magnetoresistance (TMR) of tunnel junctions with electrodes of La2/3Sr1/3MnO3 and we show how the variation of the conductance and TMR with the bias voltage can be exploited to obtain a precise information on the spin and energy dependence of the density of states. Our analysis leads to a quantitative description of the band structure of La2/3Sr1/3MnO3 and allows the determination of the gap delta between the Fermi level and the bottom of the t2g minority spin band, in good agreement with data from spin-polarized inverse photoemission experiments. This shows the potential of magnetic tunnel junctions with half-metallic electrodes for spin-resolved spectroscopic studies.Comment: To appear in Physical Review Letter

Thermochemical scanning probe lithography of protein gradients at the nanoscale

Patterning nanoscale protein gradients is crucial for studying a variety of cellular processes in vitro. Despite the recent development in nano-fabrication technology, combining nanometric resolution and fine control of protein concentrations is still an open challenge. Here, we demonstrate the use of thermochemical scanning probe lithography (tc-SPL) for defining micro- and nano-sized patterns with precisely controlled protein concentration. First, tc-SPL is performed by scanning a heatable atomic force microscopy tip on a polymeric substrate, for locally exposing reactive amino groups on the surface, then the substrate is functionalized with streptavidin and laminin proteins. We show, by fluorescence microscopy on the patterned gradients, that it is possible to precisely tune the concentration of the immobilized proteins by varying the patterning parameters during tc-SPL. This paves the way to the use of tc-SPL for defining protein gradients at the nanoscale, to be used as chemical cues e.g. for studying and regulating cellular processes in vitro

Towards an on-chip platform for the controlled application of forces via magnetic particles: A novel device for mechanobiology

In-vitro tests and analyses are of fundamental importance for investigating biological mechanisms in cells and bio-molecules. The controlled application of forces to activate specific bio-pathways and investigate their effects, mimicking the role of the cellular environment, is becoming a prominent approach in this field. In this work, we present a non-invasive magnetic on-chip platform which allows for the manipulation of magnetic particles, through micrometric magnetic conduits of Permalloy patterned on-chip. We show, from simulations and experiments, that this technology permits to exert a finely controlled force on magnetic beads along the chip surface. This force can be tuned from few to hundreds pN by applying a variable external magnetic field

Highly Sensitive Magnetic Array-based Platform for Neuronal Signal Recording

This work presents a platform for the detection of the neuronal magnetic signal arising from the propagation of the action potential along the axon, via an array of highly sensitive magnetoresistive sensors and a low noise front-end electronic setup. We report the results of calculations and experiments for estimating the limit of detection of such platform in terms of minimum detectable magnetic field. Furthermore, an experimental setup for recording the magnetic signal in a brain slice is presented. (C) 2017 The Authors. Published by Elsevier Ltd

Electrode-dependent asymmetric conduction mechanisms in K0.5Na0.5NbO3 micro-capacitors

The ultimate performance of devices employing lead-free piezoelectrics is determined not only by the intrinsic properties of the piezo, but also by processes and materials employed to create the electric contacts. In this paper, we investigate the impact of different metallic electrodes with increasing chemical reactivity (Pt, Ni, Ti, Cr), on the asymmetric behavior of the leakage current in M/K0.5Na0.5NbO3/Pt(111) micro-capacitors, where M stands for the top metallic electrode. For all electrodes we found a marked leakage asymmetry that we ascribed to the presence of a Schottky-like rectifying junction at the M/K0.5Na0.5NbO3/Pt(111) bottom interface, while the corresponding junction at the top interface is deeply affected by the creation of oxygen vacancies due to oxygen scavenging during the growth of the top metallic electrodes, leading to an almost ohmic top contact. The leakage increases with the reactivity of the electrodes, while the asymmetry decreases, thus suggesting that the creation of the top metal/K0.5Na0.5NbO3 interface generates oxygen vacancies diffusing down to the bottom interface and impacting on the rectifying behavior of the Schottky-like junction. Noteworthy, this asymmetric conduction can reflect in an asymmetric piezoelectric and ferroelectric behavior, as a sizable portion of the applied voltage drops across the rectifying junction in reverse bias, thus hampering symmetric bipolar operation, especially in leaky materials

On-chip measurement of the Brownian relaxation frequency of magnetic beads using magnetic tunneling junctions

We demonstrate the detection of the Brownian relaxation frequency of 250 nm diameter magnetic beads using a lab-on-chip platform based on current lines for exciting the beads with alternating magnetic fields and highly sensitive magnetic tunnel junction MTJ sensors with a superparamagnetic free layer. The first harmonic out-of-phase component of the MTJ response gives the imaginary part of the magnetic bead susceptibility, which peaks at the Brownian relaxation frequency. This work paves the way to on-chip implementation of Brownian magnetorelaxometry in innovative “lab-on-a-bead” assays for biomolecular recognition

Electric control of magnetism at the Fe/BaTiO3 interface

Interfacial magnetoelectric coupling is a viable path to achieve electrical writing of magnetic information in spintronic devices. For the prototypical Fe/BaTiO3 system, only tiny changes of the interfacial Fe magnetic moment upon reversal of the BaTiO3 dielectric polarization have been predicted so far. Here, by using X-ray magnetic circular dichroism in combination with high-resolution electron microscopy and first principles calculations, we report on an undisclosed physical mechanism for interfacial magnetoelectric coupling in the Fe/BaTiO3 system. At this interface, an ultrathin oxidized iron layer exists, whose magnetization can be electrically and reversibly switched on and off at room temperature by reversing the BaTiO3 polarization. The suppression/recovery of interfacial ferromagnetism results from the asymmetric effect that ionic displacements in BaTiO3 produces on the exchange coupling constants in the interfacial-oxidized Fe layer. The observed giant magnetoelectric response holds potential for optimizing interfacial magnetoelectric coupling in view of efficient, low-power spintronic devices

Towards a magnetoresistive platform for neural signal recording

A promising strategy to get deeper insight on brain functionalities relies on the investigation of neural activities at the cellular and sub-cellular level. In this framework, methods for recording neuron electrical activity have gained interest over the years. Main technological challenges are associated to finding highly sensitive detection schemes, providing considerable spatial and temporal resolution. Moreover, the possibility to perform non-invasive assays would constitute a noteworthy benefit. In this work, we present a magnetoresistive platform for the detection of the action potential propagation in neural cells. Such platform allows, in perspective, the in vitro recording of neural signals arising from single neurons, neural networks and brain slices