73 research outputs found
Elliptic Annular Josephson Tunnel Junctions in an external magnetic field: The statics
We have investigated the static properties of one-dimensional planar
Josephson tunnel junctions in the most general case of elliptic annuli. We have
analyzed the dependence of the critical current in the presence of an external
magnetic field applied either in the junction plane or in the perpendicular
direction. We report a detailed study of both short and long elliptic annular
junctions having different eccentricities. For junctions having a normalized
perimeter less than one the threshold curves are derived and computed even in
the case with one trapped Josephson vortex. For longer junctions a numerical
analysis is carried out after the derivation of the appropriate Perturbed
sine-Gordon Equation. For a given applied field we find that a number of
different phase profiles exist which differ according to the number of
fluxon-antifluxon pairs. We demonstrate that in samples made by specularly
symmetric electrodes a transverse magnetic field is equivalent to an in-plane
field applied in the direction of the current flow. Varying the ellipse
eccentricity we reproduce all known results for linear and ring-shaped
Josephson tunnel junctions. Experimental data on high-quality Nb/Al-AlOx/Nb
elliptic annular junctions support the theoretical analysis provided self-field
effects are taken into account.Comment: 30 pages, 13 figure
Brain Networks and Cognitive Impairment in Parkinson's Disease
: Aim: The aim of the present study is to investigate the relationship between both functional connectivity and brain networks with cognitive decline, in patients with Parkinson's disease (PD). Introduction: PD phenotype is not limited to motor impairment but, rather, a wide range of non-motor disturbances can occur, with cognitive impairment being one of the most common. However, how the large-scale organization of brain activity differs in cognitively impaired patients, as opposed to cognitively preserved ones, remains poorly understood. Methods: Starting from source-reconstructed resting-state magnetoencephalography data, we applied the phase linearity measurement (PLM) to estimate functional connectivity, globally and between brain areas, in PD patients with and without cognitive impairment (respectively PD-CI and PD-NC), as compared with healthy subjects (HS). Further, using graph analysis, we characterized the alterations in brain network topology and related these, as well as the functional connectivity, to cognitive performance. Results: We found reduced global and nodal PLM in several temporal (fusiform gyrus, Heschl's gyrus, and inferior temporal gyrus), parietal (postcentral gyrus), and occipital (lingual gyrus) areas within the left hemisphere, in the gamma band, in PD-CI patients, as compared with PD-NC and HS. With regard to the global topological features, PD-CI patients, as compared with HS and PD-NC patients, showed differences in multi-frequencies bands (delta, alpha, gamma) in the Leaf fraction, Tree hierarchy (Th) (both higher in PD-CI), and Diameter (lower in PD-CI). Finally, we found statistically significant correlations between the Montreal Cognitive Assessment test and both the Diameter in delta band and the Th in the alpha band. Conclusion: Our work points to specific large-scale rearrangements that occur selectively in cognitively compromised PD patients and are correlated to cognitive impairment. Impact statement In this article, we want to test the hypothesis that the cognitive decline observed in Parkinson's disease (PD) patients may be related to specific changes of both functional connectivity and brain network topology. Specifically, starting from magnetoencephalography signals and by applying the phase linearity measurement (PLM), a connectivity metric that measures the synchronization between brain regions, we were able to highlight differences in the global and nodal PLM values in PD patients with cognitive impairment as compared with both cognitively unimpaired patients and healthy subjects. Further, using graph analysis, we analyzed alterations in brain network topology that were related to cognitive functioning
The progressive loss of brain network fingerprints in Amyotrophic Lateral Sclerosis predicts clinical impairment
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterised by functional connectivity alterations in both motor and extra-motor brain regions. Within the framework of network analysis, fingerprinting
represents a reliable approach to assess subject-specific connectivity features within a given population (healthy
or diseased). Here, we applied the Clinical Connectome Fingerprint (CCF) analysis to source-reconstructed
magnetoencephalography (MEG) signals in a cohort of seventy-eight subjects: thirty-nine ALS patients and
thirty-nine healthy controls. We set out to develop an identifiability matrix to assess the extent to which each
patient was recognisable based on his/her connectome, as compared to healthy controls. The analysis was
performed in the five canonical frequency bands. Then, we built a multilinear regression model to test the ability
of the âclinical fingerprintâ to predict the clinical evolution of the disease, as assessed by the Amyotrophic Lateral
Sclerosis Functional Rating Scale-Revised (ALSFRS-r), the Kingâs disease staging system, and the Milano-Torino
Staging (MiToS) disease staging system. We found a drop in the identifiability of patients in the alpha band
compared to the healthy controls. Furthermore, the âclinical fingerprintâ was predictive of the ALSFRS-r (p =
0.0397; ÎČ = 32.8), the Kingâs (p = 0.0001; ÎČ = â 7.40), and the MiToS (p = 0.0025; ÎČ = â 4.9) scores.
Accordingly, it negatively correlated with the Kingâs (Spearmanâs rho = -0.6041, p = 0.0003) and MiToS scales
(Spearmanâs rho = â 0.4953, p = 0.0040). Our results demonstrated the ability of the CCF approach to predict
the individual motor impairment in patients affected by ALS. Given the subject-specificity of our approach, we
hope to further exploit it to improve disease management
A night of sleep deprivation alters brain connectivity and affects specific executive functions
: Sleep is a fundamental physiological process necessary for efficient cognitive functioning especially in relation to memory consolidation and executive functions, such as attentional and switching abilities. The lack of sleep strongly alters the connectivity of some resting-state networks, such as default mode network and attentional network. In this study, by means of magnetoencephalography (MEG) and specific cognitive tasks, we investigated how brain topology and cognitive functioning are affected by 24 h of sleep deprivation (SD). Thirty-two young men underwent resting-state MEG recording and evaluated in letter cancellation task (LCT) and task switching (TS) before and after SD. Results showed a worsening in the accuracy and speed of execution in the LCT and a reduction of reaction times in the TS, evidencing thus a worsening of attentional but not of switching abilities. Moreover, we observed that 24 h of SD induced large-scale rearrangements in the functional network. These findings evidence that 24 h of SD is able to alter brain connectivity and selectively affects cognitive domains which are under the control of different brain networks
Imaging spontaneous currents in superconducting arrays of pi-junctions
Superconductors separated by a thin tunneling barrier exhibit the Josephson
effect that allows charge transport at zero voltage, typically with no phase
shift between the superconductors in the lowest energy state. Recently,
Josephson junctions with ground state phase shifts of pi proposed by theory
three decades ago have been demonstrated. In superconducting loops,
pi-junctions cause spontaneous circulation of persistent currents in zero
magnetic field, analogous to spin-1/2 systems. Here we image the spontaneous
zero-field currents in superconducting networks of temperature-controlled
pi-junctions with weakly ferromagnetic barriers using a scanning SQUID
microscope. We find an onset of spontaneous supercurrents at the 0-pi
transition temperature of the junctions Tpi = 3 K. We image the currents in
non-uniformly frustrated arrays consisting of cells with even and odd numbers
of pi-junctions. Such arrays are attractive model systems for studying the
exotic phases of the 2D XY-model and achieving scalable adiabatic quantum
computers.Comment: Pre-referee version. Accepted to Nature Physic
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