We probe the presence of long-range correlations in phase fluctuations by analyzing the higherorder spectrum of resistance fluctuations in ultra-thin NbN superconducting films. The nonGaussian component of resistance fluctuations is found to be sensitive to film thickness close to the transition, which allows us to distinguish between mean field and Berezinskii-Kosterlitz-Thouless (BKT) type superconducting transitions. The extent of non-Gaussianity was found to be bounded by the BKT and mean field transition temperatures and depend strongly on the roughness and structural inhomogeneity of the superconducting films. Our experiment outlines a novel fluctuation-based kinetic probe in detecting the nature of superconductivity in disordered low-dimensional materials.The transition from superconducting to normal state in two-dimensions is known to occur via BerezinskiiKosterlitz-Thouless (BKT) mechanism. Traditionally, the signature of BKT transition is found by measuring the superfluid density where a discontinuity is observed at the critical temperature T BKT [1][2][3]. This method is limited in its scope of application for systems exhibiting interfacial/buried superconductivity like oxide heterostructures due to the inability to measure the thickness of the superconducting layer accurately. Transport-based probes including discontinuity in the power law behavior of I-V characteristics or change in the curvature of magnetoresistance from convex to concave in the presence of a perpendicular magnetic field [4][5][6][7][8][9] have their limitations as real systems contain some degree of inhomogeneity which smear out the signatures of BKT type behavior. With growing interest in lowdimensional superconductivity, it is interesting to develop new probes/techniques that are not only sensitive enough to detect BKT transition but also compare its characteristics scales with the mean field description.The BKT transition, as exhibited by ultra-thin superconducting films, is characterized by the unbinding of vortex pairs beyond T BKT . It is well known that these vortices exhibit long-range interactions which vary logarithmically with the distance between the vortices. Vortices also occur in bulk Type-II superconductors in the presence of an external magnetic field. Earlier experiments probing vortices in bulk films through measurements of flux flow noise and voltage noise have reported the presence of broad band noise (BBN) [10,11] as a function of driving current and magnetic field. The statistics of noise in these systems is non-Gaussian due to the fact that most of the noise arise from very few fluctuators. Noise measurements on quasi-2D MgB 2 films show a possibility of thermally induced vortex hopping through measurements of flux noise close to BKT transition [12]. To the best of our knowledge there are no reports on higher-order statistics of fluctuations looking into the possibility of interacting vortices in two-dimensions.Higher-order fluctuations in resistivity has been established as an useful tool to study the pres...

In this letter we present the results of systematic experimental investigations of the effect of different chemical environments on the low frequency resistance fluctuations of single layer graphene field effect transistors (SLG-FET). The shape of the power spectral density of noise was found to be determined by the energetics of the adsorption-desorption of molecules from the graphene surface making it the dominant source of noise in these devices. We also demonstrate a method of quantitatively determining the adsorption energies of chemicals on graphene surface based on noise measurements. We find that the magnitude of noise is extremely sensitive to the nature and amount of the chemical species present. We propose that a chemical sensor based on the measurement of low frequency resistance fluctuations of single layer graphene field effect transistor devices will have extremely high sensitivity, very high specificity, high fidelity and fast response times

In quantum systems, signatures of multifractality are rare. They have been found only in the multiscaling of eigenfunctions at critical points. Here we demonstrate multifractality in the magnetic-field-induced universal conductance fluctuations of the conductance in a quantum condensed-matter system, namely, high-mobility single-layer graphene field-effect transistors. This multifractality decreases as the temperature increases or as doping moves the system away from the Dirac point. Our measurements and analysis present evidence for an incipient Anderson-localization near the Dirac point as the most plausible cause for this multifractality. Our experiments suggest that multifractality in the scaling behaviour of local eigenfunctions are reflected in macroscopic transport coefficients. We conjecture that an incipient Anderson-localization transition may be the origin of this multifractality. It is possible that multifractality is ubiquitous in transport properties of lowdimensional systems. Indeed, our work suggests that we should look for multifractality in transport in other low-dimensional quantum condensed-matter systems. * aveek@iisc.ac.in arXiv:1804.04454v1 [cond-mat.mes-hall]

. (2015) Robust local and nonlocal transport in the topological Kondo insulator SmB6in the presence of a high magnetic field. Physical Review B, 92 (8). 085103. Permanent WRAP URL:http://wrap.warwick.ac.uk/87833 Copyright and reuse:The Warwick Research Archive Portal (WRAP) makes this work by researchers of the University of Warwick available open access under the following conditions. Copyright © and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable the material made available in WRAP has been checked for eligibility before being made available.Copies of full items can be used for personal research or study, educational, or not-for-profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. Publisher statement: © 2015 American Physical Society A note on versions:The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher's version. Please see the 'permanent WRAP URL' above for details on accessing the published version and note that access may require a subscription.For more information, please contact the WRAP Team at: wrap@warwick.ac.uk Robust local and non-local transport in the Topological Kondo Insulator SmB 6 in the presence of high magnetic field SmB6 has been predicted to be a Kondo Topological Insulator with topologically protected conducting surface states. We have studied quantitatively the electrical transport through surface states in high quality single crystals of SmB6. We observe a large non-local surface signal at temperatures lower than the bulk Kondo gap scale. Measurements and finite element simulations allow us to distinguish unambiguously between the contributions from different transport channels. In contrast to general expectations, the electrical transport properties of the surface channels was found to be insensitive to high magnetic fields. Local and non-local magnetoresistance measurements allowed us to identify definite signatures of helical spin states and strong inter-band scattering at the surface.

Sensing of mechanical stimuli forms an important communication pathway between humans/environment and machines. The progress in such sensing technology has possible impacts on the functioning of automated systems, human machine interfacing, health-care monitoring, prosthetics and safety systems. The challenges in this field range from attaining high sensitivity to extreme robustness. In this article, sensing of complex mechanical stimuli with a patch of taped crumpled reduced graphene oxide (rGO) has been reported which can typically be assembled under household conditions. The ability of this sensor to detect a wide variety of pressures and strains in conventional day-to-day applications has been demonstrated. An extremely high gauge factor (∼10) at ultralow strains (∼10) with fast response times (<20.4 ms) could be achieved with such sensors. Pressure resulting from a gentle touch to over human body weight could be sensed successfully. The capability of the sensor to respond in a variety of environments could be exploited in the detection of water and air pressures both below and above atmospheric, with a single device.

One of the most interesting predicted applications of graphene monolayer based devices is as high quality sensors. In this letter we show, through systematic experiments, a chemical vapor sensor based on the measurement of low frequency resistance fluctuations of single layer graphene field-effect-transistor (SLG-FET) devices. The sensor has extremely high sensitivity, very high specificity, high fidelity and fast response times. The performance of the device using this scheme of measurement (which uses resistance fluctuations as the detection parameter) is more than two orders of magnitude better than a detection scheme where changes in the average value of the resistance is monitored. We propose a number-density fluctuation based model to explain the superior characteristics of noise measurement based detection scheme presented in this article.

Detailed experimental and theoretical studies of the temperature dependence of the effect of different scattering mechanisms on electrical transport properties of graphene devices are presented. We find that for high mobility devices the transport properties are mainly governed by completely screened short range impurity scattering. On the other hand, for the low mobility devices transport properties are determined by both types of scattering potentials - long range due to ionized impurities and short range due to completely screened charged impurities. The results could be explained in the framework of Boltzmann transport equations involving the two independent scattering mechanisms.

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