U. Patel

Northern Illinois University, DeKalb, Illinois, United States

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Publications (19)35.78 Total impact

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    ABSTRACT: We have developed methods to disperse and partially size separate NbSe(3) nanowires in aqueous surfactant solutions. These dispersions can easily be formed into thin films. Optical and electrical studies show these films to display sheet resistances and transmittances ranging from (460 Ω/□, 22%) to (12 kΩ/□, 79%) depending on thickness. For thicker films, we measured the transparent conducting figure of merit to be σ(DC, B)/σ(Op) = 0.32, similar to graphene networks. Thickness measurements gave individual values of σ(Op) = 17,800 S m(-1) and σ(DC, B) = 5700 S m(-1). Films thinner than ∼ 70 nm displayed reduced DC conductivity due to percolative effects.
    Nanotechnology 07/2011; 22(28):285202. · 3.84 Impact Factor
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    ABSTRACT: In this report, we demonstrate the feasibility of applying a 250-nm focused x-ray beam to study a single crystalline NbSe(3) nanobelt under high-pressure conditions in a diamond anvil cell. With such a small probe, we not only resolved the distribution and morphology of each individual nanobelt in the x-ray fluorescence maps but also obtained the diffraction patterns from individual crystalline nanobelts with thicknesses of less than 50 nm. Single crystalline diffraction measurements on NbSe(3) nanobelts were performed at pressures up to 20 GPa.
    The Review of scientific instruments 04/2011; 82(4):043903. · 1.52 Impact Factor
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    ABSTRACT: We report on magnetoresistance oscillations in superconducting NbNx nanowires synthesized through ammonia gas annealing of NbSe3 precursor nanostructures. Even though the transverse dimensions of the nanowires are much larger than the superconducting coherence length, the voltage-current characteristics of these nanowires at low temperatures are reminiscent of one-dimensional superconductors where quantum phase slips are associated with the origin of dissipation. We show that both the magnetoresistance oscillations and voltage-current characteristics observed in this work result from the granular structure of our nanowires. Comment: Published in Phys. Rev. B
    Physical review. B, Condensed matter 07/2009; · 3.77 Impact Factor
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    ABSTRACT: The in-plane thermal conductivity $\kappa$ of the iron selenide superconductor FeSe$_x$ ($T_c$ = 8.8 K) were measured down to 120 mK and up to 14.5 T ($\simeq 3/4 H_{c2}$). In zero field, the residual linear term $\kappa_0/T$ at $ T \to 0$ is only about 16 $\mu$W K$^{-2}$ cm$^{-1}$, less than 4% of its normal state value. Such a small $\kappa_0/T$ does not support the existence of nodes in the superconducting gap. More importantly, the field dependence of $\kappa_0/T$ in FeSe$_x$ is very similar to that in NbSe$_2$, a typical multi-gap s-wave superconductor. We consider our data as strong evidence for multi-gap nodeless superconductivity in FeSe$_x$. This kind of superconducting gap structure may be generic for all Fe-based superconductors. Comment: 5 pages, 4 figures
    06/2009;
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    ABSTRACT: We investigated confinement effects on the resistive anisotropy of a superconducting niobium strip with a rectangular cross-section. When the strip's transverse dimensions are comparable to the superconducting coherence length, we find the angle dependent magentoresistances at a fixed temperature can be scaled as R(theta, H) = R(H /Hctheta) where Hctheta = Hc0 (cos^2theta+gamma-2sin^2theta)-1/2 is the angular dependent critical field, gamma = w/d is the width to thickness ratio of the strip, and Hc0 is the out-plane critical field at theta = 0 . Our results can be understood in terms of the anisotropic diamagnetic energy of a one-dimensional superconductor in a magnetic field.
    03/2009;
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    ABSTRACT: BSCCO (2212) whiskers were fabricated via a melt-quench-growth method and their morphology was characterized with scanning/transmission electron microscopy and atomic force microscopy. Four-probe magneto-transport measurements were conducted as a function of temperature and current. In low magnetic fields and currents, the resistance decreases monotonically and vanishes at a temperature of ˜ 80 K. However, at large currents and magnetic fields the resistance shows a non-monotonic dependence on temperature, even showing values that are higher than the normal state resistance for certain ranges of the parameters We attribute the observed behavior to the brick-wall morphology of the whiskers leading to a competition between normal and superconductive tunnelings that is known to take place in granular superconductive systems.
    03/2009;
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    ABSTRACT: Iron selenide (FeSex) crystals with lateral dimensions up to millimeters were grown via a vapor self-transport method. The crystals consist of the dominant α-phase with trace amounts of β-phase as identified by powder x-ray diffraction. With four-probe resistance measurements, we obtained a zero resistance critical temperature of 7.5 K and a superconducting onset transition temperature of up to 11.8 K in zero magnetic field as well as an anisotropy of 1.5±0.1 for the critical field. Magnetization measurements on individual crystals reveal the coexistence of superconductivity and ferromagnetism.
    Applied Physics Letters 02/2009; 94(8):082508-082508-3. · 3.79 Impact Factor
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    ABSTRACT: The in-plane thermal conductivity κ of the iron selenide superconductor FeSex (Tc=8.8 K) was measured down to 120 mK and up to 14.5 T (≃3/4Hc2). In zero field, the residual linear term κ0/T at T→0 is only about 16 μW K−2 cm−1, less than 4% of its normal-state value. Such a small κ0/T does not support the existence of nodes in the superconducting gap. More importantly, the field dependence of κ0/T in FeSex is very similar to that in NbSe2, a typical multigap s-wave superconductor. We consider our data as strong evidence for multigap nodeless (at least in ab plane) superconductivity in FeSex. This kind of superconducting gap structure may be generic for all Fe-based superconductors.
    Physical review. B, Condensed matter 01/2009; 80(2). · 3.77 Impact Factor
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    ABSTRACT: We investigated confinement effects on the resistive anisotropy of a superconducting niobium strip with a rectangular cross section. When its transverse dimensions are comparable to the superconducting coherence length, the angle dependent magnetoresistances at a fixed temperature can be scaled as R(theta,H) = R(H/Hctheta) where Hctheta =Hc0(cos2theta + gamma(-2)sin2theta)(-1/2) is the angular dependent critical field, gamma is the width to thickness ratio, and Hc0 is the critical field in the thickness direction at theta=0 degrees . The results can be understood in terms of the anisotropic diamagnetic energy for a given field in a one-dimensional superconductor.
    Physical Review Letters 09/2008; 101(7):077003. · 7.73 Impact Factor
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    ABSTRACT: By utilizing focused ion beam (FIB) patterning technique we were able to fabricate hole-arrays with interhole spacing down to 150 nm into superconducting niobium (Nb) films. This enabled us to have a large temperature range to explore the properties of Nb wire networks in which the superconducting strips between neighboring holes are comparable to the superconducting coherence length. We studied the anisotropy of these superconducting networks by measuring the critical temperatures and magnetoresistances at various magnetic field directions respect to the film surface. The effect of film thickness, hole diameter, interhole-spacing and the symmetry of the hole lattice on the anisotropy will be reported.
    03/2008;
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    ABSTRACT: Superconducting niobium nitride wires and ribbons with transverse dimensions down to tens of nanometers were synthesized by annealing NbSe3 nanostructure precursors in flowing ammonia gas at temperatures up to 1000 °C. Their critical temperatures increase with increasing annealing temperatures and reach 9–11.2 K when sintered at 950 °C or above. X-ray diffraction analyses identified Nb4N5 and Nb5N6 phases, dominating at annealing temperatures below and above 950 °C, respectively. Transport measurements show magnetoresistance oscillations at temperatures near the superconducting transition due to vortex-row confinement effects and voltage jumps in current-voltage characteristics at low temperatures attributed to hot-spot behavior.
    Applied Physics Letters 10/2007; 91(16):162508-162508-3. · 3.79 Impact Factor
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    ABSTRACT: We investigate the origin of the matching effect observed in superconducting Nb films containing regular arrays of holes near the zero-field critical temperature. We find “dips” in the resistance vs magnetic field curves at matching fields where the magnitude of the magnetic flux threading each unit cell is an integer number of the flux quantum. By comparing the magnetic field dependences of the resistance and critical temperature in perpendicular and parallel magnetic field directions, we find that the matching effect in Nb films containing triangular hole arrays originates from hole-induced suppression of the critical temperature rather than the widely assumed flux pinning enhancement.
    Physical review. B, Condensed matter 07/2007; 76(2). · 3.77 Impact Factor
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    ABSTRACT: The role of one-dimensional nanostructures has recently gained wide importance due to their novel properties and potential applications in electronics. Here, we report a two-step approach to synthesize one-dimensional superconducting NbN nanowires and nanoribbons by converting NbSe3 nanostructures. First, NbSe3 nanostructure precursors were prepared by sintering niobium and selenium powders in an evacuated quartz tube. Subsequently, these NbSe3 nanostructures were transformed into NbN under an atmosphere of ammonia gas at a reaction temperature of up to 1000 C. Superconducting transition temperatures up to 10 K were obtained from both magnetization and four-probe transport measurements. We also carried out morphology and structural characterizations of these NbN nanostructures. This material is based upon work supported by the US Department of Energy, under Award Numbers DE-FG02-06ER46334 and DE-AC02-06CH11357 .
    03/2007;
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    ABSTRACT: The resistance and critical current of a superconducting film containing a regular array of microscale or nanoscale holes are found to oscillate with magnetic field. These oscillations are generally interpreted as commensurate pinning. On the other hand, oscillations of the critical temperature of such films with magnetic field due to the Little-Parks effect can also induce oscillations of the resistance and critical current. We present a new method to separate the contributions of commensurate pinning from the Little-Parks effect in superconducting niobium films containing triangular lattices of holes with diameter of tens of nanometers. Remarkably, we find the oscillations of the resistance and critical current with respect to field and temperature to originate purely from the Little-Parks effect. Details on sample preparation, morphological characterization and transport measurements of the critical temperature, the resistance and critical current will be reported.
    03/2006;
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    ABSTRACT: Anomalous resistive peaks with values larger than the normal-state resistance has been observed in the resistance versus temperature and magnetic field curves of superconducting nanostructures of Al nanowires, nanoloops and nanodiscs. The excess resistance in these systems is believed to originate at the normal-superconducting (N-S) interface and induced by dynamic phase slip centers or by different critical temperatures (Tc) in the neighboring parts. Here we report on a new type of resistance anomaly which occurs only at high driving currents in disordered superconducting films. The samples were formed by sputtering niobium onto substrates containing arrays of irregularly distributed nanoscale holes. Resistance peaks appear as a function of temperature or magnetic field in the superconducting transition, with peak values up to 2% above the typical normal-state value. We attribute the observed resistance peak effect to dissipation-induced granularity which enhances contributions from the fluctuation-induced decrease of the quasiparticle density of states. The granularity of the superconducting film arises from inhomogeneous heat-transfer due to the size variation of the Nb sections between the nanoholes.
    03/2006;
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    ABSTRACT: Novel properties such as an oscillating phase boundary in the magnetic field -- temperature (H-T) diagram have been reported in superconducting films containing a regular hole array where the interhole separation is comparable to the superconducting coherence length -- the superconducting networks. Aluminum is the most common material used in fabricating such superconducting networks due to its large coherence length. However, they require low He^3 refrigeration temperatures due to aluminum's low critical temperature of about 1 K. Here, we report on the fabrication of Nb superconducting networks with interhole separation down to tens of nanometers and critical temperatures up to 9K. In addition, we present results on angular dependence of the critical temperature and magnetoresistance of the superconducting networks.
    01/2006;
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    ABSTRACT: We describe the synthesis of superconducting NbSe2 nanowires and nanoribbons by the nondestructive removal of Se from one-dimensional NbSe3 nanostructure precursors. We report scanning electron microscopy imaging, x-ray diffraction, and transmission electron microscopy analyses of the morphology, composition, and crystallinity of the converted NbSe2 nanostructures. Transport measurements on individual nanowires/ribbons confirm their superconductivity with Tc ∼ 7.2 K, and the appearance of current-induced resistance steps is attributed to localized phase slip centers, akin to those reported in other superconducting nanostructures.
    Applied Physics Letters 09/2005; 87(14):142506-142506-3. · 3.79 Impact Factor
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    ABSTRACT: The synthesis of nanoscale superconductors with controlled geometries is extremely challenging. In this paper we present results on synthesis and characterization of one-dimensional (1D) NbSe2 superconducting nanowires/nanoribbons. Our synthesis approach includes the synthesis of 1D NbSe3 nanostructure precursors followed by nondestructive and controlled adjustment of the Se composition to formulate NbSe2. The morphology, composition and crystallinity of the synthesized 1D NbSe2 nanostructures were analyzed with scanning electron microscopy, x-ray diffraction and transmission electron microscopy. Transport measurements were carried out to explore the electronic properties of these confined superconducting nanostructures.
    MRS Proceedings. 12/2004; 887.
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    ABSTRACT: Magnetoresistances and critical currents of superconducting NbSe2 and NbN nanowires with cross-section dimensions from 300 nm to 2 um were studied as a function of magnetic field. Sample specific oscillations were found with respect to applied magnetic field. The oscillations were completely reproducible, symmetric with respect to the direction of the field and independent of the field sweep direction. They disappeared at high fields, temperatures or currents. The oscillations were periodic in nature with superposition of more than one frequency as revealed in Fast Fourier Transform of the oscillations. The frequencies of oscillations were independent of temperature and current. Details about the transport measurements of resistance and critical current and origin of such oscillations will be discussed.