Granular superconductivity below 5 K in SPI-II pyrolytic graphite

Ana Ballestar, Pablo Esquinazi, and Winfried Böhlmann

Phys. Rev. B 91, 014502 – Published 7 January 2015

Abstract

We have studied the transport properties of transmission electron microscope (TEM) lamellae obtained from a pyrolytic graphite sample of grade B (SPI-II) with electrical contacts at the edges of the graphene layers. The temperature, magnetic field, input current dependence of the resistance as well as the current-voltage characteristic curves are compatible with the existence of granular superconductivity below 5 K. TEM pictures of the studied lamellae reveal clear differences of the embedded interfaces to those existing in more ordered pyrolytic samples, which appear to be the origin for the relatively lower temperatures at which the granular superconductivity is observed.

Received 20 October 2014
Revised 12 December 2014

DOI:https://doi.org/10.1103/PhysRevB.91.014502

Authors & Affiliations

Ana Ballestar, Pablo Esquinazi^*, and Winfried Böhlmann

Division of Superconductivity and Magnetism, Institut für Experimentelle Physik II, Universität Leipzig, Linnéstraße 5, D-04103 Leipzig, Germany

^*esquin@physik.uni-leipzig.de

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Vol. 91, Iss. 1 — 1 January 2015

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Images

Figure 1
Resistance vs temperature of the TEM lamella at a fixed current of $1 μ A$ . The continuous line follows a simple semiconducting-like exponential function, i.e., $R (T) = 238 [Ω] exp (E_{g} / 2 T)$ , where $E_{g} = 320$ K and $T$ is the temperature. The inset shows a TEM picture of the measured lamella. The scale bar at the bottom right represents $1 μ m$ . The crystalline domains and the borders between them, can be recognized in this TEM picture by the different gray colors. Note that the electron beam is applied parallel to the graphite sheets. The interfaces between regions with a twist angle $θ_{twist} > 0$ are located at the borders of regions with uniform gray color. Note that no sharp 2D interfaces can be recognized in this TEM picture, in clear contrast to similar TEM measurements in highly oriented graphite samples [5, 33].
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Figure 2
Resistance vs temperature of the TEM lamella at different input currents. For clarity we only show the data below 30 K, the temperature region where the non-Ohmic behavior is clearly observed.
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Figure 3
(a) Resistance vs temperature (semilogarithmic scale) at different applied magnetic fields [0 T bottom curve to 8 T upper curve (open squares symbols)] and at an input current of 5 nA. (b) Data obtained from the magnetoresistance measured at 5 nA input current and at different temperatures (not shown here). The $x$ axis is calculated as the absolute field difference $| B - B_{c} |$ multiplied by the corresponding temperature elevated to the exponent $- 1 / α$ . The curves given by the symbols are obtained at different fixed temperatures and a critical field $B_{c} = 1$ T and $α = 0.7$ . The continuous (red) line at 2.5 K was obtained decreasing the critical field to $B_{c} = 0.95$ T. The normalization factor in the $y$ axis is $R (| B - B_{c} | = 0) = 100, 300 Ω$ at all temperatures.
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Figure 4
Current-voltage characteristic curves obtained for the TEM lamella at different applied fields at 2 K. The inset shows the curve at zero field with a fit (continuous line) to the differential equation of Ref. [48], using as a free parameter the Josephson critical current $I_{c} = 82$ nA.
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Figure 5
(a) Differential conductivity measured at temperatures between 2 and 3.8 K, i.e., in the temperature range where a finite coupling between the superconducting regions exists. (b) Differential conductivity at 2 K measured at different magnetic fields. The zero field data are the same as in (a) after an average smoothing to remove the noise at low voltages. Above 1 T the nonlinear effect associated with superconducting behavior clearly vanishes.
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