Interaction between superconducting and ferromagnetic order parameters in graphite-sulfur composites

Abstract

The superconductivity of graphite-sulfur composites is highly anisotropic and associated with the graphite planes. The superconducting state coexists with the ferromagnetism of pure graphite, and a continuous crossover from superconducting to ferromagnetic-like behavior could be achieved by increasing the magnetic field or the temperature. The angular dependence of the magnetic moment m(α) provides evidence for an interaction between the ferromagnetic and the superconducting order parameters. even at room temperature [18]. The occurrence of high temperature ferromagnetism has been reported for extraterrestrial graphite [9], proton-irradiated graphite [16], and various carbon-based materials consisting of curved graphite-like sheets [5,7,12,15]. Whereas there is no general consent regarding its origin [1,2,6,13,14,17], the so far accumulated experimental evidences indicate that a structural disorder, topological defects, as well as adsorbed foreign atoms can be responsible for the occurrence of both ferromagnetic and superconducting patches in graphitic structures. In particular, it has been demonstrated that sulfur adsorption induces SC [3,4,10] in graphite within some sort of "grains" or domains [3,10]. It has been also demonstrated that the SC in graphite-sulfur (C-S) composites is highly anisotropic and associated with the graphite planes [10]. The SC domains coexist with the FM of pure graphite, and a continuous crossover from SC to FM-like behavior could be achieved by increasing either the applied magnetic field H or the temperature T [10]. An interplay between SC and FM order parameters has been theoretically analyzed for both graphite [1] and C-S composites [11]. However, little is known on this issue from the experimental side.Here we focus our attention on the highly anisotropic nature of the SC state of the C-S composites, which was explored by means of the angular dependence of the sample magnetic moment m(α, T, H), where α is the angle between the applied magnetic field H and the largest sample surface. The main conclusion of this work is that SC and FM order The graphite-sulfur sample studied in this work was thoroughly characterized in Ref. [10]. In summary, the C-S sample was prepared using graphite rods from Carbon of America Ultra Carbon, AGKSP grade, ultra "F" purity (99.9995%) (Alfa-Aesar, # 40766) and sulfur chunks from American Smelting and Refining Co. that are spectrographically pure (99.999+ %). A pressed pellet (φ = 6 mm, ~7000 lbf) of graphite was prepared by pressing graphite powder that was produced by cutting and grinding the graphite rod on the edge and side area of a new and clean circular diamond saw blade. The graphite pellet was encapsulated with sulfur chunks (mass ratio ~ 1:1) in a quartz tube under 1/2 atmosphere of argon and heat treated in a tube furnace at 400 °C for one hour and then slowly cooled (4 °C/h) to room temperature. Following this recipe a reasonable reproducibility (~75 %) was reached. X-ray diffraction measurements (θ-2θ geometry and rocking curves) ...

Resonating valence bonds and mean-fieldd-wave superconductivity in graphite

“…For example, traces of superconducting behavior have recently been reported in both graphite 2,3 and graphite-sulfur composites. [3][4][5] Highly oriented pyrolithic graphite ͑HOPG͒ has been shown to display either a partial superconducting or a ferromagnetic-like response to an applied magnetic field even at room temperature depending on heat treatment and aging. 2,3 For graphite-sulfur composites a partially superconducting state has been demonstrated below a critical temperature as high as T c = 35 K with a continuous crossover to a ferromagnetic-like behavior with increased magnetic field or temperature.…”

“…[3][4][5] X-ray analysis of the graphite-sulfur composites has shown no significant change in either the graphite or the orthorhombic sulfur structure and, with the active superconducting volume estimated to be only 0.05%, the superconductivity can be understood in the form of superconducting islands at the graphite-sulfur interfaces. [3][4][5] Large observed magnetic field anisotropy strongly suggests that the superconducting correlations are localized to the graphite planes in both graphite 3 and graphite-sulfur composites. 4 In addition to reports of traces of superconductivity, both a bulk magnetic-field-driven metal-insulator transition ͑MIT͒ and a reentrant insulator-metal transition ͑IMT͒ have been measured in graphite at low fields.…”

Giant magnetic moment enhancement of nickel nanoparticles embedded in multiwalled carbon nanotubes

“…The integer quantum Hall effect recently observed in graphene is closely related to these Dirac fermions (1,2,3). In addition to the remarkable room temperature quantum Hall effect in graphene (3), there are intriguing reports of high-temperature superconducting behaviors in both graphite (4) and graphitesulfur composites (5,6). Highly oriented pyrolithic graphite (HOPG) has been shown to display either a partial superconducting or a ferromagnetic-like response to an applied magnetic field even at temperatures above room temperature (4).…”

Suppression of the superconducting transition temperature of doped graphene due to thermal fluctuations of the order parameter

“…Recently, some evidences of SC in graphite and graphite-sulfur composites were found experimentally. [2][3][4][5] The possibility of SC in graphene was studied theoretically in Refs. 6-13. In particular, the quantum critical-point scenario of SC in undoped graphene, when the Fermi surface consists of Dirac points, was analyzed in Refs.…”