Evidence for room temperature superconductivity at graphite interfaces

In the last 43 years several hints were reported suggesting the existence of granular superconductivity above room temperature in different graphite-based systems. In this paper some of the results are reviewed, giving special attention to those obtained in water and n-heptane treated graphite powders, commercial and natural bulk graphite samples with different characteristics as well as transmission electron microscope (TEM) lamellae. The overall results indicate that superconducting regions exist and are localized at certain internal interfaces of the graphite structure. The existence of the rhombohedral graphite phase in all samples with superconducting-like properties suggests its interfaces with the Bernal phase as a possible origin for the high-temperature superconductivity, as theoretical calculations predict. High precision electrical resistance and magnetization measurements were used to identify a transition at $T_c \gtrsim 350~$K. To check for the existence of true zero resistance paths in the samples we used local magnetic measurements, which results support the existence of superconducting regions at such high temperatures.Comment: 10 pages, 9 figures, 2nd International Workshop "Towards Room Temperature Superconductivity: Superhydrides and more", Orange California May 2017. To be published in "Quantum Studies: Mathematics and Foundations" (Springer Nature

Identification of a possible superconducting transition above room temperature in natural graphite crystals

Measuring with high precision the electrical resistance of highly ordered natural graphite samples from a Brazil mine, we have identified a transition at $\sim$350~K with $\sim$40~K transition width. The step-like change in temperature of the resistance, its magnetic irreversibility and time dependence after a field change, consistent with trapped flux and flux creep, and the partial magnetic flux expulsion obtained by magnetization measurements, suggest the existence of granular superconductivity below 350~K. The zero-field virgin state can only be reached again after zero field cooling the sample from above the transition. Paradoxically, the extraordinarily high transition temperature we found for this and several other graphite samples is the reason why this transition remained undetected so far. The existence of well ordered rhombohedral graphite phase in all measured samples has been proved by x-rays diffraction measurements, suggesting its interfaces with the Bernal phase as a possible origin for the high-temperature superconductivity, as theoretical studies predicted. The localization of granular superconductivity at these two dimensional interfaces prevents the observation of a zero resistance state or of a full Meissner state.Comment: 14 pages with 21 figure

Coupled magnetic and structural transitions in La0.7Sr0.3MnO3 films on SrTiO3

The magnetic properties of three epitaxial La0.7Sr0.3MnO3 films of thickness 5, 15 and 40 nm grown on SrTiO3 (001) substrates were investigated. The structural transition of the SrTiO3 substrate induces a magnetic transition in the manganite films due to magnetoelastic coupling. Below the temperature of the structural transition additional steps in the magnetization reversal characteristics appear characterized by clearly defined coercive fields. These additional coercive fields depend on the cooling history of the sample and are related to the formation of structural domains in the La0.7Sr0.3MnO3 films induced by the substrate

Weak electron irradiation suppresses the anomalous magnetization of N-doped diamond crystals

Several diamond bulk crystals with a concentration of electrically neutral single substitutional nitrogen atoms of ≲80 ppm, the so-called C or P1 centers, are irradiated with electrons at 10 MeV energy and low fluence. The results show a complete suppression of the irreversible behavior in field and temperature of the magnetization below 30 K, after a decrease in ≲40 ppm in the concentration of C centers produced by the electron irradiation. This result indicates that magnetic C centers are at the origin of the large hysteretic behavior found recently in nitrogen-doped diamond crystals. This is remarkable because of the relatively low density of C centers, stressing the extraordinary role of the C centers in triggering those phenomena in diamond at relatively high temperatures. After annealing the samples at high temperatures in vacuum, the hysteretic behavior is partially recovered

Coupled magnetic and structural transitions in La0.7Sr0.3MnO3 films on SrTiO3

The magnetic properties of three epitaxial La0.7Sr0.3MnO3 films of thickness 5, 15 and 40 nm grown on SrTiO3 (001) substrates were investigated. The structural transition of the SrTiO3 substrate induces a magnetic transition in the manganite films due to magnetoelastic coupling. Below the temperature of the structural transition additional steps in the magnetization reversal characteristics appear characterized by clearly defined coercive fields. These additional coercive fields depend on the cooling history of the sample and are related to the formation of structural domains in the La0.7Sr0.3MnO3 films induced by the substrate

Coupled magnetic and structural transitions in La0.7Sr0.3MnO3 films on SrTiO3

The magnetic properties of three epitaxial La0.7Sr0.3MnO3 films of thickness 5, 15 and 40 nm grown on SrTiO3 (001) substrates were investigated. The structural transition of the SrTiO3 substrate induces a magnetic transition in the manganite films due to magnetoelastic coupling. Below the temperature of the structural transition additional steps in the magnetization reversal characteristics appear characterized by clearly defined coercive fields. These additional coercive fields depend on the cooling history of the sample and are related to the formation of structural domains in the La0.7Sr0.3MnO3 films induced by the substrate

Existence of a magnetically ordered hole gas at the La

The study of spatially confined complex oxides is of wide interest, since correlated electrons at interfaces might form new states of matter. Here La0.7Sr0.3MnO3/SrRuO3 superlattices with coherently grown interfaces and layer thicknesses down to one unit cell were fabricated by pulsed laser deposition. The superlattices were studied by X-ray, HRTEM, magnetization and magnetotransport measurements. For such small thicknesses La0.7Sr0.3MnO3 films are antiferromagnetic and insulating. Despite the small layer thickness, the LaSrMnO layers in the superlattices were ferromagnetic with Curie temperatures close to room temperature. Whereas the resistivity of the superlattices showed metallic behaviour and was dominated by the conducting SrRuO3 layers, the off-diagonal resistivity showed an anomalous Hall effect with ferromagnetic loop shape even far above the Curie temperature of the SrRuO3 layers as well as a positive high field slope. This indicates the presence of a highly conducting, ferromagnetically ordered hole gas at the interfaces that might be formed by a charge-transfer process. This result opens up an alternative route for the fabrication of quasi-two-dimensional systems

Magnetic manipulation in Dy/Tb multilayer upon electron-irradiation

Manipulation and control of defects triggered by an electron beam allow us to conduct defect engineering on layered materials. We investigate topologically stable helices within a [Dy(10 nm)/Tb(10 nm)]30 multilayer subjected to MeV electron(e)-irradiation up to a maximum fluence of 9.58 × 1018 e/cm2. As electrons can go through the sample homogeneously and with high penetration depth, they produce defects without doping. Our e-irradiation results indicate defect induced magnetic manipulation, which increases the blocking/freezing temperature of spin-frustrated interfaces by 4%. This increase implies an increase in the spin-cluster volume. Consequently, the reduced uncompensated pinning centres decrease the interfacial exchange bias coupling by 45%. Direct manipulation of pinning centres would thereby allow us to tailor spintronic devices in a clean way.Fil: Paul, Amitesh. Technion - Israel Institute of Technology; IsraelFil: Esquinazi, Pablo D.. Division Of Quantum Magnetism And Superconductivity, Un; AlemaniaFil: Zandalazini, Carlos Ivan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; ArgentinaFil: Setzer, Annette. Division Of Quantum Magnetism And Superconductivity, Un; AlemaniaFil: Knolle, Wolfgang. No especifíca