J. H. S. Torres

Ferromagnetic- and Superconducting-Like Behavior of Graphite

We have identified ferromagnetic- and superconducting-like magnetization hysteresis loops in highly oriented pyrolytic graphite samples below and above room temperature. We also found that both behaviors are very sensitive to low-temperature—as compared to the sample synthesis temperature—heat treatment. The possible contribution of magnetic impurities and why these do not appear to be the reason for the observed phenomena is discussed.

Reentrant Metallic Behavior of Graphite in the Quantum Limit

Magnetotransport measurements performed on several well-characterized highly oriented pyrolitic graphite and single crystalline Kish graphite samples reveal a reentrant metallic behavior in the basal-plane resistance at high magnetic fields, when only the lowest Landau levels are occupied. The results suggest that the quantum Hall effect and Landau-level-quantization-induced superconducting correlations are relevant to understand the metalliclike state(s) in graphite in the quantum limit.

Indication of superconductivity at 35 K in graphite-sulfur composites.

We report magnetization measurements performed on graphite-sulfur composites which demonstrate a clear superconducting behavior below the critical temperature T(c0) = 35 K. The Meissner-Ochsenfeld effect, screening supercurrents, and magnetization hysteresis loops characteristic of type-II superconductors were measured. The results indicate that the superconductivity occurs in a small sample fraction, possibly related to the sample surface.

Local ferromagnetism in microporous carbon with the structural regularity of zeolite Y

Magnetization M(H,T) measurements have been performed on microporous carbon (MC) with a three-dimensional nanoarray structure corresponding to that of a zeolite Y supercage. The obtained results unambiguously demonstrate the occurrence of high-temperature ferromagnetism in MC, probably originating from a topological disorder associated with curved graphene sheets. The results provide evidence that the ferromagnetic behavior of MC is governed by isolated clusters in a broad temperature range, and suggest the occurrence of percolative-type transition with the temperature lowering. A comparative analysis of the results obtained on MC and related materials is given.

Landau Level Quantization and Possible Superconducting Instabilities in Highly Oriented Pyrolitic Graphite

Measurements of the basal-plane resistivity ρa(T,H) performed on highly oriented pyrolitic graphite, with magnetic field H∥c-axis in the temperature interval 2–300K and fields up to 8 T, provide evidence for the occurrence of both field-induced and zero-field superconducting instabilities. Additionally, magnetization M(T,H) measurements suggest the occurrence of Fermi surface instabilities which compete with the superconducting correlations.

Magnetothermal conductivity of highly oriented pyrolytic graphite in the quantum limit

Instituto de F´isica, Universidade Estadual de Campinas,Unicamp 13083-970, Campinas, S˜ao Paulo, Brasil(Dated: February 2, 2008)We report on the magnetic field (0T≤ B ≤ 9T) dependence of the longitudinal thermal conduc-tivity κ(T,B) of highly oriented pyrolytic graphite in the temperature range 5 K ≤ T ≤ 20 K forfields parallel to the c−axis. We show that κ(T,B) shows large oscillations in the high-field region(B > 2 T) where clear signs of the Quantum-Hall effect are observed in the Hall resistance. Withthe measured longitudinal electrical resistivity we show that the Wiedemann-Franz law is violatedin the high-field regime.INTRODUCTION

Thermomagnetic Flux-Jump Instabilities and Second Magnetization Peak in Bi2Sr2CaCu2O8 High-Tc Superconducting Crystals

Measurements of the magnetization hysteresis loop performed on Bi2Sr2CaCu2O8high-Tcsuperconducting single crystals reveal the occurrence of pronounced jumps of the irreversible magnetization at T > 40 K. In this work we demonstrate their thermomagnetic origin. It is also shown that the low-field hollow in the magnetization hysteresis loops measured at T < 40 K and leading to the so-called “second magnetization peak,” is the manifestation of a thermomagnetic instability effect. It is suggested that the plastic vortex motion triggers the magnetic instabilities.

Vortex lattice in Bi2Sr2CaCu2O8 well above the first-order phase transition boundary

Abstract Measurements of non-local in-plane resistance originating from transverse vortex–vortex correlations have been performed on Bi2Sr2CaCu2O8+δ single crystals in a magnetic field up to 9 T applied along the crystal c-axis. Our results demonstrate that a rigid vortex lattice does exist over a broad portion of the magnetic field–temperature (H–T) phase diagram, well above the first-order transition boundary HFOT(T).