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We demonstrate a geometrical effect on the depinning line (DL) of the flux line lattice of the Bi2Sr2CaCu2O8+δ high-Tc superconductor (HTSC) micrometer ring. The DL shifts to notably lower temperatures in comparison with bulk crystals and thin flakes of the same sample. The shift is attributed to a decrease in the overall pinning potential due to a double size effect, namely (a) the ring thickness ∼ 1 μ m being smaller than the pinning correlation length and (b) the increase in the effective London penetration depth of the vortices (Pearl vortices). The large shift of the DL to lower temperatures may influence the suitability of this HTSC for applications in microstrip antennas and THz emitters. [ABSTRACT FROM AUTHOR]

Perovskite manganite La0.5Ca0.5−x□xMnO3 (LCMO) nanomaterials were elaborated using the sucrose modified auto combustion method. Rietveld refinements of the X-ray diffraction patterns of the crystalline structure confirm a single-phase orthorhombic state with Pbnm space group (No. 62). The Ca-vacancies were voluntarily created in the LCMO structure in order to study their influence on the magnetic behaviour in the system. The magnetic susceptibility was found to be highly enhanced in the sample with Ca-vacancies. Paramagnetic-to-ferromagnetic phase transition was evidenced in both samples around 254 K. This transition is, characterized by a drastic jump of the susceptibility in the sample with Ca-vacancies. The maximum of entropy change, observed for both compounds at magnetic field of 6 T was 2.30 J kg−1 K−1 and 2.70 J kg−1 K−1 for the parent compound and the lacunar one respectively. The magnetocaloric adiabatic temperature change value calculated by indirect method was 5.6 K and 5.2 K for the non-lacunar and Ca-vacancy compound, respectively. The Ca-lacunar La0.5Ca0.5−x□xMnO3 (x = 0.05) reported in this work demonstrated overall enhancement of the magnetocaloric effect over the LCMO. The technique used to elaborate LCMO materials was beneficial to enhance the magnetocaloric effect and magnetic behaviour. Therefore, we conclude that this less costly environmentally friendly system can be considered as more advantageous candidate for magnetic refrigeration applications then the commonly Gd-based compounds. [ABSTRACT FROM AUTHOR]

We report on Raman spectroscopy, temperature-dependent in-plane resistivity, and in-plane magnetoresistance experiments in highly oriented pyrolytic graphite (HOPG) implanted with As and Mn. A pristine sample was also studied for comparison. Two different fluences were applied, $$\varphi = 0.5\times 10^{16}\,\, {\hbox {ions}}/{\hbox {cm}}^{2}$$ and $$\varphi = 1.0\times 10^{16}\,\,{\hbox {ions}}/{\hbox {cm}}^{2}$$ . The implantations were carried out with 20 keV ion energy at room temperature. The Raman spectroscopy results reveal the occurrence of drastic changes of the HOPG surface as a consequence of the damage caused by ionic implantation. For the higher dose, the complete amorphization limit is attained. The resistivity and magnetoresistance results were obtained placing electrical contacts on the irradiated sample surface. Owing to the strong anisotropy of HOPG, the electrical current propagates mostly near the implanted surface. Shubnikov-de Haas (SdH) oscillations were observed in the magnetoresistance at low temperatures. These results allow the extraction of the fundamental SdH frequencies and the carriers' effective masses. In general, the resistivity and magnetoresistance results are consistent with those obtained from Raman measurements. However, one must consider that the electrical conduction in our samples occurs as in a parallel association of a largely resistive thin sheet at the surface strongly modified by disorder with a thicker layer where damage produced by implantation is less severe. The SdH oscillations do not hint to significant changes in the carrier density of HOPG. [ABSTRACT FROM AUTHOR]

Originating from relativistic quantum field theory, Dirac fermions have been invoked recently to explain various peculiar phenomena in condensed-matter physics, including the novel quantum Hall effect in graphene, the magnetic-field-driven metal–insulator-like transition in graphite, superfluidity in 3He (ref. 5) and the exotic pseudogap phase of high-temperature superconductors. Despite their proposed key role in those systems, direct experimental evidence of Dirac fermions has been limited. Here, we report the first direct observation of relativistic Dirac fermions with linear dispersion near the Brillouin zone (BZ) corner H, which coexist with quasiparticles that have a parabolic dispersion near another BZ corner K. In addition, we also report a large electron pocket that we attribute to defect-induced localized states. Thus, graphite presents a system in which massless Dirac fermions, quasiparticles with finite effective mass and defect states all contribute to the low-energy electronic dynamics. [ABSTRACT FROM AUTHOR]

We have performed high-resolution measurements of the magnetic field (0 T≤B≤9 T) and temperature (10 K≤T<140 K) dependence of the longitudinal and transverse Hall thermal conductivity of a twinned YBa2Cu3O6.95 single crystal. We have used and compared two recently published methods to extract the thermal Hall angle θH(T, B). Our results indicate that cot(θH) varies quite accurately as T4 in the intermediate temperature range ∼0.3c. It shows a well defined minimum at Tm≃20 K which resembles that observed in the c-axis microwave conductivity. The electronic part of the longitudinal and the transverse thermal conductivity show the scaling behavior for transport properties predicted for d-wave superconductors in the temperature range ∼18 K≤T≤30 K. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Measurements of the magnetization hysteresis loop performed on Bi2 Sr2 CaCu2 O8 high-Tc superconducting 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. [ABSTRACT FROM AUTHOR]

Magnetization measurements performed on a Nb thin film have revealed the existence of a “second magnetization peak” (SMP) well below the upper critical field boundary Hc2 (T). The results of this work provide clear evidence that the SMP originates from a thermomagnetic flux-jump instability. The similarity of the magnetization loops measured in Bi2 Sr2 CaCu2 O8 high-Tc superconducting crystals to those in Nb, as well as their dependence on the crystal size and field step indicate that a common mechanism is responsible for the SMP in these superconductors. [ABSTRACT FROM AUTHOR]

We have measured the nonlocal in-plane resistance in high-Tc superconducting Yba2 Cu3 O7– d films. The results obtained in zero magnetic field show a negative nonlocal resistance in the vicinity of the Kosterlitz-Thouless-type vortex-antivortex unbinding transition, as it was predicted recently by Wortis and Huse [Phys. Rev. B 54 , 12413 (1996)]. We also found that the negative nonlocal resistance takes place within a certain domain of the current-temperature diagram. Our findings indicate that nonlocal measurements in “planar” geometry can be a new method to study vortex dynamics in superconductors. [ABSTRACT FROM AUTHOR]

Magnetic relaxation caused by the creep of Josephson intergrain vortices is studied in Bi2Sr2Ca2Cu3O10 high-Tc superconductor in the temperature interval 1.8–30 K for applied magnetic fields between 15 and 25 Oe. It is found that the magnetic moment M logarithmically decays with time t. At high enough temperatures the normalized logarithmic decay rate S=−(1/MJ0)(dMJ/dlnt) linearly increases with temperature, while at T0(H), a temperature-independent relaxation rate S≈1.7·10−3 is found, manifesting the occurrence of quantum creep of the Josephson vortices. The “crossover” temperature T0 from the quantum tunneling to the classical thermally activated regime of the vortex motion rapidly decreases from T0=10 K to T0<2 K when the field is increased from H=15 Oe to H=25 Oe. No thermal enhancement of the tunneling rate was found in contrast to the quantum creep behavior of Abrikosov intragrain vortices measured in the same sample (Physica C 222 (1994) 149). [ABSTRACT FROM AUTHOR]

A letter to the editor is presented about high-temperature ferromagnetism in a polymeric phase of pure carbon that was free of ferromagnetic impurities.