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Felner
Israel FelnerThe Racah Institute of Physics My group at the Racah institute of physics was focused on various topics from which we report on two major subjects: (a) Superconducting and magnetic properties of synthetic amorphous carbon doped with sulfur. (b) Two types of anisotropy features observed in La1.85Sr0.15CuO4 single Crystals: (i) in the upper critical fields (HC2 and HC3) and (ii) in the paramagnetic Meissner effect (PME).
Superconducting and magnetic properties of amorphous carbon (a-CS)The existence of superconductivity (SC) in the commercial amorphous carbon (a-C) and in sulfur doped commercial a-C powders reported a year ago, motivated us to extend the search of SC on another a-C sources, such as synthetic a-C which was fabricated under controlled conditions. The central goal was to obtain unambiguous experimental evidence for the existence of homogeneous SC phase in sulfur-doped a-C powder, regardless its source. For that purpose we have synthesized a-C material by melting of pure sucrose (C12H22O11). Then, a mixtures of a-C and sulfur with a-C:S weight ratio of 10:1 (a-CS), was heated to 400oC under an inert gas atmosphere. The batch obtained, exhibits scattered results. In one a-CS sample, traces of a SC phase at TC= 17 K was observed. Another a-CS sample was heated once more at or at 380oC for 3 hours ((assigned as a-CS(h)) exhibited SC at 42 K. (Fig. 1). Other samples (from the same a-CS batch) exhibit pronounced peaks around 50-55 K in their zero-field-cooled (ZFC) curves (Fig. 2) and the rest did not show any significant magnetic features. (see Ref. 11)
Figure 1: ZFC and FC magnetization plots of a-CS(h) measured at 71 Oe. The peaks in both branches due to solidification of oxygen are readily observed. In Fig. 2 immediately after the first ZFC(i) and field-cooled (FC(i)) process, a second ZFC(ii) and FC(ii) run was performed under the same conditions. Unexpectedly, the peak observed in ZFC(i) is totally suppressed ZFC(ii) run. Whereas the two FC(i+ii) curves coincide which each other. Around the peaks, the ZFC(i) branch is much higher than the FC curves, thus at a certain temperature range ZFC(i)>FC. This observation is unique and to the best of our knowledge was not observed in the past. We eliminate impurities (such as magnetite and/or oxygen) as the origin of the measured properties and believe that they are an intrinsic property of the a-CS materials. We are not aware of any existing theoretical models that would be able to explain these peculiar magnetic phenomena described here.
Figure 2: Two ZFC and FC plots of a-CS (iii) measured at 500 Oe. The ZFC(i) (in black) and FCC(i) (in red) curves were measured up to 120 K. Anisotropy of HC2 and HC3 and PME in La1.85Sr0.15CuO4 single CrystalsOptimally-doped La1.85Sr0.15CuO4 single crystals have been investigated by dc and ac magnetic measurements. These crystals have rectangular needle-like shapes with the long needle axis parallel to the crystallographic c axis (c-crystal) or parallel to the basal planes (a-crystal). Two different anisotropies are accounted. (i) In both crystals, the temperature dependence of the upper critical fields (HC2) and the surface critical field (HC3) were measured and the H-T phase diagram obtained, is presented in Fig.3. Close to Tc=35 K, for the c-crystal, ?c = HcC3/HcC2 = 1.80(2), whereas for the a-crystal the ?a = HaC3/HaC2 =4.0(2) obtained, is much higher than the theoretical value 1.69. (ii) At low applied dc fields, positive field-cooled branches known as the "paramagnetic Meissner effect" are observed. Their magnitude is inversely proportional to H. The anisotropic PME is observed in both a- and c-crystals, only when the applied field is along the basal planes (H||a) (see Fig. 4).
Figure 3: Temperature dependence of the upper and surface critical magnetic fields: , and , and . Note the almost linear plots near TC of all four curves. The curve was obtained by M(T) (filled ) and M(H) (open) methods.
(a) ZFC and FC susceptibility plots of the c-crystal for H applied parallel and perpendicular to the c- axis measured at 2.3 and 1.4 Oe respectively. (b) ZFC and FC magnetization curves of the a-crystal, measured at 3.5 (blue) and 5.5 Oe (red) for H parallel to ab planes. The inset shows the FC plot measured at 8.5 Oe. For the a-crystal, the positive M/H at 25 K, decreases with H as M/H = C*H-?, where the constant C=0.0021 and ? = 1.0 ± 0.05, indicating that the M/H is inversely proportional to H. Moreover, due to the similarity in the short (H||a) and middle (H||c) dimensions of the a-crystal, we may assume the in both directions the demagnetization factors are quite similar; thus we may compare between the normalized positive and negative signals. As expected, the two ZFC curves for both orientations are quite similar to each other. On the other hand in the FC branches, the positive PME signal for H||a is twice as much as in the diamagnetic one for H||c. Remarkably, the observed PME in various SC systems which appears only at very low H values (less than 1 Oe), is exhibited here up to H of ~10?15 Oe. It is speculated that the two anisotropies are connected to each other. Specific research equipments related to Nanoscience and Nanotechnology:
List of publications in Nanoscience and Nanotechnology (2011-2012)
Three most significant publications in the last five years:
Cooperation with other researchers/universities in IsraelWithin Hebrew University:
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