As cobalt is an important ferromagnetic material and the nanofibrous shape greatly improves the magnetic properties, we are introducing cobalt nanofibers encapsulated in graphite shell to gain the advantages of the nanofibrous shape as well as to produce protected materials. An electrospun nanofiber mat consisting of cobalt acetate and poly(vinyl alcohol) has been calcined in argon atmosphere at 850 °C. Calcination of cobalt acetate in an inert atmosphere leads to the production of pure cobalt which strongly enhanced graphitization of the utilized polymer to form a graphite shell. Physicochemical characterization analyses indicated that the final product was pure cobalt nanofibers enveloped in a 10 nm thick graphite shell. The graphite shell did not affect the magnetic properties of the synthesized graphite-encapsulated cobalt nanofibers compared with the bare ones which reveal preeminent magnetic characteristics; moreover the shell modifies some of these properties to be temperature independent.
Mixture of single-crystalline MnO and Mn3O4 nanowires was synthesized by thermal evaporation of MnCl2 powders. The diameter is 50–100nm, the length is about 20μm, and the growth direction is uniformly [100] for both cubic MnO and tetragonal Mn3O4 nanowires. The temperature-dependent magnetization and magnetic hysteresis curves suggest the Curie temperature of 12 and 43K for the MnO and Mn3O4 nanowires, respectively.
Electrical transport properties of individual double‐wall carbon nanotube (DWNT) are studied. Negative differential conductance (NDC) was observed for the DWNT with a defected outer shell. Such NDC was explained in terms of the resonant tunneling through multiple quantum dots. Also observed is the Fano resonance for the low‐resistance samples. The Fano resonance was manifested by asymmetric peaks in the gate modulation and also by the zero‐bias peak in the differential conductance curve. Both NDC and Fano resonance in DWNT demonstrates the interplay of inner and outer shells via the inter‐shell hopping of electrons.
Constructing an effective field theory in terms of doped magnetic impurities [described by an O(3) vector model with a random mass term], itinerant electrons of spin-orbit coupled semiconductors (given by a Dirac theory with a relatively large mass term), and effective interactions between doped magnetic ions and itinerant electrons (assumed by an effective Zeeman coupling term), we perform the perturbative renormalization group analysis in the one-loop level based on the dimensional regularization technique. As a result, we find that the mass renormalization in dynamics of itinerant electrons acquires negative feedback effects due to quantum fluctuations involved with the Zeeman coupling term, in contrast with that of the conventional problem of quantum electrodynamics, where such interaction effects enhance the fermion mass more rapidly. Recalling that the applied magnetic field decreases the band gap in the presence of spin-orbit coupling, this renormalization group analysis shows that the external magnetic field overcomes the renormalized band gap, allowed by doped magnetic impurities even without ferromagnetic ordering. In other words, the Weyl metal physics can be controlled by doping magnetic impurities into spin-orbit coupled semiconductors, even if the external magnetic field alone cannot realize the Weyl metal phase due to relatively large band gaps of semiconductors. Furthermore, we emphasize that quasiparticles do not exist in this emergent disordered Weyl metal phase due to correlations with strong magnetic fluctuations. This non-Fermi-liquid type Weyl metal state may be regarded to be a anomalous metallic phase in the respect that a topologically nontrivial band structure appears in the vicinity of quantum criticality.
Utilizing a systematic study of transport measurements, we constructed a detailed H–T phase diagram of an MgB2 single crystal. There was no tricritical point of the surface superconductivity, the bulk superconductivity and the peak effect, in contrast to the existence of this point in Nb single crystals, as obtained from the magnetic susceptibility. We found that the surface effect was still strong up to the zero-field superconducting transition temperature and that the peak effect did not disappear on the Hc2 line because a disordered flux flow was present just below Hc2. The disappearance of the peak effect is closely related to thermal fluctuations.
We studied the low-temperature thermoelectric properties of single crystals of Mg3Sb2−xBix (0 ≤ x ≤ 2) grown by the Bridgman method. The crystals are well aligned along the hexagonal c axis as documented in the huge anisotropy of the electrical resistivity; ρ//c/ρ⊥c = 100 for x = 2. Upon increasing x, the semiconducting behaviour in ρ(T) changes to the metallic behaviour, and the Seebeck coefficient S(T) at room temperature decreases from 590 μV K−1 for x = 0 to 67 μV K−1 for x = 2. Although the thermal conductivity at 300 K is as low as 0.88 W mK−1 for x = 0.7, the large resistivity of 1 Ωcm leaves the figure of merit, ZT, at a low level of 5.8 × 10−3 at 300 K.
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