Y.W. Park

Substitutional boron-doping of carbon nanotubes ☆

The substitutional placement of boron within the lattice of carbon nanotubes yields quite different transport properties for single walled nanotubes (SWNTs) as compared to multi-walled nanotubes (MWNTs). Boron ‘‘doping’’ of the MWNTs results in an acceptor state in the local density of states (LDOS) that lies near the Fermi level and can be directly correlated with features in the thermoelectric power (TEP) of B-doped MWNT mats. Transport measurements of individual B-doped MWNTs exhibit features associated with variable range hopping. In contrast, B-doping of SWNTs results in features in the density of states further from the Fermi level, and transport of the SWNTs shows an unusual variability in rectification not observed in the MWNT case. This suggests that boron has been introduced into the lattice of these two morphologies of nanotubes in very different ways. Interest in the electrical transport properties of both

Superconductivity in the Ba1−xKxBiO3 system

Abstract For the first time, superconductivity in the Ba1−xKxBiO3 system has been investigated in the whole region of cubic phase from x=0.37 up to x=1. Samples were synthesized using the high pressure and high temperature technique. The superconducting transition temperature Tc of Ba1−xKxBiO3 samples determined from dc susceptibility measurements monotonously goes down with increasing x from the well known Ba0.63K0.37BiO3 composition with Tc=30.4 K to Ba0.24K0.76BiO3 with very low diamagnetic signal at nearly 3 K. However, following this, we have found an increase of Tc up to 9 K for x=1 according to KBiO3 nominal composition.

Magnetoresistance of Sr1-xKxBiO3: A second-family of bismuth-oxide-based superconductors

AbstractThe superconducting Sr1−xKxBiO3samples with x = 0.45 – 0.6 were synthesized by the high-pressure-high-temperature technique in a belt type apparatus (2 GPa, 700 °C, 1 h, Pt capsules) with stoichiometric mixtures of Sr2Bi2O5, Bi2O3and KO2as described earlier.1The X-ray diffraction results appear as a single perovskite-like phase. The superconductivity occurs at Tc∼ 12 K in the A. C. susceptibility measurement. The onset (zero resistivity) temperature of superconductivity in resistivity measurement of the investigated sample wasn

Anomalous superconductivity in bismuthates

T_{conset} = 12.5{text{ }}K(T_{czero} = 10.2{text{ }}K)

Thermoelectric power of Pr 0.5 Sr 0.5 MnO 3 with Ru substitution at the Mn site

n. The transition temperature region was a little bit broad and a shoulder was present about 11.3 K indicating probably the existence of crystallites of different K content. The particularly interesting point is that the resistance begins to reappear at T < 6 K at zero magnetic field. As the external magnetic field is applied, the reentrant resistance disappears and superconductivity is recovered until the applied magnetic field becomes higher than 0.65 Tesla. The superconductivity for T < 6 K is destroyed for the higher magnetic field. The Tconsetdecreases as the magnetic field increases like in the BCS type superconductors. The transition region becomes broader under the magnetic field, which indicate a kind of vortex transition as in the case of high Tccuprates.

Substitutional Doping of Carbon Nanotubes

The present paper describes the synthesis, characterization of mixedvalence bismuthates with 3- or 2-dimensional perovskite-like structures and structural criteria that influence superconductivity in these compounds. Single phase samples of Sr1−xK x BiO3 were prepared for the broad range of K-content: 0.25≤x≤0.65. For these bismuthates the symmetry of the structure changes from monoclinic to orthorhombic and finally to tetragonal upon increasing the K-content thus resulting in the decrease of the Bi-O distances and reduction of the network distortions. Superconductivity with maximum Tc=12K exists in the narrow range (x≈0.5–0.6) within the stability field of the tetragonal phase (0.33≤x≤0.65), when the 3-dimensional octahedral framework has close to the ideal perovskite structure arrangement. At the same time compositions with close to optimal Bi-valence (x=0.33 and 0.43) do not show any sign of superconductivity, probably, due to structural distortions. The layered type (Ba, K)3Bi2O7 and (Ba, K)2BiO4 bismuthates belonging to the An+1B n O3n+1 homologous series were investigated. Buckling of the (BiO2) layers in the structure of the n=2 member was revealed. The formation of the n=1 bismuthate was found by Electron Microscopy and X-ray powder diffraction studies. Both types of compounds are considered to be possible candidates for new superconducting materials among bismuthates.

Conduction in Polyacetylene Nanofibers at Low Temperatures

Abstract We report the anomalous superconductivity found in bismuthates with different compositions. In the Sr 1− x K x BiO 3 (SKBO) compound the field-induced superconductivity is observed, which shows a striking suppression of the reentrant resistance at T T c by applying an external magnetic field ( H ) or increasing the electrical transport current ( I ). The magnetotransport measurements on several different samples reveal that the reentrant resistance behavior with H and I is related to the normal state transport properties. Disordered junction barriers between superconducting grains seem to be important for the observed anomalous reentrant resistance phenomena. In the Ba 1− x K x BiO 3 system, the whole region of the superconducting cubic phase diagram from x =0.4 up to x =1 was investigated for the first time by using the high pressure and high temperature synthesis technique. The superconducting temperature ( T c ) of Ba 1− x K x BiO 3 (BKBO) samples shows a parabolic decrease of T c from the well-known Ba 0.63 K 0.37 BiO 3 composition with T c =30.4 K to Ba 0.24 K 0.76 BiO 3 with very low diamagnetic signal at nearly 3 K. However, we found that further increasing x gives an increase of T c up to 9 K for x =1. Possible physical origins of the field-induced superconductivity in SKBO and the anomalous superconducting region in high potassium concentration ( x >0.8) BKBO are discussed.

Magnetic-field-induced superconductivity in Sr1-xKxBiO3

Abstract We report a new reentrant superconducting-normal resistive transition observed in the Sr 1− x K x BiO 3 superconductor. Unlike the previous reports about the reentrant resistive transition where the superconducting region becomes reduced and finally resumes the normal state by increasing the external magnetic field ( H ) or the current ( I ), we observed the recovery of superconductivity induced by applying H or increasing I . Subsequent dc magnetization measurement shows a gradual decrease of the diamagnetic susceptibility - appearance of the paramagnetic Meissner effect - at low fields ( H 1− x K x BiO 3 are discussed.