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1.
Karayanni M. Papavassiliou G. Pissas M. Fardis M. Papagelis K. Prassides K. Takenobu T. Iwasa Y. 《Journal of Superconductivity and Novel Magnetism》2005,18(4):521-528
We report a11B NMR line shape and spin-lattice relaxation rate (1/(T
1
T)) study of pure and lightly carbon-doped MgB2−x
C
x
forx=0, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB2 exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of
boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase
ofH
c
2/c
with carbon doping with respect to pure MgB2. The spin-lattice relaxation rate 1/(T
1
T) demonstrates clearly the presence of a coherence peak right belowT
c in pure MgB2, followed by a typical BCS decrease on cooling. However, at temperatures lower than ≈10 K strong deviation from the BCS behavior
is observed, probably from residual contribution of the vortex dynamics. In the carbon-doped systems both the coherence peak
and the BCS temperature dependence of 1/(T
1
T) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping. 相似文献
2.
M. Karayanni G. Papavassiliou M. Pissas M. Fardis K. Papagelis K. Prassides T. Takenobu Y. Iwasa 《Journal of Superconductivity》2005,18(4):521-528
We report a 11B NMR line shape and spin-lattice relaxation rate (1/(T1T))(1/(T_1T)) study of pure and lightly carbon-doped MgB2-xCx_{2-x}{\rm C}_x for x = 0,0.02x = 0,0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB2_2 exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of
boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase
of Hcc2H^c_{c2} with carbon doping with respect to pure MgB2_2. The spin-lattice relaxation rate 1/(T1T)1/(T_1T) demonstrates clearly the presence of a coherence peak right below TcT_{\rm c} in pure MgB2_2, followed by a typical BCS decrease on cooling. However, at temperatures lower than ?\approx 10 K strong deviation from the BCS behavior is observed, probably from residual contribution of the vortex dynamics. In the
carbon-doped systems both the coherence peak and the BCS temperature dependence of 1/(T1T)1/(T_1T) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping. 相似文献
3.
4.
Prassides K Takabayashi Y Nakagawa T 《Philosophical transactions. Series A, Mathematical, physical, and engineering sciences》2008,366(1862):151-161
Mixed-valence phenomena associated with the highly correlated narrow-band behaviour of the 4f electrons in rare earths are well documented for a variety of rare-earth chalcogenides, borides and intermetallics (Kondo insulators and heavy fermions). The family of rare-earth fullerides with stoichiometry RE2.75C60 (RE=Sm, Yb, Eu) also displays an analogous phenomenology and a remarkable sensitivity of the rare-earth valency to external stimuli (temperature and pressure) making them the first known molecular-based members of this fascinating class of materials. Using powerful crystallographic and spectroscopic techniques which provide direct indications of what is happening in these materials at the microscopic level, we find a rich variety of temperature- and pressure-driven abrupt or continuous valence transitions-the electronically active fulleride sublattice acts as an electron reservoir that can accept electrons from or donate electrons to the rare-earth 4f/5d bands, thereby sensitively modulating the valence of the rare-earth sublattice. 相似文献
5.
6.
7.
Ganin AY Takabayashi Y Khimyak YZ Margadonna S Tamai A Rosseinsky MJ Prassides K 《Nature materials》2008,7(5):367-371
C(60)-based solids are archetypal molecular superconductors with transition temperatures (Tc) as high as 33 K (refs 2-4). Tc of face-centred-cubic (f.c.c.) A(3)C(60) (A=alkali metal) increases monotonically with inter C(60) separation, which is controlled by the A(+) cation size. As Cs(+) is the largest such ion, Cs(3)C(60) is a key material in this family. Previous studies revealing trace superconductivity in Cs(x)C(60) materials have not identified the structure or composition of the superconducting phase owing to extremely small shielding fractions and low crystallinity. Here, we show that superconducting Cs(3)C(60) can be reproducibly isolated by solvent-controlled synthesis and has the highest Tc of any molecular material at 38 K. In contrast to other A(3)C(60) materials, two distinct cubic Cs(3)C(60) structures are accessible. Although f.c.c. Cs(3)C(60) can be synthesized, the superconducting phase has the A15 structure based uniquely among fullerides on body-centred-cubic packing. Application of hydrostatic pressure controllably tunes A15 Cs(3)C(60) from insulating at ambient pressure to superconducting without crystal structure change and reveals a broad maximum in Tc at approximately 7 kbar. We attribute the observed Tc maximum as a function of inter C(60)separation--unprecedented in fullerides but reminiscent of the atom-based cuprate superconductors--to the role of strong electronic correlations near the metal-insulator transition onset. 相似文献
8.
Arvanitidis J Christofilos D Kourouklis GA Paloumpi A Papagelis K Ves S Iwasa Y Prassides K 《Nanoscale》2011,3(6):2490-2493
We present Raman spectroscopic studies of the isostructural and isoelectronic Eu(6-x)Sr(x)C(60) (x = 0, 3, 5, 6) and Ba(6)C(60) compounds. The Raman spectra of the Eu-based fullerides show dramatic changes compared to the pure alkaline-earth systems, including significant broadening, splitting and frequency shifts of the fivefold degenerate H(g) intramolecular modes of C(60). Moreover, the A(g)(2) mode exhibits an even larger downshift and a remarkable broadening. These findings are consistent with distortions of the C(60) molecular cages and a considerable electron-phonon coupling strength-strongly enhanced in the Eu containing systems-originating from the strong orbital hybridization between the metal atom and the C(60) molecule. 相似文献
9.
M.F. Craciun S. Rogge M.‐J.L. denBoer S. Margadonna K. Prassides Y. Iwasa A.F. Morpurgo 《Advanced materials (Deerfield Beach, Fla.)》2006,18(3)
Transport measurements for different electron‐doped metal–phthalocyanine (MPc) materials are reported by Morpurgo and co‐workers on p. 320. It is experimentally demonstrated that for these doped MPc materials, increasing the doping level results first in a metallic state and, subsequently, in the re‐entrance of an insulating state. The artist's impression on the inside cover shows an MPc film situated between two electrodes and exposed to a flux of alkali atoms in vacuum. The alkali atoms intercalate and donate electrons to the molecular material. 相似文献
10.
T. Yokoo T. Muranaka M. Arai K. Papagelis K. Prassides J. Taylor S. M. Bennington J. Akimitsu 《Journal of Superconductivity and Novel Magnetism》2004,17(2):199-203
Phonon density of states in Al substituted Mg1?x Al x B2 has been studied by means of inelastic neutron scattering experiments with pulsed neutron time-of-flight technique. Phonon dynamics have a significant role for realizing superconductivity in the recent discovered superconductor MgB2. We present a systematic investigation on generalized phonon density of states (PDOS) up to E = 120 meV on Al content of x = 0.1, 0.25, and 0.5. We have clearly observed a large effect on the PDOS by Al substitution; a large energy shift occurs around at 70 meV and 89 meV. We have also observed a visible increase of PDOS at a particular energy range below T c. Those phonon states are related with E 2g mode to be expected to have a strong coupling with the electron states at the Γ–A region. 相似文献