Superconductivity mostly appears in high carrier density systems and sometimes exhibits common phase diagrams in which the critical temperature Tc continuously develops with carrier density. ...Superconductivity enhanced in the lightly doped regime has seldom been reported, although it is an ideal direction towards the crossover between Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein condensate (BEC) limits, where the behavior of Cooper pairs changes dramatically. Here we report transport properties and superconducting gaps in single-crystalline lithium-intercalated layered nitrides (LixHfNCl and LixZrNCl) down to the low-doping regime, enabled by a combination of ionic gating device and tunneling spectroscopy. Upon the reduction of doping, both systems display the increase of Tc, up to 24.9 K especially in LixHfNCl, with the concomitant enhancement of two-dimensionality leading to a pseudogap state below 35.5 K as well as the increase of superconducting coupling strengths 2Δ/kBTc reaching 5.9. Such behavior in the carrier density region as low as 1020cm−3 indicates that lightly doped two-dimensional superconductors exhibit the unprecedented nature that is distinct from that in conventional superconductors, and offer a new route to access BCS-BEC crossover.
The discovery of superconductivity at 39 K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a ...low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field-temperature (H-T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB (2) are carried out and found to be in excellent agreement with our inelastic ...neutron scattering measurements. The numerical results reveal that the E(2g) in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B sigma bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high T(c) and boron isotope effect in MgB (2).
The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. ...This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The substitutional chemistry of MgB2 Cava, R.J.; Zandbergen, H.W.; Inumaru, K.
Physica. C, Superconductivity,
03/2003, Letnik:
385, Številka:
1-2
Journal Article
Recenzirano
A brief summary is given of the current status of chemical substitution experiments in MgB2 Analysis of the data in the literature shows that only in a handful of cases has partial substitution of an ...atom for Mg or B been substantiated. These cases are Al and Mn substitution for Mg, and C substitution for B. C and Al substitution have received the most attention. Oxygen incorporation is an interesting chemical issue. A summary of our work on the Mg1-xAlxB2 system is presented.
Well-crystallised TiO2 particles (P-25, 20-30 nm in diameter) were directly incorporated into surfactant-templated mesoporous silica particles (pore diameter: 2.7 nm), and the composite material with ...a high TiO2 content (60 wt%) showed molecular selective and enhanced photocatalysis for decomposition of 4-nonylphenol.
Molybdenum nitride β-Mo2N was synthesized on Si substrates by pulsed laser deposition of molybdenum metal under nitrogen radical irradiation. X-ray diffraction studies using a multiaxes ...diffractometer revealed that the crystal phase had a tetragonal structure uniaxially oriented with (100) planes parallel to the substrate surface. The composition was determined to be Mo2N0.85 by calibrated X-ray photoelectron spectroscopy. These results demonstrated that the well-crystallized tetragonal-phase β-Mo2N was synthesized by deposition, rather than a cubic γ-phase with randomly distributed nitrogen vacancies. The nitride β-Mo2N was superconducting below 5.2 K.
Single crystals of three-dimensional (3D) C60 polymer were prepared by the topotactic conversion of two-dimensional (2D) C60 polymer single crystals at a pressure of 15 GPa at 600 degrees C. The ...x-ray single crystal study revealed that the 3D C60 polymer crystallized in a body centered orthorhombic space group Immm, and spherical C60 monomer units were substantially deformed to rectangular parallelepiped (cuboidal) shapes, each unit being bonded to eight cuboidal C60 neighbors via 3 + 3 cycloaddition. The 3D C60 polymer was electron conductive, in contrast with the nonconductive behavior of 2D polymers.
A comprehensive interpretation of the microstructure and mechanism of the formation of a versatile solid acid catalyst, Cs2.5H0.5PW12O40, has been attempted by combining the new results obtained with ...solid-state NMR, XRD, SEM, and N2 porosimetry with the data reported previously. The precipitates of Cs2.5H0.5PW12O40 just formed from aqueous solutions of H3PW12O40 and Cs2CO3 consist of ultrafine crystallites in which the acid form, H3PW12O40, is epitaxially deposited on the surface of Cs3PW12O40 crystallites. Calcination of the precipitates brings about the migration of H+ and Cs+ in the solid to form a nearly uniform solid solution in which protons distribute randomly through the entire bulk, as revealed by XRD and 31P solid-state NMR. Impregnation of Cs3PW12O40 with the aqueous solution of H3PW12O40 also gives the uniform salt after calcination. Pore-size distribution evaluated by the analysis of N2 desorption isotherm showed that Cs2.5H0.5PW12O40 has mesopores as well as micropores that are interparticle voids of the crystallites. The initial heat of NH3 sorption indicated the presence of very strong acid sites on Cs2.5H0.5PW12O40. High catalytic activity of Cs2.5H0.5PW12O40 reported for solid−liquid reaction systems is thus principally attributed to the strength and number of acid sites and the mesoporous structure appropriate for the rapid diffusion of molecules.
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