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1.
Superconductivity of metallic boron in MgB2 总被引:10,自引:0,他引:10
Kortus J Mazin II Belashchenko KD Antropov VP Boyer LL 《Physical review letters》2001,86(20):4656-4659
Boron in MgB2 forms stacks of honeycomb layers with magnesium as a space filler. Band structure calculations indicate that Mg is substantially ionized, and the bands at the Fermi level derive mainly from B orbitals. Strong bonding with an ionic component and considerable metallic density of states yield a sizable electron-phonon coupling. Together with high phonon frequencies, which we estimate via zone-center frozen phonon calculations to be between 300 and 700 cm(-1), this produces a high critical temperature, consistent with recent experiments. Thus MgB2 can be viewed as an analog of the long sought, but still hypothetical, superconducting metallic hydrogen. 相似文献
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LIU Juan ZHAO Yuan YI Lin 《理论物理通讯》2008,49(2):504-506
With the help of supercell method, the first-principle calculations were performed for the study of doping crystal
Mg1-xAlxB2 and Mg(B1-yCy)2. Analyzing the
variations of the charge distribution and the partial densities of states, we found that the compounds with doping Al to MgB2 compound and/or replacing boron by carbon exhibit new covalent bond effects and unexpected electronic properties, related to
superconductivity. The study of the density of states indicates
that superconductivity decreases with the increase of Al fraction
and carbon concentration. There exists a transition of superconductor to non-superconductor with the change of Al doping fraction. The substitution of boron by carbon results in the
decrease of the transition temperature since the decrease of the
electron concentration and the lattice constant. The theoretical
predictions agree with experimental observations. 相似文献
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Superconductivity of MgB2: covalent bonds driven metallic 总被引:3,自引:0,他引:3
A series of calculations on MgB2 and related isoelectronic systems indicates that the layer of Mg2+ ions lowers the nonbonding B pi ( p(z)) bands relative to the bonding sigma ( sp(x)p(y)) bands compared to graphite, causing sigma-->pi charge transfer and sigma band doping of 0.13 holes/cell. Because of their two dimensionality the sigma bands contribute strongly to the Fermi level density of states. Calculated deformation potentials of gamma point phonons identify the B bond stretching modes as dominating the electron-phonon coupling. Superconductivity driven by sigma band holes is consistent with the report of destruction of superconductivity by doping with Al. 相似文献
5.
P. Ková? I. Hu?ek M. Kulich T. Meli?ek K. Hu?eková E. Dobro?ka 《Physica C: Superconductivity and its Applications》2010,470(5-6):340-344
Single-core MgB2/Fe ex-situ wires have been made by powder-in-tube (PIT) using: (i) commercial Alfa Aesar (AA) powder deformed by variable modes, (ii) AA powder oxidized by air milling and heat treatment and (iii) AA powder chemically treated by acetic and benzoic acid. All samples were finally annealed at 950 °C/0.5 h in Argon. The effect of deformation, oxidation and chemical treatment on the transport properties of MgB2 wires was tested. Differences in critical currents, transition temperatures and normal state resistivity are shown and discussed. 相似文献
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W.J. Feng S. Zhang Y.Q. Guo H. Yang T.D. Xia Z.Q. Wei 《Physica C: Superconductivity and its Applications》2010,470(3):236-239
Mg0.9Ti0.1B2/Cu wires have been successfully synthesized by the powder-in-tube (PIT) technique combined with the self-propagating high-temperature synthesis (SHS) method in an unsealed furnace. The analysis includes the studies of the sample microstructure, phase composition, critical transition temperature Tc and critical current density Jc. The experiments show that the PIT technique combined with the SHS method is effective in reducing the Mg volatilization and oxidation. There is little reaction between Mg and Cu in the SHS process, but with the MgCu2 and CuO phases, the reaction does occur in the inner sheath wall of the copper tube. The findings also indicate that there is little MgO in the sample. The sample has a very strong flux pinning ability at the low magnetic fields since the TiB2 phase may prevent the growth of MgB2 grains and lead to a favorable effect on grain refinement. The wires exhibit a high critical current density, e.g. 2.7 × 105 A/cm2 in the 1.0 T field at 4.2 K and 1.3 × 105 A/cm2 in the 2.0 T field at 4.2 K. 相似文献
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Iavarone M Karapetrov G Koshelev AE Kwok WK Crabtree GW Hinks DG Kang WN Choi EM Kim HJ Kim HJ Lee SI 《Physical review letters》2002,89(18):187002
The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at Delta(1)=2.3 meV and Delta(2)=7.1 meV at 4.2 K. Different spectral weights of the partial superconducting density of states are a reflection of different tunneling directions in this multiband system. Temperature evolution of the tunneling spectra follows the BCS scenario [Phys. Rev. Lett. 3, 552 (1959)]] with both gaps vanishing at the bulk T(c). The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. [Phys. Rev. Lett. 87, 087005 (2001)]]. 相似文献
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A. S. Sidorenko L. R. Tagirov A. N. Rossolenko N. S. Sidorov V. I. Zdravkov V. V. Ryazanov M. Klemm S. Horn R. Tidecks 《JETP Letters》2002,76(1):17-20
According to the crystal structure of MgB2 and band structure calculations, quasi-two-dimensional (2D) boron planes are responsible for the superconductivity. We report on critical-field and resistance measurements of 5.6-μm-thick MgB2 films grown on a sapphire single-crystal substrate. Resistivity measurements yield a temperature dependence of the fluctuation conductivity above the critical temperature, which agrees with the Aslamazov-Larkin and Maki-Thompson theory of fluctuations in layered superconductors, indicating a quasi-two-dimensional nucleation of superconductivity in MgB2. 相似文献
14.
Non-resonant microwave absorption (NRMA) studies of superconducting MgB2 and a sample containing ∼10% by weight of MgO in MgB2 are reported. The NRMA results indicate near absence of intergranular weak links in the pure MgB2 sample. A linear temperature dependence of the lower critical field H
c1 is observed indicating a non-s wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d wave symmetry is also not observed. In the MgB2 + MgO sample, much larger low field dependent absorption is observed indicating the presence of intergranular weak links.
The hysteretic behavior of NRMA is compared and contrasted in the two samples. In the pure MgB2 sample, a large hysteresis is observed between the forward and the reverse scans of the magnetic field indicating strong
pinning of flux lines. This hysteresis saturates a few degrees below T
c while in the MgB2 + MgO sample, a much slower increase of hysteresis with decreasing temperature is observed, a signature of weaker pinning. 相似文献
15.
Scanning tunneling spectroscopy in MgB2 总被引:1,自引:0,他引:1
We present scanning tunneling microscopy measurements of the surface of superconducting MgB2 with a critical temperature of 39 K. In zero magnetic field the conductance spectra can be analyzed in terms of the standard BCS theory with a smearing parameter gamma. The value of the superconducting gap is 5 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair breaking as seen in the conductance spectra in fields up to 6 T. 相似文献
16.
A.D. Bortolozo Z. Fisk O.H. Sant’Anna C.A.M. dos Santos A.J.S. Machado 《Physica C: Superconductivity and its Applications》2009,469(7-8):256-258
In this work the Nb2InC phase is investigated by X-ray diffraction, heat capacity, magnetic and resistivity measurements. Polycrystalline samples with Nb2InC nominal compositions were prepared by solid state reaction. X-ray powder patterns suggest that all peaks can be indexed with the hexagonal phase of Cr2AlC prototype. The electrical resistance as a function of temperature for Nb2InC shows superconducting behavior below 7.5 K. The M(H) data show typical type-II superconductivity with HC1 ~ 90 Oe at 1.8 K. The specific heat data are consistent with bulk superconductivity. The Sommerfeld constant is estimated as γ ~ 12.6 mJ mol?1 K?1. 相似文献
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JETP Letters - The reentrant superconductivity is the peculiar phenomenon observed in paramagnetic metal UTe2 in magnetic field parallel to the hard magnetisation axis. It is difficult to explain... 相似文献
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Yanpeng Qi Xianping ZhangZhaoshun Gao Zhiyu ZhangLei Wang Dongliang WangYanwei Ma 《Physica C: Superconductivity and its Applications》2009,469(13):717-720
Nb-sheathed Sr0.6K0.4Fe2As2 superconducting wires have been fabricated using the powder-in-tube (PIT) method for the first time and the superconducting properties of the wires have been investigated. The transition temperature (Tc) of the Sr0.6K0.4Fe2As2 wires is confirmed to be as high as 35.3 K. Most importantly, Sr0.6K0.4Fe2As2 wires exhibit a very weak Jc-field dependence behavior even the temperature is very close to Tc. The upper critical field Hc2(0) value can exceed 140 T, surpassing those of MgB2 and all the low temperature superconductors. Such high Hc2 and superior Jc-field performance make the 122 phase SrKFeAs wire conductors a powerful competitor potentially useful in very high field applications. 相似文献
20.
Results of ab initio electronic structure calculations on the compound MgB2 using the FPLAPW method employing GGA for the exchange-correlation energy are presented. Total energy minimization enables
us to estimate the equilibrium volume, c/a ratio and the bulk modulus, all of which are in excellent agreement with experiment. We obtain the mass enhancement parameter
by using our calculated D (E
F) and the experimental specific heat data. The T
c is found to be 24.7 K. 相似文献