Two-band superconductivity in MgB2

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)]].     

两带模型与MgB2的超导电性

文中基于两带模型,通过引入非电-声短程相互作用,在弱耦合极限下按实验结果选定一组参数,同时得到了MgB2超导体的临界温度、同位素效应指数、零温能隙及比热的跃变,这些结果与实验相符合。     

Three-dimensional MgB2-type superconductivity in hole-doped diamond

We substantiate by numerical and analytical calculations that the recently discovered superconductivity below 4 K in 3% boron-doped diamond is caused by electron-phonon coupling of the same type as in MgB2, albeit in three dimensions. Holes at the top of the zone-centered, degenerate sigma-bonding valence-band couple strongly to the optical bond-stretching modes. The increase from two to three dimensions reduces the mode softening crucial for T(c) reaching 40 K in MgB2. Even if diamond had the same bare coupling constant as MgB2, which could be achieved with 10% doping, T(c) would be only 25 K. Superconductivity above 1 K in Si (Ge) requires hole doping beyond 5% (10%).     

Properties of anisotropic s-wave superconductivity in MgB2

Earlier this year a new type of high-temperature superconductivity has been reported in MgB₂ with a transition temperature of 39 K. In a recent paper we proposed a simple BCS model with an anisotropic s-wave order parameter which accounts for the available thermodynamic and scanning tunneling microscopy data obtained from polycrystalline palets of MgB₂. In this work, we further investigate the physical consequences of this theory by calculating the temperature dependence of the thermal conductivity, the nuclear spin relaxation rate, and the angular dependence of the upper critical field.     

Evidence for two superconducting gaps in MgB2

We have measured the Raman spectra of polycrystalline MgB2 from 25 to 1200 cm(-1). A superconductivity-induced redistribution in the electronic Raman continuum was observed. Two pair-breaking peaks appear in the spectra, suggesting the presence of two superconducting gaps. The measured spectra were analyzed using a quasi-two-dimensional model in which two s-wave superconducting gaps open on two sheets of Fermi surface. For the gap values we have obtained Delta(1) = 22 cm(-1) ( 2.7 meV) and Delta(2) = 50 cm(-1) ( 6.2 meV). Our results suggest that a conventional phonon-mediated pairing mechanism occurs in the planar boron sigma bands and is responsible for the superconductivity of MgB2.     

Definitive experimental evidence for two-band superconductivity in MgB2

The superconducting-gap of MgB2 has been studied by high-resolution angle-resolved photoemission spectroscopy. The results show that superconducting gaps with values of 5.5 and 2.2 meV open on the sigma band and the pi band, respectively, but both the gaps close at the bulk transition temperature, providing a definitive experimental evidence for the two-band superconductivity with strong interband pairing interaction in MgB2. The experiments validate the role of k-dependent electron-phonon coupling as the origin of multiple-gap superconductivity as well as the high transition temperature of MgB2.     

On the possibility of superconductivity in a MgB2 compound

A concept of strong interaction in a single unit cell is used to predict Cooper instability in a system with hopping between main-group cations and anions. A phase diagram of ferromagnetic ordering is constructed for different degrees of (n _p )p ⁶-and (n _s )s ²-shell filling in main-group elements.     

Surface superconductivity of dirty two-band superconductors: applications to MgB2

The minimal magnetic field H(c2) destroying superconductivity in the bulk of a superconductor is smaller than the magnetic field H(c3) needed to destroy surface superconductivity if the surface of a superconductor coincides with one of the crystallographic planes and is parallel to the external magnetic field. While for a dirty single-band superconductor the ratio of H(c3) to H(c2) is a universal temperature-independent constant 1.6946, for dirty two-band superconductors this is not the case. I show that in the latter case the interaction of the two bands leads to a novel scenario with the ratio H(c3)/H(c2) varying with temperature and taking values larger and smaller than 1.6946. The results are applied to MgB(2) and compared with recent experiments (A. Rydh, cond-mat/0307445).     

Pressure and Doping Effects on Critical Temperature in MgB2 and Nonmagnetic Borocarbides within Two-Band Eliashberg Theory

Critical temperature of Mg1-xAlxB2, MgB2-xCx are calculated using two-band Eliashberg theory in intermediate e-ph coupling. Analytical calculations are conducted for different values of doping parameters x in this compounds. Pressure effects on critical temperature in MgB2 and nonmagnetic borocarbide YNi2B2C are also investigated. Results is compared with available experimental data and good agreement is achieved.     

High T(c) superconductivity in MgB2 by nonadiabatic pairing

The evidence for the key role of the sigma bands in the electronic properties of MgB2 points to the possibility of nonadiabatic effects in the superconductivity of these materials. These are governed by the small value of the Fermi energy due to the vicinity of the hole doping level to the top of the sigma bands. We show that the nonadiabatic theory leads to a coherent interpretation of T(c) = 39 K and the boron isotope coefficient alphaB = 0.30 without invoking very large couplings and it naturally explains the role of the disorder on T(c). It also leads to various specific predictions for the properties of MgB2 and for the material optimization of these types of compounds.     

价层轨道平均能与掺杂二硼化镁体系超导电性的关系

研究了不同掺杂组分二硼化镁超导体的超导电性和原子价层轨道平均能的关系,发现全部阳离子和阴离子的价层轨道平均能的平均值主要集中在9.9ev至10.35ev之间,阳离子之间与阴离子价层轨道平均能的平均值之差都集中在在3.1ev至4.4ev之间.这种关系将有助于了解晶格上电子的静电作用与超导机理之间的关系。     

AlB2 and MgB2: a comparative study of their electronic,phonon and superconductivity properties via first principles

ABSTRACT

Recently, the AlB₂-type compounds (such as AlB₂ and MgB₂) which exhibit Dirac Nodal Line (DNLs) semimetal on their electronic band structure and Phononic Weyl Nodal Straight Lines (PTWNLs) on their phonon spectrum, have received wide attentions on their novel properties. Up to date, no comparative studies have been investigated on their electronic structures, phonon spectrum, and electron phonon coupling (EPC) under the conditions of carrier doping and strain engineering. Here, we systemically investigate their above properties under carrier doping and strain engineering by first-principles calculations. The results show that the superconducting transition temperature T _c can be enhanced by electron doping and tensile strain. For AlB₂, the tensile strain of 6% can enhance T _c to 10.25?K and with the doping concentrate of 0.1 e^- per cell can enhance T _c reach to 9.89?K. Moreover, the physical quantities related to superconductivity of AlB₂ are more affected by carrier doping than MgB₂. Our results provide a theoretical reference to explore the correlation between electronic and phonon topological properties in AlB₂-type materials.     

Kohn anomaly in MgB2 by inelastic X-ray scattering

We study phonons in MgB2 using inelastic x-ray scattering (1.6 and 6 meV resolution). We clearly observe the softening and broadening of the crucial E(2g) mode through the Kohn anomaly along GammaM, in excellent agreement with ab initio calculations. Low temperature measurements (just above and below T(c)) show negligible changes for the momentum transfers investigated and no change in the E(2g) mode at A between room temperature and 16 K. We report the presence of a longitudinal mode along GammaA near in energy to the E(2g) mode that is not predicted by theory.     

Evidence for strong-coupling s-wave superconductivity in MgB2: (11)B NMR Study

We have investigated a gap structure in a newly discovered superconductor, MgB2, through measurement of the (11)B nuclear spin-lattice relaxation rate, (11)(1/T(1)). (11)(1/T(1)) is proportional to the temperature (T) in the normal state, and decreases exponentially in the superconducting (SC) state, revealing a tiny coherence peak just below T(c). The T dependence of 1/T(1) in the SC state can be accounted for by an s-wave SC model with a large gap size of 2Delta/k(B)T(c) approximately 5 which suggests it is in a strong-coupling regime.     

Critical magnetic field H c2 and electron scattering in MgB2

The correlation between the upper critical field H _c2 and the residual resistivity ρ of the MgB₂ compound is studied in a wide range of ρ values. The slope ?dH _c2/dT of the temperature dependence of H _c2 near T _c is found to increase steadily with increasing residual resistivity to 100 μΩ cm. Over the range from 0 to 50 μΩ cm, the dependence of ?dH _c2/dT on ρ is fitted well by a linear function, which is typical of a single-band superconductor. The fit is performed using the electronic parameters of the two bands (π and σ) that form the Fermi surface. The field H _c2 is assumed to depend on the electronic parameters of the σ band only. Using this approximation, the following quantities are estimated: the relative contribution of σ electrons to the total conductivity along the boron planes (which turns out to be about 1/2), the ratio of the mean free paths of electrons in the σ and π bands l _σ/l _π ≈ 1.5, and the ratios between some other parameters describing electron scattering.     

Synthesis and superconductivity of MgB2/Fe–Se–Te composites

MgB₂/FeSe_0.75Te_0.25 composites were synthesized to see the effect of the co-existence of superconducting phases on the superconductivity. MgB₂ powders were mixed with pre-alloyed FeSe_0.75Te_0.25 or elemental powder mixture, followed by sintering at 400 °C. The resulting materials were superconducting MgB₂/FeSe_0.75Te_0.25 composites without impurity phase when using pre-alloyed FeSe_0.75Te_0.25 as a starting powder. When FeSe_0.75Te_0.25 was prepared by in situ reaction from the elemental powder mixture of Fe–Se–Te, on the other hand, non-superconducting impurity phases partly formed after sintering. No double superconducting transition was observed in MgB₂/FeSe_0.75Te_0.25 composites in spite of coexistence of two superconducting phases.     

Raman scattering from a superconductivity-induced bound state in MgB2

It is shown that the sharp peak in the E(2g) Raman spectrum of superconducting MgB2 is due to a bound state caused by the electron-phonon coupling. Our theory explains why this peak appears only in the spectra with E(2g) symmetry and only in the sigma but not the pi bands. The properties of the bound state and the Raman spectrum are investigated, also in the presence of impurity scattering.