Evidence for Two Energy Gaps in High-Purity Superconducting Nb, Ta, and V

Shen, L. Y. L.; Senozan, N. M.; Phillips, N. E.

doi:10.1103/physrevlett.14.1025

Cited by 149 publications

(55 citation statements)

References 10 publications

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“…5,6 Although there have been indications of two, or multi-gap, effects in other materials, 7,8 it is in MgB 2 where the behavior has been most thoroughly investigated. Band structure calculations 9 have shown that the Fermi surface of MgB 2 consists of four sheets, mostly arising from the boron orbitals.…”

Section: Introductionmentioning

confidence: 99%

Damping of the de Haas–van Alphen oscillations in the superconducting state ofMgB2

et al. 2004

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The de Haas-van Alphen (dHvA) signal arising from orbits on the π Fermi surface sheet of the twogap superconductor MgB2 has been observed in the vortex state below Hc2. An extra attenuation of the dHvA signal, beyond those effects described in the conventional Lifshitz-Kosevich expression, is seen due to the opening of the superconducting gap. Our data show that the π band gap is still present up to Hc2. The data are compared to current theories of dHvA oscillations in the superconducting state which allow us to extract estimates for the evolution of the π band gap with magnetic field. Contrary to results for other materials, we find that the most recent theories dramatically underestimate the damping in MgB2.

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Section: Introductionmentioning

confidence: 99%

Damping of the de Haas–van Alphen oscillations in the superconducting state ofMgB2

et al. 2004

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“…Tu et al [21]) on MgB 2 samples of different forms showing that the smaller gap is a bulk property. Indirect observations of two gaps were also obtained from the temperature dependence of the specific heat in the conventional superconductors Nb, Ta and V [22] but to the best of our knowledge, a clear and direct observation for the presence of two gaps existing up to the same T c was never observed before by spectroscopic measurements. The possibility that these two gaps have different dimensionalities makes this system particularly attractive for further studies.…”

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confidence: 99%

Evidence for Two Superconducting Energy Gaps inMgB2by Point-Contact Spectroscopy

et al. 2001

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Experimental support is found for the multiband model of the superconductivity in the recently discovered system MgB2 with the transition temperature Tc = 39 K. By means of Andreev reflection evidence is obtained for two distinct superconducting energy gaps. The sizes of the two gaps (∆S = 2.8 meV and ∆L = 7 meV) are respectively smaller and larger than the expected weak coupling value. Due to the temperature smearing of the spectra the two gaps are hardly distinguishable at elevated temperatures but when a magnetic field is applied the presence of two gaps can be demonstrated close to the bulk Tc in the raw data.PACS numbers: 74.50.+r, 74.60.Ec, Two decades of the boom in the field of superconductivity has recently been boosted by the surprising discovery of superconductivity in MgB 2 [1]. In contrast to the cuprates, the first tunneling [2][3][4] and point-contact [5,6] spectroscopy measurements have unequivocally shown that this system is a s-wave superconductor and isotope effects [7,8] have pointed towards a phonon mechanism. However, the size of the superconducting energy gap has remained unclear. We report here on experimental support for the multiband model of superconductivity recently proposed by Liu et al. [9] thus showing that MgB 2 belongs to an original class of superconductors in which two distinct 2D and 3D Fermi surfaces contribute to superconductivity. Indeed, our point-contact spectroscopy experiments clearly show the existence of two distinct superconducting gaps with ∆ S (0) = 2.8 meV and ∆ L (0) = 7 meV. Both gaps close near to the bulk transition temperature T c = 39 K. Our measurements in magnetic field show directly in the raw data the presence of two superconducting gaps at all temperatures up to the same bulk transition T c indicating that the two gaps are inherent to the superconductivity in MgB 2 .Although quite scattered, the first spectroscopy measurements [2-6,10] yielded to superconducting gap values surprisingly smaller than the BCS weak coupling limit 2∆/kT c = 3.52. Moreover simultaneous topographic imaging and quasiparticle density of states mapping [11] revealed substantial inhomogeneities at the surface of the sample as well as a large scattering of the energy gap values measured at different parts of the polycrystalline sample (with ∆ ranging from 3 to 7.5 meV). This energy gap distribution can be caused by sample inhomogeneities. However, Giubileo et al. also observed a superposition of two gaps (∆ S (0) = 3.9 meV and ∆ L (0) = 7.5 meV) in some of their local tunneling spectra. The same inhomogeneity argument could of course also explain such a superposition but a much more attractive scenario would be a two-gap model. Such a model has been first developed by Suhl et al. [12] in the case of overlapping s-an d-bands in conventional superconductors (such as V, Nb, Ta). Experimental evidence for the existence of two band superconductivity was obtained by tunneling spectroscopy in Nb-doped SrTiO 3 [13]. A similar model has been recently proposed by Liu et al. for MgB 2 . It ...

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“…3) and possibly some s-d elements, 4,5 or if it exists more generally, but has not been paid sufficient attention in the past. One of the conditions for multigap superconductivity to occur is that more than one band should cross the Fermi level, a prerequisite that is commonly satisfied, e.g., in s-d metals.…”

Section: Introductionmentioning

confidence: 99%

Specific heat ofNb3Sn: The case for a second energy gap

Guritanu

Goldacker²,

Bouquet

et al. 2004

Phys. Rev. B

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We present specific-heat data for Nb 3 Sn, a well-known technically applied superconductor with a critical temperature T c Х 18 K, in the temperature range from 1.2 to 200 K in zero magnetic field, and from 1.5 to 22 K in fields H ഛ 16 T. The particularly dense and homogeneous polycrystalline sample used for this study is characterized in detail. We determine the bulk upper critical field H c2 ͑T͒ from specific-heat data, and the Sommerfeld constant ␥ from the entropy S͑T͒. We investigate in detail a low-temperature anomaly already noticed in previous investigations in zero field, and find that this feature can be quantitatively ascribed to the presence of a second superconducting gap 2⌬ S ͑0͒Х0.8k B T c , in addition to the main one 2⌬ L ͑0͒Х4.9k B T c . The signature of this minor gap, which affects 7.5% of the electronic density-of-states, vanishes in high fields.

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Evidence for Two Energy Gaps in High-Purity Superconducting Nb, Ta, and V

Cited by 149 publications

References 10 publications

Damping of the de Haas–van Alphen oscillations in the superconducting state ofMgB2

Damping of the de Haas–van Alphen oscillations in the superconducting state ofMgB2

Evidence for Two Superconducting Energy Gaps inMgB2by Point-Contact Spectroscopy

Specific heat ofNb3Sn: The case for a second energy gap

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