de Haas–van Alphen effect in MgB2 crystals

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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“…• , as measured on a large number of other crystals 38 (the minimum frequency varies approximately as 1/ cos φ).…”

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confidence: 96%

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

et al. 2004

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“…For AN217 the jump is ∼ 3%, which results from the much larger addenda relative to the contribution from this tiny sample, rather than a reduced superconducting volume fraction. Quantum oscillations were measured using piezoresistive cantilevers 10 connected to a room temperature a.c. Wheatstone bridge. The torque is related to the change in bridge resistance by a factor ≈ 10 −10 NmΩ −1 , with a noise level roughly corresponding to a torque of ∼ 10 −14 Nm.…”

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de Haas–van Alphen effect investigation of the electronic structure of Al-substitutedMgB2

et al. 2005

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We report a de Haas-van Alphen (dHvA) study of the electronic structure of Al doped crystals of MgB2. We have measured crystals with ∼ 7.5% Al which have a Tc of 33.6 K, (∼ 14% lower than pure MgB2). dHvA frequencies for the σ tube orbits in the doped samples are lower than in pure MgB2, implying a 16 ± 2% reduction in the number of holes in this sheet of Fermi surface. The mass of the quasiparticles on the larger σ orbit is lighter than the pure case indicating a reduction in electron-phonon coupling constant λ. These observations are compared with band structure calculations, and found to be in excellent agreement.

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“…26 Taking these values and E 2g ϭ620 cm Ϫ1 gives T c ϭ43 K. For the films of lower T c a lower value of must be used if the T c found from Eq. ͑1͒ is to agree with that found experimentally.…”

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Disorder-induced collapse of the electron-phonon coupling inMgB2observed by Raman spectroscopy

et al. 2003

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The Raman spectrum of the superconductor MgB 2 has been measured as a function of the T c of the film. A striking correlation is observed between the T c onset and the frequency of the E 2g mode. Analysis of the data with the McMillan formula provides clear experimental evidence for the collapse of the electron-phonon coupling at the temperature predicted for the convergence of two superconducting gaps into one observable gap. This gives indirect evidence of the convergence of the two gaps and direct evidence of a transition to an Since the discovery of superconductivity in MgB 2 ͑Ref. 1͒, a large volume of work has been directed towards developing and improving the quality of MgB 2 thin films. 2 The temperature of the onset of superconductivity (T c ) in these films has varied considerably, although there is no consensus on the mechanism behind the T c suppression. Nonetheless, theoretical models suggest that a two-band superconductivity model is needed to explain the high T c in MgB 2 . 3-6 As interband scattering increases perhaps due to impurities in the boron layer, the two gaps in MgB 2 are predicted to converge into one gap with ⌬ ϭ⌬ ϭ4.1 meV; 6 this transition is predicted to occur as the T c is suppressed below 25 K. 6 Measurements of the phonon energies in YBa 2 Cu 3 O 7Ϫ␦ helped the understanding of the relation between the crystal structure and the T c in this material. 7 Therefore, similar measurements made on MgB 2 films with different T c 's may help to elucidate the mechanisms behind the suppression of the critical temperatures in MgB 2 , while casting light on the nature of the two-band superconductivity.Numerical calculations indicate that in MgB 2 there are two phonon modes involving vibrations along the c axis (A 2u and B 1g ) and two doubly degenerate modes with vibrations only in the ab plane (E 1u and E 2g ); 8 for the P 6 mmm space group to which MgB 2 belongs, only the E 2g mode is Raman active. 9 Predictions for the frequency of the E 2g mode vary between 515 cm Ϫ1 ͑64 meV͒ ͑Ref. 3͒ and 665 cm Ϫ1 ͑82 meV͒, 10 while experimental studies have shown a broad Raman mode between 600 cm Ϫ1 and 630 cm Ϫ1 . 9,11-18 The consensus is that this mode is the E 2g mode, however, there remains controversy about this assignment, 9,14 with Kunc et al. 9 concluding that this mode was due to a ''contaminant phase at the sample surface.'' In this communication, Raman spectroscopy has been used to characterize the quality of MgB 2 films both in terms of impurity phases and disorder. The effects of a suppressed T c on the phonon mode at ϳ600 cm Ϫ1 were studied. Both magnesium stoichiometry and oxygen contamination may play a role in determining the T c . 19,20 It has been shown previously that as-grown films of MgB 2 can have significant magnesium concentration gradients through the thickness of the film. 21This effect is equally likely where a precursor boron film has been annealed in magnesium vapor. Therefore, measurement of the superconducting properties of a post annealed boron film through the thickness...

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de Haas–van Alphen effect in MgB2 crystals

Cited by 26 publications

References 21 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

de Haas–van Alphen effect investigation of the electronic structure of Al-substitutedMgB2

Disorder-induced collapse of the electron-phonon coupling inMgB2observed by Raman spectroscopy

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