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Nonadiabatic coupling effects in MgB$_2$ reexamined

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 Added by Dino Novko
 Publication date 2018
  fields Physics
and research's language is English
 Authors Dino Novko




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The unusual Raman spectrum of MgB$_2$ and its formidable temperature dependence are successfully reproduced by means of a parameter-free emph{ab initio} nonadiabatic theory that accounts for the electron-hole pair scattering mechanisms with the system phonons. This example turns out to be a prototypical case where a strong nonadiabatic renormalization of the phonon frequency is partially washed out by the aforementioned scattering events, bringing along a characteristic temperature dependence. Both electron-hole pair lifetime and energy renormalization effects due to dynamical electron-phonon coupling turn out to play a crucial role. This theory could aid in comprehending other Raman spectra characterized with unconventionally strong electron-phonon interaction.



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84 - D. Novko , F. Caruso , C. Draxl 2019
The zone-center $E_{2g}$ modes play a crucial role in MgB$_2$, controlling the scattering mechanisms in the normal state as well the superconducting pairing. Here, we demonstrate via first-principles quantum-field theory calculations that, due to the anisotropic electron-phonon interaction, a $hot$-$phonon$ regime where the $E_{2g}$ phonons can achieve significantly larger effective populations than other modes, is triggered in MgB$_2$ by the interaction with an ultra-short laser pulse. Spectral signatures of this scenario in ultrafast pump-probe Raman spectroscopy are discussed in detail, revealing also a fundamental role of nonadiabatic processes in the optical features of the $E_{2g}$ mode.
We have studied the structural and superconducting properties of MgB$_2$ thin films made by pulsed laser deposition followed by in situ annealing. The cross-sectional transmission electron microscopy reveals a nanocrystalline mixture of textured MgO and MgB$_2$ with very small grain sizes. A zero-resistance transition temperature ($T_{c0}$) of 34 K and a zero-field critical current density ($J_c$) of $1.3 times 10^6$ A/cm$^2$ were obtained. The irreversibility field was $sim$ 8 T at low temperatures, although severe pinning instability was observed. These bulk-like superconducting properties show that the in situ deposition process can be a viable candidate for MgB$_2$ Josephson junction technologies.
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Ferroelectricity in hafnia is often regarded as a breakthrough discovery in ferroelectrics, potentially able to revolutionize the whole field. Despite increasing interests, a comprehensive understanding of the many factors driving the ferroelectric stabilization is still lacking. We here address the phase transition in terms of a Landau-theory-based approach, by analyzing symmetry-allowed distortions connecting the high-symmetry paraelectric tetragonal phase to the low-symmetry polar orthorhombic phase. By means of first-principles simulations, we find that the $Gamma_{3-}$ polar mode is only weakly unstable, whereas the other two symmetry-allowed distortions, non-polar Y$_{2+}$ and anti-polar Y$_{4-}$ are hard modes. None of the modes, taken alone or combined with one other mode, is able to drive the transition: the key factor in stabilizing the polar phase is identified as the strong trilinear coupling among the three modes. Furthermore, the experimentally acknowledged importance of substrate-induced effects in the growth of HfO$_2$ ferroelectric thin films, along with the lack of a clear order parameter in the transition, suggested the extension of our analysis to strain effects. Our findings suggest a complex behaviour of the Y$_{2+}$ mode, which become unstable under certain strain conditions and an overall unstable behaviour for the $Gamma_{3-}$ polar mode for all the strain states. A robust result emerges from our analysis: independently of the different applied strain (compressive or tensile, applied along orthorhombic axes), the need of a simultaneous excitation of the three coupled modes remain unaltered. Finally, when applied to mimic experimental growth conditions under strain, our analysis show a further stabilization of the ferroelectric phase with respect to the unstrained case, in agreeement with experimental findings.
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