Seminars

25 May 2022
  • David Ruiz Tijerina (UNAM)

    25 May 2022  4:00 pm - 5:00 pm
    https://zoom.us/j/276783458

    Black phosphorus belongs to the family of van der Waals materials, and can be exfoliated down to a monolayer, know as phosphorene. While attractive due to its strongly anisotropic bands and direct band gap, phosphorene presents many experimental challenges, especially for its use in twistronic heterostructures [1, 2], which has earned it much less attention from the twistronics community than other 2D semiconductors.

    In this seminar, I will argue that the experimental effort is worthwhile. I will present an effective model for the electronic spectrum of twisted phosphorene bilayers based on the interpolation scheme introduced by Ferreira et al. [3] and Magorrian et al. [4] Based on this model, we predict three twist angle regimes where the conduction- and valence states exhibit distinct geometries. Quantum-dot-like states form below 2°, producing a rectangular SU(2) Hubbard lattice. Above 2°, these states tunnel-couple along a single direction, producing effectively 1D states, suggestive of Luttinger physics. Finally, dispersive bands are recovered at large twist angles, with anisotropic mass ratios that depend strongly on the interlayer twist. This tunability across multiple physical regimes places twisted phosphorene bilayers among the most versatile twistronic heterostructures.

    [1] ACS Applied Nano Materials 2, 3138 (2019)

    [2] Nat. Commun. 12, 3947 (2021).

    [3] Appl. Phys. Lett. 118, 241602 (2021)

    [4] Phys. Rev. B 104, 125440 (2021).

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1 Jun 2022
  • Alexei E. Koshelev (Argonne National Laboratory)

    1 Jun 2022  4:00 pm - 5:00 pm
    https://zoom.us/j/276783458

    Influence of correlated magnetic fluctuations on parameters of magnetic superconductors

    Alexei E. Koshelev

    Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue,
    Lemont, Illinois 60439, USA

    Several superconducting materials host sublattices of rare-earth local magnetic moments weakly interacting with Cooper pairs. These moments may order inside superconducting state. In this case, emerging magnetic fluctuations suppress superconducting parameters. A notable example of such a material is recently discovered iron pnictide RbEuFe4As4 with the superconducting transition at 36.7 K and the magnetic transition at 15 K. As a model for this material, we consider a clean layered superconductor containing magnetic-moments sublattice in which a magnetic order establishes inside a superconducting state without destruction of superconductivity.  We investigate the corrections to the superconducting gap and London penetration depth caused by the weak exchange interactions of Cooper pairs with correlated magnetic fluctuations [1].  The influence of nonuniform exchange field on superconducting parameters is very sensitive to the relation between the magnetic correlation length, xh, and superconducting coherence length xs defining the 'scattering' (xh < xs) and 'smooth' (xh > xs) regimes.  We quantified this 'scattering-to-smooth' crossover for the case of quasi-two-dimensional magnetic fluctuations realized in RbEuFe4As4.  In the ‘scattering’ regime, the suppression of superconductivity is similar to the case of magnetic impurities [2] and the exchange corrections are proportional to the magnetic scattering rate, which grows ∝ xh until xh ≪ xs. In the opposite limit, when xh exceeds xs, smoothening of spatial variations of the exchange field strongly diminishes its effect on superconducting parameters leading to much weaker dependence of the corrections on xh. Moreover, the gap correction may even decrease with increasing of xh in the immediate vicinity of the magnetic transition if it is located at a temperature much lower than the superconducting transition. The crossover between the regimes occurs to be unexpectedly broad: the scattering approximation becomes inaccurate already when xh is substantially larger than xs. We applied the developed theoretical framework to modelling the observed behaviour of the London penetration depth extracted from the vortex imaging in RbEuFe4As4 [3].

    Acknowledgements: This work was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

    References:

    [1] A. E. Koshelev, “Suppression of superconducting parameters by correlated quasi-two-dimensional

    magnetic fluctuations”, Phys. Rev. B 100, 014518 (2019).

    [2] A. A. Abrikosov and L. P. Gor’kov, “Contribution to the theory of superconducting alloys with paramagnetic impurities”, Sov. Phys. JETP 12, 1243 (1961); S. Skalski, O. Betbeder-Matibet, and P. R. Weiss, “Properties of superconducting alloys containing paramagnetic impurities”, Phys. Rev. 136, A1500 (1964); V. G. Kogan, R. Prozorov, and V. Mishra, “London penetration depth and pair breaking”, Phys. Rev. B 88, 224508 (2013).

    [3] D. Collomb et al., “Observing the suppression of superconductivity in RbEuFe4As4 by correlated magnetic fluctuations”, Phys. Rev. Lett. 126, 157001 (2021).

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8 Jun 2022
15 Jun 2022
  • Thierry Jolicoeur (Paris-Saclay, CNRS)

    15 Jun 2022  4:00 pm - 5:00 pm

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22 Jun 2022