 25 May 2022


David Ruiz Tijerina (UNAM)
25 May 2022 4:00 pm  5:00 pm
https://zoom.us/j/276783458Black 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. Quantumdotlike states form below 2°, producing a rectangular SU(2) Hubbard lattice. Above 2°, these states tunnelcouple 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).

 1 Jun 2022


Alexei E. Koshelev (Argonne National Laboratory)
1 Jun 2022 4:00 pm  5:00 pm
https://zoom.us/j/276783458Influence 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, USASeveral superconducting materials host sublattices of rareearth 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 RbEuFe_{4}As_{4} 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 magneticmoments 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, x_{h}, and superconducting coherence length x_{s} defining the 'scattering' (x_{h} < x_{s}) and 'smooth' (x_{h} > x_{s}) regimes. We quantified this 'scatteringtosmooth' crossover for the case of quasitwodimensional magnetic fluctuations realized in RbEuFe_{4}As_{4}. 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 ∝ x_{h} until x_{h} ≪ x_{s}. In the opposite limit, when x_{h} exceeds x_{s}, smoothening of spatial variations of the exchange field strongly diminishes its effect on superconducting parameters leading to much weaker dependence of the corrections on x_{h}. Moreover, the gap correction may even decrease with increasing of x_{h} 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 x_{h} is substantially larger than x_{s}. We applied the developed theoretical framework to modelling the observed behaviour of the London penetration depth extracted from the vortex imaging in RbEuFe_{4}As_{4} [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 quasitwodimensional
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. BetbederMatibet, 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 RbEuFe_{4}As_{4} by correlated magnetic fluctuations”, Phys. Rev. Lett. 126, 157001 (2021).

 8 Jun 2022

 15 Jun 2022

 22 Jun 2022
