Anisotropic superconductivity and the spin-vortex antiferromagnetism in Ni-doped CaKFe4As4
(Laboratorio de Bajas Temperaturas, Departamento de Física de la Materia Condensada, Instituto de Ciencia de Materiales Nicolás Cabrera, Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid)
Quasiparticle interference and vortex imaging have both been shown to be powerful tools to investigate pnictide superconductors. Here I will review new results in the recently discovered family of 1144 materials, particularly in pure and Ni-doped CaKFe4As4. The 1144 CaKFe4As4 compound is a pnictide superconducting material showing optimal superconducting critical temperature with Tc as large as 38 K [1,2]. There are no signatures of nematic, structural or magnetic transitions. Doping with Ni induces a decrease in Tc and the appearance of magnetism. Instead of the stripe like spin density wave (SSDW) antiferromagnetic order present in the 122 systems, Ni-doped CaKFe4As4 shows a spin vortex (or hedgehog) magnetic order . Here I will discuss scanning tunneling microscopy experiments in CaK(Fe0.95Ni0.05)4As4 . We have determined the superconducting density of states and observed the vortex lattice. Quasiparticle interference measurements show the reconstruction of the Fermi surface due the presence of hedgehog magnetic order and the opening of an anisotropic superconducting gap, giving experimental evidence for the coexistence of both states. Finally, I will show the recent efforts made in STM for very high magnetic fields, presenting first spectroscopic data taken at 20 T.
 K. Cho, A. Fente et al., Phys. Rev. B 95, 100502(R) (2017).
 A. Fente et al., Phys. Rev. B 97, 134501 (2018).
 W.R. Meier et al., npj Quantum Materials 3, 5 (2018).
 J. Benito-Llorens et al., Phys. Rev. B 103, L060506 (2021).
Non‑Gaussian tail in the force distribution: a hallmark of correlated disorder in the host
media of elastic objects
(Low Temperature Lab & Instituto Balseiro, Centro Atómico Bariloche, Argentina)
Coworkers: Jazmín Aragón Sánchez, Gonzalo Rumi, Raúl Cortés Maldonado,
Néstor René Cejas Bolece, Joaquín Puig, Pablo Pedrazzini, Gladys Nieva, Moira I. Dolz,
Marcin Konczykowski, Cornelis J. van der Beek, & Alejandro B. Kolton
Inferring the nature of disorder in the media where elastic objects are nucleated is of crucial
importance for many applications but remains a challenging basic-science problem. Here we
propose a method to discern whether weak-point or strong-correlated disorder dominates based on
characterizing the distribution of the interaction forces between objects mapped in large fields-of view.
We illustrate our proposal with the case-study system of vortex structures nucleated in type-II
superconductors with different pinning landscapes. Interaction force distributions are computed from
individual vortex positions imaged in thousands-vortices fields-of-view in a two-orders-of-magnitude wide
vortex-density range. Vortex structures nucleated in point-disordered media present Gaussian
distributions of the interaction force components. In contrast, if the media have dilute and randomly distributed
correlated disorder, these distributions present non-Gaussian algebraically-decaying tails
for large force magnitudes. We propose that detecting this deviation from the Gaussian behavior is a
fingerprint of strong disorder, in our case originated from a dilute distribution of correlated pinning
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