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Author: Zabolotnyy, Volodymyr B. ×

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    Intermixing-Driven Surface and Bulk Ferromagnetism in the Quantum Anomalous Hall Candidate MnBi6Te10
    (Weinheim : Wiley-VCH, 2023) Tcakaev, Abdul‐Vakhab; Rubrecht, Bastian; Facio, Jorge I.; Zabolotnyy, Volodymyr B.; Corredor, Laura T.; Folkers, Laura C.; Kochetkova, Ekaterina; Peixoto, Thiago R. F.; Kagerer, Philipp; Heinze, Simon; Bentmann, Hendrik; Green, Robert J.; Gargiani, Pierluigi; Valvidares, Manuel; Weschke, Eugen; Haverkort, Maurits W.; Reinert, Friedrich; van den Brink, Jeroen; Büchner, Bernd; Wolter, Anja U. B.; Isaeva, Anna; Hinkov, Vladimir
    The recent realizations of the quantum anomalous Hall effect (QAHE) in MnBi2Te4 and MnBi4Te7 benchmark the (MnBi2Te4)(Bi2Te3)n family as a promising hotbed for further QAHE improvements. The family owes its potential to its ferromagnetically (FM) ordered MnBi2Te4 septuple layers (SLs). However, the QAHE realization is complicated in MnBi2Te4 and MnBi4Te7 due to the substantial antiferromagnetic (AFM) coupling between the SLs. An FM state, advantageous for the QAHE, can be stabilized by interlacing the SLs with an increasing number n of Bi2Te3 quintuple layers (QLs). However, the mechanisms driving the FM state and the number of necessary QLs are not understood, and the surface magnetism remains obscure. Here, robust FM properties in MnBi6Te10 (n = 2) with Tc ≈ 12 K are demonstrated and their origin is established in the Mn/Bi intermixing phenomenon by a combined experimental and theoretical study. The measurements reveal a magnetically intact surface with a large magnetic moment, and with FM properties similar to the bulk. This investigation thus consolidates the MnBi6Te10 system as perspective for the QAHE at elevated temperatures.
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    Valence-state reflectometry of complex oxide heterointerfaces
    (London : Nature Publishing Group, 2016) Hamann-Borrero, Jorge E.; Macke, Sebastian; Choi, Woo Seok; Sutarto, Ronny; He, Feizhou; Radi, Abdullah; Elfimov, Ilya; Green, Robert J.; Haverkort, Maurits W.; Zabolotnyy, Volodymyr B.; Lee, Ho Nyung; Sawatzky, George A.; Hinkov, Vladimir
    Emergent phenomena in transition-metal-oxide heterostructures such as interface superconductivity and magnetism have been attributed to electronic reconstruction, which, however, is difficult to detect and characterise. Here we overcome the associated difficulties to simultaneously address the electronic degrees of freedom and distinguish interface from bulk effects by implementing a novel approach to resonant X-ray reflectivity (RXR). Our RXR study of the chemical and valance profiles along the polar (001) direction of a LaCoO3 film on NdGaO3 reveals a pronounced valence-state reconstruction from Co3+ in the bulk to Co2+ at the surface, with an areal density close to 0.5 Co2+ ions per unit cell. An identical film capped with polar (001) LaAlO3 maintains the Co3+ valence over its entire thickness. We interpret this as evidence for electronic reconstruction in the uncapped film, involving the transfer of 0.5e− per unit cell to the subsurface CoO2 layer at its LaO-terminated polar surface.
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    One-sign order parameter in iron based superconductor
    (Basel : MDPI, 2012) Borisenko, Sergey V.; Zabolotnyy, Volodymyr B.; Kordyuk, Alexnader A.; Evtushinsky, Danil V.; Kim, Timur K.; Morozov, Igor V.; Follath, Rolf; Büchner, Bernd
    The onset of superconductivity at the transition temperature is marked by the onset of order, which is characterized by an energy gap. Most models of the iron-based superconductors find a sign-changing (s±) order parameter [1–6], with the physical implication that pairing is driven by spin fluctuations. Recent work, however, has indicated that LiFeAs has a simple isotropic order parameter [7–9] and spin fluctuations are not necessary [7,10], contrary to the models [1–6]. The strength of the spin fluctuations has been controversial [11,12], meaning that the mechanism of superconductivity cannot as yet be determined. We report the momentum dependence of the superconducting energy gap, where we find an anisotropy that rules out coupling through spin fluctuations and the sign change. The results instead suggest that orbital fluctuations assisted by phonons [13,14] are the best explanation for superconductivity.