No abstract

MnBi2Te4/(Bi2Te3)n materials system has recently generated strong interest as a natural platform for realization of the quantum anomalous Hall (QAH) state. The system is magnetically much better ordered than substitutionally doped materials, however, the detrimental effects of certain disorders are becoming increasingly acknowledged. Here, from compiling structural, compositional, and magnetic metrics of disorder in ferromagnetic MnBi2Te4/(Bi2Te3)n it is found that migration of Mn between MnBi2Te4 septuple layers (SLs) and otherwise non-magnetic Bi2Te3 quintuple layers (QLs) has systemic consequences - it induces ferromagnetic coupling of Mn-depleted SLs with Mn-doped QLs, seen in ferromagnetic resonance as an acoustic and optical resonance mode of the two coupled spin subsystems. Even for a large SL separation (n ≳ 4 QLs) the structure cannot be considered as a stack of uncoupled two-dimensional layers. Angle-resolved photoemission spectroscopy and density functional theory studies show that Mn disorder within an SL causes delocalization of electron wave functions and a change of the surface band structure as compared to the ideal MnBi2Te4/(Bi2Te3)n. These findings highlight the critical importance of inter- and intra-SL disorder towards achieving new QAH platforms as well as exploring novel axion physics in intrinsic topological magnets.

The SOLARIS synchrotron located in Krakow, Poland, is a third-generation light source operating at medium electron energy. The first synchrotron light was observed in 2015, and the consequent development of infrastructure lead to the first users’ experiments at soft X-ray energies in 2018. Presently, SOLARIS expands its operation towards hard X-rays with continuous developments of the beamlines and concurrent infrastructure. In the following, we will summarize the SOLARIS synchrotron design, and describe the beamlines and research infrastructure together with the main performance parameters, upgrade, and development plans.

The possibility of inducing superconductivity in Weyl semimetal through coupling its surface to a superconductor was investigated. A single crystal of NbP, grown by chemical vapor transport method, was carefully characterized by XRD, EDX, SEM, ARPES techniques and by electron transport measurements. The mobility spectrum of the carriers was determined and it was found that there are four separate sharp peaks visible, which indicates that the carriers participating in the conductance have four different (almost discrete) mobilities. For the studies of interface transmission, the (001) surface of the crystal was covered by several hundred nm thick metallic layers of either Pb, or Nb, or In. DC current-voltage characteristics and AC differential conductance through the interfaces were investigated. The measurements were carried out as a function of temperature and the results were interpreted in terms of the modified Blonder-Tinkham-Klapwijk model. Upon cooling of the devices during which the metals become superconducting, qualitatively different behavior of each type of junctions was observed. The junctions with Nb and Pb show high interface transmission pointing out the Andreev reflection as a prevalent contribution to the subgap conductance. This also indicates that both Pb and Nb superconductors are promising candidates for further studies of the proximity-induced superconducting Weyl semimetals. However, in the case of In, an alloyed interface region is formed, which presumably excludes this metal from such studies.

We have performed electron transport and ARPES measurements on single crystals of transition metal dipnictide TaA$s_{2}$ cleaved along the ($\overline{2}$ 0 1) surface which has the lowest cleavage energy. A Fourier transform of the Shubnikov-de Haas oscillations shows four different peaks whose angular dependence was studied with respect to the angle between magnetic field and the [$\overline{2}$ 0 1] direction. The results indicate elliptical shape of the Fermi surface cross-sections. Additionally, a mobility spectrum analysis was carried out, which also reveals at least four types of carriers contributing to the conductance (two kinds of electrons and two kinds of holes). ARPES spectra were taken on freshly cleaved ($\overline{2}$ 0 1) surface and it was found that bulk states pockets at constant energy surface are elliptical, which confirms the magnetotransport angle dependent studies. First-principles calculations support the interpretation of the experimental results. The theoretical calculations better reproduce the ARPES data if the theoretical Fermi level is increased, which is due to a small n-doping of the samples. This shifts the Fermi level closer to the Dirac point, allowing investigating the physics of the Dirac and Weyl points, making this compound a platform for the investigation of the Dirac and Weyl points in three-dimensional materials.