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- ItemCrystal Growth of the Quasi-2D Quarternary Compound AgCrP2S6 by Chemical Vapor Transport(2021) Selter, Sebastian; Shemerliuk, Yuliia; Büchner, Bernd; Aswartham, SaicharanWe report optimized crystal growth conditions for the quarternary compound AgCrP2S6 by chemical vapor transport. Compositional and structural characterization of the obtained crystals were carried out by means of energy-dispersive X-ray spectroscopy and powder X-ray diffraction. AgCrP2S6 is structurally closely related to the M2P2S6 family, which contains several compounds that are under investigation as 2D magnets. As-grown crystals exhibit a plate-like, layered morphology as well as a hexagonal habitus. AgCrP2S6 crystallizes in monoclinic symmetry in the space group P2/a (No. 13). The successful growth of large high-quality single crystals paves the way for further investigations of low dimensional magnetism and its anisotropies in the future and may further allow for the manufacturing of few-layer (or even monolayer) samples by exfoliation.
- ItemTuning Magnetic and Transport Properties in Quasi-2D (Mn1−xNix)2P2S6 Single Crystals(Basel : MDPI, 2021) Shemerliuk, Yuliia; Zhou, Yonghui; Yang, Zhaorong; Cao, Gang; Wolter, Anja U.; Büchner, Bernd; Aswartham, SaicharanWe report an optimized chemical vapor transport method to grow single crystals of (Mn1−xNix)2P2S6 where x = 0, 0.3, 0.5, 0.7, and 1. Single crystals up to 4 mm × 3 mm × 200 μm were obtained by this method. As-grown crystals are characterized by means of scanning electron microscopy and powder X-ray diffraction measurements. The structural characterization shows that all crystals crystallize in monoclinic symmetry with the space group C2/m (No. 12). We have further investigated the magnetic properties of this series of single crystals. The magnetic measurements of the all as-grown single crystals show long-range antiferromagnetic order along all principal crystallographic axes. Overall, the Néel temperature TN is non-monotonous; with increasing Ni2+ doping, the temperature of the antiferromagnetic phase transition first decreases from 80 K for pristine Mn2P2S6 (x = 0) up to x = 0.5 and then increases again to 155 K for pure Ni2P2S6 (x = 1). The magnetic anisotropy switches from out-of-plane to in-plane as a function of composition in (Mn1−xNix)2P2S6 series. Transport studies under hydrostatic pressure on the parent compound Mn2P2S6 evidence an insulator-metal transition at an applied critical pressure of ~22 GPa.
- ItemSynthesis and Physical Properties of Iridium-Based Sulfide Ca1−xIr4S6(S2) [x = 0.23–0.33](Basel : MDPI, 2022) Vogl, Michael; Valldor, Martin; Piening, Roman Boy; Efremov, Dmitri V.; Büchner, Bernd; Aswartham, SaicharanWe present the synthesis and characterization of the iridium-based sulfide Ca1−xIr4S6(S2). Quality and phase analysis were conducted by means of energy-dispersive X-ray spectroscopy (EDXS) and powder X-ray diffraction (XRD) techniques. Structure analysis reveals a monoclinic symmetry with the space group C 1 2/m 1 (No. 12), with the lattice constants a = 15.030 (3) Å, b = 3.5747 (5) Å and c = 10.4572 (18) Å. Both X-ray diffraction and EDXS suggest an off-stoichiometry of calcium, leading to the empirical composition Ca1−xIr4.0S6(S2) [x = 0.23–0.33]. Transport measurements show metallic behavior of the compound in the whole range of measured temperatures. Magnetic measurements down to 1.8 K show no long range order, and Curie–Weiss analysis yields θCW = −31.4 K, suggesting that the compound undergoes a magnetic state with short range magnetic correlations. We supplement our study with calculations of the band structure in the framework of the density functional theory.
- ItemElastoresistivity of Heavily Hole-Doped 122 Iron Pnictide Superconductors(Lausanne : Frontiers Media, 2022) Hong, Xiaochen; Sykora, Steffen; Caglieris, Federico; Behnami, Mahdi; Morozov, Igor; Aswartham, Saicharan; Grinenko, Vadim; Kihou, Kunihiro; Lee, Chul-Ho; Büchner, Bernd; Hess, ChristianNematicity in heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from magnetic instability were declared in RbFe2As2 and its sister compounds. Here, we report a systematic elastoresistance study of a series of isovalent- and electron-doped KFe2As2 crystals. We found divergent elastoresistance on cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven toward a Lifshitz transition. However, we conclude that none of them necessarily indicates an independent nematic critical point. Instead, the increased nematicity can be associated with another electronic criticality. In particular, we propose a mechanism for how elastoresistivity is enhanced at a Lifshitz transition.
- ItemStrong surface termination dependence of the electronic structure of polar superconductor LaFeAsO revealed by nano-ARPES([London] : IOP, 2022) Jung, Sung Won; Rhodes, Luke C; Watson, Matthew D; Evtushinsky, Daniil V; Cacho, Cephise; Aswartham, Saicharan; Kappenberger, Rhea; Wurmehl, Sabine; Büchner, Bernd; Kim, Timur KThe electronic structures of the iron-based superconductors have been intensively studied by using angle-resolved photoemission spectroscopy (ARPES). A considerable amount of research has been focused on the LaFeAsO family, showing the highest transition temperatures, where previous ARPES studies have found much larger Fermi surfaces than bulk theoretical calculations would predict. The discrepancy has been attributed to the presence of termination-dependent surface states. Here, using photoemission spectroscopy with a sub-micron focused beam spot (nano-ARPES) we have successfully measured the electronic structures of both the LaO and FeAs terminations in LaFeAsO. Our data reveal very different band dispersions and core-level spectra for different surface terminations, showing that previous macro-focus ARPES measurements were incomplete. Our results give direct evidence for the surface-driven electronic structure reconstruction in LaFeAsO, including formation of the termination-dependent surface states at the Fermi level. This experimental technique, which we have shown to be very powerful when applied to this prototypical compound, can now be used to study various materials with different surface terminations.
- ItemDisorder-induced coupling of Weyl nodes in WTe2(2020) Sykora, Steffen; Schoop, Johannes; Graf, Lukas; Shipunov, Grigory; Morozov, Igor V.; Aswartham, Saicharan; Büchner, Bernd; Hess, Christian; Giraud, Romain; Dufouleur, JosephThe finite coupling between Weyl nodes due to residual disorder is investigated by magnetotransport studies in WTe2. The anisotropic scattering of quasiparticles is evidenced from classical and quantum transport measurements. A theoretical approach using the real band structure is developed in order to calculate the dependence of the scattering anisotropy with the correlation length of the disorder. A comparison between theory and experiments reveals a short correlation length in WTe2 (ξ∼5 nm). This result implies a significant coupling between Weyl nodes and other bands. Our study thus shows that a finite intercone scattering rate always exists in weakly disordered type-II Weyl semimetals, such as WTe2, which strongly suppresses topologically nontrivial properties.
- ItemSixfold fermion near the Fermi level in cubic PtBi2(Amsterdam : SciPost Foundation, 2021) Thirupathaiah, Setti; Kushnirenko, Yevhen; Koepernik, Klaus; Piening, Boy Roman; Büchner, Bernd; Aswartham, Saicharan; van den Brink, Jeroen; Borisenko, Sergey; Fulga, Ion CosmaWe show that the cubic compound PtBi2, is a topological semimetal hosting a sixfold band touching point in close proximity to the Fermi level. Using angle-resolved photoemission spectroscopy, we map the bandstructure of the system, which is in good agreement with results from density functional theory. Further, by employing a low energy effective Hamiltonian valid close to the crossing point, we study the effect of a magnetic field on the sixfold fermion. The latter splits into a total of twenty Weyl cones for a Zeeman field oriented in the diagonal, [111] direction. Our results mark cubic PtBi2, as an ideal candidate to study the transport properties of gapless topological systems beyond Dirac and Weyl semimetals.