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- ItemStrong and Weak 3D Topological Insulators Probed by Surface Science Methods(Weinheim : Wiley-VCH, 2020) Morgenstern, Markus; Pauly, Christian; Kellner, Jens; Liebmann, Marcus; Pratzer, Marco; Eschbach, Markus; Plucinski, Lukacz; Otto, Sebastian; Rasche, Bertold; Ruck, Michael; Richter, Manuel; Just, Sven; Lüpke, Felix; Voigtländer, BertThe contributions of surface science methods to discover and improve 3D topological insulator materials are reviewed herein, illustrated with examples from the authors’ own work. In particular, it is demonstrated that spin-polarized angular-resolved photoelectron spectroscopy is instrumental to evidence the spin-helical surface Dirac cone, to tune its Dirac point energy toward the Fermi level, and to discover novel types of topological insulators such as dual ones or switchable ones in phase change materials. Moreover, procedures are introduced to spatially map potential fluctuations by scanning tunneling spectroscopy and to identify topological edge states in weak topological insulators. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemTunable chirality of noncentrosymmetric magnetic Weyl semimetals in rare-earth carbides([London] : Nature Publishing Group, 2022) Ray, Rajyavardhan; Sadhukhan, Banasree; Richter, Manuel; Facio, Jorge I.; van den Brink, JeroenEven if Weyl semimetals are characterized by quasiparticles with well-defined chirality, exploiting this experimentally is severely hampered by Weyl lattice fermions coming in pairs with opposite chirality, typically causing the net chirality picked up by experimental probes to vanish. Here, we show this issue can be circumvented in a controlled manner when both time-reversal- and inversion symmetry are broken. To this end, we investigate chirality disbalance in the carbide family RMC2 (R a rare-earth and M a transition metal), showing several members to be Weyl semimetals. Using the noncentrosymmetric ferromagnet NdRhC2 as an illustrating example, we show that an odd number of Weyl nodes can be stabilized at its Fermi surface by properly tilting its magnetization. The chiral configuration endows a topological phase transition as the Weyl node transitions across the Fermi sheets, which triggers interesting chiral electromagnetic responses. Further, the tilt direction determines the sign of the resulting net chirality, opening up a simple route to control its sign and strength.
- ItemMagnetic-field- and temperature-dependent fermi surface of CeBiPt(Milton Park : Taylor & Francis, 2006) Wosnitza, J.; Goll, G.; Bianchi, A.D.; Bergk, B.; Kozlova, N.; Opahle, I.; Elgazzar, S.; Richter, Manuel; Stockert, O.; Löhneysen, H.V.; Yoshino, T.; Takabatake, T.The half-Heusler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov–de Haas (SdH) and Hall-effect measurements. Neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below TN = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field and stoichiometry. Above a certain, sample-dependent, threshold field (B>25 T), the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4f compound LaBiPt. Electronic-band-structure calculations can well explain the observed behaviour by a 4f-polarization-induced Fermi-surface modification.
- ItemElectric-field control of surface magnetic anisotropy: A density functional approach(Milton Park : Taylor & Francis, 2009) Zhang, Hongbin; Richter, Manuel; Koepernik, Klaus; Opahle, Ingo; Tasnádi, Ferenc; Eschrig, HelmutIn a recent experiment, Weisheit et al (2007 Science 315 349) demonstrated that the coercivity of thin L10 FePt and FePd films can be modified by the external electric field in an electrochemical environment. Here, this observation is confirmed by density functional calculations for the intrinsic magnetic anisotropy. The origin of the effect is clarified by means of a general and simple method to simulate charged metal surfaces. It is predicted that the coercivity of thin CoPt films is much more susceptible to electric field than that of FePt films.
- ItemCreating Weyl nodes and controlling their energy by magnetization rotation(College Park, ML : American Physical Society, 2020) Ghimire, Madhav Prasad; Facio, Jorge I.; You, Jhih-Shih; Ye, Linda; Checkelsky, Joseph G.; Fang, Shiang; Kaxiras, Efthimios; Richter, Manuel; van den Brink, JeroenAs they do not rely on the presence of any crystal symmetry, Weyl nodes are robust topological features of an electronic structure that can occur at any momentum and energy. Acting as sinks and sources of Berry curvature, Weyl nodes have been predicted to strongly affect the transverse electronic response, like in the anomalous Hall or Nernst effects. However, to observe large anomalous effects the Weyl nodes need to be close to or at the Fermi level, which implies the band structure must be tuned by an external parameter, e.g., chemical doping. Here we show that in a ferromagnetic metal tuning of the Weyl node energy and momentum can be achieved by rotation of the magnetization. First, taking as example the elementary magnet hcp-Co, we use electronic structure calculations based on density-functional theory to show that by canting the magnetization away from the easy axis, Weyl nodes can be driven exactly to the Fermi surface. Second, we show that the same phenomenology applies to the kagome ferromagnet Co3Sn2S2, in which we additionally show how the dynamics in energy and momentum of the Weyl nodes affects the calculated anomalous Hall and Nernst conductivities. Our results highlight how the intrinsic magnetic anisotropy can be used to engineer Weyl physics.