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Yan B.,Max Planck Institute for Chemical Physics of Solids | Felser C.,Max Planck Institute for Chemical Physics of Solids
Annual Review of Condensed Matter Physics | Year: 2017

Topological insulators and topological semimetals are both new classes of quantum materials, which are characterized by surface states induced by the topology of the bulk band structure. Topological Dirac or Weyl semimetals show linear dispersion around nodes, termed the Dirac or Weyl points, as the three-dimensional analog of graphene. We review the basic concepts and compare these topological states of matter from the materials perspective with a special focus on Weyl semimetals. The TaAs family is the ideal materials class to introduce the signatures of Weyl points in a pedagogical way, from Fermi arcs to the chiral magnetotransport properties, followed by hunting for the type-II Weyl semimetals in WTe2, MoTe2, and related compounds. Many materials are members of big families, and topological properties can be tuned. As one example, we introduce the multifunctional topological materials, Heusler compounds, in which both topological insulators and magnetic Weyl semimetals can be found. Instead of a comprehensive review, this article is expected to serve as a helpful introduction and summary by taking a snapshot of the quickly expanding field. © 2017 by Annual Reviews. All rights reserved.

Sun P.,CAS Institute of Physics | Steglich F.,Max Planck Institute for Chemical Physics of Solids
Physical Review Letters | Year: 2013

A distinctly temperature-dependent Nernst coefficient, ν, which is strongly enhanced over that of LaCu2Si2, is observed between T=2 and 300 K for CeCu2Si2 and Ce 0.8La0.2Cu2Si2. The enhanced ν(T) is determined by the asymmetry of the on-site Kondo (conduction electron -4f electron) scattering rate. Taking into account the measured Hall mobility, μH, the highly unusual thermopower, S, of these systems can be semiquantitatively described by S(T)=-ν(T)/μH(T), which explicitly demonstrates that the thermopower originates from the local Kondo scattering process over a wide temperature range from far above to well below the coherence temperature (≈20 K for CeCu2Si2). Our results suggest that the Nernst effect can act as a proper probe of local charge-carrier scattering. This promises to impact on exploring the unconventional enhancement of the thermopower in correlated materials suited for potential applications. © 2013 American Physical Society.

Thalmeier P.,Max Planck Institute for Chemical Physics of Solids | Takimoto T.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We discuss the conclusions on the symmetry of hidden order (HO) in URu 2Si2 that may be drawn from recent torque experiments in a rotating magnetic field by Okazaki (to be published). They are very sensitive to changes in the magnetic susceptibility induced by HO. We show that the observed twofold angular torque oscillations give evidence that HO has degenerate E-type (yz, zx) symmetry where both components are realized. The oscillations have the wrong characteristics or are absent for the one-dimensional (1D) nontrivial representations like quadrupolar B 1(x2-y2) and B2(xy) type HO or hexadecapolar A 2[xy(x2-y2)] type HO. Therefore, they may be excluded as candidates for HO. We also predict the field-angular variation of possible field-induced Bragg peaks based on the underlying E-type order parameter and discuss the expected elastic constant anomalies. © 2011 American Physical Society.

Wu S.-C.,Max Planck Institute for Chemical Physics of Solids | Shan G.,Max Planck Institute for Chemical Physics of Solids | Yan B.,Max Planck Institute for Chemical Physics of Solids | Yan B.,Max Planck Institute for the Physics of Complex Systems
Physical Review Letters | Year: 2014

Recently, the long-sough quantum anomalous Hall effect was realized in a magnetic topological insulator. However, the requirement of an extremely low temperature (approximately 30 mK) hinders realistic applications. Based on ab initio band structure calculations, we propose a quantum anomalous Hall platform with a large energy gap of 0.34 and 0.06 eV on honeycomb lattices comprised of Sn and Ge, respectively. The ferromagnetic (FM) order forms in one sublattice of the honeycomb structure by controlling the surface functionalization rather than dilute magnetic doping, which is expected to be visualized by spin polarized STM in experiment. Strong coupling between the inherent quantum spin Hall state and ferromagnetism results in considerable exchange splitting and, consequently, an FM insulator with a large energy gap. The estimated mean-field Curie temperature is 243 and 509 K for Sn and Ge lattices, respectively. The large energy gap and high Curie temperature indicate the feasibility of the quantum anomalous Hall effect in the near-room-temperature and even room-temperature regions. © 2014 American Physical Society.

Thalmeier P.,Max Planck Institute for Chemical Physics of Solids
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

In the skutterudite compounds the anharmonic "rattling" oscillations of 4f -guest ions in the surrounding Sb12 host cages are found to have significant influence on the low-temperature properties. Recently specific-heat analysis of Pr ( Os1-x Rux) 4 Sb12 has shown that the energy of crystalline electric field singlet-triplet excitations increases strongly with Ru concentration x and crosses the almost constant rattling mode frequency ω0 at about x=0.65. Due to magnetoelastic interactions this may entail prominent nonadiabatic effects in inelastic neutron-scattering intensity and quadrupolar susceptibility. Furthermore the Ru-concentration dependence of the superconducting Tc, notably the minimum at intermediate x is explained as a crossover effect from pair-forming aspherical Coulomb scattering to pair-breaking exchange scattering. © 2010 The American Physical Society.

Akselrud L.,Max Planck Institute for Chemical Physics of Solids | Grin Y.,Max Planck Institute for Chemical Physics of Solids
Journal of Applied Crystallography | Year: 2014

The fourth version of the program package WinCSD is multi-purpose computer software for crystallographic calculations using single-crystal and powder X-ray and neutron diffraction data. The software environment and the graphical user interface are built using the platform of the Microsoft.NET Framework, which grants independence from changing Windows operating systems and allows for transferring to other operating systems. Graphic applications use the three-dimensional OpenGL graphics language. WinCSD covers the complete spectrum of crystallographic calculations, including powder diffraction pattern deconvolution, crystal structure solution and refinement in 3 + d space, refinement of the multipole model and electron density studies from diffraction data, and graphical representation of crystallographic information. © 2014.

Felser C.,Max Planck Institute for Chemical Physics of Solids
Angewandte Chemie - International Edition | Year: 2013

Topologically stable magnetic screw-like nanostructures called skyrmions were designed by using the concept of topology and the guidance of theory. These particles in real space are found in non-centrosymmetric compounds such as MnSi. Skyrmions might have an enormous impact in the context of future spintronics applications. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Sun Y.,Max Planck Institute for Chemical Physics of Solids
Nature Materials | Year: 2015

Topological Weyl semimetals (TWSs) represent a novel state of topological quantum matter which not only possesses Weyl fermions (massless chiral particles that can be viewed as magnetic monopoles in momentum space) in the bulk and unique Fermi arcs generated by topological surface states, but also exhibits appealing physical properties such as extremely large magnetoresistance and ultra-high carrier mobility. Here, by performing angle-resolved photoemission spectroscopy (ARPES) on NbP and TaP, we directly observed their band structures with characteristic Fermi arcs of TWSs. Furthermore, by systematically investigating NbP, TaP and TaAs from the same transition metal monopnictide family, we discovered their Fermiology evolution with spin–orbit coupling (SOC) strength. Our experimental findings not only reveal the mechanism to realize and fine-tune the electronic structures of TWSs, but also provide a rich material base for exploring many exotic physical phenomena (for example, chiral magnetic effects, negative magnetoresistance, and the quantum anomalous Hall effect) and novel future applications. © 2015 Nature Publishing Group

Akbari A.,Max Planck Institute for Chemical Physics of Solids | Thalmeier P.,Max Planck Institute for Chemical Physics of Solids
Physical Review Letters | Year: 2012

The heavy fermion metal CeB 6 exhibits a hidden order of the antiferroquadrupolar (AFQ) type below T Q=3.2K and a subsequent antiferromagnetic (AFM) order at T N=2.3K. It was interpreted as an ordering of the quadrupole and dipole moments of a Γ 8 quartet of localized Ce 4f1 electrons. This established picture has been profoundly shaken by recent inelastic neutron scattering (G. Friemel, arXiv:1111.4151) that found the evolution of a feedback spin exciton resonance within the hidden order phase at the AFQ wave vector which is stabilized by the AFM order. We develop an alternative theory based on a fourfold degenerate Anderson lattice model, including both order parameters as particle-hole condensates of itinerant heavy quasiparticles. This explains in a natural way the appearance of the spin exciton resonance and the momentum dependence of its spectral weight, in particular, around the AFQ vector and its rapid disappearance in the disordered phase. Analogies to the feedback effect in unconventional heavy fermion superconductors are pointed out. © 2012 American Physical Society.

Thalmeier P.,Max Planck Institute for Chemical Physics of Solids
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

The effect of helical Dirac states on surface phonons in a topological insulators is investigated. Their coupling is derived in the continuum limit by assuming displacement-dependent Dirac cones. The resulting renormalization of sound velocity and attenuation and its dependence on the chemical potential and wave vector is calculated. At finite wave vectors a Kohn anomaly in the renormalized phonon frequency is caused by intraband transitions. It appears at wave vectors q<2k F due to a lack of backscattering for helical Dirac electrons. The wave vector and chemical potential dependence of this anomaly is calculated. © 2011 American Physical Society.

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