Topological Fermi-arc surface resonances in bcc iron

The topological classification of matter has been extended to include semimetallic phases characterized by the presence of topologically protected band degeneracies. In Weyl semimetals, the foundational gapless topological phase, chiral degeneracies are isolated near the Fermi level and give rise to the Fermi-arc surface states. However, it is now recognized that chiral degeneracies are ubiquitous in the band structures of systems with broken spatial inversion (P) or time-reversal (T) symmetry. This leads to a broadly defined notion of topological metals, which implies the presence of disconnected Fermi surface sheets characterized by nonzero Chern numbers inherited from the enclosed chiral degeneracies. Here, we address the possibility of experimentally observing surface-related signatures of chiral degeneracies in metals. As a representative system we choose bcc iron, a well-studied archetypal ferromagnetic metal with two nontrivial electron pockets. We find that the (110) surface presents arclike resonances attached to the topologically nontrivial electron pockets. These Fermi-arc resonances are due to two different chiral degeneracies, a type-I elementary Weyl point and a type-II composite (Chern numbers +/- 2) Weyl point, located at slightly different energies close to the Fermi level. We further show that these surface resonances can be controlled by changing the orientation of magnetization, eventually being eliminated following a topological phase transition. Our study thus shows that the intricate Fermi-arc features can be observed in materials as simple as ferromagnetic iron and are possibly very common in polar and magnetic materials broadly speaking. Our study also provides methodological guidelines to identifying Fermi-arc surface states and resonances, establishing their topological origin and designing control protocols.

Theoretical investigations of magnetic properties of MRI contrast agents and carbon nanostructures

Oleg Yazyev

The phenomenon of magnetism is one of the key components of today's technological progress. Magnetic interactions and magnetic materials are essential for the scientific disciplines of physics, chemistry and biology, making this subject truly multidisciplinary. This thesis is devoted to magnetic properties of two classes of substances. The first class represents the complexes of the ions of paramagnetic metals, primarily of the gadolinium(III) ion. These molecular compounds have an important application as magnetic resonance imaging (MRI) contrast agents for medical diagnostics. The design of more efficient MRI contrast agents requires a detailed knowledge of their magnetic properties. The other part of the thesis considers the broad class of recently discovered carbon nanostructures and materials. Their extraordinary physical properties foretell future applications of these materials in electronics, medicine and other fields. For instance, carbon nanotubes loaded with gadolinium(III) ion clusters are highly efficient MRI contrast agents. By using accurate density functional theory calculations in combination with classical molecular dynamics simulations, we determine hyperfine and quadrupole coupling constants on the nuclei of a first coordination sphere water molecule in gadolinium(III) aqua complexes. These parameters play a crucial role in the description of the key function, relaxivity, of MRI contrast agents. We found that the spin-polarization effect induced by the paramagnetic gadolinium(III) ion results in a Fermi contact hyperfine coupling of both the 1H and 17O nuclear spins and affects the dipole hyperfine coupling of the 17O nuclear spin of the inner coordination sphere water molecule. The 17O quadrupole coupling parameters of a coordinated water molecule are found to be very similar to that of neat water. We also apply the methodology of first principles molecular dynamics in order to perform realistic simulations of paramagnetic metal ions in water solution. This allows us to assess structure, dynamics and hyperfine interactions on the water molecules in the inner and outer coordination spheres of two metal ions: chromium(III) and gadolinium(III). In order to perform such calculations, we develop a novel approach for the evaluation of hyperfine coupling constants in pseudopotential electronic structure techniques. Our method takes into account the contribution of core electrons. In the second part of the thesis, we consider magnetic properties of a broad class of carbon nanostructures derived from two-dimensional graphene. We find that in metallic carbon nanotubes, an isotropic Knight shift, a hyperfine contribution to the nuclear magnetic resonance chemical shift, shows a regular dependence on the nanotube diameter. By using a more general approach, we reveal systematic dependences of magnetic interactions between arbitrarily distributed spin-polarized conduction electrons and nuclear spins in the carbon nanostructures derived from graphene. This knowledge is important for interpreting the results of magnetic resonance experiments and for evaluating the performance of carbon nanostructures as materials for alternative approaches in electronics, spintronics and quantum information processing based on electron and nuclear spins. In addition, we study magnetism in graphene induced by single-atom defects. The predicted itinerant magnetism due to the defect-induced states in graphenic materials may account for the experimental observations of ferromagnetism in irradiated graphite which has potential applications in technology. Finally, our first principles molecular dynamics study reveals the mechanisms of the irradiation-induced defect formation. We show that certain defect structures in layered carbon materials can be created selectively by irradiation at predefined conditions.

EPFL2007

Observation of a topological crystalline insulator phase and topological phase transition in Pb1-xSnxTe

Jan Hugo Dil, Chang Liu

A topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction-driven band inversion. The topological phase in the Bi1-xSbx system is due to an odd number of band inversions. A related spin-orbit system, the Pb1-xSnxTe, has long been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry-protected topological crystalline insulator phase in the Pb1-xSnxTe class of materials that has been theoretically predicted to exist in its end compound SnTe. Our experimental results show that at a finite Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi1-xSbx. Our observation of the spin-polarized Dirac surface states in the inverted Pb1-xSnxTe and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices.

2012

Advancing non-equilibrium ARPES experiments by a 9.3 eV coherent ultrafast photon source

Helmuth Berger, Philippe Bugnon, Alberto Crepaldi

The quest for investigating the non-equilibrium dynamics of the band structure of strongly-correlated materials over their entire Brillouin zone is a primary objective. However, the actual ultrafast UV light sources are not suitable for addressing several critical questions in the field. Here we report on a novel light source generating sub-250 fs, 9.3 eV photon energy light pulses at 250 kHz repetition rate, obtained via third-harmonic generation in Xe of frequency-doubled 50 fs laser pulses at 1.55 eV. By reporting the measured band dispersion of a Cu(111) crystal and the non-equilibrium dynamics of the Bi2Se3 topological insulator, we prove that this source is suitable for studying the non-equilibrium dynamics of the entire Fermi surface of several complex materials, with high signal statistics and limited space-charge effect. (C) 2015 Elsevier B.V. All rights reserved.

Elsevier Science Bv2016

Theoretical investigations of magnetic properties of MRI contrast agents and carbon nanostructures

Oleg Yazyev

EPFL2007