M. Hoesch

A stable three-dimensional topological Dirac semimetal Cd3As2

Three-dimensional (3D) topological Dirac semimetals (TDSs) are a recently proposed state of quantum matter that have attracted increasing attention in physics and materials science. A 3D TDS is not only a bulk analogue of graphene; it also exhibits non-trivial topology in its electronic structure that shares similarities with topological insulators. Moreover, a TDS can potentially be driven into other exotic phases (such as Weyl semimetals, axion insulators and topological superconductors), making it a unique parent compound for the study of these states and the phase transitions between them. Here, by performing angle-resolved photoemission spectroscopy, we directly observe a pair of 3D Dirac fermions in Cd3As2, proving that it is a model 3D TDS. Compared with other 3D TDSs, for example, β-cristobalite BiO2 (ref. 3) and Na3Bi (refs 4, 5), Cd3As2 is stable and has much higher Fermi velocities. Furthermore, by in situ doping we have been able to tune its Fermi energy, making it a flexible platform for exploring exotic physical phenomena.

Emergence of the nematic electronic state in FeSe

We present a comprehensive study of the evolution of the nematic electronic structure of FeSe using high-resolution angle-resolved photoemission spectroscopy (ARPES), quantum oscillations in the normal state, and elastoresistance measurements. Our high-resolution ARPES allows us to track the Fermi surface deformation from fourfold to twofold symmetry across the structural transition at ∼87K, which is stabilized as a result of the dramatic splitting of bands associated with dxz and dyz character in the presence of strong electronic interactions. The low-temperature Fermi surface is that of a compensated metal consisting of one hole and two electron bands and is fully determined by combining the knowledge from ARPES and quantum oscillations. A manifestation of the nematic state is the significant increase in the nematic susceptibility approaching the structural transition that we detect from our elastoresistance measurements on FeSe. The dramatic changes in electronic structure cannot be explained by the small lattice distortion and, in the absence of magnetic fluctuations above the structural transition, point clearly towards an electronically driven transition in FeSe, stabilized by orbital-charge ordering.

Spin crossover in ferropericlase at high pressure: a seismologically transparent transition?

An iron spin transition has no effect on the seismologic properties of lower-mantle minerals. Seismic discontinuities in Earth typically arise from structural, chemical, or temperature variations with increasing depth. The pressure-induced iron spin state transition in the lower mantle may influence seismic wave velocities by changing the elasticity of iron-bearing minerals, but no seismological evidence of an anomaly exists. Inelastic x-ray scattering measurements on (Mg0.83Fe0.17)O-ferropericlase at pressures across the spin transition show effects limited to the only shear moduli of the elastic tensor. This explains the absence of deviation in the aggregate seismic velocities and, thus, the lack of a one-dimensional seismic signature of the spin crossover. The spin state transition does, however, influence shear anisotropy of ferropericlase and should contribute to the seismic shear wave anisotropy of the lower mantle.

Direct observation of spin-polarized bulk bands in an inversion-symmetric semiconductor

The coupling between spin, valley and layer degrees of freedom in transition-metal dichalcogenides is shown to give rise to spin-polarized electron states, providing opportunities to create and manipulate spin and valley polarizations in bulk solids. Methods to generate spin-polarized electronic states in non-magnetic solids are strongly desired to enable all-electrical manipulation of electron spins for new quantum devices1. This is generally accepted to require breaking global structural inversion symmetry1,2,3,4,5. In contrast, here we report the observation from spin- and angle-resolved photoemission spectroscopy of spin-polarized bulk states in the centrosymmetric transition-metal dichalcogenide WSe2. Mediated by a lack of inversion symmetry in constituent structural units of the bulk crystal where the electronic states are localized6, we show how spin splittings up to ∼0.5 eV result, with a spin texture that is strongly modulated in both real and momentum space. Through this, our study provides direct experimental evidence for a putative locking of the spin with the layer and valley pseudospins in transition-metal dichalcogenides7,8, of key importance for using these compounds in proposed valleytronic devices.

Signature of Strong Spin-Orbital Coupling in the Large Nonsaturating Magnetoresistance Material WTe2.

We report the detailed electronic structure of WTe2 by high resolution angle-resolved photoemission spectroscopy. We resolved a rather complicated Fermi surface of WTe2. Specifically, there are in total nine Fermi pockets, including one hole pocket at the Brillouin zone center Γ, and two hole pockets and two electron pockets on each side of Γ along the Γ-X direction. Remarkably, we have observed circular dichroism in our photoemission spectra, which suggests that the orbital angular momentum exhibits a rich texture at various sections of the Fermi surface. This is further confirmed by our density-functional-theory calculations, where the spin texture is qualitatively reproduced as the conjugate consequence of spin-orbital coupling. Since the spin texture would forbid backscatterings that are directly involved in the resistivity, our data suggest that the spin-orbit coupling and the related spin and orbital angular momentum textures may play an important role in the anomalously large magnetoresistance of WTe2. Furthermore, the large differences among spin textures calculated for magnetic fields along the in-plane and out-of-plane directions also provide a natural explanation of the large field-direction dependence on the magnetoresistance.

Spin-polarized Fermi surface mapping

Abstract The magnetic and electronic properties of itinerant ferromagnets and their interplay have been studied in the last few years by spin resolved electron spectroscopy on one hand and by high-resolution angle-resolved photoemission experiments on the other. We discuss how the two approaches can be combined in a high resolution electron spectrometer with spin resolution for angle-scanned Fermi surface mapping experiments. We have built this new instrument, which allows an advance into a deeper understanding of magnetic thin film or multilayer systems, where band structures become intricately dense in momentum space and where the magnetization direction can change from layer to layer. Spin-resolution is thus required to arrive at a correct assignment of spectral features. A fully three-dimensional polarimeter makes the instrument ‘complete’ in the sense that all properties of the photoelectron are measured. First experiments on Ni(111) conclusively confirm previous band and spin assignments at the Fermi level and demonstrate the correct functioning of the apparatus.

Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate

The need for both high electrical conductivity and low thermal conductivity creates a design conflict for thermoelectric systems, leading to the consideration of materials with complicated crystal structures. Rattling of ions in cages results in low thermal conductivity, but understanding the mechanism through studies of the phonon dispersion using momentum-resolved spectroscopy is made difficult by the complexity of the unit cells. We have performed inelastic X-ray and neutron scattering experiments that are in remarkable agreement with our first-principles density-functional calculations of the phonon dispersion for thermoelectric Na(0.8)CoO2, which has a large-period superstructure. We have directly observed an Einstein-like rattling mode at low energy, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO2. Our results will guide the design of the next generation of materials for applications in solid-state refrigerators and power recovery.

Observation of large topologically trivial Fermi arcs in the candidate type-II Weyl semimetal WT e 2

The authors present here several advances towards a comprehensive understanding of WTe

Spin-orbit coupling in the L-gap surface states of Au(111): spin-resolved photoemission experiments and first-principles calculations

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Fermi Arcs and Their Topological Character in the Candidate Type-II Weyl Semimetal MoTe 2

. Using microfocus laser ARPES, they resolve for the first time the distinct electronic structure of both inequivalent top and bottom (001) surfaces of WTe