Huibo Cao

Bio: Huibo Cao is an academic researcher from Oak Ridge National Laboratory. The author has contributed to research in topics: Antiferromagnetism & Neutron diffraction. The author has an hindex of 38, co-authored 260 publications receiving 6262 citations. Previous affiliations of Huibo Cao include DSM & Centre national de la recherche scientifique.

Author Correction: The nature of spin excitations in the one-third magnetization plateau phase of Ba3CoSb2O9.

Abstract: The original version of this Article omitted the following from the Acknowledgements: ‘J. Ma’s primary affiliation is Shanghai Jiao Tong University.’ This has been corrected in both the PDF and HTML versions of the Article.

Spectroscopic evidence for a type II Weyl semimetallic state in MoTe2

Abstract: In a type I Dirac or Weyl semimetal, the low-energy states are squeezed to a single point in momentum space when the chemical potential μ is tuned precisely to the Dirac/Weyl point. Recently, a type II Weyl semimetal was predicted to exist, where the Weyl states connect hole and electron bands, separated by an indirect gap. This leads to unusual energy states, where hole and electron pockets touch at the Weyl point. Here we present the discovery of a type II topological Weyl semimetal state in pure MoTe2, where two sets of Weyl points (, ) exist at the touching points of electron and hole pockets and are located at different binding energies above EF. Using angle-resolved photoemission spectroscopy, modelling, density functional theory and calculations of Berry curvature, we identify the Weyl points and demonstrate that they are connected by different sets of Fermi arcs for each of the two surface terminations. We also find new surface 'track states' that form closed loops and are unique to type II Weyl semimetals. This material provides an exciting, new platform to study the properties of Weyl fermions.

Spectroscopic evidence for type II Weyl semimetal state in MoTe2

Abstract: In a type I Dirac or Weyl semimetal, the low energy states are squeezed to a single point in momentum space when the chemical potential Ef is tuned precisely to the Dirac/Weyl point. Recently, a type II Weyl semimetal was predicted to exist, where the Weyl states connect hole and electron bands, separated by an indirect gap. This leads to unusual energy states, where hole and electron pockets touch at the Weyl point. Here we present the discovery of a type II topological Weyl semimetal (TWS) state in pure MoTe2, where two sets of WPs (W2+-, W3+-) exist at the touching points of electron and hole pockets and are located at different binding energies above Ef. Using ARPES, modeling, DFT and calculations of Berry curvature, we identify the Weyl points and demonstrate that they are connected by different sets of Fermi arcs for each of the two surface terminations. We also find new surface "track states" that form closed loops and are unique to type II Weyl semimetals. This material provides an exciting, new platform to study the properties of Weyl fermions.

Direct evidence of a zigzag spin-chain structure in the honeycomb lattice: A neutron and x-ray diffraction investigation of single-crystal Na 2 IrO 3

Abstract: We have combined single-crystal neutron and x-ray diffractions to investigate the magnetic and crystal structures of the honeycomb lattice Na${}_{2}$IrO${}_{3}$. The system orders magnetically below $18.1(2)$ K with Ir${}^{4+}$ ions forming zigzag spin chains within the layered honeycomb network with an ordered moment of $0.22(1){\ensuremath{\mu}}_{\text{B}}/\text{Ir}$ site. Such a configuration sharply contrasts with the N\'eel or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form a nearly ideal two-dimensional honeycomb lattice while the IrO${}_{6}$ octahedra experience a trigonal distortion that is critical to the ground state. The results of this study provide much needed experimental insights into the magnetic and crystal structure that are crucial to the understanding of the exotic magnetic order and possible topological characteristics in the $5d$-electron-based honeycomb lattice.

Low-temperature crystal and magnetic structure of α -RuCl 3

Abstract: Single crystals of the Kitaev spin-liquid candidate $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ have been studied to determine the low-temperature bulk properties, the structure, and the magnetic ground state. Refinements of x-ray diffraction data show that the low-temperature crystal structure is described by space group $C2/m$ with a nearly perfect honeycomb lattice exhibiting less than 0.2% in-plane distortion. The as-grown single crystals exhibit only one sharp magnetic transition at ${T}_{N}=7$ K. The magnetic order below this temperature exhibits a propagation vector of $k=(0,1,1/3)$, which coincides with a three-layer stacking of the $C2/m$ unit cells. Magnetic transitions at higher temperatures up to 14 K can be introduced by deformations of the crystal that result in regions in the crystal with a two-layer stacking sequence. The best-fit symmetry-allowed magnetic structure of the as-grown crystals shows that the spins lie in the $ac$ plane, with a zigzag configuration in each honeycomb layer. The three-layer repeat out-of-plane structure can be refined as a ${120}^{\ensuremath{\circ}}$ spiral order or a collinear structure with a spin direction of ${35}^{\ensuremath{\circ}}$ away from the $a$ axis. The collinear spin configuration yields a slightly better fit and also is physically preferred. The average ordered moment in either structure is less than 0.45(5) ${\ensuremath{\mu}}_{B}$ per ${\mathrm{Ru}}^{3+}$ ion.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Weyl and Dirac semimetals in three-dimensional solids

Abstract: Weyl and Dirac semimetals are three-dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry. As three-dimensional analogs of graphene, they have generated much recent interest. Deep connections exist with particle physics models of relativistic chiral fermions, and, despite their gaplessness, to solid-state topological and Chern insulators. Their characteristic electronic properties lead to protected surface states and novel responses to applied electric and magnetic fields. The theoretical foundations of these phases, their proposed realizations in solid-state systems, and recent experiments on candidate materials as well as their relation to other states of matter are reviewed.