[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

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.

Weyl and Dirac semimetals in three-dimensional solids

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

Physical Review B

Journal of Physics Condensed Matter: Preface

Metal–organic frameworks (MOFs) are intrinsically porous extended solids formed by coordination bonding between organic ligands and metal ions or clusters. High electrical conductivity is rare in M...

Electrically Conductive Metal-Organic Frameworks.

We report the synthesis and crystal structure of a new high-temperature form of Ca3P2 The crystal structure was determined through Rietveld refinements of synchrotron powder x-ray diffraction data This form of Ca3P2 has a crystal structure of the hexagonal Mn5Si3 type, with a Ca ion deficiency compared to the ideal 5:3 stoichiometry This yields a stable, charge-balanced compound of Ca2+ and P3− We also report the observation of a secondary hydride phase, Ca5P3H, which again is a charge-balanced compound The calculated band structure of Ca3P2 indicates that it is a three-dimensional Dirac semimetal with a highly unusual ring of Dirac nodes at the Fermi level The Dirac states are protected against gap opening by a mirror plane in a manner analogous to what is seen for graphene

A new form of Ca3P2 with a ring of Dirac nodes

Design principles and predictions of new three-dimensional (3D) Dirac semimetals are presented and placed in the context of currently known materials. Three different design principles are presented (cases I, II, and III), each of which yields predictions for new candidates. For case I, 3D Dirac semimetals based on charge-balanced compounds BaAgBi, SrAgBi, YbAuSb, ${\mathrm{PtBi}}_{2}$, and ${\mathrm{SrSn}}_{2}{\mathrm{As}}_{2}$ are identified as candidates. For case II, 3D Dirac semimetals in analogy to graphene, ${\mathrm{BaGa}}_{2}$ is identified as a candidate, and BaPt and ${\mathrm{Li}}_{2}\mathrm{Pt}$ are discussed. For case III, 3D Dirac semimetals based on glide planes and screw axes, ${\mathrm{TlMo}}_{3}{\mathrm{Te}}_{3}$ and the $A{\mathrm{Mo}}_{3}{X}_{3}$ family, in general ($A=\text{K}$, Na, In, Tl; $X=\text{Se}$,Te), as well as the Group IVb trihalides such as ${\mathrm{HfI}}_{3}$, are identified as candidates. Finally, we discuss conventional intermetallic compounds with Dirac cones and identify ${\mathrm{Cr}}_{2}\mathrm{B}$ as a potentially interesting material.

Three-dimensional Dirac semimetals: Design principles and predictions of new materials

We report the crystal structure of a new polymorph of Ca$_3$P$_2$, and an analysis of its electronic structure. The crystal structure was determined through Rietveld refinements of powder synchrotron x-ray diffraction data. Ca$_3$P$_2$ is found to be a variant of the Mn$_5$Si$_3$ structure type, with a Ca ion deficiency compared to the ideal 5:3 stoichiometry to yield a charge-balanced compound. We also report the observation of a secondary phase, Ca$_5$P$_3$H, in which the Ca and P sites are fully occupied and the presence of interstitial hydride ions creates a closed-shell electron-precise compound. We show via electronic structure calculations of Ca$_3$P$_2$ that the compound is stabilized by a gap in the density of states compared to the hypothetical compound Ca$_5$P$_3$. Moreover, the calculated band structure of Ca$_3$P$_2$ indicates that it should be a three-dimensional Dirac semimetal with a highly unusual ring of Dirac nodes at the Fermi level. The Dirac states are protected against gap opening by a mirror plane in a manner analogous to graphene. The results suggest that further study of the electronic properties of Ca$_3$P$_2$ will be of interest.

Potential ring of Dirac nodes in a new polymorph of Ca$_3$P$_2$

Partial oxidation of an iron–tetrazolate metal–organic framework (MOF) upon exposure to ambient atmosphere yields a mixed-valence material with single-crystal conductivities tunable over 5 orders of magnitude and exceeding 1 S/cm, the highest for a three-dimensionally connected MOF. Variable-temperature conductivity measurements reveal a small activation energy of 160 meV. Electronic spectroscopy indicates the population of midgap states upon air exposure and corroborates intervalence charge transfer between Fe2+ and Fe3+ centers. These findings are consistent with low-lying Fe3+ defect states predicted by electronic band structure calculations and demonstrate that inducing metal-based mixed valency is a powerful strategy toward realizing high and systematically tunable electrical conductivity in MOFs.

Lilia S. Xie

Papers

Electrically Conductive Metal-Organic Frameworks.

A new form of Ca3P2 with a ring of Dirac nodes

Three-dimensional Dirac semimetals: Design principles and predictions of new materials

Potential ring of Dirac nodes in a new polymorph of Ca$_3$P$_2$

Tunable Mixed-Valence Doping toward Record Electrical Conductivity in a Three-Dimensional Metal-Organic Framework.